Q

True capacity of each 802.11b channel

When using an 802.11b access point the maximum throughput I see is approximately 5 Mbps. What is the 11 Mbps all about?
The capacity of each 802.11b channel is shared by management, control, and data frames sent to/from all radios using that channel. This is also true on 802.3 Ethernet LANs - 802.11 and 802.3 are both shared mediums, and application throughput is always less than link speed due to overhead. However, 802.11 is a higher overhead protocol than 802.3. It is not usual for an 802.11b radio with link speed of 11 Mbps to experience effective application throughput of 5.5-6 Mbps. Similarly, an 802.11a radio with a link speed of 54 Mbps will have an effective throughput of 27 Mbps.

In addition, Ethernets can operate in full duplex mode - receiving at 10 Mbps while simultaneously sending at 10 Mbps. But an 802.11 radio always operates in half-duplex mode - it cannot transmit while receiving, or vice versa, because the channel is busy. An 802.11 product would need two radios to operate in full-duplex mode, transmitting and receiving simultaneously on two different channels.

While you will never get 11 Mbps out of an 802.11b radio, there are things you can do to optimize performance. The best thing you can do to increase throughput is to make sure you have a high-quality link in the first place - 802.11 radios automatically adjust link speed in response to signal quality, so just because you're using 802.11b doesn't mean your link speed is 11 Mbps. If your card is showing a link speed of 5.5, 2, or 1 Mbps, try moving the station and/or AP to avoid sources of interference that are degrading signal.

If your card and AP BOTH support it, choose the option to use short headers. If you have a high-quality link where there is not much loss, you can also try setting the RTS threshold higher. Ready-to-Send/Clear-To-Send (RTS/CTS) is used by 802.11 to balance the cost of retransmission with the cost of handshaking. A higher threshold means there will be less handshaking and (in theory) less overhead. But a higher threshold also makes collision more likely - resulting retransmissions of course reduce effective throughput. Defaults are usually best, unless you have plenty of time and patience to fine-tune RTS parameters.

This was first published in February 2003

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