A link test is used to determine the quality of the radio link between two devices. Two types of link-test packets are transmitted during a link test: request and response. Any radio receiving a link-test request packet fills in the appropriate text boxes and echoes the packet back to the sender with the response type set.
The radio link quality in the client-to-access point direction can differ from that in the access point-to-client direction due to the asymmetrical distribution of the transmit power and receive sensitivity on both sides. Two types of link tests can be performed: a ping test and a CCX link test.
With the ping link test, the controller can test link quality only in the client-to-access point direction. The RF parameters of the ping reply packets received by the access point are polled by the controller to determine the client-to-access point link quality.
With the CCX link test, the switch can also test the link quality in the access point-to-client direction. The switch issues link-test requests to the client, and the client records the RF parameters (received signal strength indicator [RSSI], signal-to-noise ratio [SNR], and so on) of the received request packet in the response packet. Both the link-test requestor and responder roles are implemented on the access point and switch. Not only can the access point or switch initiate a link test to a CCX v4 or v5 client, but a CCX v4 or v5 client can initiate a link test to the access point or switch.
The switch shows the link-quality metrics for CCX link tests in both directions (out— the access point to the client; in— the client to the access point):
- Signal strength in the form of RSSI (minimum, maximum, and average)
- Signal quality in the form of SNR (minimum, maximum, and average)
- Total number of packets that are retried
- Maximum retry count for a single packet
- Number of lost packets
- Data rate of a successfully transmitted packet
The controller shows this metric regardless of direction:
- Link test request/reply round-trip time (minimum, maximum, and average)
The controller software supports CCX versions 1
through 5. CCX support is enabled automatically for every WLAN on the controller and
cannot be disabled. The controller stores the CCX version of the client in its client
database and uses it to limit the features for this client. If a client does not support
CCXv4 or v5, the controller performs a ping link test on the client. If a client
supports CCXv4 or v5, the controller performs a CCX link test on the client. If a client
times out during a CCX link test, the controller switches to the ping link test