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Digital Data Transmitted Over Analog Signals

Digital data have to be converted into analog signals before they can be transmitted over a analog medium such as telephone line. Modem (Modulator and demodulator) is used for this purpose. There are different techniques to get digital data into analog forms. (see Figure 2-18 of Tanenbaum (page 110)).

  • Amplitude modulation uses different amplitude (e.g. voltage) levels to represent different values such as 0s and 1s.
  • Frequency modulation uses different frequencies (tones) to represent different digital values such as 0s and 1s.
  • Phase shift modulation moves the phase (starting angle) around to represent different digital values such as 0s and 1s.

Advanced modems use a combination of these modulation techniques to transmit multiple bits per baud (per signal variation!). For example, the Quadrature Phase-Shift Keying has the following properties.

  • 30 degree phase shift at 15, 45, 75, etc. (so we have 12 different phases. remember: sin(x) and cos(x) has a period of 360 degree)
  • eight of them have a single amplitude
  • four of them (45, 135, 225, and 315) have two amplitudes

As a result, 8 single values + 4 X 2 double values = 16 values, which means 4 bit of data can be transmitted in one baud. So a 9,600 baud modem can transmit at the rate of 38,400 bps.

A well-known example of transmitting digital data over analog medium is the use of RS-232 port. RS-232 has 25 pins. See Figure 2-21 of page 115 of the textbook for an illustration of pin assignment. Figure 2-22 on page 116 gave the full detail and comparison. One can use a minimum of 3 pins to make an RS232 connection. That is, pins 2, 3, and 7. Don't forget to reverse (cross) pine 2 and 3 when connection two RS232 for direct communication.

The RS-232 starndard specifies a 25-pin DB-25 connector, including:

  • menchanical specification: 25 pins, including thickness, length, etc.
  • electrical specification: a binary '1' is indicated by a voltage of less than -3, a binary '0' is indicated by a voltage of greater than +4.
  • functional specification: what each of the pins means.

next up previous
Next: Analog Data Transmitted Over Up: Physical Layer Previous: Transmission Media

Xiannong Meng
Wed Jun 2 14:41:26 CDT 1999