BDS Fiber Optic Systems in HFC 1 — Questions and Answers
Question 1: What does HFC stand for in cable network architecture?
- High Frequency Coaxial
- Hybrid Fiber-Coaxial (Correct answer)
- Headend Fiber Connection
- High-capacity Fiber Cable
Correct answer: Hybrid Fiber-Coaxial
HFC stands for Hybrid Fiber-Coaxial, describing a network architecture that uses fiber optic cable from the headend to neighborhood nodes and coaxial cable for the final distribution to homes.
Question 2: What is the function of an optical node in an HFC network?
- It amplifies optical signals along the fiber span
- It converts optical signals to RF signals (and vice versa) for distribution over coax (Correct answer)
- It encrypts video content before transmission
- It routes IP packets between fiber segments
Correct answer: It converts optical signals to RF signals (and vice versa) for distribution over coax
An optical node (fiber node) converts downstream optical signals from the headend into RF electrical signals for coaxial distribution, and converts upstream RF signals into optical signals for return to the headend.
Question 3: What type of laser is most commonly used in HFC headends for downstream optical transmission?
- Ruby laser
- DFB (Distributed Feedback) laser (Correct answer)
- Gas laser
- LED (Light Emitting Diode)
Correct answer: DFB (Distributed Feedback) laser
DFB (Distributed Feedback) lasers are used in HFC headends because their narrow spectral linewidth minimizes chromatic dispersion and clipping distortion over long fiber runs.
Question 4: What is optical clipping distortion in an HFC transmitter?
- Physical damage to fiber from excessive bend radius
- Distortion caused when the optical modulation drives the laser below its lasing threshold, clipping the signal waveform (Correct answer)
- Loss of signal at optical splitter ports
- Reflections from fiber connector end-faces
Correct answer: Distortion caused when the optical modulation drives the laser below its lasing threshold, clipping the signal waveform
Optical clipping occurs when high composite RF input power drives the laser into its sub-threshold region, causing the negative peaks of the signal to be clipped and generating significant distortion.
Question 5: What is the purpose of an optical attenuator in an HFC fiber link?
- To amplify weak optical signals
- To reduce optical power to an acceptable receive level and prevent photodetector saturation (Correct answer)
- To split optical signals among multiple nodes
- To convert optical wavelengths
Correct answer: To reduce optical power to an acceptable receive level and prevent photodetector saturation
An optical attenuator reduces optical power in a fiber link to prevent the receiver photodetector from being saturated by excessive signal levels, which would cause distortion and errors.
Question 6: What is a fiber optical loss budget?
- The cost estimate for fiber installation
- The maximum allowable total optical loss between a transmitter and receiver for the link to function correctly (Correct answer)
- The number of fiber strands allocated per node
- The power output specification of a headend laser
Correct answer: The maximum allowable total optical loss between a transmitter and receiver for the link to function correctly
An optical loss budget is the maximum total attenuation (in dB) that a fiber link can tolerate while maintaining adequate signal quality at the receiver, accounting for fiber loss, connector loss, and splitter loss.
BDS Fiber Optic Systems in HFC 1
What does HFC stand for in cable network architecture?