SEG's Beginner's Guide to ISDN


The regular phone line which most people have in their home is an analogue line. Your voice is picked up by your telephone mouthpiece and then sent down the phone line as an analogue wave form. Regular modems work by converting your computer's digital data into audible tones which can be carried down a standard analogue phone line - a line designed to carry audible sound. At the destination, another modem converts the sound back into digital data.

To transmit data, however, it is far more efficient, and simpler to keep it in its original digital state. An ISDN line is a digital line - your computer connects to the ISDN line (via a terminal adaptor) without having to convert the data into sound first.

A basic rate ISDN line provides two 'channels', each of which can be used for separate calls, thus equivalent to having two regular lines. Each of these channels provides bandwidth (i.e. data transfer speed) of 64Kbps full duplex (i.e. both directions simultaneously - upload and download). This compares with analogue modems which provide up to (approx) 50Kbps on a good day (download) and 33Kbps (upload). An ISDN data connection is always a fixed reliable 64Kbps - it doesn't depend on conditions. You can also combine both 64Kbps channels into a single call to get twice the speed (128Kbps - See later).

As well as being used for data, an ISDN line can call carry voice calls. Whereas on an analogue line digital data has to be turned into an analogue signal, on a digital line, analogue voices have to be converted to digital signals. Many ISDN terminal adaptors provide 'analogue sockets' into which you can connect regular telephones (or fax machines, anserphones etc). BT's Highway service provides analogue sockets on the main wallbox itself.

ISDN2e vs. BT Highway

The best way to visualise the differences between BT Highway and ISDN2e is to look at some example configurations. In this case, using the Hagenuk 'Speed Dragon' which is a combined ISDN terminal adaptor and PBX (switchboard).

First, here is a standard ISDN2e line. Your terminal equipment (in this case the Speed Dragon) connects to the line. All phones or PCs connect to the Speed Dragon. You can have one phone number, or more commonly several MSNs, as shown.

ISDN2e Line

Next up, we have a Business Highway line. This works just like ISDN2e except that in addition to the digital (coloured blue) sockets on the main wallbox, there are two analogue sockets, into which you can plug a standard phone (or other analogue equipment). Each of these analogue sockets have their own unique phone number, as shown. You can still have one or more MSNs on the 'digital' side which ring phones etc.

Business Highway Line

Lastly, we have Home Highway. This uses the same wallbox as Business Highway but the main difference is that you cannot have MSN - you have just a single phone number on the 'digital' side (in addition to two analogue phone numbers/sockets on the wallbox). See 'MSN' later for more details.

Home Highway Diagram

What is Primary Rate ISDN (PRI) / ISDN30 ?

As well as ISDN2, BT provide something called ISDN30 service which is a different interface, known as 'Primary Rate Interface' (PRI). PRI provides more channels (6 or more). Equipment for Primary Rate ISDN is incompatible with Basic rate equipment and is considerably more expensive. Primary to basic converters are available, but they are not cheap.

What equipment do I need for an ISDN line ?

For data, as ISDN is fully digital, a 'Modem' is not needed. Instead a Terminal Adaptor (TA) is used which connects your computer to the ISDN line - it works much like a modem. ISDN calls connect almost instantly, compared to modem calls which can take up to 30 seconds while they negotiate. As mentioned earlier, an ISDN2 line provides two channels for data, each of which can carry data at up to 64000 bps. Each of the channels can be used for separate calls to two different destinations - just like having two separate lines. ISDN lines can carry voice, data and fax calls and can make 'analogue' calls to regular telephone lines, assuming that you have appropriate equipment.

How do I plug in a regular telephone into my ISDN line ?

As mentioned earlier, you can place regular voice calls over an ISDN line. You cannot connect your telephone straight into the ISDN line though - you have to have a terminal adaptor with an analogue port which can digitize your voice. You can also connect faxes, answerphones and modems to analogue ports.

On BT's Highway, two analogue ports are provided on the wallbox itself, each with its own phone number. In the event of a power cut then one of these continues to work on limited power. The two phone numbers allocated to the BT Highway analogue ports are not delivered to the line's digital ports - you get a different phone number for that or with Business Highway, you can have MSN which gives you several phone numbers. If your Terminal Adaptor has analogue ports, you can make use of these digital numbers for voice calls.

Apple Macintosh Users

If you use an Apple Mac, then ISDN can be used quite happily too, however in the Mac market a company called 4Sight/Wamnet has been, and still is dominant in the area of site- -to-site files transfer software (where you send a file directly to someone's computer, direct dial, rather than via the Internet). If you want to dial up 4Sight users directly, you MUST have 4Sight compatible software/hardware, and this is not standard on equipment, so do check if you need this.

How can I go faster than 64Kbps on ISDN ?

As mentioned earlier, an ISDN2e line provides two channels, each of which can send/receive data at 64Kbps. By using both channels at once, calling the same location, you can double the speed to 128Kbps. The most common protocol for using combining both channels into 128Kbps is known as Multilink-PPP and it is widely supported by TA's (Terminal Adaptors) and ISP's (Internet Service Providers). ML-PPP can often be set to use Bandwidth On Demand (BOD) - this means that the second channel is only raised once throughput reaches a certain threshold, and drops out again if throughput drops below another pre-set threshold. If your line and equipment supports "Call Bumping" then one of the 64Kbps channels can be dropped temporarily to allow an incoming voice call to be taken.

Does ISDN support Data Compression ?

The other way to increase throughput on ISDN is to use Data Compression. With modems, you've probably been using V42bis data compression. For asynchronous V120 or proprietary calls some TA's do support V42bis for data. For PPP calls on ISDN, however, another system, known as Stac™ is common (Stac is proprietary but supported by many manufacturers). Like V42bis, Stac can increase throughput substantially, given the right sort of data, however bear in mind that on the Internet much of the data you receive is already in a compressed format (e.g. GIF and JPEG images) so the benefit of Stac is not as great as it is on plain text, but for HTML type data the compression can be very effective. Few ISPs support compression for ISDN connections.

How much do ISDN calls cost ?

At present, inland calls made over an ISDN line cost the same as regular analogue calls. If you use Multilink-PPP (combining two channels to get 128Kbps throughput) then you are placing TWO calls (one on each channel) and you are charged for both calls (i.e. twice the price of using one 64Kbps channel). International calls have their own pricing structure. Check with your line supplier (e.g. BT) for current pricing.

What is CAPI ?

CAPI is an acronym for Common ISDN API. (API is in turn an acronym for Application Programming Interface). In order to understand what CAPI does, first consider how we usually talk to a comms device (a modem, a terminal adaptor etc) and why we need an improvement. CAPI is currently a system specifically for Microsoft Windows based PCs.

If you connect a regular modem or TA to your PC's serial port, a software application can open the COM port and control or use that device. This might be a fax program sending a fax, a terminal program calling a BBS (Bulletin Board) or connecting to the Internet. Because each of those programs controls the COM port directly and there is no mechanism for program co-operation, once one program has opened the port, it is unavailable for all other programs. If you've ever tried to make a modem call when your fax software is already loaded (and waiting for calls) then you will have already encountered the problem.

There are various ways to 'cope' with this currently; for instance, you can temporarily unload/disable your fax software whilst you use the modem/TA for other duties, and re-enable it when you are finished. That solution, however, is awkward and cannot be automated or used to route incoming calls to the appropriate application.

With CAPI, instead of your comms programs opening and controlling your ISDN device directly, a CAPI driver is installed on your PC which takes control of the device. The CAPI driver then acts as a switchboard for both incoming and outgoing calls, allocating the call to whichever program is appropriate. CAPI also allows concurrent usage, so as you have two channels on an ISDN2 line, two programs can be using a channel each at the same time. Once you have CAPI running, you can have many programs operating, for instance Voicemail, Euro-file transfer, Dial-Up-Networking, Fax, Call Logging etc.

For further details of CAPI, you can visit the home page of the CAPI association at

What is an ISDN Router ?

A router (or a bridge) is a piece of hardware used to link two remote networks together. This might be two offices, each with their own Local Area Network (LAN) or it might be a small LAN linking onto a large corporate LAN. More common these days, an ISDN router is used to connect a LAN to the Internet, giving everyone simultaneous shared access to the Internet. We have a separate explanation page about Bridges/Routers Here.

Caller ID

ISDN2e also supports Caller ID, i.e. the delivery of the calling telephone number. The CLI is delivered down the 'D' channel along with other call setup information. It is then up to your terminal equipment to use that information. TA's vary in what they do with the CLI information. For instance your TA might have its own LCD for CLI display, pass it onto the PC for 'screen popup' and some TA's will generate a CLI signal on its analogue ports for display by your own Caller display box or phone. On BT Highway, its own analogue ports also support Caller ID, straight to a Caller Display unit or phone. Caller ID is a subscription service. See our ISDN price list.

MSN (Multi Subscriber Numbering)

MSN is a facility whereby you can have more than one telephone number allocated to your ISDN2e or Business Highway line. MSN is NOT available on BT Home Highway (see diagrams earlier).

Each incoming number can be routed to a different extension or piece of equipment depending on the phone number called, all on the same ISDN line (up to a maximum of two calls at once, as there are only two channels per line). For instance one number might route straight through to a fax machine on an analogue port, whilst another number might go to a telephone and yet another number might go to the serial port on the TA. If you have several MSNs and also a call diversion facility then each MSN can be diverted (or not!) to a different location.

If you intend replacing your existing lines with ISDN or adding MSN to an existing ISDN line then it is not always possible to keep the same phone numbers. Once your MSN numbers are set up, you can allocate them to your equipment as you want.

Digital Select Services

In additional to the MSN and CLI services, some other services are available on ISDN2e and Business Highway lines (not Home Highway. BT Call these 'Digital Select Services'. Click here for more information.

Multiple ISDN Lines & DDI

You can of course have more than one ISDN line. Commonly, you might have multiple ISDN lines (two channels each) feeding a PBX (switchboard). In those cases, you might want to have your phone number (or numbers) deliverable across ANY of the lines, rather than tied to just one line. This requires DDI rather than MSN.

What is Home & Business Highway ?

Home Highway (Residential) and Business Highway is a variant of ISDN2e.

Like regular ISDN2e, BT Highway supports two B channels so that you can run two simultaneous calls. That can be two voice (analogue) calls, one voice + one data or two 64K data calls (giving 128Kbps). The two analogue ports on the Highway wallbox operate like normal analogue phone likes and each have their own phone number. The digital port has its own number too, giving three unique phone numbers in total. If you have Business Highway, you can also have MSN (see earlier) which can give you up to 9 more telephone numbers on the digital port. Although the Highway wallbox provides analogue ports itself, you can still use Terminal Adaptors or routers with their own analogue ports, thus making full use of your allocated numbers.

BT Highway's wallbox (NTE9) requires mains power, but in the event of a power cut, one analogue port will continue to work, on reduced levels. BT Highway supports caller ID delivery on the analogue ports as well as many of the BT Select Services which are available on analogue lines.

When BT install Highway, they will normally preserve your existing analogue linebox; they will modify the front panel slightly to allow all of your existing extensions to be 'fed' from Analogue Port 1 on the Highway box.

BT Highway Box

The picture above is of the standard Highway Wallbox (NTE9). As you can see, there are four sockets. Sockets 1 and 2 are your Highway analogue sockets. Each of those has its own telephone number. Sockets 3 and 4 are actually the same as each other; they provide two just for convenience, in case you have two pieces of terminal equipment. Into those two blue sockets you can plug your digital terminal equipment (router, terminal adaptor etc). The digital blue sockets have their own phone number (or numbers - you can have up to ten on Business Highway). Note that calls to the phone numbers which ring the highway analogue ports (sockets 1 & 2) cannot be answered by equipment connected to the digital blue sockets, even if the digital equipment has analogue ports of its own. Only the 'digital' phone numbers are delivered to the blue sockets.

What is ISDNConnect ?

ISDNConnect is BT's name for data transfer over the D-Channel. The D-channel is the third ISDN2 channel, normally used for signalling and running at 16Kbps. It is permanently online to the exchange, compared to the B-Channels which go on and offline like a conventional line. The D-channel can be used for low volume data transfer and BT's service allows a 2400bps permanently connected channel to a remote location. This might typically be an ISP and be used for checking for email or incoming web server requests which can in turn trigger a dialback call on the B channels. The service is also useful for EFTPOS applications and where low volumes of data need to be sent to remote locations regularly (e.g. Bus stops). The D-channel link is connected permanently to the single destination. ISDNConnect is available on ISDN2e and Business Highway lines. ISDN-Connect is rarely used in the UK.

How much does ISDN Cost ?

There are various options for installation cost, each one with a different rental charge and some with 'call allowances' (included calls). For a breakdown of the charges visit BT's web site. On BT's standard tariff, inland call charges are the same as on a regular (analogue) line. Remember that for the installation/rental charges you do get two channels on the line, which is effectively equivalent to two normal lines.

What is ADSL ?

ADSL is the next generation of high speed Digital Subscriber Line. The A is for Asynchronous, meaning that the upload speed (e.g. 256Kbps)is different to the download speed (e.g. 512Kbps). ADSL gives you a permanent connection to your ISP (as opposed to dial-up like ISDN or analogue lines). ADSL started trialling in the UK at the end of 1998 and was launched across the UK in 2000. For more technical details Click Here.

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