Link Quality Indicator provides statistics to judge the quality of the link.
LQI - Single metric indicating the quality of the link
Downlink Quality Index - Statistic for Downlink quality
Uplink Quality Index - Statistic for uplink quality
LQI will be available for one minute, 5 minutes and fiteen minutes period. The value ranges from 0 to 100. 100 being the best.
Its available in Session table under Link Quality tab
How are these calculated ?
Uplink Quality Index
This is a ratio of Actual Average Modulation rate of the data packets and the expected modulation rate. Expected modualtion rate differs based on the received signal RSSI . AP maintains a table of sensitivity values for each modulation rate, bandwidth per platform.
This is is sample senstivity table for 3 GHz 450i
For instance if the radio is using 5 MHz channel bandwidth and UL receive RSSI is -70 dbm, expected modulation rate is 6 X (Look at last column in the above table. RSSI higher than -75.8 and lower than -69 corresponds to 8X). Actual average modulation rate is the avarage of modualtion rate of all the packets in the time period (one/five/fifteen minutes). Lets say there were 100 packets, 70 were of 6X and 30 per 4X, actual average rate would be 5.4. Note that this can be a decimal number. That ratio is 5.4 deviced by 6 which is 90%
The main idea is that If the RSSI is good, but modulation is low , it indicates interference.
Downlink Quality Index
This is a product of two parameters
1. Ratio of Actual Average Modulation rate and Expected rate. This is similar to UL, but in DL direction
2. Beacon Quality Index - This index is based on receive beacon percentage. Since beacons are in QPSK modulation, lower beacon index indicates severe interference, so beacon quality index exponentially decreases with respect to beacon percenatge. Below picture shows the relation
If the modulate rate ratio is 90 % and beacon quality index is 90%, DL Quality index will be 81 %
LQI is product of three paramaeters
1. Uplink Quality index
2. Downlink Quality Index
3. Re-registation Quality index - Its based on number of re-registrations. When there are no re-regs, this quality index will be 100%
In our example LQI is 90% (uplink quality index) * 81% (downlink quality index) * 100%. Which is 72.
There ara three snmp tables. LUID is the key.
whispApsLQILowInterval - .184.108.40.206.220.127.116.11.18.104.22.168 - LQI data for one minute interval
whispApsLQIMidInterval - .22.214.171.124.126.96.36.199.188.8.131.52 - LQI data for five minutes interval
whispApsLQIHighInterval - .184.108.40.206.220.127.116.11.18.104.22.168 - LQI data for fifteen minutes interval
Each table has entries for
- Downlink Quality Index
- Downlink Average Actual Rate
- Downlink Expected Rate
- Uplink Quality Index
- Uplink Actual Average Rate
- Uplink Expected rate
- Beacon Quality Index
- Beacon Percent
- ReReg Quality Index
- Rereg count