nformation model but also its application to functions such as performance monitoring, multiplexer section protection, and SO on. Not only is it necessary to have a standardized information model SO that the network element and element manager can understand one another; it is also necessary to have an agreed on message set to go with it. Fortunately, this flows reasonably easily from the defini- tion of the objects themselves. However, the existence of such a message set then leads to the requirement for an agreed on information protocol stack with which to transport it. For information transferred from a network element direct to its element manager, ITU-T G.773 details several allowed protocol stacks that split into two groups: those that have a full seven-layer structure and those that have a "short stack," where layers 4, 5, and 6 are absent. Most observers favor the use of the heavier, but more flexible, seven-layer protocol stacks for SDH networks. Not only are there defined protocol stacks for information transfer direct from element manager to network element, but there is also a defined protocol stack for information transfer between individual network elements. In general, the majority of information trans- fer between network elements is actually information arising from an element manager, which is being relayed by an intermediate network element to a more remote element (see Figure 10.23). Network Site C Information relayed from Elements manager to remote elements using D4-D12 bytes of SOH (D4-D12) (D4-D12) plus G.784 protocol stack Plesiochronous STM-N Interfaces Site B Interfaces (D4-D12) (D4-D12) Information transfer between Site E element and manager using Site A G.773 protocol stacks Network element acts as the gateway To Existing between the Element Manager Element Alarm- at site A and the remote network Manager Monitoring elements at sites B, C, D, and E Schemes To Network Layer Controller Workstation Site A Fig. 10.23 Control channels and protocols within an SDH network. Chapter 10 Synchrono