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GSM Channels
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Contents
Channel Concepts
Call Setup
Burst & Multi Frames
Mapping of Logical Channels
Definitions of SDCCH
Call Set up
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Logical Channels
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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.
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Broadcast Channels (BCH)
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
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Broadcast Channels (BCH)
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
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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.
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Common Control Channels (CCCH)
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
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Common Control Channels (CCCH)
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
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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
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Dedicated Control Channels (DCCH)
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
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Dedicated Control Channels (DCCH)
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
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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.
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Call to an MS
More..
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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
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TDMA frames with Common and Dedicated Channels
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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
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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
1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )
TB TB GP
3 3 8.25ALL ZERO 142 BITS
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Bursts
Synchronization Bursts:
This burst is used for time synchronization of the MS
1 Time slot = 156.25 bits durations (15/26 = 0.577 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
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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.
1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )
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.
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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.
1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )
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
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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..
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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
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Thank you
Know More - SDCCH
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