xer can read in the header what are the STSs multiplexed into the payload area and, following the pointers, can separate them by demultiplexing the frame. It is evident that such a mechanism is very effective in giving a high multiplexing effi- ciency but, especially at high transmission speeds, is prone to errors in the presence of jitter. In particular, the low-frequency component of jitter (called generally wander) accu- mulates while the signal traverses the network, generates a random move of the pointers in the attempt to track this random change of the bit synchronism. When the signal is demultiplexed in the adaptation layer before passing the informa- tion to the client, this pointer movement can cause abrupt changes of phase generating demodulation errors. In order to minimize jitter accumulation, the SDH network does not rely on local clocks to synchronize the multiplex and demultiplexing operations, but it distributes a centralized clock via the so-called synchronization network. This is a conceptually separated overlay network distributing a clock signal to all the network elements. Due to this architecture, the overall jitter is order of magnitude smaller that in a network based on local clock gener- ation allowing high transmission speed, at the expense of network architecture complexity. Below the multiplex section layer there is the regeneration layer, dealing with the propa- gation of the signal between couples of in-line regenerators and, further below, the physi- cal layer representing physical propagation of the signal along the transmission medium. Summarizing, the physical scheme of a path in a synchronous optical network is repre- sented in Figure 3.28, where both SONET and SDH nomenclatures are used for comparison. In Figure 3.28, the main SDH network elements are also represented; they are the termi- nal equipment (TE), the digital cross connect (DXC), and the regenerator (REG). The TE simply receives as the input a line signal and a set of connection requests from the network manager thr