SSCA VoIP Architecture 2 — Questions and Answers
Question 1: In a VoIP network, what is the primary role of a Session Border Controller (SBC)?
- To convert analog audio to digital packets
- To control and secure SIP sessions at network borders (Correct answer)
- To assign IP addresses to VoIP endpoints
- To transcode audio codecs between endpoints
Correct answer: To control and secure SIP sessions at network borders
An SBC sits at the boundary between networks to enforce security policies, NAT traversal, and session control for SIP traffic.
Question 2: Which VoIP architecture component is responsible for translating E.164 telephone numbers to SIP URIs?
- Media Gateway Controller
- ENUM server (Correct answer)
- Registrar server
- Back-to-Back User Agent
Correct answer: ENUM server
ENUM (E.164 Number Mapping) uses DNS to map telephone numbers to SIP URIs and other internet resources.
Question 3: What distinguishes a Softswitch Class 4 from a Class 5 in VoIP architecture?
- Class 4 handles end-user features; Class 5 handles transit routing
- Class 4 handles transit/carrier routing; Class 5 handles end-user features (Correct answer)
- Class 4 is for residential users; Class 5 is for enterprises
- Class 4 uses SIP; Class 5 uses H.323
Correct answer: Class 4 handles transit/carrier routing; Class 5 handles end-user features
Class 4 softswitches handle inter-carrier routing and transit traffic, while Class 5 softswitches provide retail features to end users.
Question 4: In VoIP, what is the purpose of STUN (Session Traversal Utilities for NAT)?
- To encrypt RTP media streams
- To help endpoints discover their public IP address and port behind a NAT (Correct answer)
- To compress SIP message headers
- To authenticate SIP registrations
Correct answer: To help endpoints discover their public IP address and port behind a NAT
STUN allows a VoIP client behind a NAT to discover its external IP address and port mapping for media connectivity.
Question 5: Which protocol is used to carry the actual voice media (audio) in most VoIP deployments?
- SIP
- RTP (Correct answer)
- H.225
- MGCP
Correct answer: RTP
RTP (Real-time Transport Protocol) carries the encoded audio and video media streams in VoIP calls.
Question 6: What is a Back-to-Back User Agent (B2BUA) in SIP architecture?
- A SIP proxy that forwards requests without modifying them
- A SIP entity that terminates and re-originates SIP dialogs (Correct answer)
- A physical gateway connecting PSTN to VoIP
- A registration server for SIP endpoints
Correct answer: A SIP entity that terminates and re-originates SIP dialogs
A B2BUA acts as both a UAS and UAC, terminating incoming SIP dialogs and originating new ones, giving it full control over the session.
Question 7: In VoIP architecture, what does the term 'hairpinning' refer to?
- Routing media through the SBC when direct media path is not possible (Correct answer)
- Compressing SIP headers to reduce packet size
- Connecting two PSTN trunks together through the softswitch
- Looping a call back to the originating gateway
Correct answer: Routing media through the SBC when direct media path is not possible
Hairpinning occurs when media must flow back through an intermediate device (like an SBC) rather than taking a direct path between endpoints.
SSCA VoIP Architecture 2
In a VoIP network, what is the primary role of a Session Border Controller (SBC)?