I always knew that Duration/ID field is used by CSMA/CA to predict when wireless medium becomes free. However, I was confused by some publications which stated this field is set to the amount of time (in microseconds), required to transmit current frame, wait SIFS and then receive ACK. CWNA Sybex book has finally helped to understand this better.
Even though Duration/ID field tells STA how much time it has to wait before medium becomes free, it is set to the duration of SIFS + ACK. It doesn’t include the time required to transmit current frame. It kind of makes sense – to read the value from the field, STA needs to receive the frame in full and check its FCS before it can set the NAV with legitimate value.
Also, didn’t know ACK/Block ACK frames always have their Duration/ID field set to 0.
Again, makes sense. Transmission completed, all NAVs have to be reset to 0 – medium is ready for the next transmission.
Good to know: Historically this field was defined to identify STA’s association ID within PS-Poll frame (legacy power management) or Duration in any other frame. In reality, legacy power management is not being used and this field is mostly used as a Duration ONLY nowadays. However, name of the field is still Duration/ID.
Have just finished Ekahau Site Survey course in Oxford and here are my takeaways from it:
- Move away from 2.4GHz completely (inc. BYOD and Guest networks if possible) – don’t use it in modern world
- Avoid using multiple SSIDs – beacons make the air dirty (have captured 60MB of beacons on one channel in 15m as a test)
- Don’t install APs in corridors or hallways – signal from omni antennas propagates on hundreds of meters in free space
- Make sure signal strength from 2nd strongest APs meet the main requirements if voice roaming is of concern. This is an equivalent of Cisco’s channel overlap of 10-20-30% requirement, which is hard to measure or prove.
- If APs are installed in rooms/offices, put them away from windows to avoid waste of RF energy sent towards the street.
- Mirrors do not significantly affect 2.4GHz (and 5GHz) – waves are still lengthy enough to penetrate through mirrors (?).
- Adjacent channels interference is more destructive than co-channel interference, because devices on adjacent channels do not play nicely when they access the media (do not contend)
- Radio Tap header information is added by the client. RSSI and noise levels reported by clients are virtual and vendor specific. NICs do not see RF shapes to measure noise and RSSI, but spectrum analyzers do.
- Ch144 on 5GHz was introduced with 802.11ac
- ESS cheats
- Don’t use small objects to define scale, such as doors, desks and so on. This increases error level.
- Use distance between objects to define scale on one floor, then use alignment points to define scale on all others.
- Use coverage and coverage exclusion areas to make ESS reporting more accurate (coverage percentage)
- If floors are more or less identical, draw one and duplicate the rest (using image swap trick). This will help to save a lot of time as this trick copies all objects, including alignment points and scales.
- Useful Tools
- Buffalo WMR-433: Very tiny WiFi router which can reach out on tens of meters
- Theodolite: iOS app to measure azimuth, elevation, GPS coordinates – helps with outdoor deployments
- NetScout Link Sprinter 300: A link tester. Checks all layers of TCP/IP protocols suite (physical/data link, network, transport and application). Capable to send results by email.
- HORST: linux-based lightweight 802.11 analyzer with text interface
- Kismet: linux-based wireless sniffer/packet capture software
That was a great week in Oxford. Hopefully I haven’t misinterpreted the information above 🙂