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Download limits on their way?
Wednesday 02 October 2002 14:58:00 by Andrew Ferguson

That is what many people are worrying about. The news that BT Openworld is to cap the online time for its unmetered dialup service has brought the topic back into the limelight.

It is worth looking at the differences in the different services. Dialup relies on only a fraction of users being online at any one time, this is often referred to as modem contention, ratios of 10 users for every port are not uncommon. A good study of the economics of unmetered dialup was done by For the restrictions on bandwidth that have happened to BTO Satellite service users, again it is a case of limited resources. Satellites have limited bandwidth at any one time, so it must be managed carefully, you can stay connected all the time but not pulling your maximum bandwidth down continually.

So where does ADSL fit into this equation? ADSL is sold as a contended service. Contention exists at the local exchange, the route into ISPs network (BT Central), the ISPs own network and upstream bandwidth. For the vast majority of users on ADSL, the maximum level of contention at this time is in the BT Central area, contention levels of 10:1 are not uncommon. At this time it is generally a lot less than the worst case value of 50:1 that the Home products are sold as.

Concentrating on the BT Central, these cost £40,000 per year for a 155Mbps pipe, which can hold around 5000 users (true cost can be a lot higher once installation and router hardware has been purchased). Contention may start to appear though if you had just 310 users downloading at 512kbps. Potentially this means that just 5% of the user base can cause slow-downs if they download at 512kbps all the time.

So what is likely to happen in the future? Well, to a great extent it is down to the users. If people behave reasonably and do not download data every day at full speed for weeks on end then ISPs are likely to let the status quo continue. Alas if just 15-20% of users are trying to download 4GB or more a day for extended periods then everyone will start to complain of slow speeds and ISPs may be forced to either purchase more BT Centrals or do something to reduce the amount of data downloaded by some users.

If bandwidth charging is introduced what can we expect? It is hard to be certain, but we would expect that users who manage to download more than 3GB per day on average over a month may be heading for trouble. ISPs may introduce a tiered pricing system, eg. Bronze 1GB/day, Silver 2GB/day, Gold 3GB/day or more, all averaged out over a month. The period of time that charging is averaged over is important, by working to a monthly schedule it allows people to download large files now and then with no worries.

How much can we expect to pay? The problem for the ISPs is the cost of actually collecting what will be a relatively small amount and potentialy variable amount from month to month, it could be anywhere from £5 to £20 per month.

So how much bandwidth do I use? Well the table below gives some ideas. A 512kbps connection running flat out over a 24 hour period can download a maximum of 5GB.

Application KiloBytes/sec MB/hour MB in 8 hours
Audio streaming 8 30 240
Online gaming 4 15 120
Video Streaming 37 135 1080


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