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LTE L11 Throughput Troubleshooting
Techniques
Introduction
Why learn about Throughput Troubleshooting
› LTE provides data, lots of data
› Throughput is shared in time
and frequency
› Users notice throughput
problems
› Learn to troubleshoot the LTE
RAN for throughput problems
› Learn to isolate the domain
causing throughput
degradation
Scope and objectives
› Isolate throughput problems into
domains
› Pinpoint causes of throughput
degradation clearly within
domains through theory, traces
and practical examples
Objectives
Scope
› RBS Initial Checks
› Radio Analysis
› Transport Analysis
› End-2-End Analysis
> Overview
Agenda
1. Overview
2. Initial Checks
3. Radio Analysis
4. Transport Analysis
5. E2E Analysis
LTE RBS User plane
Overview
LTE RBS User Plane Overview
› User plane visualisation
› Network assumptions
› L11 software limitations
User Plane Domains
eNB S-GW
UE
Internet
FTP Server
PDCP
RLC
MAC
Physical
PDCP
RLC
MAC
Physical
GTP-U
UDP
IP
Data Link
Physical
GTP-U
UDP
IP
Data Link
Physical
GTP-U
UDP
IP
Data Link
Physical
GTP-U
UDP
IP
Data Link
Physical
IP
IP IP
FTP Serv.
FTP Client
Radio Domain Transport Domain
e2e Domain
Uu S1-U S5/S8
SGi
IP Backbone
MME
HSS
Relay Relay
S1-MME
S6a
S11
P-GW
S5/S8
S1-U
S1
Uu
Data Flow over Air (RBS/UE)
CRC
Payload
Payload Payload
CRC
Header Header Header
Payload Payload Payload
Header Header Header
PDCP
Header
PDCP
Header
PDCP
Header
PDCP
RLC
MAC
RLC
Header
RLC
Header
RLC
Header
MAC
Header
MAC
Header
PDCP PDU
RLC PDU
MAC PDU
Transport Block
PHY
Transport Block
ARQ
HARQ
RLC SDU
PDCP SDU
network assumptions
› Network configuration and integration is complete
› Data sessions have been previously verified
› Basic RBS troubleshooting has been performed:
– Node alarms verified
– MO status
– Cell availability
› See the ?Fundamental RBS Troubleshooting Techniques? for further
checks that can be performed
Release Limitations
› L11A contains some limitations that directly affect end user throughput
– One SE per TTI in UL and DL (in L11A GA)
› Each cell is treated individually, so there could be up to 3 users
simultaneously in an eNB
› SIB is scheduled the same as user data, so nothing can be scheduled
at the same time as SIB
– DUL user plane capacity limited to 150 Mbps (20MHz)
– 100PRBs UL, 150 PRBs DL.
– 16QAM UL (up to MCS24)
– MCS28 disabled in DL by default (requires CFI=1 also)
– Fixed CFI (number of OFDM symbols for PDCCH)
› Default is CFI=2 for 5MHz and less.
› CFI=3 not possible for >= 10MHz due to system limitations, this
reduces PDCCH scheduling opportunities.
Initial Checks
Initial checks
› For throughput issues, some essential checks are required:
1. Network changes & Basic troubleshooting
2. PC/Server settings
3. UE categories
4. UE subscriber profile
5. RBS parameters
6. Enabled features
NW Changes and Basic Troubleshooting
› Network/node changes can affect network throughput
› Some common examples include:
– Network configuration changes (e.g. adding/changing/removing hardware)
– RBS parameter changes (all MOs under ENodeBFunction, system
constants, EricssonOnly hidden parameters, e.g. DataRadioBearer)
– IP address plan changes
– Transport Network changes (add/reduce capacity on TN)
– DNS updates
– Hint:
› To see RBS level changes (MOs/parameters): Moshell> lgo
› To capture detailed RBS level logs: Moshell> dcg
› Basic troubleshooting checks include:
– Alarm, event and system log checks
– MO health status
PC/Server Settings
› Determine the applications or tools used in testing/monitoring throughput
› Confirm the end user PC settings:
– Laptop specification can impact throughput (processors, memory, USB bus, HDD
speed, plugged into AC power, etc)
– MTU settings in PC (1360 optimal for eNB in L11A to prevent fragmentation)
– Throughput monitors (e.g. Netpersec, only good for downlink UDP measurements,
uplink must be measured at receiving side for UDP)
– TCP enhancements in Vista (experimental), Vista should “auto-tune”.
› Confirm server settings:
– FTP server configuration
– Linux TCP setting/guide
– iperf (UDP & TCP) – be sure to use packet size 1360 for UDP (not default 1470).
– Always check first with UDP rather than TCP, as UDP is less prone to display
problems as a result of jitter variations and packet loss.
UE Categories
UE Category Maximum number of DL-SCH
transport block bits
received within a TTI
Maximum number of bits
of a DL-SCH transport
block received within a
TTI
Total number of
soft channel
bits
Maximum number of
supported layers for
spatial multiplexing in
DL
Category 1 10296 10296 250368 1
Category 2 51024 51024 1237248 2
Category 3 102048 75376 1237248 2
Category 4 150752 75376 1827072 2
Category 5 299552 149776 3667200 4
› The UE Category limits throughput
possibilities
› 5 UE Categories are defined in 3GPP
TS 36.306
› The UE-Cat is sent in the UE Capability
Transfer procedure (RRC
UECapabilityInformation)
› The COLI ue command provides
detailed capability info (KO) for
connected UEs
UE Category Maximum number of bits
of an UL-SCH
transport block
transmitted within a
TTI
Support for 64QAM
in UL
Category 1 5160 No
Category 2 25456 No
Category 3 51024 No
Category 4 51024 No
Category 5 75376 Yes
DL UL
UE Subscriber profile
› End User (EPS User) subscription data is stored in the HSS
› The EPS User Profile data is identified by its IMSI number
› The profile consists of:
– MSISDN number
– Operator Determined Barring (ODB)
– APN Operator Identifier Replacement
– Subscribed Charging Characteristics
– Aggregate Maximum Bit Rate (AMBR)
› Max requested bandwidth in Downlink
› Max requested bandwidth in Uplink
– RAT frequency selection priority
– APN configuration profile:
› Default Context Identifier (default APN for the EPS User)
› APN Configuration (every APN associated to the EPS User)
RBS Parameters RN
› RN MO parameters:
– EUtranCellFDD
› dlChannelBandwidth / ulChannelBandwidth
› (nrOfSymbolsPdcch) (Control Region Size)  NOTE: currently controlled by SC38 in L11A
› noOfUsedTxAntennas  controls whether OLSM MIMO is used (2) or not.
› partOfRadioPower  NOTE: this is the % part of RU capability independent of
SectorEquipmentFunction::confOutputPower settings
› pZeroNominalPucch  some UEs need this to be increased or ACK/NACKs are not
received successfully on PUCCH.
› pZeroNominalPusch  some UEs need this to be increased from default or lots of errors
seen on PUSCH
– SectorEquipmentFunction=Sx
› confOutputPower / fqBand (readOnly)
– DataRadioBearer
› Various parameters for RLC status reporting and retransmission. Should be set to
recommended values.
– MACConfiguration
› xxMaxHARQTx – enable (>1) or disable (1) HARQ. Recommended to use 4 HARQTx.
› tPeriodicBSRTimer – seen in UE testing to have some impact, recommend to set to 5ms.
› tTimeAlignmentTimer – seen in UE testing to have some impact, recommend to set to
5120ms
RBS PARAMTERS TN
› TN MO parameters:
– GigabitEthernet=1
› actualSpeedDuplex – if you see half-duplex, it could be a problem with auto-
negotiation
› dscpPbitMap (QoS mapping from L3 to L2)
– IpInterface=2 (rec. MO id for Signalling and Payload)
› vLan/vid (true/false and vlan id)
– IpAccessHostEt=1
› ipAddress (X2/S1 control/user plane termination)
– IpSyncRef (if NTP synchronisation is used)
› syncStatus should be OK
– Synchronization=1
› nodeSystemClock should be in LOCKED_MODE.
› syncReference should show the correct reference (NTP or GPS) active and
configured
Enabled Features
› User throughput can be limited by the available/installed licenses
› The following features directly impact end user throughput
– Downlink/Uplink Baseband Capacity
– Channel Bandwidth (5, 10, 15 and 20) MHz
– 64-QAM DL / 16-QAM UL
– Dual Antenna DL Performance Package
› To quickly check active licenses (including states):
– moshell> inv
Licensing (9/1551-LZA 701 6004 )
Expected Throughput (Simplified)
dlCyclicPrefix = 15 KHz => 7 OFDM symbols
Resource Elements (RE) per Resource Block
(7 OFDM symbols x 12 SubCarriers)
RE per SB
2 x RB
RS RE (per RB)
RS RE (per SB)
Control Region Size (CRS) in OFDM symbols
nrOfSymbolsPdcch 1 2 3 1 2 3
RE per CRS
(OFDM*12 - 4 RS Tx)
(OFDM*12 - 8 RS MIMO) 8 20 32 16 40 64
Tot Num RE per SB available for PDSCH
(best case w/o SCH/BCH) 144 132 120 288 264 240
Bits per SB - QPSK (2) 288 264 240 576 528 480
Bits per SB - 16QAM (4) 576 528 480 1152 1056 960
Bits per SB - 64QAM (6) 864 792 720 1728 1584 1440
Max Theoretical L1 Thrpt (Mbps)
20 MHz => 100 RB (64 QAM) 86.4 79.2 72 172.8 158.4 144
15 MHz => 75 RB (64 QAM) 64.8 59.4 54 129.6 118.8 108
10 MHz => 50 RB (64 QAM) 43.2 39.6 36 86.4 79.2 72
5 MHz => 25 RB (64 QAM) 21.6 19.8 18 43.2 39.6 36
Tot Num RE per SB available for PDSCH
(worst case with SCH/BCH in SB)
SCH = 24, BCH = 4 x 12 - 4 per CW 76 64 52 152 128 104
Bits per SB - (QPSK) 152 128 104 304 256 208
Bits per SB - (16QAM) 304 256 208 608 512 416
Bits per SB - (64QAM) 456 384 312 912 768 624
Tx Diversity 2x2 MIMO
84
DL Scheduling Block (SB) -> Bit calculation
(Normal CyclicPrefix)
168
16 32
168 336
8 16
Identify the domain
› Further analysis required:
– Our basic checks have come up short
– Throughput issues exist that require advanced/additional analysis
› Analysis steps to perform:
– Single UE call scenario
– Send UDP type traffic in DL/UL direction (e.g. Iperf)
– Monitor close to or on the RBS (e.g. Wireshark)
– Optionally use a radio monitor (e.g. TEMS)
› Decide - Radio or Transport analysis:
– Radio issues provide more control for LTE RAN analysis
– Transport issues blend/carry-on towards core elements
Radio Analysis
Radio Analysis
› From the domain analysis previously, we believe the Radio may be affecting
user throughput
› We’ve previously ruled out configuration and MO status using the basic checks
› The following slides will cover the various components which make up the
radio domain and help to pinpoint the source of poor throughput.
› We rely on the baseband scheduler traces and signal traces (mtd) between
blocks.
Radio Analysis
› Ericsson’s LTE Baseband provides a detailed mechanism for tracing the
complete L1 and L2 interaction, including MAC scheduling decisions and L1
decoding results.
› Using this information we can further isolate the cause of the problem and
pinpoint either:
– UE problem
› Cannot detect ACK/NACK?
› Invalid UE reports?
– Uu air interface problem
– eNB problem
› Incorrect setting or non-optimal combination of settings
› Scheduling abnormality
› Limitation in current eNB software
– eNB northbound problem
› S1 user plane
› Application Server
› Core network, SASN/SGW, etc
RADIO ANALYSIS
› To perform targeted radio analysis, it’s useful to know radio aspects
specific to the following traffic scenarios:
1. Downlink
2. Uplink
3. Both uplink and downlink
› Post-processing tools will be briefly demonstrated
Radio Analysis - Downlink
› Areas of analysis for Downlink:
– CQI (Channel Quality Index) and RI (Rank Indicator) reported from UE.
– Transmission Mode: MIMO (tm3) vs. TxD (tm2) vs. SIMO (tm1)
– MCS vs. number of assigned PRBs vs. assignable bits in scheduler
– UE Scheduling percentage of TTIs (how often is the UE scheduled)
– CFI (number of OFDM symbols for PDCCH) vs. MCS vs. % scheduling
– HARQ
– RLC retransmissions
Radio Analysis – Uplink
› Areas of analysis for Uplink:
– Uplink scheduling overview
– BSR (Buffer Status Report)
– PHR (Power Headroom Report) – is the UE at maximum power?
– Cell bandwidth vs. maximum allowable PRBs
– Link Adaptation
– MCS available and 16QAM
– PDCCH SIB scheduling colliding with UL grant
– HARQ (less important, because we can measure SINR)
Radio Analysis DL – CQI/RI and TM
› The eNB needs knowledge of the SINR conditions of downlink transmission to
a UE in order to select the most efficient MCS/PRB combination for a selected
UE at any point in time.
› Channel Quality Index (CQI):
– Is a feedback mechanism from UE to eNB
– Informs eNB of current channel conditions as seen at UE
– Directly maps to 3GPP defined modulation/code rate (TS36.213 Table 7.2.3-1)
› Defined as the highest coding rate the UE could decode at 10% BLER on HARQ
rv=0 transmission
– CQI 1-6 map to QPSK
– CQI 7-9 map to 16QAM
– CQI 10-15 map to 64QAM
› Rank Indicator (RI)
– Is a feedback mechanism from UE to eNB
– Informs eNB whether UE can successfully decode RS from 1 or 2 (or more)
antennas.
– eNB scheduler uses this feedback to transmit with either:
CQI
polling
Radio analysis DL – CQI/RI and TM
› The UE measures DL channel quality
and reports to eNodeB in the form of
Channel Quality Information (CQI)
› The average CQI (periodic-CQI
reporting) for the whole band (wide-band
CQI) is reported periodically on PUCCH
(or on PUSCH if user data is scheduled
in that TTI) with configured periodicity.
› Sub-band CQI (aperiodic-CQI reporting)
is reported when requested by the eNB.
This report is for the PDSCH. Report
sent on PUSCH.
– CQI polling is triggered on demand
by eNB based on DL traffic activity.
› When 2 antennas are configured, Rank
Indicator is also reported. Precoding
Matrix Indicator (PMI) also reported in
case of transmission mode 4 (not in
L11A).
CQI
DL
frequency
band
PUCCH
PUCCH
PUSCH
Radio Analysis DL – CQI/RI and TM
› In order to transmit with MIMO (OLSM) we should check the following:
– eNB cell is configured with two working transmit antennas.
› Check EUtranCellFDD::noOfUsedTxAntennas > 1
› L11A GA (default) system constant SC125:3 means that tm3 is used in case 2
TX antennas are defined.
› If only one TX antenna is configured, then tm1 is used
› In order to force Transmit Diversity (i.e. prevent OLSM), SC125:2 must be set
– UE CQI/RI report from UE shows RI > 1
› Rank 1: TxDiversity (transmission mode 2, tm2)
› Rank 2: MIMO (Open Loop Spatial Multiplexing in L11A) (transmission mode 3,
tm3)
› mtd peek -ta ulMacPeBl -signal LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND -dir
OUTGOING
– This signal (from L1 to MAC scheduler) shows the reported CQI and RI
– (also shows HARQ ACK/NACK for downlink data transmission)
– (also shows rxPowerReport and timingAdvanceError)
cfrPusch { cfrInfo { ri = 2, cfrLength = 22, cfrFormat = 4, cfrValid = 1, cfrExpected = 1,
cfrCrcFlag = 1 }, cfr[] = [61440, 0, 0, 0] as hex: [f0 00 00 00 00 00 00 00] }
Radio Analysis DL – CQI/RI and TM
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND UpUlMacPeCiUlL1Meas2DlIndS {
cfrPucch { cfrInfo { ri = 0, cfrLength = 4, cfrFormat = 0, cfrValid = 1, cfrExpected = 1,
cfrCrcFlag = 1 }, cfr[] = [0, 0] as hex: [00 00 00 00] }
cfrFormat=0 is a WCQI report only (ignore RI)
Valid report if cfrValid=1,cfrExpected=1,cfrCrcFlag=1
mtd peek -ta ulMacPeBl -signal LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND -dir OUTGOING
cfrFormat=4 is a SCQI + RI report
WCQI is first half octet (f => 15). Octets thereafter are
subband CQI reports for each RBG.
A number of subband CQIs follow (see next slide)
cfrPusch { cfrInfo { ri = 2, cfrLength = 18, cfrFormat = 4, cfrValid = 1, cfrExpected = 1,
cfrCrcFlag = 1 }, cfr[] = [48969, 49152, 0, 0] as hex: [bf 49 c0 00 00 00 00 00] }
Rank Indicator = 2 (indicates UE
can decode both antenna streams)
WCQI = 11. 5MHz bandwidth means 4PRBs subbands.
SCQI = F49C = 11 11 01 00 10 01 11 00
SCQI PRBs: 0-3  -1, 4-7  -1, 8-11 +1, 12-15 0, 16-19  +2, 20-23 +1, 24  -1
Radio Analysis DL – SCQI Visualisation
› From the previous slide, SCQI is visualised here..
– For 5MHz, each RBG is 4 PRBs wide (except for SCQI group 7)
– SCQI is given relative to WCQI which was 11 in this example
f
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
5 MHz
SCQI PRBs: 0-3  -1, 4-7  -1, 8-11 +1, 12-15 0, 16-19  +2, 20-23 +1, 24  -1
Sub-band 1 2 3 4 5 6 7
CQI value (  10  10 12 11  13 12  10 )
Radio analysis DL – CQI/RI and TM
› cfrFormat = 4 consists of:
– 4 bit Wideband CQI (i.e. CQI across whole bandwidth)
– Up to 13 subband CQI differentials (depends on bandwidth of cell)
› Subband CQI (3GPP TS36.211 Ch 7.2.1)
– RBG width depends on bandwidth:
› 3 & 5MHz – subband width 4 PRBs
› 10MHz – subband width 6 PRBs
› 15 & 20MHz – subband width 8 PRBs
– Subband Differential mapping, see table below:
Radio analysis DL – CQI/RI and TM
cfrFormat Report includes
0 WCQI
1 RI
2 WCQI + WPMI
3 SCQI
4 SCQI + RI
5 WCQI + SPMI + RI
6 SCQI + WPMI + RI
› 7 possible cfrFormats defined in L11A.
› Typically see reports cfrFormat 0 and 4 as described previously
› Note that PMI is not yet used (requires tm4)
Radio analysis DL – CQI/RI and TM
› Transmission Mode and MCS can be traced out with the following:
ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61
bbUeRef=201327456 HARQ idx=1 tbs={7992 0} mcs={18 0} noOfSBs={4294443008 0} rv={0 1} ndi={0
0} rmGbits={21600 0}"
MCS for each codeword. In this case, tm2 so only one MCS listed.
lhsh gcpu01024 te e trace4 UpcDlMacCeFt_DL_SCHEDULER
LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS {
sfn = 280
subframeNr = 7
l1Control {
transmissionMode = 2
prbResourceIndicatorType = 0
prbList[] = [4294443008, 0, 12, 0]dec
[ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex
commonTb { newDataFlag = 1, tbSizeInBytes = 999, l1Tb { rvIndex = 0, modType = 2
(UPDLMACPEMode64Qam), nrOfRateMatchedBits = 21600, rmSoftBits = 1237248 } }
PRB list in RBGs, for 5MHz RBG
size is 2. fff8 corresponds to 25
PRBs (last PRB is 1 less).
MCS is a combination of tbSize and modType.
999 bytes = 7992 bits then put into TS36.213 Table 7.1.7.2.1-1 for
NPRB=25. That gives ITBS of 16.
Convert ITBS to MCS using Table 7.1.7.1-1.
mtd peek -ta dlMacPeBl -signal LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND -dir INCOMING
-filter {(U16SIG)8,NEQ,(U16)0x00}
Radio Analysis DL – CQI/RI and TM
› RBG for Resource Allocation Type 0
– Defined in 3GPP TS36.213 Ch 7.1.6.1
– One bit used to represent a certain number of consecutive PRBs
– 1.4MHz is RBG size 1
– 3 & 5MHZ is RBG size 2
– 10MHz is RBG size 3
– 15 & 20MHz is RBG size 4
Radio analysis DL – CQI/RI and TM
› Example of switching transmission modes based upon RI
(bfn:3352, sfn:280, sf:5.47, bf:128) ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 :
Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=1 tbs={7992 0} mcs={18 0}
noOfSBs={4294443008 0} rv={0 1} ndi={0 0} rmGbits={21600 0}"
TM=2 transmission with MCS 18
cfrPusch { cfrInfo { ri = 2, cfrLength = 18, cfrFormat = 4, cfrValid = 1, cfrExpected = 1,
cfrCrcFlag = 1 }, cfr[] = [48969, 49152, 0, 0] as hex: [bf 49 c0 00 00 00 00 00] }
Rank Indicator = 2 received from UE. eNB will now
switch to tm3 (OLSM MIMO) transmission
WCQI 11 + SCQI.
bfn:3352, sfn:280, sf:6.47, bf:131) ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 :
Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=0 tbs={5736 5736} mcs={13 13}
noOfSBs={4294443008 0} rv={0 0} ndi={0 1} rmGbits={14400 14400}"
25 PRBs as according to previous example
TM=3 with MCS=13. TBS = 5736 => 11448 over NPRB=50
Radio Analysis DL – Assignable Bits
› If UE is sending with high CQI (in the range 10-15) and RI=2 but
throughput is still very low, then the next check should be assignable
bits.
› Assignable bits means the amount of data in the downlink buffer
available for the scheduler to schedule for this UE.
› A classic symptom of low assignable bits is that the UE is scheduled
with a high MCS but a low number of PRBs.
– The scheduler always attempts to send with the highest possible MCS and
least number of PRBs so that left-over PRBs could be assigned to another
UE.
› Another symptom is that the UE is not scheduled every TTI (and
nothing else is available to schedule).
Radio Analysis DL – Assignable Bits
› Possible causes for low assignable bits:
1. RLC STATUS messages are not being received fast enough and RLC
buffers are full.
› Until RLC STATUS ACK messages are received, already transmitted
RLC SDUs are kept in memory in UE and/or eNB
› Check for RLC DISCARDs but low (or 0) assignable bits
2. Data received from core network is not enough to fill the RLC buffers in
eNB.
› Check that non-TCP based traffic is not being sent with too large
packet size. For iperf based traffic, recommended size 1360 bytes
(default is 1470).
› Set MTU of 1360 in UE (or UE laptop).
› RLC DISCARDs will trigger TCP congestion control and lower thpt.
› In L11A the default RLC buffer size per RB is 750 IP packets
– Trace discards with lhsh gcpu00768 te e all UpDlPdcpPeFt_DISCARD
– Discards on UDP traffic will not affect throughput
– Discards on TCP traffic will trigger TCP congestion control (lower thpt.)
Radio Analysis DL – Assignable Bits
ULMA3/UpDlPdcpPeFt_DISCARD TRAFFIC_ABNORMAL Discarding DL PDCP PDU due to exceeding limits.
maxBufferedPacketsInRlc=751 totalNumNonAckedDrbPackets=751 cellId=12 bbUeRef=201327456
bbBearerRef=201327458 receiveFromTeid=3779046158 payloadLength=1506 bytes incl GTP-U header.
hoState=0"
750 is default PDCP/RLC buffer per UE in eNB (L11A)
TRAFFIC_ABNORMAL corresponds to trace1. Traffic discards for UDP are
normal, but for TCP traffic it will cause severe throughput degradation
ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61
bbUeRef=201327456 PQ lcid=1 assignableBits=0 minPduSize=56 selectedHarq=0"
ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61
bbUeRef=201327456 PQ lcid=2 assignableBits=0 minPduSize=56 selectedHarq=0"
ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61
bbUeRef=201327456 PQ lcid=3 assignableBits=8554024 minPduSize=56 selectedHarq=0"
LCID 3 is for the default bearer. LCID 1 and 2 for SRB
About 1MByte of data available for scheduling. Check for low
value of assignable bits which indicates e2e problems
affecting data available to schedule on air for eNB. Low
assignable bits for UDP traffic may indicate MTU problems.
lhsh gcpu00768 te e all UpDlPdcpPeFt_DISCARD
lhsh gcpu01024 te e trace4 UpcDlMacCeFt_DL_SCHEDULER
Radio Analysis DL – CFI and Scheduling
› Another cause of low (or lower than expected) throughput is that the
UE is not being scheduled in every TTI.
› This may be caused by:
– Limitations in current scheduler implementation
– 3GPP defined compromises between control channel efficiency and
scheduling efficiency (especially for lower number of users)
› L11A software has some limitations to be aware of:
– Only one SE per TTI is supported in L11A
– SIBs are scheduled in the same was a user data (i.e. they are sent to the
scheduler).
– When a SIB is transmitted, no user data can be transmitted in the DL at the
same time (using default parameters).
› It is possible to use (System Constant) SC43 to enable 2SE/TTI in DL
› It is possible (only for demo use) to temporarily disable SIB scheduling
Radio Analysis DL – CFI and Scheduling
› SIBs require PDCCH resources
› Typically SIBs consume 4 or 8 CCEs of PDCCH resources.
› If a UE is in good SINR conditions, the scheduler may allocate only
one CCE for that UE.
– In that case, because of limited positions in PDCCH, it is quite likely that a
PDCCH collision occurs (especially in low system bandwidths)
› If a UE is in bad SINR conditions, the scheduler may allocate a large
number of CCEs for that UE (2 or 4 or 8 CCEs)
– Depending on the configured CFI there may only be common search space
available or it may still collide with other PDCCH users.
› See Radio Analysis UL – PDCCH slides for more details
Radio Analysis Dl – HARQ
› Each transport block transmission is represented as a HARQ process.
– Each HARQ process data is held in memory until NDI is toggled (i.e. New data is to
be sent).
– This allows fast retransmission of erronerously received data.
› The schedulers representation of an HARQ process is as follows:
– Feedback status
› (ACK, NAK, DTX, PENDING)
– TBS – transport block size
– MCS – modulation and coding scheme
– RV – redundancy version. HARQ has 4 redundancy versions, rv0, rv2, rv3, rv1.
– NDI – New Data Indicator (physical layer bit toggled for new data).
› Do not confuse with newDataFlag which is scheduler internal flag where 1
means new data and 0 means retransmission.
– Number of transmission attempts (max 4 transmissions in L11A default paramters)
› In case of rank 2 spatial multiplexing there are 16 HARQ process per UE
instead of 8, but there are two processes that share the same ID
– Scheduler sees them as separate processes that are coupled to each other
Radio Analysis Dl – HARQ Example
› The following slides will show an example of tracing out downlink
HARQ
– Initial downlink grant is sent with rv=0 (MIMO, 2 codewords)
› SFN 280/subframe 8
– HARQ NACK received on both code words
› SFN 281/subframe 2 (DL Grant + 4TTI)
– First retransmission sent with rv=2
› SFN 281/subframe 6 (8 TTI past initial transmission is earliest occasion)
– HARQ ACK received on both code words
› SFN 282/subframe 0 (DL Grant ReTx + 4TTI)
Radio Analysis Dl – HARQ DL Grant
LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS {
sfn = 280
subframeNr = 8
ueAlloc[0] {
l1Control {
rnti = 61
transmissionMode = 3
prbResourceIndicatorType = 0
prbList[] = [4294443008, 0, 12, 0]dec
[ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex
swapFlag = 0
}
nrOfTb = 2
tbAlloc[0] {
tbIndex = 0
commonTb { newDataFlag = 1, tbSizeInBytes = 717, l1Tb { rvIndex = 0,
modType = 1 (UPDLMACPEMode16Qam), nrOfRateMatchedBits = 14400, rmSoftBits =
1237248 } }
macTb { dlHarqProcessId = 0, nrOfMacCtrlElem = 0 }
rlcTb { nrOfBearer = 1, bearerAlloc[0] { bbBearerRef = 201327458, lcid =
3, rbScheduledSizeInBytes = 717 } }
}
tbAlloc[1] {
...
SFN/subframe where DL PDSCH will occur.
PDCCH DL Grant sent at same sfn/subframe.
RNTI, TM, used PRBs (same for both code words)
If re-transmission, this indicates if CW0 and CW1 swapped layers
newDataFlag indicates if it is new data or not
HARQ redundancy version. rv0 used for initial transmission, rv2,
rv3, rv1 used for re-transmission.
HARQ process number. 8 HARQ processes
exist in FDD LTE L11A.
CW1 defined here.
Radio Analysis Dl – HARQ FEEDBACK
(NACK/NACK)
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND (431) UpUlMacPeCiUlL1Meas2DlIndS {
sigNo = 23070220
header {
cellId = 12
sfn = 281
subFrameNo = 2
}
nrOfPuschReports = 0
nrOfPucchReports = 1
totalNrOfReports = 1
reportList[0] {
pucchReport {
meas2DlUlReportType = 1 (ElibBbBaseCommonMeas2DlPucchReport)
bbUeRef = 201327456
isDtx { isDtx = 0 }
dlHarqInfo { dlHarqValid = 1, detectedHarqIndication = 0, dlHarqProcessId
= 0, nrOfTb = 2, swapFlag = 0 }
rxPower { prbListStart = 0, prbListEnd = 0, rxPowerReport = -1150, sinr =
0 }
timingAdvanceError { timingAdvanceError = 1 }
cfrPucch { cfrInfo { ri = 0, cfrLength = 0, cfrFormat = 0, cfrValid = 0,
cfrExpected = 0, cfrCrcFlag = 0 }, cfr[] = [0, 0] as hex: [00 00 00 00] }
}
}
}
SFN/subframe +4 from DL grant (i.e. where the
HARQ ACK/NACK is received from UE).
HARQ NACK received for DL HARQ Process 0 on both code
words.
DetectedHarqIndication: 0 => NACK/NACK, 1 => NACK/ACK,
2 => ACK/NACK, 3 => ACK/ACK, 4 => DTX (nothing received)
Radio Analysis Dl – HARQ ReTX
LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS {
sfn = 281
subframeNr = 6
ueAlloc[0] {
l1Control {
rnti = 61
transmissionMode = 3
prbResourceIndicatorType = 0
prbList[] = [4294443008, 0, 12, 0]dec
[ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex
swapFlag = 0
}
nrOfTb = 2
tbAlloc[0] {
tbIndex = 0
commonTb { newDataFlag = 0, tbSizeInBytes = 717, l1Tb { rvIndex = 2,
modType = 1 (UPDLMACPEMode16Qam), nrOfRateMatchedBits = 14400, rmSoftBits =
1237248 } }
macTb { dlHarqProcessId = 0, nrOfMacCtrlElem = 0 }
rlcTb { nrOfBearer = 0 }
} tbAlloc[1] {
tbIndex = 1
commonTb { newDataFlag = 0, tbSizeInBytes = 717, l1Tb { rvIndex = 2,
modType = 1 (UPDLMACPEMode16Qam), nrOfRateMatchedBits = 14400, rmSoftBits =
1237248 } }
SFN/subframe where DL PDSCH will occur.
PDCCH DL Grant sent at same sfn/subframe.
RNTI, TM, used PRBs (same for both code words)
Same as previous transmission
newDataFlag=0 means it’s a retransmission
HARQ redundancy version. rv2 is used for first retransmission
HARQ process number (same as before)
CW1 defined here.
Radio Analysis Dl – HARQ FEEDBACK (ACK/ACK)
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND (431) UpUlMacPeCiUlL1Meas2DlIndS {
sigNo = 23070220
header {
cellId = 12
sfn = 282
subFrameNo = 0
}
nrOfPuschReports = 0
nrOfPucchReports = 1
totalNrOfReports = 1
reportList[0] {
pucchReport {
meas2DlUlReportType = 1 (ElibBbBaseCommonMeas2DlPucchReport)
bbUeRef = 201327456
isDtx { isDtx = 0 }
dlHarqInfo { dlHarqValid = 1, detectedHarqIndication = 3, dlHarqProcessId
= 0, nrOfTb = 2, swapFlag = 0 }
rxPower { prbListStart = 0, prbListEnd = 0, rxPowerReport = -1152, sinr =
0 }
timingAdvanceError { timingAdvanceError = 0 }
cfrPucch { cfrInfo { ri = 0, cfrLength = 0, cfrFormat = 0, cfrValid = 0,
cfrExpected = 0, cfrCrcFlag = 0 }, cfr[] = [0, 0] as hex: [00 00 00 00] }
}
}
}
SFN/subframe +4 from DL grant (i.e. where the
HARQ ACK/NACK is received from UE).
HARQ ACK/ACK received for DL HARQ Process 0 on both
code words.
DetectedHarqIndication: 0 => NACK/NACK, 1 => NACK/ACK,
2 => ACK/NACK, 3 => ACK/ACK, 4 => DTX (nothing received)
Radio Analysis DL – RLC
› RLC retransmissions are triggered:
1. When HARQ fails to transmit a transport block within the maximum number of
configured retransmissions
– Default number of HARQ transmissions is 4 in L11A
2. If RLC STATUS messages are not received within the time frames configured
› RLC STATUS messages are sent between peer nodes (eNB and UE) to
inform about lost RLC packets. They can be traced out using
– mtd peek -ta dlRlcPeBl -si UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND
› Check:
– ACK_SN should be increasing, otherwise RLC buffers are not released
– NACK_SN indicates RLC retransmissions (occasionally is OK)
– DataRadioBearer::tStatusProhibit governs how often RLC STATUS messages may
be generated, default is 25ms in L11A.
› A too low value will produce too many RLC control messages
› A too high value may cause RLC buffers to become exhausted
Radio Analysis DL – RLC
0xd4205d4f=(sfn:322, sf:0.33, bf:212): UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND
(343) UpDlRlcPeRlcStatusForDlTrafficIndS {
RLC PDU {
D/C = 0 (Control PDU)
CPT = 000 (STATUS PDU)
ACK_SN = 743
E1 = 0 (A set of NACK_SN, E1 and E2 does not follow)
}
}
Indicates the SN (Sequence Number) of the last
successfully received RLC packet
NACK_SN indicates RLC retransmissions (HARQ failures)
0xd4205d4f=(sfn:324, sf:5.33, bf:212): UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND
(343) UpDlRlcPeRlcStatusForDlTrafficIndS {
RLC PDU {
D/C = 0 (Control PDU)
CPT = 000 (STATUS PDU)
ACK_SN = 787
E1 = 0 (A set of NACK_SN, E1 and E2 does not follow)
}
}
Check that ACK_SN is increasing or RLC buffers not released
Next RLC STATUS received 25ms later, check
that it’s coming regularly
DataRadioBearer::tStatusProhibit=25ms default
Radio Analysis – Uplink
› Areas of analysis for Uplink:
– Uplink scheduling overview
– BSR (Buffer Status Report)
– PHR (Power Headroom Report) – is the UE at maximum power?
– Cell bandwidth vs. maximum allowable PRBs
– Link Adaptation
– MCS available and 16QAM
– PDCCH SIB scheduling colliding with UL grant
– HARQ (less important, because we can measure SINR)
Radio Analysis UL – UPlink Scheduling
UL
› Scheduling request, SR
(PUCCH) UE requests UL
resources
eNodeB
UL scheduler
Ue
Channel
state info
› Data is transmitted (PUSCH)
› UL Grant (PDCCH)
Scheduler assigns initial
resources
› Buffer status report (PUSCH)
transmitted in UL
› UL grant (PDCCH) transmitted
(valid per UE)
› Channel sounding
macCtrlElementList[0] {
type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr)
powerHeadroomReport { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), powerHeadroom
= 127 }
cRnti { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), crnti = 8362387 }
truncatedBSR { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), bufferSize = 127 }
shortBSR { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), bufferSize = 127 }
longBSR { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), bufferSizeNr1Nr2 = 127,
bufferSizeNr3Nr4 = 39315 }
}
Radio Analysis Ul – BSR
› Buffer Status Report (BSR) is used to inform the eNB of the current
data waiting for transmission in the UE (3GPP TS36.213 Ch. 6.1.3.1)
› Values ranges from 0 up to >15000 bytes using 64 index values.
– e.g. index 0 for BS=0, index 1 for 0 < BS <= 10 and so forth.
› Can be traced out through
LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND. Expect to see high
values for maximum UL throughput. Low values indicate UE/laptop problem.
Type of MAC report, this case short BSR (6)
LSB 6 bits are the BSR index (this case >150000 bytes)
MSB 2 bits is the LCID
Radio Analysis UL – PHR
› Power Headroom Report (PHR) is used to inform the eNB of the
remaining transmit power available at the UE. (3GPP TS36.321 Ch.
6.1.3.6)
› Defined as difference between configured maximum UE output power
and estimated power used for PUSCH transmission
› Reports a index value similar to BSR with values between -23 up to 40
dB
› PH values are close to (or less than) 0 means the UE is power limited
– Ideally we look for positive values somewhat greater than 0
– When a UE is power limited, the eNB may schedule fewer PRBs in order to
reduce the required output power of the UE, this can in turn reduce
throughput.
Radio Analysis UL – PHR
macCtrlElementList[0] {
type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr)
powerHeadroomReport { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), powerHeadroom =
55 }
cRnti { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), crnti = 3637377 }
truncatedBSR { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), bufferSize = 55 }
shortBSR { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), bufferSize = 55 }
longBSR { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), bufferSizeNr1Nr2 = 55,
bufferSizeNr3Nr4 = 32897 }
}
Type of MAC report, this case PHR (3)
PHR value of 55 which corresponds to 32 <= PH < 33. In this case there is no power
limitation on the UE side.
PH Index values <= 23 indicates the UE has reached maximum transmission power
Negative values indicate the UE was power limited
See 3GPP TS36.133 Ch 9.1.8.4 for index mapping
Radio Analysis UL – PUCCH and PUSCH
› PUCCH takes a minimum 1 PRB on each side of the uplink band for
uplink control signalling, reducing the size of PUSCH
– E.g. 5MHz bandwidth, 25 PRBs available. Minimum 2 PRBs for PUCCH.
– 23 PRBs available for PUSCH
0 1 2 3 4 5 6 7 8 9
Time (ms)
Radio Frame
Cell
Bandwidth PUSCH – Used for UE data
scheduling and UL RA msgs
PUCCH – Semi-static allocation
of CQI, SR, ACK/NAK
PUCCH – Semi-static allocation
of CQI, SR, ACK/NAK
PUCCH
PUCCH
PUSCH
Radio Analysis UL – PRB Limitations
› Due to 3GPP specified design limitations in the UL it is not always possible to
utilise all free PRBs for UL transmissions
› 3GPP TS36.211 Ch 5.3.3 defines the following formula for the number of PRBs
on PUSCH for a single transmission:
– Where a, b and c are integers.
– For 5MHz:
› 23 PRBs are available for PUSCH (2 allocated to PUCCH)
› Max number of PRBs for a single PUSCH transmission is 20 PRBs.
› This corresponds to a=2, b=0 and c=1 (i.e. 3 PRBs are unavailable to be used).
› In L11A, 3 PRBs would be unused (only one SE/TTI possible).
› In later releases, 3 PRBs could be used by a second UE.
c
b
a
5
3
2 

Radio Analysis UL – Link Adaptation
Goal: Select MCS for a certain allocation size to
maintain the target BLER (10%) for the first
transmission
Inputs to Uplink Link Adaptation are:
UL interference power:
LPP_UP_ULCELLPE_CI_CELL_STATUS_REPORT_IND
outgoing from ulCellCeBl
Received power of UE (across traffical PUSCH PRBs):
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_UL_IND outgoing
from ulMacPeBl
PHR reports & HARQ CRC (BLER):
LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND
outgoing from ulMacPeBl
Input
to
Link
Adaptation
ulL1Meas2UlInd(rxPower(-140 .. 0 dB)
ulMacPe
run every subframe UE transmitts
cellStatusReportInd(interferencePower (-125 .. -80dB)
ulCellCe
run every subframe
ulL1MacCtrlInfoInd(powerHeadroom(0 .. 63dB))
ulMacPe
run every periodicPhrReport
ulL1MacCtrlInfoInd(CRC)
ulMacPe
run every subframe UE transmitts
Radio Analysis UL – Link Adaptation
LPP_UP_ULCELLPE_CI_CELL_STATUS_REPORT_IND UpUlCellPeCiCellStatusReportIndS {
sfn = 456
subFrameNo = 3
interferencePower = -1170
Cell interference level x 10 (i.e. -117.0 dBm)
High values here (>-104)
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_UL_IND (432) UpUlMacPeCiUlL1Meas2UlIndS {
sfn = 264
subFrameNo = 8
nrOfPuschReports = 1
rxPower { prbListStart=1, prbListEnd=48, rxPowerReport=-956, sinr=821854514 }
rxPwr = -95.6dBm over those PRBs (pZeroNominalPusch= -96dBm)
= ~22.9dB
)
2
(sinr
log
10
=
sinr[dB] -22
10 

LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND (433) UpUlMacPeCiUlMacCtrlInfoIndS {
sfn = 264
subFrameNo = 8
harqInfo = 1 (UpUpCommonMacCommonMacCtrlElemHarqFeedbackAck)
macCtrlElementList[0] {
type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr)
powerHeadroomReport { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr),
powerHeadroom = 55 }
HARQ ACK / PHR
Radio Analysis UL – Link Adaptation
› L11A supports up to MCS 24 in the uplink by default
– MCS21-24 are defined as 64QAM
– However, according to 3GPP TS36.213 Ch 8.6.1 if a UE does not support
64QAM then 16QAM can be used for MCS21-24.
– Check that MCS24 is selected. If not, check link adaptation inputs for
problems
› In UL, the eNB itself can directly measure SINR of the received signal
– Therefore CQI is not necessary for UL transmission
– eNB can use Received Power/SINR, PHR, UL interference and UL HARQ
BLER measurements to control MCS
Radio Analysis UL – Link Adaptation
› Check for:
– High values of UL interference
› Could there be some external interferer?
› Are the values of pZeroNominalPusch in neighbour cells too high?
– rxPower too low
› Target is EUtranCellFDD::pZeroNominalPusch. Is it set too high?
– PHR shows UE at maximum Tx power
› Is EUtranCellFDD::pZeroNominalPusch too high causing UE to exceed
maximum transmit power?
› Closed-loop power control TPC ignored by UE?
– Low values of SINR
› Is EUtranCellFDD::pZeroNominalPusch too low?
› Closed-loop power control TPC ignored by UE?
Radio Analysis UL – PDCCH
0 1 2 3 4 5 6 7 8 9
Time (ms)
Radio Frame
PDCCH carries both the
UL (PUSCH) assignment
and DL (PDSCH)
assignment.
In case many PDCCH
CCEs are used for DL
transmission (e.g. SIB
with 8 CCEs) it may be
that UL grant is not
possible to be scheduled
in this TTI for a single
UE!
PUSCH
UL subframe (4
TTI later)
DL subframe
(current)
PDSCH
PDCCH
Note: PCFICH and
PHICH multiplex into
the DL subframe red
area marked PDCCH
Radio Analysis UL – PDCCH
› PDCCH is used to signal:
– Downlink (PDSCH) assignments
– Uplink (PUSCH) grants
› In case of a downlink SIB transmission, 8 CCEs of PDCCH may be
used for downlink grant.
› To reduce processing load when decoding PDCCH, 3GPP defines
particular search spaces within PDCCH depending on:
– Number of CCEs for grant
– Number of CCEs for PDCCH
– RNTI of the UE
› Depending on these parameters, it may not be possible to allocate a
PDCCH uplink grant resource and therefore the UE may not be able to
be scheduled every TTI even if there are unused PUSCH resources.
See 3GPP TS36.213 Ch 9.1.1
See KO link: PDCCH visualisation KO
Radio Analysis UL – PDCCH
From KO link: PDCCH visualisation KO
Search space for 1 CCE completely overlaps 8
CCE search space.
In this example, DL SIB transmission
completely prevents any UL grant for this UE
RNTI 516 in subframe 5
Radio Analysis UL – HARQ
› LTE defines uplink with synchronous HARQ to reduce PDCCH
signaling load and simplify the uplink HARQ processing
› Example 1, successfully received PUSCH data:
– Subframe n: UL grant sent to UE
– Subframe n+4: PUSCH data received (rv=0)
– Subframe n+8: ACK sent, UL grant with New Data Indicator toggled
– Subframe n+12: new PUSCH data received (new HARQ process)
› Example 2, HARQ retx:
– Subframe n: UL grant sent to UE
– Subframe n+4: PUSCH data received (rv=0)
– Subframe n+8: NACK sent, NO UL grant is signaled on PDCCH
– Subframe n+12: PUSCH data received (rv=2) (same HARQ process)
– Subframe n+16: ACK/NACK, etc up to max number of retx
Radio Analysis UL – HARQ ReTx
Postponed
reTx
Colliding
ACK
Adaptive
reTx
time
PUSCH for scheduling
n+8
Assume all three non-correctly decoded (CRC not ok)
PUCCH PUCCH
PRACH Non-Adaptive
reTx
NACK
SE1 SE3
SE2
n
(N)ACK
Grant
needed
Radio Analysis UL – HARQ
› Because of the synchronous nature of Uplink HARQ, the following
scheduling priority is used:
– Random Access Message 3 (RRC Connection Request). Scheduled 6
subframes before, special case.
– Non-adaptive HARQ retransmission
– Adaptive HARQ retransmission
– New Data transmission
› Non-adaptive means no UL grant is explicitly scheduled for the
retransmission
› Adaptive means that scheduling collision occurred (e.g. collision with
PRACH) and an explicit UL grant was signalled to:
– Move the allocated PRBs to another part of the UL spectrum
– Suspend (delay) the UL HARQ retransmission for 8 TTIs later
Radio Analysis – Traffic Abnormal
› TRACE1 in baseband is defined as TRAFFIC_ABNORMAL. It should
be used to trace out abnormal conditions in baseband processing.
– Normally the output gives a good description of the problem encountered
› Some useful TRAFFIC_ABNORMAL traces:
– lhsh gcpu01024 te e trace1 UpcDlMacCeBl
– lhsh gcpu01024 te e trace1 Upc*
– lhsh gcpu00256 te e trace1 UpUlMacPeBl_Smac
– lhsh gcpu00768 te e trace1 ElibPapBlEth
– lhsh gcpu00256 te e trace1 UpUlMacPeBl_Smac
– lhsh gcpu00768 te e trace1 UpDlL1PeFt_DEADLINE_MISSED
– lh gcpu fte e trace1 .*_DISCARD
Radio Analysis – Post-Processing Tools
› 3GPP has specified L1 messages in order to reduce the bits required
for transmission on the air interface.
– These formats can be difficult to read
› For this reason, many values in the traces are presented in formats
which require conversion to human readable formats, for example:
– PRBs allocated in DL/UL grant messages
– PHR values
– BSR values
– SINR
– MIMO HARQ feedback, etc..
› Tools exist to perform these conversions and compact the data
presentation to the end user
– One such tool is bbfilter or scheduling_filter.pl
– Check the flowfox web page for details
Radio Analysis – bbfilter Downlink
$ cat decoded_dl_log.log | ./bbfilterv2.2 -bw 5 –dl
sfn|sf|mode|dlModul|mcs1|mcs2|prb|Ndf|Tbs1|Tbs2|AssBits|Harq|dlBler|cqi|ri|
280| 4|TxDi| 64QAM | 16 | 0 |25 | Y|7736| 0|8771784| | | 11| 2|
280| 5| | | | | | | | | |A | 0% | | |
280| 6|TxDi| 64QAM | 18 | 0 |25 | Y|7992| 0|8764088|A | 0% | | |
280| 7|TxDi| 64QAM | 18 | 0 |25 | Y|7992| 0|8756144|A | 0% | | |
280| 8|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8748192|A | 0% | | |
280| 9|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8736760| | | | |
281| 0|Mimo| 16QAM | 12 | 12 |25 |Y Y|4968|4968|8737384|A | 0% | | |
281| 1|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8763568|N | 0% | | |
281| 2|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8776208|N N | 2% | | |
281| 3|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8800856|N N | 4% | | |
281| 4|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8825504|N N | 6% | | |
281| 5|TxDi| 16QAM | 30 | 0 |25 | N|7992| 0|8862160|N N | 8% | | |
281| 6|Mimo| 16QAM | 30 | 30 |25 |N N|5736|5736|8862200|N N | 10% | | |
281| 7|Mimo| 16QAM | 30 | 30 |25 |N N|5736|5736|8862200|A A | 10% | | |
HARQ ACK/NACK refers to the
transmission 4 subframes earlier!
NOTE: Format modified to fit on slide, only example!
Radio Analysis – bbfilter Uplink
$ cat decoded_ul_log.log | ./bbfilterv2.2 -bw 5 –ul
sfn|sf|rxPwrPus|prb|ulTpc|sinr|ulModul|mcs|ndf|ul bsr |phr |ul tbs| ul crc |har|ulBler|
266| 6| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y | | | 25456| | A | 2% |
266| 7| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | | 25456| | A | 2% |
266| 8| -95.6 | 48| 0:1 | 22 | 16QAM | 24| N | | | 25456| ERR 3182| N | 5% |
266| 9| -95.6 | 48| 0:1 | 23 | 16QAM | 24| Y | | | 24496| | A | 5% |
267| 0| -95.7 | 48| 0:1 | 22 | 16QAM | 24| Y |>150000 | | 24496| | A | 5% |
267| 1| -95.8 | 40| 0:1 | 22 | 16QAM | 24| Y | | | 21384| | A | 5% |
267| 2| -95.6 | 48| | 23 | 16QAM | 24| Y | | | 25456| | A | 5% |
267| 3| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | | 25456| | A | 4% |
267| 4| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y | | | 25456| | A | 4% |
267| 5| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y |>150000 | | 25456| | A | 4% |
267| 6| -95.6 | 48| 0:1 | 23 | 16QAM | 24| N |>150000 | | 25456| | A | 4% |
267| 7| -95.6 | 48| 0:1 | 23 | 16QAM | 24| Y | | | 25456| | A | 4% |
267| 8| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | 32 | 24496| | A | 4% |
UL BSR and PHR values decoded
NOTE: Format modified to fit on slide, only example!
Radio Analysis - Summary
› Areas of analysis for Downlink:
– CQI / RI (Rank Indicator) reported from UE.
– Transmission Mode (MIMO, TxD, SIMO)
– MCS vs. number of assigned PRBs vs. assignable bits in scheduler
– UE Scheduling percentage of TTIs (how often is the UE scheduled)
– PDCCH CFI and scheduling impacts
– HARQ
– RLC retransmissions
› Areas of analysis for Uplink:
– BSR (Buffer Status Report)
– PHR (Power Headroom Report) – is the UE at maximum power?
– Cell bandwidth vs. maximum allowable PRBs
– Link Adaptation
– MCS available and 16QAM
– PDCCH SIB scheduling colliding with UL grant
– HARQ (less important, because we can measure SINR)
Transport Analysis
Transport Analysis
› End user data is transferred over the S1-U interface
› Several Transport Network topologies (L2/L3) provide great flexibility in design
› Several router redundancy methods are supported
› Transport network dimensioning provides insights into the peak provisioning
on the S1 link
› The LTE RBS is a QoS enabler, providing end user and transport network QoS
differentiation
› Performance management counters and tracing provide us with powerful node
observability methods
Transport Topology
Transport Topology
› No strict requirements on using a L2 switched or L3 routed LTE RAN transport
network
› No specified topology requirement
› A router is required in the network, but LTE RAN transport network does not
have to be L3
› Network design is important (number of hops for L3 vs. size of broadcast
domain for L2)
› This topology flexibility could complicate troubleshooting efforts depending on
the nodes involved (say 3PP support is required)
Transport configuration
› A 2 VLAN configuration is recommended (separating O&M and
Transport):
OAM SD
ComInf Firewall
MME Pool
S-GW Pool
CN Router/FW
S1
GW If
O&M
GW If
S1
GW If
O&M
GW If
SoIP
Server
SoIP Servers
distributed in
the network. RBS 1
CP, UP &
SOIP IP
O&M IP
Switched Ethernet
(Carrier Ethernet)
RBS n
CP, UP &
SOIP IP
O&M IP
O&M VLAN
TN VLAN
Router Path Supervision (RPS)
› RPS provides router redundancy for S1/X2 traffic
› RPS is configurable via the IpInterface MO
virtual router redundancy protocol (VRRP)
10.1.1.34 10.1.1.35
10.1.1.34
› LTE RBS supports VRRP (a router redundancy
protocol)
› VRRP uses an election method to assign
responsibility for a virtual router to one of the
VRRP routers on a LAN
› The Master VRRP router controls the IP
address(es) associated with a virtual router and
forwards packets sent to these IP addresses
› If the Master fails, one backup VRRP router will
act as the virtual router
› LTE RBS is transparent to the process, it does
not directly participate in VRRP
eNB eNB eNB
Master Backup
Transport Dimensioning
› Dimensioning of the northbound transport network will impact achievable end
user throughput rate
› LTE RBS transport network dimensioning process (mobile backhaul):
› Dimensioning is based on payload only!
Determine
bandwidth
needed for
last mile
Determine cell
thpt in a loaded
network and
avg. cell thpt
during busy
hour
Calculate agg.
bandwidth
required in
mobile
backhaul
Dimensioning methods
Method Description
Overbooking Allows more users than the dimensioned quantity, using the
rationale that only a subset of users is allocating bandwidth at
the same time.
Overdimensioning Calculates the dimensioned bandwidth required by multiplying
the average requirement by an overdimensioning factor.
Peak allocation Uses the maximum throughput capacity as the dimensioned
link capacity. The link is dimensioned for the maximum
possible bit rate.
Overprovisioning Monitors the link use. When a predefined use limit is reached
on the link, a capacity upgrade is initiated. A general rule is
that the use limit is set to 50%.
Transport Aggregation
Input (assumptions) 20 MHz Cell
Cell Peak Rate 150 Mbps
Cell Throughput in a
Loaded Network
35 Mbps
Peak load for 3x1 in a
Loaded Network
~100 Mbps
S-GW/
PDN GW
A3
A1
A1
S-GW/
PDN GW
R
B
S
R
B
S
R
B
S R
B
S
R
B
S
R
B
S
R
B
S
R
B
S
R
B
S
A1
A2 A2
Dimension for: ΣA2 × 0.8
BH displacement
factor
Dimension for peak rate
to 1 cell= 150 Mbit/s
Dimension for ‘eNodeB throughput in a
loaded network for a 3x1 configuration’
= 100 Mbit/s per eNB
Dimension for ‘Average eNodeB
throughput during Busy Hour’
= 50 Mbit/s per eNB
DUL TN Capabilities (L11)
› The DUL is equipped with two Gigabit Ethernet physical interfaces
– TN-A: Electrical Ethernet (100/1000 Mbps)
– TN-B: Electrical/Optical Ethernet (1000 Mbps)
› Only 1 interface is activated through configuration (either TN-A or TN-B)
› Maximum throughput of 173 Mbps (or 150 Mbps of User Data assuming other
overheads such as signalling)
› Up to 500 connected users
› Note:
– Wire tracing is not possible on the DUL
– You must use an external product to mirror traffic leaving the DUL
Quality of service (qos)
› Transport network QoS is a part of the complete LTE QoS concept
› QoS, in an IP transport network, is the ability to treat packets/frames differently
based on their content
› Without QoS, each packet/frame is given equal access to the network
resources
QOS for user plane Bearers
Transport
RAN
Terminal
Gateway
(Bearer Policy
Enforcer)
Service 1
(e.g. Internet)
Service 2
(e.g. P2P File Sharing)
Service 3
(e.g. IMS-Voice or MTV)
Default Bearer (QoS via MME)
Dedicated Bearer (QoS via PCRF)
Service Data Flow (SDF)
IP Address
Further Reading:
3GPP TS 23.401
Quality Class Indicators
› A Quality Class Indicator (QCI) is used to signal the QoS requirements of a bearer
› 23.401 defines 9 standard QCIs, each one with specific characteristics
› Operators may define proprietary QCIs to introduce new services
QCI Resource Type Priority Packet Delay Budget
Packet Loss
Rate
Example Services
1
GBR
2 100 ms 10-2 Conversational Voice
2 4 150 ms 10-3 Conversational Video (Live Streaming)
3 3 50 ms 10-3 Real Time Gaming
4 5 300 ms 10-6 Non-Conversational Video (Buffered Streaming)
5
Non-GBR
1 100 ms 10-6 IMS Signaling
6 6 300 ms 10-6
- Video (Buffered Streaming)
- TCP-based (e.g., www, e-mail, chat, ftp, p2p file
sharing, progressive video, etc.)
7 7 100 ms 10-3
- Voice,
- Video (Live Streaming)
- Interactive Gaming
8 8
300 ms 10-6
- Video (Buffered Streaming)
- TCP-based (e.g., www, e-mail, chat, ftp, p2p file
sharing, progressive video, etc.)
9 9
Transport Network QoS – Layer 3
Site Infrastructure, IP-backbone and RAN Transport EPC
Application
IP
PPPoE Ethernet
Ethernet
IP and Ethernet
IP Transport Network
IPx is Interface
between node and
IP transport network
Version
4
Header Length
4
DiffServ
7
Total Length
16 20 bytes
Further Reading:
RFC 2474
Differentiated Services
Code Point (DSCP)
› QCIs are mapped to IP layer Differentiated Services Code Points (DSCP)
Transport Network QoS – Layer 2
Site Infrastructure, IP-backbone and RAN Transport EPC
Application
IP
PPPoE Ethernet
Ethernet
IP and Ethernet
IP Transport Network
IPx is Interface
between node and
IP transport network
Preamble
7
DA
6
SA
6
TPI
2
TAG
2
Type
2
Data
46 to 1500
CRC
4
SFD
1
User
Priority
3bits
CFI
1bit
VLAN ID 12bits
› Use of P-Bits allows prioritizing different types of traffic
› Priority queuing, enabling some Ethernet frames to be forwarded ahead of others within a
switched Ethernet network
› Frames can be assigned to different scheduler queues
› Maintain the same value throughout the IP Network to deliver the same QoS
Further Reading:
IEEE 802.1p
Priority Bit (P-bit)
QOS Mapping
› LTE RAN Quality of Service (QoS) ensures the LTE bearer and transport level
service requirements
Ethernet
Frame
IP Packet
QoS Profile
DATA
DATA
QCI
AMBR ARP
IP Header
Ethernet Header
DSCP
Mapping:
Takes place in RBS
/ EPC
Mapping:
Edge devices
handling L2/L3
payload
P-bit
QoS Mapping
LTE RAN QoS Configuration
› LTE RAN complies with 3GPP TS 23.203 and IEEE 802.1p:
› Using these MOs we are able to map out the quality of service properties
defined in the RBS
===============================================================
MO dscp priority qci
===============================================================
QciTable=default,QciProfilePredefined=qci1 46 2 1
QciTable=default,QciProfilePredefined=qci7 20 7 7
QciTable=default,QciProfilePredefined=qci9 12 9 9
QciTable=default,QciProfilePredefined=qci5 40 1 5
QciTable=default,QciProfilePredefined=qci3 34 3 3
QciTable=default,QciProfilePredefined=default 0 10 0
QciTable=default,QciProfilePredefined=qci8 10 8 8
QciTable=default,QciProfilePredefined=qci6 28 6 6
QciTable=default,QciProfilePredefined=qci2 36 4 2
QciTable=default,QciProfilePredefined=qci4 26 5 4
===============================================================
====================================================
MO Attribute Value
====================================================
Subrack=1,Slot=1,PlugInUnit=1,ExchangeTerminalIp=1,Gig
aBitEthernet=1 dscpPbitMap t[64] =
>>> Struct[0] has 2 members:
>>> 1.dscp = 0
>>> 2.pbit = 0
... truncated ...
>>> Struct[46] has 2 members:
>>> 1.dscp = 46
>>> 2.pbit = 6
>>> Struct[47] has 2 members:
>>> 1.dscp = 47
>>> 2.pbit = 0
Transport Network Configuration (39/1553-HSC 105 50/1)
GTP-U - User Plane
› GTP-U is used as the User Plane protocol over the S1 and X2 interfaces
(defined in 29.060)
› GTP-U carries the end user data by forming tunnels towards the core
network S-GW (transports user IP payload)
› IP fragmentation is to be avoided (MTU size should be set correctly)
› Configuration aspects on the RBS are minimal (no MO models this layer)
GTP-U
UDP
IP
Data Link
Physical
GTP-U
UDP
IP
Data Link
Physical
S1-U
teid, ip address, port
teid, ip address, port
Transport Network performance
› Transport Network performance visibility is available via the GigaBitEthernet
MO and IpAccessHostEt pm counters
– Moshell> pmom GigabitEthernet|IpaccessHostEt
› The metrics include (singleton, sample and statistical metrics):
– GE Link Ingress/Egress Average usage
– GE Link Ingress Frame Error Ratio
– IPv4 Ingress/Egress Packet discard ratio
› These TN metrics can help identify:
– under-dimensioned GE links (i.e. highly utilised links)
– transport problems/errors over GE links (high frame discard ratios)
– IPv4 packet discard issues (queue capacity, errored packets, etc)
Transport Network Performance Metrics (44/1553-HSC 105 50/1)
L2 observability
L3 observability
Transport Network performance
› Additional TN Performance Management counters are available
› SCTP (Signalling transport for S1AP and X2AP)
– Sent and received data/control chunks
– Dropped chunks (buffer overflows)
– Checksum errors
› Synchronization (SoIP related counters)
– Provides the highest delay variation counters for the active IP sync reference
– Calculated in terms of the best x percentage sync frames experienced during a 100
second window (result in microseconds)
– The percentages include 1, 10, 50 %
› IpInterface (Signalling, payload and sync)
– Failed Pings to default routers (RPS)
– Discards, header and IP address errors
RBS Counters via COLI
› Detailed IP counters are available from the
transport interface (TN-A / TN-B)
› IP layer (via IpAccessHostEt MO)
– EtHostMo_getPmCounters -h 1 -t 1
› The counters show:
– Protocol errors
– Ingress/Egress Discards
– UDP protocol errors
– ICMP details
– Fragmentation details
$ EtHostMo_getPmCounters -h 1 -t 1
PM counters for host with hostFroId 1
ipAddrErrors=0
ipInDelivers=23048
ipInDiscards=-2
ipInHdrErrors=0
ipInReceivedOctets=-2
ipInReceives=321055
ipInUnknownProtos=0
ipNumFailedAt=-2
ipOutDiscards=-2
ipOutRequests=321945
ipOutRequestOctets=-2
ipReasmReqds=0
ipReasmOKs=0
ipReasmFails=0
ipFragOKs=0
ipFragFails=0
ipFragCreates=0
ipPortUnreachable=0
udpInDatagrams=0
udpInErrors=0
udpNoPorts=-2
udpOutDatagrams=0
icmpInDestUnreachs=0
<truncated>
RBS Counters via COLI
$ nssinfo tupm
****** NSS TUM2 tu_pm related data ******
PMfroId : 1
PMfroType : 66817
granularityPeriod : 900
suspectFlag : 0
par_pmMDVCounter : 186
par_pmHDVB1Pct : 0
par_pmHDVB10Pct : 1
par_pmHDVB50Pct : 5
****** END ******
$ sctphost_stat -assoc -all
sctphost_stat - START.
|-------------------------------------------------------|
|----------------------- SCTP HOST ---------------------|
|RpuId: 17
|SctpInstId: 0
|Base State: BASE_RUN
|Host State: A|C|R|X|IA
|Ext. client: CONNECTED
|Alarm Timer: NOT RUNNING
|---------------- SCTP ASSOCIATION 96 -----------------|
|-------------------------------------------------------|
|----------------Statistic (assoc level)----------------|
| [ID 7]: SCTP_STAT_SENT_CHUNKS, Count: 1|
| [ID 8]: SCTP_STAT_REC_CHUNKS, Count: 1|
| [ID 9]: SCTP_STAT_OUT_OF_ORDER_SC, Count: 0|
| [ID 10]: SCTP_STAT_OUT_OF_ORDER_RC, Count: 0|
| [ID 12]: SCTP_STAT_RETRANS_CHUNKS, Count: 0|
| [ID 13]: SCTP_STAT_SENT_CC, Count: 16264|
| [ID 14]: SCTP_STAT_REC_CC, Count: 16264|
| [ID 15]: SCTP_STAT_FRAG_USER_MSG, Count: 0|
| [ID 16]: SCTP_STAT_REAS_USER_MSG, Count: 0|
| [ID 17]: SCTP_STAT_SENT_PACKAGES, Count: 16265|
| [ID 18]: SCTP_STAT_REC_PACKAGES, Count: 16265|
<truncated>
› RBS COLI counters show detailed
performance issues
– S1AP/X2AP (SCTP MO)
– sctphost_stat -assoc -all
› SoIP (Synchronization MO)
– nssinfo tupm
Transport Network tracing
› RPS
– $ appdh info / $ appdh rps <ip inter>
– Use Wireshark to see ICMP
› QCI, ARP, AMBR values
– te e bus_send bus_receive S1AP_ASN
(e.g. InitialContextSetupRequest)
› SCTP
– $ te e all cpxSctpIC
– $ te e all Scc_SctpHost_proc
› Synchronization (SoIP using NTP)
– te e trace7 NSS_CBM_TUM2_TUREG
– $ nssinfo all
Wireshark - Protocol capture
eNB S-GW Internet
FTP Server
IP Backbone
MME
HSS
S1-MME
S6a
S11
P-GW
S5/S8
S1-U
S1
Wireshark is an open source
protocol analyser.
It provides excellent support of
the 3GPP LTE protocols.
S1 capture via Wireshark
QCI = 9
Will filter all data
and only display
S1AP protocol
messages
S1 capture via Wireshark
› Example S1 trace with user and control plane payload:
S1-U
GTP-U
L2 QoS
Pbit = 6
L3 QoS
DSCP = 46
SoIP
using NTP
S1-MME
SCTP
Transport Summary
› LTE RAN transport topology is very flexible
– a mixture of L2/L3 topologies could be implemented (could complicate analysis)
› Basic transport network redundancy is provided (in terms of L3 routers)
› Transport network dimensioning should be taken into account as statistical
gains are used (backhaul peaks should be known)
› QoS (Radio and Transport) is essential for proper network operation and
should be implemented throughout the network (i.e. in L2/L3 nodes as well)
› Basic performance management is provided in initial LTE RAN releases
› Additional observability can be secured through protocol analysis
E2E Analysis
e2e analysis
› End-2-end analysis of throughput is required when end users report
throughput issues that are not readily seen in the LTE RAN
› End user throughput investigations/analysis is done at the UE/Server side
› Typical user payload will use the TCP transport protocol
› Layer 3 IP configuration aspects are not covered
› The UDP transport protocol is not covered
lte end user Protocol stack
Telnet, FTP, TFTP, HTTP, SNMP, …..
BGP RIP
Port Number
Application Layer
OSPF EGP TCP UDP IMCP IGMP
Transport Layer
ARP IP RARP
Internet Layer
Protocol Number
Type Code
PDCP, GTP-U
Data Link Layer
TCP at a glance
› TCP will be the most used transport layer protocol (email, ftp, browsing, etc)
› TCP offers the following features:
– Stream data transfer
› TCP offers a contiguous stream of segments for applications
– Reliability
› Sequence numbers used by sender that expects positive acks (or retransmit)
› Receiver uses sequence numbers to rearrange segments (remove duplication)
– Flow control
› Receiver indicates the number of bytes it can receive (to sender)
– Multiplexing
› Achieved through the use of ports
– Logical connections
› Each connection is identified by the pair of sockets used (in receiver & sender)
– Full duplex operation
› TCP provides concurrent data streams in both directions
TCP Header
Indicates
application
Provides
reliability
rwnd - what the receiver
(sender of this segment) is
willing to accept
TCP Operation
› TCP operates in three distinct phases:
– Connection establishment
– Data transfer
– Connection termination
› TCP is known as a sliding window protocol
› Two windows are defined:
– rwnd - receive window, advertised/offered by the receiver
– cwnd - congestion window, calculated by the sender
› The TCP sender window is defined as the minimum of rwnd and cwnd
› AIMD behaviour (additive increase / multiplicative decrease)
– cwnd = cwnd + MSS*(MSS / cwnd)
– cwnd = cwnd / 2
› Sliding window in action: Sliding Window Animation
TCP Operation Cont.
› TCP congestion control:
– Slow start
› exponential growth of cwnd
› used in cold/initial start and after a timeout
– Congestion avoidance
› linear increase after congestion experienced
› congestion indicated by timeouts and duplicate acks
– Fast retransmit
› sender quickly determines congestion and retransmits
› attempts to avoid timeouts - and hence slow start
– Fast recovery
› enter congestion avoidance rather than slow start
› Senders use a retransmission timeout (RTO) based on RTT
TCP Congestion Control
Congestion
Window
3rd DUPACK
3rd DUPACK
3rd DUPACK
Time out
Pipe Capacity
Slow start threshold reached
t
ssthresh on
congestion
Fast
recovery
Fast
retransmit
Congestion
Avoidance
Slow Start
TCP Performance
› Bandwidth Delay Product (BDP) - data required to fill the TCP pipe:
– Bandwidth of link (bytes per sec) * Delay (sec) = amount of data in transit to fill pipe
› Example: A T1 (1.5 Mbps) over a satellite connection with RTT = 500ms
– (1,500,000 bits per sec / 8 bits per byte) * (0.5 sec) = 93,750 bytes
– With a rwnd of 65,535, performance is 65,535/93,750 = ~70% or 1.05 Mbps
– Hence, the BDP requires more transit data, and this is achieved with an increase in
the rwnd (the TCP scaling feature, RFC1323, can provide this increase)
› TCP Windowing (rwnd) and RTT limit the achievable throughput as follows:
–
› Hence, large receive windows and small RTT are desired
Typical LTE
RTT
TCP Tuning (Client side)
› TCP performs differently on different Operating Systems (there have been
many variations on the TCP congestion control algorithm for instance)
› On Windows Vista:
– autotuning = highlyrestricted
› netsh interface tcp set global autotuninglevel=highlyrestricted
– autotuninglevel = restricted
› netsh interface tcp set global autotuninglevel=restricted
› On Windows XP:
– SP2+: follow this Windows XP TCP configuration KO
– Pre-SP2: Use the DrTCP application
› On Linux (depends on the kernel)
– Follow this TCP Tuning Guide for kernels 2.4 -> 2.6 (i.e. 2.6.20+ are covered)
LTE RAN TCP Behaviour
› Processing (handovers, buffering, delays, scheduling, retransmissions) in the
RBS can affect TCP operation
› Affect of incorrect TCP receive window sizes:
– Lower than BDP ->
› Packet loss can lead to (retransmissions, dropped in RBS, etc):
– TCP retransmissions and delays
– Send rate (throughput) reduced up to 50%, then a linear increase until next drop or
max TCP rate reached
› TCP timeouts can lead to:
– TCP fallback into slow start
LTE RAN TCP Enhancements
› LTE RAN future adaptations to enhance end user TCP behaviour:
– AQM (Active Queue Management)
› Uses the TCP congestion avoidance algorithm
› Limits the cwnd by selectively dropping packets
– Result: smaller cwnd = smaller buffered data and delay
– Data forwarding at intra LTE handover (X2)
› Zero packet loss at handover
› Provides higher data rates for mobile users
– Result: reduces the risk of TCP fallback into slow start
Analysing e2e traffic
› The following section uses the Wireshark application to inspect e2e traffic (i.e.
FTP download from UE)
› Two scenarios are used to highlight how Wireshark can perform detailed
analysis on network traffic
› The goal is to show how e2e analysis can be performed from a UE perspective
› The following shows analysis of downlink FTP data session towards a UE
Throughput Results
› For DL throughput, select a packet from Server to UE, and select Statistics ->
TCP Stream Graph -> Throughput Graph
Scenario A Scenario B
Throughput Results Cont
› Two other ways to quickly determine the UE throughput are via Statistics ->
Summary and Statistics -> IO Graphs
Average added
for clarity
Scenario A
Time Sequence Graphs
› A great insight into the TCP performance/behaviour is found with the
Statistics -> TCP Stream Graph -> Time-Sequence Graph (Stevens)
Scenario A
Scenario B
TCP Flow Graphs
› TCP flows (Statistics -> Flow Graph) are useful in isolating a particular TCP
conversation
Scenario B
Additional TCP Analysis
› Wireshark’s Expert Info (Analyse -> Expert Info) provides a better display of
uncommon or notable network behaviour:
Scenario B
Analysis Suite
› Wireshark is a powerful tool to inspect e2e network behaviour
› Other tools can be used in conjunction with Wireshark to offer a complete e2e
analysis suite:
– iperf
› works in client/server modes
› allows UDP and TCP payload to be injected into the network
› useful to see link capacities with UDP (max DL/UL throughput)
› identifies possible TCP bottlenecks
– Netpersec
› Offers real-time display of throughput
– IXIA Chariot Endpoint
› commercial product to test IP networks
› provides automated application level analysis (HTTP, VoIP, etc)
› provides detailed results/statistics of performance
e2e Summary
› The main transport layer (OSI L4) protocol used for network services (internet)
will be the Transmission Control Protocol (TCP)
› TCP offers end user application reliable data delivery, flow control and good
bandwidth utilisation
› The LTE RAN will implement features/functions that provide better inter-work
with the TCP protocol
› Wireshark is a powerful open source network protocol analyser that can offer
advanced e2e throughput investigations using inbuilt functions
› Additional tools (like iperf) support the complete e2e analysis
Summary
Summary
› Throughput troubleshooting in
the LTE RAN can be broken into
domains
› The Radio domain includes
many uplink and downlink
specific areas of analysis and
can also be utilised to find eNB
external problems
› The Transport domain analysis
focus’ on northbound IP network
› The e2e domain analysis looks
at end user throughput concerns
More Information
› LTE PLM Troubleshooting Wiki: https://plm-
lte.rnd.ki.sw.ericsson.se/lte_trsh_wiki/index.php?n=UseCases.UseCases
› LTE Tutorial: http://utran01.au.ao.ericsson.se/flowfox/lteman.html
› LTE Observability: http://utran01.au.ao.ericsson.se/flowfox/lteobservability.html
› PLM LTE Toolbox in the field:
https://ericoll.internal.ericsson.com/sites/PLM_LTE_Toolbox_in_field/default.aspx
› LTE FlowFox: http://utran01.au.ao.ericsson.se/flowfox/index.php#lteflowfox
› LTE SW Deployment and Support EKB Community:
https://knowledgebase.internal.ericsson.com/LTE_RAN/
› 3GPP 36-series specifications (TS36.213 in particular for Radio Analysis
section): http://www.3gpp.org/ftp/Specs/html-info/36-series.htm
› LTE L11A GA (R9AJ) System Constants list
http://cdmweb.ericsson.se/WEBLINK/ViewDocs?DocumentName=1%2F19059-HRB105500&Revision=PE2-10
› LTE RAN Data Collection Guideline:
http://cdmweb.ericsson.se/WEBLINK/ViewDocs?DocumentName=60/1543-LZA7016004&Latest=true

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Ericsson LTE Throughput Troubleshooting Techniques.ppt

  • 1. LTE L11 Throughput Troubleshooting Techniques
  • 3. Why learn about Throughput Troubleshooting › LTE provides data, lots of data › Throughput is shared in time and frequency › Users notice throughput problems › Learn to troubleshoot the LTE RAN for throughput problems › Learn to isolate the domain causing throughput degradation
  • 4. Scope and objectives › Isolate throughput problems into domains › Pinpoint causes of throughput degradation clearly within domains through theory, traces and practical examples Objectives Scope › RBS Initial Checks › Radio Analysis › Transport Analysis › End-2-End Analysis
  • 6. Agenda 1. Overview 2. Initial Checks 3. Radio Analysis 4. Transport Analysis 5. E2E Analysis
  • 7. LTE RBS User plane Overview
  • 8. LTE RBS User Plane Overview › User plane visualisation › Network assumptions › L11 software limitations
  • 9. User Plane Domains eNB S-GW UE Internet FTP Server PDCP RLC MAC Physical PDCP RLC MAC Physical GTP-U UDP IP Data Link Physical GTP-U UDP IP Data Link Physical GTP-U UDP IP Data Link Physical GTP-U UDP IP Data Link Physical IP IP IP FTP Serv. FTP Client Radio Domain Transport Domain e2e Domain Uu S1-U S5/S8 SGi IP Backbone MME HSS Relay Relay S1-MME S6a S11 P-GW S5/S8 S1-U S1 Uu
  • 10. Data Flow over Air (RBS/UE) CRC Payload Payload Payload CRC Header Header Header Payload Payload Payload Header Header Header PDCP Header PDCP Header PDCP Header PDCP RLC MAC RLC Header RLC Header RLC Header MAC Header MAC Header PDCP PDU RLC PDU MAC PDU Transport Block PHY Transport Block ARQ HARQ RLC SDU PDCP SDU
  • 11. network assumptions › Network configuration and integration is complete › Data sessions have been previously verified › Basic RBS troubleshooting has been performed: – Node alarms verified – MO status – Cell availability › See the ?Fundamental RBS Troubleshooting Techniques? for further checks that can be performed
  • 12. Release Limitations › L11A contains some limitations that directly affect end user throughput – One SE per TTI in UL and DL (in L11A GA) › Each cell is treated individually, so there could be up to 3 users simultaneously in an eNB › SIB is scheduled the same as user data, so nothing can be scheduled at the same time as SIB – DUL user plane capacity limited to 150 Mbps (20MHz) – 100PRBs UL, 150 PRBs DL. – 16QAM UL (up to MCS24) – MCS28 disabled in DL by default (requires CFI=1 also) – Fixed CFI (number of OFDM symbols for PDCCH) › Default is CFI=2 for 5MHz and less. › CFI=3 not possible for >= 10MHz due to system limitations, this reduces PDCCH scheduling opportunities.
  • 14. Initial checks › For throughput issues, some essential checks are required: 1. Network changes & Basic troubleshooting 2. PC/Server settings 3. UE categories 4. UE subscriber profile 5. RBS parameters 6. Enabled features
  • 15. NW Changes and Basic Troubleshooting › Network/node changes can affect network throughput › Some common examples include: – Network configuration changes (e.g. adding/changing/removing hardware) – RBS parameter changes (all MOs under ENodeBFunction, system constants, EricssonOnly hidden parameters, e.g. DataRadioBearer) – IP address plan changes – Transport Network changes (add/reduce capacity on TN) – DNS updates – Hint: › To see RBS level changes (MOs/parameters): Moshell> lgo › To capture detailed RBS level logs: Moshell> dcg › Basic troubleshooting checks include: – Alarm, event and system log checks – MO health status
  • 16. PC/Server Settings › Determine the applications or tools used in testing/monitoring throughput › Confirm the end user PC settings: – Laptop specification can impact throughput (processors, memory, USB bus, HDD speed, plugged into AC power, etc) – MTU settings in PC (1360 optimal for eNB in L11A to prevent fragmentation) – Throughput monitors (e.g. Netpersec, only good for downlink UDP measurements, uplink must be measured at receiving side for UDP) – TCP enhancements in Vista (experimental), Vista should “auto-tune”. › Confirm server settings: – FTP server configuration – Linux TCP setting/guide – iperf (UDP & TCP) – be sure to use packet size 1360 for UDP (not default 1470). – Always check first with UDP rather than TCP, as UDP is less prone to display problems as a result of jitter variations and packet loss.
  • 17. UE Categories UE Category Maximum number of DL-SCH transport block bits received within a TTI Maximum number of bits of a DL-SCH transport block received within a TTI Total number of soft channel bits Maximum number of supported layers for spatial multiplexing in DL Category 1 10296 10296 250368 1 Category 2 51024 51024 1237248 2 Category 3 102048 75376 1237248 2 Category 4 150752 75376 1827072 2 Category 5 299552 149776 3667200 4 › The UE Category limits throughput possibilities › 5 UE Categories are defined in 3GPP TS 36.306 › The UE-Cat is sent in the UE Capability Transfer procedure (RRC UECapabilityInformation) › The COLI ue command provides detailed capability info (KO) for connected UEs UE Category Maximum number of bits of an UL-SCH transport block transmitted within a TTI Support for 64QAM in UL Category 1 5160 No Category 2 25456 No Category 3 51024 No Category 4 51024 No Category 5 75376 Yes DL UL
  • 18. UE Subscriber profile › End User (EPS User) subscription data is stored in the HSS › The EPS User Profile data is identified by its IMSI number › The profile consists of: – MSISDN number – Operator Determined Barring (ODB) – APN Operator Identifier Replacement – Subscribed Charging Characteristics – Aggregate Maximum Bit Rate (AMBR) › Max requested bandwidth in Downlink › Max requested bandwidth in Uplink – RAT frequency selection priority – APN configuration profile: › Default Context Identifier (default APN for the EPS User) › APN Configuration (every APN associated to the EPS User)
  • 19. RBS Parameters RN › RN MO parameters: – EUtranCellFDD › dlChannelBandwidth / ulChannelBandwidth › (nrOfSymbolsPdcch) (Control Region Size)  NOTE: currently controlled by SC38 in L11A › noOfUsedTxAntennas  controls whether OLSM MIMO is used (2) or not. › partOfRadioPower  NOTE: this is the % part of RU capability independent of SectorEquipmentFunction::confOutputPower settings › pZeroNominalPucch  some UEs need this to be increased or ACK/NACKs are not received successfully on PUCCH. › pZeroNominalPusch  some UEs need this to be increased from default or lots of errors seen on PUSCH – SectorEquipmentFunction=Sx › confOutputPower / fqBand (readOnly) – DataRadioBearer › Various parameters for RLC status reporting and retransmission. Should be set to recommended values. – MACConfiguration › xxMaxHARQTx – enable (>1) or disable (1) HARQ. Recommended to use 4 HARQTx. › tPeriodicBSRTimer – seen in UE testing to have some impact, recommend to set to 5ms. › tTimeAlignmentTimer – seen in UE testing to have some impact, recommend to set to 5120ms
  • 20. RBS PARAMTERS TN › TN MO parameters: – GigabitEthernet=1 › actualSpeedDuplex – if you see half-duplex, it could be a problem with auto- negotiation › dscpPbitMap (QoS mapping from L3 to L2) – IpInterface=2 (rec. MO id for Signalling and Payload) › vLan/vid (true/false and vlan id) – IpAccessHostEt=1 › ipAddress (X2/S1 control/user plane termination) – IpSyncRef (if NTP synchronisation is used) › syncStatus should be OK – Synchronization=1 › nodeSystemClock should be in LOCKED_MODE. › syncReference should show the correct reference (NTP or GPS) active and configured
  • 21. Enabled Features › User throughput can be limited by the available/installed licenses › The following features directly impact end user throughput – Downlink/Uplink Baseband Capacity – Channel Bandwidth (5, 10, 15 and 20) MHz – 64-QAM DL / 16-QAM UL – Dual Antenna DL Performance Package › To quickly check active licenses (including states): – moshell> inv Licensing (9/1551-LZA 701 6004 )
  • 22. Expected Throughput (Simplified) dlCyclicPrefix = 15 KHz => 7 OFDM symbols Resource Elements (RE) per Resource Block (7 OFDM symbols x 12 SubCarriers) RE per SB 2 x RB RS RE (per RB) RS RE (per SB) Control Region Size (CRS) in OFDM symbols nrOfSymbolsPdcch 1 2 3 1 2 3 RE per CRS (OFDM*12 - 4 RS Tx) (OFDM*12 - 8 RS MIMO) 8 20 32 16 40 64 Tot Num RE per SB available for PDSCH (best case w/o SCH/BCH) 144 132 120 288 264 240 Bits per SB - QPSK (2) 288 264 240 576 528 480 Bits per SB - 16QAM (4) 576 528 480 1152 1056 960 Bits per SB - 64QAM (6) 864 792 720 1728 1584 1440 Max Theoretical L1 Thrpt (Mbps) 20 MHz => 100 RB (64 QAM) 86.4 79.2 72 172.8 158.4 144 15 MHz => 75 RB (64 QAM) 64.8 59.4 54 129.6 118.8 108 10 MHz => 50 RB (64 QAM) 43.2 39.6 36 86.4 79.2 72 5 MHz => 25 RB (64 QAM) 21.6 19.8 18 43.2 39.6 36 Tot Num RE per SB available for PDSCH (worst case with SCH/BCH in SB) SCH = 24, BCH = 4 x 12 - 4 per CW 76 64 52 152 128 104 Bits per SB - (QPSK) 152 128 104 304 256 208 Bits per SB - (16QAM) 304 256 208 608 512 416 Bits per SB - (64QAM) 456 384 312 912 768 624 Tx Diversity 2x2 MIMO 84 DL Scheduling Block (SB) -> Bit calculation (Normal CyclicPrefix) 168 16 32 168 336 8 16
  • 23. Identify the domain › Further analysis required: – Our basic checks have come up short – Throughput issues exist that require advanced/additional analysis › Analysis steps to perform: – Single UE call scenario – Send UDP type traffic in DL/UL direction (e.g. Iperf) – Monitor close to or on the RBS (e.g. Wireshark) – Optionally use a radio monitor (e.g. TEMS) › Decide - Radio or Transport analysis: – Radio issues provide more control for LTE RAN analysis – Transport issues blend/carry-on towards core elements
  • 25. Radio Analysis › From the domain analysis previously, we believe the Radio may be affecting user throughput › We’ve previously ruled out configuration and MO status using the basic checks › The following slides will cover the various components which make up the radio domain and help to pinpoint the source of poor throughput. › We rely on the baseband scheduler traces and signal traces (mtd) between blocks.
  • 26. Radio Analysis › Ericsson’s LTE Baseband provides a detailed mechanism for tracing the complete L1 and L2 interaction, including MAC scheduling decisions and L1 decoding results. › Using this information we can further isolate the cause of the problem and pinpoint either: – UE problem › Cannot detect ACK/NACK? › Invalid UE reports? – Uu air interface problem – eNB problem › Incorrect setting or non-optimal combination of settings › Scheduling abnormality › Limitation in current eNB software – eNB northbound problem › S1 user plane › Application Server › Core network, SASN/SGW, etc
  • 27. RADIO ANALYSIS › To perform targeted radio analysis, it’s useful to know radio aspects specific to the following traffic scenarios: 1. Downlink 2. Uplink 3. Both uplink and downlink › Post-processing tools will be briefly demonstrated
  • 28. Radio Analysis - Downlink › Areas of analysis for Downlink: – CQI (Channel Quality Index) and RI (Rank Indicator) reported from UE. – Transmission Mode: MIMO (tm3) vs. TxD (tm2) vs. SIMO (tm1) – MCS vs. number of assigned PRBs vs. assignable bits in scheduler – UE Scheduling percentage of TTIs (how often is the UE scheduled) – CFI (number of OFDM symbols for PDCCH) vs. MCS vs. % scheduling – HARQ – RLC retransmissions
  • 29. Radio Analysis – Uplink › Areas of analysis for Uplink: – Uplink scheduling overview – BSR (Buffer Status Report) – PHR (Power Headroom Report) – is the UE at maximum power? – Cell bandwidth vs. maximum allowable PRBs – Link Adaptation – MCS available and 16QAM – PDCCH SIB scheduling colliding with UL grant – HARQ (less important, because we can measure SINR)
  • 30. Radio Analysis DL – CQI/RI and TM › The eNB needs knowledge of the SINR conditions of downlink transmission to a UE in order to select the most efficient MCS/PRB combination for a selected UE at any point in time. › Channel Quality Index (CQI): – Is a feedback mechanism from UE to eNB – Informs eNB of current channel conditions as seen at UE – Directly maps to 3GPP defined modulation/code rate (TS36.213 Table 7.2.3-1) › Defined as the highest coding rate the UE could decode at 10% BLER on HARQ rv=0 transmission – CQI 1-6 map to QPSK – CQI 7-9 map to 16QAM – CQI 10-15 map to 64QAM › Rank Indicator (RI) – Is a feedback mechanism from UE to eNB – Informs eNB whether UE can successfully decode RS from 1 or 2 (or more) antennas. – eNB scheduler uses this feedback to transmit with either:
  • 31. CQI polling Radio analysis DL – CQI/RI and TM › The UE measures DL channel quality and reports to eNodeB in the form of Channel Quality Information (CQI) › The average CQI (periodic-CQI reporting) for the whole band (wide-band CQI) is reported periodically on PUCCH (or on PUSCH if user data is scheduled in that TTI) with configured periodicity. › Sub-band CQI (aperiodic-CQI reporting) is reported when requested by the eNB. This report is for the PDSCH. Report sent on PUSCH. – CQI polling is triggered on demand by eNB based on DL traffic activity. › When 2 antennas are configured, Rank Indicator is also reported. Precoding Matrix Indicator (PMI) also reported in case of transmission mode 4 (not in L11A). CQI DL frequency band PUCCH PUCCH PUSCH
  • 32. Radio Analysis DL – CQI/RI and TM › In order to transmit with MIMO (OLSM) we should check the following: – eNB cell is configured with two working transmit antennas. › Check EUtranCellFDD::noOfUsedTxAntennas > 1 › L11A GA (default) system constant SC125:3 means that tm3 is used in case 2 TX antennas are defined. › If only one TX antenna is configured, then tm1 is used › In order to force Transmit Diversity (i.e. prevent OLSM), SC125:2 must be set – UE CQI/RI report from UE shows RI > 1 › Rank 1: TxDiversity (transmission mode 2, tm2) › Rank 2: MIMO (Open Loop Spatial Multiplexing in L11A) (transmission mode 3, tm3) › mtd peek -ta ulMacPeBl -signal LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND -dir OUTGOING – This signal (from L1 to MAC scheduler) shows the reported CQI and RI – (also shows HARQ ACK/NACK for downlink data transmission) – (also shows rxPowerReport and timingAdvanceError)
  • 33. cfrPusch { cfrInfo { ri = 2, cfrLength = 22, cfrFormat = 4, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [61440, 0, 0, 0] as hex: [f0 00 00 00 00 00 00 00] } Radio Analysis DL – CQI/RI and TM LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND UpUlMacPeCiUlL1Meas2DlIndS { cfrPucch { cfrInfo { ri = 0, cfrLength = 4, cfrFormat = 0, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [0, 0] as hex: [00 00 00 00] } cfrFormat=0 is a WCQI report only (ignore RI) Valid report if cfrValid=1,cfrExpected=1,cfrCrcFlag=1 mtd peek -ta ulMacPeBl -signal LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND -dir OUTGOING cfrFormat=4 is a SCQI + RI report WCQI is first half octet (f => 15). Octets thereafter are subband CQI reports for each RBG. A number of subband CQIs follow (see next slide) cfrPusch { cfrInfo { ri = 2, cfrLength = 18, cfrFormat = 4, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [48969, 49152, 0, 0] as hex: [bf 49 c0 00 00 00 00 00] } Rank Indicator = 2 (indicates UE can decode both antenna streams) WCQI = 11. 5MHz bandwidth means 4PRBs subbands. SCQI = F49C = 11 11 01 00 10 01 11 00 SCQI PRBs: 0-3  -1, 4-7  -1, 8-11 +1, 12-15 0, 16-19  +2, 20-23 +1, 24  -1
  • 34. Radio Analysis DL – SCQI Visualisation › From the previous slide, SCQI is visualised here.. – For 5MHz, each RBG is 4 PRBs wide (except for SCQI group 7) – SCQI is given relative to WCQI which was 11 in this example f 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 5 MHz SCQI PRBs: 0-3  -1, 4-7  -1, 8-11 +1, 12-15 0, 16-19  +2, 20-23 +1, 24  -1 Sub-band 1 2 3 4 5 6 7 CQI value (  10  10 12 11  13 12  10 )
  • 35. Radio analysis DL – CQI/RI and TM › cfrFormat = 4 consists of: – 4 bit Wideband CQI (i.e. CQI across whole bandwidth) – Up to 13 subband CQI differentials (depends on bandwidth of cell) › Subband CQI (3GPP TS36.211 Ch 7.2.1) – RBG width depends on bandwidth: › 3 & 5MHz – subband width 4 PRBs › 10MHz – subband width 6 PRBs › 15 & 20MHz – subband width 8 PRBs – Subband Differential mapping, see table below:
  • 36. Radio analysis DL – CQI/RI and TM cfrFormat Report includes 0 WCQI 1 RI 2 WCQI + WPMI 3 SCQI 4 SCQI + RI 5 WCQI + SPMI + RI 6 SCQI + WPMI + RI › 7 possible cfrFormats defined in L11A. › Typically see reports cfrFormat 0 and 4 as described previously › Note that PMI is not yet used (requires tm4)
  • 37. Radio analysis DL – CQI/RI and TM › Transmission Mode and MCS can be traced out with the following: ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=1 tbs={7992 0} mcs={18 0} noOfSBs={4294443008 0} rv={0 1} ndi={0 0} rmGbits={21600 0}" MCS for each codeword. In this case, tm2 so only one MCS listed. lhsh gcpu01024 te e trace4 UpcDlMacCeFt_DL_SCHEDULER LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS { sfn = 280 subframeNr = 7 l1Control { transmissionMode = 2 prbResourceIndicatorType = 0 prbList[] = [4294443008, 0, 12, 0]dec [ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex commonTb { newDataFlag = 1, tbSizeInBytes = 999, l1Tb { rvIndex = 0, modType = 2 (UPDLMACPEMode64Qam), nrOfRateMatchedBits = 21600, rmSoftBits = 1237248 } } PRB list in RBGs, for 5MHz RBG size is 2. fff8 corresponds to 25 PRBs (last PRB is 1 less). MCS is a combination of tbSize and modType. 999 bytes = 7992 bits then put into TS36.213 Table 7.1.7.2.1-1 for NPRB=25. That gives ITBS of 16. Convert ITBS to MCS using Table 7.1.7.1-1. mtd peek -ta dlMacPeBl -signal LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND -dir INCOMING -filter {(U16SIG)8,NEQ,(U16)0x00}
  • 38. Radio Analysis DL – CQI/RI and TM › RBG for Resource Allocation Type 0 – Defined in 3GPP TS36.213 Ch 7.1.6.1 – One bit used to represent a certain number of consecutive PRBs – 1.4MHz is RBG size 1 – 3 & 5MHZ is RBG size 2 – 10MHz is RBG size 3 – 15 & 20MHz is RBG size 4
  • 39. Radio analysis DL – CQI/RI and TM › Example of switching transmission modes based upon RI (bfn:3352, sfn:280, sf:5.47, bf:128) ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=1 tbs={7992 0} mcs={18 0} noOfSBs={4294443008 0} rv={0 1} ndi={0 0} rmGbits={21600 0}" TM=2 transmission with MCS 18 cfrPusch { cfrInfo { ri = 2, cfrLength = 18, cfrFormat = 4, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [48969, 49152, 0, 0] as hex: [bf 49 c0 00 00 00 00 00] } Rank Indicator = 2 received from UE. eNB will now switch to tm3 (OLSM MIMO) transmission WCQI 11 + SCQI. bfn:3352, sfn:280, sf:6.47, bf:131) ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=0 tbs={5736 5736} mcs={13 13} noOfSBs={4294443008 0} rv={0 0} ndi={0 1} rmGbits={14400 14400}" 25 PRBs as according to previous example TM=3 with MCS=13. TBS = 5736 => 11448 over NPRB=50
  • 40. Radio Analysis DL – Assignable Bits › If UE is sending with high CQI (in the range 10-15) and RI=2 but throughput is still very low, then the next check should be assignable bits. › Assignable bits means the amount of data in the downlink buffer available for the scheduler to schedule for this UE. › A classic symptom of low assignable bits is that the UE is scheduled with a high MCS but a low number of PRBs. – The scheduler always attempts to send with the highest possible MCS and least number of PRBs so that left-over PRBs could be assigned to another UE. › Another symptom is that the UE is not scheduled every TTI (and nothing else is available to schedule).
  • 41. Radio Analysis DL – Assignable Bits › Possible causes for low assignable bits: 1. RLC STATUS messages are not being received fast enough and RLC buffers are full. › Until RLC STATUS ACK messages are received, already transmitted RLC SDUs are kept in memory in UE and/or eNB › Check for RLC DISCARDs but low (or 0) assignable bits 2. Data received from core network is not enough to fill the RLC buffers in eNB. › Check that non-TCP based traffic is not being sent with too large packet size. For iperf based traffic, recommended size 1360 bytes (default is 1470). › Set MTU of 1360 in UE (or UE laptop). › RLC DISCARDs will trigger TCP congestion control and lower thpt. › In L11A the default RLC buffer size per RB is 750 IP packets – Trace discards with lhsh gcpu00768 te e all UpDlPdcpPeFt_DISCARD – Discards on UDP traffic will not affect throughput – Discards on TCP traffic will trigger TCP congestion control (lower thpt.)
  • 42. Radio Analysis DL – Assignable Bits ULMA3/UpDlPdcpPeFt_DISCARD TRAFFIC_ABNORMAL Discarding DL PDCP PDU due to exceeding limits. maxBufferedPacketsInRlc=751 totalNumNonAckedDrbPackets=751 cellId=12 bbUeRef=201327456 bbBearerRef=201327458 receiveFromTeid=3779046158 payloadLength=1506 bytes incl GTP-U header. hoState=0" 750 is default PDCP/RLC buffer per UE in eNB (L11A) TRAFFIC_ABNORMAL corresponds to trace1. Traffic discards for UDP are normal, but for TCP traffic it will cause severe throughput degradation ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61 bbUeRef=201327456 PQ lcid=1 assignableBits=0 minPduSize=56 selectedHarq=0" ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61 bbUeRef=201327456 PQ lcid=2 assignableBits=0 minPduSize=56 selectedHarq=0" ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61 bbUeRef=201327456 PQ lcid=3 assignableBits=8554024 minPduSize=56 selectedHarq=0" LCID 3 is for the default bearer. LCID 1 and 2 for SRB About 1MByte of data available for scheduling. Check for low value of assignable bits which indicates e2e problems affecting data available to schedule on air for eNB. Low assignable bits for UDP traffic may indicate MTU problems. lhsh gcpu00768 te e all UpDlPdcpPeFt_DISCARD lhsh gcpu01024 te e trace4 UpcDlMacCeFt_DL_SCHEDULER
  • 43. Radio Analysis DL – CFI and Scheduling › Another cause of low (or lower than expected) throughput is that the UE is not being scheduled in every TTI. › This may be caused by: – Limitations in current scheduler implementation – 3GPP defined compromises between control channel efficiency and scheduling efficiency (especially for lower number of users) › L11A software has some limitations to be aware of: – Only one SE per TTI is supported in L11A – SIBs are scheduled in the same was a user data (i.e. they are sent to the scheduler). – When a SIB is transmitted, no user data can be transmitted in the DL at the same time (using default parameters). › It is possible to use (System Constant) SC43 to enable 2SE/TTI in DL › It is possible (only for demo use) to temporarily disable SIB scheduling
  • 44. Radio Analysis DL – CFI and Scheduling › SIBs require PDCCH resources › Typically SIBs consume 4 or 8 CCEs of PDCCH resources. › If a UE is in good SINR conditions, the scheduler may allocate only one CCE for that UE. – In that case, because of limited positions in PDCCH, it is quite likely that a PDCCH collision occurs (especially in low system bandwidths) › If a UE is in bad SINR conditions, the scheduler may allocate a large number of CCEs for that UE (2 or 4 or 8 CCEs) – Depending on the configured CFI there may only be common search space available or it may still collide with other PDCCH users. › See Radio Analysis UL – PDCCH slides for more details
  • 45. Radio Analysis Dl – HARQ › Each transport block transmission is represented as a HARQ process. – Each HARQ process data is held in memory until NDI is toggled (i.e. New data is to be sent). – This allows fast retransmission of erronerously received data. › The schedulers representation of an HARQ process is as follows: – Feedback status › (ACK, NAK, DTX, PENDING) – TBS – transport block size – MCS – modulation and coding scheme – RV – redundancy version. HARQ has 4 redundancy versions, rv0, rv2, rv3, rv1. – NDI – New Data Indicator (physical layer bit toggled for new data). › Do not confuse with newDataFlag which is scheduler internal flag where 1 means new data and 0 means retransmission. – Number of transmission attempts (max 4 transmissions in L11A default paramters) › In case of rank 2 spatial multiplexing there are 16 HARQ process per UE instead of 8, but there are two processes that share the same ID – Scheduler sees them as separate processes that are coupled to each other
  • 46. Radio Analysis Dl – HARQ Example › The following slides will show an example of tracing out downlink HARQ – Initial downlink grant is sent with rv=0 (MIMO, 2 codewords) › SFN 280/subframe 8 – HARQ NACK received on both code words › SFN 281/subframe 2 (DL Grant + 4TTI) – First retransmission sent with rv=2 › SFN 281/subframe 6 (8 TTI past initial transmission is earliest occasion) – HARQ ACK received on both code words › SFN 282/subframe 0 (DL Grant ReTx + 4TTI)
  • 47. Radio Analysis Dl – HARQ DL Grant LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS { sfn = 280 subframeNr = 8 ueAlloc[0] { l1Control { rnti = 61 transmissionMode = 3 prbResourceIndicatorType = 0 prbList[] = [4294443008, 0, 12, 0]dec [ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex swapFlag = 0 } nrOfTb = 2 tbAlloc[0] { tbIndex = 0 commonTb { newDataFlag = 1, tbSizeInBytes = 717, l1Tb { rvIndex = 0, modType = 1 (UPDLMACPEMode16Qam), nrOfRateMatchedBits = 14400, rmSoftBits = 1237248 } } macTb { dlHarqProcessId = 0, nrOfMacCtrlElem = 0 } rlcTb { nrOfBearer = 1, bearerAlloc[0] { bbBearerRef = 201327458, lcid = 3, rbScheduledSizeInBytes = 717 } } } tbAlloc[1] { ... SFN/subframe where DL PDSCH will occur. PDCCH DL Grant sent at same sfn/subframe. RNTI, TM, used PRBs (same for both code words) If re-transmission, this indicates if CW0 and CW1 swapped layers newDataFlag indicates if it is new data or not HARQ redundancy version. rv0 used for initial transmission, rv2, rv3, rv1 used for re-transmission. HARQ process number. 8 HARQ processes exist in FDD LTE L11A. CW1 defined here.
  • 48. Radio Analysis Dl – HARQ FEEDBACK (NACK/NACK) LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND (431) UpUlMacPeCiUlL1Meas2DlIndS { sigNo = 23070220 header { cellId = 12 sfn = 281 subFrameNo = 2 } nrOfPuschReports = 0 nrOfPucchReports = 1 totalNrOfReports = 1 reportList[0] { pucchReport { meas2DlUlReportType = 1 (ElibBbBaseCommonMeas2DlPucchReport) bbUeRef = 201327456 isDtx { isDtx = 0 } dlHarqInfo { dlHarqValid = 1, detectedHarqIndication = 0, dlHarqProcessId = 0, nrOfTb = 2, swapFlag = 0 } rxPower { prbListStart = 0, prbListEnd = 0, rxPowerReport = -1150, sinr = 0 } timingAdvanceError { timingAdvanceError = 1 } cfrPucch { cfrInfo { ri = 0, cfrLength = 0, cfrFormat = 0, cfrValid = 0, cfrExpected = 0, cfrCrcFlag = 0 }, cfr[] = [0, 0] as hex: [00 00 00 00] } } } } SFN/subframe +4 from DL grant (i.e. where the HARQ ACK/NACK is received from UE). HARQ NACK received for DL HARQ Process 0 on both code words. DetectedHarqIndication: 0 => NACK/NACK, 1 => NACK/ACK, 2 => ACK/NACK, 3 => ACK/ACK, 4 => DTX (nothing received)
  • 49. Radio Analysis Dl – HARQ ReTX LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS { sfn = 281 subframeNr = 6 ueAlloc[0] { l1Control { rnti = 61 transmissionMode = 3 prbResourceIndicatorType = 0 prbList[] = [4294443008, 0, 12, 0]dec [ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex swapFlag = 0 } nrOfTb = 2 tbAlloc[0] { tbIndex = 0 commonTb { newDataFlag = 0, tbSizeInBytes = 717, l1Tb { rvIndex = 2, modType = 1 (UPDLMACPEMode16Qam), nrOfRateMatchedBits = 14400, rmSoftBits = 1237248 } } macTb { dlHarqProcessId = 0, nrOfMacCtrlElem = 0 } rlcTb { nrOfBearer = 0 } } tbAlloc[1] { tbIndex = 1 commonTb { newDataFlag = 0, tbSizeInBytes = 717, l1Tb { rvIndex = 2, modType = 1 (UPDLMACPEMode16Qam), nrOfRateMatchedBits = 14400, rmSoftBits = 1237248 } } SFN/subframe where DL PDSCH will occur. PDCCH DL Grant sent at same sfn/subframe. RNTI, TM, used PRBs (same for both code words) Same as previous transmission newDataFlag=0 means it’s a retransmission HARQ redundancy version. rv2 is used for first retransmission HARQ process number (same as before) CW1 defined here.
  • 50. Radio Analysis Dl – HARQ FEEDBACK (ACK/ACK) LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND (431) UpUlMacPeCiUlL1Meas2DlIndS { sigNo = 23070220 header { cellId = 12 sfn = 282 subFrameNo = 0 } nrOfPuschReports = 0 nrOfPucchReports = 1 totalNrOfReports = 1 reportList[0] { pucchReport { meas2DlUlReportType = 1 (ElibBbBaseCommonMeas2DlPucchReport) bbUeRef = 201327456 isDtx { isDtx = 0 } dlHarqInfo { dlHarqValid = 1, detectedHarqIndication = 3, dlHarqProcessId = 0, nrOfTb = 2, swapFlag = 0 } rxPower { prbListStart = 0, prbListEnd = 0, rxPowerReport = -1152, sinr = 0 } timingAdvanceError { timingAdvanceError = 0 } cfrPucch { cfrInfo { ri = 0, cfrLength = 0, cfrFormat = 0, cfrValid = 0, cfrExpected = 0, cfrCrcFlag = 0 }, cfr[] = [0, 0] as hex: [00 00 00 00] } } } } SFN/subframe +4 from DL grant (i.e. where the HARQ ACK/NACK is received from UE). HARQ ACK/ACK received for DL HARQ Process 0 on both code words. DetectedHarqIndication: 0 => NACK/NACK, 1 => NACK/ACK, 2 => ACK/NACK, 3 => ACK/ACK, 4 => DTX (nothing received)
  • 51. Radio Analysis DL – RLC › RLC retransmissions are triggered: 1. When HARQ fails to transmit a transport block within the maximum number of configured retransmissions – Default number of HARQ transmissions is 4 in L11A 2. If RLC STATUS messages are not received within the time frames configured › RLC STATUS messages are sent between peer nodes (eNB and UE) to inform about lost RLC packets. They can be traced out using – mtd peek -ta dlRlcPeBl -si UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND › Check: – ACK_SN should be increasing, otherwise RLC buffers are not released – NACK_SN indicates RLC retransmissions (occasionally is OK) – DataRadioBearer::tStatusProhibit governs how often RLC STATUS messages may be generated, default is 25ms in L11A. › A too low value will produce too many RLC control messages › A too high value may cause RLC buffers to become exhausted
  • 52. Radio Analysis DL – RLC 0xd4205d4f=(sfn:322, sf:0.33, bf:212): UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND (343) UpDlRlcPeRlcStatusForDlTrafficIndS { RLC PDU { D/C = 0 (Control PDU) CPT = 000 (STATUS PDU) ACK_SN = 743 E1 = 0 (A set of NACK_SN, E1 and E2 does not follow) } } Indicates the SN (Sequence Number) of the last successfully received RLC packet NACK_SN indicates RLC retransmissions (HARQ failures) 0xd4205d4f=(sfn:324, sf:5.33, bf:212): UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND (343) UpDlRlcPeRlcStatusForDlTrafficIndS { RLC PDU { D/C = 0 (Control PDU) CPT = 000 (STATUS PDU) ACK_SN = 787 E1 = 0 (A set of NACK_SN, E1 and E2 does not follow) } } Check that ACK_SN is increasing or RLC buffers not released Next RLC STATUS received 25ms later, check that it’s coming regularly DataRadioBearer::tStatusProhibit=25ms default
  • 53. Radio Analysis – Uplink › Areas of analysis for Uplink: – Uplink scheduling overview – BSR (Buffer Status Report) – PHR (Power Headroom Report) – is the UE at maximum power? – Cell bandwidth vs. maximum allowable PRBs – Link Adaptation – MCS available and 16QAM – PDCCH SIB scheduling colliding with UL grant – HARQ (less important, because we can measure SINR)
  • 54. Radio Analysis UL – UPlink Scheduling UL › Scheduling request, SR (PUCCH) UE requests UL resources eNodeB UL scheduler Ue Channel state info › Data is transmitted (PUSCH) › UL Grant (PDCCH) Scheduler assigns initial resources › Buffer status report (PUSCH) transmitted in UL › UL grant (PDCCH) transmitted (valid per UE) › Channel sounding
  • 55. macCtrlElementList[0] { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr) powerHeadroomReport { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), powerHeadroom = 127 } cRnti { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), crnti = 8362387 } truncatedBSR { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), bufferSize = 127 } shortBSR { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), bufferSize = 127 } longBSR { type = 6 (UpUpCommonMacCommonMacCtrlElemShortBsr), bufferSizeNr1Nr2 = 127, bufferSizeNr3Nr4 = 39315 } } Radio Analysis Ul – BSR › Buffer Status Report (BSR) is used to inform the eNB of the current data waiting for transmission in the UE (3GPP TS36.213 Ch. 6.1.3.1) › Values ranges from 0 up to >15000 bytes using 64 index values. – e.g. index 0 for BS=0, index 1 for 0 < BS <= 10 and so forth. › Can be traced out through LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND. Expect to see high values for maximum UL throughput. Low values indicate UE/laptop problem. Type of MAC report, this case short BSR (6) LSB 6 bits are the BSR index (this case >150000 bytes) MSB 2 bits is the LCID
  • 56. Radio Analysis UL – PHR › Power Headroom Report (PHR) is used to inform the eNB of the remaining transmit power available at the UE. (3GPP TS36.321 Ch. 6.1.3.6) › Defined as difference between configured maximum UE output power and estimated power used for PUSCH transmission › Reports a index value similar to BSR with values between -23 up to 40 dB › PH values are close to (or less than) 0 means the UE is power limited – Ideally we look for positive values somewhat greater than 0 – When a UE is power limited, the eNB may schedule fewer PRBs in order to reduce the required output power of the UE, this can in turn reduce throughput.
  • 57. Radio Analysis UL – PHR macCtrlElementList[0] { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr) powerHeadroomReport { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), powerHeadroom = 55 } cRnti { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), crnti = 3637377 } truncatedBSR { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), bufferSize = 55 } shortBSR { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), bufferSize = 55 } longBSR { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), bufferSizeNr1Nr2 = 55, bufferSizeNr3Nr4 = 32897 } } Type of MAC report, this case PHR (3) PHR value of 55 which corresponds to 32 <= PH < 33. In this case there is no power limitation on the UE side. PH Index values <= 23 indicates the UE has reached maximum transmission power Negative values indicate the UE was power limited See 3GPP TS36.133 Ch 9.1.8.4 for index mapping
  • 58. Radio Analysis UL – PUCCH and PUSCH › PUCCH takes a minimum 1 PRB on each side of the uplink band for uplink control signalling, reducing the size of PUSCH – E.g. 5MHz bandwidth, 25 PRBs available. Minimum 2 PRBs for PUCCH. – 23 PRBs available for PUSCH 0 1 2 3 4 5 6 7 8 9 Time (ms) Radio Frame Cell Bandwidth PUSCH – Used for UE data scheduling and UL RA msgs PUCCH – Semi-static allocation of CQI, SR, ACK/NAK PUCCH – Semi-static allocation of CQI, SR, ACK/NAK PUCCH PUCCH PUSCH
  • 59. Radio Analysis UL – PRB Limitations › Due to 3GPP specified design limitations in the UL it is not always possible to utilise all free PRBs for UL transmissions › 3GPP TS36.211 Ch 5.3.3 defines the following formula for the number of PRBs on PUSCH for a single transmission: – Where a, b and c are integers. – For 5MHz: › 23 PRBs are available for PUSCH (2 allocated to PUCCH) › Max number of PRBs for a single PUSCH transmission is 20 PRBs. › This corresponds to a=2, b=0 and c=1 (i.e. 3 PRBs are unavailable to be used). › In L11A, 3 PRBs would be unused (only one SE/TTI possible). › In later releases, 3 PRBs could be used by a second UE. c b a 5 3 2  
  • 60. Radio Analysis UL – Link Adaptation Goal: Select MCS for a certain allocation size to maintain the target BLER (10%) for the first transmission Inputs to Uplink Link Adaptation are: UL interference power: LPP_UP_ULCELLPE_CI_CELL_STATUS_REPORT_IND outgoing from ulCellCeBl Received power of UE (across traffical PUSCH PRBs): LPP_UP_ULMACPE_CI_UL_L1_MEAS2_UL_IND outgoing from ulMacPeBl PHR reports & HARQ CRC (BLER): LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND outgoing from ulMacPeBl Input to Link Adaptation ulL1Meas2UlInd(rxPower(-140 .. 0 dB) ulMacPe run every subframe UE transmitts cellStatusReportInd(interferencePower (-125 .. -80dB) ulCellCe run every subframe ulL1MacCtrlInfoInd(powerHeadroom(0 .. 63dB)) ulMacPe run every periodicPhrReport ulL1MacCtrlInfoInd(CRC) ulMacPe run every subframe UE transmitts
  • 61. Radio Analysis UL – Link Adaptation LPP_UP_ULCELLPE_CI_CELL_STATUS_REPORT_IND UpUlCellPeCiCellStatusReportIndS { sfn = 456 subFrameNo = 3 interferencePower = -1170 Cell interference level x 10 (i.e. -117.0 dBm) High values here (>-104) LPP_UP_ULMACPE_CI_UL_L1_MEAS2_UL_IND (432) UpUlMacPeCiUlL1Meas2UlIndS { sfn = 264 subFrameNo = 8 nrOfPuschReports = 1 rxPower { prbListStart=1, prbListEnd=48, rxPowerReport=-956, sinr=821854514 } rxPwr = -95.6dBm over those PRBs (pZeroNominalPusch= -96dBm) = ~22.9dB ) 2 (sinr log 10 = sinr[dB] -22 10   LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND (433) UpUlMacPeCiUlMacCtrlInfoIndS { sfn = 264 subFrameNo = 8 harqInfo = 1 (UpUpCommonMacCommonMacCtrlElemHarqFeedbackAck) macCtrlElementList[0] { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr) powerHeadroomReport { type = 3 (UpUpCommonMacCommonMacCtrlElemPowerHr), powerHeadroom = 55 } HARQ ACK / PHR
  • 62. Radio Analysis UL – Link Adaptation › L11A supports up to MCS 24 in the uplink by default – MCS21-24 are defined as 64QAM – However, according to 3GPP TS36.213 Ch 8.6.1 if a UE does not support 64QAM then 16QAM can be used for MCS21-24. – Check that MCS24 is selected. If not, check link adaptation inputs for problems › In UL, the eNB itself can directly measure SINR of the received signal – Therefore CQI is not necessary for UL transmission – eNB can use Received Power/SINR, PHR, UL interference and UL HARQ BLER measurements to control MCS
  • 63. Radio Analysis UL – Link Adaptation › Check for: – High values of UL interference › Could there be some external interferer? › Are the values of pZeroNominalPusch in neighbour cells too high? – rxPower too low › Target is EUtranCellFDD::pZeroNominalPusch. Is it set too high? – PHR shows UE at maximum Tx power › Is EUtranCellFDD::pZeroNominalPusch too high causing UE to exceed maximum transmit power? › Closed-loop power control TPC ignored by UE? – Low values of SINR › Is EUtranCellFDD::pZeroNominalPusch too low? › Closed-loop power control TPC ignored by UE?
  • 64. Radio Analysis UL – PDCCH 0 1 2 3 4 5 6 7 8 9 Time (ms) Radio Frame PDCCH carries both the UL (PUSCH) assignment and DL (PDSCH) assignment. In case many PDCCH CCEs are used for DL transmission (e.g. SIB with 8 CCEs) it may be that UL grant is not possible to be scheduled in this TTI for a single UE! PUSCH UL subframe (4 TTI later) DL subframe (current) PDSCH PDCCH Note: PCFICH and PHICH multiplex into the DL subframe red area marked PDCCH
  • 65. Radio Analysis UL – PDCCH › PDCCH is used to signal: – Downlink (PDSCH) assignments – Uplink (PUSCH) grants › In case of a downlink SIB transmission, 8 CCEs of PDCCH may be used for downlink grant. › To reduce processing load when decoding PDCCH, 3GPP defines particular search spaces within PDCCH depending on: – Number of CCEs for grant – Number of CCEs for PDCCH – RNTI of the UE › Depending on these parameters, it may not be possible to allocate a PDCCH uplink grant resource and therefore the UE may not be able to be scheduled every TTI even if there are unused PUSCH resources. See 3GPP TS36.213 Ch 9.1.1 See KO link: PDCCH visualisation KO
  • 66. Radio Analysis UL – PDCCH From KO link: PDCCH visualisation KO Search space for 1 CCE completely overlaps 8 CCE search space. In this example, DL SIB transmission completely prevents any UL grant for this UE RNTI 516 in subframe 5
  • 67. Radio Analysis UL – HARQ › LTE defines uplink with synchronous HARQ to reduce PDCCH signaling load and simplify the uplink HARQ processing › Example 1, successfully received PUSCH data: – Subframe n: UL grant sent to UE – Subframe n+4: PUSCH data received (rv=0) – Subframe n+8: ACK sent, UL grant with New Data Indicator toggled – Subframe n+12: new PUSCH data received (new HARQ process) › Example 2, HARQ retx: – Subframe n: UL grant sent to UE – Subframe n+4: PUSCH data received (rv=0) – Subframe n+8: NACK sent, NO UL grant is signaled on PDCCH – Subframe n+12: PUSCH data received (rv=2) (same HARQ process) – Subframe n+16: ACK/NACK, etc up to max number of retx
  • 68. Radio Analysis UL – HARQ ReTx Postponed reTx Colliding ACK Adaptive reTx time PUSCH for scheduling n+8 Assume all three non-correctly decoded (CRC not ok) PUCCH PUCCH PRACH Non-Adaptive reTx NACK SE1 SE3 SE2 n (N)ACK Grant needed
  • 69. Radio Analysis UL – HARQ › Because of the synchronous nature of Uplink HARQ, the following scheduling priority is used: – Random Access Message 3 (RRC Connection Request). Scheduled 6 subframes before, special case. – Non-adaptive HARQ retransmission – Adaptive HARQ retransmission – New Data transmission › Non-adaptive means no UL grant is explicitly scheduled for the retransmission › Adaptive means that scheduling collision occurred (e.g. collision with PRACH) and an explicit UL grant was signalled to: – Move the allocated PRBs to another part of the UL spectrum – Suspend (delay) the UL HARQ retransmission for 8 TTIs later
  • 70. Radio Analysis – Traffic Abnormal › TRACE1 in baseband is defined as TRAFFIC_ABNORMAL. It should be used to trace out abnormal conditions in baseband processing. – Normally the output gives a good description of the problem encountered › Some useful TRAFFIC_ABNORMAL traces: – lhsh gcpu01024 te e trace1 UpcDlMacCeBl – lhsh gcpu01024 te e trace1 Upc* – lhsh gcpu00256 te e trace1 UpUlMacPeBl_Smac – lhsh gcpu00768 te e trace1 ElibPapBlEth – lhsh gcpu00256 te e trace1 UpUlMacPeBl_Smac – lhsh gcpu00768 te e trace1 UpDlL1PeFt_DEADLINE_MISSED – lh gcpu fte e trace1 .*_DISCARD
  • 71. Radio Analysis – Post-Processing Tools › 3GPP has specified L1 messages in order to reduce the bits required for transmission on the air interface. – These formats can be difficult to read › For this reason, many values in the traces are presented in formats which require conversion to human readable formats, for example: – PRBs allocated in DL/UL grant messages – PHR values – BSR values – SINR – MIMO HARQ feedback, etc.. › Tools exist to perform these conversions and compact the data presentation to the end user – One such tool is bbfilter or scheduling_filter.pl – Check the flowfox web page for details
  • 72. Radio Analysis – bbfilter Downlink $ cat decoded_dl_log.log | ./bbfilterv2.2 -bw 5 –dl sfn|sf|mode|dlModul|mcs1|mcs2|prb|Ndf|Tbs1|Tbs2|AssBits|Harq|dlBler|cqi|ri| 280| 4|TxDi| 64QAM | 16 | 0 |25 | Y|7736| 0|8771784| | | 11| 2| 280| 5| | | | | | | | | |A | 0% | | | 280| 6|TxDi| 64QAM | 18 | 0 |25 | Y|7992| 0|8764088|A | 0% | | | 280| 7|TxDi| 64QAM | 18 | 0 |25 | Y|7992| 0|8756144|A | 0% | | | 280| 8|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8748192|A | 0% | | | 280| 9|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8736760| | | | | 281| 0|Mimo| 16QAM | 12 | 12 |25 |Y Y|4968|4968|8737384|A | 0% | | | 281| 1|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8763568|N | 0% | | | 281| 2|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8776208|N N | 2% | | | 281| 3|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8800856|N N | 4% | | | 281| 4|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8825504|N N | 6% | | | 281| 5|TxDi| 16QAM | 30 | 0 |25 | N|7992| 0|8862160|N N | 8% | | | 281| 6|Mimo| 16QAM | 30 | 30 |25 |N N|5736|5736|8862200|N N | 10% | | | 281| 7|Mimo| 16QAM | 30 | 30 |25 |N N|5736|5736|8862200|A A | 10% | | | HARQ ACK/NACK refers to the transmission 4 subframes earlier! NOTE: Format modified to fit on slide, only example!
  • 73. Radio Analysis – bbfilter Uplink $ cat decoded_ul_log.log | ./bbfilterv2.2 -bw 5 –ul sfn|sf|rxPwrPus|prb|ulTpc|sinr|ulModul|mcs|ndf|ul bsr |phr |ul tbs| ul crc |har|ulBler| 266| 6| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y | | | 25456| | A | 2% | 266| 7| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | | 25456| | A | 2% | 266| 8| -95.6 | 48| 0:1 | 22 | 16QAM | 24| N | | | 25456| ERR 3182| N | 5% | 266| 9| -95.6 | 48| 0:1 | 23 | 16QAM | 24| Y | | | 24496| | A | 5% | 267| 0| -95.7 | 48| 0:1 | 22 | 16QAM | 24| Y |>150000 | | 24496| | A | 5% | 267| 1| -95.8 | 40| 0:1 | 22 | 16QAM | 24| Y | | | 21384| | A | 5% | 267| 2| -95.6 | 48| | 23 | 16QAM | 24| Y | | | 25456| | A | 5% | 267| 3| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | | 25456| | A | 4% | 267| 4| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y | | | 25456| | A | 4% | 267| 5| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y |>150000 | | 25456| | A | 4% | 267| 6| -95.6 | 48| 0:1 | 23 | 16QAM | 24| N |>150000 | | 25456| | A | 4% | 267| 7| -95.6 | 48| 0:1 | 23 | 16QAM | 24| Y | | | 25456| | A | 4% | 267| 8| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | 32 | 24496| | A | 4% | UL BSR and PHR values decoded NOTE: Format modified to fit on slide, only example!
  • 74. Radio Analysis - Summary › Areas of analysis for Downlink: – CQI / RI (Rank Indicator) reported from UE. – Transmission Mode (MIMO, TxD, SIMO) – MCS vs. number of assigned PRBs vs. assignable bits in scheduler – UE Scheduling percentage of TTIs (how often is the UE scheduled) – PDCCH CFI and scheduling impacts – HARQ – RLC retransmissions › Areas of analysis for Uplink: – BSR (Buffer Status Report) – PHR (Power Headroom Report) – is the UE at maximum power? – Cell bandwidth vs. maximum allowable PRBs – Link Adaptation – MCS available and 16QAM – PDCCH SIB scheduling colliding with UL grant – HARQ (less important, because we can measure SINR)
  • 76. Transport Analysis › End user data is transferred over the S1-U interface › Several Transport Network topologies (L2/L3) provide great flexibility in design › Several router redundancy methods are supported › Transport network dimensioning provides insights into the peak provisioning on the S1 link › The LTE RBS is a QoS enabler, providing end user and transport network QoS differentiation › Performance management counters and tracing provide us with powerful node observability methods
  • 78. Transport Topology › No strict requirements on using a L2 switched or L3 routed LTE RAN transport network › No specified topology requirement › A router is required in the network, but LTE RAN transport network does not have to be L3 › Network design is important (number of hops for L3 vs. size of broadcast domain for L2) › This topology flexibility could complicate troubleshooting efforts depending on the nodes involved (say 3PP support is required)
  • 79. Transport configuration › A 2 VLAN configuration is recommended (separating O&M and Transport): OAM SD ComInf Firewall MME Pool S-GW Pool CN Router/FW S1 GW If O&M GW If S1 GW If O&M GW If SoIP Server SoIP Servers distributed in the network. RBS 1 CP, UP & SOIP IP O&M IP Switched Ethernet (Carrier Ethernet) RBS n CP, UP & SOIP IP O&M IP O&M VLAN TN VLAN
  • 80. Router Path Supervision (RPS) › RPS provides router redundancy for S1/X2 traffic › RPS is configurable via the IpInterface MO
  • 81. virtual router redundancy protocol (VRRP) 10.1.1.34 10.1.1.35 10.1.1.34 › LTE RBS supports VRRP (a router redundancy protocol) › VRRP uses an election method to assign responsibility for a virtual router to one of the VRRP routers on a LAN › The Master VRRP router controls the IP address(es) associated with a virtual router and forwards packets sent to these IP addresses › If the Master fails, one backup VRRP router will act as the virtual router › LTE RBS is transparent to the process, it does not directly participate in VRRP eNB eNB eNB Master Backup
  • 82. Transport Dimensioning › Dimensioning of the northbound transport network will impact achievable end user throughput rate › LTE RBS transport network dimensioning process (mobile backhaul): › Dimensioning is based on payload only! Determine bandwidth needed for last mile Determine cell thpt in a loaded network and avg. cell thpt during busy hour Calculate agg. bandwidth required in mobile backhaul
  • 83. Dimensioning methods Method Description Overbooking Allows more users than the dimensioned quantity, using the rationale that only a subset of users is allocating bandwidth at the same time. Overdimensioning Calculates the dimensioned bandwidth required by multiplying the average requirement by an overdimensioning factor. Peak allocation Uses the maximum throughput capacity as the dimensioned link capacity. The link is dimensioned for the maximum possible bit rate. Overprovisioning Monitors the link use. When a predefined use limit is reached on the link, a capacity upgrade is initiated. A general rule is that the use limit is set to 50%.
  • 84. Transport Aggregation Input (assumptions) 20 MHz Cell Cell Peak Rate 150 Mbps Cell Throughput in a Loaded Network 35 Mbps Peak load for 3x1 in a Loaded Network ~100 Mbps S-GW/ PDN GW A3 A1 A1 S-GW/ PDN GW R B S R B S R B S R B S R B S R B S R B S R B S R B S A1 A2 A2 Dimension for: ΣA2 × 0.8 BH displacement factor Dimension for peak rate to 1 cell= 150 Mbit/s Dimension for ‘eNodeB throughput in a loaded network for a 3x1 configuration’ = 100 Mbit/s per eNB Dimension for ‘Average eNodeB throughput during Busy Hour’ = 50 Mbit/s per eNB
  • 85. DUL TN Capabilities (L11) › The DUL is equipped with two Gigabit Ethernet physical interfaces – TN-A: Electrical Ethernet (100/1000 Mbps) – TN-B: Electrical/Optical Ethernet (1000 Mbps) › Only 1 interface is activated through configuration (either TN-A or TN-B) › Maximum throughput of 173 Mbps (or 150 Mbps of User Data assuming other overheads such as signalling) › Up to 500 connected users › Note: – Wire tracing is not possible on the DUL – You must use an external product to mirror traffic leaving the DUL
  • 86. Quality of service (qos) › Transport network QoS is a part of the complete LTE QoS concept › QoS, in an IP transport network, is the ability to treat packets/frames differently based on their content › Without QoS, each packet/frame is given equal access to the network resources
  • 87. QOS for user plane Bearers Transport RAN Terminal Gateway (Bearer Policy Enforcer) Service 1 (e.g. Internet) Service 2 (e.g. P2P File Sharing) Service 3 (e.g. IMS-Voice or MTV) Default Bearer (QoS via MME) Dedicated Bearer (QoS via PCRF) Service Data Flow (SDF) IP Address Further Reading: 3GPP TS 23.401
  • 88. Quality Class Indicators › A Quality Class Indicator (QCI) is used to signal the QoS requirements of a bearer › 23.401 defines 9 standard QCIs, each one with specific characteristics › Operators may define proprietary QCIs to introduce new services QCI Resource Type Priority Packet Delay Budget Packet Loss Rate Example Services 1 GBR 2 100 ms 10-2 Conversational Voice 2 4 150 ms 10-3 Conversational Video (Live Streaming) 3 3 50 ms 10-3 Real Time Gaming 4 5 300 ms 10-6 Non-Conversational Video (Buffered Streaming) 5 Non-GBR 1 100 ms 10-6 IMS Signaling 6 6 300 ms 10-6 - Video (Buffered Streaming) - TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.) 7 7 100 ms 10-3 - Voice, - Video (Live Streaming) - Interactive Gaming 8 8 300 ms 10-6 - Video (Buffered Streaming) - TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.) 9 9
  • 89. Transport Network QoS – Layer 3 Site Infrastructure, IP-backbone and RAN Transport EPC Application IP PPPoE Ethernet Ethernet IP and Ethernet IP Transport Network IPx is Interface between node and IP transport network Version 4 Header Length 4 DiffServ 7 Total Length 16 20 bytes Further Reading: RFC 2474 Differentiated Services Code Point (DSCP) › QCIs are mapped to IP layer Differentiated Services Code Points (DSCP)
  • 90. Transport Network QoS – Layer 2 Site Infrastructure, IP-backbone and RAN Transport EPC Application IP PPPoE Ethernet Ethernet IP and Ethernet IP Transport Network IPx is Interface between node and IP transport network Preamble 7 DA 6 SA 6 TPI 2 TAG 2 Type 2 Data 46 to 1500 CRC 4 SFD 1 User Priority 3bits CFI 1bit VLAN ID 12bits › Use of P-Bits allows prioritizing different types of traffic › Priority queuing, enabling some Ethernet frames to be forwarded ahead of others within a switched Ethernet network › Frames can be assigned to different scheduler queues › Maintain the same value throughout the IP Network to deliver the same QoS Further Reading: IEEE 802.1p Priority Bit (P-bit)
  • 91. QOS Mapping › LTE RAN Quality of Service (QoS) ensures the LTE bearer and transport level service requirements Ethernet Frame IP Packet QoS Profile DATA DATA QCI AMBR ARP IP Header Ethernet Header DSCP Mapping: Takes place in RBS / EPC Mapping: Edge devices handling L2/L3 payload P-bit
  • 93. LTE RAN QoS Configuration › LTE RAN complies with 3GPP TS 23.203 and IEEE 802.1p: › Using these MOs we are able to map out the quality of service properties defined in the RBS =============================================================== MO dscp priority qci =============================================================== QciTable=default,QciProfilePredefined=qci1 46 2 1 QciTable=default,QciProfilePredefined=qci7 20 7 7 QciTable=default,QciProfilePredefined=qci9 12 9 9 QciTable=default,QciProfilePredefined=qci5 40 1 5 QciTable=default,QciProfilePredefined=qci3 34 3 3 QciTable=default,QciProfilePredefined=default 0 10 0 QciTable=default,QciProfilePredefined=qci8 10 8 8 QciTable=default,QciProfilePredefined=qci6 28 6 6 QciTable=default,QciProfilePredefined=qci2 36 4 2 QciTable=default,QciProfilePredefined=qci4 26 5 4 =============================================================== ==================================================== MO Attribute Value ==================================================== Subrack=1,Slot=1,PlugInUnit=1,ExchangeTerminalIp=1,Gig aBitEthernet=1 dscpPbitMap t[64] = >>> Struct[0] has 2 members: >>> 1.dscp = 0 >>> 2.pbit = 0 ... truncated ... >>> Struct[46] has 2 members: >>> 1.dscp = 46 >>> 2.pbit = 6 >>> Struct[47] has 2 members: >>> 1.dscp = 47 >>> 2.pbit = 0 Transport Network Configuration (39/1553-HSC 105 50/1)
  • 94. GTP-U - User Plane › GTP-U is used as the User Plane protocol over the S1 and X2 interfaces (defined in 29.060) › GTP-U carries the end user data by forming tunnels towards the core network S-GW (transports user IP payload) › IP fragmentation is to be avoided (MTU size should be set correctly) › Configuration aspects on the RBS are minimal (no MO models this layer) GTP-U UDP IP Data Link Physical GTP-U UDP IP Data Link Physical S1-U teid, ip address, port teid, ip address, port
  • 95. Transport Network performance › Transport Network performance visibility is available via the GigaBitEthernet MO and IpAccessHostEt pm counters – Moshell> pmom GigabitEthernet|IpaccessHostEt › The metrics include (singleton, sample and statistical metrics): – GE Link Ingress/Egress Average usage – GE Link Ingress Frame Error Ratio – IPv4 Ingress/Egress Packet discard ratio › These TN metrics can help identify: – under-dimensioned GE links (i.e. highly utilised links) – transport problems/errors over GE links (high frame discard ratios) – IPv4 packet discard issues (queue capacity, errored packets, etc) Transport Network Performance Metrics (44/1553-HSC 105 50/1) L2 observability L3 observability
  • 96. Transport Network performance › Additional TN Performance Management counters are available › SCTP (Signalling transport for S1AP and X2AP) – Sent and received data/control chunks – Dropped chunks (buffer overflows) – Checksum errors › Synchronization (SoIP related counters) – Provides the highest delay variation counters for the active IP sync reference – Calculated in terms of the best x percentage sync frames experienced during a 100 second window (result in microseconds) – The percentages include 1, 10, 50 % › IpInterface (Signalling, payload and sync) – Failed Pings to default routers (RPS) – Discards, header and IP address errors
  • 97. RBS Counters via COLI › Detailed IP counters are available from the transport interface (TN-A / TN-B) › IP layer (via IpAccessHostEt MO) – EtHostMo_getPmCounters -h 1 -t 1 › The counters show: – Protocol errors – Ingress/Egress Discards – UDP protocol errors – ICMP details – Fragmentation details $ EtHostMo_getPmCounters -h 1 -t 1 PM counters for host with hostFroId 1 ipAddrErrors=0 ipInDelivers=23048 ipInDiscards=-2 ipInHdrErrors=0 ipInReceivedOctets=-2 ipInReceives=321055 ipInUnknownProtos=0 ipNumFailedAt=-2 ipOutDiscards=-2 ipOutRequests=321945 ipOutRequestOctets=-2 ipReasmReqds=0 ipReasmOKs=0 ipReasmFails=0 ipFragOKs=0 ipFragFails=0 ipFragCreates=0 ipPortUnreachable=0 udpInDatagrams=0 udpInErrors=0 udpNoPorts=-2 udpOutDatagrams=0 icmpInDestUnreachs=0 <truncated>
  • 98. RBS Counters via COLI $ nssinfo tupm ****** NSS TUM2 tu_pm related data ****** PMfroId : 1 PMfroType : 66817 granularityPeriod : 900 suspectFlag : 0 par_pmMDVCounter : 186 par_pmHDVB1Pct : 0 par_pmHDVB10Pct : 1 par_pmHDVB50Pct : 5 ****** END ****** $ sctphost_stat -assoc -all sctphost_stat - START. |-------------------------------------------------------| |----------------------- SCTP HOST ---------------------| |RpuId: 17 |SctpInstId: 0 |Base State: BASE_RUN |Host State: A|C|R|X|IA |Ext. client: CONNECTED |Alarm Timer: NOT RUNNING |---------------- SCTP ASSOCIATION 96 -----------------| |-------------------------------------------------------| |----------------Statistic (assoc level)----------------| | [ID 7]: SCTP_STAT_SENT_CHUNKS, Count: 1| | [ID 8]: SCTP_STAT_REC_CHUNKS, Count: 1| | [ID 9]: SCTP_STAT_OUT_OF_ORDER_SC, Count: 0| | [ID 10]: SCTP_STAT_OUT_OF_ORDER_RC, Count: 0| | [ID 12]: SCTP_STAT_RETRANS_CHUNKS, Count: 0| | [ID 13]: SCTP_STAT_SENT_CC, Count: 16264| | [ID 14]: SCTP_STAT_REC_CC, Count: 16264| | [ID 15]: SCTP_STAT_FRAG_USER_MSG, Count: 0| | [ID 16]: SCTP_STAT_REAS_USER_MSG, Count: 0| | [ID 17]: SCTP_STAT_SENT_PACKAGES, Count: 16265| | [ID 18]: SCTP_STAT_REC_PACKAGES, Count: 16265| <truncated> › RBS COLI counters show detailed performance issues – S1AP/X2AP (SCTP MO) – sctphost_stat -assoc -all › SoIP (Synchronization MO) – nssinfo tupm
  • 99. Transport Network tracing › RPS – $ appdh info / $ appdh rps <ip inter> – Use Wireshark to see ICMP › QCI, ARP, AMBR values – te e bus_send bus_receive S1AP_ASN (e.g. InitialContextSetupRequest) › SCTP – $ te e all cpxSctpIC – $ te e all Scc_SctpHost_proc › Synchronization (SoIP using NTP) – te e trace7 NSS_CBM_TUM2_TUREG – $ nssinfo all
  • 100. Wireshark - Protocol capture eNB S-GW Internet FTP Server IP Backbone MME HSS S1-MME S6a S11 P-GW S5/S8 S1-U S1 Wireshark is an open source protocol analyser. It provides excellent support of the 3GPP LTE protocols.
  • 101. S1 capture via Wireshark QCI = 9 Will filter all data and only display S1AP protocol messages
  • 102. S1 capture via Wireshark › Example S1 trace with user and control plane payload: S1-U GTP-U L2 QoS Pbit = 6 L3 QoS DSCP = 46 SoIP using NTP S1-MME SCTP
  • 103. Transport Summary › LTE RAN transport topology is very flexible – a mixture of L2/L3 topologies could be implemented (could complicate analysis) › Basic transport network redundancy is provided (in terms of L3 routers) › Transport network dimensioning should be taken into account as statistical gains are used (backhaul peaks should be known) › QoS (Radio and Transport) is essential for proper network operation and should be implemented throughout the network (i.e. in L2/L3 nodes as well) › Basic performance management is provided in initial LTE RAN releases › Additional observability can be secured through protocol analysis
  • 105. e2e analysis › End-2-end analysis of throughput is required when end users report throughput issues that are not readily seen in the LTE RAN › End user throughput investigations/analysis is done at the UE/Server side › Typical user payload will use the TCP transport protocol › Layer 3 IP configuration aspects are not covered › The UDP transport protocol is not covered
  • 106. lte end user Protocol stack Telnet, FTP, TFTP, HTTP, SNMP, ….. BGP RIP Port Number Application Layer OSPF EGP TCP UDP IMCP IGMP Transport Layer ARP IP RARP Internet Layer Protocol Number Type Code PDCP, GTP-U Data Link Layer
  • 107. TCP at a glance › TCP will be the most used transport layer protocol (email, ftp, browsing, etc) › TCP offers the following features: – Stream data transfer › TCP offers a contiguous stream of segments for applications – Reliability › Sequence numbers used by sender that expects positive acks (or retransmit) › Receiver uses sequence numbers to rearrange segments (remove duplication) – Flow control › Receiver indicates the number of bytes it can receive (to sender) – Multiplexing › Achieved through the use of ports – Logical connections › Each connection is identified by the pair of sockets used (in receiver & sender) – Full duplex operation › TCP provides concurrent data streams in both directions
  • 108. TCP Header Indicates application Provides reliability rwnd - what the receiver (sender of this segment) is willing to accept
  • 109. TCP Operation › TCP operates in three distinct phases: – Connection establishment – Data transfer – Connection termination › TCP is known as a sliding window protocol › Two windows are defined: – rwnd - receive window, advertised/offered by the receiver – cwnd - congestion window, calculated by the sender › The TCP sender window is defined as the minimum of rwnd and cwnd › AIMD behaviour (additive increase / multiplicative decrease) – cwnd = cwnd + MSS*(MSS / cwnd) – cwnd = cwnd / 2 › Sliding window in action: Sliding Window Animation
  • 110. TCP Operation Cont. › TCP congestion control: – Slow start › exponential growth of cwnd › used in cold/initial start and after a timeout – Congestion avoidance › linear increase after congestion experienced › congestion indicated by timeouts and duplicate acks – Fast retransmit › sender quickly determines congestion and retransmits › attempts to avoid timeouts - and hence slow start – Fast recovery › enter congestion avoidance rather than slow start › Senders use a retransmission timeout (RTO) based on RTT
  • 111. TCP Congestion Control Congestion Window 3rd DUPACK 3rd DUPACK 3rd DUPACK Time out Pipe Capacity Slow start threshold reached t ssthresh on congestion Fast recovery Fast retransmit Congestion Avoidance Slow Start
  • 112. TCP Performance › Bandwidth Delay Product (BDP) - data required to fill the TCP pipe: – Bandwidth of link (bytes per sec) * Delay (sec) = amount of data in transit to fill pipe › Example: A T1 (1.5 Mbps) over a satellite connection with RTT = 500ms – (1,500,000 bits per sec / 8 bits per byte) * (0.5 sec) = 93,750 bytes – With a rwnd of 65,535, performance is 65,535/93,750 = ~70% or 1.05 Mbps – Hence, the BDP requires more transit data, and this is achieved with an increase in the rwnd (the TCP scaling feature, RFC1323, can provide this increase) › TCP Windowing (rwnd) and RTT limit the achievable throughput as follows: – › Hence, large receive windows and small RTT are desired Typical LTE RTT
  • 113. TCP Tuning (Client side) › TCP performs differently on different Operating Systems (there have been many variations on the TCP congestion control algorithm for instance) › On Windows Vista: – autotuning = highlyrestricted › netsh interface tcp set global autotuninglevel=highlyrestricted – autotuninglevel = restricted › netsh interface tcp set global autotuninglevel=restricted › On Windows XP: – SP2+: follow this Windows XP TCP configuration KO – Pre-SP2: Use the DrTCP application › On Linux (depends on the kernel) – Follow this TCP Tuning Guide for kernels 2.4 -> 2.6 (i.e. 2.6.20+ are covered)
  • 114. LTE RAN TCP Behaviour › Processing (handovers, buffering, delays, scheduling, retransmissions) in the RBS can affect TCP operation › Affect of incorrect TCP receive window sizes: – Lower than BDP -> › Packet loss can lead to (retransmissions, dropped in RBS, etc): – TCP retransmissions and delays – Send rate (throughput) reduced up to 50%, then a linear increase until next drop or max TCP rate reached › TCP timeouts can lead to: – TCP fallback into slow start
  • 115. LTE RAN TCP Enhancements › LTE RAN future adaptations to enhance end user TCP behaviour: – AQM (Active Queue Management) › Uses the TCP congestion avoidance algorithm › Limits the cwnd by selectively dropping packets – Result: smaller cwnd = smaller buffered data and delay – Data forwarding at intra LTE handover (X2) › Zero packet loss at handover › Provides higher data rates for mobile users – Result: reduces the risk of TCP fallback into slow start
  • 116. Analysing e2e traffic › The following section uses the Wireshark application to inspect e2e traffic (i.e. FTP download from UE) › Two scenarios are used to highlight how Wireshark can perform detailed analysis on network traffic › The goal is to show how e2e analysis can be performed from a UE perspective › The following shows analysis of downlink FTP data session towards a UE
  • 117. Throughput Results › For DL throughput, select a packet from Server to UE, and select Statistics -> TCP Stream Graph -> Throughput Graph Scenario A Scenario B
  • 118. Throughput Results Cont › Two other ways to quickly determine the UE throughput are via Statistics -> Summary and Statistics -> IO Graphs Average added for clarity Scenario A
  • 119. Time Sequence Graphs › A great insight into the TCP performance/behaviour is found with the Statistics -> TCP Stream Graph -> Time-Sequence Graph (Stevens) Scenario A Scenario B
  • 120. TCP Flow Graphs › TCP flows (Statistics -> Flow Graph) are useful in isolating a particular TCP conversation Scenario B
  • 121. Additional TCP Analysis › Wireshark’s Expert Info (Analyse -> Expert Info) provides a better display of uncommon or notable network behaviour: Scenario B
  • 122. Analysis Suite › Wireshark is a powerful tool to inspect e2e network behaviour › Other tools can be used in conjunction with Wireshark to offer a complete e2e analysis suite: – iperf › works in client/server modes › allows UDP and TCP payload to be injected into the network › useful to see link capacities with UDP (max DL/UL throughput) › identifies possible TCP bottlenecks – Netpersec › Offers real-time display of throughput – IXIA Chariot Endpoint › commercial product to test IP networks › provides automated application level analysis (HTTP, VoIP, etc) › provides detailed results/statistics of performance
  • 123. e2e Summary › The main transport layer (OSI L4) protocol used for network services (internet) will be the Transmission Control Protocol (TCP) › TCP offers end user application reliable data delivery, flow control and good bandwidth utilisation › The LTE RAN will implement features/functions that provide better inter-work with the TCP protocol › Wireshark is a powerful open source network protocol analyser that can offer advanced e2e throughput investigations using inbuilt functions › Additional tools (like iperf) support the complete e2e analysis
  • 125. Summary › Throughput troubleshooting in the LTE RAN can be broken into domains › The Radio domain includes many uplink and downlink specific areas of analysis and can also be utilised to find eNB external problems › The Transport domain analysis focus’ on northbound IP network › The e2e domain analysis looks at end user throughput concerns
  • 126. More Information › LTE PLM Troubleshooting Wiki: https://plm- lte.rnd.ki.sw.ericsson.se/lte_trsh_wiki/index.php?n=UseCases.UseCases › LTE Tutorial: http://utran01.au.ao.ericsson.se/flowfox/lteman.html › LTE Observability: http://utran01.au.ao.ericsson.se/flowfox/lteobservability.html › PLM LTE Toolbox in the field: https://ericoll.internal.ericsson.com/sites/PLM_LTE_Toolbox_in_field/default.aspx › LTE FlowFox: http://utran01.au.ao.ericsson.se/flowfox/index.php#lteflowfox › LTE SW Deployment and Support EKB Community: https://knowledgebase.internal.ericsson.com/LTE_RAN/ › 3GPP 36-series specifications (TS36.213 in particular for Radio Analysis section): http://www.3gpp.org/ftp/Specs/html-info/36-series.htm › LTE L11A GA (R9AJ) System Constants list http://cdmweb.ericsson.se/WEBLINK/ViewDocs?DocumentName=1%2F19059-HRB105500&Revision=PE2-10 › LTE RAN Data Collection Guideline: http://cdmweb.ericsson.se/WEBLINK/ViewDocs?DocumentName=60/1543-LZA7016004&Latest=true