ave nonlinear responses to their drive currents and SO impose some form of pulse shaping. The elimination of the spectral components beyond the first Os allows closer channel spacing in WDM transmission without interchannel interference. NRZ and RZ Transmission: Relative Merits NRZ channels are normally preferred for low- and intermediate-datarate transmission because they are easily generated by both direct and external modulation. As an example, Modulation Formats NRZ (5 = 1) RZ with is = 1/3 NRZ is = 1) RZ with is = 1/3 Normalized Frequency (fc AT) Fig. 6.29 Normalized power spectral densities for NRZ and RZ (5 = 0.33) modulated signals showing (a) continuous constituents and (b) discrete constituents in which the NRZ and RZ pulses have the same energy. AT = bit duration. NRZ is recommended for use by the various PON standards (Keiser, 2006; ITU-T G.983.1- G.983.5; ITU-T G.984.1-G.984.6; ITU-T G.985). Equation 6.26 shows that group delay due to chromatic-dispersion broadening is directly proportional to bandwidth. Therefore, NRZ pulses experience less dispersive broadening than RZ pulses. The narrower spectral band- widths of NRZ channels provide better power spectral densities at the detector, and SO they experience lower noise penalties than RZ pulses at the same datarate. Furthermore, owing to their relatively narrow spectra, NRZ channels can be placed closer together in a WDM transmission scheme before interference between neighboring channels contributes signifi- cantly to BER. One concern with NRZ operation is if there should be long strings of consecutive 1s or Os. There is then a loss of information on the digital period and phase, which can lead in turn to a loss of clock synchronization at the receiver. As a result, the bits are not sampled at the appropriate instants. To minimize the problem, bit scramblers and descramblers are used at the transmitter and receiver, respectively, as noted in the earlier discussion of syn- chronous digital hierarchy. RZ modulation also provides advantages. The pulses 