DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 16: Modulation

Amplitude | Frequency | Phase | Summary

Introduction
This section briefly discusses modulation, the process a modem uses to convert the digital data into analogue tones which are sent over the dial up connection.

MODULATION
Is the process used to describe how the digital signal is changed so it can be transmitted across the analogue link.

Modulation methods
This refers to how the digital signal is altered so that it can be sent via the analogue PTSN. There are a number of different methods. The more complex methods allow much higher transmission rates (bits per second) than the simpler methods.

• Amplitude Modulation
  This uses a single carrier frequency, on for high, off for low. Note that there is no carrier signal being sent for approximately half the time.
  

  This modulation method is suitable only for low speed transmission. The frequency of the carrier signal used depends on the protocol standard being used.

• Frequency Modulation
  
  

  This method uses two carrier signals, one for high one for low. The higher frequency is associated with binary 1, the lower frequency with binary 0. Also called frequency shift keying, this method is used for 1200 bps modems or slower speeds.

  Higher speeds are not possible. If you increase the number of bits per second of the digital signal, the corresponding carrier tones are on for a shorter duration. Eventually, the stage is reached where only a few cycles of the carrier tone is being sent for each digital bit. In order to reliably detect the carrier signal at the receiver, up to 5 complete cycles are required. Less than this, and the carrier signal may be misinterpreted.

  To compensate for this, you might consider the option of increasing the frequency of the carrier signals. But, You just cannot raise the carrier frequency higher as the voice channel will cut it off (remember the bandwidth limits of the voice channel).
  

• Phase Modulation
  This method uses a single carrier frequency and alters the phase of the carrier. Variations of this method allow coding of multiple bits (i.e. two or four bits) to represent a single carrier change.

  Normally, a change from binary 1 to binary 0 is represented as a phase shift of 180 degrees.

  By coding two bits per phase change, this doubles the number of bits per second. This is called two level coding. This method is suitable for 2400 bps modems (CCITT V.26).

  Bit Pattern	Degrees Phase Shift
  00	45º
  01	135º
  10	315º
  11	225º

  Remember that baud rate is the number of changes per second. By moving to two bits per phase change, the number of changes per second is still the same (baud rate), but the number of bits per second is doubled. Baud rate is only equal to bit rate where single level encoding is used.

  Three bit encoding is used for CCITT V.27 modems at 4800 bps. Higher speeds are obtained by using four bits, eight phase changes, with two or four different amplitudes. Where changes in amplitude are combined with phase changes, this is called phase amplitude modulation.

Summary
Modulation is the mechanism that a modem uses to convert digital signals to analogue signals for sending across a telephone connection. The three different type of modulation are amplitude, frequency and phase, although phase amplitude modulation is a combined technique of different phase and relative signal levels that offers the greatest speed of the methods discussed above.

The limitation with sending data over a telephone connection is related to baud rate, which is a measure of the line changes per second. Once this limit is exceeded, the signal is lost and cannot be reliably detected at the receiver.

In amplitude and frequency modulation, each line change represents a bit change in the input signal. Thus, as the input signal is sped up, the baud rate limit is reached, and at that point, no increase in speed is possible.

In phase modulation, groups of bits represent a line change. The limiting factor is always the number of line changes per second, it is just that in phase modulation each line change represents more than one bit of the input signal, and in this way, higher speeds are possible.

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© Copyright B Brown. 1995-2000. All rights reserved.