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# 计算机代考|Network作业代写|COMS3200 RELIABLE TRANSMISSION

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## 计算机代考|Network作业代写|RELIABLE TRANSMISSION

As we saw in the previous section, frames are sometimes corrupted while in transit, with an error code like CRC used to detect such errors. While some error codes are strong enough also to correct errors, in practice, the overhead is typically too large to handle the range of bit and burst errors that can be introduced on a network link. Even when error-correcting codes are used (e.g., on wireless links), some errors will be too severe to be corrected. As a result, some corrupt frames must be discarded. A linklevel protocol that wants to deliver frames reliably must somehow recover from these discarded (lost) frames.

It is worth noting that reliability is a function that may be provided at the link level, but many modern link technologies omit this function. Furthermore, reliable delivery is frequently provided at higher levels, including both transport and, sometimes, the application layer. Exactly where it should be provided is a matter of some debate and depends on many factors. We describe the basics of reliable delivery here, since the principles are common across layers, but you should be aware that we are not just talking about a link-layer function.

Reliable delivery is usually accomplished using a combination of two fundamental mechanisms-acknowledgments and timeouts. An acknowledgment (ACK for short) is a small control frame that a protocol sends back to its peer saying that it has received an earlier frame. By control frame, we mean a header without any data, although a protocol can piggyback an ACK on a data frame it just happens to be sending in the opposite direction. The receipt of an acknowledgment indicates to the sender of the original frame that its frame was successfully delivered. If the sender does not receive an acknowledgment after a reasonable amount of time, then it retransmits the original frame. This action of waiting a reasonable amount of time is called a timeout.

The general strategy of using acknowledgments and timeouts to implement reliable delivery is sometimes called automatic repeat request (abbreviated ARQ). This section describes three different ARQ algorithms using generic language; that is, we do not give detailed information about a particular protocol’s header fields.

## 计算机代考|Network作业代写|Stop-and-Wait

The simplest ARQ scheme is the stop-and-wait algorithm. The idea of stop-and-wait is straightforward: after transmitting one frame, the sender waits for an acknowledgment before transmitting the next frame. If the acknowledgment does not arrive after a certain period of time, the sender times out and retransmits the original frame. Figure $2.14$ illustrates timelines for four different scenarios that result from this basic algorithm. The sending side is represented on the left, the receiving side is depicted on the right, and time flows from top to bottom. Figure 2.14(a) shows the situation in which the ACK is received before the timer expires; (b) and (c) show the situation in which the original frame and the ACK, respectively, are lost; and (d) shows the situation in which the timeout fires too soon. Recall that by “lost” we mean that the frame was corrupted while in transit, that this corruption was detected by an error code on the receiver, and that the frame was subsequently discarded.

The packet timelines shown in this section are examples of a frequently used tool in teaching, explaining, and designing protocols. They are useful because they capture visually the behavior over time of a distributed system-something that can be quite hard to analyze. When designing a protocol, you often have to be prepared for the unexpected-a system crashes, a message gets lost, or something that you expected to happen quickly turns out to take a long time. These sorts of diagrams can often help us understand what might go wrong in such cases and thus help a protocol designer be prepared for every eventuality.

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。