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Optimization setting for high latency in WLAN

Publication Date:  2014-08-30 Views:  150 Downloads:  0
Issue Description
1. The topology is shown below:

2. When do ping test from STA to Gateway, there is high latency and packets lost.
Alarm Information
None
Handling Process
In resolution to the encountered issue, we recommend to optimize the following settings on all APs:

1. To reduce the packet retransmission bigger than 1400.
#
rts-cts threshold 1400
#

2. To reduce the time of re-transmission.
#
long-retry 3
short-retry 3         
#

3. The video and voice performance takes much of the air interface resources.  To enhance the user experience, the audio and video data transmission priority (air interface resources) need to be reduced compared to default settings.
#
wmm edca ap ac-vi aifsn 2 ecw ecwmin 3 ecwmax 4 txoplimit 94
wmm edca ap ac-vo aifsn 2 ecw ecwmin 2 ecwmax 3 txoplimit 47
#

4. In addition, to enhanced Roaming Feature and prevent low rate packets encountered above, If the forced offline features is enabled, once the station receives a lower signal (-80db) , it will kick the station (1-2 RTO) and will automatically transfer to another AP with higher signal strength quickly without noticing the disconnection.
#
sta-forced-offline signal-strength switch enable
sta-forced-offline signal-strength threshold -80
#

After optimization above, the latency between stations and gateway reduces to normal range.

 
Root Cause
1. Checking the rate between STA and AP, the rate on test STA 9c:b7:0d:98:xx:xx  is unstable.
#
[CEBIN11FAP01-diagnose]dis interface radio 0 vap 0 list
ADDR               AID CHAN RATE RSSI IDLE  TXSEQ  RXSEQ   SKIP  NORML CAPS ACAPS ERP    STATE HTCAPS
9c:b7:0d:98:xx:xx    2    6   1M   47    0  51351  58608      0      0 EPSs         0    4001f P     
1c:65:9d:a8:xx:xx   1    6  64M   50    0     68  49680      0      0 EPSs         0    4001f PM   

[CEBIN11FAP01-diagnose]dis interface radio 0 vap 0 list
ADDR               AID CHAN RATE RSSI IDLE  TXSEQ  RXSEQ   SKIP  NORML CAPS ACAPS ERP    STATE HTCAPS
9c:b7:0d:98:xx:xx    2    6  98M   45    0  58533  54128      0      0 EPSs         0        f P     
1c:65:9d:a8:xx:xx    1    6  64M   55    0     71  50240      0      0 EPSs         0        f PM     
#
2. Using inSSider tool to scan the signal, there is interference between APs.


3. Base on the gathered data, there is interference between APs. When a packet sent by the AP. The AP waits for a response from the stations; if the AP doesn't detect response from the stations, the packet will be sent again with a lower rate. The lower the rate ,the higher the latency.
Suggestions
Interference is the most reason that leads to WLAN unstable, the optimization setting above is for reference.

END