What is GSM?
GSM (Global System for Mobile Telecommunication) is the standard or protocol that is used for transferring voice and data (SMS) in the mobile phone network. This technology offers speeds of a maximum of 14.4 kbps. Practical speeds achieved are lower. GSM uses circuit switching for its data transfers (both voice and text messages).

What is Circuit Switching?

In circuit switching, a dedicated path is established for a particular session of data transfer from source to destination. This path will serve that session till its end. For example, if a caller from London calls another person in Edinburgh, a path is chosen for this particular call, depending on available resources (free base stations) at that particular time. Say, the call got routed through the base stations in the following areas - London-Birmingham-Manchester-Darlington-Edinburgh. This particular route will be dedicated to this call, till it gets over. If another call originates in the meanwhile, a separate path is set up for it. In case all paths are used up in serving calls, a new call that needs to be set up, must wait for a path to get freed up.

Circuit switching has been used in landline telephone networks as well as in mobile phone networks. While the landlines had a physical path established for a call, the mobile phone calls were allotted a specific range of frequencies from the available bandwidth.

In GSM (2G), the available set of frequencies are divided into slots (FDMA - Frequency Division Multiple Access). Each of these slots are used for serving calls. The concept of TDMA (Time Division Multiple Access) came in a little later after FDMA. In TDMA, the frequency slots are divided again into time slots for supporting more number of calls simultaneously. Earlier a frequency slot was allotted completely to a call. With TDMA, the same frequency slot is allotted to multiple calls. Each call is given a fixed time frame for data transfer. The calls are served cyclically. All this happens at high speed. Therefore the voices will sound continuous to the caller and callee (However, a slight delay might be present). See the diagram below. A set of frequencies have been allotted to 3 calls - Call 1, Call 2 and Call 3 (indicated by their respective colours). The first 10 milliseconds have been allotted to Call 1. Hence the network will serve this call during this time. The next 10 milliseconds have been allotted to Call 2, and the third 10 milliseconds, to Call 3. After this, again, Call 1 will be served, then Call 2 and then Call 3 and so on.

A different set of frequencies than for voice are used for transmitting messages. Hence, you can receive SMS even when you are on a call.

The landline telephone network was used initially to access the internet. A modem had to be used, which dialled-up to connect to the internet. Voice calls and internet access could not happen at the same time. The calls usually got dropped, when the internet was being used at a particular access point. Some network providers gave the option of putting the internet connection on stand-by and answering the call, and resuming the internet activity after the call. Consumers were billed for the amount of time they used the network.

However, with GPRS, things changed. Head to What is GPRS? to understand this.

The diagram below shows a mobile phone network:-

BTS - Base Transceiver System. Each of these masts handle mobile phone calls within a certain radius around it. The area that each BTS serves is called a cell. Though these cells are circular in structure, for theoretical and analytical purposes, they are considered to be hexagonal.

BSC - Base Station Controller. Many BTSs are linked to a BSC. This BSC is in charge of allocating radio channels, administrating the frequencies, handing over the call from one BTS to another if the mobile phone user is travelling.

MSC - Mobile Switching Centre. In the mobile phone network, the MSC is equivalent to a telephone exchange in the fixed line network. It is linked with many BSCs. It is concerned with location management, handovers, routing calls to roaming mobile stations, authentication and so on. It also routes calls between the PSTN (landline telephone) and the cellular (mobile phone) network.

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