As we have seen in Chapter 2, while in the core network the topology is generally a mesh and point-to-point transmission among complex core nodes is dominant, in the metro area ring topology is frequently used. Transmission Systems Architectures and Performances TABLE 6.18 Penalty Analysis of Single Span Systems with Electronic Pre-Distortion at 10 Gbit/s Electronical Pre-Compensation Single Span System at 10 Gbit/s Notes System length 220 km Channel speed 10 Gbit/s Channel number Extended C band Channel spacing 100 GHz Modulation format Pre-compensation Ideal SN (dB) Bo/B (due to NRZ modulation) M=2@ 10 Gbit/s, NRZ modulation Required SNo (dB) Due to the very high value of Bo/Be Impairment Penalty (dB) Notes APD receiver noise This is mainly the avalanche noise (see Equation 5.29), due to the low power on the APD (-20 dBm per channel) Signal dynamic range Lithium niobate modulator Linear crosstalk AWG multistage, NRZ signal spectrum with spread of 10 GHz (first zero), Gaussian optical filter with 40 GHz bilateral width, see Figure 6.16 Nonlinear crosstalk (FWM XPM) Wide spacing (100 GHz) and use of an SSMF Residue chromatic dispersion Value due to the high input power managed and SPM interplay by electronic pre-compensation with a mixed FFE-DFE-Look-up-table system Completely cancelled by pre-distortion Polarization effects Aging System margin Normal system margin TOTAL w/o FEC FEC gain (RS 255,223) Required SNo (dB) This difference reflects the different functionalities of the two networks, the first devoted to transport of signals among faraway places, the second to collect and distribute the sig- nals generated or directed to users in the same area. In this section, we will talk about optical rings only from a transmission point of view, while networking functionalities like wavelength routing and protection have been analyzed in Chapter 3 and the functional architecture of an OADM will be detailed in Chapter 7. In the metro area there are two main technologies that coexist: CWDM and DWDM. As a rule of thumb,