Received signal strength indication and IEEE 802.11k-2008

In telecommunications, received signal strength indicator (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 the 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 relative received signal strength in a wireless 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 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 a 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. 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.

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RSSI - Receive Signal Strength Indicator

By: Srividya Iyer
Meaning of RSSI - "Receive Signal Strength Indicator", is a circuit to measure the strength of an incoming signal. The basic circuit is designed to pick RF signals and generate an output equivalent to the signal strength. The ability of the receiver to pick the weakest of signals is referred to as receiver sensitivity. Higher the receiver sensitivity, the better. There are circuits which measure the signal strength based on the output voltage. If the signal strength is good the output voltage is higher and the output voltage is poor if the signal strength is low. Some indicators are designed to flash one of their lights indicating the signal strength. Depending on the received signal power, an amplifier can be used to boost the signal.
Common devices with inbuilt RSSI are cell phones, wireless network adapters and even remote control. Antennas contain a RSSI Circuit that can help us align the antenna for maximum signal reception. In Windows XP, a software is preinstalled to indicate signal strength and data rate. There are many other tools to measure signal strength. Ranging from software preinstalled on your computer to freewares and sharewares are available for measuring signal strength. Some tools are only capable of basic signal power indication while some can even analyze data and suggest improvements and solutions. RSSI is also used while implementing Bluetooth Solutions.

Other Related Definitions:

“…RSSI is an indication of the power level being received by the antenna. Generally, the higher the RSSI level is the stronger the signal. Overall, a level of 700 or more is considered adequate” [Motorola.com]
 “…a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. This numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Receive Signal Strength Indicator (RSSI)" [IEEE 802.11 standard ]
 “…Received Signal Strength Indicator (or Indication): A signal or circuit that indicates the strength of the incoming (received) signal in a receiver. (The signal strength indicator on a cell phone display is a common example). RSSI is often done in the 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. " [Dallas Semiconductor ]
“…RSSI eliminates the need for additional hardware in small wireless devices, and exhibits favorable properties with respect to power consumption size and cost. Because of its attractiveness the research community has extensively considered the use of radio signal strength." [Yale University] 
“… Use the Rx Signal Strength indicator as a visual medium to know if you are nearer to the device. Stronger signal strength will indicate you are nearer to the device. " [Cognio Corporation] 
“…To implement a power control link the remote device must implement a Receive Signal Strength Indicator (RSSI). For a transceiver wishing to take part in a power controlled link it must be able to measure its own receiver signal strength and determine if the transmitter on the other side of the link should increase or decrease its output power level. The RSSI makes this possible." [Antenna Systems & Technology]

 802.11k makes WLANs measure up

The IEEE 802.11 standard for wireless LANs  enables interoperability between different vendors' access points and switches, but it does not let WLAN systems assess a client's radio frequency resources. Consequently, this limits administrators' ability to efficiently manage their networks.

As a proposed standard for radio resource measurement, 802.11k aims to provide key client feedback to WLAN access points and switches. The proposed standard defines a series of measurement requests and reports that detail Layer 1 and Layer 2 client statistics. In most cases, access points or WLAN switches ask clients to report data, but in some cases clients might request data from access points.
Work began on the standard in late 2002, and the IEEE expects to ratify it next year. Because 802.11k is designed to be implemented in software, existing WLAN equipment can be upgraded to support it. For the standard to be effective, both clients (WLAN cards and adapters) and infrastructure (access points and WLAN switches) will need to support it.
Here are some of the measurements 802.11k defines:
Roaming decisions.
RF channel knowledge.
Hidden nodes.
Client statistics.
Transmit Power Control (TCP)
To improve roaming decisions, access points or WLAN switches can provide a site report to clients. The standard defines a beacon request, in which an access point asks a client to go to a specific channel and report all the access point beacons it hears. The access point collects the data, and it or a WLAN switch will analyze the beacon information, looking at details such as what services and encryption types each access point supports and how strongly the client heard the access point. Then the switch or access point generates an ordered list of access points, from best to worst service, called the site report.
Currently, access points and clients cannot share channel information. With 802.11k, an access point could have a client build a "noise histogram," which will display all non-802.11 energy on that channel. An access point also can request data about channel load or how long the channel was used during a given time. An access point or WLAN switch then will know if there's too much interference or traffic on a channel to use it for WLAN services.
Hidden nodes are clients or access points that other clients or access points cannot hear. In 802.11, nodes listen to the airwaves before transmitting to avoid collisions. When a hidden node is present, multiple nodes can transmit simultaneously, creating interference that degrades WLAN performance. With 802.11k, clients track hidden nodes and access points query clients for those lists. This information tells access points about clients on the edge of their cells. Access points can use the information to direct clients to access points from which they would get better service.
Client statistics are limited today to statistics that access points or WLAN switches maintain. Today's WLANs track items such as retries, packets transmitted and packets received. With 802.11k, access points and WLAN switches can query all clients to get reports on their statistics. With both data sets, a WLAN system will have a more complete view of network performance.
TPC was defined in 802.11h to meet regulatory requirements in the 5-GHz band in Europe. With 802.11k, it is extending the use of TPC procedures in other regulatory domains and frequency bands to reduce interference and power consumption, and provide range control.
Once ratified, 802.11k would help IT build a better WLAN. IT will benefit from improved control of the airwaves, and users will get consistent network access, even when roaming; fewer disruptions to applications; and faster service as a result of improved WLAN utilization.
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IEEE 802.11k-2008

From Wikipedia, the free encyclopedia

  (Redirected from IEEE 802.11k)

IEEE 802.11k-2008 is an amendment to IEEE 802.11-2007 standard for radio resource management. It defines and exposes radio and network information to facilitate the management and maintenance of a mobile Wireless LAN.

Contents 1 Radio Resource Management 2 Protocol operation 3 See also 4 External references 5 References

Radio Resource Management

IEEE 802.11k and 802.11r are the key industry standards now in development that will enable seamless Basic Service Set (BSS) transitions in the WLAN environment. The 802.11k standard provides information to discover the best available access point.
802.11k is intended to improve the way traffic is distributed within a network. In a wireless LAN, each device normally connects to the access point (AP) that provides the strongest signal. Depending on the number and geographic locations of the subscribers, this arrangement can sometimes lead to excessive demand on one AP and underutilization of others, resulting in degradation of overall network performance. In a network conforming to 802.11k, if the AP having the strongest signal is loaded to its full capacity, a wireless device is connected to one of the underutilized APs. Even though the signal may be weaker, the overall throughput is greater because more efficient use is made of the network resources.^[1]

Protocol operation

The following steps are performed before switching to a new access point.^[1]

1. Access Point determines that client is moving away from it.
2. Informs client to prepare to switch to a new access point.
3. Client requests list of nearby access points
4. Access point gives site report
5. Client moves to best access point based on report

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Related Links:

A Study on Dynamic Load Balance for IEEE 802.11b Wireless LAN
An Empirical Analysis of Radio Signal Strength
Boeing tracks parts with Wireless LAN
Wireless Intrusion Detection Systems-Research Paper
Ericsson -Bluetooth with RSSI

Technical Resources:

Freewares
Wavemon (Linux)
WLANExpert(Win32)
XNetworkStrength(Linux)

Products and Solutions:

3Com
Apple
Cisco Aironet
Intel Wireless LAN Products
Aeroflex Solutions
Agilent Technologies
CDS Technologies
D-Link
LinkSys
Nokia

Blogs, News, Feeds, Discussion Lists:

Blogs
Sputnik
Ambient Informatics
EVDO Info RSSI
Forta Wireless
RSS Feeds
RF Design News
Wireless Articles
Radio and Antennas

Discussions
Apple-Airport

Books About:

3G Wireless with 802.16 and 802.11 (McGraw-Hill Professional Engineering) by Clint Smith and John Meyer
Optimizing Wireless/RF Circuits by Lenk, John D
Production Testing of Rf and System-On-A-Chip Devices for Wireless Communications (Artech House Microwave Library) by Keith B. Schaub and Joe Kelly
RF System Design of Transceivers for Wireless Communications by Qizheng Gu

See Also:

Other RSSI Related Resources

Thanks to :
http://en.wikipedia.org/wiki/Received_signal_strength_indication
http://en.wikipedia.org/wiki/IEEE_802.11k
http://www.networkworld.com/news/tech/2004/0329techupdate.html?page=1

http://www.birds-eye.net/definition/r/rssi-receive_signal_strength_indicator.shtml