31.5 ENHANCEMENTS OF GSM STANDARDS

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31.5 ENHANCEMENTS OF GSM STANDARDS

In its original form for data applications, GSM supports 9.6kbps data rate. The data is transmitted in the TDMA time slot. Over the last few years, many enhancements were made to the GSM standards to increase capacity and provide value added services, which are briefly discussed here.

31.5.1 GSM Phase 2

Phase 2 GSM services include advice of charge, calling line identification, and closed user groups. In addition, 1800MHz band has been allocated to GSM systems in areas where the 900MHz band is saturated.

start example

Because the spectrum allocated in the 900MHz band was not sufficient, the 1800MHz band has been allocated for GSM operation. Most of the present mobile phones are dual-band phones, so they can operate in both bands.

end example

GSM Phase 2+ addresses nearly 90 enhancements to the existing standards to introduce new services, particularly data services. The major enhancements are in the areas of speech quality improvements and enhanced data services.

Note 

In a service area, if there are four operators, two operators are allotted 900MHz band and the other two operators are allotted the 1800MHz band.

31.5.2 Speech Quality Improvements

GSM Phase I supports 13kbps speech codec-the speech quality being far from good. Speech codec for GSM was finalized in 1987, and significant advances have been made in speech coding technology in recent years. So, GSM Phase 2+ now will introduce a new codec (developed by Nokia and University of Sherbrooke, Canada) whose quality will be comparable to PSTN speech quality.

start example

Presently in the GSM system, speech is encoded at 13kbps. Half-rate codecs will encode the speech at 6.5kbps. As a result, the capacity of the GSM system can be doubled.

end example

Half-rate codec, a part of GSM Phase 1 standard but not introduced so far, can be introduced and will have the same voice quality as the existing GSM's voice quality.

Cellular operators will have two choices-improve the speech quality by introducing the new codecs in the full-rate speech channels or, using half-rate codecs, increase the capacity with the same quality of speech as that of the present full-rate codecs.

31.5.3 Enhanced Data Services

The 64kbps access speed is very common for Internet access using the PSTN. GSM users expect the same data rate support over their mobile data terminals. Particularly, such high data rate is a must to access Internet services such as multimedia Web services and large file transfers. In GSM Phase 2+, high speed circuit switched data (HSCSD) service and general packet radio service (GPRS) have been proposed, which provide high-speed data services over GSM. These are add-on features to the GSM network, with minor software-only upgrades. HSCSD is based on multislotting-dynamically combine two to eight time slots to give maximum data rate up to 64kbps for a single user. This is a connection-oriented service. If all the eight time slots of one TDMA frame are allotted to one user, it gives 76.8kbps (8 × 9.6 kbps) of theoretical data rate. In the case of GPRS, packet switching is done, and hence it is 76.8kbps of instantaneous data rate. We will study data services in detail in the chapter on wireless Internet.

start example

GSM Phase 2 enhancements include support for high data rate services. Using these enhancements, users can access the Internet through mobile phones.

end example

Using data compression techniques (based on V.42 standard), the effective data rate can be increased up to 300kbps for textual data. This data rate perhaps is sufficient for accessing the existing widely used data services over the Internet such as the Web, large file transfers, audio, and so on. But then, the data users' demands are rising very fast, and already there is enough demand for data rates such as 384kbps and even 2Mbps.

To summarize, mobile communications services have become very popular in the last decade, and though at present voice is the main application, in the future wireless data services using the mobile communication systems will catch up. Anywhere, anytime communication is now a reality-thanks to cellular mobile communication systems.



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Principles of Digital Communication Systems and Computer Networks
Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)
ISBN: 1584503297
EAN: 2147483647
Year: 2003
Pages: 313
Authors: K V Prasad

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