Types of Networks

Introduction to Telecommunications Networks
In the world of telecommunications (telecom), there are many ways and means of
exchanging information, and the nature of that information can be just as varied. As
a result, there are many different kinds of telecommunications networks: fixed and
mobile, data and voice, analog and digital. Today, networks such as the Internet allow
the transfer of many different types of data, regardless of the underlying technology.
Fixed Communication Networks
The term “fixed” usually refers to the last mile in the communication link. This
fixed part connects the user’s equipment to the telecom network using some type
of physical medium. Copper wires are the most common form of connection but
they may also include coaxial cable or, less commonly, fiber optics. In some cases
it is possible to provide a fixed connection using a radio connection. These types of
systems are often referred to as wireless local loop (WLL)

Public Switched Telephony Network (PSTN)
The PSTN (Figure 1.6) is the publicly available dial-up telephone network. It is a
complex interconnection of switching centers and end users that offers a voice connection
between any two valid subscribers. In addition, international connections
to other countries are also possible. In engineering parlance, the PSTN is sometimes
referred to as POTS, an acronym for plain old telephone system.

Connections to the PSTN
As telephone communications evolved, telephone exchanges were used to manually
connect users together. A pair of wires ran (typically overhead) from each subscriber
to the exchange where these were terminated at a switchboard. An operator then
interconnected subscribers in a manual process using short cables. As the number
of subscribers increased, the demand for physical space to house the switchboards
and operators also increased. This led to massive exchange buildings and armies of
staff required to operate the system.
The development of automatic exchanges (or switches) using electromechanical
devices simplified the interconnection of subscribers and reduced the amount of
space and manpower required. This was taken a stage further by the implementation
of digital switches. The only part of the network that remains in its original
form is the pair of wires that forms the final connection to the subscriber, commonly
known as the local loop.

Analog to Digital
Speech by its very nature is an analog signal, and early systems such as the manual
network mentioned above merely carried the signal from end to end. In modern
networks, this signal is digitized to preserve audio quality and then carried across
the network using digital links and switches (Figure 1.7).
Integrated Services Digital Networks (ISDN)
The modern digital telephony network not only carries voice, but also data at 64
kbps. This service is called an integrated digital network (IDN). As discussed above,
there remains a short local loop of analog service over a pair of wires. The next stage
is to take the digital channels all the way to the customer so that all services can be
integrated on one bearer. This is the Integrated Services Digital Network (ISDN).
The ITU-T defines ISDN as:
A network evolved from the telephony IDN that provides end-to-end
digital connectivity to support a wide range of services, including voice
and non-voice services, to which users have access by a limited set of
standard multipurpose customer interfaces.