Note: This article is computer network architecture (3) - hierarchical thinking for example of notes (good video production)

Computer Network Architecture tiered approach Case

BACKGROUND : via a specific communication network to look at the case of computer network architecture hierarchy processing method

Case are as follows : In the following network topology, the host belongs to the network N1; Web services that belong to the network N2, N1 and N2 via the router interconnect, we used to communicate between the host and server, enter the URL in the browser on the pc, a host sends a request to the Web server; Web server receives a request, it sends back a response, a response is received after the host, it resolves to a specific web content is displayed; in this process is actually the browser and the web server process process-based communication network of
Here Insert Picture Description the above is just a simple process, mainly for illustrative computer network sub-stratification approach ; then layers of system institutions how to play a role in the whole process of it?

Architecture layers play what role in the communication process

From the host side by the architecture top-down view of the order, respectively, from the application layer, network layer, transport layer, link layer and physical layer point of view

Application layer

HTTP protocol application layer required to construct a HTTP packets, so that delivery to the underlying network layer after;
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Transport Layer

Application layer HTTP request message delivered to the transport layer processing, transport layer to the HTTP request to add a TCP header to make it a TCP segment .
Here Insert Picture Description The first part of the role is mainly to distinguish the application process, and achieve reliable transmission

Network layer

运输层将TCP报文段交付给网络层处理，网络层给TCP报文段添加一个IP首部，使之成为IP数据包
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该首部的作用主要是为了使IP数据报可以在互联网上传输，也就是被路由器转发

数据链路层

网络层将IP数据报交付给数据链路层处理，数据链路层给IP数据报添加一个首部和一个尾部，使之成为帧
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假设网络N1是以太网，这是以太网帧首部的格式，该首部的作用主要是为了让帧能够在一段链路上或一个网路上传输，能够被相应的主机接受，以太网帧尾部的格式，其作用是为了让目的主机检查所接受到的帧是否有误码

物理层

数据链路层将帧交付给物理层，物理层将帧看作是比特流，由于网络N1是以太网，因此物理层还会给该比特流前面添加前导码，其作用是为了让目的主机做好接受帧的准备

Here Insert Picture Description 物理层将加有前导码的比特流，变换成相应的信号发送到传输媒体，信号通过传输媒体到达路由器

看一下路由器的处理过程

当比特流到达路由器的时候，

物理层将信号变换为比特流，然后去掉前导码后，将其交付给数据链路层，这实际上交付的是帧
数据链路层将帧的首部和尾部去掉后，将其交付给网路层，这实际上交付的是IP数据报
网络层解析IP数据报的首部，从中提出出目的网络地址，然后查找自身的路由表，确定转发端口
网络层将IP数据报交付给数据链路层，数据链路层给IP数据报添加一个首部和一个尾部使之成为帧
The Data Link layer frame delivered to the physical layer, the physical layer frame is considered as a bit stream, since N2 is an Ethernet network, so the physical layer also adds the preamble to the bitstream
The physical layer preamble will be added with the bit stream to the transmission medium

Here Insert Picture Description Through the process: the signal reaches the web server via the transmission medium, we look at the process of the Web server. PC side processing is over once the processing of the Web server of the reverse process ; when the resolution requests the server application layer, the host sends back an HTTP response message, and before the process is similar:物理层->网络链路层->网络层->链路层->物理层

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HTTP response packet requires layer encapsulated Web server,
It is then converted into a corresponding signal via the physical layer,
And then transmitted to the router via the transmission medium,
The router forwards the response packet to the host,
Host signal converted to a bit stream through the received physical layer, layer by layer, after deblocking,
Remove the final HTTP response packet

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