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T8 · T8-R02

Reading a ladder

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Learn it in 5 minutes

  • One leg at a time.
  • Method, status, CSeq, Call-ID, tags.
  • SDP addresses are where media was supposed to go.
  • A cause code is a clue, not a root cause by itself.
  • Compare a good call of the same type.
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Visual

Reading a ladder diagram
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What is going on

Start at the first request that belongs to the failure. Note who sent it and who answered. If there are two Call-IDs, you have a B2BUA. Follow the leg that actually failed, then step back one box.

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Worked example

CUCM sends INVITE to CUBE. CUBE answers 503. The carrier leg has no INVITE. The failure is on CUBE before the carrier, often dial-peer or a down interface. Do not open a carrier ticket yet.

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Quiz

1. Two Call-IDs mean the trace is corrupt.

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Ecosystem

Sister pieces sit on the ecosystem page: the glossary, the mind map, the integration map, and the sibling labs.

  • Ladder — A drawing of messages between named boxes.
  • Cause — The reason phrase or Q.850 cause on a failure.

Config. Filter the capture by Call-ID after you have identified it.

Troubleshooting. Wireshark's flow graph is a view. You still have to name the boxes.

Bridge. A Calling call may show TLS you cannot decrypt. Say that, and use the product's own call log as the other view.

Mind map, sister technologies, and future hooks live on the ecosystem page.

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Sources

Product screens move. Confirm the guide for your release before you change a live system.

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Sources and go deeper

Opened 2026-10. A row marked confirm has no link, because that page was not verified.

sources.json Markdown