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voip guide

How To Read A SIP Ladder Diagram

A SIP ladder diagram turns packet captures into a call-flow view. It shows which side sent each SIP message and how the dialog moved between endpoints.

Read From Top To Bottom

The earliest SIP message is at the top. Follow the arrows downward to see setup, ringing, answer, acknowledgements, updates, and teardown.

Common Call Flow

  • INVITE starts a call attempt.
  • 100 Trying confirms the request is being processed.
  • 180 Ringing or 183 Session Progress indicates early call progress.
  • 200 OK answers the call.
  • ACK confirms the answer.
  • BYE ends the dialog.

Where Problems Usually Show Up

  • 4xx responses often point to client, authentication, addressing, or policy issues.
  • 5xx responses usually point to server or upstream handling issues.
  • 488 can indicate media negotiation problems.
  • A missing ACK after 200 OK can cause calls to drop shortly after answer.

Pair SIP With RTP

A clean SIP ladder only proves signaling behavior. If the call connects but audio is poor, compare it with RTP packet loss, jitter, and directionality.