What does variable data rate mean?!?!?!?!?!

Dear all,

Please refer to the attached snap-shot, and notice the highlighted text (4.8kbps to 2,048kbps, 1bps variable rate)

What does that mean?
It's snaped from a catalog of a sattelite modem..

Thanks in advance,
Ahmad,

Hi

I havent seen your figure (it would be better to embed it on your message rather than attach it), but the meaning is simple

kbps = kilo-bits per second
bps = bits per second,

for example 4.8 kbps means that in 1 second your system transmit 4.8 kilo bits.

ir you think this helps you click on "helped me" for my statistics

Sal

This means that you can have any (variable) Data Rate from 4.8kbps to 2048kbps in 1bps steps ..

Regards,
IanP

I couldnt see your snapshot but if it says that this modem supports different datarates i think it means that the data coding or the data modulation is variable according to the used datarate because it's usual to have a variable datarate modem and as it mentioned in the manual so i think he tries to make you know that fact (high datarate=poor signal quality,low datarate=high signal quality)

i hope this is what you are looking for

I couldnt see your snapshot

Click to expand...

Here's it ..
Can you explain what's it inside it please?
Thanks,
Ahmad,

The data rate (C) that is achievable in a communication channel depends on SNR and channel BW.
( the well-known shannon's formula for channel capacity:
C=BWlog2(1+SNR)
).
If channel BW or SNR decreases , the maximum data rate (C) will decrease.
If we choose a data rate more than maximum data rate allowed ,we will have errors.Therefore,Our system has two ways to work error-free :
1)Increase the transmitted power (Increase SNR) , or
2)If we have restrictions in power , we must work in a lower data-rate.

Systems that work with variable data-rate always check the error rate at a given data rate, and if it exceeds an allowed level , they decrease the data-rate to gaurantee the error-free transmission.

it's that simple,

Well, and how will syncronyzation occure between Tx and Rx sides? if data rate is varying according to the SNR?
It's bit confusing..

For variable rate transmission, the data packet usually has a preamble part that indicates the data rate, modulation scheme. This part always transmits at a fixed-rate. Therefore, Rx knows what it is transmitted at the TX side.