GSM

The current generation of mobile communications technology – GSM – is known as the second generation of mobile comms. In the UK, GSM operates in two different frequency bands, 900MHz (Vodaphone and BT Cellnet) and 1800MHz (Orange and One-to-One), while in North America the 1900MHz band is used. The technology behind all of these bands is essentially the same, so understanding how one of them works means understanding how they all work.

GSM is a circuit-switched technology. It allows 25MHz for both upstream and downstream communications but this 25MHz is divided into 124 bands of 200KHz each of which have a data rate of about 270Kbps. Eight different (voice or data) conversations are allowed per band with each conversation taking up 22Kbps. Those of you who are good at mental arithmetic will have realised that this still leaves 24Kbps spare. This is used for control purposes. I will also note here that uncompressed digital speech is standardised to take up 64Kbps, but is compressed by the phone to 22Kbps before it is sent and this is the maximum speed that can be obtained by second generation mobile phones.

Since mobile comms is digital from end to end, when sending data data, you don't have convert the data signal to an analogue form before sending it. However, even so, you can't get a particularly high data rate. This is limited by the 22Kbps limit placed on the phone, although in practice, various error control methods are needed, so the actual data rate is 9600bps or 14,400bps, depending on the type of error control used.


Introduction - Modems - ISDN - DSL - cable modems - Mobile comms - Conclusion - Glossary