ing scheme are standardized in PDH. The multiplexers are connected for transmission via standard inter- faces at 2, 8, 34, or 140 Mbps to separate line terminal equipment or to a higher-order multiplexer as shown in Figure 4.24. The line interfaces of the line terminals for copper cable, optical fiber, and radio transmission are manufacturer specific SO the vendor has to be the same at both ends. 4.6.3.2 North American PDH for Higher-Order Multiplexing The North American PDH is shown in Figure 4.25. Higher-order rates are DS1C (3.152 Mbps), DS2 (6.132 Mbps), DS3 (44.736 Mbps), and DS4 (274.176 Mbps) [5]. The higher-level multiplexers are named in such a way that we know the DS levels, which are being combined. For example, M13 in Figure 4.25 has inputs from level DS1 and outputs at level DS3. Introduction to Telecommunications Network Engineering 3,152 Kbps Higher-order multiplexers build up higher data rates for optical or microwave radio transmission. 6,132 Kbps 1,544 Kbps 44,736 Kbps 3 M13 274.176 Mbps Above 1.5 Mbps, justification (stuffing) is done 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 4.25 North American PDH. As we can see in Figure 4.25 the higher-order bit rate for each multi- plexer is a little bit higher than the sum of the tributary data rates. The aggre- gate data stream at each level contains, in addition to tributary signals, framing information and the stuffing bits that are used to justify tributary data rates, which may have slightly different data rates, into the higher-order frame. In the demultiplexer these stuffing bits are stripped off and the origi- nal tributary rate is produced. 4.6.4 SDH and SONET The PDH higher-order systems were standardized more than 20 years ago. By the end of the 1980s, a lot of optical fiber cable had been installed and analog networks upgraded into digital networks. Then researchers realized