s industry are asyn- chronous (not timed), isochronous (constant delay in the timing), and synchronous (timed). SONET is plesiochronous in spite of its name (Syn- chronous Optical Network) because the communicating devices in the network rely on multiple timing sources and are, therefore, allowed to drift slightly relative to each other. This is fine, because SONET has the ability to handle this with its pointer adjustment capabilities. The devices in a SONET network have the luxury of choosing from any of five timing schemes to ensure accuracy of the network. As long as the schemes have Stratum 4 accuracy or better, they are perfectly accept- able timing sources. The five are discussed later. Line Timing: Devices in the network derive their timing signal from the arriving input signal from another SONET device. For example, an add-drop multiplexer that sits out on a customer's premises derives its synchronization pulse from the incoming bit stream, and might provide further timing to a piece of CPE that is out beyond the ADM. Loop Timing: Loop timing is somewhat similar to Line Timing; in loop timing, the device at the end of the loop is most likely a terminal multiplexer. External Timing: The device has the luxury of deriving its timing signal directly from a Stratum 1 clock source. Through-Timing: Similar to line timing, a device that is through- timed receives its synchronization signal from the incoming bit stream, but then forwards that timing signal on to other devices in the network. The timing signal then passes "through" the intermediate device. Free Running: In free running timing systems, the SONET equipment in question does not have access to an external timing signal, and must derive its timing from internal sources only. One final point about SONET should be made. When the standard is deployed over ring topologies, two timing techniques are used. Either external timing sources are depended upon to time network elements, or one device on the ring is internally timed (free running) while all the oth-