Chapter 10: Integrated Services Digital Network

ISDN was a concept developed by the consultative on International Telegraph and Telephone(CCITT).The CCITT has since changed its name to the ITU-T, but still the same, made up primarily of representatives of the world's government owned monopoly carriers.

The Application of the ISDN Interface are: Multiple channels is to provide access to possible. Telephone instead of the telephone conversation being analog from the handset to the central office where it becomes digitized. Digital fox fax machines now have digital. Analog fax use a modem, so it has to plug into the telephone that would take the analog modem tones and digitize them at 64kbps. Computer/Video Conferencing can use one of the 64kbps or bond both bearer channels together. Signaling setting up and tearing down of the switched bearer channels. Telemetry the concept is that many household devices can be connected to the data channel.

The BRI interface offers 2B+D. the Primary Rate Interface(PRI) provides 223B+D and, in this case all are 64kbps channels. What technology provides 64kbps channels. of course the answer is T1. The physical interface for PRI then is simply a T1.technically, it is extended super-frame framing (ESF). how is PRI different from an ordinary T1?  Answer is that at the physical interface it isn't any different. Channel 24 becomes the signaling channel. One might say that we have added common channel signaling to the T1. PRI is used for a PBX interface where the full signaling capability of the D channel is needed.

Signaling on the D channel -  The signaling packets on the D channel are the same for BRI and PRI. In the early days of defining the interfaces, An argument arose about another grant direct access to the signaling system by the customers. As we indicated, Signaling system 7 (SS7) is the mechanism for managing the network and it's logical simply let the D channel use SS7 packets. D channel signaling packets require a small amount of conversion to change them from D channel signaling packets to SS7 packets.

Installation Problems - Installing ISDN for other that vanilla applications can be frustrating and time consuming. The carriers come from a long tradition of providing voice services. Their entire management system and technical system is design around providing highly reliable and profitable voice services.

ISDN is digitally based, the local distribution is digital and the switching is digital. Historically, the telcos have strictly partitioned the knowledge of each technical specialty.

Therefore, specially trained ISDN technicians are required to deal with the local loop. They must know the parameters for operation and be able to test it out. In old days the installers would hook ups butt to the line and listen. He could tell about the local loop by the background noise. sorry, you cant hear digital. Therefore, the technician has to be trained in handling a special piece of test equipment to make sure the line will support the data rate.

ISDN was great technology driven service that didn't really solve a business (or home) need. it is little wonder that ISDN is not widely implemented used but there are as we have noted some clear exceptions. The most notable is video conferencing . Internet access is also a possibility, but ISDN can't compete XDSC technology in performance for the cost the ISDN primary rate is used extensively in call centers, utilizing computer telephony integration to maximize their efficiency. PRI is also used in PBX applications, where the digital PBX can make use of network control and status information provided by the PRI.

Sources: http://netcert.tripod.com/ccna/wan/isdn.html,http://en.wikipedia.org/wiki/ISDN