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

[ahmad_abdulghany]

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,

 

[Sal]

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

 

[IanP]

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

Regards,
IanP

 

[cabone]

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

 

[ahmad_abdulghany]

I couldnt see your snapshot

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

 

[amihomo]

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,

 

[ahmad_abdulghany]

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

 

[spweda]

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.