Probably the most suprising thing about the new modems is the degree to which the speed has increased. Following the evolutionary change from 28.8 kbit/s to 33.6 kbit/s, a further increase of another 5 kbit/s would not have raised much comment. However, at 56 kbit/s, these modems rival the 64 kbit/s of an ISDN line. How?
The simple answer is that the telephone system has changed, and these devices are designed to exploit this. Most users will have seen little change in their telephone service, simply because the modernisation has taken place at the exchanges and on the trunk lines - the "backbone" of the network. To explain these important differences requires an understanding of what the network was like, and what it is now.
The old telephone network was entirely analogue, relying on switches, amplifiers and transformers to convey voice from one user to another. To achieve this, the system was highly optimised for the human voice, with the aim of delivering the best service for the largest number of people. To transfer binary data down such lines required conversion to a voice-like signal. The technique used was to generate an audio-frequency sine wave - a pure note, in other words - and modulate it with the data. This method was continuously refined, offering data rates of 300 bit/s, 1200 bit/s, 2400 bit/s - using the bandwidth more and more efficiently, until today's 28.8 kbit/s and 33.6 kbit/s were reached.
In the meantime, changes to the core of the network were taking place. With the growth in computer power, the old mechanical exchanges were gradually replaced with digital "Switches", as these dedicated computers became known. Voice traffic was increasingly carried in digital form, through the network of Switches that make up the backbone of the telephone system. These changes did not reach as far as the user, however: they still had to use an analogue line to reach the local exchange.
So, despite the digital nature of the network's backbone, the telephone users, and their modems, still saw an analogue channel. This was also true when one of the users was a Service Provider - they simply had racks of analogue modems connected to multiple lines.
For many, this is the situation today. Modem users cannot make use of the higher data rates within the network, because of the limitations imposed by the local analogue lines. To exploit the system fully requires an ISDN connection at both ends of the line - a fully digital system. However, in the UK at least, this costs substantial sums of money, with the result that few have adopted it.
For Service Providers, particularly ISPs, the digital telephone connection is a more attractive proposition. Instead of using many analogue lines, they can use a single, high data-rate digital connection to the network. Such "multiplexes" can carry many telephone channels; for example, the European standard E1 line has a data rate of 2.048 Mbit/s, and can carry thirty-two 64 kbit/s channels. The service provider is then able to choose whether to connect each channel to an analogue modem or to an ISDN terminal adaptor.
It is the Service Providers' increasing adoption of such digital links that provides the opportunity for faster data transfer. While there is an analogue connection at either end of the telephone line, the whole channel will appear to be analogue. Make one of those connections digital, however, and there is suddenly an opportunity for employing new techniques. The 28.8 kbit/s and 33.6 kbit/s modems are optimised for the analogue network. The 56 kbit/s modem is optimised for this new, mixed network.
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