Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.


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January 1, 2006

 

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
 
RFID (Radio Frequency Identification) Tagging and 802.11
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
 
Measuring the Ability to Receive a Signal: RF Signal Strength Metrics
To Infinity... and Beyond!
News from the wireless marketplace...
 
Mobile WiMAX Standard Approved
Sewn into the seam of a new pair of jeans is the embedded RFID tag. Tagging of individual items of merchandise is limited only by the cost of the tag itself and per-tag pricing is moving below the sub-1-cent level. The RFID tag will ultimately rival UPC bar coding for inventory control and point-of-sale management.
Shown in closeup, held up against the light, the RFID tag itself is visible in the cloth pocket. The metal spiral is the transponder antenna.
  • Talking Prescriptions: Tags are placed in or on prescription bottles for visually impaired people. The tag contains the information that would normally go on the bottle's label, such as dosage, frequency, and so forth. A reader gets the information from the tag and then speaks the information.
  • Pets can be implanted with small RFID tags under their skin, typically on the back of the neck. If the pet is lost, animal control can read the address and phone number information from the tag.
  • Access control badges often have RFID tags in them. These tags need only be placed near the sensor instead of having to be swiped or touched to the sensor.
  • Researchers are using RFID technology to read information from seismic sensors. The sensors themselves, of course, are not RFID technology. But the sensors are attached to an RFID tag (probably active) that announces the sensor's recent data when it is interrogated.


Technology and Engineering

This month and next month, we examine three metrics of the receivability of an RF signal. Two are commonly-known: Signal Strength and Signal-to-Noise Ratio (SNR). One is less-well-known: Carrier-to-Interference Ratio (CIR). We'll discuss how these metrics interrelate to determine whether an 802.11 device can receive a transmitted signal or not. This month, we discuss signal strength.

Signal strength is the easiest of these metrics to understand. It is the raw amount of energy that is received within the frequencies on which the receiver is listening. In open space, with no obstructions between the transmitter and receiver, signal strength decreases as distance increases according to the inverse-square law, which states that a doubling of distance between the transmitter and receiver results in one-quarter the signal strength being received. By the same law, halving the distance results in four times the signal strength being received. When the line of sight between the transmitting and receiving device is obstructed (or, more correctly, when the first Fresnel Zone is obstructed), the obstructions absorb the energy, causing less of it to reach the receiver.

If signal strength were the only measure of signal receivability, then calculating signal range would be relatively simple. We would simply need to confirm that the signal strength seen by the receiver is above the receiver's receive sensitivity. To do this, we would first subtract the transmitter's transmit power from the receiver's receive sensitivity to get the maximum amount of power that can be lost before the signal is un-receivable. Then, subtract out losses for any obstructions. Finally, calculate the maximum distance for the remaining amount of power to get the maximum range. The signal strength that the receiver sees must be greater than its receive sensitivity. If this criterion is not met, the receiver won't be able to receive the signal, and so the first thing to do when 802.11 devices have connectivity problems is to measure the signal strength of the access point at the client device's location to see if the raw signal strength is high enough.

NetStumbler is a freely downloadable program available from www.NetStumbler.com

A tool like NetStumbler (shown to the left) has an advantage when performing this measurement because it can show the signal strength of individual access points. At the same time, a tool like NetStumbler has a disadvantage in that it is using the same type of 802.11 card that the clients are using. If the signal strength is so weak that the clients can't see the signal, then probably neither will NetStumbler! However, if there is interference in the area, it can occur that there is sufficient signal strength, but the packets corrupted by the interference and are not received. In these cases, NetStumbler will simply not see the access point, which might lead you to the incorrect conclusion that signal strength is insufficient.

A spectrum analyzer (shown to the right) is another way of measuring signal strength. This tool measures RF energy, regardless of whether it is 802.11 or some other type of radio. Because they don't need to demodulate packets and so forth, spectrum analyzers can show the amount of RF energy in the air (the "signal strength"), even when there is interference that would prevent a tool like NetStumbler from seeing the packets. Spectrum analyzers are also usually more precise and accurate than 802.11-card-based tools like NetStumbler. On the down-side, they are relatively expensive (thousands of dollars to tens of thousands of dollars) and take some training to interpret correctly. Finally, a spectrum analyzer can show what signal strength exists on what frequencies, but it can't differentiate between two access points on the same frequency like NetStumbler can.
Cognio IMS, a PC-based, full-featured spectrum analyzer,
can be purchased from Connect802

Measuring signal strength is the first step in determining why a wireless client is having connectivity issues. The signal strength at the client's location must be above the client's receive sensitivity. At Connect802, we prefer to design networks with a fade margin of 10 to 20 dB to account for variations in the environment. If this guideline is used, then signal strength should be higher than the receive sensitivity plus the fade margin. For example, a client with a -85 dBm receive sensitivity with a 10 dB fade margin would need a signal of at least -75 dBm for signal strength to be considered adequate.

Signal strength is only the first metric of signal receivability. Conditions can exist where signal strength is sufficient, but packets are still not received. These conditions relate to the other two metrics of signal receivaiblity, signal-to-noise ratio and carrier-to-interference ratio. We will discuss these two metrics next month.


To Infinity... and Beyond!

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