ers fail. This is an important feature if an NG-SDH transport network is used to interconnect IP routers. IP routers detect link failures by the absence of a number of periodic "hello" messages that are sent between adjacent routers; the failed link is adver- tised to the rest of the IP network, resulting in all routers updating their routing tables. This is an undesirable action, and the graceful reduction in bandwidth offered by LCAS is a desirable feature. Generic Framing Procedure Methods of transporting data over SDH networks can be achieved using a variety of methods, and a number of proprietary techniques have appeared on the market. For this reason, the ITU-T defined a Generic Framing Procedure (GFP) to provide a uniform and standardized method of encapsulating multiple client traffic types onto synchronous trans- mission channels. Compared to existing techniques such as Packet over SDH (POS) or Link Access Protocol SDH (LAPS), GFP supports more client signals, transports them more effi- ciently, and is capable of scaling to very high bitrates, such as 40 Gbit/s. GFP has seen widespread use also in Resilient Packet Rings (RPRs) and Optical Transport Networks (OTNs) for mapping data onto synchronous optical data units (ODUs), and has even been adapted for use in the Gigabit Passive Optical Network (GPON) standards. GFP Overview The GFP, defined in ITU-T G.7041, is a mechanism for mapping constant- and variable- bitrate data into synchronous SDH/SONET envelopes. GFP supports many types of proto- cols, including those used in Local Area Networks (LANs) and Storage Area Networks (SANs). Functionally, GFP can be divided into common and client-specific aspects, as illus- trated in Figure 11.17. Its common aspects include Protocol Data Unit (PDU) delineation, data link synchronization and scrambling, client PDU multiplexing, and client-independent performance monitoring. The mapping of client PDU into the GFP payload, client-specific performance monitoring, and OA&M are the responsibility of the client-specific 