3.1.     Basic cellular system

The architecture of most cellular systems can be broken down into the following six components:

a)      Mobile units: A mobile unit is basically a mobile/wireless device that contains a control unit, a transceiver and an antenna system for data and voice transmission. For example, in GSM networks, the mobile station will consist of the mobile equipment (ME) and the SIM card.

b)     Air Interface Standard: There are three main air interface protocols or standards: frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). These standards are basically the medium access control (MAC) protocols that define the rules for entities to access the communication medium. These air interface standards allow many mobile user to share simultaneously the finite amount of radio channels.

c)      Mobile Telephone Switching Office (MTSO): An MTSO is a fixed station in a mobile cellular system used for radio communications with mobile units. The serve as the central coordinating elements for all cell sites, contain the cellular processor and cellular switch. They consist of radio channels and transmitter and receiver antenna mounted on a tower.

d)     Databases: Another integral component of a cellular system is the databases. Databases are used to keep track of information like billing, caller location, subscriber data, etc. There are two main databases called the Home Location Register (HLR) and Visitor Location Register (VLR). The HLR contains the information of each subscriber who resides in the same city as the MTSO. The VLR temporarily stores the information for each visiting subscriber in the coverage area of a MTSO. Thus, the VLR is the database that supports roaming capability.

e)      Security Mechanism: The security mechanism is to confirm that a particular subscriber is allowed to access the network and also to authenticate the billing. There are two databases used for security mechanism: Equipment Identify Register (EIR) and Authentication Center (AuC). The EIR identifies stolen or fraudulently altered phones that transmit identity data that does not match with information contained in either the HLR or VLR. The AuC, on the other hand, manages the actual encryption and verification of each subscriber.

f)       Gateway: The final basic component of a cellular system is the Gateway. The gateway is the communication links between two wireless systems or between wireless and wired systems. There are two logical components inside the Gateway: 1) MTSO and 2) Interworking function (IWF). The MTSO connects the cellular base stations and the mobile stations to the public switched telephone network (PSTN) or other MTSO. It contains the EIR database. The IWF connects the cellular base stations and the mobile stations to Internet and perform protocol translation if needed.