Modem modulation methods

The modulation process modifies the characteristics of a sinusoidal carrier signal, which can be represented by the following equation:

Where

V (t) = the instantaneous voltage at a time t A = amplitude of signal f = frequency p = phase angle.

The modulation techniques used in modems include:

• Amplitude modulation or amplitude shift keying (ASK) in which the binary values are represented by two different amplitudes of tone. ASK is not used alone in modern modems.

• Frequency modulation or frequency shift keying (FSK) is commonly used for modems at data rates less than 1200 bps, where different frequencies are allocated to the logic one and the logic zero in the data message. Full-duplex modems use four frequencies, one pair for each direction of transmission. FSK modems are very reliable but need to transmit one tone for each data bit. The rate at which this can be done is called the baud rate and is limited by the bandwidth of the channel.

• Phase modulation or phase shift keying (PSK) uses changes in phase angle to represent the different data bits. For example QPSK uses four phase angles, and so one electrical transition represents two bits of information. In this way the data rate is twice the baud rate, so a 2400 bps data rate is carried at 1200 baud. Phase shift keying is used in some form in all high-speed modems.

• Quadrature amplitude modulation (QAM) uses a combination of simultaneous phase and amplitude modulation to convey even greater data rates. QAM allows data rates of four bits per baud, so a 9600 bps data rate is carried at 2400 baud. QAM systems usually incorporate trellis coding in which only some of the possible phase/amplitude combinations are valid. This means that if the presence of noise on the line causes the received signal to differ from an accepted combination, then the receiver will choose the nearest valid point. This usually also involves forward error correction as well. These systems then have high accuracy whilst transmitting many bits for each electrical signal transition.

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