For the last twenty years or so, the humble modem has been the primary means of communication between widely separated computers, once on proprietary bulletin boards and, more recently, the Internet. Although many people complain about how slow current modems are in relation to the rich multimedia nature of the Web, once you delve a bit deeper into how modems work, it's amazing that they can go as fast as they can!
Before going into those details, however, we need to understand how
the underlying infrastructure – ie the public switched telephone
network (PSTN) – works.
Note: What I'm going to say now is based on the British telephone network, but should apply in most other countries as well.
The PSTN uses high speed trunk lines that carry all data (voice and data) in digital form across optic fibres that span the country. However, for the most part, the "last mile" across this vast digital network – the wire from your local exchange to your house – is still an analogue copper line. This is known as the local loop. As I will discuss in my coverage of ISDN and DSL, it is not this copper wire that limits the speed of transfer, but the equipment at the end of the line at the local exchange.
The local loop is still analogue. What this means is that when you
make a telephone call, your voice is converted into a electrical
signal of varying strength which then gets sent down the local loop to
your exchange. Here, the analogue electrical signal is converted to a
digital signal that is then sent along the trunk lines until it
reaches the local exchange of the recipient, where it is turned back
into an analogue electrical signal and sent along the local loop to
the destination. Although this situation seems silly, since you are
performing multiple transformations, which can only degrade the signal
quality and increase complexity, the cost of upgrading the equipment
at all the local exchanges is prohibitive, so is likely to be slow to
change, although technologies like DSL are changing this.
This already silly situation then becomes ludicrous when you introduce data communications into the equation. Here, you start with a digital signal, which has to be converted into an analogue signal only to be transformed back to a digital signal again further down the chain.
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