Frame at last

 

ORGANISATIONS considering their communications needs face a sometimes bewildering array of options. Usually, their decision is based on three prime considerations: organisation needs, performance and cost.

For many companies, dial up telephone services, fax, LAN (local area network), and occasional data transfers using modems are all that is required. Even access to the Internet can be achieved via ordinary telephone lines with a modem.

Other companies in similar circumstances, but with greater data requirements, choose access via ISDN (high speed digital phone lines) because it is at least twice as fast as the highest speed modem.

Larger organisations based in more than one location often opt for the fixed cost of leased digital lines, working at speeds of 64 kilob its per second (kbps) and greater. These can carry all inter office communications voice, fax, LAN data, and file transfers - and provide major cost saving and operational benefits. All voice and fax calls are effectively - carried free of charge because the public network is avoided, and companies can use break out facilities. Operationally, companies can have direct desk to desk/dialling, and bypass the switchboard.

The number of leased line networks providing this type of integration had mushroomed since Telecom Eireann launched its DASSNET high speed network in 1990, with many companies making large savings on their phone bills. In many cases, these savings have actually paid for the capital and running costs of the networks within 18 months.

But there is still a large sector of the business community whose communications patterns don't justify fixed leased line costs. For many of them frame relay is now providing a very good alternative.

Frame relay

Frame relay is an internationally recognised, and standardised means of communications. In essence it is a very efficient protocol or convention for exchanging data, which does not overburden the data transfer with, unnecessary overhead information.

A frame relay network appears to users quite like a leased line network. However, the leased lines are "virtual" - they share a common network with other users. This results in a more efficient use of the carrier's transmission capacity, and hence the scope for lower pricing. A simple frame relay network is shown in the diagram, where the public portion is boxed in the centre.

Why use it?

Frame relay employs advanced "statistical multiplexing" techniques to allow a given network the capacity to carry considerably more traffic than it could if each user were allocated a fixed bandwidth. This is why circuits provided over frame relay are tariffed at a lower rate than the equivalent for dedicated leased lines. For example, Cornel recently installed a customer network (see diagram) with nodes in Dublin, Cork, Limerick, Waterford, and Galway. The frame relay annual charges were less than £10,000. The equivalent leased line charges would have been over £20,000 per annum, and the installation charges were almost identical.

Resilience is also normally better within the frame relay network, where automatic rerouting around failed links is provided. (This, of course, does not help if the customer's local cable is damaged.)

The remaining question concerns compressed voice traffic over frame relay. Recent independent tests have shown that packet based multiplexers achieve the best voice quality over frame relay. It is important to ensure that the committed information rate (see below) configured for a particular link is at least equal to the maximum possible voice occupancy of that link.

Getting connected

To order such a network from Telecom Eireann, it is necessary to specify a number of items:

1. The access rates at each site. This is typically 64 kbps at smaller sites and multiples of 64 kbps (e.g. 128 kbps) at the central site.

2. The layout of the virtual circuits, know as permanent virtual circuits (PVCs).

3. The committed information rate (CIR) is the guaranteed throughput rate which the net work will provide to the customer under normal conditions, that is when there are no failures in the network. In the network shown in the diagram, for example, for each virtual circuit a guaranteed rate up to 64 kbps, with up to 128 kbps in Dublin, could be defined.

4. Lower cost bandwidth can also be purchased. This is called the excess information rate (EIR), and it is used in networks which need extra capacity at certain peak times of the day. This is appropriate where average data rates fall within the guaranteed capacity and data peaks can be passed on excess capacity. Obviously, the access rate has to provide for the maximum required circuit capacity.

5. The total circuit capacity is a sum of the committed information rate and the excess in formation rate.

What next?

Frame relay services with access speeds in the range of 64 kbps to 1 Megabit per second (Mbps) will be appropriate for many businesses in Ireland for the foreseeable future.

However, it is important to keep a keen eye on other developments such as SMDS (switched multi megabit data service) where access speeds up to 25 Mbps are possible; and the ultimate back bone structure is likely to be ATM (asynchronous transfer mode). Meanwhile frame relay is a major step along the way.