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.

A Shared Risk Group (SRG) is a fundamental concept in network resiliency design. It defines a set of network resources, such as links, nodes, or line cards, that are exposed to a common point of failure. If that single point fails (e.g., a fiber cut in a shared conduit, a power outage in a facility), all resources within the SRG are likely to fail simultaneously. By identifying and configuring SRGs, network management and routing protocols (like RSVP-TE with Fast Reroute) can compute and establish physically diverse primary and backup paths, ensuring that a single failure event does not impact both paths, thereby improving overall network availability.

A. A set of fibers that share the same latency risk

SRGs are defined by a common failure risk, not performance metrics like latency. Two paths can have different latencies but still share a failure risk.

B. A set of boards that share the same failure risk

This is an example of an SRG, but it is not the general definition. An SRG can encompass a wider range of resources, including fibers, conduits, and nodes.

C. A set of nodes that share a common risk of hardware failure

This is also too specific. An SRG can be more granular than a set of nodes, such as multiple links sharing a physical conduit between two nodes.

1. Nokia 7750 SR OS Basic System Configuration Guide, Release 22.10.R1, Document Part Number: 3HE 18112 AAAC TQZZA, Edition 01.

Section: "Shared Risk Groups (SRGs)"

Quote/Paraphrase: The guide defines SRG as a mechanism to group objects that share a common risk. It states, "SRG allows the user to specify that certain LAG ports or PW-SAPs share a common risk and to ensure that the primary and secondary paths for a service do not traverse the same SRG." This confirms that an SRG is a set of resources sharing a common failure risk.

2. Nokia 7750 SR OS MPLS Guide, Release 22.10.R1, Document Part Number: 3HE 18116 AAAC TQZZA, Edition 01.

Section: "Shared Risk Link Group (SRLG)"

Quote/Paraphrase: This guide defines the closely related concept of SRLG: "A Shared Risk Link Group (SRLG) is a set of links that share a common resource, and as a result, a single failure of that resource may cause all links in the set to fail." This directly supports the definition of a group of resources sharing a common failure risk.

Network survivability mechanisms are broadly categorized into protection and restoration. Protection is a proactive strategy where backup resources, such as a secondary path and its required bandwidth, are pre-allocated and reserved at the time the primary service is established. This allows for very fast, often sub-50ms, recovery. In contrast, restoration is a reactive strategy. It initiates the process of finding and allocating backup resources only after a failure has been detected. While restoration is generally slower than protection, it is more resource-efficient because backup capacity is not dedicated and can be used for other purposes until a failure occurs.

A: Incorrectly defines restoration. Restoration is a dynamic recovery process, not a specific pre-configured scheme like 1+1 protection.

B: Incorrectly defines protection as allocating resources upon failure; this is the definition of restoration.

C: Incorrectly swaps the definitions of protection and restoration.

1. Official Vendor Documentation: Nokia 7750 SR and 7950 XRS GMPLS-UNI, RSVP-TE, and OSPF-TE for LSPs Guide, SR OS Release 22.10.R1. In the "LSP Redundancy" section, the guide distinguishes between pre-provisioned backup LSPs for fast reroute (a protection mechanism) and the signaling of secondary paths upon primary path failure (a restoration mechanism). This aligns with the definitions of pre-allocation for protection and allocation-upon-failure for restoration.

2. Peer-reviewed Academic Publication: Ramaswami, R., Sivarajan, K. N., & Sasaki, G. H. (2010). Optical Networks: A Practical Perspective (3rd ed.). Morgan Kaufmann. In Chapter 13, "Protection and Restoration," Section 13.2, the authors state: "In protection schemes, the backup path and resources are reserved for a connection at the time the connection is set up... In restoration schemes, the backup path and resources are discovered dynamically after a failure has occurred." This directly supports the definitions provided in the correct answer.

The NFM-T network map is a primary graphical interface for network monitoring and management. A core feature is its ability to visually represent the health of the network. It displays network elements (nodes) and the physical links between them, using a color-coding system to indicate the severity of the highest active alarm on each component. For instance, a link or node with a critical alarm will appear red. Furthermore, the map is interactive, offering context-sensitive menus (e.g., by right-clicking an object) that allow operators to navigate directly to relevant management functions, such as viewing detailed alarms, properties, or initiating provisioning tasks for that specific element.

A. Grouping of nodes into sub-maps is a manual process performed by the operator for organizational purposes; it is not automatically performed based on the location string attribute.

B. Nodes are grouped topologically or geographically within sub-maps created by the user. Their color represents the alarm status, but the grouping itself is not based on this dynamic state.

D. The visualization of end-to-end services is typically performed in a dedicated Service Management application or view within NFM-T, which is distinct from the primary physical network map.

1. Nokia Network Functions Manager - Transport (NFM-T) R20 User Guide, Chapter 4: "Working with network maps". This chapter details the creation and management of network maps and sub-maps, describing how network elements and links are represented. It explicitly states that the color of an object on the map reflects its alarm status.

2. Nokia Network Functions Manager - Transport (NFM-T) R20 Fault Management Guide, Chapter 2: "Alarm monitoring and management". This document describes how alarms are visualized throughout the NFM-T GUI, including the network map. Section 2.3, "Alarm severity and color coding," confirms that nodes and links change color based on the highest severity alarm present.

3. Nokia Network Functions Manager - Transport (NFM-T) R20 User Guide, Chapter 4, Section: "Navigating from the network map". This section explains that right-clicking on a managed object in the map view opens a context-sensitive menu with options to access other functions like the Alarm List, Equipment View, or provisioning windows for that specific object.

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).

A star topology features a central node to which all other peripheral nodes connect. This design is exceptionally well-suited for access networks. The primary function of an access network is to aggregate traffic from a large number of end-users or subscribers (the "spokes") and concentrate it at a central point (the "hub"), such as a DSLAM, an OLT in a PON, or a mobile base station controller. This traffic is then forwarded to the core network. The centralized management and traffic flow pattern of a star topology directly align with the aggregation requirements of access networks.

B. ASON/GMPLS networks are primarily deployed in mesh topologies to leverage path diversity for dynamic restoration and protection, which a star topology inherently lacks.

C. Backbone networks demand high resiliency and redundancy. They are built using mesh topologies to provide multiple alternative paths, avoiding the single point of failure of a star's central node.

D. SNCP (Sub-Network Connection Protection) requires physically diverse working and protection paths, a feature characteristic of ring or mesh topologies, not a simple star configuration.

1. Official Vendor Documentation: Nokia. (2022). Nokia IP Networks and Services Fundamentals, Course Number 4A0-205, Student Guide. Chapter 2: "Network Fundamentals", Section: "Physical Network Topologies". The guide explains that the star topology is a classic "hub-and-spoke" model, identifying its primary application in access network aggregation where endpoints connect to a central service aggregation point.

2. Academic Publication: P. Goransson, C. Black, T. Culver. (2016). Software Defined Networks: A Comprehensive Approach (2nd ed.). Morgan Kaufmann. Chapter 2, "The Modern Data Center", pp. 35-37. The text describes network topologies, noting that while core networks are meshed for resilience, edge/access networks often use a star or tree (extended star) topology to connect end hosts to a central aggregation switch.

3. University Courseware: MIT OpenCourseWare. (2014). 6.829 Computer Networks, Fall 2002. Lecture 15: "SONET/Optical Networks". The lecture notes discuss network topologies for protection, highlighting that ring and mesh architectures are used for protection schemes like SNCP and BLSR, while star topologies are noted for their lack of inherent redundancy, making them unsuitable for such protection use cases.

Chromatic Dispersion (CD) is a fundamental transmission impairment in optical fibers. It occurs because the refractive index of the silica glass fiber is dependent on the wavelength of the light passing through it. Consequently, different wavelengths (or "colors") of light propagate at different group velocities. For a light pulse composed of multiple wavelengths, this difference in speed causes the pulse to spread out, or disperse, as it travels along the fiber. This pulse broadening can lead to inter-symbol interference (ISI) at the receiver, limiting the transmission distance and bit rate.

A. CD is dependent on the wavelength, not the channel bandwidth. Different channels experience different CD because they operate at different center wavelengths.

B. CD is caused by the wavelength-dependent refractive index of the fiber, a property known as material dispersion, not by changes in fiber attenuation.

D. Fiber attenuation is the reduction of signal power. Inter-channel interference is typically caused by non-linear effects like Four-Wave Mixing (FWM), not attenuation.

1. Nokia, "Nokia Optical Networking Fundamentals, Student Guide," (Course Code: TTP31001-1-1), Module 2: Optical Fiber Transmission, Section 2.3.1: Chromatic Dispersion, pp. 2-15. The guide states, "Chromatic dispersion (CD) is the phenomenon where different wavelengths of light travel at different speeds in an optical fiber... This causes the different spectral components of the pulse to arrive at the end of the fiber at different times."

2. Agrawal, G. P. (2010). Fiber-Optic Communication Systems (4th ed.). Wiley. In Chapter 2, Section 2.4, "Group-Velocity Dispersion," p. 45, it is explained that "the refractive index n(ω) of the fiber material depends on the optical frequency ω... As a result, different spectral components of a pulse travel at slightly different group velocities. This phenomenon is known as group-velocity dispersion (GVD) or simply fiber dispersion." (Note: GVD is the formal term for what is commonly called Chromatic Dispersion in this context).

3. Saleh, B. E. A., & Teich, M. C. (2019). Fundamentals of Photonics (3rd ed.). Wiley. Chapter 9, Section 9.1.B, "Dispersion in Optical Fibers," p. 336. The text describes material dispersion: "Since the refractive index n(λ₀) of a dielectric medium is a function of the wavelength λ₀, the group velocity vg is also a function of λ₀... This results in pulse spreading."

The Network Functions Manager - Transport (NFM-T) is Nokia's management platform for its optical transport network portfolio, including the 1830 PSS family. Its fundamental purpose is to provide the full lifecycle management of the optical network. This includes the initial design and planning assistance, configuration and service provisioning, and ongoing monitoring and assurance. Therefore, the statement that NFM-T is used to design and manage the optical network is the most accurate and comprehensive description of its role among the given options.

A. The term "fully supports... L2 applications" is an overstatement. While NFM-T provisions L2 services over optical gear, comprehensive L2/L3 VPN service management is the domain of the NFM-P.

B. NFM-T is the domain controller for optical transport nodes. IP/MPLS nodes are managed by a different domain controller, the Network Functions Manager – Packet (NFM-P).

D. This describes a primary use case but is not a complete definition of NFM-T's role. NFM-T provisions optical services between various endpoint types, not exclusively IP nodes.

1. Nokia Network Services Platform (NSP) Optical Network Management and Control Solution Sheet: This document explicitly states that the solution enables operators to "simplify and automate the end-to-end process of designing, provisioning and troubleshooting optical services". This directly supports the "design and manage" scope described in the correct answer. (Reference: Nokia Solution Sheet, "NSP Optical Network Management and Control", 2021).

2. Nokia Network Services Platform (NSP) Data Sheet: This data sheet describes the NSP architecture, clearly delineating the roles of the domain controllers. It specifies the "NSP Network Functions Manager–Transport (NFM-T)" for managing the optical domain and the "NSP Network Functions Manager–Packet (NFM-P)" for the IP/MPLS and Ethernet domain. This confirms the separation of functions, invalidating option B and clarifying the scope limitation in option A. (Reference: Nokia Data Sheet, "Network Services Platform (NSP)", Document ID: SR2104021113EN, April 2021, Page 3).

3. Nokia Application Note, "IP-optical coordination with the Nokia Network Services Platform": This document details the use case of provisioning optical paths for IP routers. It states, "The NSP automates the creation of optical transport services that are optimized to meet the needs of IP/MPLS router-to-router connectivity." While this shows that option D describes a valid and important function, it presents it as a specific application of the broader management capabilities, not the sole or defining purpose. (Reference: Nokia Application Note, Document ID: SR1701004121EN, January 2017, Page 2).

In the context of transport networks like OTN and SDH/SONET, a trail is a logical entity that represents the complete, end-to-end transport of a client signal between two access points. Its primary function is to provide a transparent connection, meaning it delivers the client's information without modification to its content. The trail is monitored for integrity to ensure the quality of the transport service. For example, an Optical Channel (OCh) trail in an OTN network represents the full path of a specific wavelength carrying a client signal from ingress to egress.

A. This describes the process of mapping or multiplexing, which places a client signal into a transport container, rather than defining the end-to-end path (trail).

C. This describes the function of an optical amplifier or regenerator, which are network elements used to overcome signal attenuation, not the logical connection itself.

D. This describes a physical span or an Optical Multiplex Section (OMS), which is a segment of the network, not the complete end-to-end logical trail.

1. Source: ITU-T Recommendation G.805, "Generic functional architecture of transport networks"

Reference: Section 6.3, "Trail".

Quote/Paraphrase: The document states that a trail is responsible for "the transparent transfer of client characteristic information" between its access points. It is terminated by a trail termination function which monitors the integrity of the transported information.

2. Source: Nokia, "Nokia Optical Networking Fundamentals Course Student Guide"

Reference: Module 2, "OTN Fundamentals", Section on OTN Layering.

Quote/Paraphrase: The course material defines the Optical Channel (OCh) trail as the end-to-end logical connection that transports a client signal over a specific wavelength. It emphasizes the trail's role in providing a transparent path and performance monitoring from source to destination.

3. Source: ITU-T Recommendation G.872, "Architecture of optical transport networks"

Reference: Section 6.1, "Layer networks".

Quote/Paraphrase: This recommendation describes the Optical Channel (OCh) layer network, where the fundamental purpose of an OCh trail is to provide "transparent transport of a variety of client signals." This reinforces the concept of a trail as a transparent transport entity.

Logs are chronological records of all system and network occurrences, known as events. A log file, by its nature, contains a complete history. This includes events that have triggered alarms which are still active (representing an ongoing or "active" issue) as well as all other events that are purely historical (e.g., cleared faults, routine notifications). Therefore, logs serve as a comprehensive repository for both currently relevant and past event data.

B. This is an incomplete description. While the active alarm list provides a real-time status, the fault management system also maintains and reports on historical (cleared) alarms.

C. This is incorrect. The primary function of an alarm system is to provide immediate notification and status of current, active faults on the node.

D. This is conceptually incorrect. Logs record discrete, point-in-time events. They do not represent a persistent, consolidated "status" of a node; status is represented by alarms.

1. Nokia Network Services Platform (NSP) Fault Management Guide, Release 22.10

Reference for A and D: In the "Alarms vs. events" section, it is explained that an event is a single, atomic occurrence, while an alarm is a persistent condition. Logs are the records of these atomic events. This supports that logs contain all events (both related to active issues and purely historical ones) and do not represent a "status" themselves. (Document Part Number: 3HE-18278-AAAA-TQZZA, Chapter 2, "Alarm and event management").

Reference for B and C: The section "Active and historical alarms" explicitly states, "The NSP maintains a list of active alarms and a list of historical alarms." This confirms that the alarm system reports both real-time and historical information, making the statement in option B incomplete and the statement in option C incorrect. (Document Part Number: 3HE-18278-AAAA-TQZZA, Chapter 2, "Alarm and event management").