5-2 Videoconferencing over IP Networks
CHRISTINE PEREY MATTHEW FELDMAN
According to Larry Irving, assistant secretary for communications and information, in a May 8, 1996 letter to Reed Hundt, chairman of the Federal Communications Commission, The Internet now connects more than 10 million computers, tens of millions of users, and is growing at a rate of 10% to 15% a month. This growth has created opportunities for entrepreneurs to develop new services and applications such as videoconferencing, multicasting, electronic payments, networked virtual reality, and intelligent agents. Perhaps more important, it creates a growing number of opportunities for users to identify new communication and information needs and to meet those needs.
Most engineers designing networks today recall that the worlds largest system of interconnected networks had ambitious roots: to define how transmission would take place over networks connecting dissimilar computers. Chances are that few of those who developed internetworking protocols in the Department of Defense 25 years ago foresaw the variety and vast number of applications their protocols would be called on to support. Today, the protocols that together are the basis of the Internet continue to evolve to accommodate the demands of new media types.
This chapter reviews the underlying principles of Internet protocols in the context of one of the most demanding application sets to date: real-time videoconferencing and visual collaboration. It discusses the advantages and disadvantages of choosing Internet protocol (IP) networks for real-time multimedia communications and how real-time videoconferencing is achieved on IP networkstoday and in the near future.
BASIC INTERNET PROTOCOL CONVENTIONS
Protocols that together manage packets on the Internet build on several widely accepted conventions/foundations. The principal components of the network are data connecting equipment (DCE), such as routers, and data terminating equipment (DTE), such as desktop computers (also called hosts).
Without any special adjustments for the unique requirements of different media types, the network layers work in concert to transmit packets of user information. In its simplest implementation, the flow of the data from and to end-points over an IP network is monitored or verified by a simple layer 3 communications protocol (e.g., Transmission Control Protocol, or TCP).
The advantage is that there is very little communications overhead associated with components in IP networks talking to one another. TCP running over IP in effect takes care of this. Each packet allocates the maximum number of bits to the users information.
Together, protocols ratified by the Institute of Electrical and Electronics Engineers (IEEE) and the Internet Engineering Task Force (IETF) ensure that packets of data are reliably transmitted under any condition, as quickly as possible (e.g., bandwidth or network load on the segments of the networks tying together two or more points).
Several protocols have been developed to manage the unique requirements of real-time streaming data. To understand the importance of these developments, this chapter begins with a high-level discussion of videoconferencing and visual collaboration using networked multimedia desktop computers.
PRINCIPLES OF VIDEOCONFERENCING AND MULTIMEDIA COMMUNICATIONS
When a video camera and microphone pick up real-life events, the imagery and sound can be turned into digital formats for communications between properly enabled end-points over local or wide area networks. For the user to perceive the moving images and intelligible sounds, the digital information moves from transmitter to receiver (transmitters are simultaneously receiving in the case of two-way videoconferencing) in a highly consistent fashion. Compressed in real-time, the data streams over a network in such a way that frames of video can be reconstructed and synchronized with audio with the least end-to-end delay.
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