Just recently I was asked to give some advice on connecting to the Internet using ISDN. This document is derived from the email I sent in response.

See also the Web pages written by Dan Kegel, Peter Strangman, and Andrews and Arnold.

ISDN is an excellent upgrade for most systems, giving much faster throughput than is possible from a modem connection, much better than comparing V34bis with compression against a single B channel would suggest. This will go some way to explain why, and what the obstacles to overcome might be.


First, you need a connection. In the UK, this means you have to deal with BT. Some cable companies may provide ISDN, but stories indicate that they are not very well educated on the subject. Other companies (such as Mercury) can provide ISDN service, but they can't install a line; that is still a BT monopoly.

You will end up with a white wall box, with two sockets hidden on its underside. You can extend this network, using similar wiring to a computer network, if you wish. This is significantly different from POTS wiring, and requires termination resistors, etc. This wall box is the same as what is known the USA as an NT1, and is optional there. If you have a TA, this can be used immediately to replace two POTS lines. The TA plugs into one of the wall box sockets, and your analogue devices plug into the two sockets on the TA. You only need to do any more if you wish to take advantage of the extra facilities that ISDN offers. More about TAs elsewhere.

BT ISDN Pricing

The ISDN Line

A domestic line is called BRI (Basic Rate Interface), which has 2 B channels, each of 64K, and one D channel, which is useless for normal purposes, of about 16K. To connect to commercial services (CIX, Compuserve, Tel-Me, Demon) you can only use one B channel. For private connections, you can combine both Bs to get 128K. In future, I expect commercial services to offer this. With the right equipment, you can run V42bis compression over these speeds, giving an effective max of 200K. More information is available on port (serial and parallel) speeds.

A more expensive service is referred to as PRI (Primary Rate Interface), which is very similar to BRI, but have a lot more B channels available (30). BT primarily sell this for use with large voice switch boards. A special adaptor is required to break it down for use with ISDN2 (BRI) equipment.

What protocol to use?

What throughput should you expect?

ISDN Problem #1

How to use it for data

You have a number of choices to connect to ISDN. Commonest is to use a TA (Terminal Adaptor). This uses the ISDN 'protocol' to connect to other ISDN devices only; you can usually plug an analogue device, phone or modem, into the back; but with no advantages over a normal line. A very few TAs include a V34 modem emulation running over ISDN (the only one I know of the ZyXEL Elite). This lets you connect to ISDN devices and normal modem services. The last option is to use a router (see below).

ISDN Problem #2

Getting full throughput from an ISP

If you have a Demon tenner-a-month account, they let you have one IP address on their network, and won't route to any other IP addresses you may have. Given that a router will have an IP address different from your workstation, this is a problem. Some routers have built in spoofing (many types of spoofing) to handle this. With something like the ZyXEL Prestige router, it will pass through IP packets from your internal network. Apparently Demon don't mind, provided that you only have one real machine behind the router. More information (on NAT): here.

It is also possible to get 128K to Demon. Although they don't apparently support this, a normal PPP/MP connection will work, if requested to start up with both channels. It isn't possible to add/subtract a channel according to demand, as protocols for that are still being worked out.

There are a number of ISDN features I haven't mentioned, to do with use of the multiple channels, specific numbers you can allocate to different devices on your end (fax, modem, phone, answering machine, etc). And probably some others I forget. Multiple devices can cooperate to free and release multiple channels.

PC Hardware

Older PCs support serial ports at 19.2K to 38.4K. More modern systems with 16550 UART chips can run at 115.2K, but this is still slower than is required for full ISDN throughput. A parallel port driver is one solution, or a speed multiplier serial port card.

A 16550 serial port can give full throughput from a single channel (but it is marginal, if compression is used). It is inadequate for two channels, with or without compression. Parallel ports are much more variable on PCs; they may be bidirectional or not, and they may not be capable of high throughput. This is because less attention has been paid to parallel port throughput than to serial ports. If you have a bidirectional parallel port, it may give higher throughput than a 16550 serial port.

ZyXEL make a serial/parallel port card (25 GBP), using a derivative of the 16550, that they have designed to give up to 460K throughput, and will work with the parallel port on the ZyXEL Elite 2864I. This is a rate multiplying card for the serial ports. They also provide special drivers for Microsoft operating systems.

NeXT Hardware

Serial ports run at 38.4K, but you might be able to get some Turbos to 56K. Intel hardware running NeXTSTEP is worse. So you have a problem right away. The genuine fix is to use Ethernet to talk to the ISDN line, at 10M. To connect two networks, this is the only solution. If you use a TA or ISDN modem (Elite 2864I) you will be stuck with a NeXT serial port. They should work with NXFax and am (but I haven't tested these yet). Mix should be able to plug into the analogue port.

ISDN Solutions

Both the ZyXEL Omni TA (299 GBP) and Elite I (535 GBP) support fax and voice.

Using these with Demon as an ISP, and using slurp (a news transfer program for Unix systems), I see the following throughputs:

sync PPP7,300
PPP/MP (2 channels)13,300

Words and design by:
Paul Lynch
Last updated: November 26, 1996

[Home Page| Company]