ISDN


 * Integrated Services Digital Network (ISDN**)

Integrated services digital network is a switched digital transmission network that tranfers digital voice and data over traditional copper telephone lines. Using the circuit switched network the ISDN is designed to initiate and terminate calls. The quality is generally better than analog signals and the faster rate of transmission makes the use of the network preferable to older legacy networks.

In recent years ISDN networks for the use of data transmission have been largely supplanted by the use of DSL networks which transfer data at a higher bit rate over a more advanced digital switched network. For telephone systems the ISDN network is actually a preferred way of transmitting calls since the amount of ring time is shorter for contract carriers of telephone service.The number of analog lines have decreased in the last ten years as ADSL and ISDN networks have replaced the use of analog signals as method of transmitting voice and data communications. In Western Europe in countries such as France and Germany ISDN is the preferred method of transmitting voice and data communications.

ISDN operates on a two swicthed channel network :In ISDN, there are two types of channels, //B// and //D//://B channels// are used for data--this refers both to voice and data information, and//D channels// are intended for signalling and control (but can also be used for data).B stands for //Bearer// and D stands for //Delta//.There are two kinds of access to ISDN:Basic Rate Interface (BRI) - consisting of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwitdth of 16 kbit/s. Together these three channels can be designated as 2B+D, and,Primary Rate Interface (PRI) Depending upon the existing system in place that will determine which channel and interface are used to route voice and data communications.

Switched networks like B-ISDN have been in existence since the end of the Nineteenth Century. A.G. Bell, in a vision to have telephone lines interconnected, conceived of the switched network for his young telephone company in 1876. Although the network which provided universal phone service to over 90 percent of American households by 1969 was an analog technology for switching and transmission, the basic structure of switched network was in place for networks that would come online towards the close of the Twentieth Century. More recently, the B-ISDN technology began to appear to consumers at the end of the 1970s with the advent of the Automated Teller Machine (ATM). For this first time data could be combined with voice signals thus being transmitted digitally.

Switched networks like B-ISDN have been in existence since the end of the Nineteenth Century. A.G. Bell, in a vision to have telephone lines interconnected, conceived of the switched network for his young telephone company in 1876. Although the network which provided universal phone service to over 90 percent of American households by 1969 was an analog technology for switching and transmission, the basic structure of switched network was in place for networks that would come online towards the close of the Twentieth Century. More recently, the B-ISDN technology began to appear to consumers at the end of the 1970s with the advent of the Automated Teller Machine (ATM). For this first time data could be combined with voice signals thus being transmitted digitally.

A digital switched network that sends data and voice signals it uses a digital telephone line to provide guaranteed, two-way bandwidth that's switched by the telephone network. Its Basic Rate service supports data capabilities like Internet access at speeds of up to 128 kbps (before compression). It supports packet operation to provide new capabilities like Always On/Dynamic ISDN - where the user can remain continually "connected" to the Internet without having a telephone call established. Other important ISDN data applications include Work At Home/Remote Access and Videoconferencing. ISDN supports digital voice calls to other ISDN and POTS (Plain Old Telephone Service) lines as well as a host of advanced voice capabilities (like Calling Number Delivery, Threeway Calling, Call Waiting, FAX transmission, etc.) ISDN’s Basic Rate can support two simultaneous voice calls on separate digital channels as well as multiple phone numbers. ISDN supports important small and large business services such as Key System operation and Centrex. When the user wants to connect to the Internet, the ISDN Terminal Adapter (TA) places 64 kbps circuit-data call to Internet Service Providers' router. (If the call cannot complete, 56 kbps calls are used.)

It provides clean, clear voice connections, and high-quality data connections, while simultaneously supporting features such as rich voice, data, and video calls. ISDN connects quickly to the Internet, without the wait associated with using a dial-up modem. Full duplex operation - bandwidth is the same in both directions; i.e., a 1 Megabyte file can be sent just as quickly from the user to the network as from the network to the user. There's also a wide variety  of user equipment available that work with ISDN existing technologies and systems. Cable services, although still under development, will eventually provide residential customers with WAN connections of speeds between 500 Kbps and 30 Mbps.

With existing analog service, a data connection is made modem-to-modem. At all times during the connection a carrier signal is produced by the modems to indicate the presence of a connection; the connection is lost if either modem cannot detect the carrier signal. The problem with maintaining a constant analog signal is that the service provider’s equipment has to continually process the information.

As more of the compatibility issues are resolved, ISDN services will increasingly become the service of choice for businesses and residential customers. The rise in popularity should drive down equipment costs and make connection devices readily available off-the-shelf. Although other digital services are competitive, most require expensive dedicated lines or possess inherent shortcomings like slow speed. ISDN technology is standardized and provides dynamic high speed end-to-end digital connectivity over the existing worldwide telephone network. ISDN continues to evolve with broadband ISDN availability in the near future.