多媒体网络系统可用于交换不同计算机之间的离散和连续媒...教案详解.ppt

多媒体网络系统可用于交换不同计算机之间的离散和连续媒体数据，这种通信需要有合适的服务和协议来实现数据交换。 A multimedia networking system allows for the data exchange of discrete and continuous media among computers.This communication requires proper services and protocols for data transmission. 层、协议和服务 服务就是提供给应用系统的一组操作。根据 OSI模型的约定，逻辑上相关的服务放入一个 层中。因此每一层都提供了一组服务。 Layers, Protocols and Services A service provides a set of operations to the requesting application.Logically related services are grouped into layers according to the OSI reference model.Therefore , each layer is a service provider to the layer lying above. 这些服务给出了该层能提供的操作和它的服务元（服务数据元SDU）的格式（语法部分）和意义（语义部分）。不同计算机相应实例协同工作构成服务。 The services describe the behavior of the layer and its service elements (Service Data Units = SDUs).A proper service specification contains no information concerning any aspects of the implementation. 协议由一组规则组成，当进行通信时还必须后跟相应的层实例。 A protocol consists of a set of rules which must be followed by peer layer instances during any communication between these two peers. 它的构成成分包括交换数据信元（协议数 据信元，PDU）的格式（语法部分）和意义（语义部分）。不同计算机相应实例协同工作构成服务. It is comprised of the format (syntax) and the meaning (semantics) of the exchanged data units (Protocol Data Units=PDUs). The peer instances of different computers Cooperate together to provide a service. 音频和视频数据处理必须限制在一定的时限间隔内完成。数据传输（应用或传输层接口之间）都必须考虑时间范围的要求。 Audio and video data processing need to be bounded by deadlines or even defined by a time interval. The data transmission--both between applications and transport layer interfaces of the involved components--must follow within the demands concerning the time domains. 必须对端到端的抖动加以限制。对交互式应用（例如电话）来说这一点尤为重要，抖动值越大意味着需要更大的缓冲，会产生更高的端到端延时。 End-to-end jitter must be bounded. This is especially important for interactive applications such as the telephone.Large jitter values would mean large buffers and higher end-to-end delays. 为了保证数据传输能在所需的时间间隔内完成，必须有足够高的处理器性能，以保证在总线上完成数据传输；必须有保证政党处理协议所需的足够存储空间。 All guarantees necessary for achiev