On WLANs, APs and antennas convert between wired and wireless signals. As shown in Figure 1, wired network data enters an AP through a wired port. The AP converts the data into radio signals and sends the radio signals to its transmit antenna from the transmit (TX) end through a cable. The AP's transmit antenna then transmits the radio signals as high-frequency electromagnetic waves (at 2.4 GHz or 5 GHz). The high-frequency electromagnetic waves are transmitted to a wireless terminal over a certain distance. The wireless terminal receives the high-frequency electromagnetic waves through its receive antenna, and then transmits the waves to its receive (RX) end for processing. Data sent from the TX end of the wireless terminal is processed reversely according to the preceding processes, and is transmitted to the RX end of the AP.
As shown in Figure 2, signals transmitted between the transmit antenna and receive antennas are radio signals. The strength of signals gradually attenuates during wireless transmission. The following describes the basic concepts related to signal strength, including the radio transmit power, equivalent isotropically radiated power (EIRP), received signal strength indicator (RSSI), downlink signal strength, and uplink signal strength.
Figure 2 illustrates these concepts.
1 and 7: Transmit power at the radio TX end, in dBm
2 and 6: Line loss caused by connectors and feeders connected to the antennas, in dB
3 and 5: Antenna gain, in dBi or dBd
4: Path loss and signal attenuation caused by obstacles, that is, signal energy loss between the transmit antenna and receive antenna, in dB
Radio transmit power: 1 signifies the radio transmit power of the AP; 7 signifies the radio transmit power of the wireless terminal. During WLAN planning, ensure that the sum of the transmit power and antenna gain does not exceed the maximum value allowed by the country code.
EIRP: signifies the strength of signals transmitted from an antenna. The EIRP is calculated as follows:
EIRP = 1 - 2 + 3
RSSI: signifies the signal strength at a location in the coverage area of a wireless network. It is the value of the EIRP minus the transmission path loss and signal attenuation caused by obstacles. Weak signal strength means that the RSSI is lower that the edge field strength. As a result, wireless terminals receive weak signals or even fail to receive signals.
Downlink signal strength: signifies the strength of signals received by a wireless terminal from an AP. It is calculated as follows:
Downlink signal strength = 1 - 2 + 3 - 4 + 5 - 6.
Uplink signal strength: signifies the strength of signals received by an AP from a wireless terminal. It is calculated as follows:
Uplink signal strength = 7 - 6 + 5 - 4 +3 - 2.
The RSSI is calculated as follows (regardless of factors such as the interference and line loss):
RSSI = Radio transmit power + TX antenna gain – Path loss – Signal attenuation caused by obstacles + RX antenna gain
After other factors except the path loss are determined, the path loss can be determined. Then, the maximum transmission distance can be calculated according to its relationship with the path loss.
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