ericgarnel Fri, 07/06/2007 - 07:12

Please keep in mind that the 54 number is bitrate and not throughput.

You mileage will vary depending on several factors: proximity to AP, polarity, RF interference, obstacles, multipath, radio load, switch load, upstream bandwidth & congestion,etc.

Ideally, if you are not throttling the pipe elsewhere, you could hit @ 22Mbps - but that is under the best conditions.

We massage our traffic with bandwidth shaping, policing, and policy routing to provide as equitable experience for all users as possible.

google iperf for a good tool to measure actual throughput

john.preves Fri, 07/06/2007 - 15:20

Best conditions includes a G only client environment!

If you add one B client in the mix, the mechanisms in place to allow backwards compatibility kick in and you just went from 22Mbps to roughly 8.

I'm trying to find the article that explains the math, but it's pretty ugly.

Rob Huffman Fri, 07/06/2007 - 19:51

Hi John,

Here is one of the references (uggglllyyy) that you spoke of;

The throughput provided by 802.11g networks is dependent upon a number of environmental and application factors, chief amongst them being whether or not the 802.11g network is supporting legacy 802.11b clients.

802.11 networks use Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), a media access method similar to that of shared Ethernet. Also, 802.11b devices, which share the same 2.4 GHz band as 802.11g, have no means of detecting OFDM transmissions. Although 802.11b devices can sense "noise" in the 2.4-GHz band via their Clear Channel Assessment (CCA) capabilities, they cannot decode any data, management, or control packets sent via OFDM. Given this, the 802.11g standard includes protection mechanisms to provide for coexistence and backward compatibility.

When 802.11b clients are associated to an 802.11g access point, the access point will turn on a protection mechanism called Request to Send/Clear to Send (RTS/CTS). Originally a mechanism for addressing the "hidden node problem" (a condition where two clients can maintain a link to an access point but, due to distance cannot hear each other), RTS/CTS adds a degree of determinism to the otherwise multiple access network. When RTS/CTS is invoked, clients must first request access to the medium from the access point with an RTS message. Until the access point replies to the client with a CTS message, the client will refrain from accessing the medium and transmitting its data packets. When received by clients other than the one that sent the original RTS, the CTS command is interpreted as a "do not send" command, causing them to refrain from accessing the medium. One can see that this mechanism will preclude 802.11b clients from transmitting simultaneously with an 802.11g client, thereby avoiding collisions that decrease throughput due to retries. One can see that this additional RTS/CTS process adds a significant amount of protocol overhead that also results in a decrease in network throughput.

In addition to RTS/CTS, the 802.11g standard adds one other significant requirement to allow for 802.11b compatibility. In the event that a collision occurs due to simultaneous transmissions (the likelihood of which is greatly reduced due to RTS/CTS), client devices "back off" the network for a random period of time before attempting to access the medium again. The client arrives at this random period of time by selecting from a number of slots, each of which has a fixed duration. For 802.11b, there are 31 slots, each of which are 20 microseconds long. For 802.11a, there are 15 slots, each of which are nine microseconds long. 802.11a generally provides shorter backoff times than does 802.11b, which provides for better performance than 802.11a, particularly as the number of clients in a cell increases. When operating in mixed mode (operating with 802.11b clients associated) the 802.11g network will adopt 802.11b backoff times. When operating without 802.11b clients associated, the 802.11g network will adopt the higher-performance 802.11a backoff times.

When 802.11g-based networks are operating in the absence of legacy 802.11b clients, network throughput is similar to that of 802.11a. With its OFDM means of transmission and 802.11a backoff scheme, 802.11g can essentially be viewed as 802.11a applied to the 2.4-GHz band.

Note that the throughput increase for 802.11g when in mixed-mode operation is relatively modest when compared to 802.11b, and is a fraction of the throughput provided by 802.11g when not supporting legacy clients.

Check out Table 2 in this excellent doc;

Hope this helps!


john.preves Sat, 07/07/2007 - 04:44


Thanks for the good stuff. I'm going to lift that and stick it in the "file".

Rob Huffman Sat, 07/07/2007 - 05:23

Hi John,

Great idea! I always try to save good docs because I can almost never find them again :) I think your explanation and Erics were both very good and worthy of 5 points as well. It is hard to work with "wireless" without knowing what the expectations and limitations should be, so this question from Matt is a good one that pops up quite often. Once we know what to expect it is far easier to judge the performance of our deployments.

Take care and keep up the good work,



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