数据链路层

中文字数:14890,中文页数:16  英文字数:8661 页数:24

The Data Link Layer

 In this chapter we will study the design principles for layer 2, the data link layer. This study deals with the algorithms for achieving reliable, efficient communication between two adjacent machines at the data link layer. By adjacent, we mean that the two machines are connected by a communication channel that acts conceptually like a wire (e.g., a coaxial cable, telephone line, or point-to-point wireless channel). The essential property of a channel that makes it ''wirelike'' is that the bits are delivered in exactly the same order in which they are sent.
 At first you might think this problem is so trivial that there is no software to study—machine A just puts the bits on the wire, and machine B just takes them off. Unfortunately, communication circuits make errors occasionally. Furthermore, they have only a finite data rate, and there is a nonzero propagation delay between the time a bit is sent and the time it is received. These limitations have important implications for the efficiency of the data transfer. The protocols used for communications must take all these factors into consideration. These protocols are the subject of this chapter.
 After an introduction to the key design issues present in the data link layer, we will start our study of its protocols by looking at the nature of errors, their causes, and how they can be detected and corrected. Then we will study a series of increasingly complex protocols, each one solving more and more of the problems present in this layer. Finally, we will conclude with an examination of protocol modeling and correctness and give some examples of data link protocols.

数据链路层
    在这一章中，我们将研究网络第二层——数据链路层的设计原则。我们将主要研究的是在数据链路层上，两台相连的机器之间进行可靠、有效通信而涉及到的一些算法。这里的相连，意思是指两台机器通过通信信
道进行物理连接．信道意思是：概念上像一条线路〔即同轴电缆、电话线或者点到点的无线通道)；能像线路一样，这也暗示了它的一个本质特征，是接收到的比特同发送出去的顺序完今相同。
     刚开始，也许会认为这个问题很简单，觉得没有什么内容需要学习——机器A只是把比特放到线路上，而机器B也只是取走比特。不幸的是，通信线路偶尔也会出错。而且，它们只有有限的数据速率，并且：在同一数据的发送和接收之间的传输时延也不为零。这些限制对于数据的有效传输有重要的影响。  用于通信过程的协议必须把所有这些因素都加以考虑。这些协议正是本章的主题。
     在介绍完数据短路层的关键设计问题之后，我们将透过考察差错的属性，出错原因，以及如何检测和纠正这些差错来研究数据链路层的协议。然后我们由浅入深地研究一系列复杂的协议，它们各自解决越来越多的数据链路层问题，最后，我们用有关协议模型和正确性的讨论，以及数据链路协议的例子来结束本章的学习。

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