lses [36,37]. Preemphasis puts more signal energy into the high frequency components of the transmit signal to overcome the channel frequency-dependent loss characteristics. If there is a feed- back channel from the receiver to the transmitter, for example, a channel used to deliver feedback from the cards to the controller on configuration operations, it is possible to adapt the preemphasis to the channel characteristics. In order to assure that the signal is launched into the transmission line in an optimum way, without losing the shape created at the transmitter, the quality of the trace routing on the backplane PCB, near the launch point is crucial. Spurious parasitic elements due to the PCB creation processes and to other fluctuations from the design parameters must be accounted for while designing a 10 Gbit/s transmitter. An idea of the parasitic elements impacting transmission near the signal launch point and of their origin is provided in Figure 7.27. 7.5.1.1.2 Connectors Connectors are sources of signal reflections and crosstalk. Interference can be limited by shielding each connector pair with metal plates SO to avoid coupling between different signals traveling through the backplane. However, care has to be taken when adding metal to provide shielding because it can hurt connector performance by reducing the impedance, thus increasing reflections. The Line card package trace Backplane connector FIGURE 7.26 Line card Backplane trace Elements of a high-speed backplane connection between cards of a switch- ing network element. Backplane via Switching Systems: Architecture and Performances Package substrate PCB trace in Die attach R = 100 Q R =100.0 (@@@@@@@@@ Subst. Package TX via Driving amplifier FIGURE 7.27 Example of parasitic elements in the connection between the transmitter and the backplane trace, for each para- sitic element the process originating it is indicated. use of differential signaling can help to provide low crosstalk up to 50 ps rise times that is compatible with a 10 Gbit/s transmissi