【正文】 ? The Heidelberg Transport System(HeiTS) is a transport system for multimedia munication. It was developed at the IBM European Networking Center(ENC), Heidelberg. HeiTS provides the raw transport of multimedia over works. It uses the Heidelberg Continuous media Realm(HeiCoRe), which is a realtime environment for handling multimedia data within workstations. ? ⑶ METS：多媒體增強(qiáng)傳輸服務(wù) METS: A Multimedia Enhanced Transport Service ? METS是 Lancaster大學(xué)開發(fā)的多媒體傳輸服務(wù) [CCH93a]，在 ATM網(wǎng)的頂層運(yùn)行。 該協(xié)議中的傳輸協(xié)議是實(shí)時(shí)消息傳輸協(xié)議（ RMTP） 和連續(xù)媒體傳輸協(xié)議 （ CMTP） 。 ? RTP has a panion protocol RTCP (RTPControl Protocol) to convey information about the participants of a conference. RTP provides functions, such as determination of media encoding, synchronization, framing, error detection, encryption, timing and source identification. RTCP is used for the monitoring of QoS and for conveying information about the participants in an ongoing session. The first aspect of RTCP, the monitoring ,is done by an application called a QoS monitor, which receives the RTCP messages. This monitor estimates the current QoS for monitoring, fault diagnosis and longterm statistics. The second aspect of RTCP is used for “ loosely controlled” sessions. RTP and RTCP information is transmitted through separate ports. ? 3． Xpress傳輸協(xié)議 （ XTP） ? Xpress Transport Protocol(XTP) ? XTP是一高效的協(xié)議 [SDW92]。 RTCP的第一部分 （ 監(jiān)控作用 ） 由稱為 QoS監(jiān)控器的應(yīng)用實(shí)現(xiàn) ， 它接收RTCP消息 。它由 A u d i o / V i d e o T r a n s p o r t W o r k i n g Group[SCFJ94]定義 。 ? Early implementations of video conferencing applications were implemented on top of the TCP protocol. TCP provides a reliable, serial munication path, or virtual circuit, between processes exchanging a fullduplex stream of bytes. Each process is assumed to reside in an Inter host that is identified by an IP address. Each process has a number of logical, fullduplex ports through which it can set up and use as fullduplex TCP connections. 用戶數(shù)據(jù)報(bào)文協(xié)議 UDP ? User Datagram Protocol(DUP) ? UDP是 Inter網(wǎng)絡(luò)協(xié)議 IP的擴(kuò)展 ， 它支持?jǐn)?shù)據(jù)報(bào)文的多路復(fù)合 。 (2)User Datagram Protocol (UDP). 傳輸控制協(xié)議（ TCP） ? Transmission Control Protocol(TCP) ? 早期視頻會(huì)議應(yīng)用是在 TCP協(xié)議上實(shí)現(xiàn)的。 ? Distributed multimedia applications need service guarantees, otherwise their acceptance does not e through as these systems . ? ⑷ 多點(diǎn)發(fā)送 ? Multicasting ? 在分布多媒體系統(tǒng)中 ， 由于有多個(gè)用戶參加 ，這樣有時(shí)就需要把消息和數(shù)據(jù)進(jìn)行多點(diǎn)發(fā)送 ， 這對(duì)網(wǎng)絡(luò)帶寬和協(xié)議處理又提出了要求 。 ? Each session is described through its session state. This state information (start time of session, policies associated with the session, session name) is either private(., local resources) or shared among all session participants(., conference participants). ? Session management includes two steps to process the session state: an establishment and a modification of the session. During the establishment, the session manager negotiates, agrees, and sets the logical state of its own session. Further, it negotiates, agrees and sets billing policy and other policies with other session managers. Further, it permits “ publishing” a session( ., using group rendezvous service), allowing others to locate and joint a session. It negotiates and defines the transport topology with the transport subsystem. ? 根據(jù)應(yīng)用系統(tǒng)所需的功能的不同 ， 在會(huì)話管理過程中可以嵌入以下幾種控制機(jī)制： ? Dependent on the functions, which an application requires and a session control provides, several control mechanisms are embedded in session management: ? ⑴ 發(fā)言權(quán)控制 Floor Control ? ⑵ 會(huì)議控制 Conference Control ? ⑶ 媒體控制 Media Control ? ⑷ 配置控制 Configuration Control ? ⑸ 成員控制 Membership Control Transport Subsystem ? Requirements ? –User and Application Requirements ? 網(wǎng)絡(luò)多媒體應(yīng)用本身對(duì)計(jì)算機(jī)通信中的數(shù)據(jù)處理提出了新的需求，原因是它們需要大的數(shù)據(jù)吞吐量、快速數(shù)據(jù)轉(zhuǎn)發(fā)、服務(wù)保證和多點(diǎn)發(fā)送。 在會(huì)話建立階段 ， 管理程度管理和設(shè)置會(huì)話本身的邏輯狀態(tài) 。 每一代理通過特定的媒體執(zhí)行本身的控制機(jī)制 ， 例如控制操作可包括：使發(fā)聲 、 使不發(fā)聲 、 修改視頻質(zhì)量 、 開始發(fā)送和結(jié)束發(fā)送等等 。 通過創(chuàng)建一個(gè)可彼一時(shí)用的會(huì)話管理程度 ， 從而可以方便面向會(huì)議的實(shí)用工具的實(shí)現(xiàn) 。 ? Distributed conference control is based on a distributed conference state. This is achieved as follows: the initiator of the conference establishes a multicast space(., multicast tunnel in MBone) with multicast entries for distribution of information to the conference participants and the conference is established。 這意味著主席知道所有會(huì)議參加者的地址 。 靈活性由控制模式?jīng)Q定 。 ? Video is used in technical discussions to display viewgraphs and to indicate how many users are still physically present at a conference. For visual support, workstations, PCs