near to mid-term, with the long term possibly focusing on Resilient Packet Ring. The following sections describe these two approaches to the optical core in more detail. Single Wavelengths IP Router Optical Circuit IP Router Layer 2/3 Ethernet Switch Switch Figure 11.13 The peer-to-peer model Chapter 11 Optical Networking The Overlay Model The overlay model, formally known as the OTN and promoted by the ITU and car- rier communities, is characterized by devices residing on the edge of the cloud, the core network infrastructure being hidden from the routed network, and relying on UNI for signaling at the edge. The ITU's next generation of optical networks is specified under a number of recommendations, including G.872, G.709, and G.959.1. The OTN defines a new network hierarchy, the optical transport hierarchy, whose base unit is the optical transport module (OTM), which has a 64-byte frame. The OTN comprises all optical elements. The initial clients of the OTN are SDH/SONET and data services, including Ethernet, IP, ATM, and Fibre Channel. Management capabilities are defined for con- ductivity verification, performance assessment, and fault sectionalization. The OTN has three main elements: Optical channel-The optical channel is similar to SDH/SONET's path component. It defines the logical connection between stations or end-to- end networking, and it transports clients' signals between two endpoints on the OTN. Optical multiplex section-The optical multiplex section, which is similar to SDH/SONET's line component, describes the WDM portions that under- pin the optical channels and consists of many aggregated optical channels. Optical transmission section-The optical transmission section is similar to SDH/SONET's section component. It defines a physical interface that details optical parameters such as frequency, power level, and signal-to- noise ratio. The OTN reaches higher speeds by bundling wavelengths. OTMs, which can span multiple wavelengths of different capacities, are described with two suffixes: n refers