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Tks Neil

In many of the WAN analysis tools that I have used [ For T1-lines, Frame Relay etc ], true "sniffing" can be done.

The original term "sniffing" referred to the concept of having tons of air around you. You want to smell the air without causing a disturbance to the thing you are "measuring" i.e. the air, so you just intake a tiny amount of air by "sniffing" it. You perform the "analysis" i.e. determining the air quality, without having altered the item you are measuring.

With bit-error rate testers for example on an E-1 line, you can take the transmit and receive data in from a buffered high impedance test point, or you can actually put the tester "in-line" with the transmit and receive data. Inside the unit, the transmit and receive data streams [ in the in-line case ] pass through shift registers and "paralleled, copied " data is shunted off to the side where auxiliary circuitry processes the streams for display. You can even inject test patterns into the frame alignment signals without disturbing the data.

That is not really true sniffing. It is buffering and copying. This could be done in a WLAN environment, but then there would cost and programming issues. There are no reasons why this could not be done from a technical point of view. I'm sure somewhere there are drivers and/or special adapters that could allow this. I.e. monitoring of frames from the same device you are passing data through.

In the case of a T-1 or E-1 BER tester using cables leading from a high impedance test point, that is truly "sniffing". In other words, a physical sample of the transmit or receive signal [ a voltage present across a fixed value of resistance ] is taken from the main signal. This is such a small value that the main "flow" of the signal being transmitted or received is not affected. E-1 and T-1 signals are allowed to drop in voltage value from the original transmit levels of specification to allow for cable loss etc.

In large radio and satellite systems we use waveguide to pass RF signals through. What if we want to measure the physical RF power before the signal goes out the antenna ?

We can use a directional coupler. With this device, a tiny sample of the transmit or receive signal is ?sniffed? off from the main signal via a separate port [ you can see the monitor port in the following picture ]. We will know from the manufacturer what value of coupling is used at a particular frequency. We then measure the power sample at the monitor port, then add the coupling factor, and bingo, we have the value of main power flowing in the waveguide.

http://www.thetestequipmentstore.com/HP-P752A.JPG

http://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

Dave