Voice over IP

This chapter elaborates the working, architecture, signaling protocols, and media of Voice-over-IP (VoIP). VoIP over wireless (VoWLAN) must try to minimize bandwidth consumption, which depends on codecs and packetization periods. The public switched telephone network (PSTN) can be logically separated into a signaling subsystem and a media-transport subsystem. Using Internet to carry voice calls also requires implementing signaling and media transport. VoIP deployments can have various architectures. The simplest way to classify these architectures is to understand where voice transitions from the PSTN to the Internet. The Media Gateway Control Protocol (MGCP) does not explicitly define security, but can run over various security layers, such as IPsec and TLS. H248 or Megaco is a centralized protocol very similar to MGCP. Like MGCP, it has the concept of a call manager that is a central point of control for distributed-media endpoints. The chapter focuses on the distributed protocol, H323, which was designed for voice and other media such as video. H323 gateways interface non-H323 networks to the H323 network. The gateway is responsible for establishing connections. The Session Initiation Protocol is the current leader in standards-based VoIP call-signaling protocols. The majority of open-standard-based VoIP implementations use the Real-time Transport Protocol (RTP) for the media path.