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Frame calculator for co-location
The frame calculator is a tool that calculates the length of the transmit and receive times, together with the number of downlink and uplink symbols, for a given set of configuration parameters. The frame calculator can be used to verify that co-location of APs using different contention slots settings does not create overlapping transmit and receive times.
Basic rules
For co-location of AP1 and AP2, we want to ensure that AP1 stops transmitting before AP2 starts receiving, and that AP2 stops transmitting before AP1 starts receiving.
These are the rules that have to be satisfied for a correct co-location of the two APs:
AP1 Receive Start > AP2 Transmit End
AP2 Receive Start > AP1 Transmit End
Steps for co-location
Let us assume that in a cluster of multiple APs with all the same settings, one AP’s settings are modified with a different number of contention slots.
1. Obtain all configuration settings for the APs that do not change parameters (duty cycle, contention slots, max distance)
2. Input these configuration parameters into the OFDM Frame Calculator tool found in the system user interface under “Tools”.
3. Click “Calculate”
4. Note the following values from the results:
Downlink Transmit End = ____________________
Uplink Data Rcv Start = ____________________
These values will be named AP1 Transmit End and AP1 Receive Start respectively.
5. Access the AP that needs to have a different contention slots setting and use the frame calculator tool found under “Tools”
6. Input the configuration parameters for this AP (same duty cycle and max distance as the other APs, different contention slots)
7. Click “Calculate”
8. Note the following values from the results:
Downlink Transmit End = ____________________
Uplink Data Rcv Start = ____________________
These values will be named AP2 Transmit End and AP2 Receive Start respectively.
9. Check that the two following equations are both true:
AP1 Receive Start > AP2 Transmit End
AP2 Receive Start > AP1 Transmit End
10. If one or both equations are not true, adjust the duty cycle until they become true (or the max
distance if possible).
Example
Let us assume that all APs in a cluster have the following configuration parameters:
- Duty cycle: 75%
- Max range: 2 miles
- Contention slots: 3
Running the frame calculator as explained in the “Steps for co-location” section, the AP1 Transmit
End and Receive start times are:
AP1 Transmit End = 17022
AP1 Receive Start = 17386
The settings in one of the APs in the cluster are now modified by changing the number of
contention slots from 3 to 6, for example because this sector is constantly experiencing a higher
volume of VoIP traffic.
Running the frame calculator again, the AP2 Transmit End and Receive start times are:
AP2 Transmit End = 16536
AP2 Receive Start = 16900
The two equations above have to be checked for correct co-location:
AP1 Receive Start > AP2 Transmit End 17386 >16536 OK
AP2 Receive Start > AP1 Transmit End 16900 >17022 NOT OK
The second of the two equations is not true. AP1 is still transmitting when AP2 has already started
receiving. This creates interference at the AP2 receiver.
In order to avoid this interference scenario, the duty cycle of AP2 can be changed. For example,
changing the duty cycle of AP2 from 75% to 77% changes the AP2 Transmit End and Receive start
times as follows:
AP2 Transmit End = 16779
AP2 Receive Start = 17143
The two equations have to be checked again for co-location:
AP1 Receive Start > AP2 Transmit End 17386 >16779 OK
AP2 Receive Start > AP1 Transmit End 17143 >17022 OK
Now both equations are true and the APs can be co-located.