Page 127 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes 6.3.6 Deep-space Telecommunications
Many modern modems have their origin in deep space telecommunications systems of the 1960s.

Differences between deep space telecom modems and landline modems:
(a) digital modulation formats that have high doppler immunity are typically used

(b) waveform complexity tends to be low, typically binary phase shift keying
(c) error correction varies mission to mission, but is typically much stronger than most landline
modems.
6.3.7 Voice Modem

Voice modems are regular modems that are capable of recording or playing audio over the
telephone line. They are used for telephony applications. See Voice modem command set for more
details on voice modems. This type of modem can be used as an FXO card for Private branch
exchange systems (compare V.92).

6.4 Data Communication using Digital Data Connections

A communications system may be digital either by the nature of the information (also known as
data) which is passed or in the nature of the signals which are transmitted. If either of these is
digital then for our purposes it is considered to be a digital communications system. There are
four possible combinations of data and signal types:

1. Analog data, analog signal;
2. Digital data, analog signal;

3. Analog data, digital signal;
4. Digital data, digital signal.

6.4.1 Digital Data with Analog Signals

This method is used to send computer information over transmission channels that require analog
signals, like a fiber optic networks, computer modems, cellular phone networks, and satellite
systems. In each of this systems, an electromagnetic carrier wave is used to carry the information
over great distances and connect digital information users at remote locations. The digital data is
used to modulate one or more of the parameters of the carrier wave, This basic process is given
the name “shift-keying” to differentiate it from the purely analog systems like AM and FM. As
with analog modulation, there are three parameters of the carrier wave to vary and therefore
three basic types of shift keying:

1. Amplitude Shift Keying (ASK)
2. Frequency Shift Keying (FSK), and

3. Phase Shift Keying (PSK).
6.4.1.1 ASK

In amplitude shift keying, the carrier wave amplitude is changed between discrete levels (usually
two) in accordance with the digital data.

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