4. Second from the point of view of deployed length are the G.655 fibers. They are NZ+ and NZ- fibers, which trade a bit of attenuation with a substantially low dispersion in all the third window. In reality, G.655 fibers have never demonstrated their capability to support 10 Gbit/s DWDM transmission systems with performance superior to that of G.652 fibers. The continuous increase of the value of D at 1550nn of the most recent G.655 fibers has demonstrated that the optimal trade-off between D, A, and a has not been found yet. DS fibers, standardized in ITU-T recommendation G.653, have on the contrary almost disappeared from the market today. Avoiding dispersion in single-channel transmission at 1550 nm resulted in exacerbation of both XPM and FWM. Moreover, DS fibers have a higher PMD with respect to SSMF because of the core-cladding index difference two times higher than that of G.652 fibers. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices TABLE 4.4 A Few Key Specifications from ITU-T G.652 G.652 (A) G.652 (C) Cladding diameter (um) 125 + 0.7 125 + 0.7 Core-cladding concentricity (um) <0.50 <0.50 Cladding on-circularity (%) Cutoff wavelength in cable (nm) <1260 <1260 Power attenuation in cable (dB/km) <0.40 (at 1310nm) <0.40 (at 1310nm) <0.35 (at 1550nm) <0.30 (at 1550nm) <0.40 (at 1625nm) <0.40 (at 1625 nm) Chromatic dispersion parameter <3.5 at 1339 nm <3.5 at 1339 nm (ps/nm/km) <5.3 at 1360 nm <5.3 at 1360 nm <18 at 1550nm <18 at 1550nm Effective index (from LP group 1.466 at 1330 nm 1.466 at 1330 nm velocity) 1.467 at 1550 nm 1.467 at 1550 nm Mode field diameter at 1310 nm (8.6-9.5) + 0.7 (8.6-9.5) + 0.7 D3 (ps/nm2/km) <0.093 <0.093 Zero dispersion wavelength (nm) 1300 do 1324 1300 < do < 1324 PMD (ps/km / 2 <0.20 <0.50 A few key requirements from G.653 and from G.655 are reported in Tables 4.5 and 4.6, respectively. 4.2.11 Polarization Maintaining and Other Special Telecom Fibers A polarization maintaining (PM) fiber is an optical fiber in which if the input field is inje