GSM Architecture

By:•Hassan Amjad•Bilal Waheed•Mohsin Amin

Intro to GSM:

•GSM (Global System for Mobile Communication)

•  It is a standard set developed by the ETSI.

•It describe protocols for 2G digital cellular networks.

•Full duplex.

•GPRS & EDGE facilities.

GSM Variants :Variant Uplink

(MHz)Downlink (MHz)

Total Bandwidth

Duplex-frequency

Channels

GSM-400 451-458 and 479-486

461-468 and 489-496

Twice 14 MHz 10 MHz Twice 72

GSM-900 (primary band)

890-915 935-960 Twice 25 MHz 45 MHz Twice 124

Extended GSM-900

880-915 925-960 Twice 35 MHz 45 MHz Twice 174

GSM-R 876-880 921-925 Twice 4 MHz 45 MHz Twice 19

DCS-1800 1,710-1,785 1,805-1,880 Twice 75 MHz 95 MHz Twice 373

PCS-1900 1,850-1,910 1,930-1,990 Twice 60 MHz 80 MHz Twice 300

GSM Architecture

What so ever the variants are the basic architecture of GSM network remains the same as elaborated below:

•BSS (Base Station Subsystem)

•NSS ( Network Switching Subsystem )

•NMS ( Network Management Subsystem )

Architecture Diagram :

ME:•Mobile equipment •Handset + SIM

+

Handset:

• is a device that can make and receive telephone calls

•over a radio link

•Can moving around a wide geographic area.

SIM:Subscriber identification module

It contains •IMSI ( international mobile subscriber

identity )

•PIN (personal identification number )

•PUK ( personal un blocking code )

•ICCID ( integrated circuit card identification )

•IMSI ( used by operators to identify user)• IMSI = MCC+MNC+MSINi. MCC ( mobile country code)ii. MNC ( mobile network code)iii. MSIN ( mobile user identification

number )

•ICCID (SIM is internationally recognized by this number )

•Usually printed on SIM

Base Station Subsystem:

•The Base Station Subsystem is responsible for managing the radio network, and it is controlled by an MSC.

•A BSS itself may cover a considerably large geographical area consisting of many cells (a cell refers to an area covered by one or more frequency resources).

BSS consists of three element •BTS ( Base transceiver station)•BSC ( Base station controller )•TC ( Tran coder)

Base transceiver station:

•Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna.

•Time and frequency synchronizing

•Decoding, decrypting, and equalizing received signals

•Random access detection•Uplink channel measurements

Base station controller :

•The BSC manages the radio resources for one or more BTSs. It handles radio channel setup, frequency hopping, and handovers.

•The BSC is the connection between the mobile and the MSC. 

•Control of frequency hopping•Performing traffic concentration to

reduce the number of lines from the MSC•Reallocation of frequencies among BTSs•Time and frequency synchronization•Power management

Transcoder:•In the air interface (between MS and

BTS), the media carrying the traffic is a radio frequency.

•To enable an efficient transmission of digital speech information over the air interface, the digital speech signal is compressed.

• We must however also be able to communicate with and through the fixed network, where the speech compression format is different.

• Somewhere between the BTS and the fixed network, we therefore have to convert from one speech compression format to another, and this is where the Transcoder comes in.

• For transmission over the air interface, the speech signal is compressed by the mobile station to 

• 13 k bit/s (Full Rate and Enhanced Full Rate), • 5.6 k bit/s (Half Rate), • 12.2 k bit/s (Enhanced Full Rate).•  AMR (Adaptive Multirate Coding) which is

more flexible since it produces speech with bitrates similar to older solutions but adapted to link conditions.  

• However, the standard bit rate for speech in the PSTN is 64 Kbits/s. The modulation technique is called "Pulse Code Modulation" (PCM). This requires the GSM network to perform bit rate adaptation of speech.

Network switching subsystem (NSS)

NSS typically consists of following elements

• MSC (Mobile switching center)

• HLR (Home location register )

• VLR ( visitor location register )

• AUC (Authentication center )

• EIR ( Equipment identity register)

• GMSC ( Gate way mobile switching center )

HLR:•centralized network database •stores and manages all mobile

subscriptions belonging to a specific operator.

•a permanent store for subscriber until that subscription is canceled.

It stores1. Subscriber identity2. Subscriber supplementary services3. Subscriber location information4. Subscriber authentication

information

VLR :

•It is a temporary database .•Stores only information of

subscriber those are in that MSC.•Every MSC has its own VLR•When a subscriber roams into a

new MSC service area, the VLR connected to that MSC requests information about the subscriber from the subscriber's HLR.

EIR :

•Data base used to store mobile identity

•Help us blocking calls from stolen MS

AUC:

•AUC is a database connected to the HLR

•Function is to authenticate the subscribers attempting to use a network.

MSC & GMSC:•MSC performs the telephony switching

functions for the mobile network.•controls calls to and from other telephony

and data systems, such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN).

•In GMSC Gateway functionality enables an MSC to interrogate a network's HLR in order to route a call to a Mobile Station (MS).

•Any MSC can be GMSC

NMS:

NMS consists of two element

1. OMC ( Operation and maintenance center )

2. NMC (Network management center )

OMC & NMC

•OMC a computerized monitoring & maintenance center which is connected MSCs and BSCs.

•OMC is local.

• In NMC centralized control of a network is done.

•One NMC per network to control many OMCs.

•OMC and NMC functionality can be combined in the same physical network node.

GSM GEOGRAPHICAL NETWORK STRUCTURE

Local area (LA)•LA is as a group of cells. •Within the network, a subscriber’s

location is known by the LA. •The identity of the LA in which an MS is

currently located is stored in the VLR.

MSC SERVICE AREA

•An MSC service area is made up of a number of LAs and

•MSC use LA to route a call to MS

Hardware view of a sample network

Software view of a sample network

GMSK:

•Gaussian Minimum Shift Keying.•Gaussian Minimum Shift Keying (GMSK)

is a form of continuous-phase FSK in which the phase change is changed between symbols to provide a constant envelope. Consequently it is a popular alternative to QPSK.

•The RF bandwidth is controlled by the Gaussian low-pass filter.

continuous-phase :continuous-phase•In CPM the carrier phase is modulated in

a continuous manner in contrast to QPSK .

•CPM is typically implemented as a constant-envelope waveform.

•QPSK  requires a relatively large percentage of the power to occur outside of the intended band.

•CPM is attractive because the phase continuity yields high spectral efficiency.

•The primary drawback is the high implementation complexity required for an optimal receiver.

MSK:•Minimum shifting keying .•It is similar to OQPSK •OQPSK gives us square wave which is

difficult to handle.•MSK gives us half cycle sinusoidal wave for a

symbol

GMSK:

•Gaussian minimum shifting keying•MSK with Gaussian filter.•

Gaussian filter•Gaussian filters are designed to give no

overshoot to a step function input while minimizing the rise and fall time.

•Gaussian filter has the minimum possible time delay

Example

The filter smoothes the binary pulses

signal is integrated over time to obtain a continuouswaveform which captures the bit transition information

•Send this wave to I\Q modulator .•Integrate out put and send it

ADVANTAGES DISADVANTAGES

• High spectral efficiency.

• Reducing sideband power.

• Excellent power efficiency due to constant envelope.

• Good choice for voice modulation.

• Higher power level than QPSK.

• Requiring more complex channel equalization algorithms such as an adaptive equalizer at the receiver.

• Inter symbol interference

Interleaving

•Interleaving rearranges a group of bits in a particular way.

•Improve the performance of the error correction mechanisms

•The interleaving decreases the possibility of losing whole bursts during the transmission.

Interleaving