In GSM Circuit switched data, the typical maximum data rate of data throughput was officially 9.6 kbit/s, however when the overhead of control, guard data, encryption keys and error correction, are taken into account the typically user data throughput was only 1 – 1.5 kbit/s.
The first phase of the GSM evolution to higher speed data was the introduction of High Speed Circuit Switched Data (HSCSD). This gave the user the potential of data rates up to 38.4 kbit/s, which after the overhead was accounted for allowed a user data rate of 14 kbit/s. This improvement in data throughput was achieved by the use of multiple time slots, but was at the expense of the main revenue earning service (speech).
Due to the fact that data is tolerant to delays in transmission, as opposed to speech a new technique utilizing packet switching was developed (GPRS)
GPRS differs significantly from HSCSD in that it is not permanently connected to a single physical channel. Like HSCSD, GPRS utilizes multiple time slots, but the network has the ability to reduce the number of time slots used so that speech traffic is given priority. The disadvantage to the data user is that during peak hour traffic data throughput will be significantly reduced by the network in order to accommodate the higher revenue potential of speech traffic.
To the network operator the introduction of GPRS has meant a significant modification of the existing GSM network infrastructure. The advantage to the network operator is that they can now use a packet switched network for data transmission instead of dedicated circuit switched network. This has major economic advantages for the network operator. During speech transmissions, there are many instances of no information being transmitted but with data, transmission is continuous. It is for this reason that data does not need to be permanently connected as the data can always be re-arranged at the terminal end.
With users requiring faster and faster data speeds and even the ability to download multimedia applications, a new technology had to be developed due to the fact that GSM had reached it’s maximum data throughput. Also the network had to consider whether it was connected to a data or speech call. So the next step in the evolution was the development of Wideband CDMA, leading to UMTS.