Signalling rate for a 8-bit modem

[anup dhital]

the bandwidth of our telephone line is 4KHz . so for a 8- bit modem the signalling rate should be R=2*n*B=2*8*4=64 kbps
but it is seen that the signalling rate never exceed 56kbps . Why?
and why is the signalling rate not always constant?

 

[Davood Amerion]

signalling rate

anup dhital,

it is because of telephone signal is not digital.
modem must change digital signal to analoge then send it through telephone line, and vice versa.

In the centural office telephone exchange system channel signal rate is 64kbps with 8KHz sampleing rate.