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ISDN: Still a viable choice.

Understand how this "older" technology works when making purchase decisions.

Integrated services digital network (ISDN)--existent for 15 years, with actual realistic implementation even less--is certainly not a cutting-edge technology. However, ISDN provides a critical service to its customers: affordable high-speed last-mile connectivity. ISDN spawned industries such as videoconferencing and immensely helped others like SOHO (small office-home office) and Internet access.

Many technologies transmit information across the world, but few are connection-oriented and have an entrance price tag that is palatable. ISDN, a digital service product of the telephone companies, is designed to alleviate the difficulties of a network designed for voice transmissions and meet the aforementioned criteria. ISDN is based on the development of pulse code modulation (PCM), which samples existing analog signals at a rate of 64 kbps. The development of PCM led to the development of the current digital telephone network.

Information carried by ISDN is defined in the user-network interface, consisting of two types of channels based on a fixed bit rate. These rates are defined as basic rate interface (BRI) and primary rate interface (PRI). BRI is able to transmit data at 144 kbps simultaneously, while PRI operates at 1.54 Mbps. Each of these interfaces is composed of channels that determine the type of interface used.


Bearer "B" channels consist of 64 kbps, which carry the voice, data, video, facsimile, or images across the interface. Delta "D" channels are 16 kbps or 64 kbps out-of-band signaling channels used to control the ISDN circuit. An attribute of this signaling is that it does not disturb established connections and requires minimal call set-up time. In addition, there are "E" channels used for signaling circuit-switched traffic where multiple access points are encountered and "H" channels that provide user information at PRI.

BRI ISDN is configured using two 64 kbps B channels and one 16 kbps D channel, while PRI ISDN consists of 23 64 kbps B and one 64 kbps D channel. These channels can be inverse multiplexed at the user site to get information speeds--in 64 kbps chunks--from 64 kbps to 1.5 Mbps. ISDN is more than 10 times faster in information transfer than typical dialup capabilities.

Since ISDN is a digital service and capable of high rates of speed over the same pair of copper wires that service the POTS infrastructure, combining voice and data over a common medium results in improved service capacity and higher quality. ISDN originates at the customer premise equipment (CPE) and continues down a single pair of copper wires to the central office (CO).

Once at the CO, the signal is broken down into three parts (2B+D). The two B channels can be bound to the same location or different locations; the separated transmission arrives at the final destination; and, finally, the signal is received by other ISDN equipment.

When purchasing an ISDN connection, customers need to understand the choices associated with this digital service. The configuration by the local carrier (telco) dictates how the ISDN service will work. The many types of configurations or "provisioning" provided by the telco determine how the lines will operate on a day-to-day basis. The network configuration is based primarily on two parts of the telephone network--the phone company's and the customer's part. The telephone company is responsible from the demarcation point through the CO to the callee's location. The customer's part is the ISDN device (phone, computer, or videoconference equipment), wiring, and connection to the demarcation point.


In reviewing ISDN service providers, we identified 15 industry-recognized organizations. Next, a survey instrument was developed based on criteria we felt helpful to an ISDN shopper including channel configuration options, pricing structures, available speeds, and customer-service criteria which may set the service providers apart.

Channel configuration options define how the user can use and separate the ISDN "B" channels, crucial information is critical for daily-use future scalability of the customer's network.

Service criteria defines what elements are an integral part of the providers offerings. Available speed information provides the scope of offerings (BRI or PRI of 56K switched) that the service provider has.

Price is typically another critical variable for the customer. Provider pricing schemes are vastly different, and manufacturer suggested retail prices are almost always negotiable. ISDN pricing is comprised of four basic elements: Installation costs; usage charge per month (flat/variable/ongoing); customer premise equipment (CPE) cost; and ISDN speed (BRI or PRI).

ISDN meets some unique needs of the market. Developing technologies may one day shadow and replace ISDN, but the current installed base and large, available telephone infrastructure provide a sense of permanence to ISDN. With its low startup and operational costs, ISDN fits the bill. Whatever the future technology for highspeed local access, a wise choice needs to be made now in order to properly and cost-effectively implement ISDN service.

Kovac is a professor in the center for information and communication sciences program at Ball State University, Muncie, Ind. The Applied Research Institute conducts unbiased research in the telecommunications field. Research assistants for this project included Kevin Stouder, Khari Mack, Jerry Walker, and Darren White.
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Title Annotation:Technology Information
Comment:The integrated services digital network (ISDN), invented by telephone companies, offers affordable high-speed last-mile connectivity.
Author:Kovac, Ron
Publication:Communications News
Geographic Code:1USA
Date:Jun 1, 2000
Previous Article:Outsourcing WAN management.
Next Article:Wide Area Networking Buyers' Guide.

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