s is a different concept with respect to the GLSP. A GLSP consists of a sequence of consecutive labels, which, when swapped in a specific order, Telecommunication Networks FIGURE 8.9 Different types of ports in the overlay of an OXC and an LSP router. (a) Shows a locally dropped GLSP, and (b) a pass-through GLSP. ASC indicates a wavelength switch capable port. carries data from one point in a label switched network to another. In short, a GLSP is rep- resented by the distributed state needed to send data along a specific route. Because GLSPs might differ in link composition, an entity requesting labels for a GLSP needs to specify three major parameters: switching type, encoding type, and generalized payload-ID (G-PID). Switching type defines how an interface switches data. Because this is always expected to be known, a switching type needs only be specified for an interface with multiple switching capabilities. Encoding type is needed to specify the specific encoding of the data associated with the GLSP. For example, data associated with an L2SC interface might be encoded as Ethernet. The G-PID finally defines the client layer of the GLSP. In GMPLS, bidirectional GLSPs are considered the default. Bidirectional GLSPs are established through simultaneous label dis- tribution in both directions. Since generalized labels are nonhierarchical, they do not stack. This is because some supported switching media cannot stack. Given an optical link, for example, it is not possible to encapsulate a wavelength in another and then get it back again. In GMPLS, tunneling data through different layers is therefore based on GLSP nesting (i.e., encapsulating GLSPs within GLSPs). Nesting of GLSPs can be realized either within a single network layer or nesting GLSPs of a layer through a GLSP of another layer. Examples of the nesting system are reported in Figure 8.10. L Fiber = (L; L12; L13; L14) L14= (L1; L2; L3; L4) Fiber Fiber tunnels FIGURE 8.10 Examples of Generalized Labels nesting: on the left (a) labels relative to different 