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Telephone networks use circuit switching The telephone network had its beginnings in the late 1800s The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice With the advent of the computer era, the network, in the 1980s, began to carry data in addition to voice During the last decade, the telephone network has undergone many technical changes The network is now digital as well as analog
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The telephone network, as shown in Figure 91, is made of three major components: local loops, trunks, and switching offices The telephone network has several levels of switching offices such as end offices, tandem offices, and regional offices
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One component of the telephone network is the local loop, a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office The local loop, when used for voice, has a bandwidth of 4000 Hz (4 kHz) It is interesting to examine the telephone number associated with each local loop The first three digits of a local telephone number define the office, and the next four digits define the local loop number
Trunks are transmission media that handle the communication between offices A
trunk normally handles hundreds or thousands of connections through multiplexing Transmission is usually through optical fibers or satellite links
To avoid having a permanent physical link between any two subscribers, the telephone company has switches located in a switching office A switch connects several local loops or trunks and allows a connection between different subscribers
After the divestiture of 1984 (see Appendix E), the United States was divided into more than 200 local-access transport areas (LATAs) The number of LATAs has increased since then A LATA can be a small or large metropolitan area A small state may have one single LATA; a large state may have several LATAs A LATA boundary may overlap the boundary of a state; part of a LATA can be in one state, part in another state
The services offered by the common carriers (telephone companies) inside a LATA are called intra-LATA services The carrier that handles these services is called a local exchange carrier (LEC) Before the Telecommunications Act of 1996 (see Appendix E), intra-LATA services were granted to one single carrier This was a monopoly After 1996, more than one carrier could provide services inside a LATA The carrier that provided services before 1996 owns the cabling system (local loops) and is called the incumbent local exchange carrier (ILEC) The new carriers that can provide services are called competitive local exchange carriers (CLECs) To avoid the costs of new cabling, it
was agreed that the ILECs would continue to provide the main services, and the CLECs would provide other services such as mobile telephone service, toll calls inside a LATA, and so on Figure 92 shows a LATA and switching offices
Intra-LATA services are provided by local exchange carriers Since 1996, there are two types ofLECs: incumbent local exchange carriers and competitive local exchange carriers
Switching offices in a LATA
Tandem (toll) offices
Communication inside a LATA is handled by end switches and tandem switches A call that can be completed by using only end offices is considered toll-free A call that has to go through a tandem office (intra-LATA toll office) is charged
The services between LATAs are handled by interexchange carriers (IXCs) These carriers, sometimes called long-distance companies, provide communication services between two customers in different LATAs After the act of 1996 (see Appendix E), these services can be provided by any carrier, including those involved in intra-LATA services The field is wide open Carriers providing inter-LATA services include AT&T, MCI, WorldCom, Sprint, and Verizon The IXCs are long-distance carriers that provide general data communications services including telephone service A telephone call going through an IXC is normally digitized, with the carriers using several types of networks to provide service