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NRZI (Non-return to Zero, Inverted) is an encoding method that seeks to put more transitions into the bit stream. A 1 is encoded by a transition from the present voltage, either down if it is already up or up if it is presently down. Thus a string of 1s will look like a bunch of transitions, although a sequence of 0s will still look like the same voltage level hanging on indefinitely. Fig. 23.2.3 shows an NRZI encoding of 01001110. Another benefit of NRZI is that the bit stream being sent is still decipherable even if the polarity is reversed! This means that if +12 V suddenly becomes 0 and +5 V becomes 1, the exact stream of 1's and 0's is the same as before, because it is only the presence or absence of transitions that signifies a 1 or a 0. Polarity reversals can happen if wires are hooked up wrong. |
