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.
 DIGITAL SERVICES
 DIGITAL SERVICES