Connectivity by Rawn Shah

More on the new communications technologies: ISDN

We define the two forms of ISDN and examine how each works

December  1997
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We began our discussion of new communications and transmission technologies last month, describing the emergence of ISDN, digital subscriber lines (DSL), cable modems, and other high-speed transmission systems that hope to replace analog modems and T-1s. Although some of these "new" technologies have been around for well over a decade, their widespread use only began in the mid-1990s. We'll now start looking more closely at three end-user technologies primarily intended for home, small office/home office (SOHO), or small business applications: ISDN, DSL, and cable modems. First up, ISDN. (1,500 words)

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Integrated digital subscriber network (ISDN) services is a grandfather to DSL and cable modems, having been around for nearly two decades. Originally designed in the early 1980s as a low cost high speed digital data and voice service to be provided by phone companies everywhere, ISDN has had a troubled existence. Part of this can be blamed on the time-consuming, design-by-committee process of ISDN. It has nonetheless survived, and, while it's barely emerged in the U.S., it has quite a following in Europe and Japan.

Many are already familiar with ISDN, its use, and availability, so we won't go too deeply into its intricacies. There are two forms of ISDN lines in wide use: Basic rate interface (BRI) and primary rate interface (PRI). Actually, the original ISDN development called for a third form of ISDN service, known as broadband ISDN. B-ISDN as this is known has resulted in asynchronous transfer mode (ATM) and is well serviced by ATM and frame-relay technologies.


Connecting with BRI and PRI
The BRI can be viewed as a common service for small businesses and homes. It consists of three channels: two bearer channels or B-channels and one data channel or D-channel. The B-channels actually carry the bits and bytes for data communications, and the D-channel is primarily intended for signaling between the user and the ISDN telephone switch. Each B-channel provides a 64-kilobits-per-second (kbps) service. This is precisely the amount of bandwidth required to carry an uncompressed line for voice communications in digital rather than the traditional analog format. The D-channel provides an additional 16 kbps of bandwidth which goes unused by most applications. In total, an ISDN line can provide two digital voice lines, one digital voice, and one 64-kbps digital data line, or one combined digital data line at 128 kbps. In the future we will see ISDN "modems" take advantage of the D-channel in the BRI service, giving a total of 144-kbps bidirectional digital data service.

PRI ISDN is similar to BRI except that it has more channels. The typical PRI service in the U.S. has 23 B-channels (each 64 kbps) and one 64-kbps D-channel. This fits the U.S. signaling and cabling format originally used by T-1 lines. In total, a PRI line provides 1472 kbps of throughput in the B-channels; D-channel in the U.S. is reserved for switch communications. In Europe, 30 B-channels and one 240-kbps D-channel allow a data bandwidth of 1920 kbps.

An ISDN "modem," more correctly called a terminal adapter (TA), provides local users with a means to connect their analog devices or computer networking equipment to the ISDN line. Today all connections to the phone company are strictly designated with separate connectors, so that there is no confusion between the customer equipment and that which belongs to the phone company. In the U.S., the incoming ISDN line from the phone company ends with the U interface. You need to connect a small device to this interface known as the Network Termination 1 (NT-1). This device provides the proper connector, known as the S/T Interface, for ISDN devices (telephone, fax, or TA) to hook up to. In the U.S., the NT-1 is not provided by the phone company; in Europe and other parts of the world, it is. The only difference is that you need slightly different ISDN equipment depending on where you are. Many ISDN computer hardware vendors integrate the TA and NT-1 devices into a single box for the U.S. market so that you only need one connection point.

From the network layer's point of view, the two B-channels in a BRI are two separate physical channels of transmission. This poses a problem because most network point-to-point links only have one channel. Directing Internet Protocol packets over these two lines becomes a hassle if you deal with these as two separate channels. However, using a standard called Multilink PPP and an ISDN mechanism called BONDING, it is possible to combine these two channels as one logical channel. With Multilink PPP this is done between the data link and network layers. With BONDING it is an aspect of the ISDN TA and occurs in the data link layer. Both essentially perform the same tricks -- the exception is that the Multilink PPP standard is designed to operate with any two types of physical channel, such as two analog lines, two T-1s, an ISDN B-channel, and an analog modem line, etc. Unfortunately, it isn't as easy to find many applications of Multilink PPP.

One of the initial applications that ISDN was primed for was digital videoconferencing. Many ISDN vendors such as Intel, AT&T, and PictureTel have long had products for this market providing desktop or small group videoconferencing hardware and software. Although software-based videoconferencing over the Internet is now available, the ISDN hardware-based systems continue to provide a higher quality system than ones based upon analog modem connections. This of course is mostly due to the increased bandwidth.

ISDN is most often used for Internet connectivity using BRI ISDN for 128-kbps digital connections. PRI ISDN has been available almost as long, but it is often not price competitive with other forms of high-speed connection (like dedicated and frame-relay T-1s). Finally, superior transmission technologies like ATM are taking over the high-speed market for large digital-only voice and data systems.

One of the unfortunate circumstances that has led to slow adoption of ISDN is the need to use different forms of telephone switches. Although most modern switches readily provide ISDN services inherently or though special modules, there are billions of dollars worth of phone switches already in place which don't, and they would need to be replaced. While most of the communications in phone switches are pure digital, most telephone users still have analog voice sets.

Furthermore, there's been no real "killer app" for ISDN yet. Videoconferencing was a nice thought, but it didn't take as well as most ISDN adopters thought it would. Before the widespread use of the Internet, no one had use for a pricey 128-kbps connection that required specialized hardware. Now that we've reached the limits of analog modem technologies, ISDN is finally becoming popular and justifying its costs to local exchange carriers (LECs). BRI ISDN, that is.

Interestingly enough, one of the saviors of PRI ISDN might just be the analog modem industry. Having reached speeds of 33.6 kbps, analog modems have no more room to grow. Vendors have now come up with a hybrid system of analog and digital technologies. The two popular brands known as x2, from U.S. Robotics/3Com, and KFlex56, primarily from Rockwell International, provide a means of bumping analog communications to the theoretical area of 56 kbps. An analog connection is used between the user and the telephone company, and then digital voice lines connect the rest of the way to the ISP. These voice lines primarily come in the form of ISDN PRIs. One bit of warning -- the maximum bandwidth of this hybrid form is about 40 to 45 kbps and no where near 56 kbps.

U.S. LECs provide ISDN services in practically every state -- even Hawaii where the separation between islands has been problematic for communications deployment. Prices range from $20 to $150/month for a BRI line and $500 to $4000/month for a PRI line. In some cases, additional charges by the kilobyte or megabyte of data are also levied. As far as I have seen, the best deals for ISDN service come from the competitive access providers, with the exception of Pacific Bell territory.

What's in the future?
ISDN is here to stay in the U.S. and around the world. Most other "new" technologies for the small business or home user are not yet widely available or are still in flux. ISDN is mostly a dead end for high-speed communications because most of the focus on technological development has shifted to ATM and other optical carrier systems. Still, ISDN will be around for several more decades, and several new developments may become more commonplace in the business market.

Last month, we featured an interesting article on the future developments of ISDN technology titled "Is ISDN still a contender?" by Robert Lee. Please do take a look at this article as it provides a useful insight into where ISDN is going.


About the author
Rawn Shah is vice president of RTD Systems & Networking Inc. He has worked with many different aspects of the LAN world and is currently strongly investigating the world of ATM and DSL and their implementation and implication on the future of voice, video, Internet, and data networking. Reach Rawn at

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