The Integrated Services Digital Network

Karl Jeacle

Information Super Highway, eh? Seems more like a Super Footpath if you ask me. All this hype can get pretty tiresome after a while - especially if all you've got is a regular 14.4K modem. Perhaps now that Euro-ISDN has been standardised across Europe, it's time to give ISDN a look.


The Integrated Services Digital Network (ISDN) is a set of international standards for connecting voice, data, and video equipment. With ISDN, a user can make a voice call while simultaneously viewing video images or retrieving information from a computer. All these different forms of information can travel on a single ISDN circuit and be directed to an integrated voice/image/data terminal.

In 1976 the term "ISDN" appeared in the CCITT's (now ITU-T's) Orange Book list of terms. It's taken so long for ISDN to arrive that people have referred to it as "I Still Don't Need it" or "It Still Does Nothing". However, in recent years, the ISDN roll-out has finally happened, and it is now available in most parts of Europe.

ISDN comes in two flavours: BRI and PRI. The Basic Rate Interface (BRI) specifies a single access point into ISDN. Known as 2B+D, BRI consists of two bearer channels and one data/delta channel. Each bearer channel operates at 64Kbps and is a clear channel, meaning that there is no restriction on the format or type of information that passes through them. The data channel operates at 16Kbps and is used for signaling and control information. The Primary Rate Interface (PRI) is usually used to connect multiple users to ISDN. A common application would be to connect a PBX, LAN, or other multiuser switching device to an ISDN network.

As with all standards, the great thing about ISDN standards is that there are so many of them to choose from. Well, a few anyway. The European standard for PRI consists of 30 B channels of 64Kbps each and one D channel of 64Kbps. The aggregate capacity is 2.048Mbps or the equivalent bandwidth of an E1 line. The North American standard, on the other hand, consists of 23 B channels and 1 D channel giving an aggregate capacity of 1.544Mbps, the same as a T1 line.

Note that 64K in the telecoms world means 64,000bps, not the more familiar 65,536bps figure that a computer user might expect.

ISDN in theory

Figures 1 and 2 illustrate how interfaces link different types of equipment to an ISDN network. Figure 1 shows the more complex scenario which might be typical in a business environment, while Figure 2 illustrates a somewhat simpler configuration, more typical of a residential installation.

Figure 1 isdn2

Figure 2 isdn1

An "R" interface links non-ISDN compatible equipment and terminal adapter equipment. The "S" interface links ISDN compatible equipment and network terminal equipment. A "T" interface links customer premises equipment to an ISDN network while the "U" interface ties together network termination equipment or line termination equipment.

Network termination equipment can take two different forms under ISDN. NT1 describes public switched network demarcation devices such as a termination block or registered jack. This equipment will have some form of built-in intelligence under ISDN because of the functions it must perform. NT2 is the designation for customer-owned switching equipment such as a PBX or LAN. NT2 equipment can provide additional capabilities beyond NT1 such as call switching or concentration.

TE-1 equipment is ISDN compatible and can be connected directly to the network. TE-2 equipment is not ISDN compatible and requires an interface device known as a Terminal Adapter (TA). A TA can convert signals from one standard such as RS-232 to the ISDN standard.

ISDN requires a great deal of intelligence from the public switched network in order to format and transmit signals successfully. Signalling System 7 (SS7) is the series of recommendations from the ITU-T that define the content and format of signaling messages under ISDN, as well as the network design parameters necessary for transferring network control information.

ISDN in practice

In theory, ISDN can be a little tricky to figure out, but in practice, it's quite simple. A Basic Rate Interface connection consists of a pair of copper wires running from a local ISDN-capable exchange into a small wall-mounted box in your house called an NT1. The "U" interface is between the NT1 and the exchange.

The NT1 usally requires an external power supply, so it'll have to be installed near an AC power socket. In Europe, the NT1 is generally supplied (and owned) by the local Telco, but in the USA, users must provide their own. On the user side of the NT1 is a single RJ-45 socket. This is your ISDN connection, or "S/T" interface. This is a 4-wire interface, though in some cases may be 6 or 8.

Up to eight devices can be hooked up to this "S/T bus". The bus may be formed with splitters and T connectors; it is a true bus, not a star. The D channel is used to control the attachment of the one to eight devices to the two B channels. No two devices can attach to the same B channel at the same time.

ISDN uses a system called Multiple Subscriber Numbering (MSN) which allows you to allocate multiple E.164 addresses (telephone numbers) to your ISDN bus. For example, when you order your BRI connection, the Telco might provide you with two telephone numbers. If you only intend to use two 64K devices on your ISDN bus, then these two numbers might effectively map on to the two B channels and hence onto the two ISDN devices.

However, if you plan to use, say, four devices on your network, for example, a telephone, a fax machine, a computer and a videophone, then you're going to need more than two numbers. The Telco will rent you another two numbers, and you can now assign a unique number to each of your four devices. The obvious catch is that since you only have two B channels, only two devices can be active at the same time.

One case where you might have one device using both B channels is known as BONDING (Bandwidth ON Demand INteroperability Group). This is a specification which allows different ISDN vendors' equipment to interoperate when combining multiple B channels to form one large pipe. For example, BONDING supports a single 384Kbps data stream over six 64Kbps channels.

What can I use it for?

The primary use of ISDN will be to make 64 or 128Kbps data calls. These could be to a local BBS or Internet service provider. Bear in mind though, that the person you're calling must also have ISDN equipment installed to take your call.

This isn't the case for regular telephone or fax calls though. You can call and be called by normal Public Switched Telephone Network (PSTN) telephones and fax machines. So, once you have ISDN installed, there is no need for a conventional telephone connection. Although, there is a need for ISDN equipment - you can't plug normal PSTN equipment into an ISDN network.

Most demonstrations of ISDN you'll see will probably include some form of video conferencing. Most systems make use of both B channels. For example, one channel could be used for a crystal-clear audio connection, and the other for a H.261 compressed video stream and application sharing. It's worth noting that the ITU-T now has one standard which encompasses all video and audio (but not application sharing) standards pertaining to desktop video conferencing. It's called H.320, and any system complying with it, will be able to interoperate with all other H.320 systems without problems.

Unfortunately, I don't know of any H.320 video conferencing systems available for the Amiga, so if conferencing is your reason for getting ISDN, you'll have to look to the PC or Macintosh.

Another application of ISDN is as a leased line backup service in the business world. This is particularly evident in France, where a large percentage of the installed ISDN lines sit idle most of the year, waiting for an outage to occur on a leased line so they can kick into action. Naturally enough, the Telcos aren't over the moon with this scenario. Next to no revenue is generated by all those ISDN connections not making any calls. The companies, on the other hand, have no complaints - before ISDN, if a leased line went down, time and money could be lost while the Telco were busy repairing the fault.

What do I need?

At the time of writing, only one ISDN card is available for the Amiga. It's the ISDNMaster by BSC buroautomation AG (Tel: +49 89 357 130-0, Fax: +49 89 357 130-99). It's a Zorro-II card for the Amiga 2000/3000/4000, and costs about DM900 (UKP365). Alan Berney has written a review for which is available on Aminet as docs/rview/ISDNMaster.txt.

Unfortunately, Issue 54 of Just Amiga Monthly reports that BSC is leaving the Amiga market, and that most of their Amiga products will be continued on by AlfaData Technic Corp, although the status of the ISDNMaster is unclear. JAM suggests contacting Golden Image (UK) Ltd (+44 181 900 9291) who handle AlfaData products in the UK. Failing that, try AB-Union Electronic on +49 89 313 0938 (Tel/Fax).

If you can't get an internal ISDN card, then the next best thing is an ISDN Terminal Adapter. These boxes are sometimes referred to as "ISDN Modems". While this description gets the right idea across, it is technically incorrect and should be avoided. ISDN is an end-to-end digital network and no modulation or demodulation onto analog signals takes place at any stage. Basically, a TA is a box with an RS-232 port on one side and an ISDN socket on the other. They often even accept Hayes modem commands, so you can just plug in to your serial port and go.

A good example of a fully featured TA is the ZyXEL 2864I. This is a combined V.34 modem and ISDN TA. The V.34 side allows you to make 28.8 connections to other modems, while the TA side lets you make true ISDN calls. It can also send and receive faxes, and has built in voice capabilities allowing it to act as an answering machine. Truely an integrated solution! If you're serious about moving to ISDN and haven't bought a 28.8 modem yet, this might be for you.

Because ISDN calls can deliver a 128Kbps data stream, some TAs allow you to use your parallel port for connection. I presume custom software is provided that can handle this scenario. Otherwise, the TA's serial port will run up to either 115Kbps or 230Kbps. If either your computer or your TA limits you to a 115Kbps serial connection, then you will not be able to make full use of a 128K connection.

How much?

ISDN charges vary throughout Europe. The table below gives a rough idea of how costs vary from country to country. The prices listed are in UK pounds and are from July 1994.

Country         Monthly rental fee      |       Ratio   |       Total
                PSTN            BRI     |       BRI/PSTN|       B-Channels
Denmark         10.20   |       15.70   |       1.5     |       11,400
France          4.60    |       24.30   |       5.2     |       856,000
Germany         10.20   |       26.60   |       2.6     |       1,527,500
Ireland         10.10   |       35.20   |       3.5     |       1,000
Italy           3.60    |       20.60   |       5.7     |       16,200
Netherlands     9.20    |       25.50   |       2.8     |       31,000
Spain           6.00    |       31.70   |       5.3     |       78,300
Sweden          14.80   |       25.90   |       1.8     |       13,000
UK (BT)         11.00   |       28.00   |       2.6     |       100,000

In Ireland, a BRI connection costs #420 and has a monthly rental fee of #35. Calls to and from normal PSTN phones are charged at PSTN rates, while ISDN calls are charged at 1.25 times PSTN rates per B channel on national calls, and 1.5 times PSTN rates on international calls. All of these prices are subject to VAT at 21%.

Where now?

Probably the best place to start looking for more information on ISDN is the Usenet newsgroup comp.dcom.isdn. The Frequently Asked Questions (FAQ) file from the group is also well worth a read. The FAQ is posted to the group biweekly. It is also available for anonymous FTP from: as /pub/usenet/news.answers/isdn-faq.

If you have World Wide Web access, I recommend you check out Dan Kegel's ISDN Page at It has pointers to lots of ISDN related information on the Web, and links to ISDN equipment vendors.


Unfortunately, ISDN doesn't cheap. You'll have to pay more for your ISDN connection, rental, and calls than you would with a normal telephone line. ISDN equipment is more expensive too. But what you're buying is speed, flexibility, and a piece of the future.

The speed advantages is obvious, 14.4K and 28.8K are no match for 64K (or even 128K, if you can BOND). As for the flexibility, at the very least, you don't have to worry about tieing up the phone line. You can make a 64K data call on one channel, and still use the other for faxes or voice calls. As for the future - well, wouldn't it be nice to be one of the first kids on the block with ISDN?