modem

Modems

If you are using a dial-up connection mode, you are using a modem. Your computer is a 'digital' machine, that's it knows only binary language (Binary language uses '0' and '1' digits). But telephones aren't digital machines...Telephones can't understand '0' and '1' pulses. In fact telephones are 'analogic' machines. When you speak on the phone, your voice produces 'sound waves' who are transformed into 'electric waves' who 'travel' through phone cables. When these 'electric waves' reach the receiving phone set, they are transformed again into digital pulses, because the receiving computer doesn't understand analogic signals. So the main problem is: how do computers speak on the phone each other? Well, they need a 'translator', that's a modem. Modems are devices who 'MOdulate' and 'DEModulate'. The transmitting modem modulate digital data into electric waves - who can travel through cable phones - and the receiving modem demodulate those electric waves into digital data. There is a rule who limits speed of this kind of modems: the Shannon law. According to that law, modems can't exceed 33.6Kbps speed rate. But some modem's companies have developed 56Kbps modems. Well these aren't simple analogic modems. What's the trick? Easy: if your ISP uses digital lines, but you are using analogic lines, you receive always in an analogic way, but there is just one conversion from digital to analogic. On the contrary, in the classic way, there are 2 conversions: from digital into analogic (DAC) and from analogic to digital (ADC). By means of X2 your ISP send digital data (without conversions) then data are converted from digital into analogic (because your line is analogic). So by means of this kind of modems, you can receive data up to 56Kbps, but you can send data up to 33.6Kbps. In fact the main problem is: during conversions data is lost. For this reason if you avoid one conversion, the data transfer rate increase a lot. There are 2 main technologies, developed by USRobotics and by Rockwell: the 'X2' USRobotics technology and the 'K56 flex' Rockwell technology. These technologies aren't compatible. So if you want to buy let's say an 'X2' modem, your ISP must use the same 'X2' modem. Besides, there are internal modems and external modems. Well, I prefer external modems, because I can see 'lights', I can move them, I can avoid IRQ conflicts problems (as you have to plug the modem into an external serial port like Com2 for example and you haven't to think about possible hardware conflicts). However internal modems are less expensive...Well feel free to buy the kind of modem that you prefer, but buy a 33.6Kbps modem. However you could avoid using a typical modem: you could use an ISDN line, or you could receive data via satellite! Unlike the traditional way, by means of ISDN you can send/receive data in digital manner, without conversions. There are severall kind of ISDN, but generally it consist of 2 lines at 64 Kbps for data transfer. In other words you have 128Kbps available. ISDN is a digital technology, but data 'travel' on your phone line. So you don't need a special line to use ISDN.

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