ary signals. In the discussion reference is made to Figure 10.8, which graphically illustrates several Assumed bit sequence A. Neutral or unipolar NRZ (NRZ-L, unbalanced NRZ or NRS change) B. Unipolar RZ (RZ-L) C. Polar NRZ D. Polar Rz E. Biphase-L or Manchester II (Diphase NRZ) Figure 10.8 Digital data transmission waveforms. 10.8 DATA INTERFACE: THE PHYSICAL LAYER line coding techniques. Figure 10.8a shows what is still called by many today neutral transmission (also see Figure 10.4). This was the principal method of transmitting tele- graph signals until about 1960. In many parts of the world, neutral transmission is still widely employed. First, the waveform is a nonreturn-to-zero (NRZ) format in its most elementary form. Nonreturn-to-zero simply means that if a string of 1s (marks) is trans- mitted, the signal remains in the mark state with no transitions. Likewise, if a string of Os is transmitted, there is no transition and the signal remains in the 0 state until a 1 is transmitted. A "0" (zero) infers the 0 voltage line as shown in the figure. Figure 10.8c is the conventional NRZ waveform most encountered in the data industry. Figures 10.8b and 10.8d show the typical return-to-zero (RZ) waveform, where, when a continuous string of marks (1s) or spaces (Os) is transmitted, the signal level (amplitude) returns to the zero voltage condition at each element or bit. Obviously RZ transmission is much richer in transitions that NRZ. In Section 10.6.2 we discussed neutral and polar transmission systems. Figure 10.8a shows a typical neutral waveform where the two state conditions are 0 V for the mark or binary 1 condition and some positive voltage for the space or binary 0 condition. On the other hand, in polar transmission, as illustrated in Figures 10.8c and 10.8d, a positive voltage represents a space and a negative voltage a mark. With NRZ trans- mission, the pulse width is the same as the duration of a unit interval or bit. Not SO with RZ transmission, where the pulse width is less than the duration of a unit