network rather than any interface presented to a network user. A User Network Interface (UNI) has also been defined. Finally, the reason for the name "virtual container" is that unlike the STM signals that appear at the NNI, VC signals are never presented to the outside world. They exist only within pieces of SDH equipment or within STM signals; hence an SDH network element can have NNI and PDH interfaces but never VC interfaces. Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy Level 1 Payload Pointer (e.g., VC-12) Level 2 Payload Pointer (e.g., VC-4) Level 3 Payload (e.g., STM-1) Fig. 10.4 General representation of a three-layer synchronous multiplexer structure. Optical Line Terminal Fiber Proprietary Interface Nonstandard Line Code and Standard Electrical Optical Levels (G.703) Interface Standard Fiber Interface Standard Network Node Interface Optics Signal, at Standard Optical Levels Functional integration of MUX and LTE has made G.703 electrical interface superfluous Fig. 10.5 Comparison of interfaces needed in PDH and SDH environments: (a) current PDH; (b) SDH. The Basis of SONET and SDH Returning to the question of exactly what is synchronized to what, the basic problem is that it is very difficult to maintain a complete transmission network in rigid synchroniza- tion for all time. Even if we could tolerate the delays introduced by the addition of the numerous "wander buffers" necessary to accommodate the slow changes in transmission medium delay, there is no guarantee that different PTOs' networks would all be synchro- nized to the same master clock. For a network based on the SDH, this problem translates to that of how to synchronously multiplex and de-multiplex many individual VCs, which, because they have been created in disparate parts of the same, or even different, SDH net- works, might have slightly different short-term bitrates. The Concept of Pointers The solution adopted by SDH is to associate a "pointer" with each VC SO that when the VC is multiplexed, along with others