Transmission

The transmission medium can take many different forms, such as (1) copper wire,
which has been used since the early days of communication; (2) optical fiber, which
is a relatively new medium and is increasingly being used; and (3) radio, which also
has been used for many years. Each of these media can be bi-directional, that is,
allowing information to pass in both directions.

Copper may be limited by bandwidth, which limits the amount of information
that can be transmitted in a given time, compared to optical fiber, but its cost is
low. In contrast, optical fiber is relatively expensive but can carry more data in a
given time. Both copper wire and optical fiber cable are more reliable than radio,
but radio has many other advantages.

Radio transmission is a very versatile medium, ideally suited to a mobile environment,
but its bandwidth is extremely limited and therefore its total capacity is
limited. It can also be adversely affected by atmospheric conditions.

Speed of data transfer, error rates, and other key characteristics determine which
transmission medium will be used in particular circumstances. Information can be
represented in many different forms. To ensure compatibility across systems, standard
transmission systems and techniques have been specified in many instances.

Switching
To enable the placement of a call from the originator through the network and
on to its final destination, that call must pass through several switches or routers.
Switches or routers allow great flexibility in connecting transmission
resources. This gives rise to endless possibilities for end-to-end connections between
users. Users may be located nearby or be very remote, but the switches or routers
within the network will ensure that the information passes over the required transmission
resources to provide end-to-end connectivity.

The number of switches or routers needed in an end-to-end connection will
vary, and will depend on many factors, including network topology (the configuration
of the various elements), the geographical distance between the user terminals,
and the capacity of the switch or router and associated transmission media.

Signaling and Control
To ensure the establishment of end-to-end connections with the required quality
of service, control information must be passed between users and the network, and
between network elements. This control information comes in many
different forms, both simple and complex.

The information that must be included in the control information varies from
system to system, and also at different points within the same system. Control
information is required to provide services of many types, not just end-to-end connections.
Control data in telecommunications networks has traditionally been
called signaling.

Billing
Increasing network sophistication and a corresponding increase in service choices
and methods of service provision, have led to an increased requirement for flexible
and more complex billing systems. This is confusing for network operators and
customers alike, and a great deal of work is underway to provide choices that are
achievable, simple to implement, and, very importantly, reflect the requirements of
the customer. Service charges that are simple to understand, as well as flexible payment
options, are of increasing importance.