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Analysis of a Traffic Model for GSM/GPRS


In today’s mobile communications world, the 2nd generation Global System for Mobile Communications (GSM) is clearly a winning system, used by millions around the globe. Due to its limited ability to grow and satisfy packet data communication needs, its future as a communications system is shadowed by the upcoming 3rd generatio n one, the Universal Mobile Telecommunications System (UMTS). That is mainly due to the fact that GSM creators thought of it as primarily a voice system, hence, lacking the ability to deal with large amounts of data, the kind of data that is overwhelmingly taking over all communication networks.


It is widely foreseen that in the near future mobile data
accomplish an efficient way of delivering data packets, including the ones from other networks, with the minimum disturbance in the existing network.
With the implementation of GPRS in GSM networks, and the foreseen growth of data transfer in mobile networks, it is necessary to achieve a compromise between voice and data services. The objective of this work is to study the behaviour of an integrated voice/data mobile communications system, by using simulations. The simulator was fully built around the Hybrid Radio Resource Allocation (HRRA) algorithm [1].
In the next section a brief description of the algorithm is done, which allows a better understanding of the HRRA implementation described in Section III. In Section IV, results from that implementation are presented for some cases of interest, leaving general conclusions to be drawn in Section V.
According to the HRRA algorithm, either voice or packet data traffic can be carried over a traffic channel, with GPRS dedicated channels being available exclusively for data transfers. If no resources are available voice calls are blocked and lost, while packet transfers are placed on a waiting queue. In this algorithm, voice calls have priority over all ongoin g data calls, with the exception of those assigned to the packet switched dedicated traffic channels.
In our study the case of a single cell GSM/GPRS system is considered. Considering N as the number of carries in use on that particular cell, it is well known that the number of physical channels available is

n 8 N. (1)

traffic will overtake voice as the primary service provided by mobile operators. Hence, operators need to evolve from

Of these n channels, n


are assigned to control and

current GSM networks, so that they can provide the necessary packet switched multi-service communications. In order to achieve that, General Packet Radio Service (GPRS) is the solution that is being implemented.
The main reason for the limitation in dealing with large amounts of data is the circuit switched based transmission used in GSM. Due to the bursty nature of packet data, and to the scarcity of available resources, a packet switched transmission is be better suited. The GPRS concept was developed from the need to evolve GSM, in order to

signalling functions, the remaining ones, ntraffic, being used
for traffic. In the latter, both voice calls and data packets can be transmitted; however, it can happen that some of the ntraffic channels are reserved for either voice or data only, in what is known as a prioritised channel sharing scheme [2].
In our study, one will use data dedicated channels, CGPRS, these being permanently allocated to GPRS data transfer. The remaining traffic channels, Cshared, are considered to be shared ones, and they will be allocated on a demand basis. The channel sharing scheme is represented in Fig. 1.

Fig. 1: Channel sharing scheme fo r a GPRS network.