rame. This process, called justification or stuffing, adds a number of justification bits to each tributary in order to make the average tributary data rates exactly the same. In the demultiplexer these justification bits are extracted and the original data rate for each tributary is generated. At each hierarchy level the tributary signals are bit interleaved to the aggregate data stream, which means that the aggregate data stream contains one bit from tributary 1, one bit from tributaries 2, 3, and 4, and then again from tributary 1, and SO on. Additional bits are needed in the frame for frame synchronization (frame alignment) and justification, and therefore the next Transmission Plesiochronous multiplexing stages 64 Kbps standardized by ITU-T 2,048 Kbps Plesiochronous = "almost the same 8,448 Kbps data rate" 34,368 Kbps 139.264 Mbps 3 8-34 2 Mbps 3 140 30 channels 8 Mbps 120 channels 34 Mbps These multiplexers build up 480 channels 2, 8, 34 or 140 Mbps data rates for 140 Mbps 564.148 Mbps optical or microwave radio transmission. 1,920 channels 7,680 channels Above 2 Mbps, justification (stuffing) is done (not standardized) at each stage because tributary rates are allowed to be plesiochronous. Demultiplexing has to be done step by step because justification bits must be stripped off in order to locate the information content. Figure 4.24 The PDH (European standard). level has a slightly higher rate than four times the nominal tributary rate. Jus- tification bits are added to tributaries SO make their data rates equal for fram- ing. The frame also contains some spare bits that can be used, for example, for management data transmission for a network management system. Bits for far-end alarms are included in the frames just as in the 2,048-Kbps frame discussed previously. The standards for PDH ensure compatibility in multiplexing between systems from different manufacturers. The management functions are not standardized and they differ from manufacturer to manufacturer. Only the local interfaces and the multiplex