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Hi,

Need some help on this one please.

What variables might affect throughput in an 802.11 WLAN? (Choose all that apply.)

A: Absorption

B: Impedence mismatch

C: Interference

D: All of the above

Answer given: A C

Explanation: All types of interference can cause frames to become corrupted. If frames are corrupted, they will need to be retransmitted and throughput will be affected. Absorption is a propagation behavior that affects range because of attenuation. A weaker signal caused by absorption could cause radios to shift to a lower data rate and therefore affect throughput. An impedance mismatch causes reflected voltage in an RF system.

I had it as "D" because by my understanding I would have thought that an VSWR caused by an impedance mismatch could result in a decreased signal strength, which like the absorption rationale would cause a shift to a lower date rate.

Help please!

Sparky

I would have to agree. Any impedance mismatch causes a weaker signal, due to the signal being backed up into the transmitter.

My guess is this is one of those questions that has wrong answers?

I would also agree.

Thank chaps.

Yes, this question is not worded the best. While one could argue that the impedence mismatch does not occur on the "802.11 WLAN" from a medium perspective, the reality is that the WLAN signal leaves the radio and traverses to the antenna as a WLAN signal as yet to be radiated onto the RF waveform.

Is this a question from our pool or from the book? I am not seeing it in our pools.

Tom

It's from the electronic practice test that accompany the CWNA study guide (3rd ed).

Sparky

Thank you, we will notify the authors. We do not create or publish that study guide at this time, but we can inform them of the needed correction.

Tom

So... another question is why would a radio shifting to a lower data rate help an attenuated signal to, I presume, not require as many re-transmissions? 

I presume the data rate can change while signal frequency parameters remain the same as before, so how would this help if the wave paths are relatively the same, and attenuation accordingly unchanged? I suppose this is more of a Shannon, than a Nyquist discussion.

So... another question is why would a radio shifting to a lower data rate help an attenuated signal to, I presume, not require as many re-transmissions? 

I presume the data rate can change while signal frequency parameters remain the same as before, so how would this help if the wave paths are relatively the same, and attenuation accordingly unchanged? I suppose this is more of a Shannon, than a Nyquist discussion.

Switching to a lower rate, changes coding and/or modulation of the data.   Lower rates have lower SNR requirements and are therefore less sensitive to interference.

For example, 802.11n can require 6 dB better SNR than 802.11a/g.   You don't get something for nothing.