Call setup failure

Prepared: MEX/TIM Luis Batallan
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Subdirección de Calidad de
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Desempeño de la Red
CALL SETUP FAILURE
Date: 2005-06-22 Rev. A
0
CALL SETUP FAILURE
REVISION RECORD
Date Revision Sections affected Revised by
2005-06-22 A First version Luis Ricardo Batallan

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CALL SETUP FAILURE
Table of contents
1 ABSTRACT 0
2 PURPOSE OF THE DOCUMENT 0
3 SCOPE 0
4 INTRODUCTION 0
4.1 RR CONNECTION ESTABLISHMENT (RR) 0
4.2 SERVICE REQUEST (RR) 0
4.3 AUTHENTICATION (MM) 0
4.4 CIPHERING MODE SETTING (RR) 0
4.5 IMEI CHECK (MM) 0
4.6 CALL INITIATION (CC) 0
4.7 TRAFFIC CHANNEL ASSIGNMENT 0
4.8 CALL CONFIRMATION AND CALL ACCEPTED 0
5 STATISTICAL ASSESSMENT 0
5.1 RR Connection establishment 0
5.2 Service Request 0
5.3 Security Functions 0
5.4 Call Initiation 0
5.5 Traffic Channel Assignment 0
6 CONCLUSIONS AND RECOMMENDATIONS 0
7 REFERENCES 0
.
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1 ABSTRACT
After drive test evaluation by “Calidad de Servicio Región 9” has been found
some problems during Call setups in the GSM system. The problem can be
assessed from statistics point of view to determine in which part of the process
the Call Setup is failing.
2 PURPOSE OF THE DOCUMENT
The purpose of this document is to describe: the procedure used by the GSM
system for a Call Setup and how the performance for this process can be
assessed.
3 SCOPE
The TELCEL RF optimization team, which is troubleshooting Call Setup failures.
4
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INTRODUCTION
Call set-up within GSM consists of the following main steps:
• RR connection establishment
• Service request
• Authentication
• Ciphering mode setting
• TMSI-Reallocation
• IMEI Check
• Call initiation
• Assignment of a traffic channel
• User alerting
• Call accepted
The above steps are taken for both mobile originating and mobile terminating
calls. However, for mobile terminating call they must be preceded by an
interrogation phase (the system must first find out where the subscriber is located
at that moment)
The process is outlined in Figure - 1 for mobile terminated call setup, being the
process for mobile originated call setup very similar with the following differences:
• In RR Connection Establishment, there is no paging message. Only
channel request and immediate assignment.
• In Service Request, the MM message “CM Service Request” is used
instead of the RR message “Paging Response”.
• In Call Initiation, the “Setup” message is sent by the mobile and the network
replies with “Call Proceeding”.
• In Call Confirmation and Call Accepted, the messages go in the opposite
direction (compared to Mobile Terminated Call Setup)
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MS BTS BSC MSC
Paging Request (PCH)
Paging Command
Paging
Channel Request (RACH)
Channel Required
Channel Activation
Channel Activation Ack
Immediate Assign Cmd
(Immediate Assignment)Immediate Assignment
(AGCH)
SABM (Paging Response)
UA (Paging Response)
Est Ind (Paging Resp)
Paging Response
Authentication Request
Data Req (Auth Req)
Authentication Req
Authentication Response
Data Ind (Auth Resp)
Authentication Resp
Ciphering Mode Cmd
Encryption Cmd
(Ciph Mode Cmd)
Ciphering Mode Cmd
Ciphering Mode Complete
Data Ind (Ciphering
Mode Complete) Ciphering Mode Comp
Identity Request
Data Req (Identity Req)
Identity Request
Identity Response
Data Ind (Identity Resp)
Identity Resp
Setup
Data Req (Setup)
Setup
Call Confirmed
Data Ind (Call Confirmed)
Call Confirmed
Assignment Req
Channel Activation
Channel Activation Ack
Assignment Cmd
SABM
UA
Est Ind
Assignment Complete
Data Req (Ass Cmd)
Data Ind (Assign Comp)
(Assignment Complete
RF Chan Release
RF Chan Release Ack
Connect Data Ind (Connect)
Connect
Connect AckData Req (Connect Ack)
Connect Ack
RR Connection
Establishment (RR)
Done on CCCH
Service Request (RR)
Done on SDCCH
Authentication (MM)
Done on SDCCH
Cipher Mode Setting (RR)
Done on SDCCH
Identity Check (MM)
Done on SDCCH
Call Initiation (CC)
Done on SDCCH
Assignment of a TCH (RR)
Done on SDCCH and
FACCH/TCH
Establish
layer 2 link
on SDCCH
Establish
layer 2 link
on FACCH
Deactivate
SDCCH in
BTS
MS
Yes
BzzAlerting Data Ind (Alerting)
Alerting
Call Confirmation and Call
Accepted (CC)
Done on FACCH/TCH
Activate
SDCCH in
BTS
Activate
TCH in
BTS
Figure 1- Call Setup Process
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4.1 RR CONNECTION ESTABLISHMENT (RR)
MSC/VLR initiates the call set-up procedure by sending the Paging message to
the relevant BSCs. The actual information contained in the Paging message
depends on whether IMSI or TMSI is used in the Paging Request message.
The BSC detects the Paging message and translates the LAI into a Cell Global
Identity (CGI) if the cell has not been provided in the paging message. The BSC
sends the Paging Command message to the relevant BTSs. This message
contains the IMSI or TMSI, channel number (channel type and time slot number),
and paging group. In this case, the channel type is downlink CCCH (PCH). The
paging group is determined by the IMSI and two other parameters defined in
BSC and broadcast as system information.
Finally, the BTS sends the Paging Request message to the MS. It is sent on the
PCH, and it occupies four bursts.
When the paged MS receives a Paging Request message it responds by sending
a Channel Request message, thereby requesting a signaling channel for call set-
up. The Channel Request message is sent on the Random Access Channel
(RACH) and consists of 8 information bits: a random number (5 bits) and an
establish cause (3 bits).
In GSM specifications phase 2, the number of bits used to establish cause can
be 3 through 6. Accordingly, the random number will be described using 5 down
to 2 bits. The establish cause can be:
• Answer to page
• Originating call
• Location updating
• Emergency call
• Others, for example IMSI detach, SMS, supplementary services
management
When the BTS detects an access burst, it sends a Channel Required message to
the BSC to request a signaling channel for the MS. This message contains the
access delay of the access burst and the request reference parameter. BSC
compares the access delay value with the maximum permitted timing advance
parameter value to ensure that the MS is within the allowed range. BSC then
determines which channel to use and sends the identity of this channel to the
BTS in the Channel Activation message. The contents of this message are:
• Reason for allocation
• A complete description of the channel containing the channel
• Description information element
• DTX applied or not
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• MS power
• BS power
• Timing Advance (TA)
The BTS then activates the channel and confirms this by sending the Channel
Activation Acknowledge message.
The BSC then sends the Immediate Assignment command message containing
an Immediate Assign message to the mobile station.
The message tells the MS to switch to the allocated signaling channel
(SDCCH+SACCH). The message contains:
• The channel description information element (Channel Type, ARFCN, TN,
TSC, MAIO, HSN)
• The frame number when the Channel Required message was detected
• The random reference from the access burst
• The TA that the MS will use
The MS compares the random reference and the frame number with those stored
in the MS. If they correspond, the MS goes to the service request step.
4.1.1 Timers for RR Connection Establishment and Service Request (ALOHA)
Figure 2 – Timers for RR connection establishment
To avoid collisions in the BTS when more than one mobile attempt to access the
system, for example when they are paged in one message; the Aloha procedure
is used to spread out the accesses. This procedure makes use of timer T3120
(T3126 Ph2 MS) in the MS.
T3126 for
Phase 2 MS
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When the MS receives a Paging message or when it decides to access the
system for other reasons, the MS generates a random number between 0 and 8
and an integral value defined by the parameter called TX minus 1. Then it starts
timer T3120 (T3126 Ph2 MS) that expires after this random number of TDMA–
frames. When the timer expires, the MS sends the random access burst with the
Channel Request.
The MS restarts T3120 (T3126 Ph2 MS) and waits for an order (Immediate
Assignment) from the network to go to an SDCCH where the signalling can
continue. The new random number is generated in the MS, this time ranging
between 0 and TX. The parameter TX, which is broadcast on the BCCH, has a
maximum value of 50. In GSM specification phase 2, the time that the MS must
wait after the restart depends on the non-combined or combined SDCCH and the
value of TX.
Note: For TELCEL network most of MS are phase 2, so for the current parameter setting
(TX=20) the time between channel requests retries will be approx. 0.5 seconds. T3126 as
explained has a random value, but the maximum is 5 seconds.
4.1.2 Random Access Failure
If the MS does not receive an Immediate Assignment from the network before
T3120 (T3126 Ph2 MS) expires, it sends a new random access burst with a
channel request. The number of times the MS can attempt to access the system
is limited. The MS knows what the limit is from the system information read on
the BCCH.
Note: The number of times the can attempt to access is determined for the cell parameter
MAXRET, for TELCEL network the current parameter setting is between 2 and 4.
T3120 (T3126 Ph2 MS) is stopped when the Immediate Assignment message is
received.
On the network side there are two timers:
• T3113, which starts when Paging Request is sent and stops when Paging
Response is received (PAGTIMEFRST1LA, PAGTIMEREP1LA,
PAGTIMEFRSTGLOB, PAGTIMEREPGLOB)
• T3101 (default 15 decisecs.), which supervises the allocation of a channel
and hence starts when the Immediate Assignment message is sent and
stops when the SABM frame is received.
If T3113 expires before the Paging Response is received, the Paging Request
message can be sent again according to an exchange parameter set by the
operator. If T3101 expires however, the request is cancelled and the channel
released.
4.1.3
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Immediate assignment reject
When the immediate assignment is rejected by the system, on receipt of the
IMMEDIATE ASSIGNMENT REJECT message, the MS stops T3120, starts timer
T3122 with the indicated value, ("wait indication" information element set by BSC
timer 'WAIT_INDICATION') and returns to CCCH idle mode (listening to the
paging channel). The MS is not allowed to make a new attempt to establish an
RR-connection in the same cell until T3122 expires.
The IMMEDIATE ASSIGNMENT REJECT message is sent in the following
situations:
a) The random access is a request for 'Location Updating' or 'other procedures
that can be completed with an SDCCH and NECI set to 1' and no channel
available (default value for T3122=10 sec.)
b) The Central Processor in BSC is overloaded (default value for T3122=20 sec.)
c) Too many random accesses are received within 100 ms (more than 35 for APZ
212 11, more than 90 for APZ 212 25, more than 150 for APZ 212 20 and to be
defined for APZ 212 30). (Default value for T3122=200 sec.)
Figure 3 – Immediate Assignment Reject
4.1.4 Assignment Successful
For the successful assignment there are two “guard timers” for the establishment
of MS data kink connection and the establishment of MS layer 3 connection
respectively as outlined on the figure 4:
• T3107a (default 6 sec.): Guard time for establishment of MS data link
connection (assignment procedure). Started as message 'ASS_CMD' is
sent to MS and stopped by reception of 'EST_IND' from BTS.
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• T3107b (default 6 sec.): Guard time for establishment of MS layer 3
connection (assignment procedure). Started as message 'EST_IND' is
received from BTS and stopped by reception of 'ASS_CMP' or 'ASS_FAIL'
from MS.
Figure 4 – Assignment Successful
If the timers expires and the ASSIGNMENT COMPLETE has not be sent, then a
CLEAR REQUEST will be send from the BSC to the MSC invoking clear
procedure.
4.2 SERVICE REQUEST (RR)
The layer 2 message Set Asynchronous Balanced Mode (SABM) is sent to the
BTS as soon as the MS has tuned to the new channel. SABM may contain a
layer 3 service request message. In this case, the information field in SABM
contains the Paging Response message.
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The SABM message is used to start the acknowledged mode on the SDCCH
channel. In practice, this means that the counters for send and receive sequence
numbers are reset to zero.
The Paging Response message received from the MS contains the following
information:
• MS identity (IMSI or TMSI and possibly IMEI)
• The ciphering key sequence number (CKSN)
• The mobile station class mark 2, that is, the type of MS station being used
The CKSN can be used by the MSC to start ciphering without authentication.
When the Paging Response arrives at the BTS, it is forwarded to the BSC in an
Establish Indication message. This message tells the BSC that a change to
acknowledged mode has been made. BSC activates radio connection quality
supervision in the Transceiver Handler (TRH), which starts the locating function
on SDCCH. It also initiates the power control algorithm for dynamic MS power
control.
Paging Response is an Initial MS message, which means that it was initially
meant to be sent transparently to the MSC. However, the BSC adds CGI before
forwarding the Paging Response to the MSC as a Complete L3 message.
The Paging Response is also looped back to the MS in an Unnumbered
Acknowledgment (UA) frame. This confirms that only one MS is using the
signaling channel. If the paging response is not returned the following case might
occur:
Two MSs access the system at the very same frame number, with the same
establish cause and the same random reference (there are only 32). If both MSs
are strong, they must send another access burst in accordance with the Aloha
procedure, which is a way to spread out random accesses in order to avoid
consecutive collisions. However, if one of the MSs is weak, the signal does not
reach the BTS. Unaware of this, the MS receives the reply sent to the other MS a
short while after its own access attempt. The MS tunes to the signaling channel,
sends the Paging Response message and then listens for the Paging Response
to be looped back. Now the MS hears that the MS identity given is not its own
and leaves the channel.
The BSC sets up a logical SCCP connection towards the MSC by sending the
Connection Request message, including complete Layer 3 information, the
Paging Response message piggybacked. The SCCP Connection Confirm
message is returned to the BSC which means that the connection oriented
signaling is established on the A interface.
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4.3 AUTHENTICATION (MM)
Figure 5 – Authentication Successful
An exchange property in MSC/VLR defines whether authentication is to take
place or not. An Authentication Request is sent transparently to the MS.
The parameters in the Auth. Req. message are the random (RAND) number, 128
bits, and CKSN, 3 bits. When the MS receives the Auth. Req. message it stores
the CKSN, which is to be sent in the next service request message. It then
calculates the authentication parameter SRES by inserting Ki, the subscriber
authentication key that is stored in the SIM card; and RAND in algorithm number
A3 as well as Kc by inserting Ki and RAND in algorithm A8.
The SRES parameter is returned to MSC/VLR in the Authentication Response
message. The Data Indication (DI) frame is used to send layer 3 messages
transparently in the direction from BTS to BSC. In the opposite direction, the Data
Request (DR) frame is used.
Timer T3260 (default 12 sec.) on the network side supervises authentication. It is
set in the MSC when Auth. Req. is sent and stopped when Auth Resp is
received. On the first expire Auth. Req. is sent again, but on the second expire
the channel is released (CLEAR COMMAND).
4.3.1 Unsuccessful Authentication
If authentication fails that is, SRES from the MS does not correspond with the
SRES value stored in MSC/VLR; the network can distinguish between two
different ways of identification used by the MS:
• TMSI
• IMSI
If TMSI is used, the network may initiate the identification procedure, where the
MS is told to send its IMSI value. If the IMSI given by the MS differs from that
which is stored in the network, the authentication is restarted using the correct
parameters. If the IMSI provided is the expected one, the MSC should proceed
as described below.
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If the IMSI has been used or the network decides not to use the identification
procedure, an Authentication Reject message is sent to the MS. All connections
in progress are then released.
The MS receiving the Authentication Reject message deletes its TMSI, LAI, Kc,
and CKSN and switches to the “IDLE no IMSI" state. In this state, the MS is only
allowed to perform default cell selection, and only emergency calls are permitted.
This is a serious case because it means barring the SIM card from use. To
prevent this from happening because of dues to bit errors or software errors, the
GSM systems have an exchange parameter controlling unsuccessful
authentication. If this parameter is set, then the MSC does not send the
Authentication Reject message, but instead rejects the MS access to the system
by sending one of the following messages depending on the establish cause (in
brackets):
• “IMSI unknown in VLR" (Mobile originating call)
• No message, only disconnection (Mobile terminating call)
• “PLMN not allowed" (Location updating)
• “IMSI unknown in VLR" (Support services control operations)
4.4 CIPHERING MODE SETTING (RR)
Figure 6 – Ciphering successful case
If the authentication procedure is successfully performed, the ciphering mode
setting procedure can be initiated by the MSC/VLR (depending on the exchange
property setting in the MSC/VLR), which then sends the Ciphering Mode
Command to the BSC. This is a BSSMAP message, which contains the Kc.
Kc is forwarded by BSC to the BTS. BTS stores Kc and tells the MS to start
ciphering. BTS starts the deciphering. The Ciphering Mode Command message
sent from BTS to MS is non-ciphered.
The MS inserts Kc and TDMA frame number into algorithm A5, which yields a
ciphering sequence that is added to the message to be sent. The message is
called Ciphering Mode Complete, and it does not have any parameters. It only
tells the BTS that ciphering has started.
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When the BTS receives the Ciphering Mode Complete message or any other
correctly deciphered layer 2 frame, ciphering is started on the network side.
Ciphering Mode Complete is then sent in a DI frame to the MSC.
The default value for the Ciphering Timer is 30 seconds, after that if not
Ciphering Mode Complete is received on the MSC it will send a CLEAR
COMMAND. The default value for T3230 is 15 sec.
4.5 IMEI CHECK (MM)
Figure 7 – IMEI check
The exchange properties in the MSC/VLR define whether or not the IMEI check
is performed. If it is, the MSC/VLR orders the mobile station to send its IMEI
number. The downlink message, Identity Request, contains information about
what the mobile should send back. In case of an IMEI check, the mobile receives
a request to send back its IMEI number.
The same Identity Request message can be used in other situations to order the
mobile to send back its IMSI number. The IMEI sent by the mobile is checked
against information in the network’s Equipment Identity Register (EIR). If the EIR
returns the answer “white listed” the mobile is allowed to use the network. If the
mobile is “black listed” it will be rejected.
The EIR can also return the answer “gray listed” or “unknown”. In these cases the
operator can decide whether the mobile is permitted access the network.
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4.6 CALL INITIATION (CC)
Figure 8 – Call Initiation
Call initiation is started when the MSC sends the Setup message to the MS. This
message contains a request for bearer services (GSM - bearer capabilities). That
is, speech, data, fax, bit rate, etc. If the MS can handle the required service, it
sends a Call Confirmed message to the MSC. If the MS cannot handle the
requested bearer services, it sends the release complete message to the MSC
with reject cause No. 88, “Incompatible destination". The MSC then releases the
connection to the MS and also releases the connection to the calling subscriber
with cause number 88.
The call initiation procedure is supervised at the network side with the timer T303
(default value 30 sec.). This timer is set when the Setup message is sent and
stopped when the Call Confirmed message is received. If the network does not
receive the Call Confirmed message before timer T303 expires it releases the
connection to the calling subscriber with cause No. 18, “No user responding". It
also releases the connection to the MS. The Setup message is only sent once.
T303
4.7 TRAFFIC CHANNEL ASSIGNMENT
The assignment procedure is initiated by the MSC, which sends the Assignment
Request message to BSC. This message contains:
• Priority (a value between 1 and 14)
• CIC for trunk select to carry the traffic (speech or data) between MSC and
BSC
• DTX flag indicating whether DTX should be used on the downlink
• Optionally, radio channel identity. By using this parameter the MSC can
force the BSC to choose one particular radio channel for O&M reasons.
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Figure 9 – Assignment of traffic channel
The list from the locating function is then evaluated in BSC and depending on the
result, one of the following assignment procedures is started:
• Assignment to serving cell
• Assignment to another cell in the same BSC
• Assignment to an external cell
Only assignment to serving cell is described below.
The BSC software reads the value of the timing advance and calculates MS
power level from the locating function, selects an idle traffic channel and sends
the Channel Activation to the BTS.
Channel Activation is the same type of the message that is used for assigning the
SDDCH + SACCH in the RR connection establishment. But in this case the
channel type is set to Bm + ACCH meaning full rate TCH + SACCH + FACCH.
The BTS acknowledges the allocation of the speech channel by sending a
Channel Activation Acknowledge message.
The BSC then sets up a path through the group switch, and ensures that there
will be no disconnection due to missing Measurement Reports from the MS when
the MS disappears from the old channel.
The BSC sends the Assignment Command towards the MS, telling the mobile
station to switch to the new channel (TCH + SACCH + FACCH). This message is
sent on the SDCCH, and it consists of a complete channel description.
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The MS tunes to the new physical channel and sends SABM (on FACCH) to
indicate that the channel is correctly seized and that SACCH is then started in
acknowledged mode. As the BTS receives this message, it returns UA to the MS
and sends an Establish Indication message to the BSC.
The MS then sends an Assignment Complete message to the MSC indicating
that the traffic channel is up and running.
Finally, the BSC tells the BTS that the signaling channel is no longer needed by
sending the RF Channel Release message. This message is acknowledged as
soon as the connection is released.
4.8 CALL CONFIRMATION AND CALL ACCEPTED
Figure 10 – Call confirmation and call accepted
The call confirmation procedure starts as the MS sends the Alerting message to
the MSC. This message indicates that a ringing tone has been generated in the
MS and it can be used for user-to-user signaling or for invocation of
supplementary services. The message is sent transparently.
When the Alerting message is received, the MSC/VLR sends the TUP Address
Complete (ACM) message to the calling subscriber, who can now hear the
ringing tone generated in the MSC.
When the MS user answers, the Connect message is sent to the MSC. This
message contains facility, user-user signaling and a progress indicator. When the
message is received at the MSC, a Connect Acknowledgment is sent back to the
MS, and the originating exchange is informed by means of a TUP or ISUP
Answering message and also charging information.
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5 STATISTICAL ASSESSMENT
5.1 RR CONNECTION ESTABLISHMENT
Figure 11 – RR Connection establishment counters
On the RR connection establishment there are counters at BSC level for Channel
Required and the result of this request:
NOFFIEX: is incremented when capacity is requested for a received CHANNEL REQUIRED
message in BSC if establishment cause is equal to ’Answer to paging’.
NFTDIEX: Number of fetched incoming calls that has been accepted by Processor Load Control
function since there is enough capacity for the call for which the counter Number of offered
incoming calls (NOFFIEX) has been previously incremented.
NREJIEX: is incremented if an answer to paging is rejected by the Processor Load Control in
BSC/TRC function.
NOFFPRIO: Number of offered normal originating connections incremented at reception of the of
the CHANNEL REQUIRED message in BSC except if establishment cause is equal to ’Emergency
call’, ’Location update’, ’Answer to paging’ or the received Timing Advanced value is too high.
NFTDPRIO: Number of fetched normal originating calls is incremented when the Processor Load
Control function has accepted a request since there is enough capacity for the call for which the
counter Number of offered normal originating calls (NOFFPRIO) has been previously incremented.
NREJPRIO: is incremented if normal originating connections, such as originating speech or data
calls, SMS and IMSI detach, are rejected by the Processor Load Control in BSC/TRC function.
NOFFNPRIO: Number of offered location updates is incremented at reception of location update at
the Exchange Input Load Supervision function.
NFTDNPRIO: Number of fetched location updates incremented when location updates are fetched
by the Exchange Input Load Supervision function.
BSC LEVEL COUNTERS
NOFFIEX
NFTDIEX
NREJIEX
NOFFPRIO
NFTDPRIO
NREJPRIO
NOFFNPRIO
NREJNPRIO
CELL LEVEL COUNTERS
CSIMMASS
REJCSIMMASS
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NREJNPRIO: is incremented if the Processor Load Control in BSC/TRC function rejects a location
update.
After Channel Activation there two counters at Cell Level counting the Immediate
Assignment and Immediate Assignment Reject:
CSIMMASS: States the number of CS IMMEDIATE ASSIGNMENT commands sent by the BSC.
REJCSIMMASS: States the number of IMMEDIATE ASSIGNMENT REJECT commands sent by
the BSC.
Using counters from BSC level and Cell level is possible to check the difference
between the “channels required” and “immediate assignment commands sent by
the BSC”. Due to use of different level counters some inconsistent results can be
obtained (for low traffic cases), but for most of cases still useful.
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( )
[ ]%*100_
NOFFNPRIONOFFPRIONOFFIEX
SREJCSIMMASCSIMMASSNOFFNPRIONOFFPRIONOFFIEX
ASSIGNLOST
++
−−++
=
Figure 12 – Lost Assignments
The CS IMMEDIATE ASSIGNMENT REJECT message will be sent when no
channel is available for the assignment. Using the immediate assignment
counters is possible to obtain the percentage of immediate assignment rejected
due to lack of radio resources.
( )
[ ]%__
SREJCSIMMASCSIMMASS
SREJCSIMMAS
REJASSIGNIMM
+
=
Figure 13 – Immediate Assignment Reject
Figure 14 – POPTR1 Assignment failure
POPTR1 ASSIGNMENT FAILURE
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00
IMM_ASSIGN_REJ LOST_ASSIGN
0

TEM/T-
Approved Ericsson:
Approved Telcel:
2424202020202024
CALL SETUP FAILURE
Date: 2005-06-22 Rev. A
5.2 SERVICE REQUEST
Figure 15 – Service Request
When the connection is established between the MS and the BTS, it send
Establish Indication message to the BSC that could contain either “page
response” (for mobile terminated calls) or “CM service request” (for mobile
originated calls). Then the BSC will start the establishment of link to the MSC by
sending a “SCCP connection request”. The “SCCP connection confirm” message
is returned to the BSC, which means that the connection oriented signaling is
established on the A interface and then the CMSESTAB counter is stepped.
In order to check the “lost attempts” (i.e. not successful and not congested) in this
process is possible through STS counters count the total of calls attempts which
ones were “lost”.
[ ]%*100__ ⎥
⎦
⎤
⎢
⎣
⎡ −−−
=
CCALLS
CCONGSUBCCONGSCMSESTABCCALLS
ATTEMPTSSDCCHLOST
Figure 16 – Lost SDCCH Attempts
Figure 17 – POPTR1 LOST ATTEMPTS
(CM Service Request)
CMSESTAB
POPTR1 LOST SDCCH ATTEMPTS
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
17/05/05
18/05/05
19/05/05
20/05/05
21/05/05
22/05/05
23/05/05
24/05/05
25/05/05
26/05/05
27/05/05
28/05/05
29/05/05
30/05/05
31/05/05
01/06/05
02/06/05
03/06/05
04/06/05
05/06/05
06/06/05
07/06/05
08/06/05
09/06/05
10/06/05
11/06/05
12/06/05
13/06/05
14/06/05
15/06/05
16/06/05
17/06/05
0

TEM/T-
Approved Ericsson:
Approved Telcel:
2424212121212124
CALL SETUP FAILURE
Date: 2005-06-22 Rev. A
Note: Due to the system will try more than one attempt to allocate a channel in
the BTS, it is possible to obtain a little bit higher percentage of “lost attempts”
than we actually have using the above formula.
5.3 SECURITY FUNCTIONS
For security functions (Authentication and Ciphering) there are available counters
at MSC level on the Object Type Security Handling (SECHAND) in order to check
failures in these processes.
NAUTREQTOT: Number of sent authentication requests
NAUTREQSUCC: Number of successful authentications including check of SRES or XRES
NCIPATTTOT: Number of ciphering mode setting attempts
NCIPSETSUCC: Number of successful ciphering mode settings
[ ]%*100_ ⎥
⎦
⎤
⎢
⎣
⎡
=
CNAUTREQSUC
NAUTREQTOT
SUCCAUTHENT
Figure 18 – Authentication Success
[ ]%*100_ ⎥
⎦
⎤
⎢
⎣
⎡
=
NCIPATTTOT
CNCIPSETSUC
SUCCCIPHER
Figure 19 – Ciphering Success
Figure 20 – POPTR1 SECURITY FUNCTIONS
POPTR1 SECURITY FUNCTIONS
98.80
98.90
99.00
99.10
99.20
99.30
99.40
99.50
99.60
18/05/2005
20/05/2005
22/05/2005
24/05/2005
26/05/2005
28/05/2005
30/05/2005
01/06/2005
03/06/2005
05/06/2005
07/06/2005
09/06/2005
11/06/2005
13/06/2005
15/06/2005
17/06/2005
19/06/2005
AUTHENT_SUCC CIPHER_SUCC
0

TEM/T-
Approved Ericsson:
Approved Telcel:
2424222222222224
CALL SETUP FAILURE
Date: 2005-06-22 Rev. A
5.4 CALL INITIATION
For the Call Initiation there are two counters belonging to the object type “Layer 3
Call Control Counters for MSC/VLR Server” (L3CCMSG):
NL3CPROC: Number of sent Layer 3 Call Control CALL PROCCEEDING messages
NL3CCONF: Number of received Layer 3 Call Control CALL CONFIRMED messages
[ ]%
3
3
*100_ ⎥
⎦
⎤
⎢
⎣
⎡
=
CPROCNL
CCONFNL
CONFIRMCALL
Figure 21 – Call Initiation Success
Note: L3CCMSG has currently no data on TELCEL network (probably the Object
Type is not activated)
5.5 TRAFFIC CHANNEL ASSIGNMENT
Figure 22 – TRAFFIC CHANNEL ASSIGNMENT
Using the counters for “Assignment Request” and for “Assignment Complete” is
possible to obtain the failure in the Traffic Channel Assignment process.
TASSALL: Assignment attempts for all MS power classes. Successful attempts are
counted in the target cell and failed attempts are counted in the serving cell
TFCASSALL: Assignment complete for all MS power classes
TFCASSALL
TFCASSALLSUB
THCASSALL
THCASSALLSUB
TASSAL
L
(
⎟
⎠
⎞
⎜
⎝
⎛ +++−
×=
TASSALL
UBTHCASSALLSTHCASSALLUBTFCASSALLSTFCASSALLTASSALL
iedServiceDen 100
0

TEM/T-
Approved Ericsson:
Approved Telcel:
2424232323232324
CALL SETUP FAILURE
Date: 2005-06-22 Rev. A
POPTR1 Service Denied
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
15/06/2005
16/06/2005
17/06/2005
18/06/2005
19/06/2005
20/06/2005
Figure 23 – POPTR1 Service Denied
Figure 24 – Service Denied
Using the Measurements for Traffic Channels is possible to detect failures in the
channel activation on the BTS.
TFCALLS: Call attempt counter
TFMSESTB: Successful MS channel establishment on TCHs
TFCONGSAS: Number of congestion at assignment
TFCONGSHO: Number of congestion at handover
Figure 25 – POPTR1 Channel Activation Failure
[ ]%*100__ ⎥
⎦
⎤
⎢
⎣
⎡ −−−
=
TFCALLS
TFCONGSHOTFCONGSASTFMSESTBTFCALLS
FAILACTCH
Figure 24 – Channel Activation Failure
(
⎟
⎠
⎞
⎜
⎝
⎛ +++−
×=
TASSALL
UBTHCASSALLSTHCASSALLUBTFCASSALLSTFCASSALLTASSALL
iedServiceDen 100
POPTR1 Channel Activation Failure
0.00
0.50
1.00
1.50
2.00
16/06/2005 17/06/2005 18/06/2005 19/06/2005 20/06/2005
0

TEM/T-
Approved Ericsson:
Approved Telcel:
2424242424242424
CALL SETUP FAILURE
Date: 2005-06-22 Rev. A
6 CONCLUSIONS AND RECOMMENDATIONS
From the statistics for POPTR1 we can conclude the following:
- On the RR connection establishment we can see that the IMMEDIATE
ASSIGMENT REJECT is a quite high.
- There are some failures on the Channel Activation, but values below 1 % are
normal.
- For the Security Functions the rate of failure is low, so this is not an issue
- On the Traffic Channel Assignment the statistics doesn’t show too much
failures
7 REFERENCES
- 3GPP TS 04.008 (3rd Generation Partnership Project; Mobile Switching
Centre - Base Station system (MSC-BSS) Interface Layer 3 Specification)
- BSC Object Types and Counters, User Guide
- GSM Advanced System Technique Course
0

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