lar transmission that the Os are coded as absence of pulses and 1s are alternately coded as positive and negative pulses, with the alternation taking Neutral Bipolar Figure 6.10 Neutral versus bipolar bit streams. The upper diagram illustrates alternating 1s and 0s transmitted in the neutral mode; the lower diagram illustrates the equivalent in the bipolar mode, which is also called alternate mark inversion or AMI. Note that in the neutral mode, the 0 state is inactive, 0 volts. Neutral transmission is discussed in Chapter 10. DIGITAL NETWORKS place at every occurrence of a 1. This mode of transmission is also called alternate mark inversion (AMI). One drawback to straightforward AMI transmission is that when a long string of Os is transmitted (e.g., no transitions), a timing problem may arise because repeaters and decoders have no way of extracting timing without transitions. The problem can be alleviated by forbidding long strings of Os. Codes have been developed that are bipolar but with N Os substitution; they are called "BNZS" codes. For instance, a B6ZS code substitutes a particular signal for a string of six Os. B8ZS is used on subscriber loop carrier and inserts a violation after a string of 8 zeros. Another such code is the HDB3 code (high-density binary 3), where the 3 indicates substitution for binary sequences with more than three consecutive Os. With HDB3, the second and third Os of the string are transmitted unchanged. The fourth 0 is transmitted to the line with the same polarity as the previous mark sent, which is a "violation" of the AMI concept. The first 0 may or may not be modified to a 1, to ensure that the successive violations are of opposite polarity. HDB3 is used with European E series PCM systems and is similar to B3ZS. 6.5 SIGNAL-TO-GAUSSIAN-NOISE RATIO ON PCM REPEATERED LINES As mentioned earlier, noise accumulation on PCM systems is not a crucial issue. However, this does not mean that Gaussian noise (or crosstalk or impulse noise) is unimportant.7 Indeed, it will affect error perform