Receiver Sensitivity

The transmitted signal eventually reach the receiver. There, results depend on the Receive Sensitivity of that device -- i.e., the minimum power required to handle arriving frames at a given link speed. Receive Sensitivity is a given characteristic of a 802.11 device and will vary across products.

Signal strength is measured on a logarithmic scale expressed in decibels. The higher the signal, the better the performance of the wireless network. At the client side, Covera Zone records the Received Signal Strength Indicator or RSSI which as reported by the wireless client device. On a typical WLAN, RSSI will range from -20 dBm (very close to the AP) to -95 dBm (away from the AP). The following table shows the sensitivity threshold for a typical 802.11b/g network device. Assuming a 10 dBm margin to account for the inevitable RSSI fluctuations in the link budget, the current generation of 802.11b/g devices can maintain a wireless connection for a signal stronger than -84 dBm. However, a good connection will require at least -75dBm.

Table 2. Receiver sensitivity for a typical 802.11b/g wireless client NIC

Signal Strength (RSSI) Link Speed Theoretical free space range (meter) Theoretical free space range (feet)
-94 dBm 1 Mbps 1543 470
-93 dBm 2 Mbps 1375 419
-92 dBm 5.5 Mbps 1226 374
-86 dBm 6 Mbps 614 187
-86 dBm 9 Mbps 614 187
-90 dBm 11 Mbps 974 297
-86 dBm 12 Mbps 614 187
-86 dBm 18 Mbps 614 187
-84 dBm 24 Mbps 488 149
-80 dBm 36 Mbps 308 94
-75 dBm 48 Mbps 173 53
-71 dBm 54 Mbps 109 33

Given the above client sensitivity specification, we can calculate the theoretical range of an access point by making the following assumptions:

As the client moves away from the AP, the signal decreases an the wireless client/access point will automatically switch to a lower but more robust link speed. The calculated ranges may seem very optimistic when compared it to real world ranges. The reason is that in real world deployments, obstacles do exist and even a few of them will severely decrease the range. The propagation environment is the factor that has the most impact on the actual range of a wireless LAN. If an access point is located on the second floor and you get down in the basement with a laptop, then you will probably loose the connection. However. If the same access point was mounted on the roof, then you might sustain a connection hundreds of meters away.