Radio Resource Management Concepts
RRM is a collection
of algorithms which together provide a comprehensive management solution- the
key algorithmic groups to be discussed here are:
algorithm responsible for determining the RF Group Leader and members
FRA–Flexible Radio Assignment – a new algorithm charged with
identifying redundant radios resources and re-assigning the resource to a
better role – strictly applicable to 2800, 3800 series AP’s with XOR radio for
Algorithm, runs on the RF Group leader
Algorithm, runs on the RF Group Leader
Algorithm, runs on each individual controller
In addition to RRM,
there are several features which manage specific traffic types or client types
which can greatly increase the spectral efficiency and assist RRM in providing
a better experience for users. These will be discussed in context with the
RRM is organized
under the following Hierarchy:
RF Group Name ⇒ RF
Group leader(s) ⇒ RF Neighborhood(s)
For any RF Group
Name, multiple RF group Leaders may exist (a minimum of 2, one for 2.4 GHz and
one for 5 GHz will always be present). An RF Group Leader will manage multiple
It is assumed that
readers have a detailed knowledge of the following:
Readers should fully
understand the following terms used throughout this document with regard to
Cisco's RRM algorithms:
Signal: refers to RF
emanating from AP’s belonging to the same RF group or our AP’s.
signals that do not belong to our network (rogues).
Noise: any signal that
cannot be demodulated as an 802.11 signal. This can either be from a non-802.11
source (such as a microwave or Bluetooth device) or from an 802.11 source whose
signal is below sensitivity threshold of the receiver or has been corrupted due
to collision or interference.
dBm: an absolute,
logarithmic mathematical representation of the strength of an RF signal. dBm is
directly correlated to milliwatts, but is commonly used to easily represent
output power in the very low values common in wireless networking.
RSSI, or Received Signal
Strength Indicator: an absolute, numeric measurement of the strength of the
signal in a channel.
Noise floor: the ambient
RF Noise level (an absolute value expressed in dBm) below which received
signals are unintelligible.
SNR: the ratio of signal
strength to noise floor. This value is a relative value and as such is measured
in decibels (dB).
RF Group: The logical
container that an instance of RRM is configured through. All devices belonging
to a single RF Network will be configured as a member of a particular RF group.
RF Group leader: The
device where the algorithms for the RF group will be run. The RF group leader
is either automatically selected through an election process or may be manually
assigned through configuration. Two are required – one for each Spectrum band
2.4 and 5 GHz. And more may be present given the equipment and scale being
RF Neighborhood: A group
of AP’s that belonging to the same RF group which can hear each other at
=/>-80 dBm. This is a physical grouping based on RF proximity.
TPC: Transmit Power
Control is the RRM algorithm that monitors and manages transmit power level for
all AP’s in the RF group. There are two versions – each with their strenghths –
this document will cover both with recommendations.
DCA: Dynamic Channel
Assignment is the RRM algorithm responsible for selecting the operating channel
for all AP’s in the RF group.
CHDM: Coverage Hole
Detection and Mitigation–consists of the Coverage Hole Detection algorithm and
the Coverage Hole Mitigation algorithm – CHD and CHM. This also has
intersection with the HDX feature of Optimized Roaming as it relies on the
measurements obtained from CHD.
CM: Cost Metric–an RSSI
based metric which combines AP load, Co-channel interference, Adjacent channel
interference and non wi-fi sourced interference into a goodness metric used by
DCA to evaluate effective channel throughput potential.
COF: Coverage Overlap Factor – output of FRA algorithm,
represents percentage of cell covered to -67 dBm by other 2.4 GHz radios.
FRA: Flexible radio Assignment – RRM algorithm that manages
flexible radios and determines coverage redundancy and best roles for Flexible
Radios to play based on coverage requirements and density.
RRM (and RF
Grouping) is a separate function from inter-controller mobility (and Mobility
Grouping). Confusion can arise through the default use of a common ASCII string
assigned to both group names (RF Group, Mobility Group) during the initial
controller configuration wizard. This is done for a simplified setup process
and can be changed later.
It is normal for
multiple logical RF Group Leaders to exist. An AP on a given controller will
help join their controller with another controller
only if an
AP or AP's from each controller can hear one another. In large Campus
environments it is quite normal for multiple RF Neighborhoods to exist,
spanning small clusters of buildings.
How Does RRM do and
what it does?
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 (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.