Data rates if slow stations present

Eduard GV eduardgv
Thu Aug 11 12:58:33 PDT 2011


I agree with everything but the 50% airtime share thing.
If no QoS enhancement is used (802.11e), the 802.11's CSMA grants a
long term fairness in terms of access probability, not in time share.
In other words, both the fast and the slow stations will be able to
send the same number of frames, and hence they will measure the same
throughput. That is, if a 1Mbps STA competes with a 300 Mbps STA, both
will obtain about 850kbps (!!). If the 11n station activates
aggregation, multiply those 850kbps by the number of frames
aggregated. Moral: configure QoS and use 11n aggregation if you can't
avoid old 11b/g devices.

This behavior is due to the fact that slow transmissions will capture
the medium during longer periods and, in consequence, the performance
of fast links is effectively dragged down by slow links. This is
usually defined as "performance anomaly".


2011/8/5 Galen <galenz at>:
> Tobias,
> Here's the basics:
> 1) When make an 802.11n AP compatible with 802.11g, you loose some performance.
> 2) When you make 802.11n AP compatible with 802.11b, you lose even more performance.
> Basically, every layer of backwards compatibility adds to the operating overhead, even if there are no legacy devices actively using bandwidth.
> When slower devices come onto the network, the bandwidth is shared by time. Let's say that you have an 802.11n and 802.11b device both active on an AP. If they are both actively transmitting data as fast as they can, each will take around 50% of the airtime. This means that if you have an 802.11n client at 150 megabits and another at 1 megabit, each will get around 50% of the maximum throughput.
> This is of course a gross simplification, but it should give you an idea as to how things work approximately. There's so much variance in implementation, client functionality, etc. that these are VERY VERY rough approximations.
> I think that it would be helpful to provide additional context as to why you're asking this question. I could provide you with lots of links, but it would likely down you documentation.
> -Galen
> On Aug 5, 2011, at 12:52 AM, Tobias Gunkel wrote:
>> Hello,
>> I have a general question regarding the behavior of an AP if slow stations are present.
>> Imagine there are two stations - one 802.11n capable station and one that is capable of only 802.11b.
>> Now the 802.11n station connects to an AP and is able to transfer data with the lets say 150 MBit/s.
>> After a while the 11b only station connects to the AP but as the SNR is very bad it can only transfer at a 1 MBit/s rate.
>> What happens now with the data-rate of the 11n station? Will this station switch to 11b two or is an AP able to manage stations in different modes at the "same" time? At least the 11b station will occupy the channel for some time so that the 11n station will have less time-slots for data transfers so the data rate will drop at least 1 MBit/s (probably more because the 11b station needs more time for transferring 1 MBit than the 11n station).
>> Is the behavior of the AP standardized? I already had a look into the 802.11 standard but could not find a description of the behavior of AP stations (there is only a very short Annex N: AP functional description).
>> What is the behavior of hostAP in this case?
>> Thanks in advance for answers or pointers to other sites, standards, ...
>> Regards,
>> Tobias Gunkel
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