Overview Of The GSM System
and Protocol Architecture
We can use GSM as a basic framework to define and develop
the standards for handling the mobility-specific functions of
next-generation PCNs.
Moe Rahnema
IEEE Communications Magazine • April 1993
GSM and PCNs
0163-6804/93/$03.00 1993© IEEE
Global system for mobile telecommunication (GSM)
comprises the CEPT-defined standardization of the
services, functional/subsystem interfaces, and
protocol architecture, based on the use of worldwide
standards produced by CCITT and CCIR, for a pan-
European digital land mobile system primarily intend-
ed to serve users in motor vehicles. The digital mobile
radio networks, for which GSM represents the Euro-
pean standards, provide powerful message signal-
ing capabilities that facilitate and enhance
roaming, compared to the first generation analogue
systems, through automatic network location detec-
tion and registration.
GSM provides terminal mobility, with person-
al mobility provided through the insertion of a
subscriber identity module (SIM) into the GSM net-
work (mobile station). The SIM carries the personal
number assigned to the mobile user. The GSM-based
cellular mobile networks are currently in widespread
use in Europe. At the present time, the next gen-
eration of personal communication services
(PCS) beyond GSM is also being considered.
These third generation systems, known as univer-
sal personal communication networks (PCN) will be
using lower power handsets to provide personal
mobility to pedestrians, as well. The PCS low-power
handsets are expected to eliminate the need to
have different handsets for wide-area (cellular)
and local (cordless) applications. The universal PCS
will also provide a higher quality of personal-service
mobility across the boundaries of many different net-
works (mobile and fixed, wide- and local-area).
Many network capabilities, however, such as
mobility management, user security protection, and
resource allocation, addressed in GSM, are also some
of the critical requirements and issues in UPC net-
w o r k s of the future. GSM is expected to play a
major role in the specification of the standards
for UPC. In the United Kingdom, PCN is already
being designed and deployed with close adher-
ence to the GSM standards other than the differ-
ent operating frequencies (GSM operates at 900
MHz and the United Kingdom PCN operates at 1800
MHz). Generally, GSM may be viewed as a frame-
work for studying the functions and issues that
are specific to cellular type personal communication
networks, whatever the means of implementation
might be.
In applying and extending GSM to the next gen-
eration personal communication networks, how-
ever, one should be careful in differentiating some
of the implementation specifics unique to the GSM
network architecture and application from the func-
tions and issues that would be more or less gener-
ally applicable and relevant to cellular networking.
It is with this point in mind that the reader should
view GSM as a framework or platform on which
to build his or her vision of how GSM may be used
as a guide to design and build the next generation
networks. In that regard, a good understanding of
the GSM standards and network functions is
essential for the professional working on the next
generation personal communication networks. This
article is intended to assist with this objective.
The Cellular Concept
C
ellular mobile communication is based on the
concept of frequency reuse. That is, the limit-
ed spectrum allocated to the service is partitioned
into, for example, N non-overlapping channel
sets, which are then assigned in a regular repeat-
ed pattern to a hexagonal cell grid. The hexagon
is just a convenient idealization that approximates
the shape of a circle (the constant signal level
contour from an omnidirectional antenna placed
at the center) but forms a grid with no gaps or
overlaps. The choice of Nis dependent on many trade-
offs involving the local propagation environment,
traffic distribution, and costs. The propagation envi-
ronment determines the interference received from
neighboring co-channel cells which in turn gov-
erns the reuse distance, that is, the distance
allowed between co-channel cells (cells using the
same set of frequency channels).
The cell size determination is usually based on
the local traffic distribution and demand. The more
the concentration of traffic demand in the area,
the smaller the cell has to be sized in order to avail
the frequency set to a smaller number of roaming
MOE RAHNEMA is a
principal communication
engineer at Motorola Satellite
Communications.