VDV Voice Systems 2 — Questions and Answers
Question 1: Which signaling protocol is used between a PBX and the PSTN to set up and tear down calls?
- SIP
- ISDN PRI (Correct answer)
- H.323
- MGCP
Correct answer: ISDN PRI
ISDN PRI (Primary Rate Interface) is the most common digital signaling protocol connecting a PBX to the PSTN for call setup and teardown.
Question 2: What is the purpose of a codec in a VoIP system?
- To route calls between extensions
- To encode and decode analog voice into digital packets (Correct answer)
- To provide power to IP phones
- To manage call queues
Correct answer: To encode and decode analog voice into digital packets
A codec (coder-decoder) converts analog voice signals into compressed digital data for transmission and reverses the process at the receiving end.
Question 3: What does the term 'jitter' refer to in VoIP communications?
- Packet loss percentage
- Variation in packet arrival times (Correct answer)
- Total round-trip delay
- Echo on the line
Correct answer: Variation in packet arrival times
Jitter is the variation in arrival times of packets, which can cause choppy or distorted audio if not compensated by a jitter buffer.
Question 4: Which G-series codec provides toll-quality voice at 64 Kbps using PCM encoding?
- G.711 (Correct answer)
- G.729
- G.722
- G.726
Correct answer: G.711
G.711 is the standard PCM codec operating at 64 Kbps that defines toll-quality voice used in traditional PSTN and VoIP networks.
Question 5: A VDV technician is troubleshooting one-way audio on a VoIP call. Which firewall issue is the most likely cause?
- Blocked DHCP traffic
- NAT not translating RTP media streams (Correct answer)
- DNS resolution failure
- STP loop on the data switch
Correct answer: NAT not translating RTP media streams
One-way audio in VoIP is commonly caused by NAT failing to properly translate the RTP (Real-Time Protocol) media stream IP addresses.
Question 6: What is the standard impedance for analog telephone loop circuits in North America?
- 50 ohms
- 75 ohms
- 100 ohms
- 600 ohms (Correct answer)
Correct answer: 600 ohms
The standard impedance for analog telephone circuits in North America is 600 ohms, which is used for proper impedance matching to minimize signal reflections.
Question 7: Which feature allows a single phone number to ring multiple devices simultaneously in a VoIP system?
- Call parking
- Simultaneous ring / Find Me Follow Me (Correct answer)
- Direct Inward Dialing
- Hunt group
Correct answer: Simultaneous ring / Find Me Follow Me
Simultaneous ring (also called Find Me Follow Me) causes multiple registered devices to ring at once when a single number is dialed.
VDV Voice Systems 2
Which signaling protocol is used between a PBX and the PSTN to set up and tear down calls?