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
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 .
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
II. HYBRID RADIO RESOURCE ALLOCATION ALGORITHM
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 .
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.