2 Replies to “WiFi Wisdom – One”

  1. Great blog! The answer is “It is the client device responsibility to determine WHEN to roam and which AP to roam TO. A client will NEVER even try to roam until it reaches the LOWEST mandatory data rate!” Answer from your “3 steps to tuning a Cisco WLAN controller” post!

  2. Indeed you are correct! According to 802.11 the client will prioritize AP Probe Responses (while trying to connect or roam) based on 1) the SSID it wants to connect to and 2) signal strength. There ARE things we can to do encourage good roaming behavior. The last thing you want to do is connect to your “closest” AP, then go to a meeting on the other side of the building, pass 2 or 3 AP’s on the way, then be stuck at a super-low data rate on the original AP because the client hasn’t decided it wants to roam yet. Thanks 802.11!!

    According to the Cisco Wireless LAN Controller Configuration Best Practices guide we are advised to Disable Low Data Rates (http://www.cisco.com/c/en/us/td/docs/wireless/technology/wlc/82463-wlc-config-best-practice.html#pgfId-380239).

    There we find some interesting detail:

    You must carefully plan the process to disable or enable data rates. If your coverage is sufficient, it is often a good idea to incrementally disable lower data rates one by one. Management frames like ACK or beacons will be sent at the lowest mandatory rate (typically 1Mbps), which slows down the whole throughput as the lowest mandatory rate consumes the most airtime.

    Try not to have too many supported data rates so that clients can down-shift their rate faster when retransmitting. Typically clients try to send at the fastest data rate they can and if the frame does not make it through, will retransmit at the next lowest data rate and so on until the frame goes through. The removal of some supported rates means that clients who retransmit a frame directly down-shift several data rates, which increases the chance for the frame to go through at the second attempt.
    – Beacons are sent at the lowest mandatory rate, defining roughly the cell size.
    – Multicast is sent on the range between lowest and highest priority, depending on associated clients.
    – If your design does not require low data rates, consider disabling the 802.11b data rates (1, 2 ,5.5 and 11) and leave the rest enabled.
    – You might make a conscious decision to not disable all rates below 11Mbps in order to not stop the support of 802.11b-only clients.

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