To set against these problems are two new technologies. One is the new mixed analogue-digital telephone channel. The other is the increasing availibility of powerful Digital Signal Processors.
A digital link between the ISP and the telephone network is the fundamental requirement of the 56 kbit/s modem. Without it, none of this would be possible - analogue modems would still be limited to 33.6 kbit/s. The basis of this power is the close coupling between the ISP's modem and the digital network.
The fundamental data format of the network is a 64 kbit/s stream, representing 8,000 samples/s with 8 bits/sample. The digital modem is able to generate this 64 kbit/s stream, and transmit it through the exchanges without conversion or loss. A near-perfect channel is formed, stretching from the ISP, through the network, to the last exchange switch at the user's end. This switch is an important one, as it is responsible for converting the digital signal back into an analogue form, and transmitting it to the user's modem. It does this by reading data from its 64 kbit/s input stream 8 bits at a time, and sending them to a Digital to Analogue Converter. The 8 bit number is converted into an analogue signal on the telephone line. The switch repeats this process 8,000 times a second, sending an analogue waveform to the user. Note that this input data stream is the same signal that is generated by the ISP's modem. It is, therefore, able to control the shape of the analogue signal transmitted to the user's modem. It is as if the user and ISP are separated by a single length of analogue line, and not a network of lines and exchanges. This precise control over the analogue signals permits the use of modulation schemes which can overcome some of the line's limitations.
Another capability, not related to the network, is the power of Digital Signal Processors. These dedicated microprocessors are ideally suited to the demands of modern communications, running algorithms for handling synchronisation, error correction and equalisation. The trend is for ever greater processing speed at ever lower cost, allowing more and more complex algorithms to be used in low-cost modems.
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