<> Chapter I Emergence and development of computer network
Phase II ： computer - computer network 20 century 60-70 Age emergence ARPANET Packet switching technology is adopted
What is computer network ?（ From the perspective of resource sharing ）
(1) The purpose of computer network is to realize the sharing of computer resources .
(2) Interconnected computer is a host computer with independent processing capacity distributed in different geographical locations .
(3) Communication between interconnected computers must comply with common network protocols .
Two functions of computer network ： Data communication and resource sharing
How are computer networks formed ：
Logically , It is divided into resource subnet and communication subnet
From the function ： Host and terminal responsible for data processing ; Router and communication line responsible for data communication processing
Computer networks can be divided into several types ?
According to different coverage, it can be divided into ： LAN , Wide area network , Metropolitan area network , Access network
According to different users, it can be divided into ： Public network , Private network
Two ways of computer communication ：
1. customer - Server mode （c/s） method （ The customer is the requestor of the service , The server is the provider of the service ）
2. Equivalent mode （p2p） method （ When communicating, the two hosts do not distinguish which is the service requestor or the service provider .）
Performance index of computer network ：
(1) bandwidth
Bandwidth calculation ： In a network , transmission 1 The time of bit binary signal is 0.01 ms, What is the bandwidth of the communication channel ?
bandwidth = 1 Bit /0.01ms = 100000 （b/s）
(2) time delay ：
transmission delay ( Transmission delay )： When sending data , The time required for the data block to enter the transmission medium from the node .
Propagation delay ： The time it takes for electromagnetic waves to travel a certain distance in the channel .
Processing delay ： The time spent in data processing for store and forward at the switching node .
(3) throughput
<> Chapter II data communication
Multiplexing technology ：
（1） Frequency division multiplexing , It is characterized by dividing the frequency band resources of circuit or space into multiple frequency bands , And assign it to multiple users , The data of each user terminal is transmitted through the sub channel allocated to it . It is mainly used in telephone and cable TV systems .
（2） Time division multiplexing , It is characterized by segmentation according to the transmission time , Transmit different signals at different times . There are two ways ： Synchronous time division multiplexing and asynchronous time division multiplexing .
（3） Wavelength division multiplexing , It is characterized by optical frequency division multiplexing . One optical fiber is used to transmit light wave signals close to multiple frequencies at the same time .
Data exchange technology ：
Circuit switching
advantage ： Strong real-time performance , Applicable to interactive session communication , Large amount of data transmission , High reliability requirements .
shortcoming ： When a connection is established and there is no data transmission between the two sides for the time being , Waste of bandwidth resources
( The next two belong to store and forward switching mode ）
Message exchange ( Transmission by message )： There is no need to establish a connection between the sender and the receiver in advance
packet switching ( Transmission in packets )
Datagram ：
There is no need to establish in advance between packet transmissions “ Line connection ”,
Each packet sent by the source host can independently select a transmission path ;
Each packet may reach the destination host through different transmission paths in the communication subnet .
( Therefore, it may be out of order when reaching the end )
Virtual circuit ：
A virtual circuit that requires a logical connection between the sender and the receiver
No destination address is required , Source address and other auxiliary information .
The packet will not be lost when it reaches the destination node , Repetition and disorder .
Coding method for analog digital signal
amplitude modulation , frequency modulation , Phase modulation
Pulse code modulation （PCM） It is the main method of analog data digitization
It mainly includes three steps ： sampling , Quantization and coding
Digital data coding ：
Error checking ： Cyclic redundancy code check
subject ： A data communication system adopts CRC Inspection method , And generate polynomials G(x) The binary bit sequence of is 11001, The binary bit sequence received by the destination node is 10111001( contain CRC Inspection code ), Please judge whether there are errors in the transmission process ? Why? ?
The inspection code is calculated by the sender （ Sender ）：
W=D*2N （D Send data ,N Is the number of inspection codes ）
N What is it Just fill in the back 0, Then get W Pair generating polynomial G Carry out residual , The result is the value of the check code
Check whether the receiver judges whether there is an error （ Receiver ）
Received via destination node CRC Binary bit sequence of check code W yes Generating polynomial G Carry out residual , The remainder is 0 correct , Otherwise, an error occurs
<> Chapter III Computer network architecture
ISO/OSI How many layers are the reference model divided into and what are the functions of each layer ?
Divided into 7 layer , The functions of each layer are as follows
physical layer ; Realize transparent transmission of bit stream ;
data link layer ： Realize error control , Flow control, etc ;
network layer ： Realize data grouping , Routing ;
Transport layer ： Provide reliable end-to-end data transmission ;
Session layer ： Establish a connection for the communication process ;
Presentation layer ： Data structure representation , data conversion , encryption , compress ;
application layer ： Provide application entry OSI Model entry .
TCP/IP How many layers are the reference model divided into and what are the functions of each layer ?
TCP/IP reference model 4 layer
application layer , Provide users with interfaces to call and access various applications in the network
Transport layer , Provide reliable or unreliable end-to-end data transmission ;
network layer , Between adjacent nodes , Grouped data IP Address addressing and routing
Network interface layer , Responsible for sending and receiving through the network IP Datagram
ask ： Please describe the communication between the two hosts OSI Data transmission process of reference model .
（ Encapsulation and unpacking of data ）
<> Chapter IV LAN
There are three main technical factors that determine the characteristics of LAN ：
Topology connecting various devices , Data transmission medium and media access control method .
topological structure ： Star type , Ring type , Bus type
Data transmission media ： Twisted pair , Coaxial cable , Optical fiber
Media access control ：
CSMA/CD（ Carrier sense multiple access with collision detection ）( control ： Bus LAN )
Token Bus（ Token bus ）( control ： Token bus LAN )
Token Ring （ Token ring ）( control ： Ring LAN )
IEEE802 agreement
It is divided into physical layer and data link layer ( Including logical link control LLC Sublayer , Media access control MAC Sublayer )
CSMA/CD 802.3
Token Bus 802.4
Token Ring 802.5
How to solve the problem of sharing transmission line of traditional Ethernet ?
use CSMA/CD（ Carrier sense multiple access with collision detection ）
Working principle or characteristics ：
Listen before you send —— When the channel is not idle , There will never be conflict
Listen and send —— Ensure that in case of conflict , The conflicting sites will stop sending immediately
Conflict stop —— Ensure that there is no violent conflict in the system
Delayed retransmission —— The stability of the system is guaranteed
Contention period ( Shortest frame length calculation )
Contention period = 2*（ Channel length / Signal transmission rate ）* Data propagation rate
Question 1 ： It is known that the propagation rate of electromagnetic wave in cable is about 2.3×105km/s, For a data transmission rate of 10Mbps, Coaxial cable length is 1km Local area network , It can be calculated that the contention period in the coaxial cable channel is ：
Contention period = 2*（1km / 2.310^5 km/s ） * 1010001000 bit/s
= 2 0.5*10 of -5 Power * 10 of 7 Power bit/s
= 100 bits
Question 2 ：
Minimum frame length = 2*（2km / 10000km/s）*10 000 000bit/s
= 400 bit = 50 byte
How can switched LAN effectively improve the transmission rate of LAN users ?（ interconnection ）
Physical layer interconnection ( Shared bandwidth )： Repeater , Hub
Data link layer interconnection ： bridge , Switch ( Exclusive bandwidth )
Why do we need the technology of virtual LAN ?
Virtual LAN definition ： Virtual LAN (VLAN,Virtual LAN) It has some common needs , A physical location independent logical working group consisting of a number of LAN segments .
adopt Switch realization
The most common Division VLAN The method is ： Port based partitioning
<> Chapter V Network interconnection technology
Why network interconnection ?
The key is to be transparent
Form of network interconnection ：
Physical layer interconnection . Using a repeater or hub .
Data link layer interconnection . Using bridges or switches .
Network layer interconnection . Routers are used to store and forward packets between different networks .
Interconnection above the network layer . Use the gateway to provide a higher level of interconnection .
Internet Protocol （IP agreement ） What are the functions of ?
ARP Agreement function ： Used to establish IP A protocol for address to physical address translation , To support the proper determination of physical addresses .
RARP Agreement function ： Used to establish physical address to IP A protocol of address translation relation , To support those needs IP The network host of the address can obtain IP address .
ICMP Agreement function ： Used to support hosts or routers to send queries to hosts for a specific purpose .
IGMP Agreement function ： Used to help manage multicast groups .
NAT( Network address translation ) Role of ： Achieve Intranet IP Mutual conversion between address and public network address
IPV6 Characteristics of ： Address length is 128 position
Problems to be solved ： At this stage IP Not enough addresses
At this stage IPV4 Address length is 32 position
Function of port ?(TCP,UDP Use both IP agreement )
Connection oriented TCP(Transmission Control Protocol)
It provides reliable connection oriented transmission service
And unconnected UDP(User Datagram Protocol) .
Provides unreliable connectionless transmission services
Assignment of subnet mask
Example of subnet Division ：
<> Chapter VI Internet
Definition of domain name ： Relative and IP Address proposal , Is an easy to remember host name .
Domain name conversion process
Common domain name types
E-mail transmission process
FTP Using TCP agreement , Port number 21
Telnet Using TCP agreement , Port number 23
SNMP（Simple Network Management Protocol）, “ Simple network management protocol ”, Protocols belonging to the application layer
Technology
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