jueves, 29 de julio de 2010


Installing a Standard Modem

Many computers have a built-in modem or accommodate an internal modem. An internal card is usually fitted into a special modem slot or into a PCI slot in your machine. If you have a portable computer, you may be able to fit a special modem card inside the machine or you can install a PCMCIA card modem.
  • Before buying a modem you should check that it's suitable for your machine.
  • Any modem used in the United Kingdom must be approved by BABT.
If your computer doesn't have a built-in modem, or you can't fit one internally, you'll have to use an external modem. Modern modems of this type can be connected to a spare USB port on your computer, although a USB hub is necessary if there aren't enough USB outlets available. If you have an older machine without USB ports, the modem should be connected to a serial port, such as the Modem port on a 'classic' Mac OS machine, which has priority over the Printer port.

Modem Connections

A modem is usually fitted with a least one telephone connector, often a 4-way or 6-way modular connector known as an RJ11, both types of which accept a 4-way plug. Irrespective of how many many contacts are fitted, only 2 or 3 wires are normally used for the telephone circuit.
If you live in the USA, you can connect the modem to a spare telephone socket with a standard 4-way cable. However, other countries often use different connectors, so you'll need a special adaptor cable if one doesn't come with the modem. In the United Kingdom, for example, you'll need an adaptor fitted with a UK 6-way modular plug. The rest of the information in this section refers specifically to the UK's connection system, although much of it also applies to other countries.
The diagram below shows how most people connect their modem:-
This isn't ideal, since two or more phones are sitting across the line at all times, which can limit the real speed of a 56 kbit/s modem. For example, if no phones are connected, the modem runs at 48 to 50 kbit/s, but with four connected the speed often falls to 33.6 or even 28.8 kbit/s. In addition, all the extra wiring can effect the modem's performance. Worst of all, if someone picks up one of the phones whilst you're using the modem the link simply collapses and dies.
Fortunately, some modems are fitted with two sockets, allowing the modem to divert the incoming line away from the remaining phones. This is an essential feature if your modem is also used as an answering machine. Ideally, the modem should be connected directly to the incoming line, as shown below:-
Of course, the details can be varied to taste. Some users, for example, prefer to have at least one phone on the incoming line that can be used to check if the modem is working. If your modem doesn't have a socket for line diversion you can resort to some form of manual switch, although this invariably ends up in the wrong position at the wrong time.

Telephone Connections

Most telephones employ 2, 4 or 6-wire connection systems, although only two wires are needed for a basic telephone circuit. In the United Kingdom, one of the two extra wires in a 4-wire system is used as a ringing circuit. This also acts as an anti-tinkle circuit, preventing unwanted ringer noises that can be caused by another instrument on the same circuit being picked up or used for pulse-dialling. In some instances the fourth wire is used for special signalling such as recall.
Most circuits from a telephone exchange are in analogue form. However, company buildings and hotels often have their own private automatic branch exchange (PABX), also known as a PBX. These are often in the form of a digital telephone system, although the outlets appear on standard modular sockets. Worse still, the high current digital PBX signals used on these circuits can be lethal to your modem, although some modems do incorporate protective devices. So be warned:-
  • Check the phone sockets in an unfamiliar building for suitability before plugging in your modem.
In theory, those in the building should know if an outlet is suitable. Either way, if the outlet doesn't work with a standard telephone, it's almost certainly digital. If you're lucky, you'll find a second outlet, marked Data or Dataport, which is designed for a modem. Alternatively, you can plug your modem into a digital line converter and connect this to the digital socket.

Line Quality and Speed

The performance of any analogue telephone line is influenced by the following factors:-

  • Incoming Telephone Circuit
    Conventional circuits use wires from your local exchange, often fitted with loading coils or repeating coils at regular intervals. Although these improve the speech quality, they don't assist high-speed data transfer. If the distance from your local exchange exceeds 6000 metres you could be in trouble, since long cables limit a 56 kbit/s modem to 28.8 kbit/s or less. Modern installations based on cable-assisted television (CATV), commonly known as cable TV, use short runs of cable to a street box, where the data is passed onto optical cables, giving a much better digital path.

  • Number of Telephones and Other Instruments
    As mentioned above, the speed of a modem is influenced by the number of instruments on the line, and instruments with a higher REN (see above) usually have the worst effect. You may also encounter a particular device, usually of the non-approved variety, that causes problems. If in doubt, arrange for you modem to be the first item on the line. If it doesn't have an extra socket for diverting the incoming line it may be worth installing a manual switch box.

  • Quality of Wiring
    The quality of telephone wiring in your property must conform to approved standards. For example, the two 'line' conductors of the circuit must consist of a twisted-pair of wires, so as to prevent electrical interference from other circuits or equipment. If you can hear 'artifacts' on your phone, such as clicks and buzzes, then your modem may suffer, starting at a high speed but slowing down or having to re-establish broken links as it encounters problems.
C.I 18878408

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