DIGITAL DATA,ANALOG SIGNALS
We start with the case of transmitting digital data using analog signals. The most
familiar use of this transformation is for transmitting digital data through the public
telephone network. The telephone network was designed to receive, switch, and
transmit analog signals in the voice-frequency range of about 300 to 3400 Hz. It is
not at present suitable for handling digital signals from the subscriber locations
(although this is beginning to change). Thus digital devices are attached to the
network via a modem (modulator-demodulator), which converts digital data to
analog signals, and vice versa.
For the telephone network, modems are used that produce signals in the
voice-frequency range. The same basic techniques are used for modems that
produce signals at higher frequencies (e.g., microwave). This section introduces
these techniques and provides a brief discussion of the performance characteristics
of the alternative approaches.
We mentioned that modulation involves operation on one or more of the three
characteristics of a carrier signal: amplitude, frequency, and phase. Accordingly, there
are three basic encoding or modulation techniques for transforming digital data
into analog signals, as illustrated in Figure 6.2: amplitude-shift keying (ASK),
frequency-shift keying (FSK), and phase-shift keying (PSK). In all these cases, the
resulting signal occupies a bandwidth centered on the carrier frequency.
All1plitude-Shift Keying
In ASK, the two binary values are represented by two different amplitudes of the
carrier frequency. Commonly, one of the amplitudes is zero; that is, one binary digit is
represented by the presence, at constant amplitude, of the carrier, the other by the
absence of the carrier (Figure 6.2a). The resulting transmitted signal for one bit time is
where the carrier signal is A COS(211fet). ASK is susceptible to sudden gain changes
and is a rather inefficient modulation technique. On voice-grade lines, it is typically
used only up to 1200 bps.
The ASK technique is used to transmit digital data over optical fiber. For
LED (light-emitting diode) transmitters, Equation (6.1) is valid. That is, one signal
element is represented by a light pulse while the other signal element is represented
by the absence of light. Laser transmitters normally have a fixed "bias" current that
causes the device to emit a low light level. This low level represents one signal
element, while a higher-amplitude lightwave represents another signal element.
Frequency-Shift Keying
The most common form of FSK is binary FSK (BFSK), in which the two binary
values are represented by two different frequencies near the carrier frequency.
The resulting transmitted signal for one bit time is:
A signal that is less susceptible to error, is multiple FSK (MFSK), in which
more than two frequencies are used. In this case each signaling element represents
more than one bit.
We start with the case of transmitting digital data using analog signals. The most
familiar use of this transformation is for transmitting digital data through the public
telephone network. The telephone network was designed to receive, switch, and
transmit analog signals in the voice-frequency range of about 300 to 3400 Hz. It is
not at present suitable for handling digital signals from the subscriber locations
(although this is beginning to change). Thus digital devices are attached to the
network via a modem (modulator-demodulator), which converts digital data to
analog signals, and vice versa.
For the telephone network, modems are used that produce signals in the
voice-frequency range. The same basic techniques are used for modems that
produce signals at higher frequencies (e.g., microwave). This section introduces
these techniques and provides a brief discussion of the performance characteristics
of the alternative approaches.
We mentioned that modulation involves operation on one or more of the three
characteristics of a carrier signal: amplitude, frequency, and phase. Accordingly, there
are three basic encoding or modulation techniques for transforming digital data
into analog signals, as illustrated in Figure 6.2: amplitude-shift keying (ASK),
frequency-shift keying (FSK), and phase-shift keying (PSK). In all these cases, the
resulting signal occupies a bandwidth centered on the carrier frequency.
All1plitude-Shift Keying
In ASK, the two binary values are represented by two different amplitudes of the
carrier frequency. Commonly, one of the amplitudes is zero; that is, one binary digit is
represented by the presence, at constant amplitude, of the carrier, the other by the
absence of the carrier (Figure 6.2a). The resulting transmitted signal for one bit time is
where the carrier signal is A COS(211fet). ASK is susceptible to sudden gain changes
and is a rather inefficient modulation technique. On voice-grade lines, it is typically
used only up to 1200 bps.
The ASK technique is used to transmit digital data over optical fiber. For
LED (light-emitting diode) transmitters, Equation (6.1) is valid. That is, one signal
element is represented by a light pulse while the other signal element is represented
by the absence of light. Laser transmitters normally have a fixed "bias" current that
causes the device to emit a low light level. This low level represents one signal
element, while a higher-amplitude lightwave represents another signal element.
Frequency-Shift Keying
The most common form of FSK is binary FSK (BFSK), in which the two binary
values are represented by two different frequencies near the carrier frequency.
The resulting transmitted signal for one bit time is:
A signal that is less susceptible to error, is multiple FSK (MFSK), in which
more than two frequencies are used. In this case each signaling element represents
more than one bit.
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