% GSM Basics Demonstration

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GSM (Global System for Mobile Communication)

GSM defines a standard for a Public Land Mobile Network (PLMN)

GSM is a globally accepted standard for digital cellular communication

Because GSM provides a common standard, cellular subscribers can use their telephones over the entire GSM service area which includes all the countries around the world where GSM system is used

Three bands of operation :GSM900GSM1800GSM1900 (used in USA)

BSNL operates in the GSM900 & 1800 band.

GSM900 has its up-link and down-link frequencies in the 900MHz range

Up-link frequency band is from 890MHz to 915MHz (25MHz)

Down-link frequency band is from 935MHz to 960MHz (25MHz)

The 25MHz up-link and down-link frequency band is divided into 125 RF Channels or radio carriers with inter-carrier spacing being 200KHz

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GSM900 Up-link & Down-Link Frequency Band

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

200KHz

BW = 25MHz

BW = 25MHz

125 Channels of 200KHz BW 125 Channels of 200KHz BW

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0 124 0 124

890MHz 915MHz 935MHz 960MHz

45MHz

ARFCN 0

ARFCN 1

ARFCN 124

ARFCN (Absolute Radio Frequency Channel Number)

Absolute Radio Frequency Channel Number (ARFCN) is a serial Number of the Up-link and Down-link Frequencies, 0 through 124

One ARFCN corresponds to an Up-link frequency and a Down-link frequency 45MHz apart

BSNL has been allotted 31 ARFCNs

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ARFCNFREQUENCY IN MHz

Uplink Downlink

63 902.6 947.6

64 902.8 947.8

66 903.2 948.2

67 903.4 948.4

68 903.6 948.6

69 903.8 948.8

70 904.0 949.0

71 904.2 949.2

73 904.6 949.6

74 904.8 949.8

75 905.0 950.0

76 905.2 950.2

77 905.4 950.4

78 905.6 950.6

80 906.0 951.0

81 906.2 951.2

82 906.4 951.4

87 907.4 952.4

88 907.6 952.6

111 912.2 957.2

112 912.4 957.4

113 912.6 957.6

115 913.0 958.0

116 913.2 958.2

117 913.4 958.4

118 913.6 958.6

119 913.8 958.8

120 914.0 959.0

122 914.4 959.4

123 914.6 959.6

124 914.8 959.8

ARFCNs allotted to BSNL

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The access method in GSM is both FDMA & TDMA FDMA (Frequency Division Multiple Access): achieved by dividing the

available radio spectrum into 125 sub-channels each of 200KHz bandwidth so that multiple users can access the system at the same time

TDMA (Time Division Multiple Access) : each carrier is divided into 8 Time Slots so that each Frequency Channel is shared by 8 users at a time on time sharing basis

f

tCarrier-0

Carrier-1

Carrier-124

TS0 TS1 TS7

One TDMA frame

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TS TS TS TS TS TS TS TS

0 1 2 3 4 5 6 7

Time4.615 ms

TDMA frame

Physical channel # 2 = recurrence of time-slot # 2

TDMA frame

0 9.23 ms

Time-slot

(frames repeat continuously)

TS TS TS TS TS TS TS TS

0 1 2 3 4 5 6 7

GSM Time Division Multiple AccessFrame and Physical ChannelsPhysical Channels are the Time Slots in TDMA Frame

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One Time Slot has a duration of 0.577mS (148 bits) 8 timeslots (8 x 0.577 = 4.615 ms) form a TDMA frame

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GSM NETWORK ARCHITECTURE

MSC VLR

HLR

AUC

EIR

BSC BTS MS

PLMN

PSTN

ISDN

MSC – Mobile Switching Centre

VLR – Visitor Location Register

HLR – Home Location Register

AUC – Authentication Centre

EIR – Equipment Identity Register

BSC – Base Station Controller

BTS – Base Trnsceiver Station

PLMN – Public Land Mobile Network (other GSM operators)

PSTN – Public Switched Telephony Network (Land Line N/W)

ISDN – Integrated Services Digital Network (Data Network, Fax, etc.)

IN – Intelligent N/W Server (Prepaid and Virtual Private N/W)

SMSC – Short Message Service Centre

SGSN – Service GPRS Support Node

GPRS – General Packet Radio Services

MS – Mobile Station

Billing SMSC IN

SGSN Cells

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GSM NETWORK ELEMENTS

MSC (Mobile Switching Centre)

The MSC is the primary node of the GSM cellular system

Performs Call Processing like call set up, switching of the call, call termination and charging

Provides interface of the GSM network to PSTN and other PLMNs Routes calls between the GSM N/W and PSTN and other PLMNs Interrogates HLR for MSRN in order to route a call to a Mobile Station Inter-BSC Handover Paging of the MS for an Incoming Call, SMS Generates Billing data and routes Call Detail records (CDRs) to the Billing server Monitors Traffic and Load on the System and different Routes Generates Reports on Network performance, etc.

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HLR (Home Location Register)

HLR is a permanent database of the Subscriber Services

Stores user data of all Subscribers related to the GMSC International Mobile Subscriber Identity (IMSI) Users telephone number (MSISDN) Subscription information and services (like STD, ISD, Call

Forwarding, Call Waiting, SMMO, SMMT, Roaming Subscription Information, etc.)

Current VLR address and LAI (current location of the MS) Referred to for an Incoming Call for MSRN (Mobile Subscribers

Roaming Number) A subscriber’s data are deleted from the HLR database only when his

subscription is ended

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VLR (Visitor Location Register)

VLR is a temporary database of the subscribers currently present in the MSC/VLR service area

When a subscriber enters the MSC/VLR area a copy of his Subscription details is copied to the VLR database from his home HLR

Contains Subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Identity of Mobile Subscriber (MSISDN, IMSI, etc) Copy of subscriber data from HLR (so that each time the subscriber establishes a

call or has an incoming call, or for SMS, the HLR need not be queried which would increase the Call Processing Time and Signaling Load)

Generates and allocates a Temporary Mobile Subscriber Identity(TMSI) Contains Location Area Identity (LAI) – a group of cells Updates the LAI when a subscriber changes location Contains the IMEI of the MS The subscribers database form the VLR is deleted when he moves to another

MSC/VLR area

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AUC (Authentication Centre)

Authenticates the subscriber’s IMSI to receive service in the MSC/VLR area (own subscribers or from MSCs with whom we have Roaming agreement, like BSNL allover India, MTNL Delhi and Mumbai. And not subscribers from Private operators with whom we don’t have Roaming agreement)

At subscription time, each subscriber (SIM) is assigned an Authentication Key (Ki). Ki is stored in the AUC along with the subscriber’s IMSI. The same Ki and IMSI are also stored in the SIM. In an AUC the following steps are carried out :

A non-predictable random number, RAND, is generated RAND and Ki are used to calculate SRES (Signed Response) and Cipher

Key (Kc), using two different algorithms, A3 and A8 respectively RAND, SRES and Kc are delivered together to the HLR as a triplet

The MSC/VLR transmits the RAND to the MS The MS computes the Kc and SRES using RAND and the authentication key

(Ki) The SRES is sent back to MSC/VLR, which performs authentication, by

checking whether the SRES from the MS and the SRES from the AUC match. If so, the subscriber is permitted to use the network. If not, the subscriber is barred from network access.

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EIR (Equipment Identity Register)EIR The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.

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 IMEIs Black list - For all bad or stolen handsets Grey list - For handsets/IMEI’s that are faulty or non-approved mobile

equipment

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BSC (Base Station Controller)The BSC is a high capacity switch that manages all the radio-related functions of a GSM network. It also provides physical links between the MSC and BTS.

Monitors and controls several BTSs Manages channel allocation on the radio interface during a call

process Alarm Handling from the external interfaces and BTSs Performs inter-cell Call Handover Interface to OMC (Operation & Maintenance Centre) for BSS (Base

Station system) Management It contains the Cell configuration data (like Cell Identity,

Frequencies/BCCH list, Handover parameters, Neighbour cells, BTS power data, etc.)

The BTS collects data on signal strength and quality of the neighbouring cells. The BSC uses these data to allocate a channel during call setup or call handover

Several BSCs can be controlled by an MSC

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BTS (Base Transceiver Station)The BTS is the radio equipment (transceivers and antennas) needed to serve each cell in the network. A group of BTSs are controlled by a BSC.

The BTS's main function is to provide connection with the MSs over the air interface.

Consists of one or more radio terminals (called TRU – Transceiver Unit) for transmission and reception

Each Radio terminal/TRU handles an RF Channel/ARFCN Receiption of channel requests from MSs (during a call setup,

handover, etc) The BTS is responsible for the processing of signals before

transmission and after reception. This includes: Ciphering using the ciphering key (Kc) Modulation and Demodulation (GSM uses GMSK-Gaussian

Minimum Shift Keying), etc.

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The Cell

The cell is basic unit of the Cellular System. It is the Geographical area covered a BTS.

Each cell is assigned a unique number called Cell Global Identity(CGI). 404-77-80-10001

Mobile Country Code

(404 for India)

Mobile Network Code

(77 for BSNL NE)

BSC Identity Cell Identity

CGI

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Why Cellular structure

The Need of the Cell

Cellular structure increases capacity : In GSM Eight Subscribers can share an ARFCN using TDMA. And we have only 31 ARFCNs. So if we had only one BTS covering the whole service area, we would end up with only 31 x 8 = 248 Subscribers talking simultaneously. So the whole Service area is divided into many Cells covering only a small area. And cells having similar frequencies are placed a little distance away so that their frequencies don’t interfere.

Optimum Spectrum Usage : with only limited number of ARFCNs avilable the option is to re-use the frequencies a little distance away. This way we can have hundreds of cells wisely placed so that their frequencies don’t interfere (using only 31 frequencies)

Less transmission power needed

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The capacity of the Cell is limited by the number of TRUs of the BTS. If a Cell has 4 TRUs (each TRU handles one ARFCN), the capacity of the Cell becomes :

4TRU x 8TS/ARFCN = 32

Cell size ranges from some 100 m in cities to, e.g., 35 km on the country side

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3

4

5

6

7

1

2

35

6

7

4

Cluster-1

Cluster-1

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SIM (Subscriber Identity Module)

Portable Card with memory (ROM-6KB to 16KB-has A3/A8 algorithms, RAM- 128KB TO 256KB, EEPROM- 3KB to 8KB )

It contains: International Mobile Subscriber Identity (IMSI) Personal Identification Number (PIN) Pin Unlock Key (PUK) Authentication Key (Ki) Location Area Identity (LAI)

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Location Area Identity (LAI)A group of Cells is called a Location Area. It is this location information about the MS that is stored in the VLR.

LAI-1

LAI-2

LAI-3

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When an MS has an Incoming Call or an Mobile Terminated SMS, the BSC pages all the Cells in the Location Area where the MS is located.

When the MS responds to the paging message, the BTS adds its identity (CGI) to it.

All Call Setup procedure is then initiated in that particular Cell only. The MS stores the current LAI in it. When the MS moves to a new LA, it reads from the system the new LA and

updates it. It also informs the VLR its new LAI, which subsequently updates it in its database. This procedure is called Location Updation.

The LA can not be too BIG, as then for an Incoming Call or MT SMS, a lot of Cells would have to be Paged, which would increase the Processor Load on BSC.

It can not be too SMALL either, as this would result in frequent Location Updation of the Mobile, increasing again the Processor Load on BSC.

So the LAI would be a compromise between being too Big or too Small. Typically a group of adjacent cities covered by the Network would be assigned

an LAI. Next group of adjacent cities would be assigned another LAI. The format of LAI is :

404-77-80

Mobile Country Code

(404 for India)

Mobile Network Code

(77 for BSNL NE)

BSC Identity

Location Area Identity

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SUBSCRIBER IDENTIFICATION IN GSM NETWORKThere are few numbering schemes for identification of a subscriber in the GSM network.

Mobile Station ISDN Number (MSISDN)91 94 36102025

MSISDN uniquely identifies a subscriber in the GSM Network and it is the number used for dialing a Subscriber.

The HLR and VLR stores the MSISDN Number.

National Destination Code

94 for BSNL

Subscriber Number

36102025 for a subscriber

Country Code

91 for India

Number of HLR

36 for BSNL NE

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International Mobile Subscriber Identity (IMSI)

404 77 1210002025

IMSI is a unique identity allocated to each subscriber. IMSI is used by the system. All network-related subscriber information is connected

to the IMSI. The IMSI is stored in the SIM, in the HLR and VLR. IMSI has a maximum length of 15 digits.

Mobile Network Code

77 for BSNL NE

Mobile Station Identification Number

Mobile Country Code

404 for India

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Mobile Station Roaming Number (MSRN)

The MSRN is a number temporarily allocated to a Subscriber for routing a Call to him.

MSRN is allocated by the current VLR the Subscriber is in. After the Call is established the MSRN is released, and it can now

be allotted to another MS for an Incoming Call Setup. The HLR stores address of the current VLR where the subscriber

is located. When a call is made to a mobile subscriber , the HLR requests the current MSC/VLR to provide an MSRN as a temporary routing number for the called subscriber. Upon reception of the MSRN, the HLR sends it to the MSC which can now use this number to route the call to the MSC/VLR where the called subscriber is currently registered.

MSRN has the same format as of MSISDN. In an MSC/VLR some thousands of Numbers are reserved to be

used as MSRN during Call Setup. These numbers are not allotted as MSISDN to Subscribers.

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International Mobile Equipment Identity (IMEI)

The IMEI is a uniquely identity of a Mobile Equipment. The IMEI can be seen by pressing

*#06# The IMEI consists of the following parts:

IMEI = TAC + FAC + SNR + SVN TAC : Type Approval Code - determined by a central GSM body

(6 digits) FAC : Final Assembly Code - identifies the manufacturer (2

digits) SNR : Serial Number - an individual serial number of six digits

uniquely identifies all equipment within each TAC and FAC (6digits)

SVN : Software Version Number - allows the manufacturer to identify different versions of a given type of approved mobile (2 digits)

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Channel Concept

The Time slots in a TDMA Frame in GSM are called Physical Channels.

Logical Channels are Mapped in the Physical Channels. Logical Channels are used for communication between the MS

and the BTS, like transmission of user data and voice, call setup and Handover signaling, system informations like LAI, BCCH, Adjacent Cells Signal strength, SMS, Cell Broadcast Messages (site name display), etc.

Time slots (physical channels) in a TDMA Frame where the different Logical Channels are configured

TS-0 TS-1 TS-2 TS-3 TS-4 TS-5 TS-7TS-6

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LOGICAL CHANNELS

SIGNALLING

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANT CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLYBOTH UP & DOWNLINKS

TRAFFICTCH

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Channel Concept

Traffic Channel (TCH) TCH carries Voice Data One TCH is allocated for every Voice Call Occupies one TS

Broadcast Control Channel (BCCH) -- broadcasts Cell Informations like: Location Area Identity (LAI) Maximum Power allowed in the Cell Frequency used in the Cell Neighbouring Cells’ BCCH Frequencies Occupies one TS

Cell Broadcast Channel (CBCH) Downloads the Name of the BTS, like Airport, Lichubagan, etc. Each Cell has one CBCH. 1 CBCH and 7 SDCCH occupies one TS

Stand alone Dedicated Control Channel (SDCCH) Call Setup Procedure SMS – SMMT and SMMO (when the MS is in idle mode) Handover Max 8 SDCCH occupies one TS

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Channel Concept Paging Channel (PCH)

Transmits a MS identity (IMSI) for an Incoming Call or SMS (SMMT)

Random Access Channel (RACH) Receives access request from MS for Call setup, Location

Updation, SMS (SMMO)

Access Grant Channel (AGCH) Grants access to the system by allotting an SDCCH to be

used during Call setup, Mobile Originating SMS or Location Updation

Slow Associated Control Channel (SACCH) Transmission of Signal Strength and quality of the

neighbouring cells SMS – when the MS is in Busy Mode

Fast Associated Control Channel (FACCH) Used for Handover Commands

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Channel Concept

Frequency Correction Channel (FCCH) Gives information about the BCCH carrier frequency Allows MS to synchronize to the BCCH frequency used in

the Cell

Synchronization Channel (SCH) Informs the MS about the TDMA frame number Base Station Identity Code (BSIC)

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Traffic Cases – Call to an MS

PCH : the MS is paged in the LA where it is located

RACH : the MS requests for a Signaling Channel

AGCH : access is granted. An SDCCH and a SACCH is allocated

SDCCH and SACCH are used for Call Setup.

A TCH is assigned

SDCCH is released

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Traffic Cases – Call from an MS

RACH : the MS requests for a Signaling Channel

AGCH : access is granted. An SDCCH and a SACCH is allocated

SDCCH : the MS sends a Call setup Request

SDCCH : an idle TCH is allocated

Call establishes on TCH

SDCCH is released

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Thank You

% GSM Basics Demonstration

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