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.
Being Clever
Although you might be satisfied with a basic ISDN setup, you can get much more value by investing in advanced TAs or direct computer-integrated hardware for ISDN. If you're using an ISDN connection for telecommuting, for example, you could take advantage of its voice features. It has a number of flexible options for mixing voice and data traffic. For example, although a 2B+D interface provides only two B channels, that simply limits you to two devices at any one time. Up to eight devices can share access to the channels using a feature called passive bus. Passive bus uses a second kind of network terminator, called NT2, to let up to eight separate TAs share a single 2B+D circuit. TAs that support passive bus have a port labeled S/T to indicate that you're making the connection at the S/T reference point.
Figure 2 shows passive bus with a dozen computers and four fax machines sharing an ISDN circuit. You need one TA for every two pieces of terminal equipment. Whenever a computer or fax machine wants to use a B channel, its associated TA checks to see if a channel is available, and, if so, dedicates it to the requesting TE. The figure shows maximum device sharing, but the cost of additional 2B+D circuits is low enough that you're likely to have fewer devices on a single bus.
An alternative to using an external TA is to connect your computer directly, using an ISDN board. Most ISDN boards include basic software that provides the equivalent of "dumb" TA functionality. You can add options for fax, image, and even video-conferencing features. In isolation, these capabilities may not seem useful, but combined with another ISDN feature, call appearances, they let you construct sophisticated, integrated voice/data applications.
Now Appearing
You're already familiar with the call-appearance concept, although you may not know it. The traditional call-waiting and three-way-calling features, which let you put a call "on hold" while taking or making a second call, provide two call appearances: the call you're on and the call on hold. ISDN expands that capability to up to 15 calls (see Figure 3).
For incoming ISDN calls, the telephone company's central office sends a call-setup message to the TA via the D channel, indicating an incoming call. If multiple TAs are connected via passive bus, any TA can pick up the call. The TA answers the call and assigns it to an available B channel; or if both B channels are in use, it frees a channel by placing an active call on hold and making the new call active. These calls can be data and voice, in any combination. Therefore, a single TA could have as many as 15 simultaneous calls in progress, with any two of those calls actively communicating.
If the TA is a personal computer, it can act as a sort of mini Private Branch eXchange (PBX), making possible all sorts of sophisticated call-handling applications. You can transfer calls from one B channel to another (or among 23 B channels on an ISDN PRI service), join two or more calls together in a conference call, hold active calls, resume held calls, retrieve calling-number identification data, even forward calls to an unrelated telephone number that isn't an ISDN circuit.
In practice, multiple call appearances are more useful for voice connections than for data calls. Most data-capable ISDN TAs support multiple appearances only if they also support voice features. Still, the call-appearance concept is important in bandwidth-on-demand applications, where one TA might combine the two B channels to obtain a 128Kbps data pipe but then relinquish one B channel to answer an incoming call.
Will Modems Ever Die?
What happens if you need to connect with someone who isn't ISDN-capable? You can use your analog modem and a TA that supports analog voice connections (see Figure 4).
This kind of TA accepts an ordinary voice or modem audio signal through a standard RJ11 modular jack and digitizes it for transport across the ISDN interface. The TA interprets the touch-tone dialing signals put out by your telephone set or modem and generates the required ISDN call-setup signals. If the number you're calling isn't an ISDN Point of Presence (POP), the telephone equipment at the other end automatically translates the digitized audio back to analog audio. Then, the destination modem--or person--"hears" what it "heard" before ISDN came along.
This may seem like a terribly roundabout way of maintaining backward-compatibility. It is! But even with wide-spread deployment of ISDN, there will still be hundreds of thousands of modem and fax users. Some ISDN TAs include built-in analog modems (sometimes called digital modems) just to provide compatibility with existing analog fax and data devices. So, you should plan to keep your modems around at least until the end of the decade; you'll still need them occasionally. Fortunately, many TAs provide analog telephone ports without much additional cost, so it's relatively painless.
Paying the Piper
What does ISDN cost? That depends on your local telephone company, equipment budget, and Internet Service Provider (ISP). An Internet ISDN connection consists of three components: the ISDN line, the equipment (an ISDN TA and, possibly, an NT1), and the ISP's fees.
Many telephone companies are charging prices for ISDN that are similar to those for a normal business telephone line, with measured service charges for the time you actually use the circuit (plus normal long-distance charges when they apply). The cheapest service (PacBell's) costs $30 per month for local access plus message-unit charges of $.04 for the first minute and $.01 for each additional minute. If the call is long-distance, you pay long-distance digital charges, which can be two to three times higher than voice long-distance calls, although competition is rapidly bringing these costs down. Even the most expensive ISDN providers have rates below $100 per month, and many have options that eliminate message-unit charges. For example, PacBell's "Home ISDN" package charges only for message units between 8 AM and 5 PM on non-holiday weekdays.
An NT1 costs between $100 and $200, but often you can find TAs with a built-in NT1 device. ISDN TAs can cost as little as $300 for data-only units providing Hayes-compatible modem emulation. Other ISDN TAs are priced at $1000 for multimedia TAs supporting data, digital voice, and analog voice and fax, while other TAs cost $2000 or more for ISDN-capable routers that can interconnect LANs over ISDN. Stand-alone TAs often cost less than bus-specific plug-in cards and are usable across a wider range of computer systems. Unless you have a tightly coupled ISDN application that requires a plug-in card, you're better off with stand-alone devices.
The Right Choice
There seems to be little doubt that ISDN is in the Internet's future. Although some critics complain about its limitations and push alternative solutions (see the sidebar "The Last Mile"), ISDN is here now and the others aren't. If you understand how ISDN works and what it's good for, you can explore using it as an alternative Internet connection.
Keep an eye on ISDN technology. Prices are decreasing while capabilities are increasing. Look for information on significant changes in ISDN availability, equipment, and setup in future issues.