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Europe: GSM and DCS 1800

Definition of the Groupe Special Mobile (GSM) system began in 1982, under the auspices of the Committee of European Posts and Telecommunications. Now called Global System for Mobile communications, the goal of this time division-based digital cellular system was a unified pan-European system. In mid-1995 there were over 11 million customers using GSM worldwide; that number was expected to double by year-end 1996 with more than 140 service providers in 86 countries.

Prior to GSM, many independent analog systems were in use throughout Europe, with incompatible standards preventing intercountry roaming in many cases. Cellular usage was essentially regional in scope. The goal of GSM was to eliminate this fragmentation of the European cellular market. Since this was intended to be a "next generation" system, it uses digital technology with the capability of supporting data applications.

GSM was originally specified in the 900 MHz band and currently runs in that spectrum (see Table 1.2). However, in 1989 the U.K. Department of Trade and Industry defined Personal Communications Network (PCN)  to consist of GSM operating in the 1.8 GHz band. This upbanded system is referred to as Digital Cellular System 1800 (DCS 1800), with deployment anticipated by 1998.


 

  
Table 1.2: GSM Frequency Allocations
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GSM Frequency Allocations

 

 

GSM is a TDMA-based system with 8 user timeslots per frame in a 200 kHz channel. Like other TDMA systems, staggered transmit and receive timeslots allow modems to use half-duplex radios, thereby reducing their costs. The transmit/receive offset still leaves enough idle time for the mobile to participate in handovers by monitoring neighbor cell channel signal strengths in a MAHO scheme.

The GSM system uses GMSK modulation. Rate 1/2-convolutional coding with interleaving addresses Rayleigh fading. The net data rate is 22.8 kbps with error correction in what is called full-rate mode. An additional half-rate mode at 11.4 kbps is also defined by using 16 timeslots (which are half as large as the full-rate timeslots) per frame.

A form of slow frequency hopping is used by GSM to help combat the multipath burst errors characteristic of cellular environments. Each base station has its own pattern for hopping from one carrier frequency to another from slot to slot, with mobiles using that base station following suit. This frequency hopping also reduces the incidence of co-channel interference between clusters of cells.

GSM, with slow frequency hopping and coding requires an approximately 9 dB C/I ratio for effective operation. If we assume a frequency reuse factor of 3 with 3-sector antennas and 8 users per 200-kHz bandwidth, we can estimate relative system capacity for GSM to be approximately

 

[8 users / (200 kHz * 3 cells)] / [1 user / (30 kHz * 7 cells)]

or 2.8 times AMPS capacity [FALC95].

The radio data link layer is based on a LAPD-like protocol called LAPDm. LAPDm modifications (from LAPD) include using no frame flags or bit stuffing, instead relying on a "length indicator" field, as depicted in Figure 1.8. Also, the SAPI (SAP identifier) is included in the address field, shown in Figure 1.9. LAPDm also has no CRC bits for error detection, instead relying on lower layer block and convolutional coding for error detection and correction.


 

  
Figure 1.8: LAPDm Frame Format
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LAPDm Frame Format


 

  
Figure 1.9: LAPDm Address Field
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LAPDm Address Field

GSM mobility management is provided by specific layer entities which establish, maintain and release separate connections between the mobile station and the MSC under control of the higher connection management sublayer. These separate connections are for call control, short message service and the call-independent supplementary services. Each mobility management connection provides services such as encryption and authentication.

GSM mobility management is based on Signalling System 7 (SS7), an international intelligent network (IN) telephony standard. Each mobile is identified by a mobile station ISDN (MSISDN) number consisting of a country code (CC), national destination code (NDC) and subscriber number (SN). The MSISDN is used by a serving MSC to interrogate the appropriate HLR prior to providing service. The serving VLR provides a mobile station roaming number (MSRN)-similar in format to the MSISDN and in function to the TLDN-for temporary use in forwarding calls to the roaming mobile.

GSM provides the capability for a base station to autonomously handle handovers between coverage areas under its control without involvement from the MSC. This process is called internal connection handoff. Following handoffs, the original MSC handling the call retains control even though the call may be going through a new serving MSC.

Synchronous and asynchronous data services have been defined at 9.6, 4.8 and 2.4 kbps for both full-rate and half-rate operation. Interfaces to V.22bis and V.32 audio modems are also defined for GSM. ISDN and Group 3 facsimile interfaces are also included in the GSM system definition. Industry experts believe that the introduction of data capabilities and interfaces to PC (formerly PCMCIA ) cards will spur continued exponential growth in worldwide adoption of GSM. Key to this growth is "plug and play" interfaces which enable standard computer applications as well as vertical applications.

A connectionless packet data service called General Packet Radio Service (GPRS) is in standards development. This GSM capability will define the interworking between the cellular environment and those of X.25 and the Internet world. Two approaches are being considered-dedicated data channels (i.e., a voice channel shared by multiple data mobiles) and fast data channel setup for a single user. The data service objectives include a packet error rate of 10-4 with delay under one second. CCITT recommendation X.121 numbering is used for addressing mobile packet data in GSM.

A GSM-based datagram service called Short Message Service (SMS) is defined in support of Email and other messaging-type applications. This service allows transmission of datagrams which are up to 160 bytes in length at 300 bps on the reverse control channel. Higher data rates are available via traffic channels, which require a call setup; the resultant 9600 bps is better suited to longer messages.