The article refers to B station: Wang Dao Postgraduate entrance examination – computer network

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0. Introduction to computer networks

0.1 Introduction to computer Networks

0.2 Function introduction of computer network

Data communication (connectivity)
Resource sharing:
- software
- hardware
- data
Distributed processing
- Multiple computers doing different parts of the same job, such as the Hadoop platform
Improved reliability
Load balancing
…

0.3 Introduction to the composition of computer networks

One, according to the component points
- Hardware + software + protocol
Two, according to the way of work
- Edge part: users directly use
  - 1. C/S mode
  - 1. P2P mode
- Core: Serves the edge
Three, according to the functional composition of points
- Communication subnet: For implementationData communication
- Resource subnet: For implementationResource sharing/data processing

0.4 Classification of computer networks

I. Classification by distribution range
- WAN: Switching technology is used
- MAN MAN
- LAN WAN: Broadcast technology is used
- Personal regional network PAN
Two, according to users
- Public network: network provided by users such as China Telecom and China Mobile
- Private network: a network used by the user, for example, a national or military unit
Three, according to the exchange technology
- Circuit switching
- Message switching
- Packet switching
Four, according to the topology results
- Main line
- The star
- ring
- Mesh type (often used for wide area networks)
Five, according to the transmission technology
- Broadcast network:Share common communication channels
- Peer-to-peer networking: usePacket store and forward 和 routingmechanism

1. Rate-related performance indicators

Rate of 1.1

Definition: The rate at which a host connected to a computer network transmits bits of data over digital channels
unit:b/s Kb/s Mb/s Tb/s

In bytes, the value is B/s KB/s MB/s TB/s 1Byte=8 bits

1.2 the bandwidth

In a computer network, it refers to the maximum speed supported by the network equipment. The unit is the same rate, which is the highest speed under ideal conditions. For example, if the link bandwidth is 1Mb/s, it means that the maximum amount of data that can be injected into the link within a unit time is 1Mb.

A popular example is: the hen lays eggs, assuming that 1Mb/s means the hen can lay one egg per second, then 2Mb/s means the hen can lay two eggs per second!

1.3 throughput

Throughput refers to the total amount of data that passes through a network (or channel or interface) in a unit time (unit: B /s, KB /s, Mb/s, etc.).

summary

The speed is the actual network speed, the bandwidth is the theoretical network speed, and the throughput is the combined rate of one or more devices. For example, a router with 1000M broadband is connected to three mobile phones, each of which is 10Mb/s to watch movies, so the speed is 10Mb/s, and the bandwidth is 1000M of broadband. The router throughput is 30Mb/s, which is the sum of all three!

2. Delay-related indicators

2.1 time delay

Definition: Delay refers to the time it takes for data (packets/packets/bitstreams) to travel from one end of a network (or link) to the other. Also called delay or delay. It’s in seconds.

Delay includes four categories:

The name of the	describe	A formula to calculate
Send time delay	The time it takes for data to travel from the host to the channel	`Length of data sent/rate of data sent`
Propagation delay	The time taken for data to travel over a channel	`Channel length/the rate at which electromagnetic waves travel over the channel`
Queuing delay	The time it takes for data to wait in front of the router for previous data processing	No calculation mode
To deal with time delay	The time it takes for data to process requirements in the router	No calculation mode

Example:

1. Find the transmission delay:

Suppose the transmitted data is 0101010101, then the data length is 10 bits, suppose the channel bandwidth (transmission rate) is 10B /s,

Then according to the data length/transmission rate calculated by the formula, the transmission delay is 10/10=1s

2. Find the propagation delay:

Assuming that the channel length is 100m and the propagation rate of electromagnetic wave on the channel is 10m/s, the calculation formula is as follows:

Propagation delay is equal to: channel length/electromagnetic wave propagation rate in the channel = 100/10=10s

Note: using high-speed link (improve network speed), can only reduce the transmission delay, can not reduce the other three delays!

2.2 Latency bandwidth product

* * * *

Calculation formula: delay bandwidth product = propagation delay * bandwidth

The delay bandwidth product is used to describe how many bits of data exist on a link.

2.3 Round trip delay RTT

RTT definition: The time it takes from the time the sender sends data to the time the receiver acknowledges receipt!

RTT includes: propagation delay * 2 + terminal processing time (sometimes directly ignored). According to the formula, RTT does not include transmission delay, which only represents the propagation delay on the channel!

Example:

Utilization rate of 2.4

Utilization rate, as the name implies, is the efficiency of using!

In computer networks, there are two types of utilization:

Channel utilization
Network utilization

2.4.1 Channel utilization

Calculation formula: Channel utilization = Time for data to pass/(Time for data to pass + Time for no data to pass)

2.4.2 Network Usage

Calculation formula: network utilization = weighted average of all channel utilization

2.4.3 Relationship between latency and utilization

The higher the utilization, the greater the latency!

2.5 Summary of brain map

3. Layered structure

3.1 Why and what should stratification do

Take sending a file as an example. Before sending a file, complete the following five tasks:

(1) The computer initiating the communication must activate the data communication pathway.

(2) to tell the network how to identify the destination host.

(3) The computer that initiates communication should check whether the target host is powered on and connected to the network normally.

(4) initiate communication computer to make clear, the other computer in the file management program is ready to work.

(5) Ensure that errors and accidents can be resolved.

3.2 Formal understanding of hierarchical structures

Basic principles of layering

Project independence between each layer, each layer only realizes a relatively independent function;
The interface between each layer is natural and clear, easy to understand, with as little mutual communication as possible;
The structure can be divided up and down, each layer adopts the most appropriate technology to achieve;
keepThe lowerrightThe upperOf independence,The upper layer uses the services provided by the lower layer in one direction;
The whole hierarchy should facilitate standardization;

3.3 Concept Summary

The network architecture is fromfunctionDescribe the computer network structure;
Computer Network Architecture abbreviation: Network architecture isHierarchical structure;
Each layer follows a certain (or certain)Network protocolTo complete this layer function;
Computer network architectureIt’s a computer networkThe layers and their agreementsA collection of;
When layer NTH provides a service to layer N +1, the service not only contains the functionality of layer NTH itself, but also the functionality provided by the service below!
Only in theInterfaces exist between adjacent layers, and the specific details of the services provided on the next levelCompletely shielding!
The architecture isabstractAnd reality refers to some software and hardware that can run!

Brain figure summarizes

4. Computer network hierarchical structure reference model

There are two kinds of computer network hierarchies:

7-layer OSI Reference Model (Statutory standard)
Layer 4 TCP/IP Reference Model (de facto standard)

4.1 OSI reference model

Network layer and above, each layer processes the data sent by the previous layer (with a header)

The data link layer needs to add not only headers but also tails

The physical layer adds nothing, just sends data (bitstream)

The OSI reference model is divided into seven layers from the bottom up

The name of the	English	role
The application layer	Application Layer	Directly services the user’s application processes, such as E-mail, file transfer, and terminal emulation. For example, HTTP, SMTP, FTP, and DNS
The presentation layer	Presentation Layer	To convert data into a format compatible with the recipient’s system format and suitable for transmission, that is, to allow two systems to exchange information
The session layer	Session Layer	Responsible for setting up and maintaining communication connections between two computers in the computer network during data transfer
The transport layer	Transport Layer	Responsible for end-to-end communication, reliable transmission, unreliable transmission, flow control, reuse and reuse
The network layer	Network Layer	Responsible for selecting the best route, planning IP addresses (ipv4 and ipv6 changes only affect the network layer), and congestion control
Data link layer	Data Link Layer	Frame start and end, transparent transmission, error checking (error correction is handled by the transport layer)
The physical layer	Physical Layer	Define network equipment interface standards, electrical standards (voltage), how to transmit faster over physical links

4.4.1 application layer

Description: Provides services for user application processes, such as email, file transfer, and terminal emulation. Such as HTTP, SMTP, FTP, DNS, etc.

4.1.2 the presentation layer

Description: To convert data into a format compatible with the recipient’s system format and suitable for transmission, that is, two systems can exchange information;

4.1.3 the session layer

Description: Responsible for setting up and maintaining communication connections between two computers in the computer network during data transmission;

4.1.4 the transport layer

Description: Responsible for end-to-end communication, reliable transmission, unreliable transmission, flow control, reuse and reuse;

Function is introduced

Function 1: Reliable transmission, unreliable transmission
- Reliable transmission: Transfer a large file, for example, or important documents, to the sender and the receiver by first confirmation mechanism, confirmed after the connection, and then cut the file layered multiple segments or user datagram transmission (order), then one by one when receiving Fang Chenggong receives a message segment, the sender sends the next message segment, if a message send/receive failure, The sender resends! Until all segments are successfully received by the receiver!
- Unreliable transmission: for example, when simple files or messages are sent, datagrams can be directly sent to the receiver, without confirmation mechanism to confirm the connection and other processes!
Function 2: error control. For example, when transmitting files or data, some message segments are lost, or the transmission is out of order (order error). At this time, error control is needed to correct errors!
Function 3: Flow control. For example, when a large amount of data is sent to the receiver, but the receiver cannot receive the data at the same rate, the data sender needs to control the amount of data to be sent (such as slowing down the data transmission rate). This is called flow control!
Function 4: Reuse and reuse
- Reuse: Multiple application layer processes can simultaneously use the services of the lower transport layer;
- Distribution: the transport layer delivers the received information to the corresponding process in the upper application layer.

4.1.5 network layer

Description: Responsible for selecting the best route, planning IP addresses (ipv4 and ipv6 changes only affect the network layer), and congestion control;

Function is introduced

Function 1: Routing (Select the best path)
Feature 2:Flow control, that is, theThe senderSend quantity (send rate) control, if the receiving end because of the large amount of data, but when receiving, will be on the sending end flow control!
Function 3: error control, communication between the two nodes to agree some rules (such as parity check code), if the verification, the group does not meet the agreed rules, the group will be rectified or discarded (rectification failure discarded)!
Function 4: congestion control, which is different from flow control (sender), congestion control is the control of data transmission in the global, for example: all nodes are too late to receive packets, and to lose a large number of packets, the network is in a congested state, so we need to take certain measures to ease this congestion!