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A large exchange may have a hundred thousand customer lines (directly connected and/or via Remote Access Units), and several thousand 64 kb/s links to other exchanges - for instance via fiber to three exchanges which may be in the same building, or which could be a thousand kilometers distant. Since each customer must be able to call any other customer on that exchange, and since each customer's line might have to be connected to any one of the several thousand inter-exchange 'lines', the exchange has a formidable task in creating thousands of such any-to-any connections all at the same time.
Similarly the 'pipe' from the other end of the call is a 64 kb/s datastream which must be converted back into 8 bit accurate voltages 8,000 times a second with a Digital to Analog Converter. That signal is driven onto the twisted pair to the customer's phone in a way which makes it relatively easy to ignore that transmitted audio signal when the same 'line card' (the electronic circuit board which drives four or eight such twisted pair lines) simultaneously receives an audio signal from the same pair.
Twisted pair copper telephone wires
Ideally, each telephone would have two pairs going to it. One would carry audio to the ear-piece and the other would carry signals from the microphone back to the exchange. With such an arrangement, there would be no interference between the two signals. However, for reasons of economy and reliability, both signals are conveyed by the same pair of copper wires.
There are fundamental limitations, at the exchange and at the phone, which make it impossible to keep these signals perfectly isolated. Therefore, whenever the exchange sends an audio signal to the phone (with the intention of it being converted to sound by the earpiece) some of that signal will be reflected by the telephone electronics and so picked up by the exchange as if it came from the telephone's microphone.
This happens at both ends of any call involving analog telephones which are connected to the exchange by a twisted pair - but not analogue or digital mobile phones, which have separate paths for the send and receive signals.
There are a variety of other analog problems which are in practice difficult or impossible to solve for a wide range of line lengths, such as 'sidetone'. This is the degree to which the users' voice appears in their own earpiece. Too loud and they will talk too quietly. Too little and they will think the phone is disconnected. A great deal of work on standardizing the characteristics of lines and telephones is required to resolve these issues. Unfortunately different countries have decided on different electrical characteristics for their lines and phones, raising difficulties with the free trade and movement of phones across national borders. In addition, some countries - notably the USA - require very little standardization of telephone handsets, so there is wide variation between the transmitted signal of handsets, and similar variation in the sensitivity of their earpieces.
Echoes and echo cancellation
These difficulties are symptomatic of the long and varied history of telephone technology. The result can be serious mismatches in volume at each end of the call. In virtually all circumstances, there will be some echo: each phone reflects some of the signal sent to it back to the exchange. This echo is a particularly serious problem in international telephony, where the time-delays would cause speech to literally bounce between phones on opposite sides of the Earth. The only reliable approach is to quench such echoes on long distance calls by using an 'echo-canceller'. At each exchange, these circuits stop their customer's voice from being sent to the other end if the other end is sending a signal. This makes for awkward and disjointed conversation, but it is better than every word echoing back and forth for several seconds.
An analog telephone service is designed to convey audio frequencies between about 300 Hz and 3,600 Hz, which is fine for intelligible speech. The exact frequency response, and the signal to noise level (SNR), depends very much on the length and quality of the twisted pair line which drives each phone.
Faxes and modems communicate by converting their digital datastreams to and from complex audio tones which are designed to overcome the echo, noise and frequency response limitations of both twisted pair lines and the digital-to-analogue conversion stages at both exchanges at each end of the call.
Most echo-cancellers recognize the tone at the start of fax and modem calls and so do not interrupt the transmission of audio in either direction on such calls.
Generally the calls are switched within a carrier's network, and to nearby carriers, at the full 64 kb/s rate. Therefore there is no difference in sound quality between a call made from one house to the house next-door, and between a house in Melbourne and a house in Darwin, 3000 kilometers to the north. In both cases the same analogue to digital conversions take place, and the resulting numbers are conveyed without any degradation between the exchanges in the two cities.
In fact there will be one significant difference: a time delay caused by the long distances. Assuming that the path is 5000 km, and allowing for the slower speed of light in the silicon-dioxide optical fibers, and ignoring delays in probably three or so transit exchanges en-route, the round-trip delay for a sound starting in Melbourne, echoing from the analogue line and phone in Darwin, and returning is likely to be about a twentieth of a second. Unless prevented with an echo-canceller, such echoes could be distracting.
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