Ch. 5 -ISDN - Integrated Services Digital Network...Ch. 5 - ISDN - Integrated Services Digital Network 3 There is a second type of ISDN called a PRI, which stands for Primary Rate

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Ch. 5 - ISDN - Integrated Services Digital Network

1 Overview

2 Basic elements in ISDN

3 ISDN - access

4 ISDN - the bus

5 ISDN - examples

6 ISDN - between LAN’s

7 ISDN - access to Internet

8 ISDN - small office solution

9 ISDN - large office solution

10 ISDN - bandwidth on demand


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Normally the user is connected to the network by analog lines. The signals are then digitizedand inside the network all communication is digital. ISDN brings the digital network to theindividual user.

Thus, the same twisted-pair copper telephone line that could traditionally carry only onevoice, or one computer or one fax "communication" can now carry as many as three separate"connections" at the same time, through the same line. ISDN is the "magic" that makes thishappen.

The basic ISDN-to-user connection, called a BRI, which stands for Basic Rate Interface,contains three separate channels. Two of these channels called the B channels, carry usercommunication from a telephone, a computer, a fax or almost any other device. The thirdchannel called the D channel, carries call setup information for the network, but can alsocarry user data transmissions, even if it is not normally used for this purpose.

That means that two separate "communications", say, a voice call and a computertransmission, can take place at the same time through the same ISDN line. The power ofISDN enables these two transmissions to happen at the same time, through the same coppertwisted-pair telephone line that once could handle only one transmission.

ISDN allows more information to be sent, more reliably and at higher speeds, and in mostcases without changing the telephone wiring in your house or building. Another advantage isfast connection establishment. For example, with ISDN, it takes only few seconds toestablish connection with the Internet, compared to 40 seconds for an analog connection.


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There is a second type of ISDN called a PRI, which stands for Primary Rate Interface, thatis normally used by companies. PRI contains up to 32 separate channels. One of thesechannels is D channel and the rest are B channels.

ISDN is standardized by ITU. There are different configurations of ISDN. The one used inEurope is called Euro-ISDN.

There are different costs involved with ISDN. First you have a fixed fee for ISDNsubscription. Then you have an establishment cost each time you make a new connectionthrough the ISDN network. Finally you have to pay for the time you are connected.

ISDN is used for many different applications. One example is bandwidth on demand, whichmeans that ISDN is used as extra backup when the leased connection is overloaded.


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What components are needed to make ISDN work?

First you need an ISDN subscription from a service provider. Then you need a line boxcalled NT, which stands for Network Terminal equipment. In Europe this is installed by theservice provider, but in the USA it must be bought and installed by the user. You also need aterminal adapter. This adapter is sometimes called an ISDN modem or an ISDN router.Finally you need terminal equipment such as a PC or an ISDN telephone.


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Basic Rate Interface

The Basic Rate Interface, or BRI, is defined as two 64 kbps Bearer channels, and one 16kbps Data channel. The D channel normally carries call setup data but could also carry userdata across the network. The BRI interface is also referred to as a 2B+D connection.

Primary Rate Interface

The Primary Rate Interface, or PRI, is defined as several 64 kbps B channels, and one 64kbps D channel. The Primary Rate Interface has different configurations in differentcountries. In the United States the PRI consists of 23B+D configuration. This amounts to atotal bandwidth of 1.5 Mbps, and is designed for transmission through a standard NorthAmerican T-1 trunk.

In Europe and the Pacific, the transmission standard differs from the one used in the USA.The Primary Rate Interface is supplied through a standard 2 Mbps E-1 channel, and consistsof either 30B+D configuration used in Europe or 31B+D configuration used in the Pacific.Although the specifics of ISDN implementation are still slightly different from nation tonation, interconnections between any two systems in the world is now not only possible,but increasingly practical.


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There are different kinds of cable buses used in ISDN.

In short bus configuration the maximum length between the NT and terminal equipment is140 meters. The terminal equipment can be connected anywhere on the bus.

In extended bus configuration the maximum length between the NT and terminal equipmentis 500 meters. There are however some restrictions about where terminal equipment areconnected on the bus.

In both short bus configuration and extended bus configuration you can connect up to 8terminal equipments, but the maximum length between two terminal equipments is 50meters.

In Point-to-Point bus configuration only one terminal equipment can be connected to theNT. The maximum length between the NT and terminal equipment is 750 meters.


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One application is WAN backup where you use ISDN together with a leased line. When theleased line is overloaded or broken the ISDN connection becomes active.

Another application is remote access. This could refer to Internet access or access to a LANfrom a distant place.

A third application is Simultaneous voice, fax and data. This could be very convenient forpeople working from home.

ISDN could also be used as a WAN service, connecting two LANs with each other.


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One popular application for ISDN is to connect LANs. The maximum bandwidth betweenLANs is 128 kbps if BRI is used. If PRI is used the maximum bandwidth is 2 Mbps.

The ISDN call is automatically made by the router only when data is to be sent to thenetwork on the other side. The connection establishment time is very short and there arepractically no delays when the traffic flows. A timer configured by the owner of the routercan be set to disconnect the ISDN connection after some time.


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One of the major uses of ISDN is high-speed access to the Internet. More and moreapplications on the Internet, like audio and video applications, demand higher bandwidth, forsatisfactory results. The normal 28.8 kbps is not enough any longer. One alternative, forindividuals and small companies, is to use ISDN.

In the picture you can see a normal configuration used by a small company. The equipmentis connected to ISDN through an ISDN router which has a BRI connection. In this case themaximum bandwidth for Internet communication that a single user can have is 128 kbps.


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In the picture we can see two types of configurations. In the first type there is an ISDNcard installed in the PC, which can communicate directly with the NT. This is the cheapestsolution, but maximum two users can have ISDN access.

In the second type an ISDN router is used. In this configuration many users can have ISDNaccess at the same time. A PC must have a network interface card to be able to communicatewith an ISDN router. This cards are not special in any way. Normal Ethernet cards forinstance have this functionality.

One common application for ISDN in these configurations is telecommuting. The idea issimple: to "transport" as much as possible of the functionality of the office to a remote sitethrough a single ISDN BRI connection.

This functionality includes:

1. Acceptably high-speed access to the user's LAN and file servers.

2. Full access to mailboxes.

3. Access to the Internet.


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In the picture we see a large office using a PRI, ISDN connection. The advantage is that youcan use the bandwidth that you need for the moment. A PRI connection can use from 8 upto 30 B channels. You cannot however use less than 8 B channels. A typical solution for PRIis to connect some of the B channels to the PBX. The telephones which are connected to thePBX can use the ISDN network to reach public telephone service. The other B channelscould be used for data communication.


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Frame Relay and ISDN are a common internetworking team, with Frame Relay handling themain link and ISDN providing back-up capabilities, replacing traditional asynchronousconnections.

Leased lines have two major limitations. Peak periods of traffic that overload the availablebandwidth, causing congestion and delays. If the leased line is sized correctly, then a secondleased line of any capacity is overkill. In fact, normal, non-peak, traffic may be well belowthe data rate of the existing line.

A second limitation is the lack of redundancy with a single leased line. For mission-criticalapplications, as is frequently the case when leased lines are used, this can be a seriouslimitation with devastating consequences.

Separate provisions that use dial-up lines can provide the necessary backup and overflowbandwidth inexpensively and transparently. Unfortunately, integrating dial-up backup withthe existing leased lines can be a challenge. Most routers are not equipped to handle backupand overflow cost-effectively.

A solution for this problem is an ISDN Basic Rate Interface service which provides dial-upbackup and bandwidth on demand.

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Ch. 5 -ISDN - Integrated Services Digital Network...Ch. 5 - ISDN - Integrated Services Digital Network 3 There is a second type of ISDN called a PRI, which stands for Primary Rate