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Web Performance Tuning, 2nd Edition by Patrick Killelea

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How Much Bandwidth Do You Need?

Server bandwidth is the single most important factor in the performance of your web site. The math to determine what bandwidth you need is, in essence, very simple:

hits/second * average size of a hit in bits = bits/second

That is, you need some estimate of the number of hits per second you want to be able to serve. Then you need to know the average size of one of these hits. From this, you know what sort of network bandwidth you need.

Latencies Are More Important than Bandwidth

It has become clear that the number of packets is a more significant determinant of web performance than raw bandwidth once users are beyond ordinary dial-up modems. This is because each packet must be acknowledged, and the speed of light fixed, while bandwidth is increasing. It may take 20 milliseconds to send a 1500- byte packet to a PC on a DSL line, but only 12 milliseconds to get it from the network into the PC. It will take another 20 milliseconds for the acknowledgment to get back to the sender. So the 40 milliseconds latency is more than three times as important as bandwidth in this case, and it will only get more important later.

This is why it is so important to keep the number of individual items on a page to a minimum. Still, because most browsers are multithreaded, some latencies can happen in parallel. It turns out through experimentation that the best number of embedded images on a page is about the same as the number of threads the browser uses. For example, ...

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