There are four stages of computer network development: terminal-oriented computer network, computer-computer network, open standardized network, wide application of Internet and development of high-speed network technology.
China has three major networks: telecommunication network, radio and television network and computer network.
Future development trends: broadband, all-optical, multimedia, mobile, next generation network.
A computer network consists of a resource subnet and a communication subnet.
Definition of computer network: A system that uses communication equipment and lines to interconnect multiple computer systems with different geographical locations and independent functions, and uses well-functioning network software to realize resource sharing and information transmission in the network.
The function of computer network is to share software and hardware resources and exchange information among users.
Application of computer network: office automation, distance education, electronic banking, securities and futures trading, enterprise network, intelligent building, structured integrated wiring system.
Classification of computer networks:
According to the topological structure: star, bus, ring, tree, hybrid and mesh.
According to the switching mode, it is divided into circuit-switched network, message switching network and packet-switched network.
According to coverage: 10,000, MAN, LAN.
According to transmission technology, it is divided into broadcast network and point-to-point network.
ISO (International Organization for Standardization), ITU (International Telecommunication Union), IETF (Internet Engineering Task Force)
chapter two
Network protocol: A set of rules, standards or conventions established for data exchange in computer networks.
Network protocol consists of three elements: semantics, grammar and time sequence relationship.
Stratification: dividing a complex thing into several simple things.
Network architecture: the hierarchical model of computer network and the collection of its protocols.
Connection-oriented service: the connection is established at the beginning, and the address of the destination node does not need to be carried when transmitting.
Connectionless service: at the beginning, there is no need to establish a connection, and each packet must carry a complete destination node address. Different packets may choose different paths to reach the destination node, and the packets received by the node may be out of order, duplicated and lost. The protocol is relatively simple and efficient.
OSI/RM: physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer.
TCP/IP: host network layer, interconnection layer, transport layer and application layer.
Comparison between ORI/RM and TCP/IP;
* * * Same as: 1. Both of them are based on the concept of protocol stack, and the protocols in the protocol stack are independent of each other. 2. Both of them adopt the concept of hierarchical structure, and the functions of each layer are similar.
Difference: 1, OSI has 7 layers and TCP/IP has 4 layers. TCP/IP network layer provides connectionless communication, and transport layer supports two kinds. The OSI network layer supports two kinds, and the transport layer supports connection-oriented communication.
chapter three
Definition of physical layer: It provides mechanical, electrical, functional and procedural means for the reasonable activation, maintenance and removal of physical connections required for bit transmission between physical channel entities through intermediate systems.
DTE: Data terminal equipment, network equipment or workstations owned by users, such as computers and terminals.
DCE: Data communication equipment, the general name of network equipment that provides users with access points, such as modems.
Characteristics of physical channel: mechanical characteristics, electrical characteristics, functional characteristics and adjustment characteristics.
There are three kinds of electrical characteristics: 1, unbalanced mode (unbalanced transmitter+receiver+1 conductor+1 ground wire), unbalanced mode with differential receiver (unbalanced transmitter+differential receiver+1 conductor +2 ground wire), and balanced mode (balanced transmitter+)
Functional characteristics are divided into four categories: data signal line, control signal line, timing signal line and grounding line.
EIA (American Electronics Industry Association) RS-232C: It provides the technical specification for connecting remote devices through modems using public telephone network as transmission medium.
RS-422 (balanced mode), RS-423 (unbalanced mode with differential receiver)
X.2 1 mechanical properties adopt the core standard of 15.
Wired media: twisted pair, coaxial cable, optical fiber. Wireless media: radio wave, microwave, infrared ray, laser, satellite communication.
Coaxial cable is divided into baseband coaxial cable (impedance 50 ohms, supporting hundreds of devices) and broadband coaxial cable (impedance 75 ohms, supporting thousands of devices).
Optical fiber: multimode is light emitting diode LED, and injection laser diode ILD is single mode.
Data transmission rate: refers to the number of bits of binary information that can be transmitted per second, in bits per second (bps). r = 1/T * log2N(BP)
Signal transmission rate (modulation rate): indicates the number of symbols transmitted through the channel in unit time. R= 1/T (baud)
Channel capacity: indicates the capacity of the channel to transmit data, in bits per second (bps).
The channel capacity represents the maximum data transmission rate of the channel and is the limit of the channel data transmission capacity, and the data transmission rate represents the actual data transmission rate.
Nyquist announcement: C=2*H*log2N (bps), Shannon formula: C=H*log2( 1+S/N)(bps)(H: channel bandwidth, S/N: signal-to-noise ratio).
Bit error rate = number of errors/total.
Modem: digital to analog, codec: analog to digital
Amplifier: Enhance the energy in the signal, and at the same time enhance the noise component. Repeater: Regenerate the signal.
Data communication: it is a communication technology that completes the transmission, transmission, storage and processing of data coded signals through computers or other data equipment and communication lines.
Multiplexing technology: frequency division multiplexing FDM, time division multiplexing TDM, and wavelength division is a variant of frequency division.
FDM: The physical channel is divided into several sub-channels and transmits several signals at the same time.
TDM: Physical channels are allocated to multiple signals in turn according to time slices.
Sampling, quantization and coding. Word length =log2N
Three main problems of transmission line: attenuation, delay distortion and noise.
Packet switching networks are divided into virtual circuits and datagrams.
chapter four
Functions of data link layer: frame synchronization, error control, flow control and link management.
Error control: feedback retransmission, timeout timer, frame number.
Traffic control: the data link layer controls the traffic on the data link between two adjacent nodes, and the transport layer controls the end-to-end traffic from the source to the final destination.
There are two kinds of noise: instantaneous thermal noise and impact noise.
Stop-wait: sending window = 1, receiving window =1;
Go-back-N: send window >; 1, receiving window =1;
Select retransmission: sending window > 1, receiving window >; 1;
Asynchronous protocol: intra-character synchronization, asynchronous between characters; Synchronization protocol: Frame synchronization consisting of many characters and bits.
BSC: Character-oriented, divided into data message and monitoring message.
Data message:
ETX bcc
Synsyn synsoh Title STX Message ETX Bcc
Synsyn synsoh title STX message ETB BCC
ETB bcc
Monitoring message:
Synchronization confirmation
Synsynnak
SYN SYN P/S prefix station address ENQ
Synchronous synchronous EOT
HDLC: Bit-oriented, including information frame (I frame), monitoring frame (S frame) and unnumbered frame (U frame).
Frame format: flag address control information frame check sequence flag
landed cost
0111108, 8-bit, n-bit 16-bit 0111.
PPP protocol provides three functions: framing, link control and network control.
The frame format of PPP is very similar to HDLC, but PPP is character-oriented.
chapter five
Functions of network layer: routing, congestion control and Internet connection.
In packet switching mode, the communication subnet provides virtual circuit and datagram services to the terminal system.
Optimization principle: If router J is on the best route from router I to router K, then the best route from J to router K will be on the same route.
Flooding routing method: After a network node receives a data packet from a certain line, it sends the received data packet to all lines except this line.
The causes of congestion are: 1, insufficient memory, and insufficient memory to store packets arriving at the same time; 2, the processing speed of the router processor is slow, and it is difficult to complete the work of queuing and updating the routing table.
The task of congestion control is to ensure that the subnet can carry all the arriving traffic, which is a global problem. Traffic control is only related to point-to-point traffic between a specific sender and a specific receiver.
The solutions of congestion control can be divided into two categories: open loop (regardless of the current state of the network) and closed loop.
Congestion control of virtual circuit subnet: 1, admission control, 2, routing, 3, resource reservation.
Datagram subnet congestion control: 1, warning bit, 2, packet suppression, 3, hop-by-hop packet suppression.
Four characteristics of QoS: reliability, delay, jitter and bandwidth.
Integration services: Each connection has a dedicated resource. Differentiated Services: Each type of connection has dedicated resources.
Label switching: similar to virtual circuit, look up the table to get the whole line.
Multiprotocol label switching protocol.
The purpose of network interconnection is to enable users on one network to access resources on other networks, and to enable users on different networks to communicate and exchange information with each other.
Routing Information Protocol (RIP) can operate in passive state and active state.
Open Shortest Path First Protocol (OSPF) is a link-state routing protocol.
Bridges are used to connect similar types of LANs without a network layer.
The bridge works at the data link layer and the router works at the network layer.
The main service functions of the router are: 1, establishing and maintaining the routing table, and 2, providing packet forwarding function between networks.
Network management, also known as protocol converter, is used to convert high-level protocols and can support both transport layer and application layer.
IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Translation Protocol), RARP (Reverse Address Translation Protocol).
IP protocol provides an unreliable and connectionless packet transmission mechanism.
ARP: Conversion from IP address (32 bits) to physical network address (Ethernet address, DA, 48 bits).
Translation from physical network address to IP address.
IGMP (Internet Group Management Protocol): There are only two kinds of messages, query and response.
IPv6 increases the IP address length to 128 bits.