RRM- Radio Resource Management is a critical feature in the 9800 WLC which works in conjunction with the scan radio on Cisco APs (All Cisco Catalyst Wi-Fi 7 APs have a dedicated scan radio). RRM and the scan radio play a key role in defining the AP channel of operation, channel width, transmit power and detecting coverage holes in the RF environment.
The scan radio on every Cisco Catalyst Wi-Fi7 AP scans the RF environment on all the channels defined in the ‘Channel List’ of RRM configuration to collect Interference, Noise, Rogue data from the environment. The RRM algorithm running on the group leader (typically the 9800 WLC) processes the data recieved from the scan radios of the all the APs on the WLC.
Scan Radio
The primary task for the scan radio is to scan the channels defined in RRM configuration. The RRM configuration has 3 different channel lists from which we can choose 1- All channel list, Country channel list and DCA channel list. Country channel list includes all the channels from the regulatory domain where the AP is physically located while DCA channel list includes the channels selected under the DCA channel list configuration. This typically defines the channels list from which RRM can assign a channel on the AP radios.
The Channel List can be defined on 9800 WLC GUI for each band- Configuration>RRM>Band>General>Noise/Interference/Rogue/CleanAir Channels>Channel list
The DCA Channel List can be configured on 9800 WLC GUI for each band- Configuration>RRM>Band>DCA>Auto-RF Channel List
Note: DCA Channel List for 6G band can be configured on 9800 WLC GUI- Configuration>RF/radio>radio profile>RRM>DCA>DCA Channels
Scan Radio also transmits NDP packet for neighbor discovery (Cisco Indoor AP). Neighbor List is retained by the RRM leader while running RRM algorithm. (Discussed in detail under Neighbor Discovery Protocol)
How scan radio distinguishes Interference and Noise
Wi-Fi traffic on serving radio channel/other channels resulting from Rogue/Neighbor APs is distinguished as interference (on channel/off channel) while non Wi-Fi traffic is distinguished as Noise. The scan radio decodes the packet header to differentiate between Wi-Fi traffic and non Wi-Fi traffic.
AP CLI to check scan radio Interference/Noise stats (AP stats are reported after every iteration of scan)-
ap9172#show rrm receive stats
ap9172#debug rrm schedule
WLC CLI to check Interference/Noise stats (WLC stats are averaged over the duration of RRM cycle)-
WLC# sh ap name <ap name> auto-rf dot11radio <band> <slot> | begin Noise
DCA- Dynamic Channel Assignment–
DCA algorithm manages the AP channel assignment when AP radio assignment mode is set to ‘auto’. The channel assignment/channel change decision is mainly driven by interference/noise/signal energy detected by the scan radio on the serving radio channel for an extended duration of time (during the RRM cycle) resulting in RRM algorithm to trigger a channel/channel width change.
RRM triggered change on WLC can be monitored using the following WLC CLI-
Configuring AP channel logging- ap dot11 <band> rrm logging channel
Monitoring AP channel changes- show logging | begin channel change
The command lists all the RRM related changes and ‘Reason for the change’ viz. Interference/Noise/DBS/signal energy
DBS- Dynamic Bandwidth Selection-
Dynamic Bandwidth Selection allows RRM algorithm to allocate more channel width on particular AP radios in the environment such that the clients associated to that radio can benefit for the additional bandwidth. Before increasing the channel width on a particular radio the RRM algorithm takes into account the following parameters- Associated client capabilities, RF neighbor channel widths, OBSS channel overlaps, Noise, Interference, Channel utilization.
TPC- Transmit Power Control-
Transmit Power Control allows RRM algorithm to adjust transmit power levels on AP radios such that there is optimal coverage area overlap between neighbor APs without causing co-channel interference. TPC uses Tx neighbor and RF Neighbor list generated by NDP process.
Neighbor Discovery Protocol
Neighbor Discovery protocol is used by Cisco APs to determine its RF neighbors and the RSSI with which it can hear its neighbors. This data is used by TPC to determine the Tx power level for each radio to create optimal coverage areas for each radio. The scan radio (ISKU APs) sends NDP Tx frame on all channels/bands defined in the RRM channel list. The NDP Tx frame is transmitted on maximum Tx power level irrespective of what tx power level the serving radio of the same is transmitting on. The NDP Tx frame is trasmitted by the scan radio at the ‘reporting interval’ defined in the 9800 WLC RRM configuration which helps every AP to keep their neighbor list updated. The NDP Tx frame is received by the serving radio of the neighbor AP (on its serving radio channel) and is populated in its Neighbor list. The RRM algorithm evaluates the neighbor list data collected from all the APs and triggers a change in the tx power level at the end of the TPC RRM cycle which is typically 600 seconds.
AP CLI to check neighbor list-
9172#show rrm neighbor-list
AP CLI to debug rrm neighbors-
9172#debug rrm neighbor detail/tx/rx
WLC CLI to check neighbor list-
wlc#sh ap name <ap name> auto-rf dot11radio <band> <slot> | begin Nearby APs
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