or each PDH order are shown in Table 3.3. Thus a summary of the PDH rates can now be depicted diagrammatically (Figure 3.6) to show a clear picture of the various hierarchies in different regions. Major disadvantages faced by the PDH are: 1. Pulse stuffing or positive justification. As the name suggests, plesiochronous systems are not perfectly synchronous. Hence the input data stream rates are very close but not identical SO the pulses do not arrive in a synchronised manner. In order to multiplex different PDH signals at higher orders, bit stuffing is required to match the rates of the data streams. Pulse stuffing involves intentionally making the output bit rate of a channel higher than the input bit rate SO that the data rate of that particular stream matches with the other tributaries and is thus de-multiplexed/received without any errors. Table 3.3 PDH for European systems Level/order Number of channels Bit rate [Mbps] 2.048 8.448 34.368 139.264 565.992 Digital Hierarchies - PDH and SDH JAPANESE STANDARD NORTH AMERICAN STANDARD EUROPEAN STANDARD 397200 kbps 564992 kbps 97728 kbps 397200 kbps 139264 kbps 32064 kbps 44736 kbps 34368 kbps 6312 kbps 8448 kbps Primary Rate 1544 kbps 2048 kbps 64 kbps Figure 3.6 Summary of plesiochronous transfer rates 2. Add/drop facility not available. Whenever any signal is needed to be extracted from a higher order multiplexed signal, it cannot be just dropped from the signal. Instead the signal will be de-multiplexed till the needed signal is separated. The same applies when adding a signal to an already multiplexed signal. Thus no adding and dropping of signals can be done, which is a very essential part of a multiplexed system, as will be seen in the next section. 3.3.2 Synchronous Digital Hierarchy (SDH) The data rates in the plesiochronous digital hierarchy went up to as high as 140 Mbps approximately. That might seem to be satisfactory but with the advent of the optical fibre channel capacity went up to tens of gigabits and the PDH for multiplexing was not able to satisf