802.11b and 802.11g in same channel

Answered Question
Jan 9th, 2009

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

I have this problem too.
0 votes
Correct Answer by rob.huffman about 7 years 10 months ago

Hi Ken,

Yes, the "Protection Mode" is not segmented by SSID's only by the radio itself, so the "B" client on the Guest SSID will effect the Corporate "G" clients

"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)."

Hope this helps!

Rob

Correct Answer by robinjellum about 7 years 11 months ago

Protection will be enabled if:

1) There is a 11b STA associated to an 11g AP

2) An 11g AP on the same channel has protection enabled.

If your 11g (not in g-only mode) AP is on channel 11 AND you have a neighboring AP that is also on channel 11 that AP is either 11b only or is an 11b/g AP with a B client associated AND the two APs can hear each other , then your AP will go into protection mode.

There are two forms of protection RTS/CTS as stated above, or CTS-to-Self. RTS/CTS has more overhead because there is an exchange of two packets RTS and CTS, whereas CTS-to-Self is only one packet. CTS-to-Self is the method most commonly implemented. I do not know of an AP today that ships using RTS/CTS as the default protection mechanism.

A couple of other points:

Protection does not force the AP to 11b rates. It makes the 11g clients send an 11b RTS-to-Self reserving time for transmission of an 11g packet. Assuming no other traffic, with standard 11g you can get a max of 24.5 Mbps. With RTS-to-Self you will get ~16Mbps. With RTS/CTS you will get ~12Mbps.

Setting your AP to 11g-only will probably make things worse. If you AP is only sending at 11g rates the neighboring 11b AP will not be able to see those packets and will transmit at the same time causing collisions. Collisions are much mores costly that the over head of data rate protection.

Protection is enabled and disabled depending on the state of the air. If no 11b clients are associated to an 11g AP protection will not be used. If an 11g client is far enough to use 11b rates, protection will be enabled on the 11g AP because it is using 11b rates.

If you want to do something, the easiest thing to do is to set the APs closest to the neighbor to different channels. The problem is that beacons travel long distances and your neighbors AP is probably seen by many of your APs so trying to channel plan around it may be difficult.

My recommendation is to not worry about it.

Correct Answer by rob.huffman about 7 years 11 months ago

Hi Ken,

You are most welcome my friend! I don't think the non-associated B client should have any effect, have a look at this clip;

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.

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."

It is nicely described in this great doc;

http://www.cisco.com/en/US/products/hw/wireless/ps430/products_white_paper09186a00801d61a3.shtml

Hope this helps a little!

Rob

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rob.huffman Fri, 01/09/2009 - 07:12

Hi Ken,

The Home "B" client won't be associated to your Corporate AP's so they shouldn't have any effect. The bigger problem will be the competing signals on Channel 11 (Channel Overlap). You will need to move the Corporate AP to a non-overlapping Channel preferably 1 or 6.

Hope this helps!

Rob

kfarrington Fri, 01/09/2009 - 07:21

Hi Rob,

Many thx for the fast reply.

Thats interesting. As I am not really a Radio man, I must have mis-understood.

Say corporate user is associated and working using 802.11g standard. Then the home user is probing the corp network even though he is not associated using the 802.11b standard. Cant this probing and other managment traffic slow down the corp user as he is using G and the home user is using B?

Many thx, and sorry if this does not make sense? :)

Many thx

Ken

Correct Answer
rob.huffman Fri, 01/09/2009 - 07:46

Hi Ken,

You are most welcome my friend! I don't think the non-associated B client should have any effect, have a look at this clip;

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.

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."

It is nicely described in this great doc;

http://www.cisco.com/en/US/products/hw/wireless/ps430/products_white_paper09186a00801d61a3.shtml

Hope this helps a little!

Rob

Correct Answer
robinjellum Fri, 01/09/2009 - 09:26

Protection will be enabled if:

1) There is a 11b STA associated to an 11g AP

2) An 11g AP on the same channel has protection enabled.

If your 11g (not in g-only mode) AP is on channel 11 AND you have a neighboring AP that is also on channel 11 that AP is either 11b only or is an 11b/g AP with a B client associated AND the two APs can hear each other , then your AP will go into protection mode.

There are two forms of protection RTS/CTS as stated above, or CTS-to-Self. RTS/CTS has more overhead because there is an exchange of two packets RTS and CTS, whereas CTS-to-Self is only one packet. CTS-to-Self is the method most commonly implemented. I do not know of an AP today that ships using RTS/CTS as the default protection mechanism.

A couple of other points:

Protection does not force the AP to 11b rates. It makes the 11g clients send an 11b RTS-to-Self reserving time for transmission of an 11g packet. Assuming no other traffic, with standard 11g you can get a max of 24.5 Mbps. With RTS-to-Self you will get ~16Mbps. With RTS/CTS you will get ~12Mbps.

Setting your AP to 11g-only will probably make things worse. If you AP is only sending at 11g rates the neighboring 11b AP will not be able to see those packets and will transmit at the same time causing collisions. Collisions are much mores costly that the over head of data rate protection.

Protection is enabled and disabled depending on the state of the air. If no 11b clients are associated to an 11g AP protection will not be used. If an 11g client is far enough to use 11b rates, protection will be enabled on the 11g AP because it is using 11b rates.

If you want to do something, the easiest thing to do is to set the APs closest to the neighbor to different channels. The problem is that beacons travel long distances and your neighbors AP is probably seen by many of your APs so trying to channel plan around it may be difficult.

My recommendation is to not worry about it.

Nice explanation of protection and its impact. I would only add that protection only propgates one hop, so it doesn't go beyond the AP evoked into protection by the home AP.

Believe it or not, its my understanding that controllers implement RTS/CTS, not CTS to Self, and I don't think its modifiable (same with RTS retries).

As I see you work for Vocera, would you happen to know the specific reasons why 4.2.130 is mandated for badge support? Are there specific bugs resolved in this version of code?

Regards,

Bruce Johnson

robinjellum Mon, 01/19/2009 - 08:22

Bruce,

We are running at 4.2.x code and I only see CTS-Self. The only time I see RTS-CTS is after the AP retries a packet 8 times. It then follows that up with up to 32 RTSs.

-r-

kfarrington Mon, 01/19/2009 - 04:49

Hi Rob,

Many thx for this. That is most helpful indeed :)

All the best,

Ken

kfarrington Mon, 01/19/2009 - 04:59

Guys,

Sorry, here is another similar questions then. Before I was taking about a slightly different scenario, but what about this one.

I have my WLCs broadcasting two SSIDs, one for corporate and one for guest access. AP1 is broadcasting both SSIDs (corp one hidden)

The Corporate is running a G only policy

Guest is running B/G

If someone connects to the guest SSID on AP1, using 802.11b and then someone connects to corp SSID on AP1 using 802.11g will the AP1 run in protection mode and thus bring the overall thruput for corp SSID down?

Many thx indeed, for the great help,

Kind regards,

Ken

Correct Answer
rob.huffman Mon, 01/19/2009 - 05:45

Hi Ken,

Yes, the "Protection Mode" is not segmented by SSID's only by the radio itself, so the "B" client on the Guest SSID will effect the Corporate "G" clients

"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)."

Hope this helps!

Rob

kfarrington Mon, 01/19/2009 - 07:21

Hi Rob,

That is fanatstic. At least I know what the procedure is now.

Many thx as always,

Ken

kfarrington Wed, 01/21/2009 - 10:46

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 Thu, 01/22/2009 - 06:11

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 Thu, 01/22/2009 - 04:50

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 Thu, 01/22/2009 - 06:13

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 Thu, 01/22/2009 - 06:18

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 Thu, 01/22/2009 - 06:38

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 Fri, 01/23/2009 - 03:10

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 Fri, 01/23/2009 - 05:21

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.

reginald-pugh Sat, 01/24/2009 - 18:10

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 Mon, 01/26/2009 - 02:58

Thats is great. Many thx for the advice.

Whats with the new 1140 APs?

Many thx

Ken

rob.huffman Fri, 03/06/2009 - 06:10

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

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