计算机网络物理层双语.ppt

2 Physical 物理层 It is a mistake to look too far ahead .Only one link in the chain of destiny can be handled at a time. Winston Churchill 过度前瞻是错误的。命运像一串链条，一次只能处理其中的一个环节。－－温斯顿·丘吉尔 PROBLEM： Physically Connecting Hosts 物理上相连的主机 The physical layer handles the ability that send and receive bit flows over a transmission medium, and serve the data link layer. 物理层的功能：提供在传输媒体上传送和接收比特流的能力，为数据链路层服务 There are five additional problems before the nodes can successfully exchange packets. 在结点成功地交换分组之前需要解决的5个问题： 1. Encoding bits onto the wire or fiber so that they can be understood by a receiving host. 把比特编码后送到铜线或光纤上，让接收方主机能理解接收到的编码。 2. Delineating the sequence of bits transmitted over the link into complete messages that can be delivered to the end node. This is called the framing problem, and the messages delivered to the end hosts are often called frames. 将在链路上传输的比特序列描述为完整的消息，以便将此消息传给端结点，即成帧问题；传给端主机的消息通常称为帧。 3. The error detection problem. 差错检测问题 4. Making a link appear reliable (the right of bit flow and taxis of frame) 建立一个可靠链路(比特流的正确和帧的排序） 5. The media access control problem 介质访问控制问题 These five functions are implemented in a network adaptor－a board that plugs into a host’s I/O bus on one end and into the physical medium on the other end. In other words, bits are exchanged between adaptors, but correct frames are exchanged between nodes. 这五种功能是在网络适配器上实现的，网络适配器是一块板，它的一端插入主机的I/O总线，另一端插入物理介质。比特是在适配器间交换，而正确的帧是在结点间交换的。 2.1 Basic Knowledge of Data Communication数据通信的基础知识 2.1.1 The model of data communication system 数据通信系统的模型 Data communication system 数据通信系统 Source system: Source node produce data that would be sent. Transmitter encode the data and send it. 源系统： 源点：产生要传送的数据 发送器：将数据编码并发送 Destination system: Receiver receive data and decode it. Destination deliver recei