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Twisted pair Copper for Analog Telephony
Twisted Pair Cables
The characteristics of the twisted pair cables are significant, since they arguably constitute the greatest asset of an established telecommunications carrier. This network is often referred to as the Customer Access Network (CAN) - but this term may also encompass technologies other than twisted pair copper.
An analogue telephone (also known as a 'POTS' telephone, Plain Old Telephone Service) connects to the exchange or Remote Access Unit (a device which drives the twisted pair lines and is linked by fiber to a distant exchange, but which does not possess significant exchange capabilities on its own) via two copper wires which carry audio frequency signals, a 50 volt DC 'battery' voltage - and a 90 volt AC signal to ring the bell of the phone. Each wire of the pair is typically a single strand of 0.4mm diameter copper, insulated with polyethylene. The two wires are gently twisted together with one turn per 100 mm, and combined with nine other pairs into a ten pair bundle. A typical cable to serve a suburban street has five such bundles, comprising 50 twisted pair circuits.
The 50 pair cable may run for a few hundred meters along a suburban street - typically one cable buried on each side of the street. These cables may run for a few hundred meters at most before reaching a junction box or pit where each pair is connected to a pair in a much larger cable - with potentially thousands of pairs - which may run several kilometers to the exchange. Outside each home is a pit, in which a junction is made to one or more of the pairs in the cable - typically to a two pair cable which leads into the home and to the sockets for the telephone, modem or fax. In some installations the lead-in to the home is via an aerial cable - which has thicker conductors and is not twisted. In rural areas where distances of over about 5 kilometers must be spanned, thicker wires of up to 0.9mm are typically used in the underground cables, since the resistance of that length of thinner wire would cause excessive loss of signal.
Current flows to the home through one wire, through the telephone circuitry and back to the exchange on the other wire of the pair. This constitutes a 'current loop', and over time the term 'loop' or 'local loop' has come to mean everything from this actual pair of wires to the entire local access infrastructure - perhaps even to include those infrastructures which do not use current loops or copper wire at all. (Infuriating terminology is a 'feature' of the telecommunications industry. For instance 'wireless cable', 'video dialtone' and 'local loop' are often used so loosely as to be confusing or meaningless.)
Frequency response
These cables were designed to carry modest voltages and to work at frequencies up to the 4 kHz limit of telephone audio signals. The vagaries of these cables are important to ambitious technologies such as Basic Rate ISDN and xDSL which require them to operate at frequencies several hundred times higher than this - sometimes beyond 1 MHz.
Twisted pair underground cables are perfectly adequate for reliable voice communications, provided they are protected from damage by excavation and ingress of water. To this end, the larger cables are typically pressurized with nitrogen or air. Where cables are spliced, the individual connectors may be loaded with silicone grease to exclude water. Often the entire joint is sealed in a jacket, and the jacket is filled with silicone grease under pressure or with a rubbery resin that can be removed at a later date.
It would be possible to run each telephone from a single wire, with the Earth as a return path, but capacitive coupling between cables would lead to crosstalk and so to loss of privacy and security. Shielding each wire would be more bulky and costly than a pair of unshielded wires. The twisted arrangement of the unshielded pair is the key to the privacy of telephony - because it causes the cross-talk to and from each pair and all its surrounding pairs to average out to zero.
Pair gain systems
Ideally, every analogue telephone would have its own twisted pair copper cable to either the exchange or to a Remote Access Unit, which is linked by fiber to the exchange which may be hundreds of kilometers distant. Ideally, that twisted pair would be no more than about 6 kilometers long, and there would be no 'loading coils' which are sometimes installed at intervals along longer cables to compensate for these cables higher capacitance and resistance leading to excessive attenuation of higher audio frequencies.
With population growth and increased demand for additional lines per home or office for modems and fax machines, telecommunications carriers have found demand exceeding the number of available copper pairs.
One solution is to lay new twisted pair cables - but this is time consuming and expensive.
Another solution is to install a Remote Access Unit in the area, but this involves a small building or a large street cabinet - and it must be linked to the exchange by a pair of fibers and provided with mains and battery back-up power.
The third alternative is to use a 'pair-gain' system to provide two or four telephone services via a single pair to a distant group of customers. At the exchange or Remote Access Unit, the pair is driven by the pair-gain transmitter unit, and a corresponding receiver unit is located in a pit or in the customer's building. The receiver unit may be as small as a paper-back book and will be entirely water-proof. It is powered by a voltage supplied by the twisted pair (for instance 80 volts DC or so) and the transmitter and receiver carry the audio signals and signaling information (regarding ringing and the phones going off-hook) in both directions digitally using complex analogue signals with frequencies up to several hundred kHz.
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