Radio Resource Management and RF Groups

This section provides a brief overview of Cisco RRM and RF groups.

Radio Resource Management

KEY POINT

Real-time RF management is a foundation of the Cisco UWN solution.

Key RF challenges in managing a wireless environment include the following:

■ Limited nonoverlapping channels

■ The physical characteristics of RF propagation

■ Contention for the medium

■ The transient nature of RF environments

AP capacity is affected by the applications being run over the wireless network. For example, a recommended practice is to support approximately seven to eight voice calls over a WLAN (VoWLAN) (depending on the codec used), or about 20 data devices, because all clients must share the available bandwidth. The most common response to strained network capacity is to add more APs. However, wireless is a fixed resource. For example, only three channels can be used without causing interference between APs when using 802.11 b/g. To minimize cochannel interference, channels 1, 6, and 11 are usually the only ones used in medium- to high-density enterprise deployments. Adding more 802.1 b/g APs using only three channels with too much RF power can actually decrease RF performance. This situation can be somewhat improved by managing 802.11b/g RF power or using 802.11a, which provides significantly more channels than 802.11 b/g. Some enterprise designs reserve 802.1a for VoWLAN support.

The user experience in a wireless network is dependent on radio propagation and other building characteristics affecting connection speeds and error rates. The RF environment is transient; an office looks dramatically different at 10 a.m., when hundreds of people are walking around contending for network resources, than at 3 a.m., when doors are closed, no people are present, and neighboring offices are not generating RF interference. Wireless and RF propagation issues directly affect the QoS delivered to users.

KEY POINT

RRM continuously analyzes the existing RF environment and automatically adjusts the AP power and channel configurations to help mitigate cochannel interference, signal coverage problems, and so on. RRM also reduces the need to perform exhaustive site surveys, increases system capacity, and provides automated self-healing functionality to compensate for RF dead zones and AP failures.

The Cisco WLC uses dynamic RRM algorithms to create an environment that is completely self-

configuring, self-optimizing, and self-healing, via the following specific RRM functions:

■ Radio resource monitoring: Cisco lightweight APs are designed to monitor all channels. The AP goes "off-channel" for a period no greater than 60 ms to listen to channels on which it is not operating. Packets collected during this time are sent to the WLC, where they are analyzed to detect rogue APs (whether or not SSIDs are broadcast), rogue clients, ad-hoc clients, and interfering APs.

■ Dynamic channel assignment: WLCs dynamically allocate AP channel assignments to avoid conflict and minimize cochannel interference between adjacent APs. To avoid wasting scarce RF resources, channels are reused where there is no conflict.

■ Interference detection and avoidance: Interference is defined as any 802.11 traffic that is not part of the WLAN, including a rogue AP, a Bluetooth device, or a neighboring WLAN. Cisco lightweight APs constantly scan all channels, looking for major sources of interference. If the interference reaches a predefined threshold (the default is 10 percent), a trap message is sent to the Cisco WCS. When there is interference, the WLC attempts to rearrange channel assignments to increase system performance.

■ Dynamic transmit power control: The Cisco WLC dynamically controls AP transmit power based on real-time WLAN conditions. In normal instances, power can be kept low to gain extra capacity and reduce interference. The Cisco WLC attempts to balance APs such that they see their neighbors at -65 dBm (a number based on best-practices experience). If a failed AP is detected, power can be automatically increased on surrounding APs to fill the gap created by the loss in coverage.

■ Coverage hole detection and correction: Coverage holes are areas where clients cannot receive a signal from the wireless network. If clients on an AP are detected at low received signal strength indicator levels, Cisco lightweight APs send a coverage hole alarm to the Cisco WCS. This alarm indicates the existence of an area where clients are continually getting poor signal coverage without having a viable location to roam to. The Cisco WLC might also adjust AP power levels to correct the detected hole.

■ Client and network load balancing: WLAN capacity is effective only if clients can be load-balanced in such a way that they take advantage of this capacity. The Cisco WLC provides a centralized view of client loads on all APs that can influence where new clients attach to the network. The Cisco UWN can be configured to proactively "herd" existing clients to new APs to improve WLAN performance, resulting in a smooth distribution of capacity across the network.

RF Grouping

KEY POINT

An RF group is a cluster of WLC devices that coordinate their dynamic RRM calculations on a per 802.11 physical layer (PHY) type. Clustering WLCs into RF groups allows the dynamic RRM algorithms to scale beyond a single WLC and span building floors, buildings, and even campuses.

An RF domain exists for each 802.11 PHY type. RF groups are formed with the following process, as shown in Figure 9-21:

1. Lightweight APs periodically send neighbor messages over the air that include the WLC IP address and a hashed MIC derived from the time stamp, the AP's BSSID, and a shared secret. WLCs are configured with an RF domain name parameter; this parameter is pushed down to all the APs joined to the WLC and is used by the APs as the shared secret for generating the hashed MIC in the neighbor messages.

2. APs sharing the same secret can validate messages from each other via the MIC. When APs on different WLCs hear validated neighbor messages at a signal strength of -80 dBm or stronger, they inform their WLCs; the WLCs dynamically form an RF group.

3. The members or controllers of an RF group elect an RF group leader to maintain a "master" power and channel scheme for the RF group. The RF group leader analyzes real-time radio data collected by the system and calculates the master power and channel plan. The RRM algorithms try to optimize at a signal strength of -65 dBm between all APs and to avoid 802.11 cochannel interference and contention as well as non-802.11 interference. The RRM algorithms employ dampening calculations to minimize systemwide dynamic changes. The end result is dynamically calculated, near-optimal power and channel planning that is responsive to an always-changing RF environment.

The RF group leader and members exchange RRM messages at a specified updated interval, 600 seconds by default. Between update intervals, the RF group leader sends keepalive messages to each of the RF group members and collects real-time RF data. These messages use UDP port 12214 for 802.11b/g and UDP port 12115 for 802.11a; therefore, these UDP ports must not be restricted by firewalls or filters between RF group members.

Figure 9-21 RF Group Formation

3. Controllers elect an RF group leader that analyzes RF data.

Figure 9-21 RF Group Formation

3. Controllers elect an RF group leader that analyzes RF data.

same RF group at -80 dBm or stronger, they pass information to their WLC.

AP Self-Healing

AP self-healing is another benefit of RRM. An AP is determined to be lost when the neighbor APs no longer see RF neighbor messages at -65 dBm from the AP. Lost neighbor APs are reported to the WLC. RRM automatically increases power levels and adjusts channel selection on surrounding APs to fill the gap created by the loss in coverage.

It is important to note that the system must be designed and installed with a greater AP density than is otherwise required to support self-healing capabilities. Specifically, APs must be placed so that the system has at least one power level available to step up to if RF self-healing is triggered. AP self-healing works only for APs configured to be in the same RF Group.

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