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# 计算机代考|Network作业代写|CYBR371 Clock-Based Framing (SONET)

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## 计算机代考|Network作业代写|Clock-Based Framing (SONET)

A third approach to framing is exemplified by the Synchronous Optical Network (SONET) standard. For lack of a widely accepted generic term, we refer to this approach simply as clock-based framing. SONET was first proposed by Bell Communications Research (Bellcore) and then developed under the American National Standards Institute (ANSI) for digital transmission over optical fiber; it has since been adopted by the ITU-T. SONET has for many years been the dominant standard for long-distance transmission of data over optical networks.

An important point to make about SONET before we go any further is that the full specification is substantially larger than this book. Thus, the following discussion will necessarily cover only the high points of the standard. Also, SONET addresses both the framing problem and the encoding problem. It also addresses a problem that is very important for phone companies-the multiplexing of several low-speed links onto one high-speed link. (In fact, much of SONET’s design reflects the fact that phone companies have to be concerned with multiplexing large numbers of the 64-kbps channels that traditionally are used for telephone calls.) We begin with SONET’s approach to framing and discuss the other issues next.

As with the previously discussed framing schemes, a SONET frame has some special information that tells the receiver where the frame starts and ends; however, that is about as far as the similarities go. Notably, no bit stuffing is used, so that a frame’s length does not depend on the data being sent. So the question to ask is “how does the receiver know where each frame starts and ends?” We consider this question for the lowestspeed SONET link, which is known as STS-1 and runs at $51.84 \mathrm{Mbps}$. An STS- 1 frame is shown in Figure 2.9. It is arranged as 9 rows of 90 bytes each, and the first 3 bytes of each row are overhead, with the rest being available for data that are being transmitted over the link. The first 2 bytes of the frame contain a special bit pattern, and it is these bytes that enable the receiver to determine where the frame starts. However, since bit stuffing is not used, there is no reason why this pattern will not occasionally turn up in the payload portion of the frame. To guard against this, the receiver looks for the special bit pattern consistently, hoping to see it appearing once every 810 bytes, since each frame is $9 \times 90=810$ bytes long. When the special pattern turns up in the right place enough times, the receiver concludes that it is in sync and can then interpret the frame correctly.

## 计算机代考|Network作业代写|ERROR DETECTION

As discussed in Chapter 1 , bit errors are sometimes introduced into frames. This happens, for example, because of electrical interference or thermal noise. Although errors are rare, especially on optical links, some mechanism is needed to detect these errors so that corrective action can be taken. Otherwise, the end user is left wondering why the C program that successfully compiled just a moment ago now suddenly has a syntax error in it, when all that happened in the interim is that it was copied across a network file system.

There is a long history of techniques for dealing with bit errors in computer systems, dating back to at least the 1940s. Hamming and ReedSolomon codes are two notable examples that were developed for use in punch card readers, when storing data on magnetic disks, and in early core memories. This section describes some of the error detection techniques most commonly used in networking.

Detecting errors is only one part of the problem. The other part is the correction of errors once detected. Two basic approaches can be taken when the recipient of a message detects an error. One is to notify the sender that the message was corrupted so that the sender can retransmit a copy of the message. If bit errors are rare, then in all probability, the retransmitted copy will be error-free. Alternatively, some types of error detection algorithms allow the recipient to reconstruct the correct message even after it has been corrupted; such algorithms rely on errorcorrecting codes, discussed below.

One of the most common techniques for detecting transmission errors is a technique known as the cyclic redundancy check (CRC). It is used in nearly all the link-level protocols discussed in this chapter. This section outlines the basic CRC algorithm, but before discussing that approach, we first describe the simpler checksum scheme used by several Internet protocols.

## 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。