ISDN - Integrated Services

Ch. 5 - ISDN - Integrated Services Digital Network

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.

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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 and overflow cost-effectively.

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

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