802.11b and 802.11g in same channel

[kfarrington]

Hi All,

Lets say I had an enviroment like this.

I have a coporate access point in channel 11.

Then in close proximity

There is a home network that is in channel 11.

Will my G network slow down to B rates, and if i could not change channel, would I just have to put up with it?

Can I change the Radio policy for the WLAN and nic card both to G only. But would this still make a difference?

Please see attached digram?

Also, If you have a WLAN and you change the radio policy from ALL to G only, what happens exactly? Does the SSID stop be broadcast and stop accpeting traffic in the A band, and what charateristics would be different if you changed it from b/g to G only in the 2.4 band?

Many Thx indeed,

Ken

[kfarrington]

Hi Guys,

So a little info for you.

So here are my tests.

Running APs in mixed mode, and I have performed over 180 individual tests in the last two days. Using IXIA tools.

I see an average thruput when loading up the APs of approx 12 M/Bits ps over the 180 tests on different APs. I transfer 800 Mbits to every AP.

Some reach 17-18 Mbps, and some (only a couple) reach 5 Mbps, the majority between 10 and 13 Mbps.

===================================================

*** How do the stats above sound to you experts out there? ****

===================================================

I know what I have tested in the last two days may not be the same tomorrow, but thats the nature of it right? I wanted to get a benchmark.

The one thing I am thinking about is turning off 802.11b across all 300 APs, but there is no gaurenttee that this will make things better right? It may be SNR ratios that bring the performance down, and not CTS/RTS phy controls right?

Many thx

Ken

[Rob Huffman]

Hi Ken,

These stats look very good for a b/g deployment. I'm guessing the the AP's that only provided 5Mbps may have had a "B" client associated to them. If you can get away with turning off the "B" data rates, I would move ahead.

As you nicely noted, this does not guarantee better performance, but things will likely improve :) The other statement that you made about the "nature of wireless" is most correct! You have to keep on top of your deployment, due to the fluid nature of the medium itself.

Cheers!

Rob

[kfarrington]

Rob,

Hi Mate,

Your statement :

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)."

What about when you see on the WLCs lots of clients probing using protocol 802.11b.

Does that turn on mixed mode protection?

Many thx

Ken

[Rob Huffman]

Hey Ken,

Yes, that would be the case my friend :)

Page 302 of the excerpt you linked in another thread really tells it all;

http://books.google.co.uk/books?id=TLUVG9yoGx4C&pg=PA302&lpg=PA302&dq=%22may+be+deduced+by+the+reception%22&source=web&ots=1O9O_5SKxu&sig=81VU2RQVNoyaPqVWTfkeVtgb-pA&hl=en&sa=X&oi=book_result&resnum=1&ct=result

Hope this helps! It sounds like you are making great progress with this!

Rob

[kfarrington]

Man, you are most helpful.

I am turning off B tonight on one area and will retest with IXIA. Will update you tomorrow mate :)

Also, one thing I have heard on the grapevine :) is that even if you turn off 802.11b and run G only under the WLAN, client laptops can still use B rates if they are a fair distance away? and the only real way to stop that from happening is set the 11b rates to disabled.

How does that sound to you?

Cheers :)

Ken

[Rob Huffman]

Hey Ken,

Good stuff my friend! I have never heard of that anomaly (I have my doubts....but I guess anything is possible ;-).

Cheers!

Rob

[kfarrington]

So guys,

Just to finish this off in terms of test results.

I have performed some further tests, and the maximum I can get on 802.11g thruput is an avarage throughput of 19.5Mbps

This is running the following:

THE WLAN in radio policy G only

All Wireless B rates disabled withe the following

1-Disabled

2-Disabled

5-Disabled

6-Disabled

9-Disabled

11-Disabled

12-Mandatory

18-Supported

24-Supported

36-Supported

48-Supported

54-Supported

Also, please note that when all rates are left enabled and you run your WLAN in Radio policy G-Only, you see that frames still set the protection ERP information element but one thing I did not test was a packet capture when all the B rates were disabled to see if any beacons/probes had the bit set?

Any ideas on that would be good.

Cheers,

Ken

[Scott Fella]

Ken,

That is right... The radio policy in the wlan is just a policy and you will see logs generated if the policy is not meet. Data Rates specifically tells the radio what is the rates to be used. This is where you need to disable 1,2,5.5 & 11 if you don't want 802.11b clients.

[kfarrington]

Thx all guys!!!

I am liking this wifi stuff :)

[reginald-pugh]

Yes this is all good stuff. What we have done just because we still have clients using the legacy b rates. On the controller we don't require the 1 and 2 Mbps basic rates, instead we require the higher rates of 5.5 and 11 Mbps. This just to keep those customers connected and happy.

It is the l and 2 Mbps that really seems slow. If you have deployed a dense, well overlapped AP environment, you may want to require only the 11 Mbps for the b clients.

Word of Warning: Reducing your rate will reduce the AP coverage cell size and may isolate the clients on the far edges of the network. I advise migrating to this with that in mind. Some devices may need the lower rates to perform at their peak.

Then again, everybody just go purchase the new 1140 and call it a day! :-)

[kfarrington]

Thats is great. Many thx for the advice.

Whats with the new 1140 APs?

Many thx

Ken

[reginald-pugh]

The ClientLink technology  in the 1140 is reported by Cisco to enhance the throughput of the legacy a/g clients . Note not b clients bit A/G.. If you can migrate away from the legacy b , it would be ideal.

http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps10092/white_paper_c11-516389.html

[Rob Huffman]

Hi Ken,

Just thought that I would add a note about the new 1140 AP to the great tips from Reg;

See attached --

Hope this helps!

Rob

[Rob Huffman]

Hey Ken,

Hope all is well!

I thought you might enjoy this reference that we found recently (Thanks Simon!). It's pretty good :)

Here's the reference to protection mode from Matthew Gast's Definitive Guide Book (2nd Edition, Page 302):

"Only the protection frames are required to be transmitted at the 802.11b-compatible data rates. Protection does not require 802.11g stations to use a slower data rate for the payload data, as is commonly asserted."

The link below documents Gast's testing on this issue and his chart shows that "g" technology working in protected mode still experiences between 1.6 and 2.3 times the throughput of the "b" technology depending on which type of protection mechanism is used.

http://www.oreillynet.com/pub/a/wireless/2003/08/08/wireless_throughput.html

Cheers!

Rob