Wireless airtime analysis
This page is devoted to the analysis of the airtime consumption in XO networks. In other words, to getting answers to questions like: how much of the medium is consumed by management traffic, or beacons, or the path discovery mechanism.
To obtain this answers we do a little math and introduce a tool that will make this math easier to do.
Airtime and Modulation technique
The time necessary to transmit a given frame is function not only of the transmission rate; it is also affected by the modulation technique in use. An XO will transmit frames in two different mechanisms:
- For the rates 1, 2, 5.5 and 11Mbps, frames will be transmitted in CCK
- For the rates 6, 9, 12, 18, 24, 36, 48 and 54Mbps, frames will be transmitted in OFDM
Given:
- Frame size in bytes: S
- Datarate in Mbps: R
- Airtime in microseconds (us): T
For frames transmitted in CCK:
T = 192 + (S*8)/R
Note1: 192 is preamble duration (144us) + PLPC header duration (48us)
Note2: Supposing long preambles are being used in all transmissions.
For frames transmitted at more than 11Mbps (OFDM):
T = 26 + (S*8)/R
Note: 26 is signal extension time (6us) + preamble (20us)
Airtime tool
This python tool was designed to take a capture file (tcpdump format) as input and calculate the airtime consumption of a given type of traffic (specified by a wireshark filter) for a consolidation interval:
./airtime.py -f <pcap-file> -i <interval> -w <filter> -o <output_format> [--no-fcs]
