Signal-to-Noise Ratio
If noise at the radio card is high, the radio card will have difficulty recovering the signal, which results in bits errors and retransmissions. An important signal measurement is the signal-to-noise ratio (SNR). The SNR (in dB) at a particular point in the network is simply the signal power (in dBm) minus the noise power (in dBm). A signal power of –65 dBm and noise power of –90 dBm yields an SNR of 25 dB. The noise power is anything other than signals corresponding to the access point or radio card. Table 22-5 includes several SNR values and the resulting performance of an 802.11b network while using a Windows XP laptop browsing websites and downloading files. The results in Table 22-5 depict general end-user performance. The use of SNR to define the range boundary of an access point radio cell is more effective than either data rates or signal amplitudes. The higher degrees of noise cause more errors in frames and corresponding retransmissions. Note that Windows XP shows an indication of the signal strength in a series of bars on a graph, showing no bars as poor quality and five bars as the best quality.
Table 22-5 Correlation of SNR Values to Wireless LAN Performance
Table 22-5 Correlation of SNR Values to Wireless LAN Performance
