Integrated Services Digital Network (ISDN) is a digital communications technology that enables a small business or an individual to connect directly to both the Internet and other sites/users. It provides a standard interface for voice, fax, video, graphics, and data – all on a single telephone line. The term “Integrated Services” refers to its ability to deliver two simultaneous connections, in any combination of voice, fax, data and video, over a single line. Multiple devices can be attached to the line, and used as needed. It is “Digital” because it is a purely digital transmission, as opposed to the analog transmission method used by the conventional telephone lines. “Network” refers to the fact that ISDN is not simply a point-to-point connection like a leased telephone line – ISDN networks extend from the local telephone exchange to the remote user, and include all the switching equipment between. If your ISDN equipment includes analog capabilities, you can also connect to telephones, fax machines, and analog modems – even though they may be connected to standard analog telephone lines.

There are two levels of service: the Basic Rate Interface (BRI) intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and D-channels. Each B-channel carries data, voice, and other services. Each D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe.

                One of the most common uses for this technology today is videoconferencing. By using from one to four BRI lines, a videoconference application, higher connection speed translates to higher resolution and video frame rates. The telephone company infrastructure allows these connections to be made in a similar fashion to dialling a telephone.

               Other uses of ISDN includes: studio quality audio transmission for broadcast; image archives; preparation of printed materials; electronic manuals – on-line; stock quotes for brokers; and credit card authorisation. One of the key advantages of ISDN is its flexibility – you can use it in several different ways, depending on your needs.

               The ISDN is illustrated here in relation to the OSI model

              In order to understand what ISDN is, you have to understand a bit about modern telephony. ISDN is concerned (almost) entirely with the customer's part of the network. ISDN gets the data from you, to the phone company in a standard way. What they do with it in order to get it to its destination is entirely up to them. This is a very simple, important concept. If you understand this, then when someone says something like "SONET is the future of the modern telephone network" you'll know that they're talking (mostly) about what goes on inside the phone company, and between phone companies. They are probably right, but it is also true that "ISDN is the future of the modern telephone network" especially if you mean B-ISDN. They're just the future of different parts of the telephone network. 

               

         

                ISDN , therefore, was a great technology – driven service that didn’t really solve a business (or home) needs. It is little wonder that ISDN is not widely implemented or used. The ISDN primary rate is used extensively in call centers, utilizing computer telephony integration to maximize their efficiency.

sources:

        http://en.wikipedia.org/wiki/Integrated_Services_Digital_Network

        http://www.protocols.com/pbook/isdn.htm

        http://searchenterprisewan.techtarget.com/definition/ISDN

        http://www.javvin.com/protocolISDN.html