In optical network topology, the "degree" of a node refers to the number of transmission links or directions connecting it to other nodes in the network. A degree-1 node, by definition, has only a single link. This means it can only send and receive traffic from one direction, making it an endpoint or a terminal in the network topology. Such nodes are often found at the end of a spur off a main ring or mesh structure, serving as a point of traffic origination or termination.

B. The number of express channels is an internal architectural detail of a ROADM, defining how many wavelengths can pass through the node untouched, and is independent of the node's external connectivity or degree.

C. A node with east and west sides has two connections, making it a degree-2 node, which is typical for nodes in a linear chain or ring topology.

D. An In-Line-Amplifier (ILA) is a transmission element used to amplify the optical signal along a fiber span. It is a two-port device (input/output) and is considered part of the link, not a topological node with add/drop capabilities.

1. Nokia Optical Networking Certification (ONC) Program Courseware: The Nokia 4A0-205 course materials define node degree as the number of fiber-pair links connected to a Photonic Service Switch (PSS) site. A degree-1 node is explicitly described as a terminal site with a single connection to the network. (Reference: Nokia Optical Networking Fundamentals Student Guide, Module 3: WDM Topologies and Architectures).

2. Vendor Documentation: The Nokia 1830 PSS Engineering and Planning Guide discusses network topologies and node configurations. In sections describing network design, a terminal site is characterized by a single transmission link connecting it to the rest of the network, which corresponds to a degree-1 configuration. (Reference: Nokia 1830 PSS Engineering and Planning Guide, Release 11.1, Chapter 2: Network Topologies).

3. Academic Publication: In the context of WDM network design, node degree is a fundamental concept from graph theory. A research paper on optical network design states, "The degree of a node is the number of links incident to it. A node with degree one is a terminal node..." This definition is standard in the field. (Reference: "A Survey of Research on WDM Network Design and Planning," International Journal of Network Management, Vol. 17, Issue 2, pp. 133-153, March/April 2007. DOI: https://doi.org/10.1002/nem.627).