SSCA VoIP Architecture 3 — Questions and Answers
Question 1: Which VoIP component converts between PSTN TDM circuits and IP packet networks?
- SIP Proxy
- Media Gateway (Correct answer)
- Registrar
- Presence Server
Correct answer: Media Gateway
A Media Gateway (MG) performs protocol and media conversion between the circuit-switched PSTN and packet-switched IP networks.
Question 2: What is the function of RTCP in a VoIP call?
- To set up and tear down the call session
- To carry voice media packets between endpoints
- To provide quality-of-service monitoring and statistics for RTP streams (Correct answer)
- To authenticate the calling party
Correct answer: To provide quality-of-service monitoring and statistics for RTP streams
RTCP (RTP Control Protocol) runs alongside RTP to provide feedback on call quality metrics like jitter, packet loss, and delay.
Question 3: In VoIP jitter buffer design, what is the trade-off when increasing jitter buffer size?
- Higher CPU usage but lower bandwidth
- Reduced packet loss but increased end-to-end latency (Correct answer)
- Better audio quality but higher jitter
- More memory usage but fewer dropped calls
Correct answer: Reduced packet loss but increased end-to-end latency
A larger jitter buffer absorbs more delay variation and reduces perceptible jitter effects, but adds to the overall end-to-end delay.
Question 4: What VoIP architecture pattern uses a central controller to manage distributed media gateways using MGCP or Megaco/H.248?
- Peer-to-peer VoIP
- Decomposed gateway architecture (Correct answer)
- Integrated access device topology
- SIP mesh network
Correct answer: Decomposed gateway architecture
The decomposed gateway architecture separates call control (Media Gateway Controller) from media processing (Media Gateway), linked by MGCP or H.248.
Question 5: Which QoS mechanism marks VoIP packets at Layer 3 to give them priority in the network?
- 802.1p
- DSCP (Correct answer)
- VLAN tagging
- STP
Correct answer: DSCP
DSCP (Differentiated Services Code Point) marks IP packet headers at Layer 3 to indicate traffic class for priority queuing.
Question 6: What does the SIP Record-Route header accomplish in a VoIP call?
- It records the call for compliance purposes
- It forces subsequent SIP messages in a dialog to route through specific proxies (Correct answer)
- It documents the geographic route of the call
- It stores the history of previous call transfers
Correct answer: It forces subsequent SIP messages in a dialog to route through specific proxies
Record-Route causes proxies to insert themselves into the route set so they remain in the signaling path for the duration of the dialog.
Question 7: In a typical enterprise VoIP deployment, what is the role of an IP PBX?
- To route calls between the internet and PSTN only
- To provide call switching, voicemail, and features for internal and external calls (Correct answer)
- To monitor network bandwidth for VoIP traffic
- To assign SIP addresses to external customers
Correct answer: To provide call switching, voicemail, and features for internal and external calls
An IP PBX (Private Branch Exchange) manages internal extensions, call routing, voicemail, auto-attendant, and PSTN connectivity for an organization.
SSCA VoIP Architecture 3
Which VoIP component converts between PSTN TDM circuits and IP packet networks?