e mul- tiplexing process occurs as you move within the different transmission levels. It also affords very robust operations capabilities, such as service restoration. The SDH/SONET specifications define a frame format-that is, how the bits are packaged together to be transported over the fiber. As mentioned earlier, they define the nature of the physical interfaces (e.g., couplers, light sources). They define the optical carrier line rates, or transmission levels, and they define the sorts of messages that are exchanged in order to support operations, administration, maintenance, and provisioning. An important aspect of SDH/SONET is that it introduced the notion of a ring topology to address network survivability by providing rapid restoration. SDH/ SONET uses a dual-counter-rotating ring. Imagine that you have four network nodes. As shown in Figure 4.8, with a dual-counter-rotating ring, you link each of these four network nodes together by using one pair of fibers; that pair of fibers becomes the primary fiber, and information flows over it in a clockwise manner. You run another pair of fibers, which may actually be housed in the same cable as the first pair of fibers, to join the four nodes. The second pair of fibers becomes the protect fiber, which is designed to carry information in a counterclockwise manner. In theory, if a cable is cut between node A and node B, you can still move a message from A to B by reversing the information flow and going from A to D to C to B. This enables you to recover almost immediately-within 50 milliseconds-from outages that occur. The survivability of an SDH/SONET ring depends on how the cables are engi- neered; if both cables share a common housing, then it's likely that a cable cut by a construction crew will stop operating in both directions. Similarly, if a major earth- quake hits and all the streets are broken up, a counter-rotating ring will not neces- sarily ensure survivability, but for smaller-scale problems, it can very adequately handle a backup. This is one of th