Abbreviation for ISDN User Part. This is the part of the SS7 protocol that sets up and tears down phone calls. A description of the process follows:

  1. First person picks up phone and dials number
  2. The switch (SSP) servicing the first person determines where the call needs to be routed based on the number and sends an Initial Address Message (IAM) to the far end containing the voice trunk ID.
  3. The IAM is routed to the STPs the SSP is connected to and moves from STP to STP until it reaches the destination SSP.
  4. The destination SSP replies with an Address Complete Message (ACM), which returns to the originating SSP.
  5. When the receiving person picks up the phone the destination SSP sends an Answer Message (ANM) to the originating SSP and the voice trunk is opened to allow communication.
  6. No further messages are needed while the voice conversation is in progress.
  7. When one of the parties hangs up the SSP serving them sends a Release Message (REL) to the other end with a value indicating why the voice trunk is being released.
  8. The SSP receiving the REL message will then respond with a Release Complete (RLC) and the voice trunk will be dropped in both SSPs, terminating the call.

This is a simplified version of the process and does not take into account possibilities such as Local Number Portability (LNP), collect calls (LIDB), third party billing (LIDB), caller ID (CNAM), automatic call return (CLASS), toll free service (E800), or cellular (IS41).

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