 for both SDH/SONET and OTN networks. ANSI's GFP T1X1.5a is an SDH/SONET subprotocol that provides a standard means for packing voice traffic into an SDH/SONET frame, enabling equipment interoperability. Clients connect to the network through one of three types of network interfaces: User-to-Network Interface (UNI)-The UNI defines how customers can access their providers' networks. It is used to request capacity from the underlying network and shields the client from network complexities. Min- imal information is provided; the UNI provides only the name and address of the endpoint, the authentication and admission control of the client, and connect service messages. External Network-to-Network Interface (E NNI)-The E NNI provides summarized network address information along with authentication and admission control and connection service messages. The partner does not know the exact paths, but it does know the available clients that can be called. This is analogous to an electronic address book provided by one carrier to the next of all the available nodes. Internal Network-to-Network Interface NNI)-The I NNI enables devices to get topology or routing information for the carriers' network, as well as connection service messages and information necessary to option- ally control network resources. Chapter 11 Optical Networking Clients can request three types of circuits in ASTN. First, provisioned circuits are hard or permanent circuits much like a leased line, also known as a permanent connection. Second, signaled circuits are established dynamically by the endpoint requesting bandwidth. This requires network addressing information and is known as a switched connection. Finally, hybrid circuits are a cross between provisioned and signaled circuits. They have provisioned connections into the ASTN network but rely on switched connections within the ASTN network to connect with other nodes. They are also known as soft provisioned connections. The Ethernet Evolution Compared to other optical networks, Ethernet is inex