The high-level view of RRM is quite simple:
- It is a framework of services used to gather relevant over the air information and store it for analysis.
- Each AP spends time listening within its environment and collecting a variety of utilization statistics.
- The information collected drives many algorithms – note: (wIDS and rogue detection are examples outside of RRM’s algorithms).
- The information collected drives many algorithms (wIDS and rogue detection are examples outside of RRM’s algorithms).
- Each AP will gather information regarding Neighbors (Neighbor Discovery Protocol) channel conditions – Load, Interference, Noise.
- This information is collected by the RF Group Leader for the entire RF Group and used to determine the structure of the RF Domain first and break down the domain into RF Neighborhoods.
- An RF Neighborhood is a group of AP’s that can hear one another, and as such must have channel and power solutions calculated together.
- So the RF Group Leader is the designated controller that will run RRM Algorithm’s on information that it collects from Member controllers.
- It does this by first identifying groups of AP’s that are physically close enough to one another and organizing these into groups of RF Neighborhoods.
- The RF Group Leader is also the repository for the current RRM configurations (for channel and power) that will be used to configure the Algorithms for the RF Group.
The choices for monitoring are:
- All Channels— RRM channel scanning occurs on all channels supported by the selected radio, which includes channels not allowed in the country of operation. (Passive only). Country Channels–RRM channel scanning occurs only on the data channels in the country of operation. This is the default
- DCA Channels— RRM channel scanning occurs only on the channel set defined by the DCA algorithm,which by default includes all of the non-overlapping channels allowed in the country of operation. However, you can modify the channel set to be used by DCA if desired.
The Channel Scan Frequency (Wireless > 802.11a/b > RRM > General) is 180 seconds (default value). This means all channel dwells must be completed within 180 seconds. So depending on the number of channels defined by the selection in the Monitor list, the interval between dwells will increase or decrease. For instance:
- Channel List = DCA, slot =0 (2.4 GHz) – DCA defines channels 1,6,11 for a total of 3 channels. So 180 (seconds)/3(channels) =60, the AP will go off channel every 60 seconds to listen.
- Channel List = Country, slot=1 (5 GHz) in the –A regulatory domain (US) with UNii 2e enabled – gives us 22 channels defined – so 180(seconds)/22(channels) =8.18, the AP will go off channel every 8 seconds or so to listen for 50 ms.
- Neighbor Packet Frequency is also defined on the same page, the default value for 7.6 and below is 60 seconds. This means that the radio must go off channel and send a single NDP packet for every channel defined by the channel monitoring list within 60 seconds.
- Using the same example from above where Channel List = Country and slot=1 (5 GHz) this translates to 60 (seconds)/ 21 (channels)= 3 seconds, so for every three second the radio is sending an NDP packet on a channel other than the one it is currently serving.
Neighbor Discovery Protocol–NDP
- One of the most unique things about Cisco’s RRM implementation is that it uses Over The Air (OTA) messages and runs centralized even in large deployments. This gives us the advantage of being able to monitor and manage all APs and their RF experience from a single point in the network.
Not only manage – but understand how every AP relates to any other AP in the RF Group/Neighborhood. This is unique in the industry as most other implementations run AP to AP at the edge in a distributed fashion with only configuration elements being managed centrally.
Neighbor Discovery Protocol or NDP, is sent from every AP/Radio/Channel every 60 seconds or less. TheNDP packet is a special broadcast message that APs all listen for and it allows us to understand how every radio on every channel hears every other radio. It also gives us the actual RF path loss between APs.
Neighbor messages are sent to a special Multicast address of 01:0B:85:00:00:00, and are done so:
• At the Highest Power allowed for the Channel/Band
• The Lowest data rate supported in the band