 rate of 64 kbps. Only half of these bits are required for synchronization; the remainder, 32 kbps, is available for on-line error monitoring, for a data channel for remote alarms. These remote alarms tell the system operator about the status of the distant PCM terminal. HIGHER-ORDER PCM MULTIPLEX SYSTEMS 6.8.1 Introduction Higher-order PCM multiplex is developed out of several primary multiplex sources. Pri- mary multiplex is typically DS1 in North America and E1 in Europe; some countries have standardized on E1, such as most of Hispanic America. Not only are E1 and DS1 incompatible, the higher-order multiplexes, as one might imagine, are also incompatible. First we introduce stuffing, describe some North American higher-level multiplex, and then discuss European multiplexes based on the E1 system. 6.8.2 Stuffing and Justification Stuffing (justification) is common to all higher-level multiplexers that we describe in the following. Consider the DS2 higher-level multiplex. It derives from an M12 multiplexer, taking inputs from four 24-channel channel banks. The clocks in these channel banks are 6.8 HIGHER-ORDER PCM MULTIPLEX SYSTEMS free running. The transmission rate output of each channel bank is nominally 1,544,000 bps. However, there is a tolerance of +50 ppm (+77 bps). Suppose all four DS1 inputs were operating on the high side of the tolerance or at 1,544,077 bps. The input to the M12 multiplexer is a buffer. It has a finite capacity. Unless bits are read out of the buffer faster than they are coming in, at some time the buffer will overflow. This is highly undesirable. Thus we have bit stuffing. Stuffing in the output aggregate bit stream means adding extra bits. It allows us to read out of a buffer faster than we write into it. In Ref. 1 the IEEE defines stuffing bits as "bits inserted into a frame to compensate for timing differences in constituent lower rate signals." CCITT uses the term justification. Figure 6.12 illustrates the stuffing concept. 6.8.3 North American Higher-Level Multiplex The North Am