al., 2000). NRZ signaling and RZ signaling operate using intensity modulation only. The phase attributes of optical waves can also be used. We can achieve phase modulation by using Forward Error Correction more advanced modulators, which can be dual-drive Mach-Zehnder based, or by using the devices in tandem. One possibility is RZ pulses with a reduced-amplitude carrier wave (Zhu & Hadjifotiou, 2004; Zhu et al., 2002). This is called "Carrier-Suppressed Return to Zero" (CSRZ) modulation, and it suffers less from nonlinear crosstalk than does straight- forward RZ transmission. Another technique is to transmit pulses in all of the bitslots but to change their phases between 0 and TT, according to whether a 1 or a 0 is being trans- mitted (Ho, 2001; Zhu et al., 2002). This technique, known as "Differential Phase-Shift Keying" (DPSK), provides increased receiver sensitivities and a greater immunity to non- linear crosstalk. Forward Error Correction The need for error-correcting codes was discussed in Chapter 4. This section considers, in particular, how forward error correction is applied in optical transmission systems. Error-Correcting Codes As has been argued, optical transmission is subject to numerous factors that combine to worsen the measured BER. With good system and component design it is possible to ame- liorate many of the greatest problems by using the techniques described in the foregoing sections, along with the design principles from the last section of the chapter. However, high-capacity network operation is increasingly being required with BER values of 10-12 to 10-15 (ITU-T G.691), which is demanding. Even with the aid of all of the physical methodologies described in this chapter, we ultimately reach system capacities and lengths that cannot be exceeded by obvious physical means. In this section we briefly describe a method, called Forward Error Correction (FEC), in which the signal is encoded to enable bit errors to be corrected by electronic circuits after the receiver (Ait Sab, 2001). Our descri