Technology Sharing

2.5 Computer Networks

2024-07-12

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Statement: The article referencesSystem architectTutorial (Second Edition)", if there is any infringement, I will modify and delete it immediately.

A system that uses communication lines to connect geographically dispersed computer systems and communication equipment with independent functions in different forms, and relies on network software and communication protocols to achieve resource sharing and information transmission.

1. Basic Concepts of Network

1. Development

(1) Birth

In the 1960s, a remote online system centered on a single computer was used to transmit information.

(2) Formation

In the 1960s and 1970s, multiple hosts were connected through communication lines to share resources with each other.

(3) Interconnection

From the late 1970s to the 1990s, there was an open and standardized network with a unified network architecture that complied with international standards. Two important architectures emerged during this period: the TCP/IP architecture and the OSI architecture of the International Organization for Standardization.

(4) Rapid development

From the 1990s to the present, local area networks have matured, optical fiber and high-speed network technology have emerged, and the Internet, represented by the Internet, has developed.

2. Function

(1) Data communication (main function)

A communication method that uses data transmission technology to transmit information between two communication nodes according to a certain communication protocol. The transmitted information is expressed in binary data form. Features: Related to telematics.

PS: Three major communication services: telegraph, telephone, and computer network

(2) Resource sharing (main purpose)

Sharing of hardware resources, software resources, and data resources

(3) Centralized management

MIS

(4) Implementing distributed processing

Large tasks are divided into small tasks that are processed by different computers and then solved centrally.

(5) Load balancing

It means that the workload is evenly distributed to each computer system on the network. The network center is responsible for distribution and monitoring. When a computer is overloaded, the system automatically transfers the workload to a computer system with a lighter load.

3. Relevant indicators

(1) Performance indicators

  • Rate (data rate/bit rate, one of the most important performance indicators, unit b/s (bits per second))

The rate at which hosts or communications devices connected to a computer network can transmit data over a digital channel.

  • Bandwidth (two meanings)

1) Refers to the channel width of a signal. The bandwidth of a signal indicates the frequency range occupied by the various frequency components contained in a signal. The unit is Hertz (kHz, MHz, Gigahertz, etc.)

2) Computer network: Bandwidth refers to the ability of the network's communication lines to transmit data. Network bandwidth refers to the "maximum data rate" that can pass from one node to another in the network per unit time. Unit: bits per second, b/s

  • Throughput

The amount of data that passes through a network (or channel, interface) per unit time. Limited by the network bandwidth or network rated rate.

Sometimes throughput is also expressed in bytes or frames transmitted per second.

  • Latency (delay/latency, an important performance indicator)

The time it takes for data (a message, packet, or even bit) to travel from one end of a network (link) to the other.

It is divided into: sending delay, propagation delay, processing delay, queuing delay, etc.

  • Round Trip Time (RTT, an important performance indicator)

The time from when the sender sends data to when the sender receives confirmation from the receiver (the receiver sends confirmation immediately after receiving the data).

  • Utilization

Channel utilization: The probability that a channel is being used (data is passing through it), expressed as a percentage. A completely idle channel has a utilization of zero.

Network utilization: The weighted average of channel utilization across the entire network.

(2) Non-performance indicators

  • cost
  • quality
  • standardization
  • reliability
  • Scalability and Upgradeability
  • Easy to manage and maintain

4. Application prospects

The world's second largest network after the global telephone network

2. Communication Technology (Basics of Computer Networks)

1. Channel

Information transmission: The process by which the source and destination send and receive information through a channel.

(1) Classification

(2) Information transmission and processing process

1) Channel is the channel for information transmission

2) The transmitter receives the information sent by the source, encodes and modulates it, converts the information into a signal suitable for transmission on the channel, and sends it to the channel.

3) The receiver is responsible for receiving information from the channel, demodulating and decoding it, and restoring the information to the host. Not all frequency signals can be transmitted through the channel. The frequency range is the bandwidth of the channel.

(3) Shannon formula

Calculate channel capacity: the maximum transmission rate of the channel

C=B*log2(1+S/N)

C: channel capacity, b/s

B: signal bandwidth, Hz

S: Average signal power, W

N: average noise power, W

S/N: signal-to-noise ratio, dB (decibel)

To increase channel capacity, you can use a larger bandwidth to reduce the signal-to-noise ratio, or a smaller bandwidth to increase the signal-to-noise ratio.

2. Signal transformation

Transmitter signal processing: source coding, channel coding, interleaving, pulse shaping, and modulation.

Receiver signal processing: demodulation, sampling decision, deinterleaving, channel decoding, source decoding.

(1) Source Coding

Analog signal - analog-to-digital conversion - compression coding (removing redundant information) - digital signal

(2) Channel Coding

Redundant information is added to facilitate error detection and correction at the receiving end.

(3) Interweaving

In order to solve the problem of channel decoding errors caused by continuous bit errors, interleaving scrambles the data order after channel coding according to a certain rule, and restores the data order through interleaving before decoding at the receiving end.

(4) Pulse shaping

To reduce bandwidth requirements, the transmit data is converted into a suitable waveform.

(5) Modulation

The process of carrying information onto a high-frequency carrier signal that meets signal requirements.

3. Multiplexing technology

Simultaneous transmission of multiple data channels requires multiplexing and multiple access technology.

Refers to the technology of transmitting multiple data channels simultaneously on one channel.

(1) TDM time division multiplexing

(2) FDM frequency division multiplexing

(3) CMD code division multiplexing

4. Multiple access technology

A technology that transmits multiple user data simultaneously on one line and separates the data of multiple users at the receiving end.

(1) TDMA Time Division Multiple Access

(2) FDMA Frequency Division Multiple Access

(3) CMDA Code Division Multiple Access

5. Characteristics of 5G communication network

3. Network Technology

1. Local Area Network (LAN)

A computer group (i.e., a communication network) that connects several computers via transmission media within a limited geographical area. A local area network is a closed type.

(1) Network topology

  • Star structure

With the central node (control center) as the center, it is connected to the center through connecting lines. A node must pass through the central node to transmit data.

Advantages: fast transmission speed (any two nodes only need two steps to communicate), simple network structure, easy to build a network, easy to control and manage

Disadvantages: low reliability, poor network sharing capability, once the central node fails, the entire network will fail

  • Tree structure (hierarchical centralized network)

Features: Low network cost and simple structure. No loop is generated between any two nodes, each link supports bidirectional transmission, node expansion is convenient and flexible, and it is easy to inspect the link path.

Disadvantages: Any non-leaf node link failure will affect the entire network system.

  • Bus structure

Each node device is connected to a bus. All node devices transmit information through the bus.

A bus failure will affect the communication of each node.

  • Ring structure

Each node is connected by a communication link that connects end to end to form a closed loop.

Each device has equal status and information flows in a fixed direction.

Any node failure will lead to physical paralysis, which is not conducive to expansion, long system response delay and low information transmission efficiency.

  • grid

There is a communication link between any nodes. The failure of any node does not affect other nodes.

Complicated wiring, high construction cost, and complex control methods

(2) Ethernet technology (the most common LAN technology)

  • Ethernet frame structure

Ethernet frame: A data packet on an Ethernet link, with the following structure:

DMACSMACLength/TypeDATA/PADFCS

DMAC: MAC address of the destination terminal

SMAC: Source MAC address

Length/Type: 2 bytes, value greater than 1500 indicates the type of data frame; less than 1500 indicates the length

DATA/PAD: Specific data, no less than 64 bytes (if less than 64 bytes, additional padding is required)

FCS: Frame Check Field

  • Minimum frame length (64 bytes)

Purpose/Reason: To avoid data transmission conflicts on the link when a node has sent the last bit of a data packet but the first bit has not yet been transmitted to a node farther away and mistakenly believes that the line is idle.

  • Maximum transmission distance

No strict restrictions, but affected by line quality, signal attenuation, etc.

  • flow control

Purpose: Prevent frame loss when the device is blocked.

2. Wireless Local Area Network (WLAN)

(1) WLAN topology

  • Point-to-point
  • HUB type

It consists of a central node (HUB) and several peripheral nodes. Centrally controlled communication.

  • Fully distributed

No specific application.

3. Wide Area Network (WAN)

Between cities, countries or countries

A WAN consists of a communication subnet (consisting of communication node devices and the links connecting these devices) or a resource subnet (a collection of devices and their software that implement resource sharing functions in the network).

(1) Related technologies

(2) Characteristics

1) Data communication services, supporting users to exchange information over long distances using computers;

2) Wide coverage, long communication distance, and no fixed topology of the WAN

3) The telecommunications department or company will be responsible for the assembly, management and maintenance, and provide paid communication services to the whole society.

(3) Classification

  • Public transport network
  • Dedicated transport network
  • Wireless transmission network

4. Metropolitan Area Network (MAN)

Networks established in a single city

3 layers: core layer, aggregation layer and access layer

5. Mobile communication network

4. Networking Technology

1. Network equipment and its working level

(1) Hub

The simplest network device, data received from one port is forwarded to all other ports, regardless of whether the system connected to the port is ready. One port is designated as an uplink port, which can be used to connect to other hubs or routing devices.

(2) Repeater

LAN interconnection equipment, located at the physical layer of the OSI system.

(3) Bridge

The data link layer of the OSI system.

(4) Switch

The data link layer of OSI. It provides an exclusive forwarding channel for any two network nodes connected to the switch. It has the functions of automatic addressing and switching, avoiding port conflicts, and improving network throughput.

(5) Router

The network layer of OSI. Usually used for wide area networks or the interconnection between wide area networks and local area networks.

(6) Firewall

Hardware firewall: The firewall program is built into the chip.

2. Network Protocol

(1) Open Systems Interconnection Model (OSI/RM)

From low to high layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer

The same layer provides protocols, and the upper and lower layers provide services

(2) OSI protocol suite

(3) TCP/IP protocol suite

TCP/IP protocol suite: Internet Protocol IP, Transmission Control Protocol TCP, User Datagram Protocol UDP, Virtual Terminal Protocol TELNET, File Transfer Protocol FTP, Email Transfer Protocol SMTP, Online News Transfer Protocol NNTP, Hypertext Transfer Protocol HTTP

(4) Comparison between ISO/OSI model and TCP/IP model

ISO/OSI ModelTCP/IP Model
Application LayerApplication Layer
Presentation Layer
Session Layer
Transport LayerTransport Layer
Network layerInternet Layer
data link layerNetwork Interface Layer
Physical LayerHardware Layer

3. Switching technology

(1) Switch functions

Hub function; relay function; bridge function; collision domain isolation function

(2) Basic switching principle

A switch is a network device that can encapsulate and forward data packets based on MAC address identification.

Forwarding path learning; data forwarding; data flooding; link address update

(3) Switch protocol

Spanning Tree Protocol (STP): Solving Link Loop Problems

4. Routing Technology

(1) Routing principle

Receives a data packet from a network interface and determines the next address to be forwarded based on the destination address of the datagram.

(2) Router Protocol

Routing protocol: A protocol that specifies how packets are forwarded.

Classification:

  • Interior Gateway Protocol (TGP)

Routing protocols running in an autonomous system (AS)

  • Exterior Gateway Protocol EGP

Routing protocols between ASs

5. Network Engineering

1. Network planning

Including: network demand analysis, feasibility analysis, analysis of existing networks (when optimizing and upgrading existing networks)

2. Network design

Design a solution that can solve user problems, including: determining the overall network goals, determining the overall design principles, designing the communication subnet, selecting equipment, and designing network security.

3. Network implementation

Including: project implementation plan, network design acceptance, equipment installation and debugging, system trial operation and switching, user training, etc.