detail later in this chapter. Even if an effort is done to optimize the use of the transmitted high-speed channels, it is frequent in the transport network that not all the tributaries are really present in the data field (or payload) of the transmitted frame. In this case, the carried information is smaller than its maximum possible value, depending on the bit rate and the frame structure. This lack of efficiency can be at least partially corrected by means of grooming, that is, the operation consisting in the demultiplexing of a channel in the middle of its route to distribute all or part of the tributaries in other channels. This operation can be performed all in the transport layer or the cooperation of the packet layer can be required. An example of grooming in the transport layer is provided in Figure 3.15. In part (a) of the figure, the situation of a transport network receiving channels at 622 Mbit/s (syn- chronous transport module 4 [STM4] in the SDH language) and transmitting wavelength channels at 2.5 Gbit/s (STM16 in the SDH language) is shown SO that up to four STM4 can be multiplexed in a single STM16. The STM4 channels routed at a certain instant is given by the traffic matrix incorporated into Figure 3.15a: three bidirectional STM4 from A to B and two bidirectional STM4 from B to D. The traffic from A to B is routed through the direct link using a single STM16 and the traffic from B to D is also routed through the direct link using another STM16. The return channel is always routed on the same route. 4 STM4 -1 STM16 4 STM4 -1 STM16 5 STM4 1 STM4 1 STM4 3 STM4 2 STM4 3 STM4 2 STM4 1 STM16 STM16 1 STM16 STM16 Line bit-rate 2.5 Gbit/s (STM16) Line bit-rate 2.5 Gbit/s (STM16) 622 Mbit/s (STM4) traffic 622 Mbit/s (STM4) traffic FIGURE 3.15 Example of grooming in the transport network: (a) situation before the new request; (b) situation after the new request of five channels between A and D. Networks Fundamentals and Present Architectures Both the high-speed channels used for transmission are not fully 