RSSI
Encyclopedia
In telecommunications, received signal strength indicator (RSSI) is a measurement of the power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 present in a received radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 signal
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....

.

RSSI is a generic radio receiver technology metric, which is usually invisible to the user of the device containing the receiver, but is directly known to users of wireless networking
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...

 of IEEE 802.11 protocol family.

RSSI is often done in the intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...

 (IF) stage before the IF amplifier. In zero-IF systems
Direct-conversion receiver
A direct-conversion receiver , also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design that demodulates the incoming radio signal using synchronous detection driven by a local oscillator whose frequency is identical to, or very close to the carrier frequency of the...

, 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 relative received signal strength
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...

 in a wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...

 environment, in arbitrary units. RSSI is an indication of the power level being received by the antenna. Therefore, the higher the RSSI number (or less negative in some devices), the stronger the signal.

RSSI can be used internally in a wireless networking
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...

 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
IEEE 802.11 RTS/CTS
RTS/CTS is the optional mechanism used by the 802.11 wireless networking protocol to reduce frame collisions introduced by the hidden node problem...

 (CTS). Once the card is clear to send, a packet of information can be sent. The end-user
End-user
Economics and commerce define an end user as the person who uses a product. The end user or consumer may differ from the person who purchases the product...

 will likely observe a RSSI value when measuring the signal strength of a wireless network through the use of a wireless network
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...

 monitoring tool like Wireshark
Wireshark
Wireshark is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software and communications protocol development, and education...

, Kismet or Inssider
Inssider
inSSIDer is a Wi-Fi network scanner for Windows. It received a 2008 Infoworld Bossie Award for "Best of open source software in networking".-History:...

. As an example, Cisco Systems
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...

 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
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...

 chipset is made by Atheros
Atheros
Qualcomm Atheros is a developer of semiconductors for network communications, particularly wireless chipsets. Founded under the name Atheros in 1998 by experts in signal processing from Stanford University, the University of California, Berkeley and the private industry, it became a public company...

. 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 mW or dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...

. 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.

RCPI (Received Channel Power Indicator) is an 802.11  measure of the received RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

 in a selected channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...

 over the preamble and the entire received frame.

RCPI is exclusively associated with 802.11 and as such has some accuracy and resolution enforced on it through 802.11k. A similar, but not identical, metric is the RSSI (received signal strength indication), which also aims to provide some level of classification of the received signal. Both have their respective merits and uses.

Indeed, 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 can't comment on the quality of the link like other metrics such as travel time measurement (ToA
Time of arrival
Time of arrival , also called time of flight , is the travel time of a radio signal from a single transmitter to a remote single receiver. By the relation between light speed in vacuum and the carrier frequency of a signal the time is a measure for the distance between transmitter and receiver...

).
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