SBCA Network Design 5 — Questions and Answers
Question 1: What is the function of a 'guard band' in satellite frequency plan design?
- A reserved frequency buffer between adjacent carriers to reduce inter-carrier interference (Correct answer)
- A dedicated carrier used to monitor transponder health
- A backup frequency allocation activated during primary channel failure
- A time interval between TDMA bursts to allow for propagation delay differences
Correct answer: A reserved frequency buffer between adjacent carriers to reduce inter-carrier interference
Guard bands are unused frequency gaps placed between adjacent carriers to reduce spectral spillover and prevent adjacent channel interference in the transponder.
Question 2: When designing a satellite network for a geographically dispersed enterprise, what is the primary disadvantage of GEO satellite links for latency-sensitive applications?
- GEO satellites are unavailable during solar eclipse seasons
- The approximately 240ms one-way propagation delay causes issues for real-time protocols (Correct answer)
- GEO orbital slots have limited capacity for enterprise traffic
- GEO satellites require larger antennas than LEO systems
Correct answer: The approximately 240ms one-way propagation delay causes issues for real-time protocols
The approximately 35,786 km altitude of GEO satellites results in roughly 240ms one-way (480ms round-trip) propagation delay, which disrupts latency-sensitive protocols like VoIP and interactive gaming.
Question 3: Which parameter in a satellite link budget accounts for losses due to imperfect antenna illumination and edge-of-coverage beam rolloff?
- Atmospheric absorption loss
- Off-axis satellite antenna gain reduction (Correct answer)
- Polarization mismatch loss
- Intermodulation noise contribution
Correct answer: Off-axis satellite antenna gain reduction
Off-axis or edge-of-coverage gain reduction accounts for the lower satellite antenna gain experienced by earth stations located away from the center of the satellite's coverage beam.
Question 4: In a satellite broadcasting network, what is the role of a conditional access system (CAS)?
- To control which geographic areas can receive the satellite signal
- To encrypt content and control subscriber authorization to decrypt it (Correct answer)
- To manage bandwidth allocation among competing broadcast channels
- To authenticate network operations center personnel
Correct answer: To encrypt content and control subscriber authorization to decrypt it
CAS encrypts broadcast content using entitlement control messages (ECMs) and entitlement management messages (EMMs) to ensure only authorized, paying subscribers can decrypt and view the content.
Question 5: What is the primary reason satellite network designers specify a 'clear-sky' link margin in their link budgets?
- To account for transponder aging and power degradation over the satellite lifetime
- To provide a reserve of signal strength to overcome rain fade and other impairments (Correct answer)
- To compensate for errors in the satellite's orbital position
- To allow for future increases in data rate without hardware changes
Correct answer: To provide a reserve of signal strength to overcome rain fade and other impairments
Clear-sky margin is the excess signal strength above minimum threshold under ideal conditions, providing the reserve needed to maintain the link during rain fade, atmospheric absorption, and other degradation events.
Question 6: A network designer is evaluating whether to use single-channel-per-carrier (SCPC) or multi-channel-per-carrier (MCPC) for a point-to-point trunk link. When is SCPC preferred?
- When the link must carry hundreds of channels simultaneously
- When traffic is continuous and high-volume between two fixed points (Correct answer)
- When bandwidth efficiency is the primary concern over simplicity
- When the link requires frequent dynamic bandwidth reallocation
Correct answer: When traffic is continuous and high-volume between two fixed points
SCPC is preferred for dedicated high-volume point-to-point trunks where continuous full-time capacity is needed, offering simple operation without the need for multiplexing equipment.
Question 7: In a satellite network employing spread spectrum techniques, what is the primary advantage of CDMA over FDMA for the return link?
- CDMA eliminates the need for timing synchronization between terminals
- CDMA allows all terminals to transmit simultaneously on the same frequency using unique codes (Correct answer)
- CDMA requires less sophisticated modems at remote terminals
- CDMA provides higher peak data rates per terminal than FDMA
Correct answer: CDMA allows all terminals to transmit simultaneously on the same frequency using unique codes
CDMA assigns unique spreading codes to each terminal, allowing simultaneous transmission on the same frequency band without frequency planning, though all terminals contribute to aggregate interference.
SBCA Network Design 5
What is the function of a 'guard band' in satellite frequency plan design?