Introduction to the first chapter of "Computer network" notes

Three stages of Internet development

0 The first phase is the process of developing from a single network ARPANET to the Internet.

0 The second stage is the completion of the three-level structure of the Internet , including the backbone network, regional network, Campus Network (Enterprise Network).

0 The third stage is the gradual formation of multi-layered ISP structure of the Internet . (P6 figure 1-4 ISP-based multilayer Internet concept)

The component edge of the Internet (also known as End System end)

Consisting of all the hosts linked to the Internet, which are used directly by the user for communication (transmitting data, audio or video) and sharing resources;

Communication mode of the edge part

0 客户-服务器方式 (c/S mode), that is, Client/server mode. (Customer is the requester of the service, the server is the provider of the service)

0 对等连接方式 (peers), that is, the Peer-to-peer way. (Each host in a peering connection is both a client and a server.) ）

Core part

Consists of a large number of networks and routers that connect these networks. Provides connectivity and Exchange services for the Edge section

Core part switching technology circuit switching

Three stages: Connect-on-call-and- release connections . During a call, two of users occupy an end-to-end resource, and because most of the time the line is idle, the transmission rate of the line is often low.

Packet switching '

Using the storage and forwarding technology, that is, packet-to-storage packet-to -Find Routing (routing protocol) ---forwarding grouping.

The whole piece of data sent by the message
Grouping (packets) dividing messages into equal-length data segments
Header of Header (header) data segment containing important control information such as destination address , Source address , and so on.

concept map for dividing groups

Advantages of packet switching

The

Advantages	means used
Efficient	The transmission bandwidth is dynamically allocated during packet transfer, and the communication link is occupied by Segment
Flexible	Independent selection of forwarding routes for each group
Quickly	Grouped as a transmission unit, you can send groups to other hosts without first establishing a connection
Reliable	Network protocol to ensure reliability, distributed multi-router packet switching network, so that the network has a good survivability

Disadvantages of packet switching
1. Delay: Packets need to be queued when each router stores and forwards.
2. Overhead: Each group must carry control information.

Message exchange

Transmit the entire message to the neighboring node, store it all down, find the storage table, forward to the next node, delay

comparison of three types of exchange

Classification of computer networks

Classification by Network scope

广域网 WAN（Wide Area Network）: The core part of the Internet.
城域网 MAN（Metropolitan Area Network）: Used to interconnect LAN, many use Ethernet.
局域网 LAN（Local Area Network）
个人区域网 PAN（Personal Area Network）

by user category

公用网（public network）Telecommunications companies are financed to build, open to all paying people, such as Chinanet
专用网（private network）Enterprises, departments built, not open to outsiders

Performance indicator rate

b/s (bps bit per second). such as 100M Ethernet, the actual refers to 100mb/s.

Bandwidth

The highest data rate that a digital channel can transmit. Unit b/S.

Throughput

The amount of data that passes through a network (or channel, interface) within a unit of time. Its absolute upper value equals the bandwidth.

Delay (delay or latency)

Data (a message or packet, or even a bit) is transmitted from one segment of the network (or link) to the other end of time. Also called delay.
Delay classification

发送时延：主机或路由器发送数据帧所需的时间，也就是从发送数据帧的第一个比特算起，到该帧的最后一个比特发送完毕所需的时间。也成传输时延。            发送时延 = 数据帧长度（b） / 信道带宽（b/s）传播时延：电磁波在信道中传输一定距离所需划分的时间。            传播时延 = 信道长度（m） / 传输速率（m/s）处理时延：主机或路由器处理收到的分组所花费的时间。排队时延：分组在输入队列中等待处理的时间加上其在输出队列中等待转发的时间。综上：总时延 = 发送时延 + 传播时延 + 处理时延 + 排队时延。  

some of the places where time delays occur

Delay Bandwidth Product

Propagation delay * bandwidth. Represents the capacity of the link.

Round Trip Time RTT

The amount of time it takes to send data from the sender to the sender before sending the acknowledgement to the receiving party.

Utilization

A certain percentage of a channel is used (with data passing). High channel or network utilization can result in very large delays.

D0 indicates the network idle time delay, d indicates the network current delay, U represents network utilization, the following formula represents the relationship between the three
D = D0/1-u

the relationship between delay and network utilization

Formation of computer network system structure

分层---->SNA（System Network Architecture 系统网络体系结构）---> ISO（国际标准化组织）制定 OSI（开放式系统互连）--->TCP/IP协议

Agreement

A rule, standard, or constraint established for the exchange of data in a network.

Three elements of the agreement

0 syntax, structure or form of data and control information

0 semantics, which control information needs to be emitted, what actions to complete and what responses are made in a timely manner

0 synchronization, detailed description of time implementation sequence

Architecture of the five-tier protocol (bottom-up) (p28~31) Physical layer

Primarily responsible for transmitting raw binary data on physical lines;

Data Link Layer

The main responsibility is to establish the data link connection between the communication entities, assemble and extract frame data and error control and correction ;

Network layer

It is primarily responsible for creating logical links (choosing the appropriate routes, forwarding packets), and for encapsulating packets or packages into packet segments or user datagrams.

Transport Layer

0 传输控制协议TCP（Transmission Control Protocol） -Provide connection-oriented, reliable data transmission service, the data transmission unit is the message segment (segment).

0 用户数据报协议UDP(User Datagram Protocol) --Provide non-connected data transfer service, transmission Unit is user datagram.

Application Layer

Contains various protocols (HTTP,SMTP,FTP) that provide rules for application communication and interaction.

Note

1.internet（互联网）是一个通用名词，它泛指由多个计算机网络互联而成的网络）2.Internet（因特网）指的是全球最大的、开放的、由众多相互连接成的特定计算机网络，采用 TCP/IP 协议族作为通信规则，前身是美国的ARPANET。3.数据量大小单位KB表示千字节为1024，传输速率小写k/s表示的是1000.4.路由器的存储转发技术：路由器收到一个分组，先暂时存储一下，检查其首部中目的地址，找到合适的接口转发出去，把分组交给下一个路由器。路由器之间必须经常交换掌握的路由信息，以创建和维持在路由器中的转发表，使得转发表能够在整个网络拓扑结构中发生变化时及时更新。5.路由器暂时存储的是一个个短分组，而不是整个长报文，短分组是暂存在路由器中的存储器（即内存）中而不是存储在磁盘里。这就保证了较高的交换速率。6.对于高速网络链路，提高的是发送速率而不是传播速率。7.一些拥有较大主干网的ISP通常控制他们的信道利用率不超过50%。如果超过了就要准备扩容，增大线路带宽。

Examples

Books 10, 11, 15, 17, 18, 19

Introduction to the first chapter of "Computer network" notes