discrete spectral components appear as in the spectrum of RZ code in Figure 4.18. Transmission 4.4.2.4 High-Density Bipolar 3 HDB-3 was developed from AMI and standardized for European primary rate 2-Mbps systems. HDB-3 overcomes the problem of the original AMI code that occurs in the timing when a data message contains long periods of subsequent zeroes. In this coding scheme, a pulse with the same polarity as the previous one is added in such a way that no more than three sequential zeroes are allowed. In the decoder these pulses are taken away according to the AMI coding rule that they violate. 4.4.2.5 Manchester Coding Manchester coding is used in LANs. Binary digit 1 is coded as a "+ to _" transition and binary 0 as a to +" transition. The most important advan- tage of the Manchester code is that each symbol contains the timing informa- tion and the receiver needs only to detect the transition in the middle of each received symbol to extract the clock signal. Its main disadvantage is a wide spectrum because of short pulses and this is why it is suitable for LANs but not for long-distance transmission. 4.5 Regeneration In long-haul transmissions the transmitted signal is attenuated and amplifiers or repeaters are needed. Analog amplifiers amplify the signal at the input, and the signal contains both the desired message and channel noise. In every amplifier and cable section some noise is added and the S/N decreases with distance. Unlike analog amplifiers, digital repeaters are regenerative. A regenera- tive repeater station consists of an equalizing amplifier that compensates the distortion and filters out the out-of-band noise and a comparator as shown in Figure 4.19. Output of the comparator is high if the input signal is above the threshold voltage Vref and low if the input is below the threshold value. The regenerator also contains timing circuitry, which extracts the clock sig- nal from the received data, and a D-type flip-flop which decides if a digit is high (1) or low (0) at the instant of the rising e