Effect of rate-limiting on channel efficiency

[joea_5029799]

Rate limiting each client increases the amount of time needed to complete data transfer (assuming the air-interface is not the bottleneck). Unless a scheduling method such as air-time fairness is used (where clients with good link quality are given a slight advantage), the result will be increased contention for the channel and more rapidly diminishing performance as the number of concurrent users increases. A "hog" will want to consume a lot of data whether rate-limited or not. How then does Ruckus recommend reducing the amount of time that hog is contending for channel access?

Note: This topic was created from a reply on the Rate Limiting topic.

[keith_redfield]

This is a good point but I think dependent on how we actually implement the rate limiting. Given our focus on performance, I'd be surprised if this were the result. I'll see if we can get a developer to chime in.

[billkish]

It is true that rate limiting can reduce overall capacity. The primary reason for this is increased contention (as mentioned) and reduced MAC-layer efficiency due to smaller aggregation sizes. Modern WiFi is extremely reliant on the 802.11n aggregation mechanism for achieving high MAC-layer efficiency and anything that reduces aggregation sizes such as rate limiting can negatively impact overall capacity. Ruckus AP's mitigate this effect through the use of larger rate limiting buffers (essentially enforcing the specified rate over a longer average time interval) to allow the bursting that is essential to maximizing aggregation and MAC-layer efficiency. Our rate limiting and airtime fairness scheduler effectively give a bandwidth hog less frequent but longer access to the medium which results in reduced contention and higher efficiency. The downside is somewhat longer latency if rate limiting but most data applications don't notice it.

[joea_5029799]

Thank you Bill for that very detailed response. With airtime fairness, the amount of time each client uses the channel is proportioned more equitably and the impact of the hog on other users is reduced. It is interesting to note that Ruckus goes one step further to ensure that MAC-layer efficiency is improved (or restored) for the rate-limited client. Does this approach result in frame sizes or transmission intervals that are even larger than what A-MPDU would yield? Also, is there any interoperability risk associated with a "non-standard" method of aggregation? Finally, do you have any figures describing the efficiency gain of this methodology, e.g. the percentage of airtime spent sending payload packets when the larger rate limiting buffers are used versus using standard frames?

[keith_redfield]

Bill is our CTO btw...