User`s guide

In order to understand how voice communications would be treated in a data network, let’s  rst take a

look at the typical topologies of voice and data networks running separately.

Understanding Voice Networks

Public Switched Telephone Network

For the past 100 years, people have relied on the Public Switched Telephone Network (PSTN), otherwise

known as POTs (Plain Old Telephone Service), for voice communication. Although it is very reliable,

it utilizes a very basic and inef cient method for making a connection called circuit-switching. To

illustrate a circuit-switched connection, the following is the process that takes place when you make a

phone call. First, you dial the number of the party you wish to talk to. The call is then routed through

the switch at your local central of ce (CO) to the party you are calling, opening the circuit. Depending

on location, the call may be routed through multiple CO connections opening a circuit through each

one. During the call, the routed line is dedicated to the two parties. This means no other information

can travel over the line, even though there is plenty of bandwidth available.

Private Branch Exchanges

In a corporation, voice communication has traditionally been handled by proprietary platforms called

private branch exchanges (PBXs). A PBX is essentially a switch used to connect a number of phones

(extensions) to each other and to one or more outside phone lines. To illustrate how the PBX works,

when a user picks up a phone (extension) a PBX dial tone will be heard. At this point, the user can dial

any other extension on the PBX. To reach an outside line, the user typically dials a “9” (or presses a pre-

programmed button) to access the PSTN network.

A PBX was originally designed to save the cost of requiring a line for each user to the telephone

company’s central of ce (CO). In effect, the PBX acts like a mini-CO, owned and operated by the

corporation.

A limitation to the traditional PBX is that it is a location-centric platform. The networking options to

extend voice communications to other remote locations (e.g. branch of ces, sister companies, satellite

of ces, telecommuters, etc.) are few and can be costly.

One option, if the remote of ce is large enough, is to add another PBX at the remote site, and set up

a private network between the two with leased lines (tie lines) purchased from the phone company.

To make an outgoing call to the remote of ce, the user would dial an “8” (or press a pre-programmed

button) to access the tie line and then dial the remote of ce extension. Tie lines, however, are expensive

because they add extra monthly phone charges. And, the telecommunications manager may be faced

with the challenge of tying together two dissimilar proprietary PBX systems that were not designed to

be networked together.