Dynamic Rate Switching (DRS) and Performance - WLAN Foundations

Dynamic Rate Switching (DRS) and Performance - WLAN Foundations

By CWNP On 11/28/2012 - 7 Comments

An intentional part of the 802.11 standard is Dynamic Rate Switching (DRS). DRS adjusts the data rate in order to reduce retransmissions. If the data rate remains at a high rate when the client is farther from the AP, it will result in so many retransmissions that the actual throughput is greatly reduced.

These retransmissions hurt other clients as well since the wireless medium is being consumed for the transmission of the same information multiple times. For this reason, part of the 802.11 standard accommodates data rate changes. The goal is simple: maintain optimum throughput by reducing the data rate and, therefore, reducing errors and retransmissions.
If you are experiencing decreased throughput and the data rate is also being lowered, a potential solution is to increase the output power of the AP and client (using either actual power gains at both ends or greater antenna gain at one or both ends) or to move closer to the AP. If you are experiencing decreased throughput and the data rate is not being lowered, check for interference sources or for other clients connecting to the same AP that have disabled DRS.
DRS functions are not defined in the 802.11-2012 standard. As stated in the standard:
Some PHYs have multiple data transfer rate capabilities that allow implementations to perform dynamic rate switching with the opbjective 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.
In other words, the exact DRS algorithm is not defined, but the parameters for implementing it are. They are more complicated than warranted in this post, but they include consideration of protection mechanisms, the High Throughput (HT) PHY and backward compatibility.
If you are just beginning your journey into the vast land of 802.11 knowledge, the most important thing to remember for now is that DRS allows a STA to transmit at varying data rates depending on the quality of the connection and the type of devices it "sees" on the network.
Frames Are Food,
Tom Tagged with: 802.11, 802.11-2012, dynamic rate selection, DRS, dynamic rate switching

7 Responses to Dynamic Rate Switching (DRS) and Performance - WLAN Foundations

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Howard Higgins Says:
12/28/2012 at 13:24pm

Ruckus has a good article on this.   It's by our old friend Marcus.  

It's at http://www.theruckusroom.net/2012/09/rating-wi-fi.html

Tom Carpenter Says:
12/19/2012 at 13:23pm
Correct. The transmit power remains constant with the rare exception of a forced modification through Transmit Power Control (TPC), but that is not really directly related to DRS. There are indeed differences at different data rates. You will find more information on that in the CWDP and CWAP study guides and courses. The MU-MIMO whitepaper is still in the works. More likely first quarter of 2013 now. This is due to the lack of meaningful information related to it (no chipsets yet and certainly no implementations) and the need to dig dig dig.


12/18/2012 at 16:34pm


A question, when an STA and/or are transitioning to a
different data rate I assume that the transmit power for either end is
constant. Otherwise your suggestions to increase the RSSI on either side are
moot. What then is changed within the transmitted data? I believe a different chipping
sequence used for DSSS, but what about OFDM? Is there a difference in the
adjusted signal for redundancy at the expense of data rate similar to the 8 and 11 bit chipping sequences? Are there unique
differences depending on the protocol or modulation techniques?

Also, where is your whitepaper on MU-MIMO located? In another post you had hinted that you were working on one this year.


Howard Higgins Says:
11/29/2012 at 17:28pm

It would be so nice if there was a "good" algorithm combining Rate Adaptation and Roaming that everyone used.    It would be so much easier to design networks, and devices if they were more predictable in these areas.

However, I know that DAS, and probably Mesh networks have there own considerations and as long as there is no standard, individual mfgs. will be able to mix there own "secret sauce" to out-do their competitors.

The problem is, a lot of companies design poor or simplistic algorithms :-(

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