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:

Instructions

./airtime.py -f <pcap-file> -i <interval> -w <filter> -o <output_format> [--no-fcs]

Options:
 -h, --help            show this help message and exit
 -f PCAPFILE, --pcap-file=PCAPFILE
                       Capture dump
 -t TEXTFILE, --text-file=TEXTFILE
                       Capture already converted/filtered
 -i INTERVAL, --interval=INTERVAL
                       Consolidation interval in seconds
 -w FILTER, --filter=FILTER
                       Wireshark filter
 -o OUTPUT, --output-format=OUTPUT
                       Output Format [csv, lines]
 --no-fcs              don't check if frames have bad crc

Notes

• The input file (PCAPFILE) is a tcpdump capture file (captured by tcpdump, wireshark or any other compatible tool)
  • Radiotap header must be present (for the script needs the tx datarate)
  • If your capture does not provide Frame CheckSum (this is the case of the captures from XOs) you must provide the flag --no-fcs
  • By default only frames with good FCS will be computed
• The script generates a temporary file in the local directory named <caprute file>.tmp3
  • This file can be used instead of the original pcap file (using the -t option), if the filter is the same. This is meant to save processing time if you are to use the same set of data, changing only the consolidation interval.
  • The .tmp3 file can be safely removed
• The default output format will dump a line for each consolidation timeslot with the percentage of airtime consumed.
  • You can use '-o csv' to get a comma separated file that can be easily imported by a spreadsheet program
• Filters should be quoted: -w "filter expression". (see examples bellow)
  • If you do not provide a filter, all frames in the file will be computed (total airtime)
• Default consolidation interval is 1 second.
  • Fractions of a second can be used too.

Examples

1. To compute all airtime consumption for each one second interval:

./airtime.py -f capture.dump

2. To compute all traffic related to XOs in a 10 seconds consolidation interval:

./airtime.py -f capture.dump -i 10 -w "wlan contains 00:17:c4"

3. To use only icmp traffic and generate output in csv format:

./airtime.py -f capture.dump -w "icmp" -o csv

4. Only management traffic, over a file captured from an XO, consolidated at 100ms:

./airtime.py -f capture.dump -w "wlan.type == 0x0" --no-fcs -i 0.1

5. The same as above but using the pre-processed file and changing interval to 10ms:

./airtime.py -t capture.dump.tmp3 -i 0.01

6. icmp and path discovery traffic (the sum of both):

./airtime.py -f capture.dump -w "icmo or wlan_mgt.fixed.mesh_action" -o csv

Note that you need a properly patched wireshark to decode "wlan_mgt.fixed.mesh_action"

7. To have two separated set of datas (to later draw a graph on a spreadsheet):

 ./airtime.py -f capture.dump -w "icmp" -i 0.1 > output.csv
 ./airtime.py -f capture.dump -w "wlan_mgt.fixed.mesh_action" -i 0.1 >> output.csv

Note that you need a properly patched wireshark to decode "wlan_mgt.fixed.mesh_action"