CCNA Wireless Summary Notes: Understanding Radio Resource Management (RRM)

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Data Rates

Initial IEEE requirements are as follows – which are configured by default on WLC:

  • For 2.4 GHz, the 1-, 2-, 5.5-,and 11-Mbps rates are all marked as mandatory ( Must sopport all possible modulations in 802.1b)
  • For 5-GHz band, the 6-, 12-, and 24-Mbps rates are marked as mandatory

Enable 802.11a – 5GHz

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Enable 802.11b – 2.4GHz

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Enable 802.11n

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802.11n can bond one 20-MHz to an adjacent 20-MHz channel to effectively double the channel width. By default, the controller will use only a single 20-MHz channel. This is configurable.

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Enable 802.11ac

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Radio Resource Management (RRM)

This is flexible and an automatic mechanism that Cisco Wireless LAN controllers  that is used to manage Radio resources. RRM works out an optimum transmit power level and channel number for  each AP. It also detects changes and makes appropriate adjustments.

The APs should be in one RF group. RF group can traverse controllers if:

  • The controllers share a common RF group name
  • At least one AP from one controller can be overheard by an AP on another controller at a received signal strength indicator (RSSI) of –80 dBm or greater, they are close enough to belong to the same RF group. – basically they should be located near enough.
  • Up to 20 controllers and 1000 APs can join to form a single RF group.

One controller in each group is elected as an RF group leader. This can be done statically. The leader collects and analyzes the information from all members in real time.

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  • APs are set to receive and transmit on a single channel hence they can hear noise or interference on that channel.
  • APs scan other channels for noise and interference too (less than 60 ms)

What RRM can accomplish:

  • Set transmit power levels for each AP – Transmit power control (TPC) algorithm
    • AP joins the WLC and scans channels for RF conditions. It uses the RSSI values of the neighbouring APs to measure how close the AP is and sends to RF group leader for TPC algorithm calculation.
    • TPC works on one AP at a time, one band at a time. If an AP has been heard with an RSSI above a threshold (–70 dBm by default) by at least three of its neighbors, TPC considers the AP’s cell to be overlapping the cells of its three neighbors too much. The AP’s transmit power level will be decreased by 3 dB, and then its RSSI will be evaluated again.
    • Runs on both bands separately.

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  • If you select on demand, the algorithm will be run once at the next 10 min interval and frozen till next manual initiation.
  • Cisco controllers determine the transmit power level according to an index from 1 to 8

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  • It is best to match the AP and client transmit power level. To prevent such a condition, you can set minimum and maximum power level boundaries for the TPC algorithm. By default, the minimum level is set to –10 dBm and the maximum to 30 dBm

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  • Dynamically adjust AP channel – Dynamic channel allocation (DCA) algorithm
    • Adjacent APs should use different non overlapping channels.
    • When a new AP first powers up, it uses the first non-overlapping channel in each band—
      channel 1 for 2.4 GHz and channel 36 for 5 GHz.
    • Algorithm runs every 10 mins and can adjust channels.
    • Metrics used during calculation:
      • RSSI of neighboring APs
      • 802.11 interference
      • Non-802.11 noise
      • AP traffic load
      • Persistent interference
    • The RF group leader will undergo an RRM startup mode after it is elected. The startup mode consists of ten DCA iterations at 10-minute intervals, or a total of 100 minutes before the channel layout reaches a steady state.
    • DCA is not limited to 2 dimension and can also help minimize interference between floors if the APs are in the same RF group.
    • The DCA parameters also include the 802.11n channel width. By default, 20-MHz channels
      will be used. For  802.11n in the 5-GHz band and want to enable 40-MHz channels, be sure to select 40 MHz as the channel width.
    • You can specify which channels the DCA algo can dish out to APs

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  • Event-Driven RRM (ED-RRM) – Can trigger DCA based on RF events in real time

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  • Detect RF coverage holes – Coverage hole detection mechanism (CHDM) 
    • Does not run frequently, It monitors RF conditions of wireless clients and decides when to run.
    • It runs on a per band basis and on each controller separately
    • Conditions that must be met for a coverage hole to be detected:
      • Client RSSI at the AP is at or below –80 dBm.
      • The low RSSI condition must last at least 60 seconds over the past 180 seconds.
      • The condition must affect at least three clients or more than 25 percent of the clients on a single AP

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