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For basic telephony, a pair-gain system is nearly indistinguishable from a direct copper pair to the exchange. However it may not support full-speed modem access at 33.6 kb/s or above. A pair-gain system cannot support ADSL, Basic Rate ISDN or any other high-speed access technique. Pair-gain systems are widely used in areas where increased demand and multiple dwelling units have lead to full utilization of cables that were laid several decades previously. They are also used in outer suburban and rural areas. Most customers whose service relies on a pair-gain system would be unaware that they did not have a direct pair to the exchange or remote access unit.
The fact that a significant percentage of phone services do not involve a dedicated copper pair to the exchange or Remote Access Unit needs to be borne in mind when considering the deployment of technologies such as ADSL which rely on an uninterrupted copper pair. It also needs to be considered in the light of modems being potentially unable to run at speeds higher than 14 or 24 kb/s on services provided via pair-gain systems.
Analog Telephones, Modems, and Fax Machines
The electronics of the phone is powered by the 50 volt DC 'battery' voltage on the twisted pair line, and the pair simultaneously carries analogue (smoothly changing) audio signals both to and from the phone. Ideally two pairs would be used - one to carry audio in each direction - but that would almost double the cost of the cabling system.
An analogue telephone is functionally a simple device, but it must be carefully engineered to ensure reliability, safety and adequate sound quality in a wide range of operating conditions. For this reason, in most countries it is illegal to connect a device to the telephone network that does not meet stringent technical standards.
The microphone drives an audio signal onto the pair towards the exchange, and the earpiece is driven by the audio signal that is received from the exchange. A pulse-dialing system - either with a rotary dial or an electronic keypad - is a simple means of briefly connecting one wire of the pair to the other, between one and ten times in quick succession depending on the number dialed. Tone-dialing involves electronic circuits which send two tones at the same time - with four possible frequencies for each tone providing sixteen combinations.
A modem (Modulator Demodulator) is a device which can transmit and receive digital information (ones and zeros) as a series of audio tones. For instance a 300 baud (bits per second) modem transmits ones and zeros using two frequencies and simultaneously receives them using another two frequencies. Faster modems use much more complex modulation techniques, but in all cases the frequencies remain within the 300 Hz to 4 kHz (300 to 4,000 cycles per second) range which the telephone system carries. Fax machines contain a modem which can transmit or receive at a range of data rates depending on line quality - typically between 9,600 and 28,800 bits per second in one direction, and typically at a much slower rate such as 75 b/s in the other.
56 kb/s modems provide up to 33.6 kb/s upstream from the customer's site, and up to 56 kb/s downstream. This high downstream rate of 56 kb/s, or perhaps a rate lower than this but above 33.6 kb/s, will only be achieved if all of the following conditions are met:
- The telephone service is via a direct copper pair to the exchange or Remote Access Unit, ie. the service is not provided with a pair-gain system.
- There is good frequency response, low noise and low attenuation on the twisted pair copper line to the exchange or Remote Access Unit.
- The exchange or Remote Access Unit is a straightforward digital system that converts to and from analogue with the standard 8 kHz 8 bit sampling technology. (This is usually the case on modern telephone systems, but in remote areas alternative technologies such as more restrictive analogue or digital multiplexing and transmission systems may be used.)
- The customer's modem calls a number which is digitally linked (typically with Primary Rate ISDN) to the receiving device which behaves as the modem. In other words, if the other end of the call goes via another stage of digital-to-analogue conversion to an analogue twisted pair telephone line, where it connects to a modem, then the maximum data rate will be 33.6 kb/s in both directions. For 56 kb/s to be achieved, one end or the other must have a digital connection to the telephone network, and be served by a device intended to support 56 kb/s modems. The usual approach at an ISP is to have a Cisco router with one or more interface boards that connect to an E1 or T1 line, each of which carries 30 or 23 phone calls.
Analogue PABXs and certain other telephony applications use twisted pairs in a rather different way to the ordinary POTS (Plain Old Telephone Service) described above - but the cables and audio frequencies are the same. The same kind of twisted pairs cabling can also be used for other purposes, such as carrying signals for security systems and relatively high quality audio over dedicated pairs.
Twisted Pair Copper for ISDN
In principle it is possible to transmit digital data through a twisted pair by simply turning on and off the current. However this is a uni-directional approach and there are a number of technical reasons why it is best to achieve all communications with tone-like signals of 300 Hz and above.
The term Integrated Services Digital Network (ISDN) refers to an over-arching scheme to make the telephone network digital from end-to-end - to each customer's site, rather than being digital within the exchanges and relying on analogue audio signals on the twisted pairs to homes and offices.
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