SSCA Call Flows 4 — Questions and Answers
Question 1: In a SIP three-way conference call flow using a mixer, what is the role of the focus UA?
- It encrypts media between participants
- It manages conference membership, mixes audio, and handles INVITE/BYE for each participant (Correct answer)
- It acts as a back-to-back user agent only for billing
- It routes video streams to the correct participant
Correct answer: It manages conference membership, mixes audio, and handles INVITE/BYE for each participant
The focus UA (conference server) maintains a separate dialog with each participant, mixes their audio, and manages conference state via the conference event package.
Question 2: What SIP mechanism allows a UA to subscribe to call state events, such as dialog state, for a monitored extension?
- REGISTER with Expires header
- SUBSCRIBE/NOTIFY with Event: dialog package (Correct answer)
- OPTIONS polling
- INFO method with event body
Correct answer: SUBSCRIBE/NOTIFY with Event: dialog package
The SUBSCRIBE method with Event: dialog is used to subscribe to dialog state information, enabling features like BLF (Busy Lamp Field).
Question 3: In a SIP REFER-based call transfer, after the REFER is sent, what ongoing mechanism keeps the transferring UA informed of the transfer progress?
- Re-INVITE from the transfer target
- NOTIFY requests sent to the implicit subscription created by REFER (Correct answer)
- Periodic OPTIONS from the proxy
- PRACK messages from the transfer target
Correct answer: NOTIFY requests sent to the implicit subscription created by REFER
REFER creates an implicit subscription; the recipient sends NOTIFY requests carrying the SIP response status (e.g., 100, 200) of the transfer attempt.
Question 4: Which SIP call flow scenario uses the UPDATE method instead of re-INVITE?
- Modifying session parameters before the dialog is confirmed (during early dialog) (Correct answer)
- Transferring a call to a third party
- Registering a UA with multiple contacts
- Sending DTMF tones mid-call
Correct answer: Modifying session parameters before the dialog is confirmed (during early dialog)
UPDATE is used to modify session parameters during an early dialog (before the final 200 OK), since re-INVITE can only be used after the dialog is confirmed.
Question 5: In a SIP call flow with PRACK, what is PRACK used to acknowledge?
- The final 200 OK response
- Reliable provisional responses (1xx) when the 100rel extension is used (Correct answer)
- NOTIFY requests from a subscription
- ACK messages sent by the UAC
Correct answer: Reliable provisional responses (1xx) when the 100rel extension is used
PRACK (Provisional ACK) is sent to reliably acknowledge provisional responses such as 180 Ringing when the 100rel extension is negotiated.
Question 6: A SIP proxy receives an INVITE with a Max-Forwards value of 0. What should the proxy do?
- Forward the INVITE anyway to the next hop
- Return a 483 Too Many Hops response (Correct answer)
- Return a 408 Request Timeout
- Drop the request silently
Correct answer: Return a 483 Too Many Hops response
When Max-Forwards reaches 0 at a proxy that is not the final destination, the proxy MUST return 483 Too Many Hops.
Question 7: In SIP call flows, what is the significance of the tag parameter in the From and To headers?
- It specifies the authentication realm
- It uniquely identifies the dialog endpoint to distinguish multiple dialogs (Correct answer)
- It indicates the transport protocol used
- It signals call priority level
Correct answer: It uniquely identifies the dialog endpoint to distinguish multiple dialogs
The From-tag (generated by the UAC) and To-tag (generated by the UAS) together with the Call-ID form the unique dialog identifier.
SSCA Call Flows 4
In a SIP three-way conference call flow using a mixer, what is the role of the focus UA?