en by real-time commands. Besides protection, this could be a point if a GMPLS (generalized multiprotocol label switching) control plane should take the control of all the networks (see Chapter 8). Parallel design is common and widely used with a significant amount of industry design and implementation information available, including several standards. While the main disadvantages of this design technique are as follows: Significant board and system space can be required, especially with wide or high card count design that is typical of DWDM systems, where, for example, more than one hundred transponder cards can be foreseen in a fully equipped system. Transmission Systems Architectures and Performances Implementation costs can be high because of multiple terminations and devices, wide connectors, and high board trace counts on the backplane itself. Line-to-line signal skew must be matched and controlled. As far as serial design is concerned, its main advantages are as follows: Serial data offer an adjustable approach to data rates, allowing longer cable lengths. Fewer ground lines and signal lines are required to transmit the data from one point to another. Serial design offers significant space savings over equivalent parallel designs. This point could be a key in highly compact DWDM designs, where real estate sparing is a system value proposition. While the weaknesses of serial design can be summarized as follows: A time delay is incurred, when serializing and deserializing data blocks, that has to be controlled accurately if real-time protection switching is to be performed. Moreover, if design margins are not sufficient, it could be very difficult to upgrade the system to advanced network control requiring more and more a real-time response. Serial devices are generally more expensive than parallel backplane drivers. Higher speeds require greater attention to signal path layout and impedance matching. Once the bus physical wiring is defined, there are still several ways allowing multiple cli- ents to access