Referenceable material on RF duty saturation

[Andrew McHale]

Does anyone have a link to a credable referenceable document/whitepaper on duty cycle saturation?

More and more often I find myself having to inform customers their duty cycle is too high due to an excessive use of SSID's. I have heard the value of 30% mentioned on multiple courses as a key value to stay under but this is very hard to justify in a report without some science or brand behind it.

The document doesn't have to be 802.11 specific because, as we know, duty cycle is a measure of RF energy regardless of the technology generating it.

Thanks in advance!

Andrew

[Scott Fella]

There are a few out there that you can reference.

Here are some

http://phasedcoexistence.blogspot.com/2011/11/high-channel-utilisation-part-one.html?m=1

http://zone.ni.com/devzone/cda/epd/p/id/6566

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[Andrew McHale]

Thanks for the links Scott but they don't provide guidance as to what to keep utilisation under. I think my customers are happy as to why channel utilisation is crucial, and how to keep it down, the question is what level is acceptable.

Trying to tell someone that a channel over 30% utilised (in sprectrum terms) is saturated is not easy because they want to know whats wrong with the other 70%.

Am I wrong with this figure of 30%? As mentioned, I'm sure I've heard this number from more than one different WLAN instructors in my time, but I can't find anything to support it. And I'm talking about Duty Cycle as seen by a spectrum analyser, rather than what the Cisco AP's report as Channel Utilisation.

In fact, that raises another good question. Does Cisco's CU as reported by their AP's differ from Duty Cycles as seen in a SA (I'd imagine so), and if so, how?

Thanks for anyone who can help explain this stuff!

[Scott Fella]

There is not magic number for that. If you have an non 802.11 interferer that is 100% duty cycle and high power, your channel utilization would show low.... Because no other device can communicate. If you have the same device but very low power, you will see higher channel utilization because devices can still communicate.

What you need to do is look at throughput when you start having a lot of SSID's. 3-4 is the number I try to consolidate SSID's to and also eliminate the lower data rates when possible.

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[Andrew McHale]

The particular customer I am working with now (although I want this ratification for future use also) has 14 (yes, f.o.u.r.t.e.e.n) active SSID's!!!

If I understand your description correctly, the utilisation reported from the Cisco AP's is how much 802.11 traffic it detects, hence if the RF is taken up fully by a non-802.11 device the AP's will actually report a 0 utilisation? But when the AP can see 802.11 traffic on the channel it reports that as utilisation hence the value will be higher, even though the situation is actually "better" than the 0 reading?

Thanks for the continued clarification!

[Scott Fella]

You need to stick to recommendations. I tell my customers what I want all the time and if they don't go with my recommendation, then the risk goes on their hands. There is no magical number because it also depends on what data rate that can affect throughput. If they had 14 SSID's and high density deployment, the 2.4ghz would not work:) it's really hard to come up with an number and show them, but you always can use iperf during high utilization time when there are a lot of users and then another sample after hours with minimal users and then when there is almost no users. This data can help visually show them the issues at hand.

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