izontal cards exist. The way in which the cards fit into the sub rack is important since it is determined by the cards dimension. Larger cards, fitting horizontally into the subrack, contain a more efficient electronics design (since more components are on the same card), but make the system less flexible. Different cards, either belonging to the same subrack or to different subracks can exchange information though the back plane. Let us assume that no switching capability is embedded into the system (e.g., no embed- ded ethernet switch or no SONET/SDH ADM). In this case, the system does not need the capability to demultiplex the TDM frame of the incoming signals in order to access to the tributaries in the deterministic case or to the packets in the statistical case. On the other hand, the incoming signal is very high capacity. A standard ultra-long-haul system can manage more than 1.2 Tbit/s capacity shared among the wavelength multiplexed 10 Gbit/s channels. In this condition, almost all the DWDM systems are designed to transfer the 10 Gbit/s channels from a card to the other using fiber patchcords on the front panel. For this reason, almost all the racks designed to accommodate DWDM and CWDM systems comprise slides and other facilities to route the external fiber patchcords in an ordered way. This can be very important to facilitate the system maintenance, due to the fact that a long- haul DWDM system can have up to 600 patchcords in a single terminal to manage its 150 wavelength channels connecting transponders to the clients and to the mux-demux. This means also that the backplane, differently from what happens in switching equip- ment, does not carry the signal from one card to the other, but only the commands and other information that are passed from a generic card to the system controller. In the architecture of a generic DWDM system we can classify the cards in three classes: Transmission Systems Architectures and Performances 1. Signal cards 2. Control cards 3. Support cards 6.5.1.1 Signal Cards Signa