Architecture of GSM

Basicly, GSM consists three main subsystems:

>> SSS - Switching SubSystem (commutation network subsystem);

>> RSS - Radio SubSystem;

>> OMS - Operation and Maintenance Subsystem;


SSS

This subsystem is actually the commutation network. The basic elements of this subsystem are:

> MSC - Mobile-services Switching Center;
> VLR - Visitor Location Register;
> HLR - Home Location Register;
> AC - Authentication Center;
> EIR - Equipment Identification Register;

VLR and HLR are databases. As we can see from the Picture 1, MSC can establish connections with other networks, like PSTN (Public Switched Telephone Network), ISDN (Integrated Services Digital Network), etc. For these purposes, the MSC uses IWF (InterWorking Functions).


RSS

This is the Radio subsystem, which actually provides the mobile communications between the users of GSM. The RSS subsystem is consists of:

> BSC - Base Station Controllers;
> BTS - Base Transceiver Stations;
> MS - Mobile Station (mobile phone, this is user side);

Every BSC is consists of two sub-elements: BCE - Base Station Central Equipment, and TCE - TransCoding Equipment. Maximum of 16 BTS-s can be connected to one BSC. One BTS can serve up to 24 TDMA (Time Division Multiple Access) systems, but one BCE can serve maximum of 48 TDMA.


OMS

The OMS subsystem has the main role in the GSM system, because it operates with other two subsystems, RSS and SSS, and provides maintenance for them. The architecture of the GSM system is shown on the Picture 1.

Picture 1: Architecture of GSM


SIM Card

SIM - Subscriber Identity Module. The SIM card is placed in mobile phone (MS), and this is users identification. Another parameters for users of GSM are:

> IMSI - International Mobile Subscriber Identification;
> TMSI - Temporary MSI;
> MSRN - Mobile Station Roaming Number;
> MSISDN - Mobile Subscriber ISDN number;
> LMSI - Local MSI;



Hierarchy of areas

Location Area (LA) is territory of one or more cells. Every LA has its own LAI - Location Area Identity. One or more LA-s creates area which is under control of one VLR, and this area is called VLRA - Visitor Location Register Area. One or more VLRA-s creates an PLMNA - Public Land Mobile Network Area. One or more PLMNA-s are included within the state PLMN (or state GSM area).


Signal processing

GSM uses RPE-LTP (Residual Pulse Excited - linear Long Term Predictive) coder or RELP - Residual Excited Linear Prediction coder. Samples are taken on every 20 ms, and they are digitalized with 260 bits, so, the speed of data transfer for speech is 13 kbps (kilo bits per second). Adding the redundant bits for every sample, the number of bits per sample then is 456, and the speed of data transfer is 22.8 kbps. These bits are followed into modulator. GSM uses digital phase modulation, like GMSK - Gaussian Minimum Shift Keying.