SSCA VoIP Technologies & Network Integration 5 — Questions and Answers
Question 1: What is the purpose of the Record-Route header in SIP?
- To store the history of previous SIP registrations
- To force subsequent in-dialog requests to traverse specific proxies (Correct answer)
- To list codec preferences in order of priority
- To identify the physical location of the SIP UA
Correct answer: To force subsequent in-dialog requests to traverse specific proxies
Proxies insert themselves into the Record-Route header so that future requests within the dialog continue to pass through them.
Question 2: Which mechanism in ICE (Interactive Connectivity Establishment) is used when direct peer-to-peer media is not possible through NAT?
- STUN reflexive candidates
- TURN relay candidates (Correct answer)
- SRTP key exchange
- SIP PRACK method
Correct answer: TURN relay candidates
TURN (Traversal Using Relays around NAT) provides relay candidates that route media through a server when direct connectivity is impossible.
Question 3: In VoIP network design, what is the recommended maximum packet loss rate for acceptable voice quality?
- Less than 1% (Correct answer)
- Less than 5%
- Less than 10%
- Less than 15%
Correct answer: Less than 1%
Packet loss above 1% begins to noticeably degrade voice quality; most QoS designs target sub-1% loss for voice traffic.
Question 4: What does the SIP REFER method allow a UA to do?
- Transfer a call or direct another UA to contact a third party (Correct answer)
- Refuse an incoming call with a custom reason
- Query the media capabilities of a remote endpoint
- Renegotiate codec parameters mid-call
Correct answer: Transfer a call or direct another UA to contact a third party
REFER instructs the recipient to initiate a new contact with a URI provided in the Refer-To header, enabling call transfer scenarios.
Question 5: Which SIP feature allows a provisional response (1xx) to be acknowledged to avoid retransmissions over unreliable transports?
- SIP UPDATE method
- Reliable Provisional Responses (PRACK) per RFC 3262 (Correct answer)
- SIP SUBSCRIBE/NOTIFY mechanism
- SIP session timers (Session-Expires)
Correct answer: Reliable Provisional Responses (PRACK) per RFC 3262
RFC 3262 defines PRACK (Provisional Acknowledgment) to reliably deliver provisional responses when using unreliable transports like UDP.
Question 6: When deploying VoIP over a WAN link, which technique is used to reduce the bandwidth consumed by small VoIP packets?
- Voice Activity Detection (VAD)
- Header compression (cRTP) (Correct answer)
- Traffic shaping with WFQ
- Echo cancellation
Correct answer: Header compression (cRTP)
Compressed RTP (cRTP) reduces the combined IP/UDP/RTP header from 40 bytes down to 2-4 bytes, significantly lowering WAN bandwidth usage.
Question 7: What is the function of a SIP outbound proxy?
- It registers SIP UAs on behalf of the domain
- It routes outgoing SIP requests from UAs to the next hop in the network (Correct answer)
- It converts SIP to H.323 for legacy endpoints
- It provides WebRTC-to-SIP media translation
Correct answer: It routes outgoing SIP requests from UAs to the next hop in the network
An outbound proxy is the first hop that receives and forwards all outgoing SIP requests from a UA, typically configured in the UA's settings.
SSCA VoIP Technologies & Network Integration 5
What is the purpose of the Record-Route header in SIP?