nous digital hierarchy (SDH). The first level of multiplexing can be looked at in more detail along with the construction of its frame. Famously known as T1 in North America and Japan, the digital signal 1 (DS1) consists of 24 basic voice channels (64 kbps) multiplexed together to form a single frame. A frame consists of 24 X 8 = 192 bits, plus one extra bit for framing, making it a total of 193 bits in all in every 125 us. The 193rd bit is used for frame synchronisation. Thus the data rate of the T1 is calculated as 193 bits /125 us = 1.544 Mbps. This rate is also known as the primary rate. When a T1 system is only for data just 23 frames carry the data/real information and the last frame is used for synchronisation and a signalling pattern. The frame structure of the T1 (DS1) system is shown in Figure 3.5. Details of the higher order PDH systems in North America are shown in Table 3.2. Europe and the rest of the world (excluding North America and Japan) use the PDH hierarchy in which 30 basic voice channels (64 kbps) are multiplexed together to form a primary system (first order of multiplexing). Four primary systems are combined/multiplexed to form an output having 120 channels (second order of multiplexing). Similarly, four 120-channel systems can be multiplexed to give an output of 480 channels (third order of multiplexing). Four 480-channel systems are multiplexed to give an output of 1920 channels (fourth order of multiplexing) and four 1920-channel systems are multiplexed to give Transmission Network Planning and Optimisation Table 3.2 PDH for North American systems Level/order Number of channels Bit rate [Mbps] 1.544 6.312 44.736 274.176 193 Bit frame (125 usec) channel 1 channel 2 channel 3 channel 24 Bit 1 is a 7 Data Bits per Bit 8 is for signalling Framing code channel per sample Figure 3.5 The T1 carrier frame structure an output of 7680 channels (fifth order of multiplexing). All levels beyond the fifth level are now levels of SDH systems and are taken care of by their multiplexers. The bit rates f