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802.11n DRS


Does anyone have a document or outline exactly what the signal strength in dbm is need to the various 802.11n data rates.

Also an explanation on DRS in the real world would be great I have read few things on the in books but would be great to hear some real world experience.

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Craig Beck

8/22/2022 - Mon

Well, there are other factors, and I don't think signal strength is even considered.

What *should* trigger a rate drop is some percentage increase in the number of errors/retransmits occurring.

Here's what the first part of IEEE 802.11-2012 specification's section 9.7 says:
9.7 Multirate support
9.7.1 Overview
Some PHYs have multiple data transfer rate capabilities that allow implementations to perform dynamic rate
switching with the objective of improving performance. The algorithm for performing rate switching is beyond
the scope of this standard, but in order to provide coexistence and interoperability on multirate-capable PHYs,
this standard defines a set of rules to be followed by all STAs.
Only the data transfer rates of the mandatory rate set of the attached PHY are guaranteed to be supported when
a STA for which dot11OCBActivated is true transmits a management or data frame. Higher layer protocols
may negotiate a rate outside the mandatory rate set.
A STA that transmits a frame shall select a rate defined by the rules for determining the rates of transmission of
protection frames in 9.23 when the following conditions apply:
— The STA’s protection mechanism for non-ERP receivers is enabled.
— The frame is a protection mechanism frame.
— The frame initiates an exchange.
Otherwise, the frame shall be transmitted using a rate that is in accordance with rules defined in 9.7.5 and 9.7.6.

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You can download your own copy of that 2,793 page document from
though you have to choose a User Type (e.g. Academic/Student or whatever) from the picklist, (entering an email address is optional) and click the ACCEPT button.

Likewise for their 536 page 802.11n-2009 document
Craig Beck

The receive strength is a factor in rate-selection.  Faster rates can't work at poorer receive signal strengths.  This is usually an important factor is cell-size design.

Have a look at these...

(See the rate vs reach paragraph specifically)


Interesting... Juniper seems to have misinterpreted the specification... those dBm levels are the minimums to be used when testing adjacent channel and non-adjacent channel rejection levels as described on pages 318 to 320 of the 802.11n spec... it doesn't say anywhere in the 11n spec that the rates must be raised/lowered at those input sensitivity levels.
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William Peck
Craig Beck

No, but if you actually test it you'll see that certain rates can't be maintained at longer distances and that is proportional to receive strength.

Have a look at some specs for different APs.  They should list receive strength indicators for each data-rate...


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Craig Beck

That is correct - however I've not said that the values are exact across all vendors.  All I've said is that the data-rate is selected according to signal strength.

The point I was making is that the data-rate is affected by the received signal strength, and that is correct.