" Computer Retail Week reported earlier this year that 28.8 kbps modems would capture 30% of the sales of new modems in 1995, and would account for more than half of all new modems purchased in 1996. This greed for speed is obviously driven by demands to use imaging applications over the net; video applications are clearly on the horizon. It seems reasonable to expect that users of today's fastest modems will soon be searching for higher-speed devices.

But the next step forward in high-speed telecommunications will require a radical re-work of users' phone systems--it will involve more than a simple trade-up of one modem for another. Today's modems are analog devices that work on analog phone lines, that is, lines which transmit signals over a continuous range of voltage levels. The throughput of modems, i.e., the rate at which information can be transferred, is limited by two factors: (a) the rapidity with which the sender (or receiver) can change (or detect) voltage levels, and (b) the extent to which the sender (and receiver) can make fine distinctions in the voltages being sent and received. Formally, this limitation is known as the Shannon-Hartley law, which states that Rmax = B x log2(1 + SNR) where B is the bandwidth (i.e., the range of frequencies) in the signal being transmitted, and SNR is the "signal-to-noise" ratio, i.e. the ratio between the voltage in the signal being transmitted, to the average fluctuation of the voltage on the line when no signal is being transmitted.

For analog phone lines, B is approximately 3000 Hz (i.e., 3000 cycles per second) and the SNR is typically 1000. Under these conditions, the maximum rate at which data can be transmitted is ~31 kbps. This is the theoretical limit, and is rarely achieved in practice. Thus, modems which operate at 28.8 kbps are already pushing the limiting capabilities of existing phone lines, and higher speed modems (for example, the new 33.6 kbps modems) are unlikely to operate frequently at their maximum rated capacity.

Therefore, to achieve higher data transmission speeds, users will need phone systems with greater bandwidth or higher signal-to-noise or both.

ISDN, which means Integrated Services Digital Network, is a suite of hardware and software standards which, when implemented on your phone line, allow it to operate at the desired higher bandwidth. The term itself refers to the fact that a suite of standards includes the capability to use your phone system for a wide variety of tele-services and with more flexibility than that which is available with ordinary analog lines, a.k.a. POTS (plain old telephone system).

ISDN is a digital phone system. Instead of transmitting your voice as a smoothly varying voltage, it converts the sound into a pattern of discrete voltage levels. Transmitting your voice or your modem's output over phone lines as digitally encoded signals is nothing new. The switches at most Central Offices digitize all the signals coming in to them, so that they can transmit them at high speed over digital trunk lines. In fact, practically the only portion of the phone system which is not digital at present are the signals being sent from individual residences to local telco Central Offices. ISDN merely provides for the digitization to occur within your local phone set itself or within your computer communications device, rather than at the CO's switch. That's why some people call ISDN ``the digitization of the last mile."

But the last year has seen the ceiling being reached in the speeds achievable with analog modems, and the prices of ISDN computer communications devices drop down to the realm of analog modems. Now consumers in some areas are not only embracing the new phone system, but waging civic battles to ensure its availability!

There are three steps to making the transition from POTS to ISDN. Proceeding outward from your computer, you must (a) replace your modem; (b) have the phone company reconfigure your phone line so that it transmits digital and not analog signals; and (c) sign up for Internet access with a company that provides ISDN service.

In practice, you will probably begin by investigating the availability of an Internet Service Provider (ISP) in your area.

It is important to understand that not everyone can obtain ISDN service at the present time--at least, not for the same price. One reason is that the telephone Central Office in your town or neighborhood may not be equipped with a switch that can interpret incoming digital signals.

The second obstacle to uniform access to ISDN is the quality of individual phone lines. If your phone line is too long or too eroded, the digital signals will become indecipherable. The phone company anticipates that residences within approximately 18,000 feet of a telephone company Central Office will have sufficiently clear signals to be useful for ISDN. However, this distance estimate is based on the assumption of a certain line quality; if your phone line is degraded, then ISDN service might be limited to shorter runs of cable. Thus the test for ISDN suitability is not a simple distance measurement, but an electronic ``loop test" which is carried out by the phone company before scheduling the conversion of a line to ISDN service. The bad news is that if your line fails the loop test the only way to obtain ISDN service is to purchase signal repeaters, which cost nearly $1500 each, and must be purchased in pairs!

Assuming that your location does qualify for ISDN service, then (and only then) it makes sense to invest in ISDN equipment. Not much is needed. You must of course replace your analog modem. The analog modems commonly in use nowadays transmit data by transforming a digital signal from your computer into an analog form before transmission to your Central Office. The replacements for analog modems on an ISDN line do not, or course, need to make this digital-to-analog conversion, but do perform many other handshaking, formatting, and error-checking functions. The simplest of these products are called ``Terminal Adapters"; they are sometimes also called ``digital modems", but that phrase (though catchy) is technically meaningless.

In many if not most cases, these details are arranged with the phone company by your Internet Service Provider, relieving you of the need to sort through the numerous choices of ISDN telephone configurations that are possible.

If your only need for ISDN is to obtain higher speed access to the Internet at the lowest cost, the choice is simple: you want a single `B' channel, configured for Voice calls only. While the use of a Voice channel for transmitting data might seem peculiar at first, the procedure is exactly analogous to sending data over a POTS line with an analog modem. This practice is sometimes called ``data-over-voice" (DOV) ISDN.