Dot11 Guru

802.11, Design, Troubleshooting

Blindly disabling lower data rates

Blindly making changes to a wireless network is dangerous, but sometimes required. When considering disabling lower data rates, here are some factors to consider.

  • Lower data rates = larger cell size.  This could be a requirement for the site, maybe they have some coverage problems.  What maybe better, instead of disabling the lower rates, set them to supported. 
  • Lower mandatory data rates = Higher channel utilization.  Management and control frames, like beacons that are sent every 100~ms, are always sent at the lowest mandatory data rate.  If you don’t have a survey of the site and are completely blind, I would just set the lower rates as supported.  You will reduce the amount of management overhead on the wireless network.
  • Devices using lower data rates are more likely to send less re-transmissions. 😲 This is because of the error coding built into the different types of modulation used.  They do however consumer more time transmitting their frames over the air – because the data rate is slower
    • 6.5Mbps uses BPSK and has a coding rate of ½.  This means that only half of the transmission is the data.  The other half is for error correction
    • 65Mbps uses 64-QAM, and has a coding rate of 5/6.  This means that the majority of the transmission is data, and only a small part is for error correction.  The chances are higher for interference and retransmissions

What is more true, is that a client might be using a low data rate because it experienced many re-transmissions at a higher data rate (similar to how TCP windowing works).  If clients are associating at lower data rates, it could be a sign that the site has some kind of interference or poorly designed WLAN

  • Devices using low data rates will increase channel utilization.  It can affect roaming, but it’s more likely that the client is unable to roam because there isn’t another AP in the area with a good enough signal to roam to.  And that is why the client stays on the other AP with a bad signal and low data rate.

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