iled standards, due to the fact that they have to fit into the central office space in a better way. A diffused rack standard is the so-called 19' rack, that was born in the United States, but is now diffused also in Europe and the far east [46-48]. A pictorial representation of a 19' rack is presented, besides the rack standard measures, in Figure 6.45, while Figure 6.46 shows as the subracks being mounted one over the other into the rack. Subracks on the contrary, as far as they fit into the standard rack, are largely different from system to system. Both European Standardization Institute (ETSI) in Europe and American Standardization Institute (ANSI) in the United States have defined standard subracks, but several nonstandard solutions are well accepted by the market as far as lower power and real estate sparing is demonstrated are demonstrated. Transponders Transponders Redundant power supply Control Service mux/demux channel unit FIGURE 6.43 DWDM subrack: this is a metropolitan system with small cards, often long-haul and ultra-long-haul systems have higher subracks. (Published under permission of PGT Photonics s.p.a. from Arecco. F., City8TM System Configuration Manual V 1.0. Internal PGT doc, p. 25, 2003.) Transmission Systems Architectures and Performances FIGURE 6.44 Cards allocation and backplane in a DWDM subrack. (Published under permission of PGT Photonics s.p.a. from Arecoo. F., City8TM System Configuration Manual V 1.0. Internal PGT doc, p. 25,2003.) 23.62 Top view 23.62 19.56 19.11 70.31 63.00 Left side view Front view FIGURE 6.45 Standard measurements of the 19' rack. Telecommunication Networks FIGURE 6.46 Two superimposed subracks ready to be mounted inside a 19' rack. (Published under permission of PGT Photonics s.p.a. from Arecoo. F., City8TM System Configuration Manual v 1.0. Internal PGT doc, p. 25, 2003.) The cards can fit into the subrack in different ways: vertically (like in Figure 6.44) is the most frequent solution since it allows better cooling of the system, but also subrack with hor