This animated TECHTip tutorial is available here.

This is a self-repeating tutorial on RLT [Release Link Trunking] where, after transferring a call, unused “trombone” (like the musical instrument with dual slides) trunks are released (disconnected).

1. Caller dials a CRBT [Color Ring Back Tone] service subscriber.

2. Incoming call initiates IN [Intelligent Network] and routes the call via SS7-ISUP-IAM to the CRBT server via the SCP [Service Control Point].
   • A Q.931-SS7-ISUP-ISDN [Integrated Services Digital Network] User Part IAM [Initial Address Message] is sent with the Routing label, CIC [Circuit Identification Code] and Message Type Code. Examples of Message Type Codes include: Called Number, Calling Number, DPC [Destination Point Code], OPC [Origination Point Code].

   • As the IAM passes through switching centers, circuits are reserved.
3. CRBT server generates color tone, which is sent to the caller via SS7.
4. CRBT server dials the called party via SS7-ISUP-IAM, “tromboning” on another SS7 link.

Once the IAM [Initial Address Message] is processed at the DPC [Destination Point Code] (switch), an SS7-ISUP ACM [Address Complete Message] is sent to the OPC [Origination Point Code] (switch). The ACM is used also used to indicate the start of billing.

Upon off-hook connect, the reserved trunks are activated and an ANM [ANswer Message] completes the call setup/connect and communication (conversation) begins.

Upon RELease or disconnect (on-hook), a series of SS7-ISUP messages are sent to teardown (release) the trunks, terminate billing, and provide network management and testing for the next call.

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