Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

The example in Figure 12.1 shows the following steps for a voicemail appli-

cation [1]. The high-level message flow is as follows:

1. A caller on the PSTN or mobile network places a call to a SIP phone.


2. The VoIP gateway sends the INVITEmessage to its outgoing SIP server.


3. The SIP server forks the call to the called party and to the unified mes-
   sage (UM) server.


4. The called party does not answer the call.


5. The UM server takes the call after a timeout, and the caller leaves a
   message that is stored in the UM store.


6. The UM server sends an e-mail notification to the called party.


7. The UM server sends a NOTIFYmessage to SIP devices and while also
   using the other facilities for message waiting and the notification for
   message waiting will be delivered:
   ■■ To the SIP phone as a NOTIFYmessage
   ■■ To the IM client (not shown in Figure 12.1) as a NOTIFYmessage
   ■■ Via e-mail to the mail client
   ■■ Can be displayed on the browser by accessing the UM store via
   HTTP
   ■■ Can be announced via PSTN phone when the called party calls the
   mailbox

The voice message can be retrieved by PSTN, PBX, or SIP phones and also

by Web browsers that have an RTSP-enabled media player (native or plug-in).

For simplicity we have not shown other network elements that may be

involved in this voicemail application, such as various VoIP gateways; SIP

servers performing registration, rendezvous, and routing; and component

servers, such as VoiceXML voice portals and IVRs.

These network elements are part of the SIP infrastructure and can be reused

without modification.

RTSP-Enabled Voice Message Retrieval

The Real-Time Streaming Protocol (RTSP), defined by RFC 2326 [2], allows the

remote recording and replay of various media across the Net, and provides

similar functionality to the familiar players for audio/video. The most popu-

lar Internet media players (such as RealPlayer or QuickTime) support RTSP.

Users will quickly appreciate the advantage of listening to voicemail by

using an RTSP-enabled media player instead of a phone. Lengthy voicemail

messages can be replayed selectively, so as to listen to certain parts with rele-

vant information, instead of having to replay all messages in sequence with

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