VDV Voice Systems 4 — Questions and Answers
Question 1: Which protocol does SIP use for transporting voice media (audio packets) between endpoints?
- TCP
- RTP over UDP (Correct answer)
- FTP
- SCCP
Correct answer: RTP over UDP
SIP uses RTP (Real-Time Protocol) carried over UDP to transport the actual voice media between endpoints due to UDP's low latency characteristics.
Question 2: What is the acceptable maximum one-way voice delay recommended by ITU-T G.114 for toll-quality calls?
- 50 ms
- 100 ms
- 150 ms (Correct answer)
- 400 ms
Correct answer: 150 ms
ITU-T G.114 recommends a maximum one-way delay of 150 ms for toll-quality voice to prevent noticeable conversational degradation.
Question 3: A technician measures 24 VDC on a tip and ring pair at a 66-block. What does this indicate?
- The line is on-hook and active (Correct answer)
- The line is off-hook (in use)
- The line has no battery — dead
- The line has ring voltage present
Correct answer: The line is on-hook and active
A standard on-hook analog POTS line carries approximately 48 VDC (–48V); reading 24 VDC suggests the line is active but at half battery, which may indicate a high-resistance fault.
Question 4: What is the purpose of a Music on Hold (MOH) server in a PBX system?
- Record voicemail greetings
- Stream audio to callers placed on hold (Correct answer)
- Detect DTMF tones during a call
- Provide dial tone to analog extensions
Correct answer: Stream audio to callers placed on hold
An MOH server streams audio content — music, announcements, or promotional messages — to callers who have been placed on hold.
Question 5: Which feature in a VoIP system allows an extension to be answered from a different phone without transferring?
- Call forwarding
- Call pickup (Correct answer)
- Call park
- Barge-in
Correct answer: Call pickup
Call pickup allows a user at one extension to answer a call ringing at a different extension by dialing a pickup code.
Question 6: What is the role of LLDP-MED in a VoIP deployment?
- Encrypt voice traffic
- Allow IP phones to discover VLAN and QoS configuration from the switch (Correct answer)
- Provide failover routing for VoIP calls
- Assign MAC addresses to IP phones
Correct answer: Allow IP phones to discover VLAN and QoS configuration from the switch
LLDP-MED (Media Endpoint Discovery) allows IP phones to automatically receive voice VLAN IDs, QoS policies, and network policy information from the connected switch.
Question 7: A customer reports that callers hear echo during calls. Which device is most likely causing the echo if it is near-end echo?
- The remote caller's headset
- The hybrid circuit at the CO or gateway where 2-wire meets 4-wire (Correct answer)
- A loose connector in the IDF
- The voicemail server
Correct answer: The hybrid circuit at the CO or gateway where 2-wire meets 4-wire
Near-end echo is typically caused by impedance mismatch in the hybrid circuit that converts between 2-wire analog and 4-wire circuits at the gateway or CO.
VDV Voice Systems 4
Which protocol does SIP use for transporting voice media (audio packets) between endpoints?