In IMS, what allows early media (ringback) before 200 OK?

Interview
1

Q. In IMS, what allows early media (ringback) before 200 OK?

a) MEGACO Subtract command
b) RTCP
c) SIP 183 Session Progress with SDP
d) SIP PRACK

Check 🔎

:arrow_right: Correct answer: Letter c.

In IMS, early media—such as ringback tones or announcements before the call is accepted—is enabled by the SIP 183 Session Progress response with SDP.

  • The 183 Session Progress provides the media parameters (via SDP) before the final 200 OK.
  • This allows RTP streams to begin flowing even while the session is still being established.
  • It’s crucial in VoLTE for user feedback (e.g., network-generated ringback tones).

Q. In IMS, what allows early media (ringback) before 200 OK?

a) :x: MEGACO Subtract command
→ Incorrect. MEGACO handles media gateway control, not SIP early media signaling.

b) :x: RTCP
→ Incorrect. RTCP provides quality feedback, not media establishment.

c) :white_check_mark: SIP 183 Session Progress with SDP
→ Correct. 183 with SDP allows the establishment of a media path before call acceptance.

d) :x: SIP PRACK
→ Incorrect. PRACK is used for reliable provisional responses, but doesn’t itself enable early media.