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This section covers the following topics in depth:
Receiver Start of Packet Detection Threshold (RX-SOP) determines the Wi-Fi signal level in dBm at which an AP radio will demodulate and decode a packet. The higher the RX-SOP level, the less sensitive the radio is and the smaller the receiver cell size will be. By reducing the cell size, we ensure that the clients are connected to the nearest access point using the highest possible data rates. This is ideal for high density environments such as stadiums and large auditoriums where there are a large number of client devices connected per AP. In a high density environment, the smaller the cell size, the better.
As the RX-SOP value increase, only packets of a higher RSSI value are decoded by the AP’s radio. This decreases the cell size as shown in the figure above, but ensures that all clients are connected with a good RSSI value.
The following is an example of an AP receiving 802.11 packets. All frames received with weaker RSSI than configured RS-SOP will be classified as non-WiFi frames and will not get decoded by the radio, only packets with acceptable RSSI values are decoded. Packets that are not decoded are treated as non-WiFi interference and detected at the AP as noise.
In this example, the RX-SOP threshold is set to low, so authentication requests sent at -84 dBm are not decoded, but when the same message is sent at -36 dBm, it is decoded.
802.11 Band | High Threshold | Medium Threshold | Low Threshold | Auto |
---|---|---|---|---|
5 GHz | -76 dBm | -78 dBm | -80 dBm | Use radio default |
2.4 GHz | -79 dBm | -82 dBm | -85 dBm | Use radio default |
The RX-SOP default threshold value is Auto, which means that the RX-SOP threshold is set to the radio’s default value.
Note |
To create a new RF Profile, from the WLC GUI, go to Wireless > RF Profile, and then click New.
Enter a RF Profile name and select a band. A separate profile must be applied to the 2.4 GHz band and 5 GHz band.
Select the High Density tab and choose the desired RX-SOP threshold.
Note | Existing RF profiles can be modified to include an RX-SOP Threshold. You need not create a new profile solely for RX-SOP. |
The RF Profile must now be applied to an AP Group. Go to WLANs > Advanced > AP Groups, then either add a new group or modify an existing group.
Under the RF Profile tab, select the pre-band RF Profiles with the desired RX-SOP thresholds.
To complete the AP Group configuration, additional steps such as adding WLANs and selecting APs are required. These steps are not listed as part of this guide.
config 802.11<a/b> rx-sop threshold <level> auto Revert 802.11a radio receiver SOP to auto high Set 802.11a radio receiver SOP to high low Set 802.11a radio receiver SOP to low medium Set 802.11a radio receiver SOP to medium
The Sticky Client Problem:
A client is referred as "Sticky" when it does not roam or move to a nearby AP connection that has stronger signal strength. As illustrated in the above figure, when Optimized Roaming is disabled, the client will continue to stay with the AP at very weak signal strength, for example—lower than -86 dB. Lower signal strength will lead to increase in Airtime interference, thus reducing the overall performance.
Solution—Cisco Optimized Roaming:
Note | The client will be allowed to rejoin the AP when the client RSSI value has increased to 6 dBm above the disassociation threshold. For example, if the Data RSSI threshold is -80 dBm, the client will be allowed to re-associate once the RSSI values is increased to -74 dBm. |
To configure Optimized Roaming on a global level, go to Wireless > Advanced > Optimized Roaming.
On selecting the page, Optimized Roaming Mode, Optimized Roaming Interval, and Data Rate Threshold appear. Different values can be selected for the 802.11a/n/ac and 802.11b/g/n radios.
To change the Optimized Roaming RSSI thresholds, go to Wireless > 802.11a/n/ac or 802.11b/g/n > RRM > Coverage.
The Data RSSI value and the Coverage Exception level percentage can be configured using the WLC GUI or WLC CLI (See the WLC CLI command in the next section). The received data packet count can only be configured via the WLC CLI.
These values can also be set via an RF profile. If an RF Profile is applied, the RF Profile values will override the globally set values.
To view the Optimized Roaming statistics, go to Monitor > Statistics > Optimized Roaming.
config advanced 802.11<a/b> optimized-roaming <enable/disable> enable Enable 802.11a OptimizedRoaming disable Disable 802.11a OptimizedRoaming
config advanced 802.11<a/b> coverage data rssi-threshold <dBm> -60 to -90
config advanced 802.11<a/b> coverage data fail-percentage <percent> <percent> 1 to 100
config advanced 802.11<a/b> coverage packet-count <num-packets> <num-packets> 1 to 255
config advanced 802.11<a/b> optimized-roaming interval <seconds> Configure the reporting interval of 802.11a/b Optimized-Roaming
config advanced 802.11<a/b> optimized-roaming datarate <mbps> Enter a rate of either 6, 9, 12, 18, 24, 36, 48 or 54, or 0 for disable
Sample Configuration:
Note | Per WLAN Coverage Hold Detection must be enabled on all WLANs that participate in Optimized Roaming. |
The 8.0 release now supports the ability to configure Dynamic Channel Assignment in an RF Profile. This is very powerful because of the following:
RF Profiles allow Network Administrators to apply different RF characteristics to an AP Group. This allows for a completely customized RF environment for access points that are connected to the same physical WLC.
In the following example, the APs covering a large meeting room with many clients are assigned a High Density RF Profile. The High Density RF Profile uses 40 MHz channels to minimize channels from being reused, lower the TX power to shrink the cell size, and disable lower data rates to remove low performing clients.
The APs in RF Profile 2 are deployed in a less density enterprise office space. Their cell sizes are larger, 80 MHz channel widths are used, and the Tx power is greater.
We can now assign channels to an RF Group. This gives the network administrator the ability to select the channels that DCA can select by group of APs. The administrator can configure the first floor of a building to use only UNII-1 channels, the second floor to use UNII-2, and the third floor to use UNII-3.
You can either edit an existing RF profile or create a new RF profile. Go to Wireless > RF Profile > Edit RF Profile Name > RRM tab.
Select the channels that you would like configured for this RF Profile.
Different channel widths can be assigned to different RF groups. This allows network administrators to select 20/40/80 MHz channel widths per group of APs.
You can either edit an existing RF profile or create a new RF profile. Go to Wireless > RF Profile > Edit RF Profile Name > RRM tab.