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Concept of TDMA Frames AndChannels 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

UPLINK

DOWNLINK


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 slotsTDMA 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 MHz124 channels (200 kHz)downlink

890-915 MHz124 channels (200 kHz)uplink

frequ

ency

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

guardspace

guardspacetail user data TrainingS S user data tail

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMAwww.arcadianlearning.com

LOGICAL CHANNELS

TRAFFIC SIGNALLING

FULL RATEBm 22.8 Kb/S

HALF RATELm 11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLYBOTH 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 8.25FREQUENCYCORRECTION BURST - FB

3 3 8.25 39 64 39SYNCHRONISATION BURST - SB

3 6 41 36 68.25ACCESSBURST - 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



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 RATEBm 22.8 Kb/S

HALF RATELm 11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLYBOTH 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

AIRINTERFACE

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 - 8KEncoding - 13 bit Encoding (104 Kbps)RPE/LTP - Regular Pulse Excitation/Long Term PredictionRPE/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 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




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 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3

3 57 bits 26 1 1 57 bits 3


GSM Protocol Suite www.arcadianlearning.com

BTS

Radio interface

HLR

MSCVLR

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 ProtocolRadio Subsystem Management (RSM)Operation and Maintenance procedure (OAM)


Presentation of A-bis Interface Messages exchanges between the BTS and BSC.

Traffic exchangesSignaling 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

PCMLINK 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

Signaling Protocol Model

Presentation on the A-InterfaceBSSMAP - 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 presentationwww.arcadianlearning.com

OAM

LAPD

BTS

MTP2

SCCP

MTP3

LAPD

OAM

RR

DTAP

BSSMAP

BSSAP

BSC

MTP1

MTP3

MTP2

SCCP

MTP2

MTP3

SCCP

BSSAPDTAP/

BSSMAP

TCAP

MM

CM MAP

NSS

RR

MM

CM

MS

UmInterface

A bisInterface

AInterface


SCCP Ref=R2

TRX:TEI=T1

Channel ID = N1

SCCP Ref=R1

DTAP

DLCI: SAPI=3

DLCI: SAPI=0

Channel=C1Link: SAPI=3

Link: SAPI=0PD=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 discriminatorTI: Transaction Identifier for RIL3-CC protocolDLCI: Data Link connection IdentifierSAPI: Service Access Point Identifier on the radio InterfaceTEI: 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 telephonyprimary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz.

Emergency numbercommon number throughout Europe (112); Mandatory for all service providers; Free of charge; Connection with the highest priority (preemption of other connections possible).

Multinumberingseveral 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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Education

mobile channel

base channel

rate channel

bidirectional channel

broadcast control channel

synchronisation channel

paging channel rach

particular traffic channel