The Telephone Network

The telephone network began in the late 1800s which was referred to as plain old telephone system (POTS). It was orignally analog, but with the advancement in computer technology the network started to carry data as well as voice in 1980's. It is now both digital and analog.

Major Components

The telephone network is made of three major components - the local loops, the trunks and the switching office.

The local loop connects the subscriber to the nearest end office (or local central office) through a twisted-pair cable. It has a bandwidth of 4000Hz.

The trunk is a transmission media that connects switching offices. It handles a lot of connections through multiplexing. these are usually optical fibers or satellite links.

The switching office establishes a connection between two subscribers. The connection between two subscribers is not permanent and will only be made upon request. Connections are limited by the total bandwidth of a transmission media therefore having permanent idle lines would limit the services of the network.

LATA

The local telephone network is referred to as Local Access Transport Areas (LATA). LATAs are made up of multiple local loops connected to a tandem office. Services of Comon Carriers (telephone companies) within a LATA are called intra-LATA services. These carriers are refered to as Local Exchange Carriers (LEC). LEC has to types. The Incumbent Local Exchange Carriers (ILEC) is the original company that set up the LATA. To avoid cost for new cabling, Competitive Local Exchange Carriers (CLEC) were allowed to use the LATA of the ILEC for their own services.

Services between LATAs (Inter-LATA services) are handled by Interexchange Carriers (IXC), commonly referred to as long-distance companies. LECs are also allowed to become IXCs. To allow multiple IXCs to use a LATA, a Point of Presence (POP) switching office is created for each IXC.A caller, who needs to connect to a receiver in another LATA, first connects to an end switch then, either directly or through a tandem office, to a POP of the caller's choice. The call then goes from the POP in the caller's LATA to the POP of the same IXC in the reciever's LATA then down to the switching offices and finally to the telephone of the receiver.

Signals

Voice communication used analog signals in the past, but is now moving to digital signals. On the other hand, dialing started with digital signals (rotary) and is now moving to analog signals (touch-tome).

Services

A common carrier may provide Analog and/or Digital services.

Analog Services

Analog Switched Service is the familiar dial-up service most often encountered when a home telehpone is used. Analog Switched services include multiple optional services. Local Call service is normally provided for a flat monthly rate. Toll call services (calls that need to pass through the tandem offices) are charged per call. 800 services are free for the caller and is charged to the callee. 800 services are usually used by companies to encourage customers to call. 900 services are charged to the caller and is normally more expensive than toll call services. This is because the carrier charges two fees - first is charge of the carrier, second the charge of the callee, for example a software company that charges for technical support.

Analog Leased Service offers customers the opportunity to lease a dedicated line that is permanently connected to another customer. Although calls still pass through the switching office, no dialing is needed.

Digital Services

Switched/56 Service is the digital version of the analog switched service. It allows a data rate of up to 56 Kbps. Sender and reciever must both subscribe to this service for connection to happen. Because the line is already digital, the users do not need a modem. All they need to use is a Digital Service Unit (DSU) which changes the rate of the digital data sent by a subscriber to 56Kbps and encodes it in a format used by the carrier.

Digital Data Service is the digital version of an analog leased line with a data rate of up to 64Kbps.

Forouzan, B. (2003). Data Communications and Networking. New York: McGraw Hill.