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The TCP/IP Guide  9  Networking Fundamentals       9  Network Performance Issues and Concepts

Understanding Performance Measurement Units 1 2 3 Simplex, Full-Duplex and Half-Duplex Operation

The Effect of Asymmetry

In addition to the above, bear in mind that many networking technologies, especially ones used for Internet access, are asymmetric, meaning that they offer much higher bandwidth in one direction than the other. Usually, this is arranged so that more bandwidth goes down to the user than from the user to the network, since most Internet users download far more than they upload. However, it's always important to find out if a speed rating is for both directions, or for only one direction, and if so, what the other direction's speed is. Common technologies with asymmetric performance include 56k modems, ADSL, cable modems, and satellite Internet access. Beware, because the marketing droids who sell these technologies will often try to hide the asymmetry of their services, usually highlighting only the bigger download figure and avoiding mention of the slower uploads.

Asymmetry can also have unexpected effects on network performance, because most communications, even if they seem unidirectional, are not. The most common case is when an Internet access technology has much higher download bandwidth than upload bandwidth. When using TCP/IP to download data, acknowledgments must be sent regularly. If the upstream bandwidth is too low, this may make it impossible to fully exploit the download bandwidth of the link.

Finally, take into account that there are many different ways of measuring and assessing performance. Synthetic benchmark programs are often used to measure throughput, and can produce impressive performance scores—which usually have little to do with how a network will actually operate. Such metrics are best used for comparison purposes, by showing that one network or system is faster than another, rather than paying too much attention to the actual number they produce. Even when doing comparisons, however, caution is wise.

Understanding Performance Measurement Units 1 2 3 Simplex, Full-Duplex and Half-Duplex Operation