Minimum Data rate: what is best?

Answered Question
Feb 26th, 2010

Hello

I have a scenario where the minimum 802.11b/g data rate has been set at 1mbps mandatory on a WiSM template stored on WCS.

We think that this is allowing clients to try to attach to the wireless network with a very tenuous connection, which is resulting in very poor performance.

We also think that this poor performance will be impacting the wireless experience for all the other users of the wireless access point who are close enough to talk at a more reasonable speed.

Are we correct in this thinking?

If we change the manadatory connection speed back to 5.5mbps what effects, if any will we see as we have concerns that we will create some coverage holes and if we did, would they be huge?

Is there a way to model it?

If we do, would the increase in network performance for the many outweigh the inconvenience of a covergae hole?

thank you for any help/feedback, I am trying to build a case to get the data rate back to 5.5mbps at least but am enountering mgmt resistance.

thanks

Bryn

I have this problem too.
1 vote
Correct Answer by Kayle Miller about 6 years 9 months ago

Bryn,

     There are a number of factors that can impact/influence the data rates you select; such as AP density, client devices, and protocols supported. Also it's important to understand the difference between Mandatory and Supported when configuring the data rates.

     Generally speaking it's ok to set 5.5mbps to Mandatory, which means that you must achieve a 5.5mbps or higher conenction in order to associate to the access point once associated you can step down to 1mbps or 2mbps if they are configured as supported.

     Personally without knowing what your wireless design looks like in terms of density of access points and with out knowing what types of clients your running: laptops, pda's, vocera badges, VOIP phones. I'd set 5.5mbps as manadatory and set the rest to supported and test from there, keep in mind that the coverage holes are more than just data rates they are signal levels. The thing to remember with coverage holes is that your wireless network needs to be a 2-way communication between the client and the AP, in many installations you can encouter scenarios where the client device (ie: phone or laptop) can hear the AP just fine but the AP never hears the client device, this results in huge performance issues and coverage holes.  This happens because the majority of client devices are limited to a maximum transmit power of 25 to 30 mw, While an AP is capable of 100mw (802.11b), 30mw (802.11g), 40mw (802.11a).

     So as you progress thru this issue it's important to consider not just the data rates, but the overall design and make sure that your not encountering a true coverage issue.

Hope this helps

Kayle

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Correct Answer
Kayle Miller Fri, 02/26/2010 - 07:53

Bryn,

     There are a number of factors that can impact/influence the data rates you select; such as AP density, client devices, and protocols supported. Also it's important to understand the difference between Mandatory and Supported when configuring the data rates.

     Generally speaking it's ok to set 5.5mbps to Mandatory, which means that you must achieve a 5.5mbps or higher conenction in order to associate to the access point once associated you can step down to 1mbps or 2mbps if they are configured as supported.

     Personally without knowing what your wireless design looks like in terms of density of access points and with out knowing what types of clients your running: laptops, pda's, vocera badges, VOIP phones. I'd set 5.5mbps as manadatory and set the rest to supported and test from there, keep in mind that the coverage holes are more than just data rates they are signal levels. The thing to remember with coverage holes is that your wireless network needs to be a 2-way communication between the client and the AP, in many installations you can encouter scenarios where the client device (ie: phone or laptop) can hear the AP just fine but the AP never hears the client device, this results in huge performance issues and coverage holes.  This happens because the majority of client devices are limited to a maximum transmit power of 25 to 30 mw, While an AP is capable of 100mw (802.11b), 30mw (802.11g), 40mw (802.11a).

     So as you progress thru this issue it's important to consider not just the data rates, but the overall design and make sure that your not encountering a true coverage issue.

Hope this helps

Kayle

BRYN JONES Mon, 03/01/2010 - 01:11

Kayle

thank you for your input, I think this information is very helpful and I will be using it.

regards

Bryn

George Stefanick Mon, 03/01/2010 - 19:36

Great response! I just wanted to piggy back on your comment Kayle.

When selecting a high mandtory rate, you pull in the cell as you mentioned. But you also lessen the overall channel utilization on your entire network. By standard, your managment frames are sent on the lowest mandatory rate. What does that mean !? That means your beacons are sent at a HIGH / FASTER rate which allows them to go a much faster at 5.5 then say 1 or 2 (when 5.5 is set to man).

Take an AP add 10 SSIDs and open AirMagnet. You will see channel utlization about 10%. Then axe 1 and 2 and set man at 5.5, you will see your channel dip down to 1 - 2 %...

Cheers!

EvaldasOu Wed, 12/11/2013 - 23:30

Hi George,

Beacons - sent at lowest mandatory rate;

Multicast - sent at highest mandatory rate;

What happens if configuration has not two, but three or more mandatory rates configured? We just look at the first one, and at the last one, skipping all the other?

How this scenario would work in this configuration? :

802.11a Operational Rates                        

802.11a 6M Rate.............................. Mandatory                                                          

802.11a 9M Rate.............................. Supported                                                          

802.11a 12M Rate............................. Mandatory                                                          

802.11a 18M Rate............................. Supported                                                          

802.11a 24M Rate............................. Mandatory                                                          

802.11a 36M Rate............................. Supported                                                          

802.11a 48M Rate............................. Supported                                                          

802.11a 54M Rate............................. Supported   

                                                       
vlad.mihailov Thu, 12/12/2013 - 07:00

Am I wrong assuming multicast is sent at the lowest data rate as well? It would be strange to send it at data rates higher than what cell may be operating at.. I'd suspect they'd be sent either at the lowest mandatory data rates or at the rate of the slowest associated client (and that last is a bit hard to believe)

Scott Fella Thu, 12/12/2013 - 07:05

In the autonomous world multicast is sent on the lowest mandatory rate, however with a WLC, it's the opposite... Highest rate. So you set two mandatory rates, the lowest one is for your beacons and the highest is for your multicast. You set these depending on your RF environment. If you know you have 48mbps everywhere and you require multicast, then set the highest rate to 48mbps or 36mbps.

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EvaldasOu Thu, 12/12/2013 - 07:07

Scott,

So if you have 3 mandatory rates configured as in my example above... the middle one acts just as supported? Nothing more?

Saravanan Lakshmanan Sun, 12/15/2013 - 03:44

downstream traffic with wlc:

for unicast data traffic, based on negotiated rssi it selects datrates, when there are data retries then it starts switching to next available supported/mandatory rate downward until it hits lowest mandatory. for multicast data traffic on the ap radio, it starts at highest mandatory, also if there is an edge/slow client that always uses lowest mandatory then that ap radio adapts to that rate for all multicast apps. if required, configure only one mandatory datarate to avoid this situation. we've to remember multicast on wireless is best effort due to no ack and no qos.

use mediastream feature, its reliable, uses unicast datarates to send multicast.

can use rf profile to change mandatory datarate per ap radio based on design requirement.

George Stefanick Sun, 12/15/2013 - 07:40

Can you explain further what you mean by this "also if there is an edge/slow client that always uses lowest mandatory then that ap radio adapts to that rate for all multicast apps. if required, configure only one mandatory datarate to avoid this situation."

Not sure I grasp the comment radio adapts to that rate for all multicast apps ..

Thanks

Scott Fella Sun, 12/15/2013 - 07:10

Saravanan explains it well. You really don't need three mandatory rates, two at the most.

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