Figure 1: Network architecture
Figure 1 shows the architecture of a basic GSM network. The network is usually divided into three parts :
- Mobile stations (MS)
- Base station subsystem (BSS)
- Network switch subsystem (NSS) or core network (CN)
A mobile station (MS) is equipment within the network that is used in applications involving motion. It can be a hand-held device or any kind of device installed in a mobile vehicle like a car, bus, boat, or airplane. Each MS has a unique international mobile station equipment identity (IMEI) number. The IMEI is often used for preventing a stolen cell phone from accessing the GSM network. It isn’t used, however, for identifying the subscriber. Each MS requires a subscriber identification module (SIM), which stores a unique international mobile subscriber identity (IMSI). The MS can be locked or unlocked to a SIM provided by the operator. Each IMSI contains a unique mobile country code (MCC), a mobile network code (MNC), and a ciphering key (Kc) for authorization with the authentication center (AuC) in the GSM network subsystem.
Base station subsystem
The base station subsystem (BSS) consists of two components: a base transceiver station (BTS) and a base station controller (BSC). The BTS is used for communication with the MS over an air interface called the Um (the name comes from it being a mobile analog to the U interface of an ISDN ). The BTS consists of radio channels and single or multiple transceiver (TRx) antennas mounted on a tower (for an outdoor BTS) or printed circuit board (PCB) patch antenna (for an indoor BTS).
One BSC can handle multiple BTS simultaneously via its Abis interface. The BSC provides basic functions like RF power control, cell configuration, and a link between the BTS and mobile switching center (MSC) via the A interface. The BSC also provides intra-cell BSC handover (changing between TRx within the same cell), inter-cell intra-BSC (changing between BTS controlled by the same BSC), and inter-cell inter-BSC (changing between BTS controlled by different BSC via the E interface).
Network switch subsystem/Core network
The network subsystem consists of a mobile switching center (MSC), a home location register (HLR), a visitor location register (VLR), an authentication center (AuC), and an equipment identity register (EIR).
The MSC provides call routing functions and mobile management for calls and short messages directed to subscribers. The MSC connects to the Public Switched Telephone Network (PSTN) for access to external networks.
The home location register (HLR) stores the data related to each subscriber registered in the network and provides the current location of each user. Data exchange between the MSC and HLR is performed via the C interface. The HLR is also used to exchange data for servicing the general packet radio service (GPRS) support node (SGSN) via the Gr interface. The HRL sends all the information required to support the service for the subscriber to the SGSN. An optional signaling path between the HLR and gateway GPRS support node (GGSN) is also used to retrieve information about the location and supported service to the subscriber (via Gc interface) .
The visitor location register (VLR) is a database that temporarily stores the information of a subscriber who is roaming the coverage area controlled by the associated MSC via the B interface and is internal to the MSC.
The authentication center (AuC) is a database that handles the authentication and encryption keys for every single subscriber in the HLR and VLR via the F interface. The AuC retrieves mobile subscriber information from the equipment identity register (EIR), a database that stores the IMEI information of the mobile station to identify stolen phones. The EIR is also accessible from SGSN to provide the verification status of the IMEI retrieved from mobile station via the Gf interface .
In this blog post, we’ve covered all the components that comprise a basic GSM network so you can get an general idea of how a GSM network works. For additional, detailed information on GSM networks, refer to the ETSI standard for 2G of digital cellular networks.
In my next blog post in this series, I’ll discuss mobile data service on the GSM network.