数据通信与网络10-数据链路控制-3.ppt

第十章 数据链路控制 10.1 Line discipline 10.2 Flow control 10.3 Error control 10.4 Summary Introduction In the physical layer, we have transmission but we do not have communication. ☆ No way of controlling which of several devices attached to that line will receive it; ☆ No way of knowing if the intended receiver is ready and able to receive it; ☆ No way of keeping a second device on the line from transmitting at the same time and thereby destroying our signal. Introduction Communication requires at least two devices working together, which requires a great deal of coordination for an intelligible exchange. Data link control deals with the design and procedures for communication between two adjacent nodes: node-to-node communication. Introduction The most important functions in the data link layer are line discipline, flow control and error control. Collectively, these functions are known as data link control. Introduction Line discipline: Who should send now? Flow control: How much data can be sent in some time interval? Error control: error correction by coordinates retransmission. 10.1 Line discipline 10.1.1 ENQ/ACK(询问/确认) 10.1.1 ENQ/ACK ENQ/ACK coordinates ① which device may start a transmission and ② whether or not the intended recipient is ready and enable. Session can be initiated by either station on a link as long as both are of equal rank. Peer-to-Peer 10.1.1 ENQ/ACK 发起方发送一个(ENQ)帧(接收方是否可以接收数据?)。 接收方如果已经就绪，则回答一个(ACK)帧;如果没有，必须回答一个否认(NAK)帧。即使是否定的也要求一个响应，使发起方知道接收方不能接收但已经收到了询问。 a.如果响应是肯定的，发起方开始发送数据。 b.如果对ENQ帧的响应是否定的，并且三次尝试都失败了， 发起方断开连接并在另一个时刻重新开始整个过程。 若在一定的时间内既没有收到(ACK)帧也没有收到(NAK)帧，发起方就假定询问帧(ENQ)丢失，于是重新发送一个(ENQ)副本。发起方在放弃之前为建立链路进行三次尝试。 当数据传送完毕，发送方以传输结束(EOT)帧结束传输。 10.1.1 ENQ/ACK 10.1.1 ENQ/ACK 10.1.1 ENQ/ACK 10.1.1 ENQ/ACK review 询问/确认(ENQ/ACK)方式主要用在不会有非预期的接收方接收的传输系统中，即，两个设备之间存在一条专用链路，因此惟一能够接收传输的设备也就是发送方所预期的接收设备。 ENQ/ACK负责协调哪个设备可以开始传输，以及确定预期的接收方是否已经就绪并可以进行接收。 The Poll/Select method works with topologies where one device is designated as primary and t