Received signal strength indication
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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.
[edit] RSSI in 802.11 implementations
In an IEEE 802.11 system RSSI is the relative 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 Wireshark, Wildpacket, Kismet or Inssider.
RSSI measurements are unitless and in the range 0 to 255, expressible as a one-byte unsigned integer. The maximum value, RSSI_Max, is vendor dependent. For example, Cisco Systems cards have a RSSI_Max value of 100 and will report 101 different power levels, where the RSSI value is 0 to 100. Another popular Wi-Fi chipset is made by Atheros. An Atheros based card will return an RSSI value of 0 to 127 (0x7f) with 128 (0x80) indicating an invalid value.
There is no specified relationship of any particular physical parameter to the RSSI reading. The 802.11 standard does not define any relationship between RSSI value and power level in mW or dBm. Vendors provide their own accuracy, granularity, and range for the actual power (measured as mW or dBm) and their range of RSSI values (from 0 to RSSI_Max).
The subtlety of 802.11 RSSI comes from how it is 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 Received Channel Power Indicator. RCPI is a functional measurement covering the entire received frame with defined absolute levels of accuracy and resolution.
RSSI is stored on the TX/RX descriptor and is measured by baseband and PHY for each individual packet.
[edit] See also
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