hostapd 802.11n Throughput Optimizations

[Galen P Zink]

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