THE TELEPHONE
SYSTEM:
The traditional telephone system
(even if it someday gets multi gigabit end to-end fiber) will still not be able
to satisfy a growing group of users. People now expect to make phone calls from
airplanes, cars, swimming
pools, and while jogging
in the park. Within a few years they will also expect to send e-mail and surf the Web from all
these locations and more. Consequently, there is a tremendous amount of
interest in wireless telephony.
[The telephone system is the most basic application of the multiplexing.
It includes common carriers that offer local and long distance service to
subscribers. These carriers also include local companies. The line connecting a
subscriber to that of a network is called a Service line.]
Wireless
telephones
come in two basic varieties: cordless phones
and mobile phones (sometimes
called cell phones). Cordless phones are devices consisting of a base
station and a handset sold as a set for use within the home. These are never
used for networking, so we will not examine them further. Instead we will
concentrate on the mobile system, which is used for wide area voice and data
communication.
Telephone services are basically
categorised in two classes:
1: Analog services.
2: Digital
services.
[I] ANALOG
SERVICES:
Analog services are further subdivided into –
Analog switched services and Analog leased services.
(a)
Analog
switched services: It is the most familiar dial
up service generally used in a home telephone. It uses two wire or four wire twisted pair
cables to connect the subscriber’s handset to the network through an
exchange. The connection is called Local loop. (Sometimes referred to as
‘Public switched telephone network’ or PSTN). The bandwidth of the signal is
usually between 0 to 4000 Hz. When the caller dials a number, the call is
conveyed to a switch or a series of switches at the exchange. The appropriate switches
are then activated to link the caller’s line to that of the person being called.
The switch connects the two lines during the duration of the call.
(b) Analog leased services: It
offers customers the opportunity to lease a line, sometimes called a dedicated
line that is permanently connected to another customer. Even though the
connection always passes through the switches in the telephone network,
subscribers experience it as a single line because the switch is always closed,
i.e. no dialling is needed.
Telephone carriers also offer a
service called the Conditioning
which means improving the quality of a line by lessening attenuation, signal
distortion or delay distortion. Conditioned lines are analog, but their quality
makes them usable for digital data communication if they are connected to
modems.
[2] DIGITAL SERVICES
[2] DIGITAL SERVICES
