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The telecommunications industry is very used to the concept of contention - it has formed the basis of network dimensioning since the days of Alexander Graham Bell. In a typical enterprise, 200 – 300 people may be sharing a single ISDN30 and if everyone on a particular BT exchange picked up the phone and tried to make a telephone call at exactly the same time most would not get through. Telephone behaviour is predictable and I cannot remember the last time I got ‘network busy’ when I tried to make a phone call. In telecoms, companies do not describe their products using contention (10:1 ISDN2?) so why do they when it comes to broadband?

Well, as with a lot of things it started with BT who launched broadband back in the late 1990s with two products; one where they guarantee there were no more than 49 other people sharing your bandwidth and one where there were no more than 20. The industry was taken a little by surprise in October 2005 when BT Wholesale abandoned the concept of contention in favour of ‘Expected End User Experience’ (BTW BRIEFING 024/05, 06.10.05) and announced that actually the experience of their customers was more like 5:1 and 2.5:1 not 50:1 and 20:1. Most ISPs that had dimensioned (oversold) their networks to match BT’s original specifications were caught out and realised they were now the bottleneck. Only a few premium business ISPs were able to match and advertise the new BTW performance parameters.

So here are some of the common facts and myths around ADSL contention.

1. All ISPs operate to the same contention dictated by BT.

MYTH - Although the BT network is dimensioned to 50:1 Home and 20:1 Office products the actual customer experience through their network is a lot better than this (see below). In general actual contention depends on the ISP and how much they oversell their own network. General purpose ISPs that sell to home and business have very peaky traffic and to compete with the free ISPs they have to sell their capacity many times over. Choose a business-only ISP that has more predictable users and an uncongested network.

2. It is only possible to dynamically control contention ratios with Datastream and LLU.

FACT - With Datastream and LLU the service provider controls both the local loop (the line between the exchange and the end-user) and the virtual path (the leased line from the exchange to the SPs network). Consequently they can decide how many end-users share an amount of bandwidth. If you choose an ISP that interacts with multiple suppliers on a capacity basis rather than per line you will be able to set your own contention ratios.

3. On IPStream only contention ratios of 50:1 and 20:1 are available.

MYTH - BT no longer report the performance of their exchanges and backhaul in terms of contention, they use expected throughput. See table below.

Source: BT Wholesale

4. It is much more expensive to have very low contention ratios and coverage is patchy.

FACT – Because you are reserving bandwidth for your own use or sharing it with few people you are costing the SP more and therefore the charge goes up. The cost for a 1:1 circuit will often exceed that of an equivalent leased line and because it is only available on LLU and Datastream it is probably available in only 30% - 50% of BT’s exchanges. That said, the number of exchanges enabled is increasing every month and in terms of businesses covered it is probably as high as 60%.

Source: Comms Business June 2007.