Beacons, data-rates and throughput

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Feb 24th, 2009
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Hi everyone,

I try to figure out, how much throughput is used by beacons, when using different data rates. It has no practical reason - I just want to understand the theory.

I understand, that beacon frames are sent at the lowest configured data-rate. So, a beacon frame needs longer on the air, when using 1MBit/s than 54 MBit/s :-)

When allowing 1Mbit as the base data-rate, the medium is longer busy by beacons, than with higher data-rates. I want to calculate the time the medium is busy by beacons per second.

So here are my questions and assumptions:

1.) Does the AP has to wait for an IFS before sending a beacon (DIFS, SIFS, PIFS)?

2.) If yes to the above, is it necessary to do "backoffing" as well?

3.) If the beacon interval is 0.1s, then 10 beacons are sent per second. I assume 100byte per beacon, which makes 1k per second by beacons.

4.) On a data-rate of 1MBit/s (125000byte/s), the 1k of beacons need 0,008 seconds for transmission in total.

Are these assumptions correct?

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dennischolmes Fri, 02/27/2009 - 10:38
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Pretty much correct. Also bear in mind the random duration time value before the media is released for reuse. This is the number that Meru manipulates to maximize throughput on their WLAN solution. A direct 802.11 standard violation I migh add.

Johannes Luther Tue, 03/31/2009 - 04:15
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Thanks for your reply.

Does the AP needs to wait for a backoff timer, when sending beacons? Or is it allowed to send beacons without backoffing?

Also what Interframe Space so I have to consider, when sending beacons (DIFS, SIFS ...)?

dennischolmes Tue, 03/31/2009 - 04:51
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I think you're trying to make a mountain out of a mole hill here. To get into a real discussion of Interframe Space and 802.11 packets would require a lot more reading than just what I could type here. Might I suggest that you read the IEEE 802.11 Handbook, A Designer's Companion, by Bob Ohara. Bob is a friend and former founder of Airespace. It is an excellent resource for those wanting detailed information about 802.11 networks and should be read by anyone seeking higher certifications such as CCIE Wireless or CWNE.

Johannes Luther Tue, 03/31/2009 - 05:42
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I just want to understand that stuff.

I should find that in the IEEE paper. But anyway, thanks for the book hint. I was already studying the CiscoPress books, but with no luck.

dennischolmes Tue, 03/31/2009 - 05:49
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You're quite welcome and you will get a lot more out of Bob's book than anything on Cisco press.


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