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Understanding ISDN Communications

What is ISDN?

Most of today's telephone lines are analog lines. ISDN stands for Integrated Service Digital Network - in plain English, a digital telephone line. A digital telephone line can be used more efficiently for data communications than an analog telephone line.

To understand why this is, we need to understand how a digital phone line is different from an analog phone line. Imagine communicating with a friend through a kids' 2-cans-and-a-string play telephone. When you listen in one can, you can hear your friend's voice in a fuzzy sort of way. If she talks louder, you will hear her voice louder. That's because the sound travels along the string as complex wave impulses of varying intensity.

Now imagine talking to your friend over that same can-and-string phone using a code, like Morse code. You'd pluck the string that was stretched between you so that it either vibrated or stayed still at various intervals. The sequence of plucks and no plucks would stand for different letters. This is a form of digital communications, which uses either a signal (on) or no signal (off). It's a much simpler signal than voice (analog), even though at first glance it would seem harder to spell out all the words and keep track of what was being said. But because it's such a simple signal, there's less detail to get lost or blurred in the conversation. If you could pluck the string to send the code really fast, and understood the code well enough to decode it at the same speed as it was being sent, you'd have a very fast and accurate communications system running over a very primitive device. This would be a digital communications system, like ISDN.

Most of our modern telephone system was already digital before ISDN was ever offered by the telephone companies. When you make a call, your voice is converted to a digital signal at your local telephone company switch, is sent to the telephone company's central office, is then transmitted to the switch nearest the party you are calling, and only there is it converted back into an analog signal so that your friend can hear your voice normally. All ISDN changes in this scheme is that the line that goes into your telephone starts out as a digital, not an analog line. All the conversions are done before the signal ever leaves your home. When you are using an ISDN line to communicate with your Internet service provider, your and your provider's lines are digital all the way from one end to the other. Time lost and errors made in making the conversions at the switches from analog to digital and back again are avoided.

What Can ISDN Do For Me?

ISDN Internet access is very similar to regular analog dial-up access. However, your access speed will be either 64 kbps (for one "B" channel) or 128 kbps (for two "B" channels). The only other way to get this kind of speed is through a different kind of digital connection, frame relay. Equipment, installation, and monthly service costs for ISDN are a mere fraction of the costs of frame relay. ISDN is affordable for individual users as well as for businesses.

ISDN is also simple to use. Just like with an analog modem connection to the Internet, you will dial into your Internet service provider. You will not need to have any special wiring run to your home or business. Aside from purchasing an ISDN "modem", you will not need to invest in any additional equipment or software.

The only other contender in the high-speed arena for individual and small business users is the new 56K modem technology. While 56K is even simpler and less expensive to use than ISDN, it can only take advantage of higher speeds when it downloads information to your computer. ISDN can both send and receive data at high speed. Also, ISDN, especially in 128 kbps mode, still substantially surpasses 56K speeds.

Why is ISDN More Expensive and Harder To Get Than Regular Phone Service?

The switches that we were talking about in the last section are just large computers. Telephone companies have a local switch in every area with a certain number of telephone lines. Depending where you live that could be one switch for every neighborhood, or even one switch for every few blocks in a neighborhood. Unfortunately, like all computers these telephone switches are likely to become obsolete, running only older software. All the telephone company would have to do to offer ISDN service from a certain switch is to run the right software on it, but some switches can't run the necessary software. Those switches have to be replaced if the areas they serve are to get ISDN. This process of upgrading and replacing means that the telephone company passes on the cost to people who want this new service. To find out whether your area has ISDN, call your telephone company. Generally, even if your local switch has ISDN capability, you will need to live within 3.5 miles of it in order to get the service. Telephone companies are working to extend this range.

In the Minneapolis/St. Paul area, ISDN is sold in three packages. The least expensive package costs $39/month, but the calls are metered. That means that you will be charged for each minute of your calls (about 1 to 2 cents per minute. The most practical service for most home users is US West's 200 hour service, which costs a flat $68/month for the first 200 hours of monthly service. The same per-minute charges would apply if the 200 hour limit was exceeded. Finally, you can purchase an unlimited flat rate plan for $184/month. Whichever plan you decide on, it will cost you $110 to install if you are within 18,000 feet of the switch, and $210 to install if you are farther from your local switch. If you are farther, US West will only install the line if it believes that it can deliver the proper signal quality.

What Kind Of Equipment Will I Need For ISDN?

Other than the ISDN line, you will need a TA/NT1 unit, sometimes mistakenly referred to as an ISDN modem. The TA stands for "Terminal Adapter", which controls the transmissions on the line. NT1 stands for "Network Termination Unit", a device that supplies power to the line and turns your normal 2-wire line into a 4-wire interface for the digital connection. For a single user, the TA/NT1 unit normally is connected directly into a PC; for a LAN, the TA/NT1 unit usually connects to a router on the LAN. A PC card with a TA/NT1 unit can cost as little as $300 - $400. A router with a TA/NT1 unit costs about $900 to start. Some of TA/NT1 units also have an analog modem built in to allow users to dial into non-ISDN services.


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