A network engineer is troubleshooting high packet loss and throughput degradation on an 8 … — CompTIA Network+

A network engineer is troubleshooting high packet loss and throughput degradation on an 8 km 10GBASE-LR single-mode fiber link connecting two core switches. The switch SFP+ modules report optical receive power levels comfortably within receiver sensitivity thresholds ( $-4\text{ dBm}$ received vs. a receiver sensitivity range of $-14\text{ dBm}$ to $+0.5\text{ dBm}$ ). Interface counters show zero FCS or CRC frame errors. An Optical Time-Domain Reflectometer (OTDR) test reveals a high-amplitude reflective Fresnel spike with an Optical Return Loss (ORL) of $-22\text{ dB}$ at $3.2\text{ km}$ , while overall fiber end-to-end attenuation remains normal at $0.4\text{ dB/km}$ . Concurrently, packet captures exhibit repeated TCP Out-of-Order and Fast Retransmission packets. Which of the following physical layer anomalies is the primary cause of the degradation?

A contaminated or damaged optical connector at 3.2 km causing high back-reflection that destabilizes the transmitter laser diode.Cevap
B
A severe macrobend in the optical fiber at 3.2 km inducing non-reflective localized power loss.
C
A speed and duplex auto-negotiation mismatch between the two 10GBASE-LR transceiver interfaces.
D
An incorrect TCP Maximum Segment Size (MSS) negotiation occurring during the initial three-way handshake.

Cevap

A contaminated or damaged optical connector at 3.2 km causing high back-reflection that destabilizes the transmitter laser diode is the primary cause of the network degradation.

The correct answer identifies that a contaminated or damaged connector end-face creates an air gap causing Fresnel reflection. In high-speed single-mode fiber systems (

10\text{ Gbps}

and above), back-reflection toward the optical transmitter destabilizes the laser source through optical feedback, leading to physical bit stream corruptions, frame loss, and TCP retransmissions despite adequate optical receive power.

Adım Adım Çözüm

Analyze the physical layer OTDR measurement data.

The OTDR indicates normal linear optical attenuation (

0.4\text{ dB/km}

) and sufficient receiver power (

-4\text{ dBm}

), eliminating complete fiber breaks or high macrobend signal absorption as the root cause. However, it highlights a large reflective event (Fresnel spike with ORL of

-22\text{ dB}

) at

3.2\text{ km}

High reflectance events in single-mode fiber (SMF) typically indicate air gaps, dirty end-faces, or mismatched polish types (e.g., UPC plugged into APC) at connection points.

Correlate physical reflection (ORL) with transmitter stability and packet capture symptoms.

High Optical Return Loss (ORL) causes optical power to reflect straight back into the transmitting laser cavity (optical feedback). This destabilizes the laser diode's operational wavelength and output phase (laser jitter/mode hopping), introducing bit-level errors.

When bit-level signal degradation occurs dynamically due to laser instabilities, receiver clock recovery fails, causing lost frames and triggering TCP Fast Retransmissions without necessarily registering structured Layer 2 CRC errors.

Evaluate alternative explanations.

Macrobends cause non-reflective power loss (low receiver power), 10GbE fiber does not support half-duplex negotiation, and TCP MSS misconfigurations are pure Layer 4 issues incapable of generating OTDR reflective spikes.

Only optical back-reflection at a dirty or damaged connector matches both the OTDR reflective spike and the packet capture loss symptoms while maintaining normal receiver power levels.

Anahtar Kavram

Optical Time-Domain Reflectometry (OTDR) Fresnel Reflection and Optical Return Loss (ORL) Troubleshooting

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