SSCA VoIP Architecture 5 — Questions and Answers
Question 1: What is the purpose of a TURN server in VoIP NAT traversal?
- To translate SIP URIs to phone numbers
- To relay media through the server when direct or server-reflexive connectivity fails (Correct answer)
- To terminate TLS connections for SIP signaling
- To balance load across multiple SIP proxies
Correct answer: To relay media through the server when direct or server-reflexive connectivity fails
TURN (Traversal Using Relays around NAT) relays media packets through the TURN server when direct NAT traversal methods cannot establish a path.
Question 2: Which VoIP signaling protocol uses a master/slave model where a call agent controls dumb endpoints called Media Termination Points?
- SIP
- H.323
- MGCP (Correct answer)
- XMPP
Correct answer: MGCP
MGCP (Media Gateway Control Protocol) uses a centralized call agent (controller) that issues commands to endpoint devices (Media Termination Points).
Question 3: In VoIP, what does 'early media' refer to?
- Media negotiated before the SDP offer is sent
- Audio or video played before a call is answered, such as ringback tone or announcements (Correct answer)
- Compressed media sent to save bandwidth during call setup
- The first RTP packet transmitted in a session
Correct answer: Audio or video played before a call is answered, such as ringback tone or announcements
Early media is audio or video transmitted in the 180/183 provisional response stage before the called party answers the call.
Question 4: What VoIP architecture element provides features like music-on-hold, conferencing, and IVR by manipulating the media plane?
- SIP Registrar
- Media Resource Function (MRF) (Correct answer)
- ENUM server
- SIP Redirect Server
Correct answer: Media Resource Function (MRF)
A Media Resource Function (MRF) processes and mixes media streams to provide conferencing, announcements, IVR, and other media-related services.
Question 5: What is 'SIP forking' and what two types exist?
- Splitting a call into two legs; inbound and outbound forking
- Sending an INVITE to multiple destinations; parallel (simultaneous) and sequential (one at a time) (Correct answer)
- Routing calls to backup servers; primary and secondary forking
- Dividing bandwidth between two codecs; hard and soft forking
Correct answer: Sending an INVITE to multiple destinations; parallel (simultaneous) and sequential (one at a time)
SIP forking allows a proxy to forward an INVITE to multiple destinations simultaneously (parallel) or one after another (sequential) to find the called party.
Question 6: Which layer of the OSI model do VoIP QoS mechanisms like 802.1p VLAN priority tags operate at?
- Layer 3 (Network)
- Layer 2 (Data Link) (Correct answer)
- Layer 4 (Transport)
- Layer 7 (Application)
Correct answer: Layer 2 (Data Link)
802.1p priority marking is carried in the 802.1Q VLAN tag in Ethernet frames, operating at Layer 2 (Data Link layer).
Question 7: In VoIP security architecture, what attack does a SIP proxy defend against by validating the 'Max-Forwards' header?
- SIP registration hijacking
- Infinite routing loops where requests circulate endlessly between servers (Correct answer)
- Toll fraud through unauthorized call generation
- RTP media injection attacks
Correct answer: Infinite routing loops where requests circulate endlessly between servers
Max-Forwards limits how many times a SIP request can be forwarded, preventing routing loops from consuming server resources indefinitely.
SSCA VoIP Architecture 5
What is the purpose of a TURN server in VoIP NAT traversal?