d against this reference model. Figure 2-56 depicts an example of O-UNI and O-NNI in a private optical network environment.19 The ODSI, OIF, and other stakeholders have defined an open O-UNI that enables user devices to request optical services from the network (see Fig- ure 2-57). This model is called an overlay model. The end-user device sets up static lightpaths (circuits) over the network. User protocols and network protocols are considered distinct; this hides the service provider's optical topology from the customer. ODSI completed the specification and its vali- Optics Primer Figure 2-55 Enterprise Collapsed view of Intranet U.S. carriers' metro/regional/long- haul network. IP Control Plane Optical Control Plane (such as Route Selection) Optical Optical Optical Layer Routing and Control: Lightpath Provisioning, Restoration Intelligent Optical Lightpath Switched Network Optical Switch dation testing. The work was then submitted to official standards groups. In the OIF, final issues were being resolved at the end of 2001, with testing to follow. ANSI T1X1 and the ITU-T are in the requirements phase of this UNI work. Another set of activities is underway in the IETF, which anticipates a peer-to-peer model. In this scenario, the network acts as a single collection of devices that exchange topology information, routing information, and SO on (for example, using Open Shortest Path First [OSPF], Resource Reser- vation Protocol [RSVP], or CR-PPD). This model is supported by the GMPLS architecture and protocols. It is rather unlikely that U.S. carriers will subscribe to this kind of external control in the near future because this may well frustrate their traffic engineering, provisioning, security/access- control, and perhaps even billing efforts. In this scenario, there is a single protocol run by both user routers and the network's optical switch for cir- cuit setup and management. This model derives directly from the MPLS model and its predecessors, Cisco's Tag Switching and Ipsilon's Flow Switching. Besides th