VoIP Voice and Fax Signal Processing

2.5 VoIP SIGNALING

VoIP signaling is essential to set up, tear down, and manage VoIP voice and fax calls, as well as various call features. Once a call is established, RTP/UDP packets of voice and fax are transmitted for the media. In this section, an overview on H.323, SIP, and MGCP signaling is presented. While writing this book, SIP signaling was popularly adapted by several deployments. SIP signaling is given in more detail to provide an immediate introductory-level explanation. Refer to [ITU-T-H.323 (2006), ITU-T-H.248 (2005), Andreasen and Foster (2003), Rosenberg et al. (2002), Sparks (2003)] for more details on VoIP signaling. SIP end points are called user agents (UAs), and they can work in the point-to-point mode without a SIP proxy. In a service provider environment, VoIP calls are made through SIP servers. MGCP end points are called media gateways (MGs). MGCP calls are made through the media gateway controller (MGC). MGCP customer premises equipment always requires MGC support to establish VoIP calls. H.323 end points are called terminals, and they can work in the point-to-point mode. In a service provider network, H.323 gatekeepers are used to establish VoIP calls.

2.5.1 VoIP–H.323 Overview

The ITU-T-H.323 standard (2006) was used in early packet media applications for the transmission of real-time audio/voice and video and data communications over packet-based networks. It specifies the components, protocols, and procedures providing multimedia communication over ...