Radio Ressource Management

Author:	Herbert Haas
Address:	herbert AT perihel DOT at http://www.perihel.at/
Revision:	0.5
Date:	2008-05-22
Copyright:	Copyright (c) 2008 Herbert Haas.

Abstract

This document summarizes important facts about the Radio Resource Management in Cisco's WLAN Controllers (WLCs). I made these notes once when I was trying to solve some site-survey optimization problems. Of course the major authority about this technology is Cisco so you might consult their documentation database for further information. If you find any mistakes please send me an E-Mail, thanks!

Contents

1 Upgrading to 4.1.185.0
2 Aggressive Load Balancing
3 Group Leader
4 Neighbor Messages
5 Dynamic Channel Assignment Algorithm (DCA)
6 Transmit Power Control Algorithm (TPC)
- 6.1 Example
7 Coverage Hole Detection and Correction Algorithm (CHA)
- 7.1 SNR Threshold Details
- 7.2 Clients with stupid roaming logic
8 Additional Issues

1 Upgrading to 4.1.185.0

TPC-Threshold verification:

show advanced [802.11b|802.11a] txpower

The new default value is -70 dBm (= only increase TX-PWR, when AP is weaker than this)
Proved to be optimal under a range of conditions.
Value should be the same on all the controllers in an RF group! (Because upon dynamic change of the group leader, we want to ensure that the TPC and DFS results per AP remain basically the same.)

Change it via:

config advanced [802.11b|802.11a] tx-power-control-thresh 70

Check:

show advanced [802.11a|802.11b] profile global

Shows global coverage threshold of 12 dB and 16 dB for 2.4 GHz and 5 GHz respectively.
On older versions (before 4.1.185) reduce coverage threshold to 3 dBm because the CHA algorithm was triggered even when a single client falls below the 12 dB default threshold. On newer versions we have additional conditions (25 percent and at least 3 clients).

2 Aggressive Load Balancing

Check:

show load-balancing

The default window is now 5.
The amount of clients that need to be associated before load-balancing upon association will take place.

Also values of 10-12 are practical:

config load-balancing window 10

3 Group Leader

Group Leader priority based on the Group Identifier information element in each Neighbor Message. Contains:

counter value (each controller maintains a 16-bit counter that starts at 0 and increments following events such as an exit from an RF Group or a WLC reboot).
Controller MAC address.

Calculates

Transmit Power Control (TPC) for all APs
Dynamic Channel Assignment (DCA) for all APs

But the Coverage Hole algorithm (CHA) is run per WLC.

Algorithm is re-run at 600 sec interval
If an existing neighbor AP is not heard for 60 minutes (before 4.1.185: 20 minutes), the AP is pruned out of the neighbor list.
Use the On-Demand button for finer control, and deterministic RRM behavior. The algorithms are then dormant until the next time on-demand option is used; the system is in a freeze state.

4 Neighbor Messages

Are sent at full power and lowest data rate
Are sent every 60 seconds (Neighbor Packet Frequency).

If not all APs can hear each other then Sub-Groups are created for those APs who can hear each other’s messages.

APs in the same logical RF sub-Group can share a single WLC, each be on a separate WLC, or be on a mix of WLCs.

Each controller must have at least one AP hear another AP from any other WLC. It must be possible for a Neighbor Message to pass betweem each two controllers (so intermediate WLCs which have neighboring APs can be utilized)

Each AP compiles and maintains a list of up to 34 neighboring APs (per radio) that is then reported up to their respective controllers.

Each WLC maintains a list of 24 neighbors per AP radio from the Neighbor Messages sent by each AP.

Once at the controller level, this per-AP, per-radio neighbor list of up to 34 APs is then pruned, which drops the ten APs with the weakest signals.

WLCs then forward each AP neighbor list up to the RF Group Leader, the WLC elected by the RF Group to perform all RRM configuration decision making.

Note: RF Grouping works per radio type!!!

WLCs will add neighbors to their lists given they are heard at greater than or equal to -80 dBm and will only then remove them once their signals dip below -85 dBm.

RRM supports a maximum of 20 controllers and 1000 access points per RF Group.

5 Dynamic Channel Assignment Algorithm (DCA)

Each sub-group (consisting of APs that hear each other) is configured separately (because sub-groups cannot interfere with each other)

The RF Group Leader considers the following parameters:

Load - Every AP measures the percentage of total time occupied by transmitting or receiving 802.11 frames.
Noise - APs calculate noise values on every serviced channel.
Interference - APs report on the percentage of the medium taken up by interfering 802.11 transmissions (this can be from overlapping signals from foreign APs, as well as non-neighbors).
Signal Strength - Every AP listens for Neighbor Messages on all serviced channels and records the RSSI values at which these messages are heard. This AP signal strength information is the most important metric considered in the DCA calculation of channel energy.

The Group Leader checks if another channel schema will result in a 5 dB better SNR on the worst performing AP.

Note:

Clearly, when the DCA-Algorithm changes the channels, clients will be briefly disconnected. Therefore consider the on-demand option (instead of the 10-minute update interval)
Utilization is considered: A less-used AP will have a higher likelihood of having its channel changed

6 Transmit Power Control Algorithm (TPC)

Can only reduce the power (but the CHA may increase it again)

Exceptions:
- There is no 3rd neighbor => fallback to default power (TX_max)
- The 3rd neighbor dissappears AND a new weaker 3rd neighbor appears
Only works when an AP has at least 3 neighbors (i. e. at least four APs must see each other)
WLC will reduce powers such that the third loudest neighbor AP will then be heard at a signal level of TX power thresh (-70dBm per default) or lower.

transmit power is calculated using:

Tx_Max for given AP + (Tx power control thresh – RSSI of 3rd highest neighbor above the threshold).

Hysteresis: 6 dBm before decreasing, 3 dBm before increasing.

6.1 Example

Based on the three steps/conditions defined in the TPC algorithm, the example in this section explains how the calculations are made to determine whether the transmit power of an AP needs to be changed. For the purpose of this example, these values are assumed:

The Tx_Max is 20
The current transmit power is 20 dBm
The configured TPC threshold is -65 dBm
The RSSI of the third neighbor is -55 dBm

Thee TPC algorithm results in:

Condition one: is verified because there is a third neighbor, and it is above the transmit power control threshold.
Condition two: 20 + (-65 – (-55)) = 10
Condition three: Because the power has to be decreased one level, and a value of ten from condition two satisfies the TPC hysteresis, the Tx power is reduced by 3dB, which brings the new Tx power down to 17dBm.

At the next iteration of the TPC algorithm, the AP’s Tx power will be lowered further to 14dBm. This assumes all other conditions remain the same. However, it is important to note that the Tx power will not be lowered further (keeping all things constant) to 11dBm because the margin at 14dBm is not 6dB or higher.

7 Coverage Hole Detection and Correction Algorithm (CHA)

Based on client statistics, therefore CHA is run per WLC independently
Is triggered when clients fall below the SNR threshold
Configured number of clients (Client Min. Exception Level=3 per default) must fall below this threshold for at least 60 seconds. Then the TX power of the corresponding APs is increased.
WLC runs CHA on each band on each AP per default every 180 seconds

7.1 SNR Threshold Details

Note: The SNR threshold varies based on

AP power

coverage profile of the WLC

The actual SNR threshold value is:

SNR_threshold = ABS ( AP_TX_Power - 17 dBm - coverage_profile )

The coverage profile is user defined (default: 12 dB)

7.2 Clients with stupid roaming logic

Some clients fail to roam to an available better AP. Then the WLC might detect a coverage problem although there is none. Eventually the CHA is triggered unnecessarily.

Workaround:

Increase the Client Min. Exception Level to a higher number (e. g. 3 => 10)
Decrease the tolerable client SNR value (e. g. 12 dB => 3 dB)

8 Additional Issues

8.1 Interesting configuraion options

Avoid Foreign AP Interference allows the co-channel interference metric to be included in DCA algorithm (enabled by default)
Avoid Cisco AP Load allows the DCA to weight APs for a channel change depending of the AP load. Since the load is frequently changing we usually do not want to include this metric in the RRM calculation. (Therefore this field is NOT selected.)
Power Neighbor Count - The minimum number of neighbors an AP must have for the TPC algorithm to run.

8.2 Profile Thesholds for Traps

Only used for notification! These settings do not affect the RRM algorithms!
Therefore any change of these parameters (Clients, Coverage Exception Level, ...) is harmless.

8.3 Debugging RRM

> debug airewave-director ?

   all            Configures debug of all Airewave Director logs
   channel        Configures debug of Airewave Director channel assignment protocol
   error          Configures debug of Airewave Director error logs
   detail         Configures debug of Airewave Director detail logs
   group          Configures debug of Airewave Director grouping protocol
   manager        Configures debug of Airewave Director manager
   message        Configures debug of Airewave Director messages
   packet         Configures debug of Airewave Director packets
   power          Configures debug of Airewave Director power assignment protocol
   radar          Configures debug of Airewave Director radar detection/avoidance protocol
   rf-change      Configures logging of Airewave Director rf changes
   profile        Configures logging of Airewave Director profile events