d or drop wave- lengths from a single fiber without using a SONET terminal, as shown in Figure 5-22. Figure 5-23 shows the traditional approach, which entails using SONET terminals (transponders) at the intermediate drop-off point. Additional flexibility will come from crossconnect capability at the optical layer, combined with DWDM and optical add/drop multiplexer (OADM) functionality (see Figure 5-24). Photonic networking achieves multiplexing and crossconnecting strictly in the optical domain, significantly reducing the number of O-E-O conversions and the cost of the network. However, full photonic networking is not available today at the commercial level, and not all of the network elements discussed here were available or broadly deployed in early 2002. Tracking the technology evolution, OC-192 SONET systems have emerged recently that can transport an OC-192 (10 Gbps) on one fiber pair. These can be deployed discretely (as a simple SONET/SDH system) or in 4 OC-48 4 OC-48 Figure 5-22 1B AA OADM. Demux OC-192 Optical Amplifier OC-48 Chapter 5 Figure 5-23 ADM with back-to- back transponders (O-E-O). Optical Amplifier Optical Channel Figure 5-24 All-optical transmission system. WDM Ring Optical ADM Optical Crossconnect WDM Ring conjunction with a WDM system. There are a number of issues, though, that impact TDM-based data rates at 10 Gbps, including Polarization mode dispersion (PMD) Caused by a combination of intrinsic effects (birefringence and mode coupling) and extrinsic effects on cable (bends and twists). Studies show, though, that transmission up to 250 miles is possible at OC-192. Stimulated Brillouin Scattering (SBS) Limits power that can be injected into a single-mode fiber. As more OC-48/OC-192 systems are placed over a fiber, the output power of the EDFAs can cause SBS Dense Wavelength Division Multiplexing (DWDM) Systems problems. Techniques have recently been developed to boost the SBS threshold. Self-phase modulation (SPM) Introduces chirping and in turn interacts with fiber dispersion to cause pu