Baseband Signalling

1. The simplest way to send digital information through wires is to encode everything in binary and use the same system for encoding bits that is used within the digital logic components of a computer. That is, simple choose one voltage to represent 0 and another to represent 1.

2. If d(t) is a function whose value at potin t is the value of the binary digit we wish to represent on a wire at time t (i.e. d(t)'s value is always either 0 or 1), then the value of the voltage used as a signal to represent the bit inside a typical digital logic circuit is V x d(t) for some voltage V.

   

3. On a network transmission line, it is more common to use two voltages of equal magnitude but opposite polarity to represent 0 and 1. This reduces the direct current component of the signal. As a result, in such a transmission system

   s(t) = 2 x V x (d(t) - 1/2)

   

   This scheme is knows as Non-Return to Zero (NRZ) signalling.

4. When sending a long sequence of zeroes or ones, the voltage used in NRZ transmission remains fixed for a long period of time.

   

   This causes two problems:

   • The signal will again have a direct current component (although I must admit that I have too little background to really explain why this is bad).

   • The receiver and sender will have to have precisely synchronized and highly accurate clocks since the signal itself will provide no information to enable the receiver to detect any drift in its clock.

5. Several codes are used that avoid these problems by making a voltage transition at some point in the process of sending each bit. For in "manchester encoding", the direction of the voltage change in the middle of the time interval associated with the transmission of a bit is used to encode the value of the bit.

   = 0

   = 1

   

6. Encoding techniques similar to NRZ and manchester encoding can be used with optical fiber. With fiber, variations in voltage are replaced with variations in light intensity.

   • At the sending end of the fiber, an electrical signal produced by the computer is used to control a light emitting diode or laser producing a beam of light whose intensity varies to reflect the bits being transmitted.

   • At the receiving end, a photo detector converts the light back into an electrical signal.