re, a basic channel capacity is 56 Kbps. Figure 4.23 The 1.544-Mbps PCM frame. Introduction to Telecommunications Network Engineering In transatlantic connections, E1 frames are adapted to the T1 frame structure and transcoding between u-law and A-law PCM is carried out. Each time slot in E1 is transmitted further in one time slot of T1. 4.6.3 Plesiochronous Transmission Hierarchy A primary rate of 1.5 or 2 Mbps is usually too slow for transmission in trunk or even in local networks. This was noticed in the early 1970s and the ITU-T, at that time CCITT, standardized the higher data rate systems for transmission in the latter half of the 1970s. The digital systems of those days carried primarily analog information and end-to-end synchronization was rarely required. The first standardized digital higher-order transmission hier- archy is known as plesiochronous digital hierarchy (PDH). We review first the European hierarchy of higher-order multiplexing. 4.6.3.1 European PDH for Higher-Order Multiplexing The higher-order multiplexers of PDH are allowed to operate according to their own independent clock frequencies. These standards are based on ple- siochronous operation ("almost the same data rate"), which allows a small frequency difference between tributary signals that are multiplexed into a higher aggregate rate. For example, at 2,048 Kbps the frequency tolerance was standardized at 50 ppm, and at 8,448 Kbps the allowed tolerance is 20 ppm. This means that, for example, the data rate of a 2,048-Kbps system may deviate by 100 bps. The basic principle of the European standard for higher-order multi- plexers is that each multiplexer stage takes four signals of a lower data rate and packs them together into a signal at a data rate that is a little bit over four times as high, as shown in Figure 4.24. In addition to tributaries, aggregate frames contain frame alignment information and justification information. The tributary frequencies may differ slightly and their frequencies must be justified to the higher-order f