This GSM tutorial covers GSM basics, network architecture, network elements, system specifications, applications, GSM burst types, GSM frame structure or frame hierarchy, logical channels, physical channels, GSM physical layer or speech processing, GSM mobile phone network entry or call setup or Power ON procedure, MO/MT call, VAMOS, AMR, MSK/GMSK modulation, physical layer, protocol stack, mobile phone basics, RF planning, PS call downlink and PS call uplink.
GSM is the short form of Global System for Mobile Communications. It is called 2G or Second Generation technology. It is developed to make use of same subscriber units or mobile phone terminals throughout the world. There are various GSM standards such as GSM900, EGSM900, GSM1800 and GSM 1900; they mainly differ based on RF carrier frequency band and bandwidth. This gsm tutorial covers network architecture, network elements, various interfaces, specifications, GSM frame structure or GSM frame hierarchy, GSM burst types, GSM physical layer, GSM physical channels, GSM logical channels and their functions, logical channel mapping, GSM mobile network entry procedure, GSM MO call and GSM MT call, VAMOS basics, AMR basics and MSK & GMSK modulation types.
GSM Network Architecture
GSM network is consists of Mobile station, Base station subsystem and Network and operation subsystem. Following figure depicts complete GSM system network architecture.
Fig.1 GSM Network Architecture
Mobile Station- This Mobile station is GSM mobile phone equipment which houses DSP,RF chip and SIM(subscriber Identity Module). This SIM is enough to carry to avail the service of GSM network. SIM contains subscriber related all the information, network with which subscriber is subscribed with and encryption related information.
Base station Subsystem- Base station subsystem houses Base Transceiver station-BTS and Base station controller-BSC. This subsystem take care of radio control related functions and provides GSM air interface for GSM mobile phones to connect with GSM network. To provide GSM service, region/city on earth is divided into various cells. The cell size is usually about 100m to about 35 km. BTS coverage is limited to this cell. Like this many BTSs cover entire region. All this BTSs are interfaced with one BSC in various ways mesh, star etc. This BSC takes care of radio frequency assignments to the mobile phones, takes care of handoff within BSS i.e. between one BTS and the other BTS.
Network Subsystem (NSS) - This subsystem provides interface between cellular system and circuit switched
telephone network i.e. PSTN. It performs switching and operation & maintenance related functions. NSS takes care of
call processing functions such as call setup, switching, tear down and also hand over between BSCs. NSS takes care of security
and authentication related functions. There are various network elements in this subsystem as mentioned in GSM network architecture above.
They are explained below. These are basically database elements.
HLR-Home Location Register, it stores permanent and temporary subscriber related information.
VLR- Visitor Location Register, it stores visitor subscriber related information about its facilities, the network it is subscribed to, and its home location and so on.
AUC- Authentication center, used to authenticate activities in the system. It holds encryption (A5 key) and authentication keys (A3 key)in both HLR and VLR.
EIR- Equipment Identification Register, it helps in security as it keeps track of equipment type available in Mobile Station or Terminal.
Air interface between Mobile station and BTS
Abis interface between BTS and BSC
A interface between BSC and MSC
SS7 interface between MSC and PSTN
Refer GSM network interfaces page of gsm tutorial for detailed description.
GSM system Specifications
Access Method- TDMA/FDMA
Uplink frequency band- 890 to 915 MHz
Downlink frequency band- 935 to 960 MHz
System Bandwidth- 200 KHz
No. of frequency channels or ARFCN (Absolute Radio Frequency Channel Number)-124
Users per channel-8
Spectral efficiency-1.35 b/s/Hz
Data rate per user- 33.6 kbps (270.833 kbps Gross data rate for 8 users/8users)
The GSM tutorial cover almost all the aspects of GSM technology other than network interfaces, architecture and system specifications mentioned on this page. Refer the links mentioned on the left side panel for the same.
GSM Tutorial RELATED LINKS
GSM protocol stack which covers GSM layer 1, layer 2 and layer 3 basics.Read More
GSM Physical layer or GSM layer 1 modules in depth.READ MORE
GSM Mobile originated call flow,READ MORE
GSM Mobile Terminated call flow,READ MORE
GSM downlink packet switched data call, READ MORE
GSM uplink packet switched data call, READ MORE
Circuit Switching(CS) versus Packet Switching(PS), READ MORE