The metro area network, also known as the aggregation network, is architecturally defined by its position and function within a hierarchical network model. Its primary role is to aggregate traffic from multiple access networks (which directly connect to end-users and businesses) and transport this traffic to the high-capacity core network for long-haul transmission. This intermediate position between the access and core domains is the most precise and fundamental characteristic of a metro network from a network design and engineering perspective.

A. This is a descriptive but less precise definition. The access network is more directly responsible for providing connections to customers; the metro network aggregates these access points.

C. This statement is factually incorrect. Modern metro networks utilize a diverse range of technologies, including pure photonic nodes like Reconfigurable Optical Add-Drop Multiplexers (ROADMs), not just OCS/SWDM.

D. This is incorrect as it falsely restricts the technology to only photonic nodes. Metro networks often include electrical/packet-based switching (OCS/SWDM) and its architectural description is imprecise.

1. Nokia White Paper: "The metro network reimagined" (2021).

Reference: Page 3, Section "The metro network defined".

Quote: "The metro network connects users and content in a metropolitan area. It aggregates all traffic from the mobile, residential and business access networks and directs it to the metro/regional core, where it is either switched locally or forwarded to the backbone core for long-haul transport." This text explicitly defines the metro network by its function of aggregating traffic from access networks and connecting to the core, which directly supports its position "between access and core domains."

2. Nokia Data Sheet: "Nokia 1830 Photonic Service Switch (PSS) portfolio" (2022).

Reference: Page 2, Section "Addressing the full spectrum of optical networking needs".

Content: The document describes how the 1830 PSS family has platforms optimized for different network segments, including access, metro, and core. It consistently presents the metro network as the intermediate layer that connects the access edge to the long-haul core, reinforcing the architectural hierarchy.

3. Mukherjee, B. (2006). Optical WDM Networks. Springer.

Reference: Chapter 1, Section 1.2, "Network Segments: Long-Haul, Metro, and Access".

Content: This foundational academic text on optical networking describes the network architecture in segments. It defines the metropolitan-area network (MAN) as the segment that bridges the gap between the high-capacity long-haul (core) backbone and the lower-speed, end-user-facing access networks. This aligns perfectly with the definition of being located between the access and core domains.