components at the harmonic frequencies of the data rate. The harmonic frequencies are multiples of the data rate. An external clock signal is always needed for the timing of the receiver. 4.4.2.2 Return to Zero (RZ) RZ each symbol is cut into two parts. The first half of the symbol represents the binary value and the rest of the symbol is always set to zero. Because pulses are shorter than in the case of NRZ the spectrum is wider, as we saw in Figure 4.2, and the spectrum of a random data has strong discrete frequency components at the harmonic frequencies of the data rate. With the help of Introduction to Telecommunications Network Engineering Power density spectrum Unipolar NRZ-signal Unipolar RZ-signal Pseudoternary signal, AMI or HDB-3 Manchester code Figure 4.18 Common line codes and their power spectrums. these components, timing information can be extracted from the signal spec- trum and an external clock is not necessarily needed. However, because RZ code has high low-frequency content and a wide spectrum (see Figure 4.18), it is never used in long-distance transmission. Another problem is that syn- chronization is lost if the data content is all zero for a long period of time. 4.4.2.3 Alternate Mark Inversion (AMI) If every other mark or 1 of the NRZ or RZ symbols is transmitted as an inverted voltage polarity, an AMI signal is produced. The advantage of this is that no dc component is present on the transmission line. The dc component is unwanted because it does not carry any information; it merely wastes power. With the help of this kind of code we can avoid the problem caused by transformers on the line. Transformers are needed on copper cable lines for matching impedance, for overvoltage or surge protection, and for other purposes. Direct current does not propagate through transformers. AMI code is used in American telecommunications network in primary rate 1.5-Mbps transmission systems. We may extract the timing information by rectifying the AMI signal into an RZ signal in the receiver and then the 