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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|Circuit Switching

Circuit switching is used in the public switched telephone network (PSTN). Circuit switching has three stages. Firstly, a circuit is set up from one end device to another. This circuit may involve several switches along the route. Next, the data is transferred. Finally, the circuit is disconnected. In circuit switching, there is a dedicatêd path between the end devices.

Setting up the circuit before any data can be sent takes some time. Let us assume that circuit switching is in use in the network shown in Hig. 2.19. If computer 8 needs to connect to computer 7 , the complete circuit between them needs to be set up. Computer 8 will send a request to switch $E$ to be connected to computer 7 . Switch $\mathrm{E}$ will have knowledge of available routes and will be able to choose the best one. A dedicated path is set up between switches $\mathrm{E}$ and $\mathrm{D}$. Switch $\mathrm{D}$ then asks for access to computer 7 . If computer 7 permits access, then an ACK will be sent back to computer 8 . Only now can the data be transferred. When the data is being sent, the only delay involved will be the time taken for it to propagate through the network. When data transfer is finished, more signalling takes place to ‘tear down’ the circuit (get rid of it) and make all the links that have been used available again for future use.

If there is only a little data to transfer, circuit switching is inefficient because of the time taken to set up the circuit. Also, a transfer will hog a transmission path that no other devices can use until the circuit is released. Despite these disadvantages, circuit switching has been very successfully used for voice transmission.

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Message Switching

In message switching, the circuits are permanent but it is the data that is switched. There is no need to set up a dedicated path before sending the data. If an end device needs to send a message, it adds the address of the destination to the message before sending it off. At every switch in the communication path, the message is briefly stored before being passed on to the next switch. For obvious reasons, this technique is known as store and forward.

Let us assume that message switching is in use in the network shown in Fig. 2.19. Computer 3 wants to send a message to computer 7. Computer 3 appends the address of computer 7 to the message. The message is store briefly by switch B, which then forwards it to switch F. After having briefly stored the message, switch F forwards it to switch D. Switch D stores the message before forwarding it to computer $7 .$

For computer data transfer, message switching is more efficient than circuit switching. However, it is no longer used because of the large amounts of storage space required on the switches. Furthermore, the delays involved are unacceptable today.

# 计算机网络概论代考

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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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