I. Overview
Introduction
A computer network is a collection of interconnected and autonomous computer systems that can realize resource sharing and information transmission .
In terms of functional composition, the computer network consists of:
Communication subnet (composed of various transmission media, communication equipment and corresponding network protocols)
Resource subnet (a collection of equipment and software that implement resource sharing functions)
composition.
Performance
1. Rate
Refers to the rate at which the host connected to the computer network transmits data on the digital channel, also known as the data rate.
Unit: b/s
2. Bandwidth
The highest data rate is bandwidth.
Unit: b/s
3. Delay
Refers to the total time required for data (a message or packet) to be transmitted from one end of the network (or link) to the other end.
It consists of four parts: transmission delay + propagation delay + processing delay + queuing delay
Where sending delay = packet length/channel width
Propagation delay = channel length/transmission rate of electromagnetic waves on the channel
4. Delay bandwidth product
Refers to how many bits the sender has sent when the first bit sent by the sender is about to reach the end.
Also known as the link length in bits.
Delay bandwidth product = propagation delay × channel bandwidth
5. Round trip delay
Refers to the total time delay from the sender sending data to the sender receiving the acknowledgment from the receiver.
Round trip delay = propagation delay × 2 + end processing time
6. Throughput
Refers to the amount of data passing through a certain network (or channel or interface) per unit time.
7. Channel utilization
Refers to what percentage of the time a certain channel has data through.
Second, the principle of stratification
Why stratify?
Because layering can divide a large complex system into several layers that are easier to implement.
The basic principle:
1. Each layer implements a relatively independent function, reducing the complexity of the large system.
2. The interface between the layers is naturally clear, which is conducive to understanding, and there is as little mutual communication as possible.
3. Use the most suitable technology for each layer function.
4. Maintain the independence of the lower layer from the upper layer, and the upper layer unilaterally uses the services provided by the lower layer.
Data unit
Each layer has its own data unit to transmit, and its name, size, and meaning are different.
We call it the Protocol Data Unit PDU.
It consists of two parts:
1. Service data unit SDU: The data that should be transmitted in order to complete the function required by the user.
2. Protocol control information PCI: Information that controls the operation of the protocol.
The PDU of each layer is the SDU of the next layer;
The next layer gets the PDU of the previous layer, plus its own PCI, it becomes its own PDU.
entity
The elements that are active in the nth layer are called n-layer entities.
Entities at the same level are called peer entities.
protocol
The agreement is established between peer entities and is horizontal.
For example, there is an agreement between the transport layer of node A and the transport layer of node B, but there is no agreement with the network layer of node B.
The agreement consists of three parts:
Syntax: specifies the format of the transmitted data
Semantics: specifies the function to be completed
Synchronization: specifies the conditions and timing relationships for performing an operation
Three, OSI seven-layer model
Cleverly remember:
Internet of Things Shuhui Trial
Shuhui is alone.
1. Physical layer
Convert the bit stream, that is, the 01 sequence into an electrical signal.
2. Data link layer
Assemble the IP datagrams from the upper layer-the network layer into frames.
Functions: framing, error control, flow control, transmission management.
3. Network layer
Responsible for selecting the best route for routing, planning IP addresses (ipv4 and ipv6 changes will only affect the network layer), congestion control.
4. Transport layer
Responsible for end-to-end communication, reliable transmission, unreliable transmission, flow control, multiplexing and demultiplexing.
5. Session layer
Responsible for setting up and maintaining the communication connection between two computers in the computer network during data transmission.
6. Presentation layer
The data is converted into a format compatible with the receiver's system format and suitable for transmission, that is, the two systems can exchange information.
7. Application layer
Provide services directly for users' application processes (such as e-mail, file transfer, and terminal emulation). Such as HTTP, SMTP, FTP, DNS, etc.
Four, TCP/IP four-layer model
The fourth layer is especially concerned with IP, and the core is the Internet layer.
5. Comparison of the two models
with:
1. All adopt a layered architecture with low coupling.
2. All based on independent protocol stack.
3. Both solve the interconnection of heterogeneous networks.
different:
1. OSI precisely defines three main concepts: services, protocols, and interfaces, which are very consistent with object-oriented thinking.
2. TCP/IP is more practical.
3. TCP/IP considers the interconnection issues of a variety of heterogeneous networks, and regards the Internet Protocol IP as a single important level.
4.
OSI TCP/IP Network layer Connectionless+connection-oriented no connection
Transport layer Connection-oriented Connectionless+connection-oriented