GSM

Unlike IS-136 or IS-95, GSM was designed from scratch as a complete system,
including air interface, network architecture, interfaces, and services.
In addition, the design of GSM included no compatibility with existing analog
systems. The reasons for this included the fact that multiple analog systems
were used in Europe and it would have taken great effort to design a
system that would provide backward compatibility with each of them. The
lack of compatibility also meant that carriers had a greater impetus to build
GSM coverage as extensively and as quickly as possible.
In the following sections, we spend some time describing the GSM architecture
and functionality. The main reason is because GSM is the foundation
of a number of more advanced technologies such as the General Packet
Radio Service (GPRS) and the Universal Mobile Telecommunications Service
(UMTS). An understanding of GSM is necessary in order to understand
those technologies.