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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 ...


Most telephone networks these days are fully digital between the exchanges. Voice - and hence the signal between dial up modems - is transmitted at quite a low bit rate. This may be only 64kbps. That's why modems can never match the speed of ADSL. If the connection is modem-to-modem, then the modem creates an analog signal. That goes to the exchange, ...


The bandwith for the transmission is limited to view kHz. This bandwidth was splitted into upstream and downstream. The required math to use even noise to transmit date was applied in later standards. Most of the users of a modem demand more downstream than upstream. Therefore downstream speed is higher.

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