PRESENTATION BYR. N. DUBEY

M.TECH (COMMUNICATION SYSTEM)

OUTLINE

Why Cellular? Evolution of GSM Standards

Elements of GSM Network

Why “cellular”?   Radio spectrum is very limited, that’s why we have only 10-25MHz dedicated to wireless communication

Such narrow bandwidth allows 100-400 channels of reasonable quality, which is not rational and commercially not profitable to develop network for such small number of mobile subscribers. Genius idea lead to division of the whole geographical area to relatively small cells, and each cell may reuse the same frequencies by reducing power of transmission.

Each cell has its own antenna (base station), and all base stations are interconnected using microwave or cable communication.

EVOLUTION OF GSM STANDARDS.( 1 )

1980s Analog Mobile System AMPS, in America.

Early 1980s European nations were developing cellular

solutions, but no common standard available. 1982

CEPT ( Conference of European Posts and Telegraph ) formed a study group called the Groupe Special Mobile ( GSM ).

PSTN

GSM ARCHITECTUREMS

BTS

BSC MSC ExchangeA

MS Mobile Station.BTS Base Transceiver StationBSC Base Station Controller.MSC Mobile Switching Center.PSTN Public Switched Telephone Network.

ExchangeB

Fixed Subs.

Microwaveor OFCOFCOFC/MW

BTS

MS

MS

EVOLUTION OF GSM STANDARDS.( 2 )

Objective of GSM Good speech quality. Low terminal and service cost. Support for international roaming. Ability to support handheld terminals. Support for range of new services and facilities. Spectral Efficiency. ISDN Compatibility.

EVOLUTION OF GSM STANDARDS.( 3 )

1989 GSM responsibility transferred to ETSI ( European

Telecommunications Standards Institute ). Global System for Mobile Communication.

1990 Phase I of GSM Specifications published.

Mid 1991 Commercial services started.

1997 Commercial services available in 110 countries.

8

ELEMENTS OF GSM NETWORK

MS BTS BSC MSC

HLR

VLR

EIR

AuC

OMC SMS

Um Abis A

X.25

MS Mobile Station.BTS Base Transceiver StationBSC Base Station ControllerMSC Mobile Switching Center.

HLR Home Location Register.VLR Visitor Location Register.

EIR Equipment Identity RegisterAuC Authentication Center.SMS Short Message Service.

The Mobile stationA subscriber will use a mobile station (MS) to make and receive calls via the GSM network

The MS is composed of two distinct functional entities

1. Subscriber identity module (SIM)- a removable smart card which includes the subscriber’s unique international mobile subscriber identity (IMSI) number which is used to identify each individual subscriber within the GSM. The SIM also contain the subscriber’s secret authentication key.

2. Mobile equipment (ME)-which is essentially the mobile phone itself without the SIM

•Terminal equipment (TE)- performs functions that are specific to a particular service, for example a fax machine. The TE does not handle any functions that are specific to the GSM system.

• Mobile termination (MT)- carries out all the functions relating to the transmission of information over the GSM radio interface.

The Base Station Subsystem composed of BTS and BSC

A BTS performs all the transmission and reception functions relating to the GSM radio interface along with a degree of signal processing.

The BTSs are used to form the coverage cells in GSM and it is their position that determines the network’s coverage and capacity.

BSC- controls multiple BTSs and manages radio channel setup, and handovers.

Management of radio interface which include the allocation of radio channels to MSs on call set-up, determining when a handover is required and identifying a suitable target BTS and controlling the transmitted power of an MS to ensure that it is just sufficient to reach its serving BTS

MOBILE TELEPHONY ( 1 )

BASESTATION

CONTROLLERXchg.

BXchg.

A

Call Through via BS 1.

Cell 1

Cell 2

BS 1

BS 2

MOBILE TELEPHONY ( 2 )

BASESTATION

CONTROLLERXchg.

BXchg.

A

Cell 1

BS 1

Cell 2

BS 2

Call Through via BS 1.

MOBILE TELEPHONY ( 3 )

BASESTATION

CONTROLLERXchg.

BXchg.

A

Cell 1

BS 1

Cell 2

BS 2

Call Through, but now through BS2.Call has been HANDED OVER FROM BS1 TO BS2 without call interruption.

Mobile switching centre MSC performs all the switching function needed for the operation of the MS in the group of cells its services. The function of the MSC include call routing and call control, procedures needed for internetworking with other network, procedure related to MSs mobility management such as location updating while roaming and authentication to prevent unauthorized access, as well as procedure required to implement handovers.

GSM network databases

In a cellular network where subscribers are free to roam throughout the coverage area, the network must also possess some way to track MSs so that it is able successfully to route incoming calls to them. All of these functions are supported using a combination of databases or registers.

Home Location Register (HLR) contains all the subscriber information for the purposes of call control, and location determination. There is logically one HLR per GSM net work, although it may be implemented as a distributed database.

Visitors Location Register (VLR) is only a temporary storage while the particular subscriber is located in the geographical area controlled by the MSC/VLR. Contains only the necessary information provision of subscribed services.

Authentication Center (AuC) is a protected database that stores the security information for each subscriber (a copy of the secret key stored in each SIM).

Equipment Identity Register (EIR) is a list of all valid mobile equipment on the network.

Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM

The functional blocks associated with the management of the GSM network are the operations and maintenance centre (OMC), the network management centre (NMC) and the administration centre (ADC)

OMC provides the means by which the operator controls the network. Each OMC will typically be in charge of a subsystem, e.g. the BSS or the Network Switching Subsystem, NSS (i.e. the MSC, HLR, VLR, etc.)

NMC is concerned with the management of the entire network and it generally has a wider operational role than an OMC.

ADC is concerned, with the administrative functions required within the network.

As we have mentioned above radio spectrum is very limited resource shared by all users. The method to divide up the bandwidth among as many users as possible, chosen by GSM, is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA).

FDMA divides frequency bandwidth of the (maximum) 25 MHz into 124 carrier frequencies. Each Base Station (BS) is assigned one or more carrier frequencies.

Using a TDMA scheme each carrier frequency is divided in time, which forms logical channels.  

Time Division Multiple Access (TDMA) - the users take turns (in a round robin), each one periodically getting the entire bandwidth for a little burst of time. 

GSM : Radio Link Aspects

ITU Allocation for GSM 900 Band 890 - 915 Mhz for Uplink ( MS to BSS ) Band 935 - 960 MHz for Downlink ( BSS to MS )

25 MHz bandwidth divided into 125 Carriers, spaced 200 KHz apart 8 channel per carrier (total 8 x 125 = 1000 channel) actually 124 carriers are used total of 8 burst during a frame cycle, which is defined

as the information format during one time slot on the TDMA channel

Frequency Division Multiple Access (FDMA) - the frequency spectrum is divided among the logical channels, with each user having exclusives possession of some frequency band.