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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Automatic Repeat Request Retransmission Mechanisms

If an error occurs when continuous $\mathrm{RQ}$ is in use, there is a choice of $\mathrm{ARQ}$ retransmission mechanisms. These mechanisms are go-back- $N$ and selective retransmission. In go-back- $N$, after an error, the receiver sends a NAK. This means that the block having that sequence number should be sent again. The sender sends that block again and then sends the following blocks, even though these may already have been transmitted successfully. In Fig. 2.16, block $N$ is positively acknowledged but block $N+1$ is negatively acknowledged. By the time the NAK for block $N+1$ arrives, the sender has already sent out blocks $N+2$ and $N+3$. Since blocks $N+2$ and $N+3$ are out of sequence, the receiver ignores these and waits for block $N+1$ to arrive again. Having retransmitted block $N+1$ (which this time is received with no errors), the sender retransmits blocks $N+2$ and $N+3$

Selective retransmission is illustrated in Fig. 2.17. When selective retransmission is in use, the sender’s response to a NAK for block $N+1$ is to retransmit block $N+1$ only, but not blocks $N+2$ and $N+3$. Despite the fact that blocks $N+2$ and $N+3$ are now out of sequence (because block $N+1$ has not arrived intact), the receiver accepts them. When the sender has retransmitted block $N+1$, it sends blocks $N+4$ and $N+5$ and so on.

The disadvantage of the go-back- $N$ method is that some blocks will be retransmitted unnecessarily. This is a waste of bandwidth. The disadvantage of selective retransmission is that the receiver needs plenty of buffer (temporary storage) capacity in order to store temporarily data blocks that have been received out of sequence. Go-back- $N$ is the more popular of the two methods, since buffer capacity is finite.

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Forward Error Correction

The idea behind FEC is to add enough redundant bits to the data block to be transmitted so that the receiver can correct errors without having to ask for a retransmission. It is, of course, essential to use FEC instead of ARQ if only a simplex link is available because in such a situation it is impossible to ask for a retransmission. When there is a duplex link, FEC is often used in combination with ARQ.

Two-dimensional parity offers a very simple form of FEC (see Fig. 2.18). In twodimensional parity, we not only add a parity bit to each character but also add a row of parity bits after a block of characters. The row of parity bits is actually a parity bit for each ‘column’ of characters. The row parity bits plus the column parity bits add a great amount of redundancy to a block of characters. Unfortunately, such a system can correct only single-bit errors. So, in practice, we need a more sophisticated system, as it is quite possible that there will be more than one error in a block.
Usually, special error correcting codes known as Reed-Solomon codes are used for FEC. The applications in which these codes are used include wireless and mobile communications and digital subscriber line (DSL) modems. The sophisticated mathematical techniques used by Reed-Solomon codes are beyond the scope of this text. Please refer to the following Web site if you need further information: http://www.4i2i.com/reed_solomon_codes.htm.

# 计算机网络概论代考

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Forward Error Correction

FEC 背后的想法是向要传输的数据块添加足够的冗余位，以便接收器可以纠正错误而无需请求重新传输。当然，如果只有单工链路可用，则必须使用 FEC 而不是 ARQ，因为在这种情况下，不可能请求重传。当有双工链路时，FEC 常与 ARQ 结合使用。

## 有限元方法代写

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## 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。

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