hostapd 802.11n Throughput Optimizations

[Galen P Zink]

I spent some time analyzing the wireless frames and discovered that I  
was only seeing HT20, not HT40. This has now been corrected.

However, I am still struggling with the throughput quality versus the  
AR5008 access point I am testing against. UDP is able to nearly match  
my reference access point, but I have to push it 'harder' (higher  
bitrate in, higher packet losses) to approach the same values. TCP  
throughput is lagging even more, significantly behind the reference  
access point, by around 40-50% slower.

In my comparisons, I am using iperf against a wired gigabit ethernet  
machine. Hostapd is acting only as a transparent bridge.

Ideas for optimizations?

-Galen

On May 31, 2009, at 2:36 AM, Galen P Zink wrote:

> I'm trying to optimize 802.11n throughput with hostapd. Before I begin
> tweaking with the ath9k drivers, I'd like to make sure hostapd is
> setup for optimal performance.
>
> Software: Debian unsable with 2.6.29-2 kernel; latest compat-wireless
> bleeding edge snapshot; <1 day old hostapd snapshot. Dedicated system
> for wireless testing.
> Hardware: 2 GHz Core Duo CPU, 2 GB RAM, AR5008 miniPCI-e card (3
> antennas)
> Objective: Maximize 802.11n throughput at 5 GHz, assuming complete
> control of all stations on the access point. 40 MHz channels desired.
>
> Right now, hostapd appears to be yielding lower-than-expected
> performance, approximately half that of an AR5008 access point. Given
> the AR5008 card I have is more powerful and has (arguably) better
> antennas, and a vastly faster CPU, it seems such a large gap may
> likely be a software issue.
>
> I have appended my hostapd.conf file below. I started with the
> template and adjusted items as I saw appropriate. I did not try to
> change too much. I welcome comments and suggestions!
>
> I would greatly appreciate comments on the best ways to debug /
> monitor hostapd. -dd is not enough resolution. It would be preferable
> if I could do something less granular than reviewing every wireless
> frame, of course; something like broad statistics on frame size, re-
> transmits, bitrates, signal to noise levels, etc. would be very nice.
>
> I also am hoping for some clarity on the ht_capab flags as well. I
> expected [GF] to ban all legacy clients, but it doesn't. What does it
> do? Is there a way to enforce 802.11n only? Does HT40+ versus HT40-
> have any impact on client / radio compatibility? Are there any
> disadvantages to enabling [LDPC] - I'd expect only benefits, unless
> the CPU were overloaded? I believe ath9k still does not support STBC;
> is there any harm to having it on, or is STBC somehow handled by
> hostapd?
>
> My apologies in advance if I missed documentation on the above items
> anywhere...
>
> -Galen
>
> ----------------------------------------------------------------
>
> interface=wlan0
> bridge=br-test
> driver=nl80211
> logger_syslog=-1
> logger_syslog_level=2
> logger_stdout=-1
> logger_stdout_level=2
> dump_file=/tmp/hostapd.dump
> ctrl_interface=/var/run/hostapd
> ctrl_interface_group=0
> ssid=80211n-testing
> hw_mode=a
> channel=44
> beacon_int=100
> dtim_period=2
> max_num_sta=2007
> rts_threshold=2347
> fragm_threshold=2346
> preamble=1
> macaddr_acl=0
> auth_algs=3
> ignore_broadcast_ssid=0
> wmm_enabled=1
>
> # Low priority / AC_BK = background
> wmm_ac_bk_cwmin=4
> wmm_ac_bk_cwmax=10
> wmm_ac_bk_aifs=7
> wmm_ac_bk_txop_limit=0
> wmm_ac_bk_acm=0
>
> # Normal priority / AC_BE = best effort
> wmm_ac_be_aifs=3
> wmm_ac_be_cwmin=4
> wmm_ac_be_cwmax=10
> wmm_ac_be_txop_limit=0
> wmm_ac_be_acm=0
>
> # High priority / AC_VI = video
> wmm_ac_vi_aifs=2
> wmm_ac_vi_cwmin=3
> wmm_ac_vi_cwmax=4
> wmm_ac_vi_txop_limit=94
> wmm_ac_vi_acm=0
>
> # Highest priority / AC_VO = voice
> wmm_ac_vo_aifs=2
> wmm_ac_vo_cwmin=2
> wmm_ac_vo_cwmax=3
> wmm_ac_vo_txop_limit=47
> wmm_ac_vo_acm=0
>
> ieee80211n=1
> ht_capab=[HT40-][SHORT-GI-40][LDPC][GF][DELAYED-BA][DSSS_CCK-40][MAX-
> AMSDU-7935][TX-STBC][RX-STBC123]
>
> own_ip_addr=127.0.0.1
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