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

2. GSM Architecture and Interfaces
Unit 1
GSM Architecture and Interfaces
SYLLABUS
Introduction
GSM frequency bands
GSM PLMN
Objectives of a GSM PLMN
GSM PLMN Services, GSM Subsystems
GSM interfaces
Mapping of GSM layers onto OSI Layers.
9/19/2018

3. GSM FREQUENCY BANDS
GSM system frequencies include two bands at 900MHz and 1800 MHz commonly referred as GSM-900 and DCS-1800 Systems.
For primary band in GSM-900 systems,124 radio carriers have been defined and assigned in two sub bands of 25MHz each in the MHz and MHz ranges with channel widths of 200kHz.
9/19/2018

4. GSM PLMN
GSM PLMN
9/19/2018

5. OBJECTIVES OF A GSM PLMN
To provide the subscriber a wide range of services and facilities both voice and non-voice.
To introduce a mobile radio system.
To provide certain services and facilities exclusive to mobile situations.
To provide facilities for automatic roaming, locating and updating of mobile subscribers.
To provide wide range of MSs.
To provide efficient use of the frequency spectrum.
To allow for a low cost infrastructure and terminal.
9/19/2018

6. GSM PLMN SERVICES Bearer services Teleservices Supplementary services
9/19/2018

7. GSM SUBSYSTEMS
Operational – External networks to/from NSS to/from BSS to/from MS to/from subscriber.
Control – OSS to/from service provider.
9/19/2018

8. GSM Subsystems
9/19/2018

9. GSM Reference Model
9/19/2018

10. GSM Subsystem Entities
The functional entities of the GSM and their logical interconnection are:-- MS
BSS
NSS
Operation and Maintenance Subsystem (OMSS)
9/19/2018

11. MS
Types of MSs
9/19/2018

12. continued
An MS has number of identities including International Mobile Equipment Identity (IMEI), International Mobile Subscriber Identity (IMSI) and ISDN number.
IMSI Contains:
Mobile country code(MCC), Mobile Network Code(MNC), Mobile Subscriber Identification(MSIN), National Mobile Subscriber Identity(NMSI).
IMEI Contains:
Type Approval Code(TAC), Final Assembly Code(FAC), Serial number(SN), Spare(SP).
9/19/2018

13. BSS
It is the physical equipment that provides radio coverage to prescribed geographical areas, known as cells.
BSS consists of a control function carried out by the BSC and the transmitting function performed by the BTS.
The BTS is the radio transmission equipment and covers each cell.
BTS contains Transcoder Rate Adapter Unit (TRAU).
In TRAU specific speech encoding and decoding is carried out.
9/19/2018

14. NSS
The MSC performs the necessary switching functions required for the MSs located in an associated geographical area, called an MSC area.
MSC area in GSM
9/19/2018

15. Operation and Maintenance Subsystem (OMSS)
The OMSS is responsible for handling system security based on validation of identities of various telecommunications entities.
The AuC is accessed by the HLR to determine whether an MS will be granted service.
The EIR provides MS information used by the MSC.
The Operational maintenance Center (OMC) is the functional entity through which the service provider monitors and controls the system.
9/19/2018

16. GSM INTERFACES The Radio Interface (MS to BTS)
Abis Interface (BTS to BSC)
A Interface
Interfaces between Other GSM Entities
9/19/2018

17. The Radio Interfac ( MS to BTS )
The Um interface is the most important in any mobile radio system, in that it addresses the demanding characteristics of the radio environment.
The radio interface uses the Link Access Protocol on Dm channel (LAPDm).
The logical channel types that are supported are:--
Speech traffic channels(TC H)
Broadcast channels (BCCH)
Common control channels (CCCH)
Cell broadcast channel (CBCH)
Dedicated control channel (DCCH)
9/19/2018

18. Abis Interface ( BTS to BSC)
The interconnection between the BTS and BSC is through a standard interface, Abis.
This interface supports two types of communication links : traffic channel and signaling channel.
There are two types of messages handled by the traffic management procedure part of the signaling interface – transparent and nontransparent.
Transparent messages are between the MS and BSC-MSC and do not require analysis by the BTS.
Nontransparent messages do require BTS analysis.
9/19/2018

19. A Interface ( BSC to MSC)
The A interface allows interconnection between the BSS radio base subsystem and the MSC.
the signaling transport uses message transfer part (MTP) and Signaling Connection Control Part (SCCP) of SS7.
Error free transport is handled by a subset of the MTP, and logical connection is handled by a subset of the SCCP.
9/19/2018

20. Interfaces between other GSM Entities
Information transfer between GSM PLMN entities uses the MAP.
The major procedures supported by MAP are:--
Location registration and cancellation
Handover procedures
Handling supplementary services
Retrieval of subscriber parameters during call setup
Authentication procedures.
9/19/2018

21. MAPPING OF GSM LAYERS ONTO OSI LAYERS Mapping of GSM onto OSI layersMS
BSS
MSC
OSI Ref mod CM MM RR
LAPDm
TDMA/FDMA CM MM RR
BSSAP
SCCP
LAPDm
MTP
TDMA/FDMA 3 2 1
Mapping of GSM onto OSI layers
9/19/2018

22. Radio Link Features in GSM Systems
Unit 2
Radio Link Features in GSM Systems
SYLLABUS
Introduction
GSM frequency bands
GSM PLMN
Objectives of a GSM PLMN
GSM PLMN Services, GSM Subsystems
GSM interfaces
Mapping of GSM layers onto OSI Layers.
9/19/2018

23. INTRODUCTION
The GSM system uses a number of interference reducing mechanism These include
Adaptive Power Control(APC)
Discontinuous Transmission(DTX)
Slow Frequency Hopping(SFH)
9/19/2018

24. RADIO LINK MEASUREMENTS
In GSM the MS uses the BS identity code (BSIC)
NCC – Network Color Code(3 bits)
BCC - Base Transceiver Color Code (3 bits)
BSIC
NCC
BCC
Fig: GSM BSIC
9/19/2018

25. RADIO LINK FEATURES OF GSM
Dynamic Power Control
Discontinuous Transmission(DTX)
Slow Frequency Hopping(SFH)
9/19/2018

26. Dynamic Power Control
The GSM network is designed so that the MS is instructed to use only the minimum power level necessary to achieve effective communication with the BTS
The receive power level in dBm is mapped to a value between 0 and 63
For the BTS the power output is nominally controlled in 2-dB steps to provide better co channel interference performance.
Both MS and BTS power control is performed in 2dB steps
The use of minimum transmitting power to access the network helps to increase the battery life of the mobile set and reduce interference
9/19/2018

27. DISCONTINUOUS TRANSMISSION (DTX)
The transmitter is active only 50 percent of the time then the average interference can be reduced by 3dB
DTX activity factor v of the transmitter to the occupancy
E.g if v=0.4,Poccup =0.8 then the effective channel occupancy
Pactive=0.4*0.8 = 0.32
Poccup is used to determine the capacity
DTX in GSM feature in which speech is transmitted only when there is speech available to transmit
This helps to reduce interference in MSs.a Voice Activity Detector is used to initiate the switching Process.
9/19/2018

28. SFH
SFH used in GSM to improve performance in multipath fading environment and to reduce the required S/I ratio.
The mobile radio channel is a frequency selective fading channel
GSM uses SFH to improve signal quality in SFH the hop rate is less than the message bit rate
FH provides frequency diversity to overcome Rayleigh fading due to multipath propogation
Fig: DTX in GSM
9/19/2018

29. SFH Fig: Baseband Frequency Hopping Implementation TRX1 Baseband of BS
9/19/2018

30. FUTURE TECHNIQUES TO REDUCE INTERFERENCE IN GSM
Channel borrowing or effective channel management
Advanced antenna technology
9/19/2018

31. CHANNEL BORROWING Dynamic Channel Allocation(DCA)
Hybrid Channel Assignment (HCA)
Channel Borrowing without Locking (CBWL)
9/19/2018

32. CHANNEL BORROWING
A channel is borrowed from the pool by a BS for use on a call
The channel assignment is based on interference consideration
The interference level of the idle channels is measured and by means of the signal level from the preferred BS the resulting S/I ratio is estimated.
Channel licking is used to prevent an increase in cochannel interference
The CBWL exhibits better performance in light as well as in heavy traffic loads.
9/19/2018

33. SMART ANTENNAS
Adaptive antennas would be considerably beneficial in the following areas
Coverage
Capacity
Signal Quality
Portable terminal transmit power
9/19/2018

34. SMART ANTENNAS
Way to reduce interference is to use smart or intelligent antennas
A smart antennas evaluates signal conditions continuously of each signal that is transmitted or received.
It belongs to two basic classes
Switched beam
Adaptive
9/19/2018

35. GSM LOGICAL CHANNELS AND FRAME STRUCTURES
UNIT 3
GSM LOGICAL CHANNELS AND FRAME STRUCTURES
Syllabus
GSM Logical channels
Allowed Logical Channel Combinations
Multi-frames
GSM Frame Structures
GSM Bursts
Data Encryption in GSM
Mobility Management
9/19/2018

36. GSM LOGICAL CHANNEL
Physical Channel
Logical Channel
9/19/2018

37. 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.
9/19/2018

38. USER INFORMATION( TRAFFIC) SIGNALLING INFORMATION (CONTROL)
LOGICAL CHANNELS
USER INFORMATION( TRAFFIC)
SIGNALLING INFORMATION (CONTROL)
9/19/2018

39. 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
9/19/2018

40. 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
9/19/2018

41. 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.
9/19/2018

42. 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
9/19/2018

43. 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
9/19/2018

44. 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
9/19/2018

45. 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.
9/19/2018

46. 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
9/19/2018

47. 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
9/19/2018

48. 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. .
9/19/2018

49. Dedicated Control Channels (DCCH)….
SDCCH (Stand alone Dedicated Control Channel)
Call Set up
Authentication
Transmission of SMS
Location Update
9/19/2018

50. 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
9/19/2018

51. 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
9/19/2018

52. 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
9/19/2018

53. Normal Burst Tail bit 3 57 Data bits 26 bit Training seq 57 Data bits
8.25
Bit GP
Stealing Flags
9/19/2018

54. 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
9/19/2018

55. 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
9/19/2018

56. GSM Mobility Management
GSM architecture overview
Network layout
Protocols
Addresses & identifiers
Location management
Call delivery + location update
Security
Handover management
9/19/2018

57. 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
9/19/2018

58. 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
9/19/2018

59. 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
9/19/2018

60. LOCATION REGISTRATION
Geographic based
Time Based
ON/OFF Based
9/19/2018

61. MOBILE IDENTIFICATION
Mobile identification is used to identify the MS when the VLR does not recongnize the TMSI sent by the MS.
9/19/2018