Received Signal Strength Indication(RSSI)

In telecommunications, Received Signal Strength Indication (RSSI) is a measurement of the power present in a received radio signal.

RSSI is generic radio receiver technology metric, which is usually invisible to the user of device containing the receiver, but is directly known to users of wireless networking of IEEE 802.11 protocol family.

RSSI is often done in the intermediate frequency (IF) stage before the IF amplifier. In zero-IF systems, it is done in the baseband signal chain, before the baseband amplifier. RSSI output is often a DC analog level. It can also be sampled by an internal ADC and the resulting codes available directly or via peripheral or internal processor bus.

RSSI in 802.11 implementations
In an IEEE 802.11 system RSSI is the received signal strength in a wireless environment, in arbitrary units. RSSI can be used internally in a wireless networking card to determine when the amount of radio energy in the channel is below a certain threshold at which point the network card is clear to send (CTS). Once the card is clear to send, a packet of information can be sent. The end-user will likely observe an RSSI value when measuring the signal strength of a wireless network through the use of a wireless network monitoring tool like Network Stumbler or Inssider (for Windows Vista).

RSSI measurements will vary from 0 to a maximum of 255 depending on the vendor. It consists of a one-byte integer value. A value of 1 will indicate the minimum signal strength detectable by the wireless card, while 0 indicates no signal. The value has a maximum of RSSI_Max. For example, Cisco Systems cards will return an RSSI of 0 to 100. In this case, the RSSI_Max is 100. The Cisco card can report 101 distinct power levels. Another popular Wi-Fi chipset is made by Atheros. An Atheros based card will return an RSSI value of 0 to 127 and a value of 0x80 indicates an invalid number

The subtlety of 802.11 RSSI comes from how it's sampled; RSSI is acquired during the preamble stage of receiving an 802.11 frame. To this extent 802.11 RSSI has (for the most part) been replaced with RCPI; a functional measurement covering the entire received frame with defined absolute levels of accuracy and resolution.

RSSI is stored on TX/RX descriptor and was measured by baseband and PHY for each individual packet.
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