Modem Standards-Cont

™   56K Modems o    If one side is an ISP and the signal does not have to pass through a PCM converter , quantization is eliminated in one direction and data rate can be increased to 56 Kbps

Uploading o    Transmission  of  data  from  the  subscriber  to  the  ISP(UPLOADING)  follows the following steps: o    Digital data is modulated by Modem at site A o    Analog data is sent from the modem to the switching station at site A on the local loop o    At the switching station, data is converted to digital signal using PCM o    Digital data travel through the digital network of the telephone network of the telephone company and is received by the ISP computer o    The limiting factor is again step 3 o    However  user  does  not  need  high  data  rate  since  in  this  direction  only  small blocks of data is sent    DOWNLOADING o    Tx of data from ISP to the modem at site A follows these steps: o    Digital  data  is  sent  by  the  computer  of  ISP  through  the  digital  telephone network o    At the switching station, digital data is converted to analog using inverse PCM o    Analog data is sent from the switching station at site A to the modem on the local loop o    Analog data is demodulated by modem at site A o    Note that in this direction there is no quantization of data using PCM. o    The limitation when uploading is not an issue an here a o    Data can be sent at 56 Kbps o    This  is  what  user  is  looking  for  since  larger  files  are  typically  downloaded from internet ŠRESULT The maximum data rate in the uploading direction is still 33.6 Kbps but the data rate in downloading direction is now 56 Kbps    Why 56Kbps? -Switching   stations   of   the   telephone   company   use   PCM/Inverse   PCM   for digitizing voice -8000 samples/sec * 7 bits/sample = 56 Kbps