GSM Channels New

7/27/2019 GSM Channels New

1/24

GSM Channels


2/24

Contents

Channel Concepts

Call Setup

Burst & Multi Frames

Mapping of Logical Channels

Definitions of SDCCH

Call Set up


3/24

Logical Channels


4/24

Broadcast Channels (BCH)

Frequency Correction Channel (FCCH)

Downlink Channel

BTS: Transmits a carrier frequency (Pure sine wave of 67.7 Khz)

This Solve 2 purpose :

a> Make sure that this is BCCH Carrier

b> To allow the MS to synchronize to the frequency

MS: After Switch on MS Scan for this channel, since it has no information

to which frequency to use. FCCH carrier enables a mobile to tune its

frequency to that being broadcast by the BTS.


5/24


Syncronization Channel (SCH)

Downlink Channels

BTS: Transmits TDMA Frame number + Base Station

Identity Code (BSIC= NCC + BCC )

MS: MS decodes the BSIC if the chosen BTS is GSM

Base station within a cell


6/24


Broadcast Control Channel (BCCH)

Downlink Channels

BCCH contains the detailed Network and cell specific informationsuch as :

Frequency used by Cell and its Neighboring cells.Frequency HSN

Paging Groups

LAI

Max output power allowed in the cell


7/24

Common Control Channels (CCCH)

Paging Channel (PCH)

Downlink Channels

BTS: Broadcast the paging message to indicate the IncomingCalls or Incoming SMS. Paging message also includesthe MSs identity number IMSI/TMSI

MS: MS listens to the PCH. If it identifies its own mobile

subscriber identity number on the PCH, it will respond.


8/24


Random Access Channel (RACH)

RACH is transmitted Uplink only

When mobile is paged , it replies on RACH requesting asignaling channel.

RACH can also used if the MS wants to make a contact the NW/

Originating calls


9/24


Access Grant Channel (AGCH)

Downlink channel

AGCH is answer to the RACH

NW assigns a signaling channel (SDCCH) to the MS. This

assignment is performed on the AGCH


10/24

Dedicated Control Channels (DCCH)

Stand Alone Dedicated Control Channel (SDCCH)

SDCCH is bi-directional Channel

System Signaling

Call Setup

Authentication

Location Update

Assignment of Traffic channels and

Transmission of Short messages


11/24


Slow Associated Control Channel (SACCH)

SACCH is transmitted in both Uplink and Downlink directions

SACCH is associated with each SDCCH and also with TCH

Uplink : MS Sends the averaged measurement on its own BTS and

neighboring BTSs

Downlink: MS receives information regarding information

concerningTransmit power to use

Instructions on Timing Advance


12/24


April 25, 2007

Fast Associated Control Channel (FACCH)

While Calls in progress and HO is required FACCH is used

FACCH works in Stealing mode meaning that one 20ms

segment of speech is exchanged for signaling information

necessary for the HO

Cell Broadcast Channel (CBCH)

CBCH is used in Downlink only

It is used to carry Short Message Service Cell Broadcast(SMSCB) and uses the same physical channel as the SDCCH


13/24

Traffic Channels (TCH)

Traffic channels are Bi-directional logical channel that transfer the user

speech or data.

Full Rate TCH ( TCH/F) :

This channel carries information at a gross rate at 13Kbit/s *

* Now it is 22.8 Kbit/s with latest R9.1

Half Rate TCH (TCH/H) :

This Channel carried information at a gross rate at 6.5Kbit/s*

* Now it is 11.4 Kbit/s with latest R9.1

Enhance Full Rate :

The speech coding in EFR is still done at 13Kbit/s, but the

coding mechanism is different that is used for normal FR.

EFR gives better speech quality at the same bit rate than

normal FR.


14/24

Call to an MS

More..


15/24

Relationship Between Burst & FrameBurst : Physical content of a TS is called Burst.There are 5 types of Bursts

each having 15/26 ms duration and 156.25 Bits.

Hyperframe: In GSM system every TDMA frame is assigned a fixed number,

which repeats itself in a time period of3 HOURS 28 MINUTES 53

SECONDS 760 MILLISECONDS. This time period is referred to

as Hyperframe.

Superframe : =51x26 Multiframes. So, Duration =51x26x8x15/26=6Sec 120ms

Multiframe :There are two types of multiframe.

26 TDMA Frame Multiframe :Used to carry TCH, SACCH and

FACCH

Duration =26 x 8 x 15/26 =120ms

51 TDMA Frame Multiframe : Used to carry BCCH,CCCH,SDCCHand SACCH.

Duration =51 x 8 x 15/26 =235.38ms


16/24

TDMA frames with Common and Dedicated Channels


17/24

Bursts

Normal Bursts:This burst is used to carry information on :

Traffic channel

SDCCH Channel

Broadcast Control Channel

Paging Channel

Access Grant Channel

SACCH & FACCH Channel

1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )

TB Encrypted bits Training Sequence Encrypted bits TB GP

3 57 26 57 3 8.25

F : One Stealing Bi:=0 Indicates 57bit packet contains user data or speech

:=1 Indicates burst stolen for FACCH Signalling

RxQual derived from the 26 bit midable from the TDMA frame

F F


18/24

Bursts

Frequency Bursts:

All 148 bits(142+6) are coded with 0. The output of GMSK Modulator is a fixed

frequency signal exactly 67.7 Khz above the BCCH carrier frequency.

Thus the MS on receiving this fixed frequency signal fine tunes to the BCCH frequency

and waits for the Sync burst to arrive after 1 TDMA Frame .i.e =15/26*8=4.615ms


TB TB GP

3 3 8.25ALL ZERO 142 BITS


19/24

Bursts

Synchronization Bursts:

This burst is used for time synchronization of the MS


TB TB GP

3 3 8.25

SCH DATA

39 Bits

SCH DATA

39 Bits

Extended Training Sequence

64 Bits

39Bit x 2=78 Bits :Are decoded to arrive 25-SCH control bits and that contains

the information of the NCC ,BCC & TDMA FN

64 Bits : Long training seq. of 64 Bits are identical for all BTS


20/24

Bursts

Access Bursts:

This burst is used only for initial access by the MS to the BTS which applies 2 cases :

For connection setup when idle state where a CHAN_REQ message is sent using

access burst

For HO when MS send HND_ACC message.


TB

8

SCH Sequence RACH Data TB

41 Bits 36 Bits 3

Guard Band

68.25 Bits

36Bit Contains : BSIC+CHAN_REQ or HND_ACC

41Bit Contains : Fixed bit sequence allow BTS to recognize it is Access Burst

86.25 bits GP: Long GP enables BTS to get Propagation delay information.


21/24

Bursts

Dummy Bursts:

To enable the BCCH frequency to be transmitted with a constant power level, dummy

burst are inserted.

This burst is transmitted on CHGR=0 when no other type of burst is to be sent.

Thus it makes possible for MS to perform the power measurement on the BTS in order

to determine which BTS to use for initial access or which to use for HO

CCCH is replaced by the dummy page, when there is no paging message to transmit.


TB

3

Mixed Bits Training Sequence TB Guard BandMixed Bits

58 26 3 8.2558

58Bits: Coded with pseudo random bit seq. to prevent confusion with Freq correction burst


22/24

Call Setup-Mobile Terminating Call

MS BTS BSC MSC

1>Paging(LAI+IMSI/TMSI)

2>Paging CommandImsi/Tmsi+PG+TRX+CG+TN3>Paging Req(Imsi on PCH)

4>Channel Req(On RACH)5>Channel Reqd (Access Delay)

6>Channel Actn (MSPwr,BSPwr,TA)

7>Channel Activation Ack

8>Imm Assign Cmd(On AGCH , Freq

+TS+ SDCCH SubChannel No+TA8>Immediate Assign

9>Estblish Ind (Paging Resp)

IMSI+MS Class9>Conn Req (Paging Resp: BSC

add CGI)

10>Auth Req (128 bit RAND+

3bit CKSN)10>Auth Req (128 bit RAND+3bitCKSN)10>Auth Req (128 bit

RAND+3bitCKSN)

11>Auth Response (MS Calculate

SRES & Kc with its own Ki stored in

SIM by appling algorithm A3&A8)

11>Auth Response (SRES)11>Auth Response (SRES)

SABM (Paging Resp:IMSI/MS Class)

UA(Paging Resp) Unnumbered Ack

Frame which confirms only 1 MS is

using Sig Channel

PCH

RACH

AGCH

SDCCH

SDCCH

Next..


23/24

Call Setup-Mobile Terminating Call

MS BTS BSC MSC

14>Setup (Req for Services I.e.

Speech/Data/Fax etc)15>Call Confirmed

17>Channel Activation

(BSC Allocated Idle TS for Traffic)

20>Assign Comp (MS tune to TCH

and send Ind that Chan is Seized)

14>Setup

14>Setup15>Call Confirmed 15>Call Confirmed

16>Assignment Req

(MSC send CIC to BSC)

18>Channel Activation Ack

19>Assignment Cmnd (BSC send

message on SDCCH to MS telling to go

TCH)

19>Assignment Cmnd (BSC send

message on SDCCH to MS telling to

go TCH)

20>Assign Comp (MS tune to TCH

and send Ind that Chan is Seized)20>Assign Comp (MS tune to TCH

and send Ind that Chan is Seized)

21>RF Chann Realease

21>RF Chann Realease Ack22>Alert (MS Send Alert to MSC as

soon as the ringing is started in MS)22>Alert (MS Send Alert to MSC as

soon as the ringing is started in MS)22>Alert (MS Send Alert to MSC as

soon as the ringing is started in MS)

23>Connect (When MS Sub Answer

the Conn message sent to MSC)23>Connect (When MS Sub Answer

the Conn message sent to MSC)23>Connect (When MS Sub Answer

the Conn message sent to MSC)

24>Connect Ack24>Connect Ack24>Connect Ack

SDCCH

SDCCH

SDCCH

TCH

TCH

TCH

TCHExit..

K M SDCCH


24/24

Thank you

Know More - SDCCH

Documents

GSM Channels New