A-MPDU transmission in net80211 on FreeBSD 8
Alexander Egorenkov
egorenar at googlemail.com
Sun Feb 14 20:30:28 UTC 2010
Wow, 160-180 Mb/s. That's great :-)
No, it's HT40 but my AP is not very good, it sends to me only A-MSDUs but no
A-MPDUs (don't know why, i guess because it's cheap), and there are many
WLANs in my neighbourhood in 2.4GHZ and no WLANs in 5GHZ but unfortunately i
have an 2GHZ only AP. Furthermore, i don't have any documentation about the
Ralink chip i have, only Linux source code from Ralink which i study in
order to learn how the device works :-) But it's OK, i'm making progress.
For which devices did you develop 802.11n drivers ? And what AP did you use
in your tests ?
Alex.
On Sun, Feb 14, 2010 at 8:57 PM, Sam Leffler <sam at errno.com> wrote:
> That's great to hear. I take it this is HT20? I believe you can get 40-60
> Mb/s @ MCS15 w/o AMPDU and 80-100 Mb/s w/ AMPDU. In HT40 I've gotten
> 160-180 Mb/s for TCP netperf (5GHz) and 220+ Mb/s for unidirectional streams
> (netperf -t UDP_STREAM). Note that once you get to these higher rates
> things like TCP window sizes become important and doing things like TSO in
> s/w can give noticeable speed boosts.
>
> Sam
>
> Alexander Egorenkov wrote:
>
>> Hi,
>>
>> thanks for your help, I implemented A-MPDU Tx in my Ralink driver now and
>> it works very good.
>> I have average Tx rate about 4.5~5 MBytes/s now :-) The Ralink chip that i
>> have implements the frame queue in hardware so i had only to assign sequence
>> numbers, set BA window size and MPDU density and it worked, lucky me :-)
>>
>> Alex.
>>
>> On Sun, Jan 31, 2010 at 8:20 PM, Sam Leffler <sam at errno.com <mailto:
>> sam at errno.com>> wrote:
>>
>> Alexander Egorenkov wrote:
>>
>> Why doesn't 802.11 stack assign sequence numbers to A-MPDU frames ?
>>
>>
>> Because if net80211 does the assignment it may be wrong. As the
>> comment says, if tx aggregation causes frames to be q'd above the
>> h/w then by the time they are sent OTA the pre-assigned seq# may be
>> invalidated by other frames going out ahead of it.
>>
>>
>> When sequence numbers are not assigned to A-MPDU frames, then BA
>> doesn't work with my AP.
>> I tried to assign sequence numbers to A-MPDU frames in my device
>> driver and then BAs worked
>> with my AP.
>>
>>
>> That is what the comment says to do.
>>
>>
>> And what is meant by aggregation queue ? Where is that queue anf
>> how do i use it ?
>>
>>
>> The aggregation q is the mechanism used to hold frames waiting for
>> additional frames to aggregated into an A-MSDU/A-MPDU. The queue is
>> typically wherever the aggregation work is done. Some devices do
>> this in h/w, others require the host handle this. When done in the
>> host it can be done in the driver or above. The intent has always
>> been to have net80211 implement tx aggregation that a driver can
>> fallback on but I never did the work. All the 11n drivers I've done
>> have either handled tx aggregation in h/w or in the driver.
>>
>>
>> Thanks.
>>
>>
>> On Sun, Jan 31, 2010 at 4:43 AM, Sam Leffler <sam at errno.com
>> <mailto:sam at errno.com> <mailto:sam at errno.com
>>
>> <mailto:sam at errno.com>>> wrote:
>>
>> Alexander Egorenkov wrote:
>>
>> Sorry, i posted the wrong comment.
>> Here is the comment which i don't understand:
>>
>> /*
>> * NB: don't assign a sequence # to potential
>> * aggregates; we expect this happens at the
>> * point the frame comes off any aggregation q
>> * as otherwise we may introduce holes in the
>> * BA sequence space and/or make window accouting
>> * more difficult.
>> *
>> * XXX may want to control this with a driver
>> * capability; this may also change when we pull
>> * aggregation up into net80211
>> */
>>
>> Thanks.
>>
>>
>> What is unclear?
>>
>> Sam
>>
>>
>>
>> On Wed, Jan 27, 2010 at 8:04 PM, Alexander Egorenkov <
>> egorenar at googlemail.com <mailto:egorenar at googlemail.com>
>> <mailto:egorenar at googlemail.com
>>
>> <mailto:egorenar at googlemail.com>>> wrote:
>>
>> Hi,
>>
>> i'm implementing a device driver for a 802.11n NIC under
>> FreeBSD 8
>> und experimented with A-MPDU transmission. I looked into
>> net80211 code
>> and there is some code which implements this feature
>> but it
>> worked not very
>> well for me.
>> I noticed e.g. that sequence numbers are not assigned to
>> A-MPDU frames
>> and found this comment in file ieee80211_output.c :
>>
>>
>> /*
>> * Check if A-MPDU tx aggregation is setup or
>> if we
>> * should try to enable it. The sta must be
>> associated
>> * with HT and A-MPDU enabled for use. When
>> the policy
>> * routine decides we should enable A-MPDU we
>> issue an
>> * ADDBA request and wait for a reply. The
>> frame being
>> * encapsulated will go out w/o using A-MPDU,
>> or possibly
>> * it might be collected by the driver and
>> held/retransmit.
>> * The default ic_ampdu_enable routine handles
>> staggering
>> * ADDBA requests in case the receiver NAK's
>> us or we are
>> * otherwise unable to establish a BA stream.
>> */
>>
>> Can somebody elaborate this description to me please.
>>
>> Thanks.
>>
>> ALex.
>>
>>
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