6 Weeks Industrial Training In Telecom In Chandigarh

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Concept of TDMA Frames
And
Channels In GSM

Concept of TDMA Frames and
Channels
 GSM combines FDM and TDM: bandwidth is
subdivided into channels of 200khz, shared by up to
eight stations, assigning slots for transmission on
demand.
f
t
c

GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz

GSM delays uplink TDMA frames
T1 T2 T3 T5 T6 T7T4 T8
R T
R T
R1 R2 R3 R5 R6 R7R4 R8
Uplink TDMA
Frame
F1 + 45MHz
Downlink TDMA
F1MHz
The start of the uplink
TDMA is delayed of
three time slots
TDMA frame (4.615 ms)
Fixed transmit
Delay of three time-slots

1 2 3 4 5 6 7 8
higher GSM frame structures
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
4.615 ms
546.5 µs
577 µs
guard
space
guard
spacetail user data TrainingS S user data tail
3 bits 57 bits 26 bits 57 bits1 1 3
GSM - TDMA/FDMA www.arcadianlearning.com

LOGICAL CHANNELS
TRAFFIC SIGNALLING
FULL RATE
Bm 22.8 Kb/S
HALF RATE
Lm 11.4 Kb/S
BROADCAST COMMON CONTROL DEDICATED CONTROL
FCCH SCH BCCH
PCH
RACH
AGCH
SDCCH SACCH FACCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNEL
BCCH -- BROADCAST CONTROL CHANNEL
PCH -- PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL
FACCH -- FAST ASSOCIATED CONTROL CHANNEL
DOWN LINK ONLY
UPLINK ONLY
BOTH UP &
DOWNLINKS

Broadcast Channel - BCH
 Broadcast control channel (BCCH) is a base to
mobile channel which provides general information
about the network, the cell in which the mobile is
currently located and the adjacent cells
 Frequency correction channel (FCCH) is a base to
mobile channel which provides information for
carrier synchronization
 Synchronization channel (SCH) is a base to mobile
channel which carries information for frame
synchronization and identification of the base
station transceiver

Common Control Channel -
CCH
 Paging channel (PCH) is a base to mobile channel
used to alert a mobile to a call originating from
the network
 Random access channel (RACH) is a mobile to
base channel used to request for dedicated
resources
 Access grant channel (AGCH) is a base to mobile
which is used to assign dedicated resources
(SDCCH or TCH)

Dedicated Control Channel -
DCCH
 Stand-alone dedicated control channel
(SDCCH) is a bi-directional channel allocated
to a specific mobile for exchange of location
update information and call set up
information

Dedicated Control Channel -
DCCH
 Slow associated control channel (SACCH) is a bi-
directional channel used for exchanging control
information between base and a mobile during the
progress of a call set up procedure. The SACCH is
associated with a particular traffic channel or stand alone
dedicated control channel
 Fast associated control channel (FACCH) is a bi-
directional channel which is used for exchange of time
critical information between mobile and base station
during the progress of a call. The FACCH transmits
control information by stealing capacity from the
associated TCH

TAIL BIT
ENCRYPTION BIT
GUARD PERIOD
TRAINING BITS MIXED BITS
SYNCHRONISATION BITSFIXED BITS
FLAG BITS
3 57 1 26 1 57 3 8.25NORMAL BURST
- NB
3 142 3 8.25
FREQUENCY
CORRECTION
BURST - FB
3 3 8.2539 64 39SYNCHRONISATION
BURST - SB
36 41 36 68.25
ACCESS
BURST - AB
DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms

0 1 2 3 4 5 6 2043 2044 2045 2046 2047
0 1 2 3 4 48 49 50
0 1 2 24 25
0 1 2 3 24 25
0 1 2 3 4 48 49 50
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
1 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS )
1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S )
LEFT (OR) RIGHT
1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms )
1 SUPER FRAME = 26 MULTI FRAMES
1 SUPER FRAME = 51 MULTI FRAMES
1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms )
TDMA FRAME NO.
0 1
0 1
HIERARCHY OF FRAMES
1 2 3 4 155 156
1 TIME SLOT = 156.25 BITS
( 0.577 ms)
(4.615ms)
(4.615 ms)
1 bit =36.9 micro sec
TRAFFIC CHANNELS
SIGNALLING CHANNELS

GSM Frame
0 1 2 3 4 5 6 7
3 57 1 26 1 57 3 8.25
0 1 2 12 24 25
Full rate
channel is
idle in 25SACCH is
transmitted
in frame 120 to 11 and 13 to 24
Are used for traffic data Frame
duration =
120ms
Frame
duration =
60/13ms
Frame
duration =
15/26ms

 114 bits are available for data
transmission.
 The training sequence of 26 bits in the
middle of the burst is used by the receiver
to synchronize and compensate for time
dispersion produced by multipath
propagation.
 1 stealing bit for each information block
(used for FACCH)

LOGICAL CHANNELS
TRAFFIC SIGNALLING
FULL RATE
Bm 22.8 Kb/S
HALF RATE
Lm 11.4 Kb/S
BROADCAST COMMON CONTROL DEDICATED CONTROL
FCCH SCH BCCH
PCH
RACH
AGCH
SDCCH SACCH FACCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNEL
BCCH -- BROADCAST CONTROL CHANNEL
PCH -- PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL
FACCH -- FAST ASSOCIATED CONTROL CHANNEL
DOWN LINK ONLY
UPLINK ONLY
BOTH UP &
DOWNLINKS

Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH authenticate
SDCCH switch to cipher mode
SDCCH request for location updating
SDCCH authenticate response
SDCCH cipher mode acknowledge
SDCCH allocate TMSI
SDCCH acknowledge new TMSI
SDCCH switch idle update mode
Location update from the mobile

RACH send channel request
AGCH receive SDCCH
SDCCH do the authentication and TMSI allocation
SDCCH require traffic channel assignment
SDCCH send call establishment request
SDCCH send the setup message and desired number
FACCH switch to traffic channel and send ack (steal bits)
FACCH receive alert signal ringing sound
FACCH acknowledge connect message and use TCH
TCH conversation continues
FACCH receive connect message
Call establishment from a mobile

Receive signaling channel SDCCH on AGCH
Receive alert signal and generate ringing on FACCH
Receive authentication request on SDCCH
Generate Channel Request on RACH
Answer paging message on SDCCH
Authenticate on SDCCH
Receive setup message on SDCCH
FACCH acknowledge connect message and switch to TCH
Receive connect message on FACCH
Receive traffic channel assignment on SDCCH
Mobile receives paging message on PCH
FACCH switch to traffic channel and send ack (steal bits)
Call establishment to a mobile

GSM speech coding
AIR INTERFACE
UPLINK
890 - 915 MHz
DOWNLINK 935 - 960 MHz
MOBILE
BASE TRANSCEIVER STATION

Transmit Path
BS Side
8 bit A-Law
to
13 bit Uniform RPE/LTP speech Encoder
To Channel Coder 13Kbps
8 K sps
MS Side
LPF A/D RPE/LTP speech Encoder
To Channel Coder 13Kbps
8 K sps,
Sampling Rate - 8K
Encoding - 13 bit Encoding (104 Kbps)
RPE/LTP - Regular Pulse Excitation/Long Term Prediction
RPE/LTP converts the 104 Kbps stream to 13 Kbps

GSM Speech Coding
 GSM is a digital system, so speech
which is inherently analog, has to be
digitized.
 The method employed by current
telephone systems for multiplexing voice
lines over high speed trunks and is
pulse coded modulation (PCM). The
output stream from PCM is 64 kbps, too
high a rate to be feasible over a radio
link.

GSM Speech Coding
 Speech is divided into 20 millisecond
samples, each of which is encoded as
260 bits, giving a total bit rate of 13
kbps.
 Regular pulse excited -- linear predictive
coder (RPE--LPC) with a long term
predictor loop is the speech coding
algorithm.

 The 260 bits are divided into three classes:
 Class Ia 50 bits - most sensitive to bit errors.
 Class Ib 132 bits - moderately sensitive to bit errors.
 Class II 78 bits - least sensitive to bit errors.
 Class Ia bits have a 3 bit cyclic redundancy code added for
error detection = 50+3 bits.
 132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136.
 Class Ia + class Ib = 53+136=189, input into a 1/2 rate
convolution encoder of constraint length 4. Each input bit is
encoded as two output bits, based on a combination of the
previous 4 input bits. The convolution encoder thus outputs 378
bits, to which are added the 78 remaining class II bits.
 Thus every 20 ms speech sample is encoded as 456
bits, giving a bit rate of 22.8 kbps.

 To further protect against the burst errors common to
the radio interface, each sample is interleaved. The
456 bits output by the convolution encoder are
divided into 8 blocks of 57 bits, and these blocks are
transmitted in eight consecutive time-slot bursts.
Since each time-slot burst can carry two 57 bit
blocks, each burst carries traffic from two different
speech samples.
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3
3 57 bits 261 1 57 bits 3

GSM Protocol Suite www.arcadianlearning.com

BTS
Radio interface
HLR
MSC
VLR
BSC
RR
MM + CM
SS

Link Layer
 LAPDm is used between MS and BTS
 LAPD is used between BTS-BSC
 MTP2 is used between BSC-
MSC/VLR/HLR

Network Layer
 To distinguish between CC, SS, MM and RR
protocol discriminator (PD) is used as network
address.
 CC call control management MS-MSC.
 SS supplementary services management MS-
MSC/HLR.
 MM mobility management(location management,
security management) MS-MSC/VLR.
 RR radio resource management MS-BSC.
 Messages pertaining to different transaction
are distinguished by a transaction identifier
(TI).

Application Layer protocols
 BSSMAP between BSC and MSC
 DTAP messages between MS and MSC.
 All messages on the A interface bear a
discrimination flag, indicating whether the
message is a BSSMAP or a DTAP.
 DTAP messages carry DLCI(information on
type of link on the radio interface) to
distinguish what is related to CC or SMS.
 MAP protocol is the one between neighbor
MSCs. MAP is also used between MSC
and HLR.

Q.921
Radio Interface
Q.931
Q.921
MAP
TCAP
CCS7 MTP
CCS7 SCCP
Mobile Application Part
Q931 BSSAP
SCCP
CCS7 MTP
A Interface
A-Bis Interface
Um
Base Station System
GSM Functional Architecture and Principal Interfaces

GSM protocol layers for signaling
CM
MM
RR
MM
LAPDm
radio
LAPDm
radio
LAPD
PCM
RR’ BTSM
CM
LAPD
PCM
RR’
BTSM
16/64 kbit/s
Um Abis A
SS7
PCM
SS7
PCM
64 kbit/s /
2.048 Mbit/s
MS BTS BSC MSC
BSSAP
BSSAP

Protocols involved in the radio
interface
 Level 1-Physical
 TDMA frame
 Logical channels multiplexing
 Level 2-LAPDm(modified from LAPD)
 No flag
 No error retransmission mechanism due to real time
constraints
 Level 3-Radio Interface Layer (RIL3) involves three sub
layers
 RR: paging, power control, ciphering execution, handover
 MM: security, location IMSI attach/detach
 CM: Call Control(CC), Supplementary Services(SS), Short
Message Services(SMS),

LAPDm on radio interface
 In LAPDm the use of flags is avoided.
 LAPDm maximum length is 21 octets of
information. It makes use of “more” bit to
distinguish last frame of a message.
 No frame check sequence for LAPDm, it
uses the error detecting performance of
the transmission coding scheme offered
by the physical layer

ADDRESS CONTROL INFORMATION 0-21 OCTETS
SAPI
N(S) N(R)
LAPDm Message structure

LAPDm on radio interface
 The acknowledgement for the next expected frame in
the indicator N(R ).
 On radio interface two independent flows(one for
signaling, and one for SMS) can exist simultaneously.
 These two flows are distinguished by a link identifier
called the SAPI(service access point identifier).
 LAPDm SAPI=0 for signaling and SAPI=3 for SMS.
 SAP1=0 for radio signaling, SAPI=62 for OAM and
SAPI=63 for layer 2 management on the Abis interface.
 There is no need of a TEI, because there is no need to
distinguish the different mobile stations, which is done
by distinguishing the different radio channels.

Protocols involved in the A-bis
interface
 Level 1-PCM transmission (E1 or T1)
 Speech encoded at 16kbit/s and sub
multiplexed in 64kbit/s time slots.
 Data which rate is adapted and synchronized.
 Level 2-LAPD protocol, standard HDLC
 Radio Signaling Link (RSL)
 Operation and Maintenance Link (OML).
 Level 3-Application Protocol
 Radio Subsystem Management (RSM)
 Operation and Maintenance procedure (OAM)

Presentation of A-bis
Interface
 Messages exchanges between the BTS and
BSC.
 Traffic exchanges
 Signaling exchanges
 Physical access between BTS and BSC is PCM
digital links of E1(32) or T1(24) TS at 64kbit/s.
 Speech:
 Conveyed in timeslots at 4X16 kbit/s
 Data:
 Conveyed in timeslots of 4X16 kbit/s. The initial user
rate, which may be 300, 1200, … is adjusted to 16
kbit/s

FLAG ADRESS CONTROL INFORMATION 0 – 260 OCT FCS FLAG
SAPI TEI
N(S) N(R)
LAPD message structure

LAPD
 The length is limited to 260 octets of
information.
 LAPD has the address of the destination
terminal, to identify the TRX, since this is a
point to multipoint interface.
 Each TRX in a BTS corresponds to one or
several signaling links. These links are
distinguished by TEI (Terminal Equipment
Identities).
 SAPI=0, SAPI=3, SAPI=62 for OAM.

Presentation of the A-ter
interface

BSC
TRAU
MSC
OMC
OAM
Transcoding
LAPD TS1
Speech TS
CCS7 TS
X.25 TS2
Speech TS
CCS7 TS
X.25 TS2
PCM
LINK PCM
LINK

Presentation on the A-ter
interface
 Signaling messages are carried on specific timeslots (TS)
 LAPD signaling TS between the BSC and the TCU
 SS7 TS between the BSC and the MSC, dedicated for
BSSAP messages transportation.
 X25 TS2 is reserved for OAM.
 Speech and data channels (16kbit/s)
 Ater interface links carry up to:
 120 communications(E1), 4*30
 92 communications(T1).
 The 64 kbit/s speech rate adjustment and the 64 kbit/s data
rate adaptation are performed at the TCU.

Presentation of the A interfacewww.arcadianlearning.com

Presentation on the A-Interface
BSSMAP - deals with procedures that take place logically between the
BSS and MSC, examples:
Trunk Maintenance, Ciphering, Handover, Voice/Data Trunk
Assignment
DTAP - deals with procedures that take place logically between the MS
and MSC. The BSS does not interpret the DTAP information, it simply
repackages it and sends it to the MS over the Um Interface. examples:
Location Update, MS originated and terminated Calls, Short
Message Service, User Supplementary Service registration,
activation, deactivation and erasure

Inter MSC presentation www.arcadianlearning.com

O
A
M
L
A
P
D
BTS
MTP2
SCCP
MTP3
L
A
P
D
O
A
M
R
R
D
T
A
P
B
S
S
M
A
P
BSSAP
BSC
MTP1
MTP3
MTP2
SCCP
MTP2
MTP3
SCCP
BSSAP
DTAP/
BSSMAP
T
C
A
P
MM
CM M
A
P
NSS
R
R
MM
CM
MS
Um
Interface
A bis
Interface
A
Interface

SCCP Ref=R2
TRX:TEI=T1
Channel ID = N1
SCCP Ref=R1
DTAP
DLCI: SAPI=3
DLCI: SAPI=0
Channel=C1
Link: SAPI=3
Link: SAPI=0
PD=CC
TI=a
TI=b
PD=MM
PD=RR
TI=A
MS BSC MSC
Channel=C2 Channel ID = N1
Radio Interface Abis Interface A Interface
PD: protocol discriminator
TI: Transaction Identifier for
RIL3-CC protocol
DLCI: Data Link connection
Identifier
SAPI: Service Access Point
Identifier on the radio
Interface
TEI: Terminal Equipment
Identifier on the Abis I/F

Bearer Services
 Telecommunication services to transfer
data between access points
 Specification of services up to the terminal
interface (OSI layers 1-3)
 Different data rates for voice and data
(original standard)
 Data service
○ Synchronous: 2.4, 4.8 or 9.6 kbit/s
○ Asynchronous: 300 - 1200 bit/s

Tele Services
 Telecommunication services that enable voice communication
via mobile phones.
 All these basic services have to obey cellular functions, security
measurements etc.
 Offered services.
 Mobile telephony
primary goal of GSM was to enable mobile telephony
offering the traditional bandwidth of 3.1 kHz.
 Emergency number
common number throughout Europe (112); Mandatory for all
service providers; Free of charge; Connection with the
highest priority (preemption of other connections possible).
 Multinumbering
several ISDN phone numbers per user possible.

Performance characteristics of
GSM
 Communication
 mobile, wireless communication; support for voice and data
services
 Total mobility
 international access, chip-card enables use of access points
of different providers
 Worldwide connectivity
 one number, the network handles localization
 High capacity
 better frequency efficiency, smaller cells, more customers
per cell
 High transmission quality
 high audio quality and reliability for wireless, uninterrupted
phone calls at higher speeds (e.g., from cars, trains)
 Security functions
 access control, authentication via chip-card and PIN

Disadvantages of GSM
 No full ISDN bandwidth of 64 kbit/s to the
user
 Reduced concentration while driving
 Electromagnetic radiation
 Abuse of private data possible
 High complexity of the system
 Several incompatibilities within the GSM
standards

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