A computer network architecture of the computer network Getting Started
1.1 Cause the computer network architecture is formed:
The computer network is a complex system , now give a simple example of computer network applications: two computers connected to the network to transfer files, the first to establish a path between two computers, there is less work to be done after:
1) through the first two computers communicate instructions to ensure that these two data can be calculated in the correct transmission and reception of
2) to tell the network how to receive data
3) to initiate communication with the computer to ensure whether the other computers can communicate
4) to initiate communication with the computer you must figure out whether the other computer file management application is ready to receive documents and store files
5) if an error occurs during transmission, should be a reliable way to ensure that the recipient can eventually receive the correct data
from the above it can be seen computer networks have a lot of details that need to be considered, in order to solve these problems, the designer of computer networks proposed the concept of layering, dividing the problem into different parts, one by one to resolve in the implementation process.
1.2 Development history of computer network architecture
IBM first proposed a computer network architecture, namely Systems Network Architecture (System Network Architecture) This structure has been in use since IBM
after other companies have also proposed a different architecture of their company name
International Organization for Standardization ISO 1977 proposed the famous OSI basic reference model (Open systems Interconnection reference model), referred to as OSI, it is divided into seven computer network, it is also known as seven-layer protocol architecture
After the OSI standard to make some big companies and governments have expressed support, but after many years in the past due to the OSI standard is too complex and did not consider market conditions to be eliminated, today's largest Internet coverage around the world and does not use OSI standards. The main reason for the failure is the development of OSI OSI who are academic experts, they are not considered in the development of OSI market conditions resulting in no one wants to use OSI
Now we use networks using TCP / IP architecture , TCP / IP architecture is developed by commercial companies, due to its good to meet the needs of the market and are widely used large-scale, TCP / IP architecture only four, simple to use
II. Computer network architecture at all levels
In the past few decades, various companies and organizations a lot of computer network architecture model, each model will become the computer network into different levels, which is two influential OSI model and the TCP / IP model , OSI computer network into seven layers, TCP / IP model computer network is divided into four layers. Since the standards are not unified computer network system is not good levels of learning, so teaching is generally the OSI and TCP / IP were taking and compromise, to teach five levels, namely: the application layer, transport layer, network layer, data link path layer, the physical layer
OSI, TCP / IP, five agreements comparison:
2.1 Application Layer:
Application Layer Introduction:
网络应用层是通信用户之间的窗口，为用户提供网络管理、文件传输、事务处理等服务。其中包含了若干个独立的、用户通用的服务协议模块。网络应用层是OSI的最高层，为网络用户之间的通信提供专用的程序。应用层的内容主要取决于用户的各自需要，这一层设计的主要问题是分布数据库、分布计算技术、网络操作系统和分布操作系统、远程文件传输、电子邮件、终端电话及远程作业登录与控制等。至2011年应用层在国际上没有完整的标准，是一个范围很广的研究领域。在OSI的7个层次中，应用层是最复杂的，所包含的应用层协议也最多，有些还在研究和开发之中
应用层协议的代表包括：Telnet、FTP、HTTP、SNMP、DNS等。
应用层主要功能：
应用层为操作系统或网络应用程序提供访问网络服务的接口
2.2 运输层：
运输层简介：
运输层建立在网络层和会话层之间，实质上它是网络体系结构中高低层之间衔接的一个接口层。用一个寻址机制来标识一个特定的应用程序（端口号）。运输层不仅是一个单独的结构层，它还是整个分层体系协议的核心，没有运输层整个分层协议就没有意义。运输层的数据单元是由数据组织成的数据段（segment）这个层负责获取全部信息，因此，它必须跟踪数据单元碎片、乱序到达的数据包和其它在传输过程中可能发生的危险
运输层为上层提供端到端（最终用户到最终用户）的透明的、可靠的数据传输服务，所谓透明的传输是指在通信过程中运输层对上层屏蔽了通信传输系统的具体细节
运输层协议主要是：传输控制协议TCP(Transmission Control Protocol）和用户数据报协议UDP(User Datagram protocol）。
运输层的主要功能:
一、格式化信息流
二、提供可靠传输，为实现可靠传输，传输层协议规定接收端必须发回确认，并且假如分组丢失，必须重新发送，即耳熟能详的“三次握手”过程，从而提供可靠的数据传输。
2.3 网络层：
网络层简介：
网络层也称通信子网层，是高层协议之间的界面层，用于控制通信子网的操作，是通信子网与资源子网的接口。在计算机网络中进行通信的两个计算机之间可能会经过很多个数据链路，也可能还要经过很多通信子网。网络层的任务就是选择合适的网间路由和交换结点，确保数据及时传送。网络层将解封装数据链路层收到的帧，提取数据包，包中封装有网络层包头，其中含有逻辑地址信息源站点和目的站点地址的网络地址
IP是第三层问题的一部分，此外还有一些路由协议和地址解析协议（ARP）。有关路由的一切事情都在第三层处理。地址解析和路由是三层的重要目的。网络层还可以实现拥塞控制、网际互连、信息包顺序控制及网络记账等功能
在网络层交换的数据单元的单位是分割和重新组合数据包（packet）
网络层协议的代表包括：IP、IPX、OSPF等
网络层主要功能：
网络层主要功能是基于网络层地址（IP地址）进行不同网络系统间的路径选择。
网络层为建立网络连接和为上层提供服务，应具备以下主要功能：
一.路由选择和中继
二.激活，终止网络连接
三.差错检测与恢复
四.排序，流量控制
数据链路层的典型设备：
网关、路由器
2.4 数据链路层：
数据链路层简介：
在物理层提供比特流服务的基础上，将比特信息封装成数据帧（Frame），起到在物理层上建立、撤销、标识逻辑链接和链路复用以及差错校验等功能。通过使用接收系统的硬件地址或物理地址来寻址。建立相邻结点之间的数据链路，通过差错控制提供数据帧（Frame）在信道上无差错的传输，同时为其上面的网络层提供有效的服务。数据链路层在不可靠的物理介质上提供可靠的传输。该层的作用包括：物理地址寻址、数据的成帧、流量控制、数据的检错、重发等。
数据链路层协议的代表包括：SDLC、HDLC、PPP、STP、帧中继等。
数据链路层的主要功能：
1）链路层的功能是实现系统实体间二进制信息块的正确传输
2）为网络层提供可靠无错误的数据信息
3）在数据链路中解决信息模式、操作模式、差错控制、流量控制、信息交换过程和通信控制规程的问题
4）为网络层提供数据传送服务，这种服务要依靠本层具备的功能来实现。链路层应具备如下功能：
一.链路连接的建立，拆除，分离
二.帧定界和帧同步。链路层的数据传输单元是帧，协议不同，帧的长短和界面也有差别，但无论如何必须对帧进行定界
三.顺序控制，指对帧的收发顺序的控制
四.差错检测和恢复。还有链路标识，流量控制等等。差错检测多用方阵码校验和循环码校验来检测信道上数据的误码，而帧丢失等用序号检测。各种错误的恢复则常靠反馈重发技术来完成
数据链路层的典型设备：
二层交换机、网桥、网卡
2.5 物理层：
物理层简介：
The physical layer OSI layered architecture is the most important and most basic level, it is built on the basis of the transmission medium, from the establishment, maintenance and cancellation effect physical connection, implement the physical interface between devices. Receiving and transmitting a physical layer of a series of bits (bit) stream, and does not consider the significance of the information structure information. The physical layer includes a predetermined equipment connected to the network which describe various mechanical, electrical, functional
The main function of the physical layer:
1) providing a data transfer path for data terminal device
data path may be a physical medium, it may be connected in a plurality of physical media. A complete data transfer, comprising activating a physical connection, data transfer, terminating the physical connection. The so-called activation, regardless of how many physical media is involved between the two to be connected in communication with data terminal equipment to form a passage.
2) transmit data
Physical layer entity is adapted to be formed needs data transmission, data transmission services, including the following:
First, to ensure the correctness of the data bit transmission
II provides a transparent bit transfer to the data link layer
three providing sufficient bandwidth (the bandwidth is by bits per second (bIT) number), to reduce the congestion of the channel. Transmission of data to meet the point, point to multipoint, serial or parallel, half-duplex or full-duplex, synchronous or asynchronous transmission required
to complete some physical management layer, such as a data terminal device, data communication in and switching equipment to complete the establishment of a data link between the devices, holding and removal operation
Typical equipment physical layer:
Fiber optic, coaxial cable, twisted pair, and the repeater hub