Q

Wireless speed

I have a 10BaseT Hub with 55mbs Wireless Access Points connected to it. What speed will the wireless computers receive will they receive the 55Mbs that the access point sends out or will it be the 10 mbs that the hub sends to the access point? If your wireless computer is very close to your 802.11a access point, it may see a data link rate of 54 Mbps. However, actual throughput will be limited by the bottleneck. On paper, the bottleneck...

appears to be the (shared) 10BaseT hub. In practice, this may or may not be true. Standard 802.11a access points have a top link speed of 54 Mbps, but effective application throughput is much lower.

First, WLAN distance and speed are inversely related - the further a station is from the access point, the slower the link speed. For example, in 802.11a tests conducted by Atheros, the data link rate was 36 Mbps at 65 feet, dropping to 6 Mbps at 225 feet. Obstructions further reduce range.

Second, the capacity of each 802.11 radio channel is consumed by management, control, and data frames sent and received by all radios using that channel. While 802.3 CSMA/CD Ethernet LANs also use a shared medium, the overhead associated with 802.11 CSMA/CA Wireless LANs is higher. So, even if a Wireless LAN and an Ethernet LAN were operating at approximately the same link speed, the WLAN would be the bottleneck.

Third, Ethernet NICs operate in either full or half duplex mode - a 10BaseT card operating in full duplex mode receives at 10 Mbps while simultaneously sending at 10 Mbps. An 802.11 radio operates in half-duplex mode - it does not transmit while receiving, or vice versa.

Finally, application throughput is affected by many additional factors like message length, TCP windowing, contention for resources, and capabilities of the sending and receiving hosts. So, you can see, there is no simple answer for how fast your wireless computers will actually receive data, even if they do "connect" at 54 Mbps.

This was first published in August 2002

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