SIP Trunking “Flash” Tutorial - SIP-T-SIP for Telephones: IP-to-PSTN via SS7 - RFC 3372

(This animated tutorial is available at http://blog.tmcnet.com/cross-talk/)

This is an introduction to SIP Trunking and connections for telephones and discussion of SIP telephony network with respect to the PSTN-Public Switched Telephone Network via SS7-Signaling System 7.  This tutorial will present the various SS7 protocols used in PSTN signaling from a SIP phone.  Later, other call processing scenarios will be presented.  The following are the steps in the SIP signaling process to complete a call via the PSTN-Public Switched Telephone Network.  There are others but this is a start.  First, the SIP device sends an INVITE.

The GMC-Gateway Media Controller converts IP-Internet Protocol packets to CAS-Channel Associated Signaling TDM-Time Division Multiplexed channels signaling and media transmission with the PSTN.

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 (shown in green) are reserved. 

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.

RTP-Realtime Transport Protocol or SRTC-Secure RTP (encrypted media) is the Session Layer protocol used for the voice transmission with UDP-User Datagram Protocol and then IP for transmission.

During the call, various Call Progress messages such as CPG-Call ProGress Message, COT-COTinuity Message, SUS-SUSpend Message, RES-RESume Message, FOT-FOrward message Transfer, INR-INformation Request message, INF-INFormation Message and others may be sent.

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.

This presentation is part of the SIP-VoIP Essentials online course available from NACSE at MailScanner has detected a possible fraud attempt from "www.tmcnet.com" claiming to be http://www.nacse.com or the version for Channel Partners called NVBE or also onsite.  Please email Tom Cross cross@gocross.com for more details on all delivery and customizing options.