Overview

This chapter will describe the concept of the Integrated Services Digital Network (ISDN) and will focus specifically on the following topics:

• Its original goals

• How it can be used

• Some of the alternatives

The world's telephone companies conceived ISDN in the early 1980s as the next generation network. The existing voice networks didn't deal well with data for the following reasons:

• One had to use modems to transmit data.

• The data rates were around 9600 bps.

• Connections (worldwide) were unreliable.

Not only would the connection drop without notice, but also the error rate was high enough to require a complex protocol to recover from errors. The back end of the network, that is, the interoffice trunks, were practically all digital, with more being installed daily. The switching systems were becoming digital just as quickly. It was expected that the only part of the network that would still be analog was the local loop (our infamous last mile). It was a logical step to provide digital capability already in the network directly to the customer.

This created what is always called the local loop problem. The problem is that much of the local loop plant was installed between 40 and 60 years ago and had been designed for normal voice communications only. The local loop problem therefore is how to run high-speed digital data on the local loop. Several solutions to the local loop problem are discussed in this book, and the ISDN solution is a little different from the...