September 2008
Intermediate to advanced
550 pages
18h 10m
English
Content preview from VoIP Voice and Fax Signal ProcessingBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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8.5 CALLER ID IN VoIP
VoIP enables many of the CLASS family of telephone services offered by telephone companies. Several VoIP call combinations and systems are given in Chapter 2. In this section, an overview on Type-1 on-hook caller ID generation in a VoIP system is given. For continuity of the description, a functional representation of VoIP call is shown in Fig. 8.6. Various voice interfacing VoIP adapters and soft switches are associated to establish the VoIP call. VoIP signaling protocols such as SIP, H.323, MGCP, and Megaco are used to set up, tear down calls, carry user information required to locate users, and negotiate capabilities. Plain old telephone service (POTS) interfaces connected to analog interfaces on these VoIP systems are called FXS. In Fig. 8.6, FXS-A (referred as User phone-A) and FXS-B (referred as User phone-B) are end-point interfaces of different VoIP adapters connected to the IP network. The VoIP adapter samples voice from the telephone interface at 8 kHz, compresses samples, and generates IP packets with proper headers. The VoIP adapters are registered to the soft switch and subscribe to the specified call feature services. Soft switch and VoIP systems work together to emulate a PSTN central office for call feature establishing. Depending on the supported VoIP signaling, the soft switch will be working as a SIP—proxy server, MGCP—Media Gateway Controller, and H.323 Gatekeeper.
Figure 8.6. VoIP adapters and caller ID working functional representation. ...
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