the traditional TDM hierarchy is described as DS levels 0 through 4. Europe simply uses The transmission of multiple telephone calls is not the only require- digits to indicate the digital order. These digital streams, produced ment for a T1 or E1 network. The network also has to transfer by multiplexing equipment, are by design independent of the target signalling information. Signalling information is auxiliary informa- transmission medium. In fact, in an end to end circuit, many differ- tion (digits, commands, acknowledgement signals, etc.) sent from ent types of media may be encountered. exchange to exchange to control switching and management oper- The OkHz to 4kHz nominal voice band channels are first con- ation of the network. verted to digital information by PCM techniques and then stacked 21/6 Time division multiplexing (multiplexed) onto higher bit streams. Each of the individual di- and often far away, and whose clocks have neighbouring frequen- gitised 64kbit/s channels is referred to as DSO levels (USA) or 0 cies (plesiochronous tributaries) or, in the best case, equal (syn- order systems (Europe). Table 21.2 summarises the American and chronous tributaries), but whose relative phases can certainly be European digital hierarchies. In the USA 24 voice band analog anything and even vary, because the lines have different propagation channels are combined or multiplexed to form a DS1 signal delays which further depend on temperature. (1.544Mbit/s), also called a digroup (for digital group). The rest of The multiplexing of the four tributaries requires perfect isochron- the digital hierarchy uses 3.152Mbit/s for 48 channels (DS1-C), ism between the bits. It is therefore necessary to bring them all to 6.312Mbit/s for 96 channels (DS2), 44.736Mbit/s for 672 channels exactly the same rate. This is generally done with the aid of retime (DS3), and 174.176Mbit/s for 4032 channels (DS4). buffers, capable of storing an entire frame. The tributary writes the frame in the buffer at its own data rate, while the high