access point ,wireless client data rate

Unanswered Question
Apr 11th, 2008

hi every body!

I was reading cisco press book ccnp bcmsn by David Hucaby ccie 4594.I will quote from the book, chapter#17 pg no 462 3rd pargraph

" one interesting side effect of the different data rate is that higher data are only possible at shorter distance form Aps. The higher data rate use a more complex modulation than the lower data rates,requring better signal quality that is only available closer to the AP."

Based on above excerpt , Can i say Ap use different modulation let say with two clients A and client B, given that A has 11Mbps while B has 5.5 Mbps?

What determines data rate signal quality or distance? for example , we have an Ap and two wireless clients A and B.

A and B are at equidistant from Ap. Between B and ap , there is fish aquarium which absorbs some signal. The end result is client A has better signal quality than client B as fish aquarium absorbs some signal, though both clients are at equal distance from Ap.

If distance between Ap and Client determines the date rate, then it should be same at both clients right?

If the signal quality determines the data rate , then client A should have better data rate than client B does right?

simpy put is it distance or signal quality that determines data rate?

thanks ! sorrry i know it is kinda long winded.


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Joseph W. Doherty Sat, 04/12/2008 - 03:55

It's really the overall quality of the signal that should determine the transmission rate. Both distance and in-between materials attenuate the signal decreasing the signal to noise ratio. There are other factors that can also impair the signal, such as multipath and other radio interference.


PS:

BTW, wireless devices should step down their transmission rate automatically as necessary, but I've seen the situation where I've had to manually step down the transmission rate of a wireless client that had a very weak signal with the associated AP to improve its effective transfer rate.

sarahr202 Sat, 04/12/2008 - 13:11

thanks for your reply.I know sources which may interfere with signal. all i want to know when wireless client do lower their transmission rate what they take into account signal quality or distance to access point.

you mentioned in your reply about wireless device being connected to ap despite very poor signal quality and you had to manually step transmission rate. Depending upon vendors, some wireless device will latch on ap until , it can not hear ap.

thanks for you reply .

Istvan_Rabai Sun, 04/13/2008 - 02:29

Hi Sarah,


An addition to Joseph's post:


When the signal level of the transmitter goes down (due to higher distance or other attenuating factors), the RATIO of the noise in the received signal gets higher.

This is what we call the signal quality goes down.


So it gets harder for the receiver to sort out which is the signal from the transmitter (useful signal) and which one is the noise.


This is why the devices should change the modulation, where the signal levels can be seen and differentiated more clearly for the receiver while having the same noise level.


Changing the modulation to have a more clearly visible signal implies lower data rates.


This is the physical background behind the lower data rates.


It is possible to have different data rates within the cell of the same AP.


So Client A can have 11 Mbps and client B can have 5.5 Mbps at the same time.


Data rates can be determined on a per frame basis.


Cheers:

Istvan




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