 pair of core optical switches, there may be multiple data links. It could be both wasteful and confusing to establish a separate in-band sig- naling session within each link. It is more efficient to manage the links as a group using a single out-of-band signaling session. Typically, therefore, an optical switch completely separates its control plane from its data plane, and control connections to other switches go via a lower-performance, nonoptical network. Performance Characteristics One great flexibility of MPLS networks is their capability to define a hierarchy of LSPs over which data is passed. Chapter 7 This enables more transitory, low-bandwidth LSPs, starting and finishing close to the network edge, to use preexisting high-bandwidth LSPs that span the core of a provider network. The requirement for high-bandwidth, long-distance LSPs, coupled with their long lifetime, forms a natural fit with optical technologies. Consequently, LSPs in optical networks are typ- ically long lived and stable. On the other hand, optically switched LSPs can be slow to set up compared with electronically switched LSPs because micromirrors need time to be physically adjusted and to wait for the result- ing movement vibrations to damp away. The stability and slow setup of opti- cal MPLS networks have influenced the development of GMPLS. Bidirectional LSPs Trunks through the core of an optical network are typically bidirectional. GMPLS is needed for them to set up bidirectional LSPs. Management Issues By their nature, the mechanisms that optical switches use to switch traffic flows-such as micromirrors-do not notice if the traffic flow disappears altogether. Without additional hardware and software support, therefore, a break in the core of an optical network might not be detectable until the egress of that network, where the egress LSR tries to convert the signal back to packet form and finds it to be missing. This is known as the loss of light (LoL) problem. Fault localization in an optical network requires that the switches ca