Neon-komputadór

Modems

Whilst no user PCs in the Ministry of Foreign Affairs and Cooperation have modems installed, it is still worth knowing about them, because outside the Ministry, modems are a standard piece of equipment in any contemporary computer system. Most modems are used by individuals to make a dial-up connection to an Internet Service Provider or another computer with a modem installed. Physically, modems either exist like an adapter card, for the internal version or a rather small and plain looking box with indicator lights on the external version. The external version is invariably more expensive and more complex than the internal version. External modems require their own power cable and an RS232 or USB cable to plug into the computer, whereas an internal modem runs of the computer's power supply and obviously needs no connecting cable. External modems are easier to transport between one computer and another of course - and certainly easier to reset is it hangs (simply turn it off and on again). Obviously, monitoring the behaviour of the modem is easier as well with external modems.

A modem (an abbreviation for modulator-demodulator) allows computer to computer connections by connecting to a telephone line and converting the digital signal of computer to the analog signal of the 'phone system to send down a line and from analog to digital when it is received. The rate that a modem sends data is referred to as bits-per-second, the frequency that signals are sent is referred to as the baud rate. A modem that has a baud rate of 2400 baud per second and sends 6 bits per baud has a data transmission rate of 14,400 bits per second. Because it is difficult to send data above 2,400 baud on normal 'phone lines a modem may have be set at speeds below it's optimal data transmission rate. Contemporary modems are available at transmission speeds of 28,000 bps, 33,600bps and 56,000 bps. Nearly all contemporary modems incorporate the technology of facsimile machines (fax-modems), through the standards established through the International Telecommunications Union of the United Nations.

Modems that claim to operate at 56,000bps require some reading of the fine print. Even though the telephone line is capable of carrying data at more than 56kbps, due to requisite conversions the maximum effective speed is about 35kbps. However this is only the case through analog 'phone lines - which is not the case in advanced industrial nations, particularly in the urban regions, where the 'phone system is largely digital. The many users, because their Internet Service Provider has a digital connection to the telephone network will discover that they can download files at 56K, but can only upload them at 33.6K, although a new ITU standard, V.90plus, also allows uploads faster than 33.6kbps.

There are several features that are advertised with modems that should be considered before purchase and installation. Modems with MNP10EC support are particularly useful where 'phone lines are of poor quality, or for those who wish to use their modem through a mobile phone. Make sure that you get MNP10EC, rather than just MNP10. Call-waiting support is a nice feature - it allows you several seconds to answer the 'phone and pass on a message without disconnecting the modem connection. Finally, it should be obvious for those who have USB ports or PCI slots on their computer modems that can connect to these ports is useful.