The digital hierarchy levels

A range of digital systems with increasing capacity has been defined, the systems of each order being composed of four systems of the immediately lower order. In America the traditional TDM hierarchy is described as DS levels 0 through 4. Europe simply uses digits to indicate the digital order. These digital streams, produced by multiplexing equipment, are by design independent of the target transmission medium. In fact, in an end to end circuit, many differ­ent types of media may be encountered.

The 0kHz to 4kHz nominal voice band channels are first converted to digital information by PCM techniques and then stacked (multiplexed) onto higher bit streams. Each of the individual di­gitised 64kbit/s channels is referred to as DSO levels (USA) or 0 order systems (Europe). Table 21.2 summarises the American and European digital hierarchies. In the USA 24 voice band analog channels are combined or multiplexed to form a DS1 signal (1.544Mbit/s), also called a digroup (for digital group). The rest of the digital hierarchy uses 3.152Mbit/s for 48 channels (DS1-C), 6.312Mbit/s for 96 channels (DS2), 44.736Mbit/s for 672 channels (DS3), and 274.176Mbit/s for 4032 channels (DS4).

Table 21.2 American and European digital hierarchy

The European telephone system is based on 30-channel blocks and uses transmission rates of 2.048Mbit/s to carry 30 channels (1st order), 8.448Mbit/s to carry 120 channels (2nd order), 34.368Mbit/s for 480 channels (3rd order), 139.264Mbit/s for 1920 channels (4th order), and 565.148Mhit/s for 7680 channels (5th order).

Figure 21.4 illustrates the coupling of the hierarchical systems. The high order services are structured around a point to point plesiochronous sectioned concept. This concept has a layered ap­proach in which the 2Mbit data stream is supported by a 8Mbit server network. In turn this is supported by a 34Mbit server network which is itself supported by a 140Mbit server network. Starting from the second order, the equipment no longer contains an analog to digital converter and only deals with digital frames. The equip­ment essentially consists of multiplexers, combining into new frames the frames of four systems of the preceding order, called tributaries.

During the construction of an nth order multiplexer starting from frames with order n-1, we are confronted with the problem of anisochronism of the tributaries. In effect, the frames to be grouped come from different equipments which are geographically distinct

and often far away, and whose clocks have neighbouring frequen­cies (plesiochronous tributaries) or, in the best case, equal (syn­chronous tributaries), but whose relative phases can certainly be anything and even vary, because the lines have different propagation delays which further depend on temperature.

The multiplexing of the four tributaries requires perfect isochronism between the bits. It is therefore necessary to bring them all to exactly the same rate. This is generally done with the aid of retime buffers, capable of storing an entire frame. The tributary writes the frame in the buffer at its own data rate, while the higher order multiplexer empties the buffer at the new internal rate.

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