The GSM system is based on the protocols used for ISDN, and it was designed from the beginning to support fax and data.
Outgoing fax or data call
When you make an outgoing fax or data call, the mobile sets up a call of fax or data type, and the connection is digital straight through to the GMSC, at which point a modem rack places the call.
Incoming fax or data call
For incoming fax or data calls, the call type is detected simply by the incoming number. All calls to your fax number are assumed to be fax calls, and similarly data calls are defined as calls to the data number!
How fast can data go through a mobile phone?
If the handset is in a position to accept the call, everything goes much as a voice call would.
The data speed was originally restricted to 9600 and this provides acceptable performance for text browsing, or for email. This is partly because the signalling and data correction overhead does not use any of the data capacity: you get to use all the 9600 for the data.
There is facility in the GSM standard for 14.4, 19.2 and 28.8 connections, and these are available for some handsets on Orange (but not any other UK networks), the latter two by using more than one channel at a time. Unfortunately, with the launch of GPRS, Orange raised the price of High Speed Circuit Switched Data (HSCSD) connections to the point where they are unlikely to be cost-effective.
HSCSD simply uses two timeslots to give twice the throughput, and uses less error-correction on good links to squeeze a bit more through. The GSM specification allows for connections using three links for the downstream side, and one for the upstream side, but no UK network supports this at present.
Video conferencing is likely to drive the demand for higher data rates, and these are part of the “Third Generation” mobile networks coming on stream. 3 is the first of these Universal Mobile Telephony System (UMTS) networks to go live in the UK, and makes streamed video to and from from a mobile phone a possibility. The snag is that all five networks paid a huge price in the auction to get a 3G lience, and they are not sure that they’ll get their money back from offering the service!
In the meantime, the other networks offer General Packet Radio System (GPRS), which gives always-on connections – at a rather high price. See here for more details