 failure Monitoring and control of channels transmission Transmission itself is carried out by transmission systems that physically connects the near to the far ends via fiber optics. These systems will be analyzed in detail in Chapters 6 and 9. Generally, transmission systems transmit a set of very high-speed signals (10 Gbit/s or more) over different optical wavelengths (WDM). The channels coming from the packet networks are not SO fast; thus, TDM multiplexing is performed in different steps to create a TDM hierarchy. The scheme of a simple TDM hierarchy is represented in Figure 3.14: the channels received from the packet network are multiplexed in a faster TDM frame where every data Bit rate Payload rate Frame efficiency 155.5 Mbit/s, 150.3 Mbit/s, 96.6% Progressive 622.0 Mbit/s, 601.3 Mbit/s, 96.6% multiplexing STM 4 2.488 Gbit/s, 2.405 Gbit/s, 96.6% STM 16 STM 64 9.953 Gbit/s, 9.621 Gbit/s, 96.6% FIGURE 3.14 Scheme of a simple TDM multiplexing hierarchy. A subset of the SDH hierarchy is shown. Telecommunication Networks field is filled with the frames of the channels to be multiplexed, which are called multiplex- ing tributaries. This operation is repeated several times, passing through a set of intermediate-speed TDM channels up to achieving the speed compatible with transmission systems. The operation of multiplexing is not performed in a single step for different reasons. Besides the fact that it would be technically difficult to multiplex together a huge num- ber of slow channels in a single very fast channel, in real networks, the speed of the chan- nels created by the packet network is not unique; on the contrary the packet network can create channels at different speeds following the needs. The creation of a TDM hierarchy allows channels of different bit rate to be received from the packet network and to be accommodated at different levels of the hierarchy. The most important standards defining TDM multiplexing hierarchies are SDH/SONET and optical transport network (OTN) that we will analyze in some 