Note:
Signal versus Link Quality
Link quality
- Fundamentally, the best way to measure link quality is to derive the Signal to Noise Ratio (SNR) of
the desired received signal. For that to happen, a demodulator in the receive chain needs to provide information
about the confidence of the detected symbol - also known as soft detection. However, with modern CMOS
transceivers used in WiFi type products, soft detection is not provided. Instead, in a typical application, there are two
measures of the Rx signal quality: Received Signal Strength Indication (RSSI) and integrity check of the demodulated
data by either Cyclic Redundancy Check (CRC) or PN code correlation strength.
Signal quality
- The RSSI can be used as a measure of signal quality when there is no interference. But, in the
case where the receiver experiences interference, the RSSI may falsely indicate sufficient signal strength even if the
desired signal is completely jammed by the interfering signal. This is because the RSSI cannot distinguish the desired
signal from the interfering signal. In this situation, a data integrity check can be used to detect if the demodulated
data is corrupted, and then subsequently adjust the transmit power. Using a CRC check as a measure of link quality
implies that a few bit errors occur in the data or voice transmission before the transmit power can be increased. There
is a lot of controversy about the term signal quality used in WLAN networks but the most likely definition of “signal
quality,” or “PN code correlation strength” is that it is some metric of the correlation between the correct symbol-
stream and the actual symbol-stream received. For example, the PHY might count the average number of “wrong” bit
positions over a window of some number of symbols, where zero “wrong” bit positions equals 100% signal quality and
more “wrong” bit positions results in lower signal quality.
Signal strength and RSSI
- In reality, there are four units of measurements used to represent RF Signal Strength
namely mW( milliwatts), dBm (“dB”-milliwatts), RSSI (Received Signal Strength Indicator), and a percentage
measurement. Signal strength defined in the IEEE 802.11 recommendation for WiFi type devices is based on the
Received Signal Strength Indicator (RSSI) and is intended to be used as a ‘relative value’ within the WiFi chipset. The
1-byte value can have values ranging from 0 to 255, but vendors prefer to use arbitrary scales from 0 to RSSI_Max
but in fact no vendor actually measures the 256 different signal level values so each adopts and uses their own
specific maximum RSSI value (RSSI_Max). For example on the WiFi adaptor we use, Linksys (Cisco) chooses to
measure 101 separate values for RF energy so their RSSI_Max is 100. Note that the RSSI value is not associated
with any particular power scale (e.g. mW) and it is also not required to be of any particular accuracy. The RSSI value
is used internally by the microcode in the adapter and by the device driver and this is why vendors are not forced to
use a compatible standard. As a result, the signal strength numbers reported by an 802.11 device or adaptor will not
be consistent between two vendors, and should not be assumed to be particularly accurate or precise.
Note: WiFi Ping Test
The signal level might change depending on the distance between the Access Point and the test set causing
the WiFi connection to drop. Furthermore, if the link quality is below 60%, the connection might drop too.
If the connection drops the test set will automatically search for the connection and re-connect. Always
ensure the connection is active before performing the Ping test.
MX100/120 e-Manual D07-00-004 Rev A04
Page 85 of 115
