short description about gsm with reference

7/30/2019 Short Description About GSM with Reference

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1. General Description.

GSM(Global System for Mobile communication) is the evolution of Groupe Special

Mobile(Later transferred to an ETSI technical committee) established by CEPT in 1982 . In the

beginning GSM was exclusively for speech communication. But later on Short Message

Service(SMS), High Speed Circuit Switched Data (HSCSD) and the General Packet Radio Service

(GPRS) was added on GSM[1].

Mainly GSM is operate in the 900Mhz frequency band. The key features with GSM were

Terminal mobility & roaming.

2. Fundamental Components of a GSM Network.

Figure 2.1 : Main Components & Basic Interfaces of a GSM Network

MS : Mobile Station PSDN : Public Switched Data Network

BTS : Base Transceiver Station ISDN : Integrated Digital Service Network

BSC : Base Station Controller PSTN : Public Switched Telephone Network

MSC : Mobile Switching Center PLMN : Public Land Mobile Network

VLR : Visited Location Register HLR : Home Location Center

AuC : Authentication Center EIR : Equipment Identity Register

Um Interface : Interface between MS & BTS

Abis Interface : Interface between BTS & BSC

A Interface : Interface between BSC & MSC


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3. Um Interface(Air Interface)-Physical Layer.

3.1 Multiple Access, Duplexing Method & Frequency allocation.

Um interface is the interface between MS & BTS. All the information generated from MS &

Information from outside of the network will pass through this interface. Frame structure,

Protocols, Control/Signal/Traffic Channels has been defined according to the ETSI standards.

Figure 3.1.1 : Downlink, Uplink frequency allocation in GSM 900Mhz

Figure 3.1.2 : Air interface Frame Structure of GSM

Each GSM channel's bandwidth is 200khz. Frequency Division Duplexing(FDD) is using when

transmitting information in Downlink & Uplink separately. In each direction multiple access has

been achieved by using Time Division Multiple Access(TDMA). In air interface GSM frame's

duration is 4.615ms & its consist with 8 time slots. Each time slot can be assumed 156.25 bits wide

& it's structure is varying according to the burst type. Therefore GSM channel bit rate is

approximately 271 kbits/Sec.

Multiple Access Method TDMA/FDM

Duplex Method FDD

Channel Spacing 200khz

Modulation GMSK

Channel Bit Rate ~271 kbits/Sec

Types of Bursts;1. Normal Burst 3. Frequency Correction Burst 5. Access Burst

2. Synchronization Burst 4. Dummy Burst

Channel Spacing

for 1 Channel : 200khz

Down Link925Mhz880Mhz 960Mhz915Mhz

Up Link

Downlink & Uplink separation

for 1 channel : 45Mhz

1 2 3 4 5 6 7 8

Frame Duration : 4.615ms

0.577ms


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Apart from above in one cell more than one channel frequency is configured & following

parameters also has defined.

MA List : Set of frequencies allocated to a cell .

Frequency Hopping : As stated in earlier in one cell there are few frequencies configured & they

will hop among those frequencies.

HSN(Hopping Sequence Number) : This is the algorithm used during the Frequency Hopping.

MAIO(Mobile Allocation Index Offset) : This index defined in which frequency number

Frequency Hopping should start.

3.2. Logical Channels

Logical channels are the channels assigned between MS & BTS on physical channels to transmit

information in dedicated or random access mode.

Two Types of Logical Channels;

1. Traffic Channels(TCH).

Channels used for transmission of payload data. Data can be circuit switched or packet switched.

2. Signaling Channels.

The control & management of network is done through these channels. There are different type of

signaling channels for different purposes.

Group Channels Sub-Channel Function Direction

Traffic channel TCH

TCH/F, Bm Full-rate TCH MSBSS

TCH/H, Lm Half-rate TCH MSBSS

Signaling

channels(Dm)

BCH

BCCH Broadcast control MSBSS

FCCH Frequency correction MSBSS

SCH Synchronization MSBSS

CCCH

RACH Random access MSBSS

AGCH Access grant MSBSS

PCH Paging MSBSS

NCH Notification MSBSS

DCCHSDCCH

Stand-alone dedicated

Control MSBSS

SACCH Slow associated control MSBSS

FACCH Fast associated control MSBSS

Table 3.2.1 : Traffic & Signaling Channels in GSM[2]


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The above signaling & traffic channels will be mapped in to physical frames & there are few

frames types has been defined to map logical channels .

1. Multi frame(Two Types).

26 - Multi frame : Consist of 26 TDMA frames.

TCH & SACCH, FACCH logical channels used this frame.

51 - Multi frame : Consist of 51 TDMA frames.

All the control channels not associated with TCH(except SACCH & FACCH) used this frame.

2. Super frame : Consist of 1326 TDMA frames(Cycle Period: 6.12s).

3. Hyper frame : Consist of 2048 Super frames(Cycle Period: 3h 28m 53.760s).

Synchronization of user data encryption done in this frame.

4. BTS, BSC & Abis interface(BSS-Base Station Subsystem)[3].

BTS is configured with all the parameters defined in above & build the Um interface between MS

& BTS. Some functions of BTS;

Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna &Decoding, decrypting, and equalizing received signals from the antenna.

Transcoding and rate adaptation. Time and frequency synchronizing. Voice through full- or half-rate services. Random access detection. Timing advances & Uplink channel measurements.

BSC controls the management of all the BTS connected to it. Some functions of BSC;

Control of frequency hopping in BTS. MS Handover between Cells. All the configurations of BTS(Frequency, HSN, MAIO etc.) Time and frequency synchronization. Power management between BTS & MS.

Abis create the interface between BTS & BSC. Physical interface could be E1/T1 or IP. This interface

is not totally standardized & could be varied among the vendors.

5. NSS(Network Switching Subsystem).

NSS carries out call switching and mobility management functions for mobiles connected to

network of base stations. MSC is the primary service delivery node of this subsystem and it set up

& release end to end connection , handles mobility , charging etc.


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As illustrated in Figure 2.1 main blocks of the NSS are MSC, HLR, VLR, AuC & EIR.

Home Location Register(HLR)

HLR has a record of all the subscribers registers with particular network operator. Some of the

recorded data are permanent administrative data & some are temporary data.

Subscriber &

Subscription data

IMSI(International Mobile Subscriber Identity) : MCC+MNC+MNIC

MSISDN(Mobile Subscriber Integrated Service Digital

Network-Number) : CC+NDC+SN

Service subscriptions

Service restrictions (e.g., roaming restrictions)

Information on the subscribers equipment (if available)

Authentication data (subject to implementation)

Tracking & Roaming

Information

Mobile Station Roaming Number (MSRN) Current VLR address (if available)

Current MSC address (if available)

Local Mobile Subscriber Identity (LMSI) (if available)

Table 5.1 : Mobile Subscriber data in HLR[4]

Visitor Location Register(VLR)

A VLR is responsible for a group of location areas and stores the data of all users that are currently

located in this area. VLR is containing Some of the data copied from HLR for fast access & some of

the temporary data.

Subscriber and

subscription data

IMSI

MSISDN

Parameters for supplementary services

Information on the subscribers equipment (if available)

Authentication data (subject to implementation)

Tracking and

routing information

MSRN

TMSI(Temporary Mobile Subscriber Identity)

LMSI (if available)

LAI of LA where the MS was registered (used for paging and call

setup)

Table 5.2 : Mobile Subscriber data in VLR[5]

Authentication Center & Equipment Identity Register(AuC & EIR)

Both of them are security related registers and respectively EIR & AuC take care of Subscriber

identification & authentication. EIR stores IMEI numbers of the terminals.


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6. Mobile Station(MS) & Services.

MS is the terminal device connected to mobile network. Main part of the MS is the Subscriber

Identity Module(SIM). SIM of a subscriber turns a piece of mobile equipment into a complete

mobile station with network usage privileges. A SIM card contains its unique serial number

(ICCID), international mobile subscriber identity (IMSI), security authentication and ciphering

information, temporary information related to the local network[6].

Mainly GSM network facilitate Voice, SMS, Data services to the subscribers. In voice services GSM

used Full Rate(FR), Half Rate(HR), Enhanced Full Rate(EFR) & Adaptive MultiRate(AMR) codecs

for voice encoding[7].

GSM provide two types of data services which are Circuit Switched Data & Packet Switched Data.

In Circuit Switched Data connection reserves a certain amount of bandwidth between two points

till release the connection[8].

Most commonly known Packet Switched Data service is General Packet Radio Service(GPRS). The

advantage of this type of service is that bandwidth is only used when there is actually data to

transmit[9].

SMS is the text messaging service provide on the GSM network. The Short Message Service

Center(SMSC) which is connected to MSC is handling this function. EMS & MMS are the

advanced messaging services.

Reference

[1] -"Chapter 1:Introduction" from, GSM - Architecture, Protocols & Services (Third Edition)-2009

By Jrg Eberspcher (Technische Universitt Mnchen, Germany)

Hans-Jrg Vgel (BMW Group Research & Technology, Germany)

Christian Bettstetter (University of Klagenfurt, Austria)

Christian Hartmann (Technische Universitt Mnchen, Germany)

[2] -"Chapter 4: Air Interface" from, GSM - Architecture, Protocols & Services (Third Edition)-2009

[3] - http://en.wikipedia.org/wiki/Base_station_subsystem

[4],[5] - "Chapter 3: System architecture & addressing" from, GSM - Architecture, Protocols &

Services (Third Edition)-2009

[6],[7] - http://en.wikipedia.org/wiki/Subscriber_identity_module

[8],[9] - http://en.wikipedia.org/wiki/GSM_services

short description about gsm with reference

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