Post on 28-Jan-2015
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1
Global System for Mobiles
GSM
2
TOPICS
• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
3
TOPICS
• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
4
Telecom Basics
• Communication– Voice and Data– Analog and Digital– Circuit Switched and Packet Switched– Media - Copper Wire, Co-axial cable, Air, Optical
Fibre– Networks -PSTN, ISDN, PDN and Mobile
Networks
5
Background to GSM
• 1G : Advanced Mobile Phone Service (AMPS)• Analog, Circuit Switched, FDMA, FDD
• 2G : Digital Advanced Mobile Phone Service (D-AMPS)• Digital, Circuit Switched, FDMA, FDD
• 2G : Global System for Mobile (GSM)• Digital, Circuit Switched, FDMA and TDMA, FDD
• 2G : Code Division Multiple Access (CDMA)• Digital, Circuit Switched, CDMA, FDD
6
GSM History
7
1982: Groupe Spécial Mobile (GSM) created
1984: Description of GSM features
1985: List of recommendations settled
1987: Initial MoU (Memorandum of Understanding) aside the drafting of technical specifications was signed by network operators of 13 countries:
1988: Validation and trials, of the radio interface.
1991: First system trials are demonstrated at the Telecom 91 exhibition.
1992: Official commercial launch of GSM service in Europe. First Launch in Finland
1993: The GSM-MoU has 62 signatories in 39 countries worldwide.
1995: Specifications of GSM phase 2 are frozen.
1999: GSM MoU joins 3GPP (UMTS)GPRS Trials begins
2000: 480M GSM Network operators WorldwideFirst GPRS Networks roll out
End 2002: 792M GSM Net work Operators Worldwide
Development of the GSM Standard
ver2.2
12 SERIESOPERATION ANDMAINTENANCE
01 SERIESGENERAL 02 SERIES
SERVICE ASPECTS
03 SERIESNETWORK ASPECTS
04 SERIESMS-BSS INTERFACE AND
PROTOCOLS
05 SERIESPHYSICAL LAYER ON THE
RADIO PATH.
06 SERIESSPEECH CODINGSPECIFICATIONS
07 SERIESTERMINAL ADAPTERS
FOR MOBILE STATIONS
11 SERIESEQUIPMENT AND TYPE
APPROVAL SPECIFICATIONS
10 SERIESSERVICE INTERWORKING
09 SERIESNETWORK
INTERWORKING
08 SERIESBSS TO MSC INTERFACES
GSM Specifications
ver2.2
Increasing GSM Data Rates
Transmission TimeGPRS = General Packet Radio ServiceHSCSD = High Speed Circuit Switched DataEDGE = Enhanced Data rate for GSM EvolutionUMTS = Universal Mobile Telecommunication System
10 sec 1 min 10 min 1 hour0
UMTS
E/GPRS
ISDN
PSTN
GSM
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web photoe-mail
web photo
video clipreportphoto
web photoe-mail
video clipreport
video clipreport
video clipreport
video clipreport
10
thro
ug
hp
ut
kbp
s
10 k
100 k64 k
1 M
2 M
1 k1998 1999 2000 2001 2002
Time frame
UMTSUMTS
GPRSGPRS
HSCSDHSCSD
9.69.614.414.4
packet
GPRS = General Packet Radio ServiceHSCSD = High Speed Circuit Switched DataEDGE = Enhanced Data rate for GSM EvolutionUMTS = Universal Mobile Telecommunication System
EDGEcircuit
Wireless Data Technology Options
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Circuit mode
Packet mode
A→FD→H
C→G
C→G
C→G
C→G
C→G
D→H
D→H
A→FD→H
D→H
A→F
A→F
A→F
A
B
C
D
E
F
G
H
Circuit-Switched or Packet-Switched
13
What are the types in GSM Network?
• GSM-900 (Channels 125 operating band 900Mhz carrier spacing 200khz spacing 45Mhz)
• GSM -1800 (Channels 374 spacing 95Mhz)
• GSM -1900(Used in USA)
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Multiple Access Technique• Multiple Access – Achieved by dividing the available radio
frequency spectrum, so that multiple users can be given access at the same time.
• FDMA - Frequency Division Multiple Access– ( eg: GSM each Frequency channel is 200KHz)
• TDMA - Time Division Multiple Access– ( eg: GSM each frequency channel is divided into 8
timeslots)• CDMA - Code Division Multiple Access
– (eg: IS95- Each User data is coded with a unique code)
15
Duplex Technique
• Duplex - How the up link and Down link of a user is separated
• FDD - Frequency Division Duplex– (eg:In GSM the up link and down link of a user is
separated by 45MHz )
• TDD - Time Division Duplex – (the up link and down link of a user will be at the same
frequency but at different Time )
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Frequency band Uplink 890 - 915 MHz Downlink 935 - 960MHzDuplex Frequency Spacing 45MHzCarrier separation 200KHzFrequency Channels 124Time Slots /Frame(Full Rate) 8Voice Coder Bit Rate 13KbpsModulation GMSKAir transmission rate 270.833333 KbpsAccess method FDMA/TDMASpeech Coder RPE-LTP-LPC
GSM System specifications
17
Uplink 890 MHz to 915 MHzDown Link 935 MHz to 960 MHz25 MHz divided into 125 channels of 200 KHz bandwidth
890.0 890.2 890.4 914.8 915.0
935.0 935.2 935.4 959.8 960.0
UP
DOWN
Access Techniques
18
Time Division Multiple AccessEach carrier frequency subdivided in time domain into 8 time slots Each mobile transmits data in a frequency, in its particular time slot - Burst period = 0.577 milli secs.8 time slots called a TDMA frame. Period is .577 * 8 = 4.616 milli secs
0 1 2 3 4 5 6 7
4.616 ms
0.577 ms
Access Techniques ...
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GSM in comparison with other Standards• GSM gives mobility without any loss in Audio quality
• Encryption techniques used gives high security in the air Interface and also use of SIM.
• Bit Interleaving for high efficiency in Transmission.
• Variable Power (Power budgeting- extend battery life)
• Minimum Interference.
• Features-CCS7 Signaling
– SMS (Short Message Services)
– Emergency Calls
– CELL Broadcast
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TOPICS
• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
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AuC
MS
MS
BTS
BTS
BTS
BSC
BSC
MSC
MSC
VLR
VLR
GMSC
HLR
PSTN
EIR
Um
Abis
Abis
A
A
OMC Server
Um
GSM - Network Structure
B
E
E
X.25
C
F
H
X.25
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GSM Network
OMC
AUC
HLR
MSC
EIRVLR
BSC
BTS
MS
ExternalPSTN &PDN N/W
SS
BSS
SwitchingSystem
Base StationSystem
MS Mobile Station
BTS Base transceiver System
BSC Base Station Controller
MSC Mobile Switching Center
HLR Home Location Register
VLR Visitor Location Register
EIR Equipment Identity Register
AUC Authentication Center
OMC Operation And Maintenance Center
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GSM Architecture
HLR
VLR
EIR
AUC
MSC
BSC
BSC
SMSC
PSTN
VMSC
MobileStation
GSM
Air interface
OMCR
TRAU
Base Station System
Network and switching subsystem
A interface SS7 / speech
SS7
X.25
BTSBTS
BTS
BTS
BTSBTS
Abis interface
A interface
OMCS
25
GSM utilizes two bands of 25 MHz. 890-915 MHz band is used for uplink while the 935-960 MHz is used for downlink.
The frequency bands are divided into 200 KHz wide channels called ARFCNs (Absolute Radio Frequency Channel Numbers) i.e. there are 125 ARFCNs out of which only 124 are used.
Each ARFCN supports 8 users with each user transmitting / receiving on a particular time slot (TS).
Fundamentals
124
123
…….
……
2
1
124
123
…….
……
2
1
0
1
2
3 4
5
6
7
0
1
2
3 4
5
6
7
Data burst = 156.25 bit periods = 576.9s
960 MHz
959.8MHz
200KHz
935 MHz
935.2 Mhz
915 MHz
200KHz
45 MHz
Downlink (TDMA frame) = 8 TS
Uplink (TDMA frame)
Delay
TS: Time slot
914.8 MHz
890.2 MHz
890 MHz
DOWNLINK
UPLINK
Therefore Therefore 1 TDMA1 TDMA frame = 156.25 x 8 = 1250 bits frame = 156.25 x 8 = 1250 bits and has a duration of 576.92and has a duration of 576.92s x 8 = 4.615 mss x 8 = 4.615 msThe technology
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Mobile Station (MS)
• Hand portable unit
• Contains Mobile Equipment(ME) and Subscriber Identity Module (SIM)
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Mobile Equipment(ME)
• Frequency and Time Synchronization• Voice encoding and transmission• Voice encryption/decryption functions• Power measurements of adjacent cells• Display of short messages• International Mobile Equipment Identifier (IMEI)
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SIM• Portable Smart Card with memory (ROM-6KB to 16KB-A3/A8
algorithm, RAM- 128KB TO 256KB, EEPROM- 3KB to 8KB )• Static Information
– International Mobile Subscriber Identity(IMSI)– Personal Identification Number (PIN)– Authentication Key (Ki)
• Dynamic Information– Temporary Mobile Subscriber Identity(TMSI)– Location Area Identity (LAI)– Phone memories, billing information– Ability to store Short Messages received
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Base Transceiver Station (BTS)• Handles the radio interface to the mobile station.• Consists of one or more radio terminals for
transmission and reception• Each Radio terminal represents an RF Channel• TRX and MS communicates over Um interface• Received data transcoding• Voice encryption/decryption• Signal processing functions of the radio interface• Uplink Radio channel power measurements
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Base Station Controller (BSC)• Provides all the control functions and physical links between
the MSC and BTS • External Interfaces
– ‘Abis’ interface towards the BTS– ‘A’ interface towards the MSC
• Monitors and controls several BTSs• Management of channels on the radio interface• Alarm Handling from the external interfaces• Performs inter-cell Handover• Switching from ‘Abis’ link to the ‘A’ link• Interface to OMC for BSS Management
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Mobile Switching Center (MSC)
• Performs call switching • Interface of the cellular network to PSTN• Routes calls between PLMN and PSTN• Queries HLR when calls come from PSTN to mobile
user• Inter-BSC Handover• Paging• Billing
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Home Location Register (HLR)• Stores user data of all Subscribers related to the
GMSC– International Mobile Subscriber Identity(IMSI)– Users telephone number (MS ISDN)– Subscription information and services– VLR address– Reference to Authentication center for key (Ki)
• Referred when call comes from public land network
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Visitor Location Register (VLR)
• Database that contains Subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
• Identity of Mobile Subscriber• Copy of subscriber data from HLR• Generates and allocates a Temporary Mobile
Subscriber Identity(TMSI)• Location Area Code• Provides necessary data when mobile
originates call
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Authentication Center (AuC)
• Stores Subscriber authentication data called Ki, a copy of which is also stored in in the SIM card
• Generates security related parameters to authorize a subscriber (SRES-Signed RESponse)
• Generates unique data pattern called Cipher key (Kc) for user data encryption
• Provides triplets - RAND, SRES & Kc, to the HLR on request.
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EIR (Equipment Identity Register)
• EIR is a database that contains a list of all valid mobile station equipment within the network,
where each mobile station is identified by its International Mobile Equipment Identity(IMEI).
• EIR has three databases.,– White list - For all known,good IMEI’s– Black list - For all bad or stolen handsets– Grey list - For handsets/IMEI’s that are
on observation
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GSM Entities and Signaling Architecture
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GSM Protocols• CM - Connection Management• MM - Mobility Management• RR - Radio resource • LAPDm - LAPD for mobile• LAPD - Link Access Procedure for D channel• BTSM - BTS Management Part• BSSAP - BSS Application Part (BSC - MSC)• DTAP - Direct Transfer Application Part (MS - MSC)• MAP - Mobile Application Part• MTP - Message Transfer part of SS7• SCCP - Signalling Connection Control Part of SS7• TCAP - Transaction Capabilities Application Part• ISUP - ISDN User Part
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Functional Plane of GSM
MS BTS BSC MSC/ HLR GMSC VLR
MS BTS BSC MSC/VLR HLR GMSC
CC
MM
RR
Trans
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TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTIFIERS USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
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Subscriber Identity -MSISDN
• The MSISDN is a GSM directory number which uniquely identifies a mobile subscription in the Public Switched Telephone Network (PSTN).
• Calls will be routed from the PSTN and other networks based on the Mobile Subscribers’ MSISDN number.
• MSISDN= CC + NDC + SN– CC= Country Code (91)– NDC= National Destination Code(98370)– SN= Subscriber Number (12345)
42
International Mobile Subscriber Identity [IMSI]• Subscriber always identified within the GSM network
by the IMSI • This is used for all signaling in the PLMN stored in
SIM and HLR/VLR• The IMSI consists of three different parts
– MCC = Mobile Country Code(3 Digits)– MNC = Mobile Network Code(2 Digits)– MSIN = Mobile Station Identification Number(Upto 10 digits)
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Temporary Mobile Subscriber Identity [TMSI]• The TMSI is used for the subscriber’s confidentiality.• It should be combined with the LAI to uniquely
identify a MS.• Since the TMSI has only local significance (that is,
within the MSC/VLR area), the structure may be chosen by each administration.
• The TMSI should not consist of more than four octets.
44
Mobile Station Roaming Number[MSRN]• HLR knows in what Service area the subscriber is
located. • In order to provide a temporary number to be used
for routing, the HLR requests the current MSC/VLR to allocate a Mobile Station Roaming Number(MSRN) to the called subscriber and to return it.
• At reception of the MSRN, HLR sends it to the MSC, which now can route the call to the VLR where the called subscriber is currently registered.
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International Mobile Equipment Identity [IMEI]• The IMEI is used for equipment identification. An IMEI • uniquely identifies a mobile station as a piece or
assembly of equipment.• IMEI = TAC + FAC + SNR + sp
– TAC= Type Approval Code (6 digits),determined by GSM body– FAC= Final Assembly Code (2 digits), identifies
themanufacturer– SNR= Serial Number (6 digits), uniquely identifying all
equipment within each TAC and FAC– sp = Spare for future use (1 digit)
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Location Area Identity• LAI identifies a location area which is a group of cells..• It is transmitted in the BCCH. • When the MS moves into another LA (detected by
monitoring LAI transmitted on the BCCH) it must perform a LU.
• LAI = MCC + MNC + LAC – MCC= Mobile Country Code(3 digits), identifies the country– MNC= Mobile Network Code(1-2 digits), identifies the GSM-
PLMN– LAC= Location Area Code, identifies a location area within a GSM
PLMN network. The maximum length of LAC is 16 bits,enabling 65536 different location areas to be defined in one GSM PLMN.
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TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
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Channels : differentiating between Physical and Logical channels
Physical channels : The combination of an ARFCN and a time slot defines a physical channel.
Logical channels : These are channels specified by GSM which are mapped on physical channels.
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Physical channel:One timeslot of a TDMA-frame on one carrieris referred to as a physical channel.There are 8 physical channels per carrier inGSM,channel 0-7(timeslot 0-7)
Logical channel:A great variety of information must be transmitted between BTS and the MS,for e.g.user data and control signaling.Dependingon the kind of information transmitted we refer to different logical channels.These logicalchannels are mapped on physical channel.
Channel concept
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Logical Channels on Air interface
LOGICALCHANNELS
COMMONCHANNELS
DEDICATED CHANNELS
BROADCAST CHANNELS
COMMONCONTROLCHANNELS
DEDICATED CONTROL CHANNELS
TRAFFIC CHANNELS
FCCH BCCHSCH SDCCH SACCH FACCH
PCH AGCHRACH TCH/F TCH/EFRTCH/H
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Frequency plan and importance of BCCH
B3
B2
B9
B6 B4
B1
Sectored antennas
MS ( monitoring the broadcast radio B1 in ‘idle
mode’ )
F S B B B B …..
F S …..
…..
I
F0 F50F2 F3 F4 F5 F10 F11F1
•F,S,B exist in time slot 0 of each frame
B7
B8
B5
B10
B11
B12
BPL frequency plan:
Broadcast frequencies :
15 Broadcast channels = 48-62
15 Hopping channels = 32-46
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Broadcast channels BCH
• Broadcast Channel-BCH – Alloted one ARFCN & is ON all the time in every cell. Present in TS0 and
other 7 TS used by TCH.
• Frequency correction channel-FCCH – To make sure this is the BCCH carrier.– Allow the MS to synchronize to the frequency.– Carries a 142 bit zero sequence and repeats once in every 10 frames on
the BCH.
• Synchronization Channel-SCH– This is used by the MS to synchronize to the TDMA frame structure within
the particular cell.– Listening to the SCH the MS receives the TDMA frame number and also
the BSIC ( in the coded part- 39 bits).– Repeats once in every 10 frames.
53Back
1. The MS is monitoring the BCCH and has all the decoded information stored on the SIM ( including the LAC)
2. As soon as the mobile is on a TCH it sends the signal strength indication on the corresponding SACCH
3. The BSC monitors the signal strengths and on analysis sends a ‘handoff request’ on FACCH. The handoff process is completed on the FACCH.
4. After the completion of call, the MS starts monitoring the BCCH again. On finding the LAC (stored on SIM) and that decoded from the BCCH to be different , the MS requests a ‘Location Update’ through SDCCH.
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Broadcast channels BCH ...• BCH
– The last information the MS must receive in order to receive calls or make calls is some information concerning the cell. This is BCCH.
– This include the information of Max power allowed in the cell.– List of channels in use in the cell.– BCCH carriers for the neighboring cells,Location Area Identity etc.– BCCH occupies 4 frames (normal bursts) on BCH and repeats
once every Multiframe.– This is transmitted Downlink point to multipoint.
• Cell Broadcast Channel - CBCH– Used for the Transmission of generally accessible information like
Short Message Services(SMS)
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What information does Broadcast Control channel
(BCCH) contain?
Serves as a Beacon for the Cell
Country Code (CC) and the Network Code (NC)
Location Area Identity (LAI)
List of neighboring cells which should be monitored by MS
List of frequencies used in the cell
Cell identity
Back
56
Common Control Channels CCCH• CCCH-
– Shares TS-0 with BCH on a Multiframe.
• Random access channel-RACH:– Used by Mobile Station for requesting for a channel. When the
mobile realizes it is paged it answers by requesting a signaling channel (SDCCH) on RACH. RACH is also used by the MS if it wants to originate a call.
– Initially MS doesn’t know the path delay (timing advance), hence uses a short burst (with a large guard period = 68.25 bits).
– MS sends normal burst only after getting the timing advance info on the SACCH.
– It is transmitted in Uplink point to point.
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Common Control Channels CCCH ..• Access Grant Channel-AGCH
– On request for a signaling channel by MS the network assigns a signaling channel(SDCCH) through AGCH. AGCH is transmitted on the downlink point to point.
• Paging Channel-PCH– The information on this channel is a paging message
including the MS’s identity(IMSI/TMSI).This is transmitted on Downlink, point-to-multipoint.
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Dedicated Control Channels-DCCH• Stand alone dedicated control channel(SDCCH)• AGCH assigns SDCCH as signaling channel on
request by MS.The MS is informed about which frequency(ARFCN) & timeslot to use for traffic.
• Used for location update, subscriber authentication, ciphering information, equipment validation and assignment of TCH.
• This is used both sides, up and Downlink point-point.
59
Dedicated Control Channels-DCCH• Slow associated control channel-SACCH
– Transmission of radio link signal measurement, power control etc.
– Average signal strengths(RXLev) and quality of service (RXQual) of the serving base station and of the neighboring cells is sent on SACCH (on uplink).
– Mobile receives information like what TX power it has to transmit and the timing advance. It is associated with TCH or SDCCH
• Fast associated control channel-FACCH– Used for Hand over commands and during call setup and
release. FACCH data is sent over TCH with stealing flag set
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Traffic Channels-TCH
• TCH carries the voice data.• Two blocks of 57 bits contain voice data in the normal
burst.• One TCH is allocated for every active call.• Full rate traffic channel occupies one physical
channel(one TS on a carrier) and carries voice data at 13kbps
• Two half rate (6.5kbps) TCHs can share one physical channel.
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TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
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GSM Radio Link
• Speech Coding -Done at Transcoder of BSC and MS– The Linear Predictive Coder uses RPE-LTP(Regular Pulse
Excitation- Long Term Prediction)– Converts 64kbps voice to 13kbps(260 bits every 20ms)
• Channel Coding - Done at BTS and MS– Uses Convolution Coding and CRC (Cyclic Redundancy
Check)– Converts 13 kbps to 22.8 kbps (456 bits per 20ms)
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GSM Radio Link
• Bit Interleaving - Done at BTS and MS• Encryption - Done at BTS and MS
– EX OR data with cipher block, which is generated by applying A5 Algorithm to the Ciphering Key(Kc)
• Multiplexing - Done at BTS • Modulation - Done at BTS and MS
– GMSK(Gaussian filtered Minimum Shift Keying)– Phase change of +90 for 0 and -90 for 1
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Speech Coding
BP A/D SPEECHENCODER
CHANNEL CODING
LP D/A SPEECHDECODER
CHANNEL DECODING
BAND PASS
300 Hz - 3.4 kHZ
Every 125 s value issampled from analog signal and quantised by 13 bit wordData rate = 13/125*10 -6
= 104 kbps
Every 20ms 160 samples takenData rate = 160 * 13/20ms = 104 kbps
Linear Predictive Coding & Regular Pulse Excitation Analysis
1. Generates 160 filter coeff2. These blocks sorted in 4 sequence 1,5,9,…37 / 2,6,10----38/ 3,7,11…39/8,12,16…403. Selects the sequence with most energy
So data rate = 104/4 = 26 kbps
Long term prediction analysis1. Previous sequences stored in memory2. Find out the correlation between the present seq. And previous sequences3. Select the highest correlation sequence4. Find a value representing the differencebetween the two sequences.
Reduces data rate = 26 kbps/2 = 13 kbpsie 260 bits in 20ms
50 132 78
1A 1B 2
1A = Filter Coeff block ampl, LTP params1B = RPE pointers &pulses2 = RPE pulse & filterparams
50 3 132 4
3 crc bitsFour 0 bits for codec
378 coded bits
Conv coding rate = 1/2 delay = 4
78
456 bits in 20 ms = 22.8 kbps57 x 8 = 456
To modulator
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Interleaving
57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57
57 57 57 57
57 57 57 57
57 57 57 57
57 57 57 5757 57 57 57
57 57 57 57 57 57 57 57
57 57 57 57
Encoded speech blocks - Diagonal Interleaving
Even bitsOdd bits
Tb 3
Coded Data 57
F 1
Training Sequence26
F 1
Coded Data 57
Tb 3
Gp8.25
Bn-4 Bn-3 Bn-2 Bn-1 Bn Bn+1 Bn+2 Bn+3
57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57
57 57 57 57
57 57 57 57
57 57 57 57
57 57 57 5757 57 57 57
57 57 57 57
Encoded control channel blocks - Rectangular Interleaving
Even bitsOdd bits
Bn-4 Bn-3 Bn-2 Bn-1 Bn Bn+1 Bn+2 Bn+3
66
Burst• The information format transmitted during one
timeslot in the TDMA frame is called a burst.• Different Types of Bursts
– Normal Burst– Random Access Burst– Frequency Correction Burst– Synchronization Burst
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Normal Burst
T3
Coded Data57
S1
T. Seq.26
S1
Coded Data57
T3
GP8.25
Tail Bit(T) :Used as Guard TimeCoded Data :It is the Data part associated with the burstStealing Flag :This indicates whether the burst is carrying
Signaling data (FACCH) or user info (TCH).Training Seq. :This is a fixed bit sequence known both to
the BTS & the MS.This takes care of the signal deterioration.
156.25 bits 0.577 ms
68
T3
Training Sequence41
Coded Data36
T3
GP68.25
Random Access Burst
T3
Fixed Bit Sequence142
T3
GP8.25
T3
Coded Data39
Training Sequence64
CodedData 39
T3
GP8.25
Freq. Correc. Burst
Synchronization Burst
156.25 bits 0.577 ms
156.25 bits 0.577 ms
156.25 bits 0.577 ms
69
Transmission on the radio channels
• A timeslot has a duration of .577 m seconds (148 Bits)• 8 timeslots(8 x 0.577 = 4.62 ms) form a TDMA frame• If a mobile is assigned one TS it transmits only in this time slot• and stays idle for the other 7 with its transmitter off, called bursting• The start on the uplink is delayed from downlink by 3 TS periods• One TS = duration of 156.25 bits, and its physical contents is • called a burst
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
DownlinkBTS > MS
UplinkMS > BTS
Offset
70
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
MS1 near
MS2far
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
AtBTS
0 1 2 3 4 5 6 7MS1 near
0 1 2 3 4 5 6 7MS2far
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
AtBTS
Timing Advance
71
Frames Types On Um Interface• TDMA Frame
– 8 Time slots (Burst Period)– Length is 4.62 ms(8 * 0.577ms)
• 26-TDMA Multiframe– 26 TDMA Frames (24 TCH, SACCH, Idle)– 120 ms (26 * 4.62ms)
• 51-TDMA Multiframe– 26 TDMA Frames (FCCH, SCH, BCCH, SDCCH, CCCH)– 235.6 ms (51 * 4.62ms)
72
Frames Types On Um Interface• Super Frame
– 51* 26 TDMA Frames– 6.12 S
• Hyper Frame– 2048 * 51* 26 TDMA Frames– 3 Hours, 28 Minutes, 53 Secs and 760 ms
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TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
74
Mobility Management
• Mobility Management (MM)• Location updating- normal,periodic, IMSI attach• Paging• Security Management
– Preventing unauthorized users- authentication– Maintaining Privacy of users- ciphering
• Providing roaming facility• MM functionality mainly handled by MS, HLR,
MSC/VLR.
75
Network Attachment
Cell Identification MS scans complete GSM frequency band for
highest power Tunes to highest powered frequency and looks
for FCCH. Synchronizes in frequency domain Get training sequence from SCH which follows
FCCH. Synchronizes in time domain. Accesses BCCH for network id, location area and
frequencies of the neighboring cells. Stores a list of 30 BCCH channels
76
Network Attachment….. PLMN Selection
Get the operator information from SIM.
Cell Selection Selected cell should be a cell of the selected
PLMN Signal strength should be above the threshold. Cell should not be barred
Location Update Register with the network by means of location
updation procedures.
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MS BTS BSC (G)MSC VLR HLR
Action
Channel Request (RACH)
Channel Assignment (AGCH)
Authentication Request (SDCCH)
Authentication Response (SDCCH)
Comparison of Authentication params
Accept LUP and allocTMSI (SDCCH)
Ack of LUP and TMSI (SDCCH)
Entry of new area and identity into VLR and HLR
Channel Release (SDCCH)
MS Location Update (registration)
Location Update Request (SDCCH)TMSI + old LAI
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Security - Authentication
Authentication center provides RAND to MobileAuC generates SRES using Ki of subscriber and RANDMobile generates SRES using Ki and RANDMobile transmits SRES to BTSBTS compares received SRES with one generated by AuC
MSKi RAND
A3
SRES
RAND
SRES
SRES
Auth Result
AuCBTSMS
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Security - Ciphering
Data sent on air interface ciphered for security A5 and A8 algorithms used to cipher dataCiphering Key is never transmitted on air
MSKi RAND
A8
Kc
MS NetworkUm interface
A5 A5
Kc Kc
Data DataCiphered
Data
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TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
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Communication Management (CM)• Setup of calls between users on request• Routing function i.e. Choice of transmission
segments linking users• Point to Point Short message services
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Mobile Originated Call
• Request for Service• Authentication• Ciphering• Equipment Validation• Call Setup• Handovers• Call Release
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MS
MS
BTS
BTS
BTS
BSC
BSC
MSC
VLR
GMSC
HLR
PSTN
EIR
AuC
Req for dedicated channel for signaling (RACH)
Give SDCCH
Allocates SDCCH using the AGCH
Sends call set-up request including dialled digits on SDCCH
Call set-up forwarded to BSC
Call set-up forwarded to MSC
Authentication request(SDCCH)
Authentication response(SDCCH)
Authentication response(SDCCH)
Assigns TCH Req
Activate TCHTCH assigned
Assn complete
Release SDCCH
SDCCH released
ACM
Ring tone over FACCH
Ring alert
Called Sub answers
Connect message
Ring tone ceases over FACCHSpeech path enabled
Mobile Originated Call
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Mobile Terminated Call
• Paging• Authentication• Ciphering• Equipment Validation• Call Setup• Handovers• Call Release
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Mobile Terminated CallMS
MS
BTS
BTS
BTS
BSC
BSC
MSC
VLR
GMSC
HLR
PSTN
EIR
AuC
Land to Mobile call(MSISDN)
Query forVLR info
Reply(MSRN)
Routeto MSC
Query VLR for LAC andTMSIPaging
the area(+TMSI)
TMSI Pagedon PCH
Paging
Ch. REQover RACHAllocate SDCCHover AGCHPage RESP on SDCCH ( TMSI + LAI)
Page RES Page RES
Authentication and Ciphering procedure done as seen in Location Updation
Assign. REQ
Assignment CMD(=TCH) on SDCCH
* MS tunes* Assgn CMP* Phone rings
Connect traffic Ch.to trunkfrees SDCCHAssgn CMP
Network Alerting
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TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT
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Radio Resource Management
• Establish maintain and release stable connections between MS and MSC
• Manage Limited Radio and Terrestrial resources• Handover process is the sole responsibility of the RR
Layer• Functions of RR layer are performed by MS and BSC
and partly by MSC
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Radio Resource Management
• Power Control• Hand over Control• Discontinuous Transmission• Frequency Hopping
89
BTS commands MS at differentdistances to use different power levelsso that the power arriving at the BTS’s Rx isapproximately the same for each TS
- Reduce interference- Longer battery life
Power Control
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HandoverMeans to continue a call even a mobile crosses the border of one cell to anotherProcedure which made the mobile station really roamHandover causes
RxLev (Signal strength , uplink or downlink)RxQual (BER on data)O & M interventionTiming AdvanceTraffic or Load balancing
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Handover Types
• Internal Handover (Intra-BSS)– Within same base station - intra cell– Between different base stations - inter cell
• External Handover (Inter-BSS)– Within same MSC -intra MSC– Between different MSCs - inter-MSC
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Handover Types
BSC
BSC
BSC
BSC
MSC
MSC
GMSC
C-1 C-2
C-3
C-4
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Periodic Measurement Reports (SACCH)
Periodic Measurement Reports
HO requiredActivate TCH(facch)with HoRef#
if 1. Check for HO passed2. Channel avail in new BTS
Acknowledges and alloctes TCH (facch)
HO cmd with HoRef#Receives new BTS data(FACCH)
MS tunes into new frequency and TS and sends HO message to new BTS (facch)
Periodic Measurement Reports (SACCH)
HO performed
Release TCH
Cell 1
Cell 2
BSC
BTS 1
BTS 2
Intra BSC handover
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Discontinuous Transmission
• Discontinuous Transmission(DTX) allows the radio transmitter to be switched off most of the time during speech pauses.
• A Silence Indicator Block is transmitted at 500bps, which generates a comfort noise
• Down Link interference is decreased.• Up link battery is saved
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Frequency Hopping• Frequency Hopping permits the dynamic switching of
radio links from one carrier frequency to another.• Base Band Hopping
– At the BTS each the timeslot is shifted to another transceiver, which is transmitting at the hop frequency. User will be connected to different Transceivers depending on hop sequence.
• Synthesis Hopping– At the BTS transceiver changes the frequencies used.
The user will be connected to only one transceiver.• Decreases the probability of interference• Suppresses the effect of Rayleigh fading
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Wireless Data
98 99 2000 2001
GSMDATA
GSMDATA
HSCSDHSCSD
GPRSGPRS
EDGEEDGE
UMTSUMTS
SIMToolkit
SIMToolkit
WAPWAP
Data Application
Time
Circuit Switched technology
Packet Switched technology
Technology for Applications
SMS Data: 160 -numeric charactersUser Data Rate : 9.6kbpsOne time slot over the air interface
High Speed Circuit Switched DataUser Data Rate:14.5kbpsUse multiple timeslots (max=8), hence max rate = 115.2kbps.Needs a duplexor in MS for simultaneous Tx and Rx
Add-on to GSM network : PCU; Packet Segmentation/re-assembly and scheduling• Radio channel access control and management• Transmission error detection and retransmission.• Power controlSGSN: GPRS mobility• Encryption• Charging GGSN : Interface to the PDN, Internet
Max user data rate : 21.4 kbpsDynamic rate adaptation to suit the radio conditions at that time ( 9.05 kbps, 13.4 kbps, 15.6 kbps 21.4 kbps)
W@P Gateway W@P ServiceW@P
Fone
InternetMobile Network
Surf the Internet while on the move
W@P Gateway :• Adaptation of the information to the mobile• Compression of the data• Buffering of the information
Enhanced Data rate for GSM Evolution• EDGE is an enhancement of GPRS and CSD technologies.• Based on the current GSM technology - same TDMA frame structure, same bandwidth (200 kHz).• Uses 8-PSK modulation instead of GMSK.• Requires good propagation conditions. • Allows upto 48 kbps (EGPRS) and upto 28.8 kbps (ECSD) on every radio channel• EDGE helps GSM-Only operators to compete with UMTS.
Universal Mobile Telecommunication Standards • Innovative Service Architecture : VHE Concept - providing the us the same look and feel of its personalized services independent of network and terminal.• Global Convergence : Fixed/Mobile, Telecom/Datacom, public/private• Mobile Multimedia driven market.• Wideband bearers - 2GHz band ( 5 MHz per carrier), -max. 2Mbps
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References• Wireless and Personal Communication Systems.
Vijay.K.Garg and Wilkes • Overview of the GSM System and Protocol
Architecture, IEEE Comm. Magazine, Moe Rahnema.
• The GSM System for Mobile Communications- Michel Mouly & Marie-Bernadette Pautet
• Overview of the GSM Comm- John Scourias.
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Location Updates
Location Updates can be classified into two:
Periodic Location Updates:
This occurs as per the timer set by the network operator. If the MS does not perform this update the MSC marks the MS as ‘Detached’ on the VLR.
Location Update on a handover:
This occurs if during a handover the MS is moved into a new Location Area Code (LAC).