often uses symbols per second and bauds interchangeably. It would be preferable, in our opinion, to use "symbols" for the output of a coder or other conditioning device. For the case of a channel coder (or encoder), bits go in and symbols come out. There are more symbols per second in the output than bits per second in the input. They differ by the coding rate. For example, a 1/2 rate coder (used in FEC) may have 4800 bps at the input and then would have 9600 symbols per second at the output. 10.7.4.1 Period of a Bit, Symbol, or Baud. The period of a bit is the time duration of a bit pulse. When we use NRZ (nonreturn-to-zero) coding (discussed in Section 10.7.5), DATA COMMUNICATIONS the period of a bit, baud, or symbol is simply 1/(bit rate), 1/(baud rate) or 1/(symbol rate). For example, if we were transmitting 9600 bits per second, what is the period of a bit? It is 1/9600 = 104.16 usec. For 2400 baud = 1/2400 = 416.6 usec; for 33.6 kbps = 1/33,600 = 0.0297 usec, or 29.7 nsec. 10.7.5 Digital Data Waveforms Digital symbols may be represented in many different ways by electrical signals to facil- itate data transmission. All these methods for representing (or coding) digital symbols assign electrical parameter values to the digital symbols. In binary coding, of course, these digital symbols are restricted to two states, space (0) and mark (1). The electrical parameters used to code digital signals are levels (or amplitudes), transitions between different levels, phases (normally 0° and 180° for binary coding), pulse duration, and fre- quencies or a combination of these parameters. There is a variety of coding techniques for different areas of application, and no particular technique has been found to be optimum for all applications, considering such factors as implementing the coding technique in hardware, type of transmission technique employed, decoding methods at the data sink or receiver, and timing and synchronization requirements. In this section we discuss several basic concepts of electrical coding of bin