isdn - integrated services

5 - ISDN - Integrated Services Digital NetworkNormally the user is connected to the network by analog lines.. The basic ISDN-to-user connection, called a BRI, which stands for Basic Rate

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

Ch 5 - ISDN - Integrated Services Digital Network

Normally the user is connected to the network by analog lines The signals are then digitized and inside the network all communication is digital ISDN brings the digital network to the individual user

Thus, the same twisted-pair copper telephone line that could traditionally carry only one voice, 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 this happen

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 user communication from a telephone, a computer, a fax or almost any other device The third channel called the D channel, carries call setup information for the network, but can also carry 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 computer

There is a second type of ISDN called a PRI, which stands for Primary Rate Interface, that

is normally used by companies PRI contains up to 32 separate channels One of these channels is D channel and the rest are B channels

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

There are different costs involved with ISDN First you have a fixed fee for ISDN

subscription Then you have an establishment cost each time you make a new connection through 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, which means that ISDN is used as extra backup when the leased connection is overloaded

Ch 5 - ISDN - Integrated Services Digital Network

What components are needed to make ISDN work?

First you need an ISDN subscription from a service provider Then you need a line box called NT, which stands for Network Terminal equipment In Europe this is installed by the service provider, but in the USA it must be bought and installed by the user You also need a terminal 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

Basic Rate Interface

The Basic Rate Interface, or BRI, is defined as two 64 kbps Bearer channels, and one 16 kbps Data channel The D channel normally carries call setup data but could also carry user data 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 64 kbps D channel The Primary Rate Interface has different configurations in different

countries In the United States the PRI consists of 23B+D configuration This amounts to a total bandwidth of 1.5 Mbps, and is designed for transmission through a standard North American 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 consists

of 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 to nation, interconnections between any two systems in the world is now not only possible, but increasingly practical

Ch 5 - ISDN - Integrated Services Digital Network

There are different kinds of cable buses used in ISDN

In short bus configuration the maximum length between the NT and terminal equipment is

140 meters The terminal equipment can be connected anywhere on the bus

In extended bus configuration the maximum length between the NT and terminal equipment

is 500 meters There are however some restrictions about where terminal equipment are connected on the bus

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

meters

In Point-to-Point bus configuration only one terminal equipment can be connected to the

NT The maximum length between the NT and terminal equipment is 750 meters

One application is WAN backup where you use ISDN together with a leased line When the leased 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 LAN from a distant place

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

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

Ch 5 - ISDN - Integrated Services Digital Network

One popular application for ISDN is to connect LANs The maximum bandwidth between LANs 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 the

network on the other side The connection establishment time is very short and there are practically no delays when the traffic flows A timer configured by the owner of the router can be set to disconnect the ISDN connection after some time

One of the major uses of ISDN is high-speed access to the Internet More and more

applications on the Internet, like audio and video applications, demand higher bandwidth, for satisfactory results The normal 28.8 kbps is not enough any longer One alternative, for individuals and small companies, is to use ISDN

In the picture you can see a normal configuration used by a small company The equipment

is connected to ISDN through an ISDN router which has a BRI connection In this case the maximum bandwidth for Internet communication that a single user can have is 128 kbps

Ch 5 - ISDN - Integrated Services Digital Network

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

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

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

This functionality includes:

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

In the picture we see a large office using a PRI, ISDN connection The advantage is that you can use the bandwidth that you need for the moment A PRI connection can use from 8 up

to 30 B channels You cannot however use less than 8 B channels A typical solution for PRI

is to connect some of the B channels to the PBX The telephones which are connected to the PBX can use the ISDN network to reach public telephone service The other B channels could be used for data communication

Ch 5 - ISDN - Integrated Services Digital Network

Frame Relay and ISDN are a common internetworking team, with Frame Relay handling the main link and ISDN providing back-up capabilities, replacing traditional asynchronous connections

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

A second limitation is the lack of redundancy with a single leased line For mission-critical applications, as is frequently the case when leased lines are used, this can be a serious

limitation with devastating consequences

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