User Guide

Configuring RF Auto-Tuning 293
Nortel WLAN Security Switch 2300 Series Configuration Guide
Channel and Power Tuning
RF Auto-Tuning can change the channel or power of a radio, to compensate for RF changes such as interference, or to
maintain at least the minimum data transmit rate for associated clients. A radio continues to scan on its active data
channel and on other channels and reports the results to its WSS switch.
Periodically, the switch examines these results to determine whether the channel or the power needs to be changed.
Power Tuning
By default, the switch evaluates the scan results for possible power changes every 300 seconds (5 minutes), and raises or
lowers the default power level if needed.
A radio can temporarily increase its power level from the default at any time to correct either of the following
client-related symptoms:
• Increase in the number of duplicate packets received from a client. This symptom indicates that the AP radio’s
acknowledgements (ACKs) are not reaching the client.
• Increase in the number of retransmissions of unicast packets from the radio to a client.
The radio uses two methods to treat these symptoms. First, if the data rate at which the radio is sending packets to the
client is above the minimum data rate allowed, the radio lowers the unicast data rate with the client down to the next
valid rate.
If the retransmissions still exceed the threshold, the radio continues to lower the data rate, one setting at a time, until
either the retransmissions fall under the threshold or the minimum allowed data rate is reached.
If the retransmissions still exceed the threshold after the minimum allowed data rate is reached, the radio increases
power by 1 dBm. The radio continues increasing the power in 1 dBm increments until the retransmissions fall to or
below the threshold.
After the retransmissions fall to or below the threshold, the radio reduces power by 1 dBm. As long as retransmissions
remain within the allowed threshold, the radio continues reducing power in 1 dBm increments until it returns to its
default power level.
Channel Tuning
By default, the switch evaluates the scan results for possible channel changes every 3600 seconds (1 hour). WSS
Software uses the following parameters to determine whether to change the channel on a radio:
• Received signal strength indication (RSSI)
• Amount of noise on the channel
• Packet retransmission count, which is the rate at which the radio receives retransmitted packets.
• Utilization, calculated based on the number of multicast packets per second that a radio can send on a channel while
continuously sending fixed-size frames over a period of time.
• Phy error count, which is the number of frames received by the AP radio that have physical layer errors. A high
number of Phy errors can indicate the presence of a non-802.11 device using the same RF spectrum.
• Received CRC error count. A high number of CRC errors can indicate a hidden node or co-channel interference.
The thresholds for these parameters are not configurable. RF Auto-Tuning also can change a radio’s channel when the
channel tuning interval expires, if a channel that has less disturbance is detected. Disturbance is based on the number of
neighbors the radio has and each neighbor’s RSSI.