SSCA Call Flows 2 — Questions and Answers
Question 1: In a SIP re-INVITE scenario, what is the primary purpose of sending a re-INVITE mid-call?
- To terminate the call and start a new session
- To modify session parameters such as codec or media direction (Correct answer)
- To authenticate the remote party again
- To reset the RTP sequence numbers
Correct answer: To modify session parameters such as codec or media direction
A re-INVITE is sent during an established session to renegotiate session parameters, such as changing the codec, putting the call on hold, or updating media addresses.
Question 2: Which SIP response code indicates that a call transfer via REFER was accepted and the transfer is in progress?
- 200 OK
- 202 Accepted (Correct answer)
- 180 Ringing
- 301 Moved Permanently
Correct answer: 202 Accepted
202 Accepted is returned by a UA that has accepted the REFER request and will attempt to contact the refer-to URI.
Question 3: During a SIP call hold using RFC 3264, what attribute is placed in the SDP to indicate the hold?
- a=inactive
- a=sendonly (Correct answer)
- a=recvonly
- a=hold
Correct answer: a=sendonly
A UA placing a call on hold sends a re-INVITE with SDP containing a=sendonly, meaning it will send but not receive media.
Question 4: In a SIP forking scenario, what happens when multiple 200 OK responses arrive from different branches?
- The proxy accepts the first and sends CANCEL to the others (Correct answer)
- The UAC sends ACK to all of them
- The proxy forwards all 200 OKs to the UAC simultaneously
- The proxy ignores all but the last response
Correct answer: The proxy accepts the first and sends CANCEL to the others
When the first 200 OK is received from a forked branch, the proxy or UAC accepts it and cancels the remaining pending branches with CANCEL.
Question 5: What SIP method is used by a transferring UA to instruct the transfer target to contact a third party?
- INVITE
- NOTIFY
- REFER (Correct answer)
- UPDATE
Correct answer: REFER
The REFER method is used to instruct the recipient to contact a third-party URI, which is specified in the Refer-To header.
Question 6: In a SIP attended transfer, what is established before the actual transfer is completed?
- A REGISTER transaction with the transfer target
- A separate consultative call leg between the transferring UA and the transfer target (Correct answer)
- A three-way conference bridge
- An OPTIONS ping to verify availability
Correct answer: A separate consultative call leg between the transferring UA and the transfer target
In an attended transfer, the transferring party first establishes a separate call with the transfer target to consult before sending REFER to complete the transfer.
Question 7: Which header in a NOTIFY request carries the result of a REFER-triggered INVITE to the transfer target?
- Event: refer (Correct answer)
- Subscription-State
- Refer-To
- Contact
Correct answer: Event: refer
NOTIFY requests resulting from a REFER subscription carry the Event: refer header along with a message body containing the SIP response status of the transfer attempt.
SSCA Call Flows 2
In a SIP re-INVITE scenario, what is the primary purpose of sending a re-INVITE mid-call?