BDS RF Technology and Spectrum Management 2 — Questions and Answers
Question 1: What is the mid-split upstream frequency range proposed for DOCSIS 3.1 Extended Spectrum?
- 5–42 MHz
- 5–85 MHz (Correct answer)
- 5–204 MHz
- 5–300 MHz
Correct answer: 5–85 MHz
Mid-split architecture extends the upstream return path from the traditional 5–42 MHz sub-split to 5–85 MHz, providing additional upstream capacity for operators upgrading their plants.
Question 2: A cable technician measures excessive composite triple beat (CTB) distortion. What is the most likely cause?
- Too many splitters in the signal path causing excessive loss
- Amplifiers being driven beyond their linear operating range (Correct answer)
- Insufficient tilt at the amplifier input
- Incorrect torque on coaxial connectors
Correct answer: Amplifiers being driven beyond their linear operating range
CTB distortion is caused by third-order nonlinearities in amplifiers that are over-driven; reducing the output level or increasing the amplifier's operating headroom will reduce CTB products.
Question 3: What is the function of an equalizer in a cable amplifier?
- To maintain constant output power regardless of input level variations
- To pre-emphasize higher frequencies to compensate for the frequency-dependent attenuation of coaxial cable (Correct answer)
- To reduce amplifier noise figure across the passband
- To block ingress from entering the amplifier's return path port
Correct answer: To pre-emphasize higher frequencies to compensate for the frequency-dependent attenuation of coaxial cable
Cable attenuation increases with frequency, so equalizers pre-tilt the frequency response at the amplifier input to compensate, ensuring a flat response at the amplifier output.
Question 4: What is the DOCSIS 3.1 maximum downstream channel width for an OFDM channel?
- 6 MHz
- 24 MHz
- 96 MHz
- 192 MHz (Correct answer)
Correct answer: 192 MHz
DOCSIS 3.1 OFDM downstream channels can be up to 192 MHz wide, dramatically increasing capacity compared to the 6 MHz QAM channels used in earlier DOCSIS versions.
Question 5: What is PNM (Proactive Network Maintenance) in a DOCSIS 3.1 cable system?
- A scheduled monthly maintenance window for amplifier servicing
- Tools and techniques that use built-in DOCSIS measurement capabilities to detect impairments before subscribers are affected (Correct answer)
- A software update system for CMTS firmware
- A billing system for tracking subscriber data usage
Correct answer: Tools and techniques that use built-in DOCSIS measurement capabilities to detect impairments before subscribers are affected
PNM leverages DOCSIS 3.1 wideband spectrum analysis and pre-equalization data from cable modems to proactively identify and locate network impairments before they cause service outages.
Question 6: What causes common path distortion (CPD) in a cable plant?
- Two amplifiers operating out of phase with each other
- Non-linear mixing products generated at corroded or loose metallic contacts in the signal path (Correct answer)
- Upstream modems transmitting on the same frequency
- Excessive signal level at the CMTS input
Correct answer: Non-linear mixing products generated at corroded or loose metallic contacts in the signal path
CPD occurs when corroded connectors or loose metallic contacts act as a diode junction, generating intermodulation products between downstream carriers that fall in the upstream return band.
BDS RF Technology and Spectrum Management 2
What is the mid-split upstream frequency range proposed for DOCSIS 3.1 Extended Spectrum?