Handbook of Local Area Networks, 1998 Edition:Applications of LAN Technology Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Bandwidth Allocation Exhibit 5-2-3 illustrates the following types of bandwidth allocation: •  Total bandwidth. This is the maximum bandwidth available for user data and network management overhead. •  User-available bandwidth. This is the total bandwidth minus the network management overhead. •  Application-specific bandwidth. This is the bandwidth requested by the application. •  Actual used bandwidth. This is the minimum bandwidth of either what the application requested or what is available over the end-to-end network during a particular session (whichever is lower). •  Allocated bandwidth. This is portion of the total user-available bandwidth that can be reserved; it only applies when a reservation mechanism is in place, requested, and granted. Exhibit 5-2-3.  Examples of Bandwidth Allocation IP Network Advantages Compared with other wide area network communications protocols, IP has four principal advantages for videoconferencing: •  Low management architecture overhead and high carrying capacity make IP cost-effective. •  No guaranteed quality of service and low overhead make IP bandwidth scalable (64K bps to 100M bps and beyond). •  IP architecture provides for many simultaneous virtual circuits, thereby enabling multiple services and multiple connections (through well-recognized sockets) at the same time. •  Packet structure and network design make it possible for both broadcasting and multicasting to occur in the same network, without requiring packet redundancy. Using extensions of IP to modify the payload format and reduce overhead associated with packet acknowledgment between end-points, the communications between end-points dedicated to user information is high. In some cases as much as 95% of the bandwidth can be assigned to user traffic. The actual bandwidth a data stream uses is a function of the bit rate the end-points have agreed to send upon call establishment. When two end-points have only very low bit rate capacity, owing to the local bus architecture or modem technologies over a POTS (plain old telephone services) line, the IP-based videoconference session operates within this constraint. On the other hand, when the same software (e.g., user application) is used over a high-bandwidth connection, a videoconference can take advantage of the increased capacity without any change in the application or communications protocol stack. Networks without support for Internet Protocol Independent Multicasting (PIM) must re-create a data stream for each of the desktops to which the user wishes to communicate simultaneously. This puts extra pressure on the source (i.e., the host has to create, manage, and transmit the same packets more than once) and on the shared resources connecting all users, whether they are multimedia communications enabled or using traditional transactional functions between clients and servers. Multicasting on IP enables multiple participants (so-called multipoints) to experience the same real-time conference at the same time. The importance of this feature cannot be underestimated. IP Network Disadvantages Many practitioners believe that connectionless communications and best-effort delivery of packets is inappropriate for isochronous network traffic (e.g., where data needs to be sent sequentially and arrive at its destination at specific intervals), such as that generated by digital video and audio. The principal disadvantages of IP network for videoconferencing are basically: •  Lack of guaranteed bandwidth, unless all the network components are controlled by a central manager that has chosen to implement networkwide bandwidth reservation schemes. •  Lack of international standards that would support interoperability of products from different vendors for intra- and intercompany communications. The most common wide area network solution for videoconferencing has been integrated services digital network (ISDN). With a dedicated connection (i.e., a communications circuit) between two end-points, bandwidth is guaranteed and, consequently, the quality of service remains consistent throughout a session. The next most-common wide area network for point-to-point desktop videoconferencing has been POTS, which despite its low bandwidth ensures, like ISDN, a dedicated circuit for a conference between two end-points. Users find that they prefer running business applications over networks in which the bandwidth allocated, requested, and used are all the same (i.e., the circuit-switched environment). And there are many competitive, standards-compliant offerings for desktop videoconferencing over basic rate ISDN. The natural consequence of a large—and especially a standards-compliant—installed base is that there are more people with which a system can interoperate without modification of any software or hardware. Until more vendors deliver standards-compliant, interoperable solutions for IP videoconferencing, the installed base will be confined to pockets of proprietary products that interoperate among themselves, but do not permit end-point application independence. Though specifications for session setup, management, and compression are under development (e.g., IETF Audio/Visual Transport Working Group and the International Telecommunications Union’s [ITU] H.323), they are not expected in commercial products until early or mid-1997. Standards and interoperability are essential for vendor independence, but they may be sacrificed for exceptional functionality and quality until such a time that vendor independence is possible. Compared to other protocols on the local area network for managing desktop videoconferencing (e.g., IsoEthernet or the specialized multimedia operating system, such as MOS, by First Virtual Corp., for 25M-bps ATM to the desktop), IP communications over 10BaseT Ethernets are prone to suffering from network contention and congestion. The relatively low bandwidth available for each user on a 10BaseT network has, to date, been unsuitable for business quality videoconferencing using inexpensive software codecs. 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