Computer network (final review)

Chapter One

mind Mapping

Overview

Definition: A collection of objects or objects that are connected or combined in a certain way in order to achieve a certain goal: such as a network of water, electricity, transportation, etc.
A comprehensive view of computer networks: use communication equipment and lines to interconnect multiple computer systems (self-made and independent) with different geographical locations and independent functions, and complete network software (ie, network communication protocols, information exchange methods, and network operations). System, etc.) A system that realizes resource sharing and information transmission in the network.

• The composition of the computer network: communication subnet, resource subnet.
• The resource subnet consists of: host, terminal, terminal controller, networked peripherals, server and various software resources and information resources
• The communication subnet is composed of routers, communication control processors, communication lines and other communication equipment, and completes communication processing tasks such as network data transmission and forwarding.
• Local area network rate: 10M-1000Mbps.
  Typical features: short distance, small communication delay, high data rate, and low bit error rate.
• Divided by topological structure:
  1. Bus type advantages: high channel utilization, all PCs are hung on one line, simple and easy to expand, and easy to network. The transmission rate is high, and it is mostly used in local area networks.
  Insufficiency: Difficulty in fault diagnosis and isolation, and serious conflicts.
• Star shape: Advantages: Simple networking and easy centralized management .
  Insufficient central node load is heavy and it is easy to form bottleneck.
• Loop: Short cable length and high overall efficiency. The fault diagnosis is difficult, the loop is difficult to expand, and the control is complicated.
• Tree shape: simple structure, low cost, easy to expand, convenient for positioning and isolation. Depression at the root node becomes a bottleneck.
• Mesh: localized faults and easy to expand. Network construction is difficult and the control mechanism is complicated.
  Classified by network exchange method
• Circuit switching: link is established in advance, and the bit stream reaches the end point
• Message exchange: There is no need to establish and monopolize a dedicated physical channel, and only occupy one segment of the network link each time. When the message arrives at a node, it is first stored in the forwarding
• Packet exchange of messages: Divide a longer message into shorter fixed-length data segments, the basic unit (packet). The header contains address information. When the message arrives at a node, it is first stored in the forwarding
• Advantages, efficient, flexible, fast and reliable. Disadvantages: queuing, delay, overhead.

The main performance index of computer network

• 1. The rate of data transmission (bit rate, a bit is a 0 or 1 in a binary number) M (sixth power of ten = mega) G (nine power of ten) T (twelve power of ten)
• 2. Bandwidth originally refers to the bandwidth of the bandwidth. In the computer network, it refers to the " highest data rate" unit bit/s that the digital channel can transmit . Note that in the storage space K=2 ^10, M=2 ^20, G= 2 ^30, T=2 ^40. One byte = 8bit (B means byte byte, b means bit)
• 3. Throughput means the amount of data passing through a network in a unit time (actual amount)
• 4. Delay: The sending delay is the time required to send a data frame, that is, the total time required for the data frame to enter the transmission medium from the beginning of the frame to the end of the frame. Calculation formula = data frame length (bit)/channel bandwidth (bit/s)
• Propagation delay: The time it takes for electromagnetic waves to propagate in the channel for a certain example. Propagation delay = channel length / signal propagation rate on the channel
• The delay-bandwidth product refers to the length of the link in bits, which is how many bits can be accommodated in such a link. Delay bandwidth product = propagation delay * bandwidth.
• Utilization rate: Set the network utilization rate as U. The network delay is D, and the minimum network delay is D0
  U=90%, D=D0/(1-U)=>D/D0=10

Computer network architecture

• Standardization work and related organizations: ISO (International Organization for Standardization)
• ITU-T: International Telecommunication Union
• NIST: National Institute of Standards and Technology
• ANSI: American National Standards Institute
• IEEE: Institute of Electrical and Electronics Engineers
• EIA: Electronics Industry Association
• Internet Association: ISOC includes: RFC ()
• Network layering: The network is divided into a series of layers according to function, and each layer completes a specific function.
• The advantages of layering: each layer is independent of each other , and the interaction between adjacent layers is only through the interface, and the complexity of the entire problem is reduced. The structure can be separated , and each layer can be realized by the most suitable technology. Each layer has simple functions, easy to implement and maintain , and good flexibility is conducive to standardization
• Excessively detailed: The "responsibility" of each function of the advantage network is clear and easy to implement. Disadvantages: increase the complexity of connection, increase the amount of information exchanged between layer interfaces, increase system overhead-reduce the response speed and work efficiency of networked computers.
• Several basic concepts of the network architecture
  1. Source: the sender of data during the communication process
  2. Target: the receiver of the data during the communication process.
  The two-way nature of computer network communication determines that a network node has the role of source and target at the same time.

• Protocol: A set of rules, standards, or agreements established for data exchange in a computer network. Protocol always refers to a certain layer of protocol. To be precise, it is a collection of related communication rules and conventions established for communication between equivalent entities. The three elements are grammar, semantics, and timing.
• The service is vertical, and the agreement is horizontal.

OSI/RM model

PDU (Protocol Data Unit) SDU (Data Field)

• Physical layer function: transmit the original data bit stream on the physical medium.
• Data link layer: error-free transmission between adjacent nodes-frame data
• Network layer function: transmit packets from the source to the sink through the communication subnet
• Transport layer: Provide end-to-end data transmission services
• Session layer: Establish organization and synchronization session between two processes that communicate with each other, session activity management, and dialog control.
• Presentation layer: Provides the presentation transformation of data or information grammar to ensure that computers with different presentation methods can communicate with each other

TCP/IP

• Features: open protocol standards
• Independent of specific computer hardware and operating system
• Independent of specific network hardware, it can run on different local area networks, metropolitan area networks and wide area networks
• The heterogeneous network is a protocol system, which is suitable for different models and different types of networks.
• Standardized high-level protocols can provide a variety of reliable user services.
• 
  

Comparison of the two models

Chapter two

mind Mapping

Main indicators of data communication and channel limit capacity

• Channel: the line that transmits information

• Bit rate: The data transmission rate refers to the number of binary bits that can be transmitted per second.

• Symbol: a signal encoding unit on the time axis

• Baud rate: The number of transmitted symbols per second or the number of pulse signal changes per second. If it is a pulse signal, B=1/T.

• Nyquist criterion: On an ideal low-pass noise-free channel with a bandwidth of H, the maximum baud rate of a signal symbol is B=2H

• The relationship between bit rate and baud rate S=B*log2N (N is the number of symbol states, log2N is a few binary numbers that can represent the number of symbol states, that is, the number of bits)

• Maximum bit rate: S=2Hlog2N

• Channel capacity: the maximum data transmission rate of the channel, which characterizes the ability of a channel to transmit data

• Shannon's theorem: The bandwidth is limited and the channel limit is interfered by Gaussian noise, and the information transmission rate is error-free. C=H*log2(1+S/N)

• Signal to noise ratio: db=10*log10S/N

Multi-channel communication foundation and physical layer

Multiplexing technology

• Multiplexing: Multiple information resources share a common channel

• Frequency division multiplexing (FDM): Prerequisite: When the available bandwidth of the transmission medium exceeds the sum of the bandwidth required for each given signal. Method: modulate multiple signals on different carrier frequencies. (Frequency shift, superposition)

• Time Division Multiplexing (TDM): Prerequisite: When the data transmission rate that the transmission medium can achieve exceeds the sum of the data transmission rates of each signal. Method: The physical channel is divided into several time slices according to time and allocated to multiple signals in turn. Result: Each signal is transmitted exclusively on the channel in its own time slice. Time division multiplexing can be divided into synchronous TDM and asynchronous TDM. (Mostly used to transmit digital signals or analog signals)

• Data transmission rate: frame length * frequency per second

• Wavelength division multiplexing: Principle: The entire wavelength band is divided into several wavelength ranges, and each signal occupies a wavelength range for transmission.

• (Code Division Multiple Access CDMA) The chip sequences are orthogonal to each other. The normalized inner product of the vectors S and T must be zero (S T=1/m and SiTi=0), so S (T) is not =0, S S =1, S (S) non=-1

Transmission and modulation technology of analog data and digital data

• Terminology: information, data, signal (electrical and magnetic), transmission

• 

• Analog transmission system: use amplifiers to compensate for signal attenuation during transmission

• Digital transmission system: Insert a transponder or repeater into the transmission medium to compensate for the attenuation of the signal and suppress interference during transmission.

• Data encoding and modulation: is to convert data into a certain point or optical signal form (analog or digital signal) suitable for transmission on the channel

• Baseband transmission: According to the inherent frequency band of the original signal, it usually means that the digital signal that has not been modulated is transmitted on the channel.

• Frequency band transmission: modulate the baseband signal into a signal with a higher frequency range that is convenient for transmission in an analog channel.

• 

• Three elements of communication: Shingen, Shinjuku, Shindo

• 

• Continuous wave digital modulation (digital data-analog signal): amplitude modulation (ASK), frequency modulation (FSK), phase modulation (PSK). f(t)=Asin(wt+&)

• Pulse digital modulation (analog data-digital signal): three steps (sampling, quantization, coding) sampling: according to the Nyquist theorem, as long as the sampling frequency is not less than twice the highest frequency of the effective signal, the sampled value contains the original signal All information.

• Digital signal encoding of digital data (digital data-digital signal): unipolar pulse (with or without voltage) and bipolar pulse (voltage positive and negative).

• The level of return-to-zero encoding always returns to zero at the end of a symbol, and the level of non-return-to-zero encoding remains constant during the entire time of the symbol.

• Manchester code: from low to high jump 0, from high to low jump to 1.

• Differential Manchester encoding: If there is a jump at the start bit, it becomes 0. If not, it means 1.

Point-to-point communication and synchronization

• Simplex communication: information is transmitted from one end to the other in a fixed manner
• Half duplex: data can be transmitted in both directions, but not at the same time
• Full duplex: data transmission in both directions at the same time
• Data transmission can be divided into serial transmission and parallel transmission according to different interface types. Serial transmission can be divided into synchronous transmission and asynchronous transmission (the difference is that the sender is enough to send a synchronous clock signal if it is synchronous transmission).
• Asynchronous transmission: Each character consists of four parts: start bit, data bit, parity bit, and stop bit.

Transmission medium

• Channel: A physical channel established between a source and a sink for signal transmission, also known as a physical channel.
• Medium: Refers to the physical medium used to link computers in a computer network.
• The difference between the two: the channel is established on the transmission medium, but includes the transmission medium and communication equipment. The same transmission medium can rely on multiplexing technology to provide multiple channels, and one channel allows one signal to pass.
• Wired transmission: twisted pair, coaxial cable and optical fiber.
• Wireless transmission media include: radio, microwave, infrared and laser, etc.
• Twisted pair: shielded twisted pair (STP) and unshielded twisted pair (UTP).
• Optical fiber: divided into multi-mode and single-mode optical fiber. Features: light weight, bandwidth, high transmission rate, low bit error rate, high-performance and reliable transmission quality.
• Weaknesses of wireless media: susceptible to interference, poor confidentiality and security.

Examples of physical layer functions and protocols

• Physical layer function: Provides the ability to transmit and receive bit streams on physical transmission media.
• The basic characteristics of the physical layer protocol: mechanical characteristics, electrical characteristics, regulatory characteristics, and functional characteristics.
• DTE and DCE: data terminal equipment, data circuit termination equipment.
• Physical layer equipment: repeaters and hubs. Copy the bit signal between the cable segments, and the signal is shaped, amplified, and regenerated. The expanded LAN is still a network in the network layer.
• Repeater: Realize the expansion of the LAN segment, signal shaping, amplification, regeneration, and extend the network distance on the physical layer. The advantages are simple to install, easy to use, and inexpensive. **Disadvantages: The network length cannot be extended without limitation. **Features: The object processed by the repeater is the data bit pulse signal, and it cannot identify the frame format of the data link layer or the packet format of the network layer.
• Note: There is a difference between repeater (data transmission) and amplifier (analog transmission).
• Hub (HUB): Disadvantages The collision domain has increased, but the total throughput has decreased

Chapter 3 Link Layer

mind Mapping

• Noise in transmission: thermal noise, cross-modulation noise, crosstalk, pulse interference (the main source of interference in digital transmission)
• The link is a passive point-to-point line segment with no exchange points in the middle. The path is formed by connecting multiple links in series. The data link is a link + communication protocol.
• Functions: 1. Assemble the datagrams handed over by the network layer into frames 2. Transmit data in units of frames on the link between two adjacent nodes 3. The receiving end performs error detection and discards if there is an error Error frame.
• Purpose: To provide transparent and helpless data transmission between adjacent nodes.
• Functions: framing (synchronization, delimitation, transparent transmission), error control, retransmission strategy, flow control, addressing, media access control and link management.
• Basic service: The data delivered by the source node's network layer is correctly transmitted to the adjacent destination node, and handed over to the destination node's network layer. Three kinds of services: 1. No connection service without confirmation 2. No connection with confirmation 3. Link-oriented (reliable)
• Computer networks are divided into point-to-point networks and broadcast networks according to transmission technology. The difference between the two: a media access control sublayer is separated from the broadcast network.

Framing

• The key question: How does the receiver distinguish the boundary of the frame from the bit stream, that is, the beginning and the end of the frame.
• Frame synchronization method: 1. Byte counting method, 2. Use character padding of the first delimiter method 3. Use bit padding of the first and last mark method 4. Code violation law
• Byte counting method: Disadvantages If the length field of the frame is wrong, the synchronization information is completely lost, and synchronization must be re-established.
• The beginning and end delimiter method using character padding: A specific ASCLL character sequence is used to represent the beginning and end of the frame and the control field. Confirmation: 1. Poor compatibility 2. Frame length digits need to be an integer multiple of 8, and arbitrary long digits cannot be transmitted 3. If the "DLE insertion and deletion technology" is not used (for the DLE in the data, insert a DLE at the back, and appear in pairs DLE is a character)", the data transmission will be opaque.
• Use bit stuffing to mark the beginning and end: use a special bit combination to mark the beginning and end of the frame. Advantages: bit-oriented communication protocol, can convey any bit length, strong versatility (zero bit insertion and deletion technology)
• Coding Violation Law: A strategy that violates coding rules is used to mark the boundary of a frame.