ISDN 101 • An Introduction to ISDN

By Bill Ranney

You hear about ISDN every day. Your neighbor down the street has a new ISDN telephone and your buddy in the business next door has an ISDN Internet link. What is ISDN? Is it time to jump on the ISDN band wagon? Read on to see if ISDN is for you.

ISDN stands for Integrated Services Digital Network. Great, but what does that really mean? Well, the DN part of ISDN can simply be thought of as a robust digital communications network. While the current telephone network has an analog connection to your home or business, generally with one pair of wires for each phone or fax, ISDN is an end-to-end digital network, right up to your desk top. In fact, ISDN puts 3 digital channels on that single pair of wires formerly used for only one telephone. So what, you say. Well, the capabilities (the IS part) that ISDN can offer are amazing, ranging from voice and data transmission to full color video, stereo quality sound, and high speed imaging services. Imagine data transmission rates up to 128,000 bits per second (almost five times as fast as the new 28.8 kbps [kilobits per second or 28,800bits per second] modems), or 64 kbps fax services which are over 6 times as fast as most of today’s fax machines. Or how about data connections on demand to your remote facility, without having to pay for expensive leased lines. ISDN can offer all this and more.

Some rich features (more IS) that ISDN offers for voice calls are call forwarding on busy or no answer, call holding, call transfer, caller ID & caller ID blocking, conference calling with up to 8 callers on a single call, distinctive ringing, speed calling and more. ISDN voice services are like having a fully featured digital PBX for your telephone, without all the up front costs. These features may or may not be at an additional fee, depending on your carrier. What if the party you’re calling doesn’t have an ISDN telephone? Don’t worry, since the carrier network will do all the necessary translation required and you still have universal service.

ISDN can also give you 64 kbps or 128 kbps digital dial-up Internet data connections. Many Internet service providers are now offering dial-up ISDN links as a way to give their clients faster Internet connections. No more finicky modem connections at less than satisfactory speeds. You can also have an ISDN dial-up data or voice/data connection to any remote location you wish, such as a field sales office or a technical support center in a far off city. ISDN services are perfect for the work at home telecommuter who needs simultaneous voice and data access to the home office. In the case of transporting data, you do need to have ISDN at both ends of the connection.

The basic building block of ISDN, which can be used for both residential and business services, is called a Basic Rate Interface, or BRI for short. This BRI service offers the user three digital channels;

2 B (Bearer) channels and 1 D (Delta) channel all on one pair of wires. The B channels are 64,000 bit per second digital channels that can be used for voice, data, voice and video conferencing, and video or image transfer. The D channel is a 16,000 bit per second channel that is used for call control, call supervision and call feature activation, such as a conference call. The D channel can also be used for packet switched data transmission, such as X.25 services. This base service can be expanded to PRI (Primary Rate Interface) service, which is 23 B channels plus one D channel. PRI service generally is transported into a business on a T1 span. The D channel still carries the signaling and the 23 B channels can carry whatever digital traffic you wish to transport.

Costs for ISDN are surprisingly low. Rates range from a low of $24.82 per month from PAC Bell (which I understand was just raised to $32) to a high of $69.00 per month from US West. Most carriers also have measured service charges on top of their monthly fees, usually a few cents per minute. One time installation charges range from $58.00 up to $147.00, depending on your carrier. Still, ISDN is a good bet if you want these enhanced services. And if you are using modems or point-to-point leased lines to carry intermittent data traffic, ISDN may be an excellent money saving substitute.

More than just a circuit from your local Telco is needed to make ISDN work. First of all, one needs to have an NT1 Network Terminating interface with the ISDN network. The junction between the NT1 unit and the carrier’s copper wires is known as the U interface. This NT1 equipment provides the physical and electrical termination of the twisted pair coming from the telephone company. It will allow the telephone company to perform line maintenance functions such as loop back testing and performance monitoring. The NT1 box also acts like a modem in that it codes your digital signal and place sit on the copper wires for transmission to the telephone company. For many applications, the NT1 interface equipment is integrated into other ISDN equipment. The NT1 can also be purchased as a stand alone box or your ISDN carrier may want to provide it (which is not recommended, due to the cost).

On the subscriber side of the box, the NT1 provides what is called the S/T interface. There are two distinct types of equipment that can connect to the S/T interface. The first is a TE1 or Terminal Equipment type 1. The other type is a Terminal Adapter, or TA. TE1 type equipment is ISDN-ready equipment such as an ISDN telephone, integrated ISDN voice/data terminals, and digital ISDN fax machines. A TA unit is a box that will allow you to hook up (or adapt) non-ISDN equipment such as routers, PC’s, LAN’s, modems, and plain old telephones to the ISDN network. Both of these are described below.

TE1 – ISDN telephones: These ISDN digital telephone shave many robust features incorporated into the design. Usually, these phone shave an LCD display screen that will show you the caller ID on inbound calls. You can control multiple calls using a separate button for each call. For example, if you get multiple calls, each caller’s number will be displayed. You then can either put the first call on hold and answer the second call or the second call will be transferred to voice mail. Conference calling is also supported by these telephones. Prices range from several hundred dollars per telephone and higher, depending on the features. By the way, if you purchase an ISDN telephone with an integrated NT1 interface, you can only put one telephone on the circuit. A better solution is to purchase the NT1 interface separately and then you can hook up to 8 telephones or TE1 devices to the ISDN circuit.

Voice/Data terminals: These devices are ISDN telephones with a data port to allow PC access to your ISDN link. You can now transport voice and data simultaneously over your single ISDN circuit. So, if you are on a voice call and need to transfer a data file, send a fax or access the Internet at the same time, you have that capability.

ISDN TA equipment: TA (Terminal Adapter) equipment is used when you are primarily interested in a data or Internet link. There are many different brands available with many different functions. Some have an RS-232 interface for a computer link and some come with an Ethernet port for a LAN connection. Some TA’s have one or two jacks for an analog telephone, fax or modem. You can get TA equipment with multiple computer links, V.3 5 interfaces, a built-in bridge or router, integrated modems, frame relay access, and numerous other features, all for a price, of course. One exciting feature that is usually available, called the BONDING protocol, allows you to aggregate the band width of the two B channels for a total transfer rate of 128 kilobits per second. You will pay your measured rate for both channels, however, when you bond two of them together. For the real band width fanatics, there is equipment available to bond up to 8 B channels from 4 BRI circuits for a total band width of 512 kbps.

ISDN does have some limitations. First of all, you can only be about 3.4 miles (18,000 feet) from the central office without the use of expensive repeaters. This distance is increasing as equipment manufacturers develop enhanced products. For instance, Adtran recently announced their new Total Reach product line, which should be available this fall, that will increase the maximum distance to 5.8 miles. ISDN is not available everywhere, either, because many telco central offices around the US are not provisioned for ISDN. Furthermore, new services may supplant ISDN with greater speeds. One new technology called ADSL, currently being tested by US West and GTE, will give the user a 6 megabit per second link, or almost 100 times the speed of basic ISDN. Cable operators are also investigating cable modem technology, which will give the user vastly increased speeds over ISDN. It may be many, many years before these alternative technologies are readily available to the public, however, so ISDN remains a viable solution.

Ordering ISDN used to tax the patience of everyone. Don’t just contact your local central office to place an order because they will be clueless. Many carriers have set up ISDN order centers with specially trained agents to assist you with the services they offer and provide you with their costs.

After speaking with your carrier, but before ordering your ISDN service, it is imperative that you have your ISDN TE1 or TA equipment picked out and that you have chosen the services you want, so your ISDN line can be provisioned correctly by the carrier. From there it is a matter of waiting while the carrier installs your service (sometimes many weeks) and then hooking up your new ISDN telephone or TA gear. You, along with many others, can then enjoy faster access to the exciting Information Superhighway.

Bill Ranney is president of Transnet Engineering, Inc., and provides customer premise equipment such as T1 channel banks, CSU/DSUs, ISDN telephones and terminal adapters, voice and data multiplexers, and routers and bridges to businesses throughout the USA. Mr. Ranney can be reached at 303-413-0665.

[From Connection Magazine, May 1996]

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Peter DeHaan is the president of Peter DeHaan Publishing, Inc., ( the publisher and editor of Connections Magazine and AnswerStat, and editor of Article Weekly.