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ISDN for LAN Internetworking

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What is ISDN?

Integrated Services Digital Network (ISDN) is an extension of the public telephone network, designed to carry digitised voice calls, or data, from one subscriber to another. Its main advantages over the conventional telephone network are better voice quality, higher data speeds, lower error rate, faster call setup times and greater flexibility.

ISDN call charges are similar to conventional telephone calls which, combined with the high data rates available, make ISDN a good choice for LAN internetworking, particularly when communication is intermittent.

This application note gives a short description of ISDN technology, including Euro-ISDN. It also covers why ISDN is suitable for LAN internetworking and the types of ISDN LAN internetworking equipment. Some example applications are then covered, and, finally, some specific country differences in the uses and tariffs for ISDN.

ISDN Technology

In ISDN, the conventional analog telephone signal is replaced by a 64 Kbps digital data stream, called a B channel. In an ISDN voice call this B channel carries the digitized speech; in internetworking applications we use it to carry data. To make, receive and generally control calls, an additional signalling channel, called the D channel is used.

ISDN is available on two main types of interface, a basic rate interface (BRI) or a primary rate interface (PRI). The difference is in the number of B channels which each can support.

ISDN Numbering

The following is what needs to be ordered from the local PTT to access ISDN.


  • Basic Rate ISDN line (ISDN2)
  • Auxiliary working -- this associates both B channels with the same ISDN "telephone" number
  • Optional for added security is Calling Line Identification (CLI)


  • Basic Rate Line with ITR6 or DSS1 - comes complete with all required features
  • If using Euro-ISDN then request standardanschluss

Basic Rate ISDN

Basic rate is the service which corresponds directly to the conventional telephone line installed in a home or small office. It is delivered over a pair of copper wires and the ISDN plug is similar to a normal telephone jack.


By using the benefits of the digital technology, a single BRI can improve on a single phone line -- in each BRI there are two B channels, which are each 64 Kbps pipes, and the D channel which runs at 16 Kbps.

This means that a single BRI can have two telephones plugged into it, which can each call a different destination at the same time. Alternatively, a more sophisticated product like a router could simultaneously use both B channels to two destinations.

Primary Rate ISDN

An ISDN Primary Rate Interface provides up to 30 independent 64 Kbps B channels and a separate 64 Kbps D channel to carry the signalling. It is intended for larger commercial users, and would generally be attached directly to a PABX, or high volume data networking product.


Since there is a substantial difference between the bandwidth of BRI and PRI, many telecommunications operators, including BT, provide intermediate services which are a subset of Primary Rate. These can offer, for example, six B channels.

The support of the BONDING (Bandwidth ON Demand INteroperability Group) consortium's recommendations for inverse multiplexing, allows the PRI card to aggregate from 2 to 30 B channels and therefore back up a high speed line. Data rate adaption allows connection to ISDN terminal adapters attached to the ISDN network and to 56 Kbps ISDN interfaces.

PPP Multilink Protocol is a method of utilising two or more B channels so as to make maximum use of the available bandwidth, thereby controlling WAN cost spend.


Many national telecommunications operators (TOs) have introduced their own implementations of ISDN. While this satisfied the need of national communications, the international interoperability of services and equipment could not be guaranteed. Pan-European approvals have just become possible for both ISDN equipment and equipment where there is a recognised harmonised standard e.g. X.25, through the CE approval.

In a Memorandum of Understanding (MoU), 26 TOs from 20 European countries have undertaken to introduce an ISDN service which conforms to an agreed European standard. This service is called Euro-ISDN.

Euro-ISDN has the following core set of features:

  • Basic Rate and Primary Rate access
  • An audio service corresponding to the current PSTN and an unrestricted 64 Kbps digital service
  • Five supplementary services:
    • Calling Line Identification Presentation (CLIP)
    • Calling Line Identification Restriction (CLIR)
    • Direct Dialing In (DDI)
    • Multiple Subscriber Numbering (MSN)
    • Terminal Portability (TP)

The Evolution of ISDN

In most countries in the 1950s, there were two main types of public networks: the telephone network and the telex network.

The telephone network was based on analog techniques which had not changed much since the beginning of the century.

The telex network was based on a 50-baud digital network. It had developed because the telephone network, with converters (now called modems) to change the telex signal from analog to digital, was prone to unacceptable error rates.

The next phase of development was inspired by the rapid rise of computers and the need to communicate between them. Using the standard analog telephone network with modems, communication speeds of up to 100 baud were possible. This has now typically increased to 9600 baud.

However, due to this limited speed and the long call set up time, new specialised networks for computer communications became necessary. These were either circuit switched, like the old telex network, or packet switched. Packet-switched networks were the choice in most European countries, with the exception of the Nordic countries. In 1976, the X.25 standard was developed for packet-switched services. Typical speeds for these networks are 9600 bps or 4800 bps.

In parallel with this, the main telephone networks were being converted from analog speech into a digital transmission to remove distortion. This was done by sampling the analog data at 8 kHz and coding it into 8 bits. This gave a binary stream of 64 Kbps. In the 1970s, all major telephone networks were replaced with this new digital system, using cables which could carry 30 such channels interleaved (24 in the US).

However, this new digital system did not extend to the "local loop" to the end customer. This part of the link was still the old analog system. But the basic infrastructure was there to carry both data and voice up to 64 kbits/s, giving an Integrated Digital Network.

The next obvious step was to extend this all the way to the customer so that all services could be integrated. This gave rise to the Integrated Services Digital Network or ISDN.

The CCITT-defined standards for ISDN services and interfaces to be used throughout Europe in 1984. This was enhanced by standards drawn up by the European Telecommunications Standards Institute (ETSI) in 1988. Further work by the European signatories to a memorandum of understanding committed them to set up core ISDN services conforming to ETSI standards by December 1993. The term Euro-ISDN is used to describe the implementation of these services.

LAN Internetworking

The four main technologies for linking LANs are Leased Line, X.25, Dialup, and ISDN.

Leased Lines have fixed rental costs based on speed and distance and are usually installed for a minimum of one year. Costs are the same, whether there is any data being transmitted or not. This means that they are suitable for constant access, but can be prohibitively expensive for ad hoc communication.

X.25 charges are based on line speed, call duration and volume of data sent. The data charge dominates the overall costs, making X.25 good for interactive applications, but expensive for file transfer.

Dialup services using the PSTN are charged at the same rate as telephone calls. They are limited by the speed and can be poor quality, especially for international calls. The call setup time can also be quite long. They are mainly used for short, infrequent communication.

ISDN is tariffed like a phone call, based on call duration, time of day and distance. As there is no volume data charge, as with X.25, it is most cost effective for file transfer. Another major advantage is that adding a new site becomes much easier and more cost-effective than with a leased line solution. The inbuilt security of ISDN services make it a better option than Dialup.

Spoofing for ISDN

In typical client server environments, particularly Novell Netware, when a connection between a server and client is idle, packets are often sent between network nodes.

In a LAN-only environment these packets do not cause a problem but when used across a WAN link, such as ISDN, they can significantly increase the cost of operating the network. "Spoofing" is a method of responding locally to these "keep-alive" packets, thereby optimizing WAN costs.

ISDN Benefits

The main benefits of ISDN are:

  • Quality. ISDN connections are very low error rate digital pipes.
  • Flexibility. ISDN can be thought of as a configurable leased line. Connections can be established at any time between any two locations where ISDN is available. It offers a very fast (almost transparent) call set-up, so its dialup nature is transparent to most users.
  • Economy. ISDN is tariffed like a telephone call. Usage costs are identical to the telephone service. In general, ISDN is extremely cost-effective for intermittent LAN to LAN connectivity.
  • Availability. ISDN is now very widely available across Europe, particularly Euro-ISDN.

ISDN Internetworking Equipment

There are different internetworking devices which can make use of ISDN.

Terminal Adapters (TAs)

These are external devices which connect a conventional data interface, such as X.21, to an ISDN circuit, allowing non-ISDN equipment to use the ISDN. Terminal adaptors are widely used by internetworking manufacturers who do not have an approved native ISDN interface for their devices.

A disadvantage of this solution is that not all the information from the D channel passes through the TA, so the non-ISDN equipment cannot take full advantage of ISDN facilities, such as Calling Line Identification.

ISDN Bridges

Because of its simplicity, bridging is one of the most popular ways of linking LANs. The big problem with ISDN bridging is controlling the bridge's use of the ISDN network. Bridges are simple to set up and use because they will forward data, such as broadcasts, by default. Over ISDN, this means that calls will be made to send largely unnecessary data. Over a period of time, this can prove very expensive.

To avoid this, bridges can be configured to block broadcasts from specific addresses, and to understand particular protocols. But the bridge loses its major benefit of simplicity. Bridges are ideal for ISDN backup.

ISDN Routers

Routing is a far more effective way in which to utilise ISDN for LAN internetworking. This is the approach being taken by all the major internetworking vendors.

Data is only sent over the ISDN network when it is really needed. There are no unnecessary broadcast messages to transmit, so the bandwidth is used more efficiently than with bridges and the configuration can actually be simpler! As the network grows, the additional configuration is easy compared to setting up filters on bridges to block out all unnecessary traffic.


In many businesses today, there is an increasing need to connect home office users into the central LAN to enable resource and application sharing.

ISDN PC cards allow individual workstations to connect to a central site that contains an ISDN router. This gives home users access to the resources of the entire corporate LAN.

ISDN Internetworking Applications

The following applications show how versatile internetworking with ISDN can be.

Routers from Shiva support all the applications detailed below, as well as many others.

Connection Directly into ISDN

The ISDN Router can be connected directly into the ISDN socket (NT1) supplied by the telephone operator.


Connection can also be made into an ISDN bus, where other ISDN equipment is sharing the ISDN line. Each piece of equipment (up to a maximum of eight) will have a different extension number and all will contend for the two B channels available.

Use of ISDN Router through ISDN PABX

This is a common application where you already have several ISDN lines in your office, and want to share the use of outgoing lines, or where a primary rate line comes into your PABX. ISDN supports direct dialling through a PABX, allowing incoming data calls to be sent directly to the router.


Bandwidth on Demand

The main advantage of ISDN is that bandwidth is available when required and only paid for when used. When data is being sent, the connection is open and when it is idle, the connection is automatically closed.


If access to a remote location is required for less than, say 20 minutes every business hour, then ISDN is more cost-effective than a leased line in all major European countries.

Leased Line Backup


A very common use of ISDN, particularly in the UK, is as a backup to leased lines. In the figure above, the leased line is the primary link *. The ISDN connection * is kept in reserve. If the primary link fails, the ISDN link will be used automatically. When the primary link becomes available again, the ISDN connection is terminated. ISDN Bridges are ideal for this application.

Bandwidth Augmentation

ISDN can be used to 'augment' other services, such as leased lines, where the cost of another leased line would not be justified. When the leased line * reaches full capacity, the ISDN connection * is made. When the requirement falls back below the leased line capacity, the ISDN connection is then closed. This process is automatic. ISDN Bridges are ideal for this application.


Bandwidth Aggregation


ISDN routers can dynamically allocate extra B channels for high bandwidth applications. As the bandwidth increases, extra B channels can be opened, giving a much higher effective link speed. Router (1) above can aggregate extra B channels to Router (2) as required. This process is sometimes referred to as Inverse Multiplexing.

Remote PCs Access to the Corporate LAN

ISDN provides a cost-effective way to connect remote PC users with individual workstations to the corporate LAN. The PC requires an ISDN card and appropriate protocol support, such as IP. Connection is made via an ISDN router. If the router supports primary rate ISDN, then up to 30 PCs can have simultaneous access to the LAN.


ISDN in Europe

ISDN for LAN interconnection is still a small part of the total ISDN market, although it is growing strongly. The suitability of ISDN for LAN interconnection is closely related to the different tariffing and availability in each country.


Germany is the largest market for ISDN in Europe. The key reason for this is low tariffing. As an alternative to a leased line, ISDN can be used for up to 14 hours in the day before a leased line would be more cost-effective.

The German national ISDN protocol is called 1TR6. Since there is such a large installed base of 1TR6-based ISDN, DBP Telekom will offer network termination equipment that supports both 1TR6 and Euro-ISDN to ease the transition.


Under 1TR6, a service providing semi-permanent connections is available. With this service, a fixed tariff is applied to calls to predefined numbers, overriding the normal ISDN charges. This service is not available under Euro-ISDN.

Connection is possible to Datex-P -- the X.25 service -- via new packet mode services on both the D and B channels. Both basic rate and primary rate services are available.

In Germany an ISDN service offering "mailed-up" links to a fixed destination is available. This is called leased line ISDN.


The commercial ISDN network in France is called Numeris and provides both basic rate and primary rate services. It is well established and second only to Germany in number of installations. The current protocols used are either VN3 (which will be withdrawn) and VN4 which is being introduced to provide a transition to Euro-ISDN. Current tariffing in France means that ISDN is more cost-effective than a leased line when usage is up to approximately 4 hours in the day.

Due to the widespread use of the Transpac X.25 network in France, a service has been available since 1993 to provide X.25 over the ISDN D channel. A new service provides access over the B channel, but only using a separate unit to provide inter-working.


The ISDN market in the UK has been slower to develop than in France and Germany. The main reason is that the tariffs for leased lines are considerably cheaper. ISDN has therefore been used mainly to back up leased lines. Tariffs are being changed to make ISDN a more attractive option. It is currently cost effective to use ISDN instead of leased lines when usage is less than 6 hours in the day.

Both BT and Mercury provide ISDN services in the UK. BT's basic rate service (ISDN 2) is based on Euro-ISDN. Mercury has an indirect basic rate service (ISDN 132) via BT's connections. Both BT and Mercury provide primary rate services.

See Also