Is ISDN still a contender?

The latest ISDN implementations: how they work and the challenges they face

By Robert E. Lee

November  1997
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Although ISDN has had a bumpy history, telcos and corporations certainly aren't giving up on it yet. In this overview of the most current ISDN implementations -- including ISDN over POTS and Always On/Dynamic ISDN -- we detail how each works to improve the speed of communications. We also ponder the difficulties each face as they attempt to make their way towards mass-market acceptance. (2,100 words)

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ISDN, Integrated Services Digital Network, is an often-maligned technology because it has been promoted as the networking technology of the future for the last 17 years. Starting at the base data rate of 144 (kilobits per second (Kbps), ISDN has had the capability to jump well past even the most advanced analog technologies coming to market today -- notably 56-Kbps technology. Although it has been a long and tortuous road, often riddled with horror stories of installation delays and nightmarish service interruptions, ISDN still has what it takes to be a prime choice for high-speed networking.

The latest implementations of ISDN -- ISDN over POTS, Always On/Dynamic ISDN, and PCS GSM 1900 wireless technologies -- are setting networking and voice applications on fire. While ISDN growth is headed towards 1.5 million lines in the United States this year, it still lags behind countries like Japan and Germany where there are more than eight million lines installed. These new developments should remedy that situation and push the U.S. into a leadership position over the next few years. Before we get into the details behind these new services, let's look at the basis of ISDN as a voice and data service.


What is ISDN?
Category 3 telephone lines (the basis for ISDN) have the capacity to transport data at rates greater than the predominate 28.8-Kbps technology available today. The explanation for the increase in data rates is two-fold: First, there have been improvements to the analog error correction and transmission technologies, and second, the use of the digital technologies built to create the early central office transports some 25 years ago. On the digital front, ISDN is based on the fact that voice sampling was designed around a 64-Kbps rate, opening the door to data transmissions much faster than any other technology at the time. Add to that the time multiplexing of the circuit, and designers were able to create two types of ISDN circuits: basic rate (BRI) at 144 Kbps and primary rate (PRI) at 1.544 megabits per second (Mbps). These rates are the standard for the U.S. and a few other countries. While there is another standard for primary rate of 2.048 Mbps, the basic rate remains the same everywhere.

At the core of the technology is the division of this bandwidth into channels: bearer channels and a data channel (Figure 1). In this context, it is the bearer channels that actually carry the high-speed data at 64 Kbps, while the data channel carries messages about the bearer channels. As each ISDN call is established, a series of messages are passed between the terminal adapter at your premise and the central office telephone systems that will interconnect you to your destination. These messages allow for each call to use a different service of ISDN -- voice, switch 56, clear channel 64, or X.25 packet data -- without requiring any central office reconfigurations of the circuit.

This messaging service allows the replacement of dedicated circuits with ISDN lines because the circuit can be dynamically redefined for each call. As you can begin to imagine, replacing a 56-Kbps circuit that remains idle for most of the day with a circuit that can provide 128 Kbps on demand for the data application, turn right around on the very next call and handle a fax transmission, then split into two remote access calls, and finish up with a video conference call is a very powerful capability.

ISDN is a dynamic technology and one that the industry, despite many previous reports, just won't give up on. That's why there are three new services available based on ISDN: ISDN over POTS, Always On/IP, and PCS GSM 1900. Each of the services puts ISDN in front of competing technologies like xDSL and cable modems. Right now, we'll take a detailed look at the first two, while reserving an in-depth report on PCS GSM 1900 -- the wireless digital phone services launched this past year in the U.S. -- for another article. (The early word on CS GSM 1900, however, is that ISDN service that will extend to the 38.4-Kbps wireless data rate. This will service portable computers and allow portable phones to become messaging workstations and text-only network clients.)

POTS, as you may know, stands for Plain Old Telephone Service. ISDN can now be delivered over an active POTS line, providing a sorely missed feature for ISDN users -- a phone line that will work when the power goes out. In addition to now providing this analog line with a BRI capabilities, ISDN over POTS can add multiple BRI lines to the circuit. That boosts the available bandwidth on a single Category 3 phone line to at least three bearer channels and a data channel. Under creative conditions, this line capacity could yield a data rate of 184 Kbps without compression. A typical configuration, using this approach would provide two BRI lines linked to a router, giving the router 256-Kbps raw data speed (1 Mbps with compression) -- all on the single two wire circuit.

This technology is made possible by a new piece of equipment provided by the telephone companies. Based on the same principles of digital subscriber lines (DSL), this device will multiplex the circuit into analog and digital circuits. While the technology is new and untried, manufacturers have claimed that it will be possible to put a single POTS line and up to six BRI lines onto a single wire pair. The telephone companies are, on the other hand, taking a conservative approach, entering the market with single or dual BRI configurations.

Presently, telephone companies are going to make use of the technology to relieve line congestion going to your premise. If you request an ISDN line, and your site is out of wire pairs, then this device can be installed to add this feature. Pacific Bell is at the forefront of implementing this technology, expecting to deploy it across at least 300 impacted customers in the state of California. Don't expect to request this feature, but do question your telephone company if it indicates ISDN is not available in your area because of a lack of wire pairs -- this technology will solve that problem.

While this represents the best news many organizations have heard about getting ISDN delivered into areas that are out of wire pairs, this technology imposes an old limitation -- that of distance. The ISDN line cannot exceed 18,000 feet from the telephone company central office. On the distance front, if wiring is not a concern, new repeater technology from Adtran, the market leader for ISDN repeaters, places the repeater at the end of the ISDN circuit. This placement allows for repeaters to be installed during normal circuit installation activities, without a crew entering a manhole in the middle of the circuit. This keeps installations on time, makes them easier to troubleshoot, and increases the distance from the central office to 30,000 feet.

Always On/Dynamic ISDN
Whoa, what an exciting technology! Is it the "always on" part or the word "dynamic" packs the biggest punch? In reality, it's both -- the power they represent individually and combined.

What can Always On/Dynamic ISDN do? To begin, remember that the D Channel is a messaging and X.25 packet data channel. The telephone companies have finally recognized that this resource has gone unused, perhaps because some basic changes are required in order to use the channel more effectively. Enter Always On/Dynamic ISDN. Always On refers to the ability of the ISDN terminal adapter (TA), the device at your end of the line, to open an X.25 circuit to your Internet service provider or corporate network and keep that circuit open permanently at a 9.6-Kbps data rate. Immediately, you can see that this open circuit allows for the free flow of data between the two ends, without additional dial-ins or authentication.

While 9.6 Kbps seems slow, in a complete digital circuit, the reliability of the line plus the fact that this is a true data rate (unlike analog modems that carry a 20 percent overhead) gives a higher throughput than the analog modem counterpart. But when the data rates exceed this capacity, the Dynamic ISDN feature enters the picture. When either end of the connection sees that more speed is required, a message is sent to the TA at your site, asking that one or two B Channels be brought up to increase the speed of the transfer.

In the telephone network, the X.25 packet data is routed out to the existing X.25 network the phone companies maintain. When the Dynamic ISDN feature is invoked, the B Channels at your site then dial through the normal phone network to make an ISDN connection to the ISP or corporate site, dropping the X.25 connection temporarily. This step invokes the per minute billing rate you will typically pay with ISDN, but the call will be dropped as soon as the transfer needing additional bandwidth is completed, reverting back to the X.25 connection. This is where the Dynamic aspect of this feature comes in.

One way to get around the per-minute calling cost when the B Channels are invoked is the use of Centrex lines. Centrex is a brand name used in many markets to refer to a virtual phone system in your office provided by your telephone company's equipment. With Centrex, each phone extension, analog or ISDN, is actually terminated at the phone company, not on your site. As a result, you can link multiple sites and incur no usage charges for calls between sites because all of your locations are seen as one logical site. This service is very dependent on local phone company tariffs, but is worth investigating.

Where's the downside?
Let's see, major manufacturers, including Shiva, Eicon, ITK, Jetstream, and Cisco are bought into this, with Ascend and 3Com quickly following suit. No downside there. All of the telephone companies are testing and planning deployments. Again, no downside there.

Oops, it seems that the ISPs aren't there yet. In order to make this technology work, you have to have both ends of the connection on board. Both Don Norman, FasTrak ISDN transaction services product manager at Pacific Bell, and Pete Castleton, director, ISDN product manager at Bell Atlantic, see the ISP market as a critical path to the success of this product. While each company can deliver solutions now, it is necessary to do two things to make it a reality in the timeframe they both agree on, which is market availability in the first half of 1998.

The first is the establishment of tariffs that price the X.25 packet services competitively. Pacific Bell has positioned the service as a normal part of the packet service option for BRI services, which carries a flat $5.00 per month fee. Bell Atlantic sees a longer-term issue where the service needs to be priced at a flat rate to a limit of packets then charged for the excess packets, in order to place the pricing in parity with the current X.25 network pricing schemes. There is incentive towards the flat rate only because the service provides relief to the congestion of the voice circuits due to ISDN and analog data calls.

The second is the educating of the ISP market to see the value-added services that can be delivered by this service. While there will be an impact to network management as IP addresses are permanently assigned or kept for long periods of time while users are logged into the network, the overall effect is an increase in the number of users per incoming port or inbound capacity because users are more effectively serviced.

For corporate networks that want to extend their remote access solutions to something more permanent and more reliable, this technology can be brought up as soon as the right equipment, ISDN lines and X.25 links, are installed at the central and remote sites.

The overall effect of the Always On/Dynamic ISDN will be to even out the information flow over remote access networks, whether it is a corporate network or the Internet. ISDN over POTS will increase the availability of ISDN to even more sites, making the choice for ISDN more prevalent.


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About the author
Robert E. Lee is a technology consultant, speaker, columnist, and author who has been in the computer industry for 20 years. He specializes in networking, Internet strategies, systems analysis and design activities, and has participated in the Windows NT and Internet Information Server betas since the start of those products. In addition to several other recent feature stories, Rob wrote the June 1997 SunWorld news story, "Cisco throws its support behind Microsoft's directory service vaporware."

[ Books]You can learn more about Robert E. Lee's The ISDN Consultant: A Stress-Free Guide to High-Speed Communications and Serving the Net: Using the Power of Microsoft Internet Information Server at Books. Reach Robert at

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