raneous noise. has benefited from the same advances in microelectronics as In order to provide a constant response to sounds within this range, the rest of the computing industry. This has allowed it to change telephone circuits are 'loaded' with extra inductance to offset the Serial character Serial bit transmission Serial character Encoding (EBCDIC Coding) Decoding first Serialise Deserialise Input Process Modem Modem Process Input Analogue Communications Channel Data Link Figure 39.1 A complete end to end data communication system using a modem link 39/4 Modems cables natural capacitance. Add to this the effect of switching and the variation in route from call to call and it can be readily appreci- ated that it is not an environment conducive to digital transmission, 00000000000000000000 where acceptable bit error rates are typically less than one in a million. The humble dial up modem must deal with all of this, but is limited as a result to a maximum speed of around 30kbit/s for today's dial up line according to Shannon's Law. Contrast this with a digital KiloStream circuit delivering 64kbit/s and digital Mega- Stream circuits delivering 2.048Mbit/s. An analogue signal has an amplitude, a frequency and a phase. Each can be varied to modulate the waveform (Figures 39.2 to 39.4). In practice, simple amplitude modulation is not used on its own because of its sensitivity to noise. Crackles on the line may not impair the ability of a human ear to understand speech, but they certainly impair a computers' ability to understand a digital signal if it is corrupted by them during transmission. Frequency modula- Figure 39.3 Frequency modulation: (a) carrier; (b) tion changes the frequency of the basic carrier wave (unmodulated modulating waveform; (c) modulated waveform signal) according to the state of the digital signal generated by the computing or terminal equipment. modem is ready, it responds with Clear To Send (CTS) and turns on its Data Carrier Detect (DCD). 0000000000000000000 Having raised CTS, transmit data 