VeEX VePal CX350, Руководство пользователя

Страница: 62 / 81

network key is required to complete the connection. The WiFi function supports WEP encryption. The key can either be 10
characters or 26 characters. Note: If the user enters the wrong network key, the test set will still connect to the Access Point;
however, it will not be able to connect to the web or perform the Ping test.
Note:
Select the manual selection to enter the parameters manually if the SSiD did not detect the AP that the user is searching for.
Connection
Once connected, the Connection screen will display the following information on the connection:
SSiD
BSSiD MAC address
Channel number
Encryption type - WEP, WPA, Disabled
Type - Adhoc, Infrastructure
Signal strength is constantly updated so the value varies.
Link Quality is constantly updated so the value varies.
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. However, 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 measurement 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. 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
9
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.
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 adaptor 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.
Ping
- Before performing the Ping test, ensure the WiFi connection is Up and the WiFi icon (at the top of the screen) has turned
green.
Go back to top Go back to TOC
CX350 e-Manual D07-00-037 RevC01 Page 62 of 81