In an IEEE 802.11 system, RSSI is the relative received
signal strength in a
wireless environment, in arbitrary units. RSSI is an indication of the power level being received by the receiving radio after the antenna and possible cable loss. Therefore, the greater the RSSI value, the stronger the signal. Thus, when an RSSI value is represented in a negative form (e.g. −100), the closer the value is to 0, the stronger the received signal has been. 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,
Kismet or
Inssider. As an example,
Cisco Systems cards have an RSSI maximum 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 standardized 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
milliwatts or
decibels referenced to one milliwatt (dBm). Vendors and chipset makers provide their own accuracy, granularity, and range for the actual power (measured as milliwatts, which can be expressed in terms of decibels relative to one milliwatt) and their range of RSSI values (from 0 to RSSI maximum, in arbitrary signal units "asu"). One subtlety of the 802.11 RSSI metric comes from how it is sampledRSSI is acquired during only the preamble stage of receiving an 802.11 frame, not over the full frame. More recent work was able to reproduce these results using more advanced techniques. Nevertheless, RSSI does not always provide measurements that are sufficiently accurate to properly determine the location. However, RSSI still represents the most feasible indicator for localization purposes as it is available in almost all wireless nodes and it does not have any additional hardware requirements.
Received channel power indicator For the most part, 802.11 RSSI has been replaced with
received channel power indicator (
RCPI). RCPI is an 802.11 measure of the received
radio frequency power in a selected
channel over the preamble
and the entire received
frame, and has defined absolute levels of accuracy and resolution. RCPI is exclusively associated with
802.11 and as such has some accuracy and resolution enforced on it through
IEEE 802.11k-2008. Received signal power level assessment is a necessary step in establishing a link for communication between wireless nodes. However, a power level metric like RCPI generally cannot comment on the
quality of the link like other metrics such as travel time measurement (
time of arrival). == Uses in indoor localization ==