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Internet Connection Speeds

Updated 18/3/02

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From a response on the CONet discussion forum by Horace Mitchell - reproduced with his permission.

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Some Web terminology


Original issue: >> Our ISP maintains that due to our set up [community internet cafe sort of thing] and ADSL being a shared service, we should keep the more expensive ISDN.

I think your ISP is either confused or disingenuous! ADSL isn't "shared" it's contended, not quite the same thing. As I understand it the current BT contention ratio for business ADSL is 20:1, which means that the worst case would be twenty users all doing high intensity downloads concurrently.

In reality of course the amount of traffic per user depends mainly on the rest of the connection to whatever distant server they are connecting to. For example when you use your local ADSL link to reach some website or other you get a potential 2 meg (or whatever) download performance between you and the local exchange, and perhaps the same between the exchange and your ISP. What you get between the ISP and the rest of the net varies enormously, some ISPs have very high capacity relative to user activity, others much lower. Then your data crosses the Internet to the distant website's ISP. What
performance that achieves is in the hands of God. And then what you get locally from the distant website depends on lots of factors - their connection to the Internet, their server performance, the number of concurrent users . . . .

And none of this is distant dependent - the website might be next door but you are still crossing the Internet and dependent on all the same factors.

So on your 2 meg (or whatever) line you might get a very slow connection to website X even when you only have one internal user connecting and there are no other BT customers contending.

The worst case (in very round figures), might be like this:

- BT actually reaches 20:1 on your connection (that's the maximum they will allow, not the number they actually have as contending customers at any one time);

- all twenty of you are connected concurrently;

- all twenty are trying to download an intensive data stream across a well-tuned and high capacity ISP link;

- the Internet routings for all twenty are in very good shape and delivering high performance;

- all the distant websites concerned have excellent Internet connections and their servers are delivering high performance with relatively few concurrent users.

In this worst case your notional 2 meg is shared with the other 20 users so each of you gets 100kbps (I did say round numbers!). This means you are now getting less than you could get with your 2 x 64kbps ISDN link.

Assuming BT does really have 20 contended users on your connection, no doubt from time to time you might hit this in a transient way. If it happens a lot BT will have twenty unhappy customers and may have to scale down the contention ratio. But I expect they would do that quietly in any case. And in a more likely scenario, one of the contending customers has a high download intensity, another is mainly doing email, a third is mainly managing a website etc.

If you find that a 2 meg ADSL link aint fat enough BT will happily rent you a dedicated link at a much higher rate than 2 x 64 ISDN [the current connection of the enquirer]. They call it a leased line, but they've been using DSL to support leased lines for yonks.

By the way I believe the contention ratio for consumers on the standard 512/256 consumer ADSL service is 50:1. I think BT and its resellers are also being somewhat disingenuous when they talk about ten to forty times the performance compared with a conventional modem, since the download performance we see depends on all the other factors rather more than it depends on our local connection. When the net is busy and your chosen website is busy a faster local connection doesn't make much difference. Feels good to know that you are not the cause of the bottleneck though.