This discussion has so far been entirely concerned with the downstream channel, from ISP to user. What of the upstream? With the present systems, this is still limited to the original speed of 33.6 kbit/s. A possible reason for this asymmetry is the requirement for synchronisation.
All the higher speed modem systems transfer data synchronously. Both transmitter and receiver use clocks running at the symbol rate. The transmitter encodes its clock signal with the data, so that it may be recovered by the receiver. The receiver then synchronises its own clock to the recovered timing signal, allowing it to decode the incoming data. With conventional modem modulation schemes, the two modems must synchronise with one another. With 56 kbit/s coding, the situation is more complicated.
The ISP's modem, being connected to a digital line, can easily synchronise to the 64 kHz network clock. For the receiver in the user's modem, the procedure is also fairly straight-forward: it must synchronise to the 8 kHz signal generated by the exchange switch, usually during the training sequence. However, the procedure for the upstream connection is not easy. In order to exploit all the techniques described above, the user's modem would have to accurately synchronise itself with the exchange's clock. Note that, in this case, it is the transmitter which is required to vary its clock; the exchange cannot synchronise with the modem. This is a serious problem, as the user's modem has no means of measuring the delay through the analogue line, and therefore cannot tell how it should adjust its clock. Without synchronisation, none of the 56 kbit/s techniques will work, and so 33.6 kbit/s is the highest speed that can be achieved.
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