GSM Mobile Network – mobile network of 2nd generation
Data Transmission in GSM Network and 2.5 Generation of Mobile Systems

Basic transfer rate between a MS and a GSM network is 13.2 kbps for voice call in both directions. This channel can be used not only for transferring calls but also for data transmission based on circuit switching, CSD (Circuit Switched Data), with nominal transfer rate of 9.6 kbps. Later, the transfer rate has been increased up to 14.4 kbps, mainly by reducing redundant information in the form of protective codes.

However, considerable increase in transfer rates is ensured by the systems belonging to 2.5 generation, either by using:

- transmission based on packet switching, known as GPRS (General Packet Radio Service), with the theoretical available bit rate up to 171 kbps or
- transmission based on circuit switching, known as HSCSD (High Speed Circuit Switched Data), with the theoretical available bit rate up to 115 kbps

A further increase of transfer rates is possible by utilization of technique denoted as EDGE (Enhanced Data for GSM Evolution). Compared with conventional modulation in GSM system, there is used the modulation with a higher number of states, namely 8-PSK (Phase Shift Keying). Theoretical achievable bit rate of data transmission using EDGE is 473.6 kbps.

Infrastructure of GSM network with 2.5G systems is supplemented by a data node, SGSN (Serving GPRS Support Node), which communicates with the radio part of GPRS network. For the data transfer to other packet networks, such as the Internet, a data gateway GGSN (Gateway GPRS Support Node) is subsequently implemented. The GGSN operates as a router. Simplified infrastructure of 2.5G mobile network can be seen in the following figure.

Simplified infrastructure of 2.5G mobile network