ISDN and Windows NT

As enterprise applications become more sophisticated and require more bandwidth, modem WAN connections can seriously bottleneck. Unfortunately, the current crop of high-speed modems, running at 28.8K bits per second (Kbps), are about as fast as modems will ever get. That's because a physical speed barrier exists at 30Kbps which can't be broken without abandoning the modem's analog signaling for something completely different (see the sidebar " The Truth About High-Speed Modems").

That "completely different" something is digital signaling through Integrated Services Digital Network (ISDN). Using the same copper phone lines that modems use, ISDN delivers an enormous speed improvement (up to 128Kbps) and provides essentially perfect transmission reliability. And ISDN can mesh into other digital technologies, such as Frame Relay and Asynchronous Transfer Mode (ATM), making possible future speeds several times higher than 128Kbps. One of Windows NT's Remote Access Services (RAS) connectivity options is ISDN, letting you bump up your remote networking speeds by adding a serial ISDN interface to your NT Server. Or, you can connect your entire LAN to an ISDN WAN via a dedicated ISDN-capable bridge or router (see the sidebar "ISDN Hardware Solutions").

The "Integrated" part of ISDN's name describes the combining of voice and data services through the same medium: ISDN can connect computers directly to the telephone network without first converting their signals to analog audio signals, as modems must. This integration brings with it a host of new capabilities combining voice, data, fax, and sophisticated switching. Because ISDN uses existing local telephone lines, it's equally available to home and business customers. And ISDN offers a huge improvement in access speed at only a fractional increase in cost over those of a modem.

ISDN service is available today in most major metropolitan areas and may well be deployed throughout the US by the time you read this article. ISDN is operable worldwide, making it a workable solution for interconnecting far-flung LANs. Because every call is a switched telephone call, ISDN works as an impromptu solution for telecommuters and temporary business connections. To find out whether ISDN will work for you, you need to understand the capabilities it offers, how it delivers them, and what it costs in equipment and fees.

The Basics
ISDN provides a raw data rate of 144Kbps on a single twisted-pair telephone line. To better suit voice applications, this 144Kbps channel is partitioned into three subchannels: two 64Kbps bearer (B) channels, so named because they bear the voice or data traffic you're trying to move, and one 16Kbps data (D) channel, which communicates dialing information and incoming call events. Each B channel can carry a separate telephone call and usually has its own telephone number, called a Directory Number (DN). You can combine the two B channels to form a single 128Kbps data channel through a process called bonding.

Figure 1 shows a minimal ISDN setup connecting two computers. The incoming twisted pair enters a telephone-company-provided box called a network terminator (NT1), which breaks the 144Kbps channel into the B and D subchannels. (If you're wondering how ISDN squeezes 144Kbps into the same twisted pair that modems struggle with at 28.8Kbps, see the sidebar "How ISDN Does It".)

You need only a minimal ISDN setup to connect two computers
The B channels carry customer voice or data signals. The D channel carries signals between your ISDN equipment and the phone company's central office. These three channels are called the Basic Rate Interface (BRI) or 2B+D. You can buy ISDN in bulk: 23 B channels with a single 64Kbps D channel (23B+D). This service, called the Primary Rate Interface (PRI), inherits most of the capabilities and limitations of BRI, so what you learn about 2B+D applies to PRI's 23B+D service, as well.

In figure 1, a single four-wire cable carries the 2B+D channels to another box called the Terminal Adapter (TA). Unlike the NT1, which provides only a single function (creating the 2B+D channels), the TA can do many things. Its job is to connect your Terminal Equipment (TE)--computers, fax machines, LANs, or telephone sets--to one or both of the B channels. Depending on the types of TE you want to connect, the TA might be cheap or expensive, simple or complex. In this example, the TA is a separate unit, but it could easily be contained within the computer as an add-in card or an integrated feature or integrated with the NT1 into a single box. ISDN's current popularity is stimulating the introduction of new TAs almost monthly.

Figure 1 also shows the external ISDN reference points, labeled R, S/T, and U. R, S, T, and U are simply consecutive letters of the alphabet, chosen by the International Telegraphic Union (ITU), a standards-setting body, as the next available designations in the set of ITU standards. Each interface point requires an electrically different device connection and cabling. The U reference point is the incoming unshielded twisted pair (UTP); the S/T reference point is a four-wire UTP cable.

A typical TA for data-only applications might simply emulate a pair of ordi-nary--and very fast--Hayes-compatible modems, translating standard modem setup and dialing commands into ISDN call-setup commands. You connect your computer to this kind of TA with a normal RS-232 cable and use your usual modem or fax software set to 64Kbps (or as high as it can go). The TA provides automatic rate adaptation to match whatever data rate your computer supports with ISDN's 64Kbps channel. This means that if your computer can't communicate faster than 38.4Kbps, it will still work fine under ISDN and can even connect properly to a remote computer operating at another speed.

The example in figure 1 is a minimal ISDN setup. Even this simple configuration gives you the equivalent of two 64Kbps modems, two telephone lines, and virtually guaranteed reliable data transport. The last item is an important advantage that ISDN has over analog modems, which suffer from maladies ranging from intermittent line noise to speed mismatches and protocol conflicts.

Because ISDN is purely digital, it's easier to deliver data intact from end to end, largely eliminating the effects of noise. And because the 64Kbps channel is essentially a pure "bit pipe," with no rate negotiation or handshaking involved, there are no modem speed or protocol differences to cause conflicts. In fact, because the negotiation phase with ISDN is so simple, ISDN takes only a second or two to dial and establish a connection. Modems may take as long as a minute to accomplish the same thing. These benefits alone are worth the cost of two high-speed modems, which is about what a bare-bones TA costs.

Windows NT treats the basic TA the same as it would a modem, using RAS and Point-to-Point Protocol (PPP) to carry network traffic. From your point of view, then, the ISDN connection setup is identical to that for a PPP-modem- RAS connection. NT also supports the PPP multichannel extension, called Multichannel Point-to-Point Protocol. MPP lets you combine the two 64Kbps D channels into one 128Kbps bonded channel. (This is also called inverse multiplexing and is usually set up to provide bandwidth on demand--adding the second channel only when network traffic warrants it. Bandwidth-on-demand is a cost-saving feature.) Each D-channel connection is treated as a separate phone call, so having two channels up costs twice as much as one if your ISDN connection has per-minute usage fees. For flat-rate ISDN calls, you can permanently bond the D channels.

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