First, the composition of computer network
The first stage of computer development ARPA
ARPANET was the precursor to the Internet
The second stage tertiary structure
Backbone network – Regional network – Campus network (enterprise network)
Stage 3 – Multi-level ISP architecture
ISP: Internet service provider.
Computer network is a decentralized, independent function of the computer network, through communication equipment and lines connected, by the functional software to achieve resource sharing and information transfer system.
Computer network composition: hardware, software, and protocols
Way to work
1. Edge users directly use C/S mode and P2P mode
2. The core serves the edge
Function of
1. Resource subnet application layer, presentation layer, session layer, and transmission layer to realize resource sharing and data processing
2. Communication subnet network layer (router), link layer (switch, bridge), physical layer (hub, repeater) to achieve data communication
The upper three layers encapsulate and process the data, while the lower three layers send it on its way
The transport layer is designed to bridge the gap between the services required by the upper three layers and those provided by the network layer, and to mask some of the details of the communication subnet.
A computer network is an interconnected, autonomous collection of computers.
Classification of computer networks
1. By distribution scope: WAN, MAN, LAN, PAN
2. According to users: public network, private network
3. By switching technology: circuit switching, message switching, packet switching
4. Topological structure: bus, star, ring, mesh (commonly used in wide area network)
5. By transmission technology:
Broadcast networks share common communication channels
Point-to-point networks use packet store-and-forward and routing mechanisms
Standardization work
1. Statutory standard OSI
2. Standard TCP/IP
ISO OSI model, HDLC protocol
ITU specifies communication rules
Institute of Electrical and Electronics Engineers IEEE Academic Institute, IEEE802 standard, 5G
The Internet Engineering Task Force IETF is responsible for the Specification of the Internet related standards RFC XXXX
Rate: data rate/data transfer rate/bit rate, the rate at which a host connected to a computer network transmits bits of data over a digital channel
Bits 1/0 bit
b/s,kb/s,Mb/s,Gb/s,Tb/s
1Mb/s=10^6b/s
1Gb/s=10^3Mb/s
1kB=2^10B
1MB=2^10KB
1GB=2^10MB
1TB=2^10GB
Bandwidth originally refers to the frequency band width of a channel, the difference between the highest and lowest frequency, in Hertz HZ
In computer networks, bandwidth is used to describe the capacity of the network’s communication lines to transmit data, “maximum data rate”
A link is also called a channel, and the medium can be twisted pair or coaxial cable.
Propagation rate 2*10^8m/s, electromagnetic wave 1US can spread to the front 200 meters
The link bandwidth is 1Mb/s, and the host sends 1bit data to the link within 1us.
The link bandwidth is 2Mb/s, and the host sends 2 bits of data to the link within 1us.
throughput
The amount of data passing through a network (channel, interface) per unit of time, the sum of all links
Delay: The time it takes for data to travel from one port of a link to the other
1. Transmission delay data length/channel bandwidth (transmission rate) to check whether the channel bandwidth is provided
2. Propagation delay channel length/electromagnetic wave rate in the channel
3. Wait in a queue for the output link to be available
4. Deal with experimental errors and find the exit
Latency bandwidth product = propagation latency * bandwidth
Bit=s*b/s
The length of a link is also called the number of bits in a link in unit of bit
Round-trip delay RTT The total delay experienced when sending data from the sender to the receiver.
The larger the RTT, the more data you can send before receiving an acknowledgement
RTT includes 1. Round-trip propagation delay 2. Terminal processing time
utilization
Channel utilization Yes Data passage time /(Yes + None) Data passage time
Network usage Weighted average value of channel usage
Cognitive hierarchy
1. Entities Active elements at the NTH layer are called n-layer entities, and entities at the same layer are called peer entities
2. Rules, standards and agreements between the peer entities. (horizontal)
3. Interface (access service point SAP) : entry for upper layer to use lower layer services
4. The lower level of service provides function invocation for the upper level (vertical)
SDU service data unit
PCI protocol control information
PDU data unit: data unit transmitted between peer layers
7-layer OSI Reference Model Statutory Standard (theoretical success, market failure)
Layer 4 TCP/IP reference model fact standard
A 5-tier architecture
PDU=PCI+SDU
The PDU at the upper layer is the SDU at the lower layer
End to end on the upper four floors and point to point on the lower three
Application layer supports various network applications FTP,SMTP,HTTP
Presentation layer The presentation of information exchanged in two communication systems 1. Data format conversion 2. Data encryption and decryption 3. Data compression and recovery
The session layer provides the presentation layer entity/user process with the ability to establish connections and sequentially transfer data over connections
This is a session, and it’s also a synch.
Function 1 Establish, manage, and terminate a session
Function 2 When the communication fails, the communication is resumed from the verification point or synchronization point for data synchronization
Syntax: Specifies the format for transferring data
Semantics: Specifies the functionality to be accomplished
Synchronization: Specifies the order of operations
The transport layer is responsible for the communication between two processes, that is, end-to-end communication. Unit: data packet TCP or UDP
1. Reliable and unreliable transmission 2. Error control 3. Flow control 4
The network layer transfers packets from the source to the destination and provides communication services. Packet routing and forwarding unit of data from the source host to the destination host: datagram
Route selection 2. Error control 3. Traffic control 4
The data link layer assembles the data packets transmitted from the network layer into frames
1. Framing (defining the start and end of a frame)
2. Error control frame error + bit error
3. Flow control
4. Access control: Controls channel access
Physical layer: transparent transmission of bitstream on physical media
Physical layer transmission unit bit
1. Define interface features
2. Define transmission modes simplex, half-duplex, and duplex
3. Define the transmission rate
4. Bit synchronization
5. Bit coding
Main protocol Rj45,802.3
Connection-oriented three phases: 1. Establish a connection. 2
No direct data transmission
The OSI and TCP/IP reference models are similar
1. The stratification
2. The concept of an independent protocol stack
3. Heterogeneous network interconnection is implemented
The difference between
1. OSI defines three-point services, protocols, and interfaces
2. OSI was invented first, and reference model was invented before protocol
3. Heterogeneous network interconnection is considered at the beginning of TCP/IP design, and IP is an important level