Link Quality Indicator

Products PMP
1

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.

GUI

Its available in Session table  under Link Quality tab

GUI.PNG
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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

Sample Sensitity.PNG
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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

Beacon Quality Index.PNG

If the modulate rate ratio is 90 % and beacon quality index is 90%, DL Quality index will be 81 %

LQI

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.

SNMP OIDs

There ara three snmp tables. LUID is the key.

whispApsLQILowInterval  -  .1.3.6.1.4.1.161.19.3.1.13.1 -  LQI data for one minute interval

whispApsLQIMidInterval  -  .1.3.6.1.4.1.161.19.3.1.13.2 -  LQI data for five minutes interval

whispApsLQIHighInterval  -  .1.3.6.1.4.1.161.19.3.1.13.2 -  LQI data for fifteen minutes interval

Each table has entries for

  1. LQI
  2. Downlink Quality Index
  3. Downlink Average Actual Rate
  4. Downlink Expected Rate
  5. Uplink Quality Index
  6. Uplink Actual Average Rate
  7. Uplink Expected rate
  8. Beacon Quality Index
  9. Beacon Percent
  10. ReReg Quality Index
  11. Rereg count
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