Communications Technologies

Remember when Teletype machines ran at 110 baud? Were you around when 300 baud modems were considered "high-speed"? Since the development of the World-Wide Web, and the subsequent explosive growth in the number of Internet users, demand for higher speed access has been insistent. It was only a few short years ago that 14.4K modems were the ultimate speed demons. They were rapidly supplanted by 28.8K and then 33.6K modems. Modem technology hit the wall at 56K, since higher speeds cannot be carried over analog lines. Of course, it didn't help that there were two competing standards for 56 K modems; X2 and 56Flex. Your choice of which to purchase was often dictated by the equipment installed by your ISP. The recent ratification of the ITU V.90 specification should eventually provide equipment interoperability, although many users will never experience the advertised speed.

 In order to break the 56K speed limit, the only choice is to use digital communication technology. Although slow to see widespread deployment, ISDN (Integrated Services Digital Network) provides powerful telephony capabilities as well as speeds of up to 128K and beyond. Cable modems promise phenomenal speeds, at least on downloads, but the technology is still very new and not widely deployed. The Telcos are fighting back with ADSL (Asynchronous Digital Subscriber Line) service which can be carried over existing copper telephone lines. Although slower than cable modem speeds, ADSL does not preclude the continued use of the telephone line for analog conversations.

The veracity of the previous paragraph has diminished recently, especially in the larger markets. Both cable modems and ADSL are being aggressively marketed in the Atlanta area. Media One, and even little Smyrna Cable, have cable modems on the market. In certain areas of town, ADSL is available from Bell South. The limited coverage reminds me of the ISDN situation, but at least it is available. Not only that, Bell South is pushing what it calls "Digital Television", which will directly compete with the cable providers. If I wanted a single provider for both internet access and television programming, I'd probably go with Bell South over Media One. I would definitely choose it over Smyrna Cable, but it's only available for single-family dwellings. On second thought, maybe the situation hasn't really improved very much.

The speed performance of these two technologies is surprisingly close, according to recent testing. It also uncovered interesting variations in the speed provided by the competing technologies. Cable modems, for example were slower during the evening while ADSL was slower during the day. These variations make sense, when you think about it, but it's still interesting to see how close the numbers are. Since I don't want neighbours seeing what I'm sending or receiving, I'd probably go for the ADSL option.

With the new technologies on the horizon, and actually deployed in limited areas, why do I continue to suggest the use of ISDN? Depending on individual requirements, ISDN provides a powerful yet flexible mechanism for voice/data communication using a mature technology. For a number of years ISDN was a solution in search of a problem. It was also a victim of the classical "chicken-and-egg" scenario: Telcos didn't want to deploy it without a committed market, and customers were not willing to commit to a product which was not widely available. Telephone switch vendors interpreted the standards differently making equipment interoperability next to impossible to achieve. 

Further discussion depends upon an understanding of the basics of ISDN. It is a digital transmission technology which uses 64K B (Bearer) channels. This rate derives from the fact that conversion of analog signals (human conversation) to digital signals (for transmission over trunk lines) involves digitizing the analog signal 8,000 times per second. Each sample encodes the analog signal intensity as an 8-bit value; 8,000 samples/sec times 8 bits/sample = 64,000 bits/second. Older switch systems would perform "bit-robbing" in order to maintain DC (Direct Current) balance on analog lines, resulting in a data rate of 56,000 bit/second (hence the theoretical upper limit for modems.)

Analog telephones use "in-band" signaling; the same channel carries both data (the voice conversation) and signaling (dialing) data. ISDN uses "out-of-band" signaling; a separate channel is used to perform switching functions. The signaling channel never needs to carry the same amount of information as the data channels and so the speed is lower. ISDN is provisioned in one of two forms: PRI (Primary Rate Access) and BRI (Basic Rate Access). PRI provides 23 B channels and 1 D (signaling) channel. BRI provides 2 B channels and 1 D channel. The PRI D channel operates at 64K while BRI uses a 16K D channel.

Using this information we can calculate the bandwidth of a PRI. 23 B channels running at 64K plus one 64K D channel totals 1,536,000 bits/second. Note that this is almost indistinguishable from the 1.544 MB/sec T1 (or DS-1) rate. From this you might expect that a BRI with 2 64K B channels and 1 16K D channel would require a bandwidth of 144K bits/second. Actually, a BRI channel also includes a maintenance channel which operates at 48K, resulting in an actual bandwidth of 192K bits/second. While this might not sound spectacular compared to the speeds claimed for ADSL, ISDN is a switched technology while ADSL is "always on".

The switching capabilities of ISDN are what set the technology apart. Each B channel can be used for data or voice. Both channels could be used for voice, both could be used for data, or one could carry voice while the other carries data. Because signaling data is carried out-of-band on the D channel, you can receive notification of an incoming call (a "call appearance") even when both B channels are in use. Depending on the type and sophistication of the equipment connected to the ISDN line, you could even drop a B channel already in use in order to "accept" the incoming call.

As previously mentioned, residential ISDN is typically provisioned as a BRI. The RJ-45 jack installed by the Telco provides a two-wire interface known as the U interface. In order to accommodate multiple devices in a bus configuration, the two-wire U interface has to be converted to the four-wire S/T bus. The S/T bus uses one pair of wires to carry data in each direction. Devices such as Group 4 fax machines and ISDN telephones connect to the S/T bus. The device which performs the translation is known as an NT1 (Network Terminator 1). This device can be either a standalone unit or can be integrated into ISDN equipment which connects directly to the U interface. Finally, analog devices can be accommodated through a device known as a TA (Terminal Adapter). The following diagram shows these various elements.

Now that you know some of the capabilities of ISDN, it is time to map capabilities to your requirements. With most of the solutions mentioned here, you have a choice as to whether to use a built-in or stand-alone NT1. It should be noted that it is generally less expensive to pay the slight premium for a built-in NT1 than to pay extra for a stand-alone device. About the only exception to this rule would be in the case where you need to power an analog device such as a telephone. Since analog telephones require a 48 V power supply, and no voltage is supplied on an ISDN line, stand-alone NT1s power the analog telephone from the AC power line. In the case of power failure, these units often provide battery backup to permit the continued operation of the telephone.

Some common configurations for the B channels include dedicating both channels to data, both to voice, one to each, or more flexible mechanisms. The following sections will discuss how to apply ISDN technology to some typical scenarios. Representative products for the application will also be mentioned. If you detect a pro-Cisco bias, it is because I believe that they design and manufacture some of the finest networking equipment in the world. There are other products which could be used in place of those mentioned, but that would represent an individual choice.

Dial Backup

In networking scenarios where the primary link from a remote site is a high-speed serial connection, ISDN can provide a dial-on-demand backup. While not as speedy as the primary link, the backup would at least provide minimum connectivity in cases where the primary link fails. Consider the situation where a company is using a frame-relay backbone to connect all their sites. The link could be provisioned on a fractional T1 at speeds from 64K to 1,536K. If the frame-relay link failed, the remote site could use ISDN to dial-in to the central site and continue operation.

In this scenario, the ISDN link would never normally be activated. If the remote site was a fair distance from the central site, long-distance costs could be prohibitive for full-time operation. Using the link only when the primary link failed would be cost effective if it could keep the enterprise operational. A Cisco 1604 would be perfect for this scenario. It incorporates an NT1 and can be equipped with a high-speed serial interface. The LAN interface is provided by a 10-Base-T ethernet port.


The small office/home office requirements are very different. In these situations the emphasis is on leveraging a single line for a multitude of purposes. The primary use for an ISDN adapter in this scenario would be to provide digital access to information services such as an ISP (Internet Service Provider). Far more useful than fax/modem line splitters, ISDN adapters can use the DN (Dialed Number) of an incoming call to direct the call to a particular port, either analog or digital.

A number of vendors provide products for this market. A representative model would be the Cisco 766M. This device incorporates an NT1, a 10-Base-T ethernet port, two analog ports as well as an S/T bus interface. This last interface permits the connection of additional devices such as Group 4 fax machines. The adapter provides for flexible configuration of the various ports, directing incoming calls to the appropriate destination.


This category is similar to the SOHO, except that the telecommuter will typically need to connect to the office LAN. The ISDN line is also not expected to support multiple devices, perhaps only one analog telephone. The level of sophistication required in an adapter for this situation need not be as high as that needed in the SOHO application. This situation could effectively utilize an intelligent NT1 such as the IBM 7845 and use a simple ISDN "modem".

ISDN modems are not modems in the true sense of the word, as they have no need to modulate/demodulate a signal; they communicate digitally. They do, however, typically present a modem-like interface to the host computer, even down to supporting the Hayes AT modem command set. These devices can be either plug-in cards or external devices connected to the serial port. Since they communicate over the S/T bus, the NT1 is responsible for arbitrating access to the B channels.


I just spent the weekend installing and configuring another type of device which generally fits in this category. It's an ISDN modem from Bay Networks, a Netgear XM128U which incorporates an NT-1 and has a serial interface. The serial interface is the single biggest limitation to performance on this type of device. The UART chips in most PCs have a maximum speed of around 115,000 bps, which is the bottleneck when communicating with the modem. There is a compression feature available, and it's implemented in the software driver so effective speeds of over 400,000 bps are claimed. I don't see anywhere close to this, but it's still a much better performer than my so-called "56K" modem. While not what I would consider an optimum solution, it does have an AT command interface as well as two POTS ports. The biggest advantage of a device such as this is price: approximately $180, which is quite a deal for getting ISDN connectivity. Click here for more information on this unit.

Remote Support Specialist

This is a completely different category of user. Although remotely located, they require full access to office facilities, even including such elements as access to the PBX (Private Branch Exchange) for voice mail or ACD (Automatic Call Dispatch). Proprietary solutions are the rule in this market, utilizing the ISDN channels in a manner totally transparent to the user. An example of this type of product is the M-LAN solution from Northern Telecom. At each end of an ISDN link, the M-LAN provides both a 2616 Meridian telephone jack and a LAN jack. The system acts as an office extender, providing the remote user with all the features of a cubicle at the office.


As I hope you can discern by now, there are a number of ISDN capabilities which are simply not available in other technologies. While ADSL and cable modem technologies might suffice for internet browsing from the home, they do not begin to address the requirements of business. Some of the available ISDN adapters exhibit functionality approaching that of a PBX. With the rapid growth in the number of SOHOs and remote workers, I expect to see increasing penetration of ISDN in the advanced connectivity marketplace.

Copyright © 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015 by Phil Selby