RSSI （Received signal strength indication）
RSSI用来评估信号的相对强度，IEEE 802.11标准规定RSSI取值范围是0-255，值越大信号越强。每个芯片厂商可以定义自己的RSSI最大值，比如CISCO的最大值是100，其RSSI取值范围是0-100。
OpenWrt计算Signal时，将其值减去255。
dbm
贝是衡量一个数和标准数之间的关系：lg（a/b）。对于功率，分贝是1/10贝。对于振幅（位移，速度，加速度，声压），分贝是1/20贝。声音的分贝是声压和2*10^-5pa的比值。通常3分贝是2倍的比值。dbm是功率，m表示milliwatts毫瓦。功率和毫瓦的比值。
对于无线接收信号强度：
| Signal Strength | TL;DR | Required for | |
|---|---|---|---|
| -30 dBm | Amazing | Max achievable signal strength. The client can only be a few feet from the AP to achieve this. Not typical or desirable in the real world. | N/A |
| -67 dBm | Very Good | Minimum signal strength for applications that require very reliable, timely delivery of data packets. | VoIP/VoWiFi, streaming video |
| -70 dBm | Okay | Minimum signal strength for reliable packet delivery. | Email, web |
| -80 dBm | Not Good | Minimum signal strength for basic connectivity. Packet delivery may be unreliable. | N/A |
| -90 dBm | Unusable | Approaching or drowning in the noise floor. Any functionality is highly unlikely. | N/A |
https://en.wikipedia.org/wiki/DBm
https://en.wikipedia.org/wiki/Received_signal_strength_indication
https://www.metageek.com/training/resources/understanding-rssi.html
https://zhuanlan.zhihu.com/p/22821588