Wireless support 802.11b concerns

[Kia Seng Ronald Tan]

I have a customer have intention to get all medical devices on wireless however some of devices are on 802.11b. Their understanding is that if they use wireless AP running a/b/g/n, if any 802.11b client connects , all would be downgraded to 802.11b even if some client are on 802.11 g/n

A) how can we help them managed this?
B) what options can they do to avoid this gross degradation?


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[Stephen Rodriguez]

Having a mixed environment doesn't slow down all the clients to b rates. What happens is the other clients have to wait longer to be able to transmit. But when they do they still transmit at the best rate they can.

For an example, think of a class. You have some students that speak fast and some that speak slowly. When the fast talkers have their turn they speak fast and get their thought out quickly. Now a slower talker has their turn, they still get the thought out, it just takes longer for them to do so.

Steve

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[Daniel Ordóñez Flores]

Dear Stephen.

So the throughput is not been affected? or we are talking about just rates?

[Stephen Rodriguez]

It is...but it isn't.  The 'n' clients still send at 'n' rates, they just have to wait to send if a 'b' client is sending.  Pure perception

HTH,
Steve

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[Daniel Ordóñez Flores]

I'm readig a book called 642-732 CUWSS by Jerome Henry. He saying in chaptet one that clients type "b" affect the performance of the network and said this

"After determining the type of protocol support, set your costumer expectations and explain that the data rate (or Modulation and coding scheme) is just the link speed, no the throughput (download speed). 802.11b at an 11Mbps offers in average a 5.5 throughput. 802.11g at 54Mbps offers an average a 20Mbps  throughput  but degrade to 6Mbps throughput as soon as a 802.11b devices is in the cell ans protection mode is enable."

So the degrate depends of the type of protocol are you using? or always exists this degrate?

[George Stefanick]

When a single b client enters the network. The cell degrades because of RTS/CTS and CTS-self overhead that g clients are required to do  AND due to b clients sending their frames much slower. This is what Jerome mentions as modulation. Because the 802.11b devices use a simple modulations DBPSK, DQPSK and CCK. If im a g client I need to worry about the b client. Because the b client doesnt understand OFDM, the modulation 802.11g uses. The g client needs to add overhead so the b client knows there is a transmission coming.

Here is another way of think about it .. You own a Ferrari and I own a Yugo. We're racing around a track. I will cause you to be slower cause as you circle the trap you will need to slow down and go around me each time. Just having me on the track makes the race laps slower.

To get more technical. Onces the g client sends the RTS/CTS or CTS-self, the g client steps up to OFDM modulation and punches the frame at a higher rate. The reason for the RTS/CTS or CTS-self, is the g client sends this at 1 MB PHY, so that the b client understand there is a coming frame. The b client reads either preamble and or reads the frame and looks at the duration ID.

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
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‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."

[George Stefanick]

I run a very large wireless network in Helathcare. I know the 802.11b problem very well. There are many medical devices still in use that only support 802.11b.

My first comment, try and replace the device(s) with one that supports 802.11g minimally and remove the 802.11b devices from your network.

Here is a high level overview.  When an 802.11b client connects to your ap or when an 802.11b association is close proxcimity to your network your APs that hear this traffic flip the protection bit from 0 to 1. This indicates in the beacon that 802.11g clients that trggier a frame MUST proceed that transmissiom with an RTS/CTS or CTS-self. This added frame exchange causes addiitonal overhead. In addition, when an 802.11b client wants to talk, it talks very slow.

Remember wifi is a halfduplex medium. Only 1 device can transmit at a time on a channel.

Hope this helps

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
__________________________________________________________________________________________
‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."