Question 1

Which of the following about the Optical channel Transport Unit (OUT0 in ONT network is correct?

Accepted Answer

D

Explanation: The Optical channel Transport Unit (OTU) is a fundamental component of the Optical Transport Network (OTN) layer model, as defined by the ITU-T G.709 standard. The OTU frame encapsulates an Optical channel Data Unit (ODU) and adds specific overhead for transport over a single optical channel (wavelength). A primary and critical function enabled by this overhead is Forward Error Correction (FEC). The OTU layer calculates and adds FEC parity bytes to the frame, which allows the receiver to detect and correct a significant number of bit errors incurred during transmission, thereby improving the overall bit error rate performance and extending the unregenerated reach of the optical signal.

Question 2

Consider two EDFA amplifiers in cascade. Each EDFA has an average input signal power of 10 mW and an average noise power of 1 mW. What is the OSNR of the system at the receiver.

Accepted Answer

C

Explanation: The Optical Signal-to-Noise Ratio (OSNR) for a system with a cascade of N identical amplifiers, where each amplifier's gain compensates for the preceding fiber span's loss, is calculated by dividing the input signal power by the total accumulated noise. The total noise is the sum of the noise generated by each amplifier. Given: Input Signal Power (Pin) = 10 mW Noise Power per amplifier (PASE) = 1 mW Number of amplifiers (N) = 2 The total accumulated noise power (Pnoisetotal) is N PASE = 2 1 mW = 2 mW. The system OSNR in linear scale is: OSNRlinear = Pin / Pnoisetotal = 10 mW / 2 mW = 5 Converting to decibels (dB): OSNR (dB) = 10 log10(5) ≈ 6.99 dB, which is rounded to 7 dB.

Question 3

Which of the following statements about the management plane data Communications Network (DCN) is correct?

Accepted Answer

C

Explanation: The management-plane Data Communications Network (DCN) is an out-of-band, usually Ethernet-based, network whose sole purpose is to give the Operations/Element Management System a path to every network element (NE). Each NE has a dedicated management (craft/OOB) port that attaches directly to the DCN switch or router; DCN traffic is not transported through other NEs and is not carried in-band with user traffic. Therefore, the only statement that accurately reflects DCN operation is that it “can be connected to each NE directly.”

Question 4

Which of the following is NOT an advantage of super-channels?

Accepted Answer

D

Explanation: A super-channel is a high-capacity optical signal formed by combining multiple optical carriers into a single manageable entity. Key advantages include simplified network management (managing one logical channel instead of many) and improved spectral efficiency. The improved efficiency is achieved by packing the constituent carriers closer together than the standard ITU-T grid allows, minimizing unused spectrum. However, no optical transmission system is free from physical layer impairments. Signals are always affected by linear effects like Chromatic Dispersion (CD) and non-linear effects like Self-Phase Modulation (SPM) and Cross-Phase Modulation (XPM). In fact, the close carrier spacing in super-channels can increase inter-carrier non-linear impairments. Advanced Digital Signal Processors (DSPs) are essential to compensate for these impairments, but they do not eliminate them. Therefore, "impairment-free transmission" is not an advantage; it is an impossibility.

Question 5

Consider the exhibit, where the two highest ports are OT line ports. Which of the following statements is correct? https://kxbjsyuhceggsyvxdkof.supabase.co/storage/v1/object/public/file-images/NOKIA_4A0-255/page_12_img_1.jpg

Accepted Answer

D

Explanation: The exhibit displays a single Optical Transponder (OT) card with two line-side ports. This hardware configuration is designed to support an intra-card line protection scheme, commonly referred to as "Y-cable protection". In this scheme, the client signal is internally bridged to both line-side transmitter ports. To make this protection functional, an external 1x2 Optical Protection Switch (OPS) is required. The OPS is placed between the two line ports of the OT and the single line fiber, where it selects the signal from either the working or the protection port for transmission. The diagram shows the prerequisite hardware (the OT with dual line ports) for this type of protection but lacks the switching component, making the statement that an OPS is required the only correct conclusion.

Question 6

Consider the exhibit shows the standard 8-step network design process. Suppose that you have used
this process to design a Green filed network, which has been commissioned and is in operation. The
customer now requests an extension to the network to accommodate more steps and traffic. Which
steps are not required when designing such an extension?
4A0-255 practice exam questions

Accepted Answer

C

Explanation: When designing an extension to an existing (brownfield) network, the process is constrained by the current operational environment. "Analyze Available Resources" (Step 3) and "Analyze Cost" (Step 8) are treated less as distinct analytical steps and more as initial constraints that frame the design problem. The existing resources are a known baseline, and the budget for the extension is typically a predetermined business constraint. The primary design effort focuses on the iterative steps of analyzing new traffic (Step 2), defining the topology extension (Step 4), and evaluating its performance and impact (Steps 5, 6, 7) within these established boundaries.

Question 7

Which of the following statements about the recovery time for protection and restoration combined (PRC) is correct?

Accepted Answer

A

Explanation: Protection and Restoration Combined (PRC) is a resilience mechanism in GMPLS-controlled networks. It leverages a pre-computed, pre-signaled protection path to provide very fast recovery for the first network failure. This initial recovery action is a protection switchover, which is designed to be as fast as traditional protection schemes like Sub-Network Connection Protection (SNCP) or 1+1 protection, typically completing within 50 milliseconds. For subsequent or concurrent failures, PRC then relies on slower, dynamic restoration mechanisms to establish new paths. Therefore, its initial recovery speed is comparable to SNCP.

Question 8

Suppose that for a given number of buffer credits and for an given average FC frame size, the
maximum FC link length is 50 km.
If the average FC frame size is doubled, which of the following is correct?

Accepted Answer

D

Explanation: The initial configuration has the exact number of buffer credits needed for a 50 km link with the original frame size. When the average frame size is doubled, the number of credits required to keep the link saturated is halved. Consequently, the original credit allocation is now excessive. Buffer credits are a finite, shared resource within a switch ASIC. Over-allocating credits to one port unnecessarily consumes this limited resource, potentially preventing other ports on the same ASIC from obtaining the credits they need to achieve their own maximum throughput over their respective distances. Therefore, to optimize the switch's overall performance and achieve maximum aggregate throughput, the credits on this link need to be adjusted down to the new, lower requirement.

Question 9

Where is guardband mandatory for EMF?

Accepted Answer

C

Explanation: A guardband is mandatory between 10G and 200G channels due to significant differences in their electro-magnetic field (EMF) characteristics, specifically their modulation formats. Legacy 10G channels typically use Non-Return-to-Zero (NRZ) On-Off Keying, which has a very different spectral shape and power profile compared to modern 200G coherent channels using formats like DP-8QAM or DP-16QAM. The sharp spectral edges of the 10G signal can cause severe cross-channel interference and non-linear penalties on the adjacent, spectrally efficient, and more sensitive 200G signal. The guardband provides the necessary optical isolation to maintain the required optical signal-to-noise ratio (OSNR) and performance for the 200G channel.

Question 10

Which of the following statement about guaranteed restriction (GR) in generalized Multiprotocol label Switching (GMPLS) is correct?

Accepted Answer

A

Explanation: Guaranteed Restoration (GR), also known as pre-planned restoration, is a GMPLS recovery mechanism where a backup Label Switched Path (LSP) is pre-calculated and pre-signaled before a failure occurs. Critically, network resources (such as bandwidth and wavelength) are reserved along this backup path during its signaling. Upon failure of the primary LSP, the head-end node is notified and activates the already established and resource-reserved backup LSP. This pre-provisioning of the path and its resources is what "guarantees" the restoration, providing a high degree of survivability with faster recovery times than dynamic restoration.

Free Nokia 4A0-255 Actual Exam Questions