Wifi – How do mixed wireless clients (b/g/n) effect total throughput of an access point


Say you are planning a network based around access points that support the full range of 802.11 protocols on 2.4Ghz(b/g/n). How will a mixed set of clients effect the total throughput possible through the access point?

For example, say we have the following scenario:

  • 1 Access Point
  • 1 b client with an 11 Mbps connection transmitting 5 Mbps
  • 1 n client with a 135 Mbps connection can transmit ? Mbps

Can the second client transmit 65 Mbps (ie the first client uses 'half the spectrum')? Or can he use only ~5 Mbps (ie the lowest common denominator sets the rate)? Or is reality something else completely?

Best Answer

It depends on the specifics of the situation and the capabilities of the access point.

An 11Mbps client will tie up 100% of the spectrum space it is using when it transmits 11Mbps of data. It will tie up the channel 50% of the time to transmit 5.5Mbps. However, modern 802.11n access points are capable of sustaining multiple streams. So while an 802.11b client will tie up one stream, others will be free to run at higher speeds.

Note that this applies also if an 802.11a/g/n client is forced to use a reduced rate due to signal strength issues. If the rate is dropped to, say, 27Mbps, then 27Mbps will fill the channel. If its transmit rate is 27Mbps and it's transmitting 5Mbps of data, it will consume the channel roughly 5/27th of the time.

However, there is a critical complicating factor. If there simply exists an 802.11b client on the channel, all traffic must be preceded by a burst (called a 'preamble') sufficient to let the 802.11b client know that the channel is in use. The 802.11b preamble, naturally, is much longer than the 802.11n preamble. So the mere existence of an 802.11b client will slow down the network for everyone.

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