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]