's IP results in a best-effort service, that is, packets section, we will illustrate the process of setting up a call may be delivered with an arbitrary delay or may even be using corresponding protocols in more detail (Figure 3.6-8). lost. Transmitted over the best-effort network and suf- Figure 3.6-2 shows a typical architecture for real-time fering from variable throughput, delay, and loss, data audio and video communication over IP networks. packets have to be delivered by a deadline to become Transport protocols, including UDP, TCP, and Real-Time useful. Excessive delay severely impairs communication Transport (RTP)/Real-Time Transport Control Protocols interactivity; packet loss results in glitches in audio and (RTCP) are built on top of the IP layer. Audio and video poor picture quality and frozen frames in video. The codecs are applied on the content encapsulated or to be heterogeneity of today's Internet also poses a major encapsulated in the payload. The upper-layer applications challenge for media delivery to users with various con- call audio and video codecs to perform data compression. nection speeds, where scalability is highly desirable. Signaling protocols, such as session initiation protocol The challenges that the industry faces, in conjunction (SIP), are used for call setup and control. The signaling with the commercial promise of the technology, have and transport protocols are described in more detail in attracted considerable effort in research and product the following sections. development. In this chapter, we will first describe an architecture for real-time communication, followed by 3.6.2.2 Signaling protocols topics on how to improve the QoS. In Section 3.6.2, we will describe the basic system architecture as well as two The SIP, originally developed by the IETF Multi-Party categories of the most important protocols: signaling and Multimedia Session Control Working Group, is the most Communications Engineering Desk Reference; ISBN: 9780123746481 Copyright © 2007 Elsevier Inc. All righ