【Computer Network】【Wangdao Notes】Chapter 1 Concept, Composition, Function and Classification of Computer Network

1.1.1 Concept composition function classification

concept

• Computer network: It is a system that connects scattered computer systems with independent functions through communication equipment and lines, and realizes resource sharing and information transmission by software with complete functions.
• interconnected, autonomous collection of computers
  • interconnected - interconnected
    • Communication Links (Wired & Wireless)
  • Autonomy - no master-slave relationship
    • independent of each other and cannot control each other

Function

1. Data communication - connectivity communication

   • Email
   • message broadcast

2. resource sharing resource sharing

   1. hardware - printers, fax machines computing resources, disks, printer
   2. software application software
   3. Data - Baidu Library

3. Distributed processing - distributed processing where multiple computers each take on different parts of the same job

   • Hadoop platform
   • cloud computing
   • edge computing

4. Improve reliability

   • Alternative machine - detour

5. Load Balancing - more intimacy between computers

information sharing

easy accessibility from anywhere (files, databases)

Search capability (www)

remote computing

composition

component

1. hardware
   • host-end system
   • Link - twisted pair, fiber optic…
   • Communication equipment - routers, switches…
   • end systems: Host, PC, Mainframe, Client, Workstation, Server
   • intermediate systems: Communications: Switch, Router
   • interface: Network interface card(NIC), Modem
   • Medium: Twisted pair, Coaxial cable, Fiber, Wireless
2. software
   • installed on end system
   • applications: HTTP, SMTP, FTP, Telnet
3. Protocol [core]
   • protocol: CSMA/CD, TCP/IP, UDP, PPP, ATM

Way of working

1. Edge part - host (directly used by users)
   1. C/S mode - client/client server
   2. P2P way - P2P peer-to-peer way
2. Core - serving the edge
   1. network
   2. router

Functional composition

1. Communication subnet - enables data communication
2. Resource subnet - enables resource sharing/data processing (inside the host)

Classification

Distribution range by scale

• WAN - wide area network
• MAN
• LAN - local area network
• Personal Area Network PAN
• Internet
  • a worldwide system of computer networks

user

classified by boundary

• public network extranet (public networks)
• private network (army, etc.) intranet (private networks)

switching technology

• circuit switching circuit switching
• message exchange
• packet switching

topology topology

• Bus type bus topology
  • as a linear bus
  • connected by single cable
• Star-shaped star&tree topology
  • most commonly used in Ethernet LANs
• Ring ring topology
  • single ring
  • dual ring
• Mesh (commonly used in WAN Internet)

transmission technology

• Broadcast network - shared common communication channel
• Peer-to-peer networking - using packet store-and-forward and routing

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1.1.2 Standardization work and related organizations

standardized work

Classification

• statutory standards

  • Formal and legal standard OSI formulated by authoritative organizations

• de facto standard

  • Some companies' products occupy the mainstream TCP/IP

RFC： Request for comments

​ RFC has risen to four stages of formal Internet standards:

1. Internet-Drafts – not yet RFC documents (email)
2. Proposed Standard - started as an RFC document
3. Draft standard – IETF, IAB review
   • IEIF: internet engineering task force
4. internet standards

related organizations

• International Organization for Standardization ISO *****
  • OSI reference model, HDLC protocol
• International Telecommunication Union ITU
  • Make communication rules
• International Institute of Electrical and Electronics Engineers IEEE
  • Academic institutions, IEEE802 series standards, 5G
• Internet Engineering Task Force IETF
  • Internet-related standards development

1.1.3 Rate-related performance indicators

rate

• Data Rate/Data Transfer Rate/Bit Rate

bandwidth

• Originally refers to the frequency bandwidth of a new number (the difference between the highest frequency and the lowest frequency unit: Hz)
• Now refers to the ability of network communication lines to transmit data
  • Refers to the "highest data rate" that can pass from one point in the network to another point per unit time
  • Unit: bit/second b/s
  • The maximum speed supported by the network device

• Link bandwidth: sender

throughput

• Indicates the amount of data passing through a network/channel/interface per unit time
  • Unit: b/s
• Limited by the bandwidth of the network/rated rate of the network
• Sum of all links – actual amount

1.1.4 Delay-delay-bandwidth product RTT utilization

Latency / Latency / Delay

• Refers to the time required for data (message/packet/bit stream) to be transmitted from one end of the network/link to the other
• Unit: s

1. sending delay/ transmission delay
   • Send the first bit of the packet -> the last bit of the packet is sent
   • = data length/channel bandwidth (send rate)
   • High-speed link refers to the increase in sending rate
2. propagation delay
   • = channel length/propagation speed of electromagnetic waves on the channel
   • Electromagnetic Waves – Bits
3. queuing delay
   • Waiting for out/in link time
4. processing delay
   • error detection
   • find the exit

delay-bandwidth product

• = propagation delay * bandwidth
• unit: bits
• link length in bits
  • Data capacity at this moment

Round trip time delay RTT

• From the time the sender sends data, to the time when the sender accepts the confirmation from the receiver
• The larger the RTT, the more data can be sent before an acknowledgment is received
• include:
  • Round trip propagation delay = propagation delay * 2
  • end processing time

Utilization

1. channel utilization
   • link
   • = time with data passing/(with + no) data passing time
2. network utilization
   • Channel Utilization Weighted Average

1.2.1 Layered structure protocol interface service

reason for stratification

• Things to do before sending files
  • The initiating computer needs to activate the data communication path
  • Tell the network how to identify the destination host
  • The initiating computer needs to find out that the destination host is powered on & connected to the network normally
  • The initiating computer checks to see if the other file management program is ready
  • Ensure errors and surprises can be resolved

The upper layer requests the service, the lower layer provides the service

hierarchical structure

1. Entities: Active elements in the nth layer are called n-layer entities

   • Entities at the same layer are called peer entities

2. protocol: A rule, standard, or convention established for the exchange of data between peer entities in a network [horizontal]

   1. Syntax: Specifies the format of the transmitted data
      • Data form + how to split
   2. Semantics: specify the function to be completed
      • meaning of each paragraph
   3. Synchronization: specify the order of various operations

3. Interface (access service point SAP): the entrance for the upper layer to use the lower layer service

4. Service: the function call provided by the lower layer for the adjacent upper layer [vertical]

• SDU service data unit: the data that should be transmitted in order to complete the functions required by the user (useful part)
• PCI Protocol Control Information: Information that controls the operation of the protocol
• PDU protocol data unit: data unit transmitted between peer layers (downward transmission)

concept

1. network architecture
   1. It is a functional description of the computer network structure
   2. It is a collection of various layers and protocols of a computer network
   3. abstract (implementation - refers to some software hardware that can run)
2. hierarchical structure
3. Each layer follows a certain/some network protocol
4. When layer n provides services to layer n+1, the service not only includes the functions of layer n itself, but also includes the functions provided by the lower layer services
5. There are only interfaces between adjacent layers, and the specific implementation details of the services provided are completely shielded from the upper layer

1.2.2 OSI reference model

• Open System Interconnection
• Statutory standards first theory and then practice

source

The International Organization for Standardization (ISO) proposed the Open Systems Interconnection (OSI) reference model in 1984

• The data link layer also adds a tail

model structure

application layer

• All programs that interact with users to generate network traffic

Typical service:

• File TransferFTP
• Email SMTP
• World Wide Web HTTP

presentation layer

• A representation (syntax & semantics) for handling information exchanged between two communication systems

Function:

1. Data Format Conversion (Translator)
2. Data encryption and decryption
3. Data Compression and Recovery

session layer

• Provide the presentation layer entity/user process to establish a connection and transmit data in an orderly manner on the connection

• Session establishment synchronous SYN

Function:

1. Establish, manage, terminate sessions
   • Opening a web page is establishing a session
2. The use of checkpoints allows the session to resume communication from the checkpoint/synchronization point when the communication fails to achieve data synchronization
   • Good for transferring large files

Protocol: ADSP, ASP

transport layer

• Responsible for the communication between two processes in the host , that is, end-to-end communication.

• Transmission unit: segment/user datagram

Function:

1. Reliable transmission, unreliable transmission
   • Reliable transmission: establishment of connection, confirmation mechanism
   • Unreliable transmission: very small data
2. error control
3. flow control
4. multiplexing
   • Multiplexing: Multiple application layer processes can use the services of the transport layer below at the same time
   • Separation: the transport layer delivers the received information to the corresponding processes in the application layer above

(can be reserved)

protocol:

• TCP
  • HTTP(Web), FTP(file transfer), Telnet(remote login), SMTP(email)
• UDP: no flow control, no congestion control
  • streaming media, teleconferencing, DNS, Internet telephony
  • QQ

Network layer

Transfer packets from the source to the destination, and provide communication services for different hosts on the packet switching network.

Transmission unit: datagram

Function:

1. Routing - Best Path
2. flow control
3. error control
4. congestion control

data link layer

Assemble the datagram passed down from the network layer into a frame

Transmission unit: frame

Function:

1. Framing (defines frame start & end)
2. error control
3. flow control
4. Access/Access Control
   • Control access to channels

protocol:

• SDLC
• HDLC
• PPP
• STP

physical layer

Transparent transmission of bit streams over physical media

Transmission unit: bit

Function:

1. Define interface properties
2. Define transmission mode simplex/half duplex/duplex
3. Define the transmission rate of 100M network...
4. bit synchronization
5. bit coding

protocol:

• Rj45
• 802.3

1.2.3 TCP/IP Reference Model Layer 5 Reference Model

Practice first, theory after protocol stack

Similarities between OSI and TCP/IP

1. all layered
2. Based on an independent protocol stack concept
3. Can realize heterogeneous network interconnection

• Transport layer: exists for reliable transmission
  • Therefore, a connection needs to be established first
• TCP/IP pays more attention to interconnection issues, so there is no connection at the network layer

Five-layer reference model