GSM Summary & Key Points

With the increase in demand for the need for communications whist away from a fixed line, GSM specifications were developed in order to provide greater capacity, improved quality and significantly greater security. Also the need to be able to use your mobile phone in other countries became of paramount importance.

GSM provided significantly greater capacity by the use of a combination of FDMA and TDMA. Whereas in previous technologies such as TACS and MTN the modulation technique was analogue and thus not very secure, GSM utilizes digital encoded modulation (See section 2.1.7.2), thus making it virtually impossible for interception by the casual eaves dropper. As a result of this certain Government agencies found it impossible to intercept calls and decode them in real time (DEA, See Appendix 2). However, in recent years details of the encryption algorithms have been made available to these agencies, and then leaked on the internet, thus reducing the security of the GSM system.

A further security improvement brought about by GSM was the use of SIM cards. The SIM card holds data relating to the user, authentication codes, and billing details.

The GSM radio air interface (Abis) is split into two levels, Physical Channels which are the radio bearers and Logical Channels which contain all the control and speech information in data streams. These logical channels are described in GSM Section 2.1.3.1. The net outcome is that speech only occupies a small amount of these data streams, hence the use of TDMA on the FDMA radio bearers.


Mobility means that the MS is moving from one location to another. Thus the system has to know where the MS is in order to direct calls. The MS continually provides a location update so that the network knows where to find the MS. Also because the MS is moving the network has to handover from BSC to another BSC, or another sector in the BSC. Consequently the MS has to continually measure the signal levels and quality in order to know which BSC has the appropriate availability in terms of capacity and quality. In this way the network is able to maintain connectivity and quality of service to the user.

With this increased mobility, users started to demand the ability to send data from their PCs and thus the mobile internet was born. In earlier technologies data transmission was achieved by the use of an analogue modems, but with GSM digital modems are now incorporated into the some MSs, thus providing greater data throughput.

The success of GSM data transmissions has meant that users require faster and faster speeds. On fixed lines the technique known as packet switched data transmission was available, but this was no use for GSM as the human ear cannot tolerate the delays introduced by such a technology. Initially data on the GSM network used a technique known as circuit switched (GSM-CS), which meant that the MS was permanently connected to a specific circuit, which was necessary for speech transmission. In order to increase the data rate a new technology based around packet switching had to be developed and thus the evolution of the GSM network continues into EDGE.


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