an be transported very comfortably using the SDH standard. No compatibility between the signal and the hardware, i.e. the network equipment, is required. Moreover, the signal does not need to be compatible in any sense as it is being carried in a container. The container just keeps the signal and adds, if necessary, any redundant bits. In a similar context this can be thought of as someone filling up his/her knapsack until it fills completely. In this way many signals of less capacity can be added to the container, but there is a set standard up to which the number of signals can be filled. The payload, which carries all the information, fills up the majority of the space but the housekeeping bits, known as the overhead, take up some amount. Generating the STM-1 when the source incoming signal is 2.048 Mbps can be pictorially represented as in Figure 3.23. Space for 3 AU-3 or 1 AU-4 pointers Figure 3.22 Administrative unit group Transmission Network Planning and Optimisation 9 Bytes 261 Bytes STM 1 TUG-3 TUG-2 TUG-12 VC-12 STM 1 TUG-3 TUG-2 TUG-12 VC-12 Figure 3.23 Generation of an STM-1 signal from a 2.048 Mbps signal Multiplex Paths in the SDH There is not a particular path that any signal has to follow to be transported as an STM-n signal. In- stead signals of different speeds take up different paths on their way to the final signal. The source signals received are assembled in the corresponding containers, provided with the POH and pointer and converted into an STM-1 signal via the different multiplex steps as shown in Figure 3.24. Source sig- nals with bit rates higher than 139.264 Mbps are multiplexed into the STM-1 frame in one step, while those with lower bit rates are multiplexed in two steps. This multiplex scheme complies with ITU-T G.707 and includes optional multiplex paths. The VC-3 can, for example, be multiplexed via TU-3 into VC-4 or the AU-3 path can be selected. A distinction is made between the lower order and higher order paths. For SDH signals there are two levels, which are used to set up t