802.11g & 802.11b Throughput Question

[JasonPia]

If an AP 1200 is set up to support both standards but there are no 802.11b clients connected to the AP, do you get the maximum 22mbps 802.11g throughput?

I have read "Capacity Coverage & Deployment Considerations for IEEE 802.11g" but it doesn't answer that specific question (unless I missed it) or at least doesn't explicitly answer the question.

I ask because all our clients are 802.11g but we occasionally need to temporarily attach 802.11b clients. I can put up with the reduced throughput on these occasions but if it is all the time I will have to consider installing a second 802.11b only AP.

Thanks, Jason

[#2]

Sorry, I can't find specifically where I saw it, but I do recall reading in Cisco documentation that using a mixed b/g setting will slow down the entire network regardless of any "b" activitiy on channel. The reason has to do with a protocol change. Since b-devices cannot hear g-devices, the entire network switches to a request-to-send/clear-to-send protocol. This about doubles the number of packets being sent. I imagine that some low speed overhead packets are sent by the AP as well. Net result is about 8 Mbps throughput for a g-device, instead of the typical 22 Mbps throughput on a 54 Mbps network.

-- Gary

[JasonPia]

I thought this was a really straight forward question and I would be inundated with replies!

Thanks for your comments Gary. Following your response I decided to get my finger out and try things out for myself. I think I have it sussed but I thought I would share for anyone else who is in the same position or who wishes to challange my findings.

Having 802.11b support enabled in the AP DOES NOT reduce throughput to 8Mbps AS LONG AS THERE ARE NO 802.11B CLIENTS CONNECTED TO THE AP.

As soon as you connect an 802.11b client the throughput drops from 22Mbps to 8Mbps.

I tested with a file that was 173,000,000 bytes in size and it took approx 80 secs to transmit with no 802.11b clients but after attaching an 802.11b client to the AP the transfer time using my 802.11g client increased to 220s. No changes were made to the AP between transfers.

I suspect the confusion with what you said Gary is that there doesn't have to be any throughput on the 802.11b for it to decrease the overall AP speed BUT it does have to be attached to the AP. Just having 802.11b support in the AP doesn't effect its trhoughput on its own.

Hope this helps someone.

Cheers, Jason