In most areas, ISDN service prices start at twice as much as those for plai n old telephone system (POTS) connections, but ISDN offers much more than twice the service. It opens a world of applications that have been difficult to implement using POTS technology. Applications requiring high-speed access from remote offices or home users to corporate offices or Internet service providers (ISPs) can be implemented with ISDN technology.

ISDN vs. POTS ISDN Basic Rate Interface (BRI) consists of three channels: Two B (Bearer) channels carry digital data at 64 Kbps and a 16-Kbps D (Delta or Data) signaling channel carries information about incoming or outgoing calls, along with which and how many B channels are being used. In some areas, it's also possible to use the D channel for packet data. The analog nature of POTS subjects connections to noise, which results in retran smissions, lower connect speeds and unreliable data connections.

The most noticeable difference with ISDN data connections is the fast call setup time--frequentl y less than one second--versus 30 seconds or more for most analog modem connections. Unlike POTS, which uses the data-transmission channel for signaling, ISDN signaling is digital, eliminating noise and enabling connections that are virtually error free. ISDN has a guaranteed transmission rate of 56 Kbps or 64 Kbps per channel, depending on the carrier and switch.

Additionally, some commonly implemented protocol-defining standards for ISDN devices allow equipment to combine each B channel for aggregated transmission of up to 128 Kbps (or 112 Kbps for 56 Kbps limited switches) on a 2B BRI line. For very-high-capacity installations, ISDN Primary Rate Interface (PRI) operates over a T1 link and offers 23 B channels and one D channel. PRI typically is used in large corporate environments with many remote ISDN connections and at ISP sites.

Besides data communications, ISDN permits advanced

One of the disadvantages of ISDN services has been its availability. That is changing, however, as telephone companies rapidly deploy the technology. Most heavily po pulated cities in North America, as well as most Intern et service providers, have ISDN offerings.

Another drawback: ISDN equipment works only with other ISDN equipment; ISDN can't speak with POTS-based technology. But you don't necessarily need another line for analog service if your equipment includes analog ports or onboard analog devices, such as a modem.

NT1--the Power Middleman An NT1 unit is required for all ISDN equipment. It consists of a box the size of a small modem and a power supply. Like a channel service unit/data service unit (CSU/DSU), an NT1 acts as an interface between your equipment and the local telco switch. Since ISDN doesn't operate from its own power source, it must be powered by your local utility connection. Many vendors offer equipment with built-in NT1 units. There are advantages and disadvantages to this.

NT1 units allow up to eight different devices to be daisy-chained to the ISDN circuit, enabling all eight to use it simultaneously or to contend for t he circuit as need arises, depending on the equipment. Thus a n ISDN phone, IP