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CCIE Collaboration: Call Control and Signaling

CCIE Collaboration · Work through your exam, one topic at a time.

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Call Control and Signaling

Domain 3: Call Control and Signaling

This domain tests CUCM digit analysis and route selection, SIP, H.323, and MGCP signaling, codec negotiation, DTMF transport, and mobility features. The goal is to predict which pattern matches a dialed string and which signaling path completes the call.

Core rules

  • Partitions hold patterns; a CSS is an ordered list of partitions assigned to a line or device. CUCM evaluates the patterns reachable through the CSS and chooses the best match, typically the longest match and then the more specific pattern.
  • Route groups and route lists provide trunk selection and failover; route patterns and translation patterns transform digits. Translation patterns can discard and prefix digits before routing.
  • SIP early offer carries SDP in the INVITE; delayed offer waits for SDP in the 200 OK. SIP profiles normalize headers and can enable PRACK/100rel.
  • DTMF can use RFC 4733, SIP INFO, or in-band; mismatched DTMF methods are a common cause of IVR failures.
  • H.323 uses gatekeeper RAS and fast or slow start; MGCP separates call control from gateway media handling.
  • Locations provide call admission control; AAR reroutes calls through PSTN when CAC rejects them. Device mobility and extension mobility change dial-plan and device settings for roaming users.

Worked example

A user dials 9-1-408-555-1212 and the SIP provider expects +14085551212. Create a translation pattern for 9.1[2-9]XX[2-9]XXXXXX that discards the pre-dot and prefixes +, or a route pattern 9.@ with discard pre-dot and prefix +1 for ten-digit dialing. Use the more specific pattern for 9.1... to avoid double-prefixing. Reasoning: the leading 9 is an access code, the NANP number must become +E.164, and the pattern specificity prevents the generic pattern from also matching.

Traps

  • Assuming the first partition in a CSS wins; CUCM uses best-match logic across the CSS.
  • Forgetting that a SIP 183 without SDP can break early-offer cut-through.
  • Mixing DTMF methods between CUCM, CUBE, and the IVR.
  • Ignoring transcoder/MTP needs when regions force G.729 but an endpoint only supports G.711.
  • Breaking H.323 calls by mismatching fast-start and slow-start expectations.