BDS Customer Premises Equipment and Drop Systems 2 — Questions and Answers
Question 1: A cable modem shows a downstream power level of +12 dBmV. What should the technician do?
- No action needed — this is within the optimal range
- Add a signal attenuator at the modem input to bring the level into range (Correct answer)
- Add an amplifier to boost the signal further
- Replace the drop cable with a lower-loss type
Correct answer: Add a signal attenuator at the modem input to bring the level into range
A downstream level of +12 dBmV exceeds the DOCSIS upper limit (typically +15 dBmV) and is above the optimal range; a small attenuator at the modem input will bring it into specification.
Question 2: What is the purpose of a high-pass filter (HPF) installed at a customer's set-top box or cable modem?
- To block upstream return path signals from unauthorized devices (Correct answer)
- To reduce signal levels that are too high for CPE equipment
- To extend the downstream frequency response to higher channels
- To filter AC power line interference from the coaxial cable
Correct answer: To block upstream return path signals from unauthorized devices
High-pass filters (set-top box filters or traps) block the 5–42 MHz upstream return band, preventing unauthorized or defective CPE from injecting noise or interference into the return path.
Question 3: What causes a 'stairstep' pattern in the upstream power levels across multiple modems in a node service area?
- All modems are functioning correctly and the pattern is normal
- Ingress or noise in a portion of the plant is causing some modems to increase their transmit power to overcome the impairment (Correct answer)
- The CMTS is intentionally varying modem power for load balancing
- The node's optical transmitter is operating at reduced power
Correct answer: Ingress or noise in a portion of the plant is causing some modems to increase their transmit power to overcome the impairment
A stairstep pattern in upstream modem transmit power indicates a localized impairment (noise, ingress, or high loss) in a branch of the plant where modems must compensate by raising their power.
Question 4: What is the maximum recommended number of passive splitters in a standard residential drop installation?
- 1 (one 2-way split only)
- 2 (providing up to 4 outputs) (Correct answer)
- 4 (providing up to 16 outputs)
- No limit if signal levels are adequate
Correct answer: 2 (providing up to 4 outputs)
Best practice limits residential drops to two cascade splitter stages (typically yielding up to 4 outlets) to keep signal levels above the minimum threshold without requiring in-home amplification.
Question 5: What is DOCSIS ranging and why is it important?
- A process where the CMTS measures the physical distance to each cable modem for inventory purposes
- A process where modems adjust their upstream transmit timing and power so all signals arrive at the CMTS at the correct time and level (Correct answer)
- A diagnostic tool that measures the RF noise floor in the return band
- A cable modem feature that automatically selects the best upstream channel
Correct answer: A process where modems adjust their upstream transmit timing and power so all signals arrive at the CMTS at the correct time and level
Ranging is the DOCSIS initialization process where the CMTS measures round-trip delay and signal level, instructing each modem to adjust its timing offset and transmit power so all upstream bursts arrive synchronized at the CMTS.
Question 6: What type of bonding/grounding is required at the cable entry point of a residential installation per US code?
- No bonding is required for low-voltage coaxial cable
- The coaxial shield must be bonded to the electrical service entrance ground electrode system per NEC 830 (Correct answer)
- The cable should be connected to a separate dedicated ground rod
- Grounding is only required for commercial installations
Correct answer: The coaxial shield must be bonded to the electrical service entrance ground electrode system per NEC 830
NEC Article 830 requires the coaxial cable shield to be bonded to the premises grounding electrode system at the point of entry, protecting against lightning-induced voltage differences.
BDS Customer Premises Equipment and Drop Systems 2
A cable modem shows a downstream power level of +12 dBmV.
What should the technician do?