Session Initiation Protocol (SIP Tutorial: SIP to ISDN Q.931 Call Flow (Brief)) SIP Subscriber Network SIP Client VOIP Network Company Network Alice Proxy 1 GW 1 PBX C

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This call flow diagram was generated with EventStudio Sequence Diagram Designer 2.5 (http://www.EventHelix.com/EventStudio).

LEG: Brief
This article is based on the call flow presented in http://www.iptel.org/info/players/ietf/callflows/draft-ietf-sipping-pstn-call-flows-02.txt and is reproduced here as per the copyright statement at the end of this document. Alice is a SIP device while Carol is connected via a Gateway (GW 1) to a PBX. The PBX connection is via a ISDN trunk group.

allocate Port 49172

Alice's PC allocates a port for receiving RTP data. This port number will be included in the SIP Invite. Alice dials Carol's telephone number (918-555-3333) which is globalized and put into a SIP URI. The message contains information about the RTP port number and the supported voice codecs. Proxy 1 indicates to the SIP client that it is trying to establish the call. Proxy 1 looks up the telephone number and locates the gateway that serves Carol. Carol is identified by her extension (444-3333) in the Request-URI sent to GW 1. The host portion of the Request-URI in the INVITE is used to identify the context (customer, trunk group, or line) in which the private number 444-3333 is valid. Otherwise, this INVITE message could get forwarded by GW 1 and the context of the digits could become lost and the call unroutable. GW 1 indicates to the Proxy that it is trying to establish the call. The GW routes the call. Since Carol is served by an ISDN PBX, the Gateway initiates a Q.931 call setup with the PBX. The ISDN PBX responds with Call Proceeding. This message indicates that the call is in the process of being setup. The ISDN PBX passes call progress information to the Gateway. This message indicates that the called subscriber is being rung.