SSCA VoIP Technologies & Network Integration 4

Question 1

What is the function of STUN (Session Traversal Utilities for NAT) in VoIP?

Accepted Answer

To allow a UA behind NAT to discover its public IP address and port

Answer

STUN enables a UA behind NAT to learn its externally visible IP and port so it can include correct contact information in SIP and SDP.

Question 2

Which SIP response code indicates that a request has been forwarded to another URI and the client should retry there?

Accepted Answer

302 Moved Temporarily

Answer

A 302 Moved Temporarily response instructs the UAC to retry the request at the Contact URI provided in the response.

Question 3

In SDP, which field specifies the media type, port, transport protocol, and codec payload types for a media stream?

Accepted Answer

m= (media)

Answer

The m= line in SDP describes the media type (audio/video), port, transport (RTP/AVP), and the list of supported payload types.

Question 4

What is the primary difference between a SIP B2BUA and a SIP proxy?

Accepted Answer

A B2BUA terminates and re-originates both legs of a call; a proxy only forwards requests

Answer

A B2BUA (Back-to-Back User Agent) acts as a UAS on the incoming leg and a UAC on the outgoing leg, giving it full control over the call.

Question 5

Which protocol is used to carry DTMF tones out-of-band in SIP/RTP calls?

Accepted Answer

RFC 2833 / RFC 4733 (RTP Named Telephone Events)

Answer

RFC 2833 (updated by RFC 4733) defines RTP Named Telephone Events as the standard method for carrying DTMF tones out-of-band in RTP streams.

(SSCA) Sip School Certified Associate Practice Test