Department of Electronic EngineeringDepartment of Electronic EngineeringELE 52PMC
Personal Mobile Communication
GSM Public Land Mobile Network GSM Public Land Mobile Network (PLMN)(PLMN)
Michael [email protected]/~mf
Lecture 4
2
Introduction
The European Telecommunication Standards Institute (ETSI) standardised the Global System for Mobile communications (GSM).
ETSI originally defined GSM as a European digital cellular telephony standard.
The GSM interfaces defined by ETSI lay the ground-work for a multivendor network approach to digital mobile communication.
GSM offers users good voice quality, call privacy, and network security.
3
GSM Statistics
Number of Countries/Areas with GSM System is 205 (Feb 2004)
GSM Total Subscribers - 1024.3 million (end Feb 2004)
SMS messages sent per month - 45.6 Billion SMS forecast for 2004 - 547.5 Billion GSM accounts for 72.5% of the World's digital market
and 72.0% of the World's wireless market
Ref.: [5] GSM Association
4
GSM Subscribers Growth
Blue bars = in-year net gains2001 is an estimate
5
GSM Subscribers by Region
6
Others ….
GSM/PSTN/ISDN
MSC
BTS
BSC
BTS PSTN
BTSISDN
SS#7MS
MS
MS
MS
MS
MS
7
Communication Path
OSI Protocol Reference Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
PhysicalBits
F A C Data Unit ( I Field ) FCS F
AP DataPH
Data UnitSH
Data UnitTH
Data UnitNH
AP DataAH
AP DataAP “X” AP “Y”
8
OSI of GSM
Transmission
RR
MM
CM
OAM
UserOperator
1. Radio transmission (Physical)1. Radio transmission (Physical)
2. Radio Resource Management2. Radio Resource Management
3. Mobility Management3. Mobility Management
4. Communication Management4. Communication Management
5. Operation Administration5. Operation Administration & Maintenance& Maintenance
9
System Elements
MS (Mobile Station)– MT (Mobile Terminal)
– TE (Terminal Equipment)
BS (Base Station)– BTS (Base Transceiver Station)
– BSC (Base Station Controller)
MSC (Mobile Switching Centre)
10
Maximum RF Power for MS
I 20
II 8
III 5
IV 2
V 0.8
Class Max. RF Power (W)
11
Functional Architecture
BSCMSC
HLR
VLR
MS
OAM
BTS
Base Station Subsystem(BSS)
Um
Air Interface
A-bis Interface
A Interface
MAP
MAP
12
Internal Environment
NSSBSSMS
OSS
Control flowUser data flow
OSS Operation SubsystemBSS Base Station SubsystemMS Mobile StationNSS Network & Switching Subsystem
13
External Environment
PSTNNSSBSS
OSS
Control flowUser data flow
OSS Operation SubsystemBSS Base Station SubsystemPSTN Public Switch Telephone NetworkNSS Network & Switching Subsystem
BSS
14
MTTE
MS
BTS
BTS
BTS
BSC
BSS
MTTE
MS
MTTE
MS
BTS
BTS
BTS
BSC
BSS
MTTE
MS
MSC
NMC
OMC
ADC
OMC
HLR
AUC
VLR
EIR
MSCA
Um
A: Interface between BSC and MSCA-bis: Interface between BTS and BSCUm: Air Interface between MS and BTS
A-bis
AUC: Authentication CentreEIR: Equipment Identity RegisterHLR: Home Location RegisterVLR: Visitor Location Register
15
Functions of MSC
Gateway to other networks (PSTN, ISDN, etc.), Paging of MS, MS location updating, and MS mobility management assisted by:
– Home Location Register (HLR), and
– Visitor Location Register (VLR).
16
Gateway MSC (GMSC) Key Role
GMSCUser
HLR
MSCVLRDirectory
NumberRoutingNumber
RoutingNumber
DirectoryNumber
GMSC handling of a domestic call
17
Radio S
ubsystem
RPE-LTPencoder
RPE-LTPdecoder
13 bit uniform to 8 bit A-Law
8 bit A-Law to 13 bit uniform
MS
C
PSTN/
ISDN
104 kbps
13 kbps
8000samples/s
13-bit uniformcode
Analoguespeech
8000 samples/s13-bit uniform
code
8000 samples/s8-bit A-Law
ITU-T G.711, G.712,G.713, G.714
RPE-LTPencoder
RPE-LTPdecoder
13 bit uniform to Analogue
O/P
Analogue to 13 bit uniform
I/P
System Description
18
Radio Access & Demod
GMSK
De-Interleaving
Forward Error
CodingInterleaving
Radio Access & ModGMSK
InterleavingForward
Error Coding
Error Correction
Radio Access & ModGMSK
Error Correction
De-Interleaving
Radio Access & Demod
GMSK
Dup Dup
Encoded speech456 bits/frame
Encoded speech456 bits/frame
Radio Subsystem
19
Digital Radio Link ProcessSpeechCoding
ChannelCoding
BitInterleaving
Encryption
Modulation
Burst Forming& Multiplexing
SpeechDecoding
ChannelDecoding
BitDeinterleaving
Decryption
DelayEqualisation
Demodulation
Demultiplexing
20
Transmission
Speech Coding– Regular Pulse Excitation (RPE)
– Linear Prediction Coding (LPC)
Data Service Modulation - GMSK GSM channels
– Physical channels
– Logical channels
Multiple-access scheme (TDMA/FDMA, FH)
21
Speech Coding
A 4-kHz analogue signal is first converted to 64 kbps and then to 13 kbps using RPE-LPC (Regular Pulse Excitation - Linear Prediction Coding).
RPE generates the impulse noise to simulate the nature of speech.
LPC compresses the speech waveform by using a filter with 8 transmitted coefficient.
There are two modes of voice transmission in GSM; continuous (normal) and discontinuous.
22
GSM Speech Processing - BS
8-bit A-Lawto 13-bituniform
Converter
RPE-LTPSpeechEncoder
13 x 8,000 = 104 kbps
DigitalSpeech Signal(64 kbps)
To ChannelEncoder
13 kbps
RPE-LPC: Regular Pulse Excitation - Linear Prediction CodingRPE-LTP: Regular Pulse Excitation - Long-Term Predictor
MSside
MSCside
23
GSM Speech Processing - MS
Low-PassFilter
RPE-LTPSpeechEncoder
To ChannelEncoder
A/DConverter
AnalogueSpeech Signal(4 kHz BW)
13 kbps
13 x 8,000 = 104 kbps
RPE-LPC: Regular Pulse Excitation - Linear Prediction CodingRPE-LTP: Regular Pulse Excitation - Long-Term Predictor
MS BS
24
0 1 2 3 4 5 6 7
260 260
456 bit456 bit
20 ms20 ms
D4
D1
D2
D3
D5
D6
D7
D8
D4
D1
D2
D3
D5
D6
D7
D8
Tail Bits3
57 bit data(first 20 ms)
Training Sequence26
57 bit data(second 20 ms)
Tail Bits3
Guard Period8.25
Normal Burst (NB)
1 1
57 bits each
Speech (13 kbps)
Speech coder
Channel encoder
TDMA frame (4.615 ms)
20 ms at 13 kbps
160 Samples (2,080 bits) 160 Samples (2,080 bits)
25
Data Services
The highest data rate is 9600 bps and has two different modes.
Transparent mode (T) employs FEC:– 2400 bps (intermediate rate is 3.6 kbps)
– 4800 bps (intermediate rate is 6 kbps)
– 9600 bps (intermediate rate is 12 kbps)
Non-Transparent mode (NT) employs ARQ.
26
Modulation
Gaussian Minimum-Shift Keying (GMSK) with BT = 0.3 normalised bandwidth Gaussian filter.
Transmission rate: (1/T) = 270 kbps Baseband: B = 270 x 0.3 = 81 kHz The GSM RF channel is 200 kHz which means the
bandwidth efficiency is 270 kbps / 200kHz = 1.35 b/s/Hz
27
GMSK Modulation
coscos
sinsin
GaussianFilter
GaussianFilter
dt dt
-sin t
cos t
cos [t + (t, n)]cos [(t, n)]
sin [(t, n)]
(t, n)
Phase pulse shaping
28
GSM Channels
Physical channels– Full rate
– Half rate
– One-eighth rate
Logical channels– Downlink common channels
– Uplink common channel
– Signalling channels
29
Physical Channels
Physical channel in a TDMA system is defined as a timeslot with a timeslot number TN in a sequence of TDMA frames.
GSM employs TDMA combined with FH and hence the physical channel is partitioned in both time and frequency.
FH is known to be very efficient in combating channel fading.
30
TCH (Traffic Channels)
TCH/F 22.8 kbps (Full Rate)– TCH/F9.6
– TCH/F4.8
– TECH/F2.4
TCH/H 11.4 kbps (Half Rate)– TCH/H4.8
– TCH/H2.4
TCH/8 (One-eighth Rate)
31
Downlink Common Channels
FCCH - Frequency Correction Channel SCH - Synchronisation Channel BCCH - Broadcast CCH PAGCH - Paging and Access Grant Channel CBCH - Call Broadcast Channel
32
0 1 2 3 4 5 6 7
1 TDMA Frame = 8 Timeslots
1 Timeslot = 156.25 Bit Duration
Tail Bits3
Encrypted Bits58
Training Sequence26
Encrypted Bits58
Tail Bits3
Guard Period8.25
Normal Burst (NB)
Tail Bits3
Fixed Bits142
Tail Bits3
Guard Period8.25
Frequency Correction Burst (FCB)
Tail Bits3
Encrypted Sync Bits39
Extended TrainingSequence 64
Encrypted Sync Bits39
Tail Bits3
Guard Period8.25
Synchronisation Burst (SB)
Tail Bits8
SynchronisationSequence 41
Encrypted Bits36
Tail Bits3
Guard Period68.25
Access Burst (AB)
33
Uplink Common Channel
RACH - Random Access Channel is used by MS to access a call.
RACH/F - Full rate, one timeslot every 8 BP. RACH/H - Half rate, using 23 timeslots in the 51 x 8
BP cycle. The frame is 8 BP (Burst Periods) of 4.615 ms duration.
34
Signalling Channels
CCCH - Common Control Channel ACCH - Associated CCH SACCH - Slow Associated CCH FACCH - Fast Associated CCH SDCCH - Stand-alone Dedicated CCH
35
Multiple-Access Scheme
A combination of FDMA and TDMA A total of 124 FDMA channels of 200 kHz bandwidth
each. The Uplink frequency band 935 - 960 MHz (25 MHz)
and Downlink 890 - 915 MHz (25 MHz). The duplex separation is 40 MHz. TDMA employs 8 timeslots forming a frame of 4.16
ms, 0.577 ms per timeslot.
36
TDMA/FDMA
F1 F’1F0
Cell Rx Cell Tx
F2 F’2
45 MHz
Amplitude
Frequency
37
GSM FDMA/TDMA
ch #1 ch #2 ch #3 ch #4 ch #5 ch #6 ch #8ch #7
ch #1 ch #2 ch #3 ch #4 ch #5 ch #6 ch #8ch #7
ch #1 ch #2 ch #3 ch #4 ch #5 ch #6 ch #8ch #7
Frequency 1
Frequency 2
Frequency 124
Fre
quen
cy D
omai
n
Time Domain
TS0 TS3 TS4 TS5 TS6 TS7TS1 TS2
38
Frequency Hopping
GSM employs a slow frequency hopping rate defined in bits per hop.
The regular rate is 217 hops/s, with 270 kbps transmission rate, the result is 1200 bits/hop.
It is worth mentioning that FH improves the propagation performance when considering the effect of frequency selective fading.
39
Radio Resource Management
Radio channels (resources) are allocated for call setup, handover, and release on a call basis.
Management involves three functions: location, handover, and roaming.
Three link protocols are used for RR functions:– Radio Link Protocol (LAPDm)
– Link Access protocol (LAPD)
– Message Transfer Protocol (MTP)
40
Mobile-Originating Call
Mobile StationMobile Station Base StationBase Station
Random Access
Immediate Assignment
Exchange of Call Setup Information
Traffic Channel Assignment
Data Flow
41
Mobile-Assisted Handoff (MAHO)
MAHO algorithm is carried out within the MS. MS scans for another RF carrier under direction from a
BS. BS requests MS to measure the signal strength of a
specific RF carrier. Upon request, MS forwards the result to BS. BS initiates handover on the basis of:
– Signal strength at MS from the candidate BTS– Signal strength at the candidate BTS from MS
42
Mobility Management
Location updates, handovers, and roaming Cell selection Authentication:
– PIN checked locally by SIM– GSM network RAND and SRES
Encryption User identity protection - Security management, SIM
(MS side) and AUC (network side).
43
Cell Selection
C1 = A - max(B, 0)C1 = A - max(B, 0)A = received level average - p1A = received level average - p1B = p2 - maximum RF power of the MSB = p2 - maximum RF power of the MSp1 = a value between -110 and -48 dBmp1 = a value between -110 and -48 dBmp2 = a value between 13 and 43 dBmp2 = a value between 13 and 43 dBmMS maximum power = 29 to 43 dBmMS maximum power = 29 to 43 dBm
MS chooses the best cell depending on:MS chooses the best cell depending on:1. The level of the signal received by MS,1. The level of the signal received by MS,2. The maximum transmission power of MS2. The maximum transmission power of MS3. Two parameters p1 and p2 specified by the cell3. Two parameters p1 and p2 specified by the cell
Both values of p1 and p2 are broadcast from the cell.Both values of p1 and p2 are broadcast from the cell.
44
Authentication
Authentication protects the network against unauthorised access.
Phase One - PIN is checked locally by the SIM. Phase Two - A 128-bit RAND is sent from the network
to MS.– RAND mixes with MS’ secret parameter Ki in A3 process
algorithm producing a 32-bit SRES.
– SRES (signed result) is sent from MS to network for verification.
45
Authentication Computation
Ki RAND
A3
SRES
Ki RAND
A3
SRES
RAND
Equal?
MS Network
46
User Identity Protection
SIM (MS side) and AUC (network side) are the repositories of the subscriber’s key Ki.
The key Ki never transmits over the air. Both sides perform A3 and A8 computations.
47
Communication Management
CM layer provides telecommunication services (speech, fax, and data) via RR and MM layers.
The management functions of CM layer are:– Call Control
– Service Management
– Short Message Service
48
0 1 11 241312SACCH 0 1 2 49 50
0 1 2 7
TB3 58 Encrypted bits 56 bits Training Seq. 58 Encrypted bits
TB3
GP8.25
0 1 2 49 50
24 250 1
0 1 2 3 2046 2047
1 hyperframe = 2048 superframes=2,715,648 TDMA frames (3 hours, 28 minutes, ..)
1 superframe = 1326 TDMA frames (6.12 s)
1 timeslot = (156.25 bits duration ~ 0.577 ms
1 TDMA frame = 8 timeslots (4.615 ms)
1 multiframe = 26 TDMA frames (120 ms) 1 multiframe = 51 TDMA frames (235 ms)
e.g. TCH/FS
e.g. BCCH
Idle/SACCH
49
Objectives of GSM PLMN
To provide the subscriber with a wide range of services and facilities, both voice and nonvoice, that are compatible with those offered by existing networks (e.g. PSTN, ISDN)
To introduce a mobile RS that is compatible with ISDN To provide certain services and facilities exclusive to
mobile situations To give compatibility of access to the GSM network for
a mobile subscriber in a country that operates the GSM system
50
Objectives of GSM PLMN - cont.
To provide facilities for automatic roaming, locating, and updating of mobile subscribers
To provide services to a wide range of mobile stations, including vehicle mounted stations, portable stations, and handheld stations
To provide for the efficient use of the frequency spectrum
To allow for low-cost infrastructure, terminal, and service cost
51
GSM PLMN Services
A telecommunication service supported by the GSM PLMN is defined as a group of communication capabilities that the service provider offers to the subscribers
GSM PLMN offers 3 basic telecommunication services:– bearer services
– teleservices, and
– supplementary services
Please refer to Reference [1] for a list of services
52
Bearer Services Support
Data services x x
Alternate speech/data x x
Speech followed by data x x
Clear 3.1 kHz audio x x
Unrestricted digital information (UDI) x x
Packet Assembler/Disassembler (PAD) x
3.1 kHz external to PLMN x
Others x
Service GSM ISDN
53
Teleservices Support
Circuit speech (telephony) x x
Emergency call x x
Short message point-to-point x x
Short message cell broadcast x x
Alternate speech/facsimile group 3 x x
Automatic facsimile group 3 x
Voice-band modem (3.1 kHz audio) x x
Messaging teleservices x
Service GSM ISDN
Paging teleservices x
Others x
54
Access methodAccess method TDMA/FDMATDMA/FDMA
Frequency band (MS to BS)Frequency band (MS to BS) 890 - 915 MHz890 - 915 MHz
Frequency band (BS to MS)Frequency band (BS to MS) 935 - 960 MHz935 - 960 MHz
Channel bandwidthChannel bandwidth 200 kHz200 kHz
ModulationModulation GMSKGMSK
Bit rateBit rate 270.833 kbps270.833 kbps
FilterFilter BT = 0.3 (Gaussian)BT = 0.3 (Gaussian)
Voice channel codingVoice channel coding RPE-LPC Convolutional 13 kbpsRPE-LPC Convolutional 13 kbps
GSM Specification
Frequency HoppingFrequency Hopping Slow hopping (217 hops/s)Slow hopping (217 hops/s)
55
Cont.
Handoff methodHandoff method MAHOMAHO
Adaptive equalisationAdaptive equalisation yes (up to 16 yes (up to 16 s time dispersion)s time dispersion)
Users per channelUsers per channel 88
MS power levelMS power level 0.8, 2, 5, 8, 20 Watts0.8, 2, 5, 8, 20 Watts
Number of channelsNumber of channels 124124
Frame IntervalFrame Interval 8 timeslots = 4.615 ms8 timeslots = 4.615 ms
InterleavingInterleaving 40 ms40 ms
Associated control channelAssociated control channel Extra frameExtra frame
TimeslotTimeslot 0.577 ms0.577 ms
56
MSMS Mobile Station Mobile StationTETE Terminal Equipment Terminal EquipmentMTMT Mobile Terminal Mobile TerminalBSBS Base Station Base StationBTSBTS Base Transceiver Station Base Transceiver StationBSCBSC Base Station Controller Base Station ControllerMSCMSC Mobile Switching Centre Mobile Switching CentreHLRHLR Home Location Register Home Location RegisterVLRVLR Visitor Location Register Visitor Location RegisterNMCNMC Network Management Centre Network Management CentreOMCOMC Operation and maintenance Centre Operation and maintenance CentreADCADC Administration Centre Administration CentreAUCAUC Authentication Centre Authentication CentreEIREIR Equipment Identifier Register Equipment Identifier Register
57
MAHO Mobile Assisted HandoffMAHO Mobile Assisted HandoffGMSCGMSC Gateway MSC Gateway MSCRPERPE Regular Pulse Excitation Regular Pulse ExcitationLPCLPC Linear Prediction Coding Linear Prediction CodingBPBP Burst Period Burst PeriodRSRS Radio Service Radio Service
58
References
[1] Garg, Vijay K and Wilkes Joseph E, “Wireless and Personal Communication Systems”. Prentice Hall PTR
[2] Parsons J D and Gardiner J G, Mobile Communication Systems, Blackie USA Halsted Press
[3] Lee, William C. Y., Mobile Cellular Telecommunications Systems, McGraw-Hill, Inc.
[4] Gibson, D. Jerry, The Communications Handbook, 1997 CRC Press, Inc.
[5] GSM World from GSM Association: www.gsmworld.com