 section seeks to answer the following question: how do the packet or MPLS networks request services from the time slot or lambda- switched optical transport network? There are two main possibilities, with others acting as hybrids between the two: the peer model and the overlay model. 8.7.1 Models Peer Model In the peer model shown in Figure 8-10, GMPLS is used to set up the whole path, including both the packet and transport elements, and the client's requests are spliced into the requests needed to set up the internal tunnel. Although this has the advantage of providing a single man- agement layer to the service provider, there are some significant consider- ations to make: The service provider may want to keep topology information of the optical core private. Much of the equipment available for the optical core today uses proprietary signaling rather than GMPLS. Service providers may want to keep the paths set in the transport core as stable as possible and managed on a overall network basis for protection and restoration purposes. Thus, they would not want the service requests of the packet network to be fulfilled automatically. Overlay Model In the overlay model (see Figure 8-11), the optical and packet networks are managed independently and may potentially be sig- naled by different protocols. One solution is for the Optical Transport GMPLS Figure 8-10 GMPLS peer model. Router Router Chapter 8 MPLS/ MPLS/ Figure 8-11 Other GMPLS/ Other Optical protocol Proprietary Signaling protocol Internetworking Forum (OIF) UNI Control channel overlay model. (Out-of-Band) UNI-C UNI-C UNI-N UNI-N UNI LMP UNI LMP Network (OTN) to provide a UNI that enables client network devices to request connections across it dynamically. A signaling protocol needs to be defined to bridge the UNI (see Fig- ure 8-12). An internal tunnel between endpoint UNI-N nodes is set up using either GMPLS or some proprietary signaling mechanism. This tunnel can be pro- visioned ahead of time or on demand in response to the client request. The data pa