 because it is a single-wavelength solution. SDH/SONET is a TDM scheme, SO it does not take advantage of WDM and the resident ability to carry multiple wavelengths (hun- dreds today and thousands in the future). As a result, operators have to manage two layers today-SDH/SONET and WDM-and riding on top of those, both Layer 2 and Layer 3 networks. To enable a network to request additional capacity to accommodate changes with very large traffic flows, two approaches are emerging: The overlay model-The overlay model, shown in Figure 11.12, calls for maintaining two discreet and separately provisioned networks: a Layer 1 optical network and the client network. In this model, devices reside at the edge of the network cloud, and core network infrastructure is hidden from the routed network. The overlay model relies on the User-to-Network Routed Network Circuit-Switched Optical Core Infrastructure Signaling Figure 11.12 The overlay model The Optical Core: Overlay Versus Peer-to-Peer Networking Models Interface (UNI) for signaling at the edge. The ITU and ANSI support this model. The traditional voice carriers are driving the development of a pro- tocol that will transport all types of traffic: nonvoice over SDH/SONET as well as its high-speed successor, the Optical Transport Network (OTN). The OTN would operate across a new standard, the Automatic Switched Optical Network (ASON). (The OTN and ASON are described later in this chapter.) The peer-to-peer model-The peer-to-peer model, shown in Figure 11.13, argues for a single network in which the equipment on the network edge decides how bandwidth is allocated within the network's core. The network is fully visible to all devices, but edge devices need to know about only their closest optical switches. Routers and optical switches are peers in this model. This approach is promoted by the data community, including the IETF and the IEEE. This IP-centric approach is based on Generalized Mul- tiprotocol Label Switching (GMPLS). GigE and 10GigE are promoted as solutions for the 