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The physical layer (PHY) rate is the speed at which client devices communicate with the AP. When you move a computer connected to the AP within the WLAN coverage area, the adapter properties dialog in Windows displays the varying connection speed, which may be as high as 450 or 300 Mbps when you are close to the AP or as low as 1 Mbps when you are 50 meters away from it. The displayed speed is the actual PHY rate at which the client was connected to the AP during an active survey. This is unlike the Expected PHY Rate available for passive surveys, where the PHY rate is not measured, but rather estimated based on the signal level. The actual PHY rate that you observe may be lower or higher than the expected rate, depending on the specific adapter and AP equipment being used.
The PHY rate that is measured is the rate at which the adapter is connected to the AP at any given spot along the survey path. As you move along the path, the adapter typically roams to the AP that provides the strongest signal within your WLAN.
It is important to remember that some adapters let you adjust roaming thresholds; these roaming settings might affect the roaming behavior and, therefore, the measured PHY rate. For example, consider two APs located 20 meters away from each other with the signal level ranging from -30 dBm next to the APs to -70 dBm in the middle of the distance between the APs. As you walk from the first to the second AP, some clients will roam as soon they pass the middle point, while others will not roam until they are only a few meters away from the second AP. For this reason, the PHY rate visualization is heavily dependent upon one’s walkabout path and its direction. Walking from the first to the second AP might produce a picture that would be different from the one produced by walking in the opposite direction.
Double-clicking on the Actual PHY rate legend on the status bar allows you to configure the color scheme and change its value range.
Suggested Solutions
When low actual PHY rate areas are discovered, the following solutions are suggested:
| § | Verify that the set of supported rates and standards of the adapter being used for active surveys is at least as good as that of the AP. |
| § | Low PHY rates might be caused by non-aggressive roaming. Check the adapter roaming settings and repeat the survey in the questionable zones. Try to walk very slowly to give the adapter time to roam and establish a good quality link to the AP. |
| § | Increase the signal level, as it is directly related to the PHY rate. See the suggested solutions for increasing signal level in the Signal Level chapter. |
| § | Check your AP capabilities. If you are using newer 802.11n equipment, make sure that the maximum MCS indices, Short GI, and 40 MHz channel bandwidth are allowed in the device configuration. |
| § | If you are using legacy 802.11 a/b/g equipment, consider upgrading to 802.11n. |
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