er order Table 21.2 American and European digital hierarchy multiplexer empties the buffer at the new internal rate. Order Number of telephone Total bit rate 21.4 The T carrier framing and coding channels at 64khit/s (Mbit/s) formats Europe Europe Europe 21.4.1 0.064 0.064 The superframe format 2.048 1.544 The T carrier was designed to carry 24 independent digitised voice DS1-C 8.448 3.152 channels, with each channel encoded as a 64kbit/s data stream. One of the earliest framing formats was the D4 framing pattern. This T1 34.368 6.312 frame format evolved principally to carry voice streams, and data to 139.264 44.736 be transmitted over a T carrier system must conform to this format. 565.148 274.176 The frame consists of 193 bits, with the last bit always being a framing bit. The first 192 bits correspond to 24 conversations, or channels, sampled with PCM type methods and generating 8-bit The European telephone system is based on 30-channel blocks words. The combined signal is word interleaved, providing one and uses transmission rates of 2.048Mbit/s to carry 30 channels (1st frame. order), 8.448Mbit/s to carry 120 channels (2nd order), 34.368Mbit/s A superframe (CCITT calls it a multiframe) is a repeating se- for 480 channels 3rd order), 139.264Mbit/s for 1920 channels (4th quence of 12 such frames and thus contains 12 framing/signalling order), and 565.148Mbit/s for 7680 channels (5th order). bits. In order to keep track of the frame structure, at least 1 bit in 15 Figure 21.4 illustrates the coupling of the hierarchical systems. bits of the combined stream (information plus signalling) must be a The high order services are structured around a point to point 1, and at least 3 bits in 24 bits of the stream must be 1s. The plesiochronous sectioned concept. This concept has a layered ap- bandwidth used on voice frequency (VF) signalling is minimised by proach in which the 2Mbit data stream is supported by a 8Mbit putting signalling information only in the LSB in the sixth and server network. In turn this is suppor