1. Subject Name: GSM
Subject Code: 10EC843
Prepared By: Gopika D K, Shruthi N
Department: ECE
Date

2. GSM LOGICAL CHANNELS & FRAME STRUCTURES
UNIT 3
GSM LOGICAL CHANNELS & FRAME STRUCTURES
11/9/2018

3. GSM LOGICAL CHANNEL
Physical Channel
Logical Channel
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4. Physical Channel
Each timeslot in TDMA frame is called a physical channel.
There are 8 physical channels per carrier in GSM.
Physical channels may be used to carry speech data or signaling information.
Logical Channel
The Information carried by physical channels is known as logical channel.
There are a several type of logical channels available in GSM.
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5. USER INFORMATION( TRAFFIC) SIGNALLING INFORMATION (CONTROL)
LOGICAL CHANNELS
USER INFORMATION( TRAFFIC)
SIGNALLING INFORMATION (CONTROL)
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6. Logical Channels Logical Channels Common Channels Dedicated Channels
Control
Channels
Traffic
Channels
Broadcast
Channels
Common
Control
Channels
TCH (Full-Rate)
TCH (Half-Rate)
TCH (EFR)
SDCCH
SACCH
FACCH
FCCH
SCH
BCCH
RACH
PCH
AGCH
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7. Common channels are used to broadcast different information to MS & for setting up signaling channels between the MSC/VLR & the MS
Common
Channels
Broadcast
Channels
Common
Control
Channels
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8. Broadcast Channels
FCCH (Frequency Correction Channel) – Provides the frequency correction information used by the mobile station.
SCH (Synchronization Channel) – Contains the Base Station Identity Code (BSIC) and the TDMA frame number used for synchronization of the mobile station to the frame structure of a new BTS.
BCCH (Broadcast Control Channel) – Used to broadcast general information, related to network, to all mobile stations.
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9. Broadcast Channels…..
FCCH:-
This channel do not contain any information, this have a stream of 142 zeros.
So, the frequency that have this sequence helps MS to identify the BCCH frequency among all transmitted frequency
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10. SCH(Synchronization Channel):- BTS sends TDMA frame number on SCH by which MS synchronize it self with GSM system.
SCH
Down link
Down link
TDMA Frame Number
TDMA Frame Number
BSIC
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11. Broadcast Control Channel(BCCH):- System Type 900/1800 LAI
Neighbor Cell BCCH Information
Frequency hopping used or not?
Type of Training sequence used
Max power allowed in cell
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12. Common Control Channels (CCCH)
PCH (Paging Channel) – Used to page the mobile station. PCH
information is transmitted over the downlink It contain IMSI or
TMSI.
RACH (Random Access Channel ) – Used by a mobile station to
Request access to the system. RACH information is transmitted
over the uplink.
AGCH (Access Grant Channel) – Used to assign a SDCCH for
call setup. AGCH information is transmitted over the downlink.
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13. It used when MS want to uplink first time Used for Emergency Call
RACH:-
It used when MS want to uplink first time
Used for Emergency Call
Answer to paging
Initiate O/G call
Location update request
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14. It show which SDCCH is allotted to MS
AGCH (Access Grant Channel) – Used to assign a SDCCH for call setup. AGCH information is transmitted over the downlink
Down link
It show which SDCCH is allotted to MS
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15. Dedicated Control Channels (DCCH)
SDCCH (Stand alone Dedicated Control Channel) – Carries
signaling information during call setup
SACCH (Slow Associated Control Channel) – Transmits call
Control data and measurement reports during the call.
FACCH (Fast Associated Control Channel) – Carries urgent
Signalling information for handover etc. .
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16. Dedicated Control Channels (DCCH)….
SDCCH (Stand alone Dedicated Control Channel)
Call Set up
Authentication
Transmission of SMS
Location Update
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17. ALLOWED LOGICAL CHANNEL COMBINATIONS
Channels are grouped into
26-multiframe - payload / voice – summarizes the bursts of TCHs and associated SACCHs and FACCHs
51-multiframe – signaling data – puts together all bursts of traffic channels without SACCHs and FACCHs
GSM uses certain predefined pattern of channel combinations:
CC1: TCH/F + FACCH/F + SACCH/TF
CC2: TCH/H (0,1) + FACCH/H(0,1) + SACCH/TH(0,1)
CC3: TCH/H(0) + FACCH/H(0) + SACCH/TH(0)+TCH/H(1)
CC4: FCCH + SCH + BCCH + CCCH
CC5: FCCH + SCH + BCCH + CCCH + SDCCH/4(0,1,2,3) + SACCH/C4 (0,1,2,3)
CC6: BCCH + CCCH
CC7: SDCCH/8 + SA
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18. Communication Systems GSM – frames, multiframes, superframes
Why 26, 51:An active call transmits/receive in 25 frames, except the last one
In this last frame, it can monitor the BCCH of this (and neighbor) cell
This particular numbering allows to scan all BCCH slots during a superframe
Important slots while call is active: frequency correction FCCH and sync SCH - needed for handover
Why multiframes - determine how BCCH is constructed, e.g. which specific information transmitted on BCCH during a given multiframe
Superframes are composed of multiframes
Used as input parameter by encryption algorithm
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19. TDMA Bursts in GSM 3 8.25 FB 1 142 fixed bits 3 SB 3 39 data 64 bit
Training seq
8.25
Dummy
Burst 3
26 bit
Training seq 3
8.25 8
41 bit
Training seq
36 data 3
68.25
Access
Burst
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20. Normal Burst Tail bit 3 57 Data bits 26 bit Training seq 57 Data bits
8.25
Bit GP
Stealing Flags
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21. DATA ENCRYPTION IN GSM network MS Authentication
What signed response (SRES) are you able to derive from the input challenge RAND by applying the A3 algorithm with your personal key Ki (Ki is per subscriber)?
RAND
network
A3 algorithm Ki MS
A3 algorithm Ki
SRES Ki
RAND (128bit)
SRES
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22. DATA ENCRYPTION IN GSM BTS MS Encryption
Digital technology – easy to encrypt voice data
A5 derives a ciphering sequence of 114 bits for each burst independently
XOR 114 bits of a radio burst with 114 bits of a ciphering sequence generated by A5
A5 algorithm
Kc (64 bits) MS
frame number
(22 bits)
BTS Kc
S1(114)
deciphering
S2(114)
ciphering S1
ciphering S2
deciphering S1
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23. GSM Mobility Management
GSM architecture overview
Network layout
Protocols
Addresses & identifiers
Location management
Call delivery + location update
Security
Handover management
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24. GSM network layout GSM Network (PLMN) MSC region MSC region MSC region
PLMN: Public Land Mobile Network
MSC: Mobile Switching Center
BTS: Base Transceiver Station
BSC: Base Station Controller
GSM Network (PLMN)
MSC region
MSC region
Location area
Location area
BSC
BSC
MSC region
BTS
BTS
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25. GSM network layout PSTN ISDN OMC MSC GMSC BSC HLR EIR VLR AUC BTS A
Abis E
B,C
EIR
HLR
AUC
VLR
BTS Um
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26. GSM MAP protocol GSM MAP similar to IS41 MAP
MAP uses Transactions Capabilities Part (TCAP) of the SS7 stack
MAP functions:
Updating of location information in VLRs
Storing routing information in HLRs
Updating and supplementing user profiles in HLRs
Handoff of connections between MSCs
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27. LOCATION REGISTRATION
Geographic based
Time Based
ON/OFF Based
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28. MOBILE IDENTIFICATION
Mobile identification is used to identify the MS when the VLR does not recongnize the TMSI sent by the MS.
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