【正文】 nts are separate from the session manager and they are responsible for decisions specific to each type of media. This modularity allows a replacement of agents. Each agent performs its own control mechanism over the particular medium, such as mute, unmute, change video quality, start sending, stop sending , etc. ? Shared Workspace Agent ? 共享工作空間代理在共享的應(yīng)用系統(tǒng)間傳送共享對象（遠(yuǎn)程指坐標(biāo)、圖形或正文對象） ? The shared workspace agent transmits shared objects (., telepointer coordinate, graphical or textual object) among the shared applications. ? 每個會話過程都由它的會話狀態(tài)描述 。 狀態(tài)信息包括會話開始 、 會話策略 、 會話過程名等 ，它們或是私有的 （ 如局部資源 ） ， 或由多個會話參加者共享 。 ? 會話管理分兩步處理會話狀態(tài)：會話建立和會話修改 。 在會話建立階段 ， 管理程度管理和設(shè)置會話本身的邏輯狀態(tài) 。 并且它與其他會話管理程序一起管理和設(shè)置付費策略和其他策略 。它還可以公布一個會話 （ 例如使用組匯合服務(wù) ） ， 使其他參加者可以定位和加入該會話 。它與傳輸子系統(tǒng)一起管理和定義傳輸拓?fù)浣Y(jié)構(gòu) 。 ? 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)用本身對計算機(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)保證和多點發(fā)送。 ? Networked multimedia applications by themselves impose new requirements onto data handling in puting and munications because they need (1) substantial data throughput, (2)fast data forwarding, (3)service guarantees, and (4)multicasting. ? ⑴ 數(shù)據(jù)吞吐量 ? Data throughput ? 傳輸音頻和視頻數(shù)據(jù)即使在壓縮模式下也需要大數(shù)據(jù)吞吐量。 ? Audio and video data resemble a streamlike behavior, and they demand, even in a pressed mode, high data throughput. ? ⑵ 快速數(shù)據(jù)轉(zhuǎn)發(fā) ? Fast Data Forwarding ? 當(dāng)不同的應(yīng)用系統(tǒng)在同一個端系統(tǒng)內(nèi)共存時 ， 每個中能對數(shù)據(jù)移動有不同的數(shù)據(jù)移動要求 （ 無錯數(shù)據(jù)傳輸 、 帶時間限制的數(shù)據(jù)傳輸 ） ， 并且需要快速的數(shù)據(jù)轉(zhuǎn)發(fā) 。 ? Fast data forwarding imposes a problem on endsystems where different applications exist in the same endsystem, and they each require data movement ranging from normal, errorfree data transmission to new timeconstraint traffic types traffic. ? ⑶ 服務(wù)保證 （ QoS） ? Service Guarantees ? 在有些分布式多媒體應(yīng)用系統(tǒng)中 ， 必須使用資源管理來保證某些服務(wù)功能 ， 否則多媒體系統(tǒng)將不能提供可靠連續(xù)媒體傳輸 。 ? Distributed multimedia applications need service guarantees, otherwise their acceptance does not e through as these systems . ? ⑷ 多點發(fā)送 ? Multicasting ? 在分布多媒體系統(tǒng)中 ， 由于有多個用戶參加 ，這樣有時就需要把消息和數(shù)據(jù)進(jìn)行多點發(fā)送 ， 這對網(wǎng)絡(luò)帶寬和協(xié)議處理又提出了要求 。 ? Multicast is important for multimediadistributed applications in terms of sharing resources like the work bandwidth and the munication protocol processing at endsystems. ? Processing and Protocol Constraints ? Transport Layer ? 支持多媒全傳輸協(xié)議必須新的特征 ， 提供以下功能：時間信息 ， 半可靠性 、 多點發(fā)送 ， 基于 NAK（ 非確認(rèn) ， NoneAcKnowledgement） 的錯誤恢復(fù)機(jī)制和位率控制 。 ? Transport protocols, to support multimedia transmission, need to have new features and provide the following functions: timing information, semireliability, multicasting, NAK(NoneAcKnowledgment)based error recovery mechanism and rate control. ? Inter Transport Protocols ? Inter協(xié)議包括兩種傳輸協(xié)議： TCP和 UDP。 ? The Inter protocol stack includes two types of transport protocols(1)Transmission Control Protocol (TCP)。 (2)User Datagram Protocol (UDP). 傳輸控制協(xié)議（ TCP） ? Transmission Control Protocol(TCP) ? 早期視頻會議應(yīng)用是在 TCP協(xié)議上實現(xiàn)的。 TCP在交換全雙工字節(jié)流的進(jìn)程之間提供可靠的串行通信路徑或虛擬線路。每一進(jìn)程假定都放在由 IP地址標(biāo)識出的Inter終端中。每一進(jìn)程都有一些邏輯的全雙工端口，通過這些端口可以建立進(jìn)程，并用作全雙工 TCP連接。 ? Early implementations of video conferencing applications were implemented on top of the TCP protocol. TCP provides a reliable, seria