Why download and upload speed is different in 56k modems?
56kbps (PCM) became possible once most dial-up ISPs were using digital services (e.g., T1 lines) for their incoming telephone connections. This meant that the entire path between a network server and your computer was digital, EXCEPT for the local loop between your modem and your central office.

That local loop has codecs (A/D and D/A converters) at each end, one in your central office, and one in your modem. The difference in data throughput is based on the relative performance of those codecs.

The phone company uses standard 8-bit &micro;-law codecs for their local loops. However, your high-speed modem has the flexibility to use a much higher-resolution (12-bit or more) and higher-speed (oversampling) ADC. With enough "training", the DSP in the modem can actually figure out exactly which PCM code the ISP sent for each audio sample, which gives you 7 bits1 &times; 8000 samples/second = 56 kbps downlink speed. The ADC in the central office can't do the same thing for the uplink direction, so you're limited to the 33.6 kbps of a purely analog connection in that direction.



1 You don't get 8 bits per sample for a number of complex reasons, including things like robbed-bit signaling and FCC spectral power limits.