The concept of pointers

Date: 2015-10-07; view: 356.

The solution adopted by SDH is to associate a pointer with each VC so that when it is multiplexed, along with others, into a larger VC, its phase offset in bytes can he identified relative to some reference point in this larger VC. (See Figure 42.7.) Furthermore, there is also a mechanism for allowing the value of this pointer to change if, for some reason, there is a loss of synchronisation, and the smaller capacity VC is running either slightly slower, or slightly faster than the larger VC. In fact, each of the smaller capacity VCs has its own pointer, which can change independently of any of the others. Although the use of these pointers still entails some input buffers, these are very much smaller than would be required if there were no mechanism for changing the phase of a small capacity VC within a larger one, hence the problem of excessive delays can be contained. We now have a picture of an SDH network as one in which the majority of VCs, both large and small, are well synchronised to each other, but, at the same time, there are a few which are not so well synchronised, and every so often, the increasing strain of their asynchronism has to be relieved by a byte sized slip relative to the majority of the other VCs in the network. Nevertheless, we retain our ability to locale the management and control information in the VCs POH bytes, because the pointer value associated with VC is recalculated whenever a slip occurs.

The pointer mechanism described above is at the very heart of the SDH standard. It is this mechanism that enables us to construct networks that are nearly, hut not completely synchronous, and yet still allows us to easily locate each traffic channel (VC), together with its associated management and control information i.e. POH, but without incurring large penalties in transmission delay. It could be argued that SDH networks are not really synchronous at all, but are actually very tightly controlled asynchronous networks. However, the fact that we have quantised the slips due to this asynchronism means that it is now possible, at any time, to locate and route any of the traffic paths within an SDH network. This, together with network management software, gives us the traffic routing flexibility that was very difficult to achieve using PDH based equipment. In terms of actual network hardware, it opens the way to the production of economically viable drop and insert multiplexers and cross connects.

Figure 42.5Difference in synchronisation between PDH and SDH: (a) PDH; (b) SDH

Figure 42.6General representation of a 3 layer synchronous multiplexer structure

Figure 42.7Pointers allow small VC to have arbitrary phase with respect to large VC

Exerise 1. Learn the following technical words and word-combinations:

42.1

42.2

42.3

Exercise 2 Read the text 42.1- 42.3

Exercise 3 Find the Russian equivalents for the following English technical word-combinations: