The main task of program-controlled telephone exchange is to realize the call connection between users. It is basically divided into two parts: voice equipment and control equipment. Voice equipment mainly includes various interface circuits (such as subscriber line interface and trunk line interface circuit). ) and switching (or connecting) networks; The control equipment mainly includes the marker and transmitter in the crossbar switch, while in the program-controlled switch, the control equipment is an electronic computer, including central processing unit (CPU), memory and input/output equipment. Program-controlled switch is essentially a switch controlled by computer "stored program". It programs various control functions and methods, stores them in the memory, and controls and manages the work of the whole switching system by scanning external data and stored programs.
1, switching network
The basic function of the switching network is to establish a connection path between the calling party and the called party through the connection command of the control part according to the user's call requirements. In crossbar switches, various electromechanical connectors (such as crossbar connectors, coding connectors, reed connectors, etc.) are used. ). At present, in program-controlled switches, the space-division switching network composed of electronic switch arrays and the time-division connection network composed of memory circuits are mainly adopted.
2. User circuit
The function of subscriber circuit is to realize the connection between various subscriber lines and switches, which is also called subscriber line interface circuit (SLIC). According to different switching systems and application environments, there are many types of subscriber circuits. For PBX, there are mainly analog subscriber line circuit (ALC) connected with analog telephone and digital subscriber line circuit (DLC) connected with digital telephone and data terminal (or terminal adapter).
Analog subscriber line circuit is an interface configured to adapt to the analog subscriber environment, and its basic functions are:
Battery feeding: the switch provides DC power to the telephone through the subscriber line;
Overvoltage protection: prevent the switch from being damaged by voltage shock or overvoltage on the subscriber line.
Ringing: the ringing stream is transmitted to the phone of the called user.
Supervision: Monitor the on-off state of the subscriber line with the help of scanning points, so as to detect off-hook, on-hook, dialing pulse and other subscriber line signals, and transmit them to the control equipment to indicate the user's busy and idle state and connection requirements.
CODEC: Codec and filter are used to complete analog-to-digital and digital-to-analog exchange of voice signals, so as to interface with digital switching network in PBX.
Mixing: 2/4-line conversion is carried out on the subscriber line to meet the requirements of coding, decoding and digital exchange of four-line transmission.
Test: Provide a test port to test the user circuit.
These seven functions are usually represented by the acronym BORSCHT. For analog program-controlled switches, coding and decoding functions are not needed; In digital program-controlled switches, except for some small switches with specific applications that use incremental modulation, most of them use PCM coding and decoding. Digital subscriber line is an interface set to adapt to the digital user environment. It is mainly used to connect with various data terminal equipment (DTE) such as computers, printers, VDU, telex, etc. through line adapters (LAM) or digital telephones (SOPHO-SET).
3. Access the repeater
Access repeater is an interface circuit between trunk and switching network, which is used to connect trunk of switch. Its function and circuit are closely related to the switching system and signal mode of the inter-office trunk.
Analog Relay Interface Unit (ATU), whose function is to realize the interface between analog relay and switching network, has the following basic functions:
Send and receive line signals indicating trunk status (such as idle, occupied, response, release, etc.). ).
Forward and receive the transmitter signal representing the called number.
Power supply and signal tone of the call.
The received line signal is provided to the control device.
In the simplest case, if the repeater of one switch is connected to another switch through a solid trunk line and adopts subscriber loop signaling, then the function and function of the analog repeater is equivalent to a "telephone". If other more complicated signaling methods are adopted, the repeater should realize the corresponding voice and signaling transmission and control functions.
The function of digital relay interface unit (DTU) is to realize the interface between digital relay and digital switching network. It transmits relay signaling through PCM-related time slots to complete the basic functions similar to analog repeater. However, because digital trunk transmits PCM group digital signals, there are some special problems in digital communication, such as frame synchronization, clock recovery, code exchange, signaling insertion and extraction. That is, to solve the connection problems of signal transmission, synchronization and signaling coordination.
The basic functions of digital relay interface unit include frame and multiframe synchronization code generation, frame adjustment, even zero suppression, code conversion, alarm processing, clock recovery, frame synchronization search, and inter-office signaling insertion and extraction. Just like Luo Songtang of analog subscriber circuit, the above eight functions of digital relay unit can also be summarized as gazpacho.
4. Control equipment
The control part is the core of the program-controlled switch, and its main task is to execute stored programs and various commands according to the requirements of external users and internal maintenance and management, so as to control the corresponding hardware to realize the switching and management functions.
Microprocessor is the main part of the control equipment of program-controlled exchange, which can be divided into centralized control and decentralized control according to its configuration and control mode. In order to better meet the requirements of software and hardware modularization, improve processing capacity and enhance the flexibility and reliability of the system, the degree of decentralized control of program-controlled switching systems is increasing day by day, and partial or complete decentralized control methods have been widely adopted.
computer network
Computer network is a system that uses communication equipment and lines to interconnect multiple computer systems with different geographical locations and independent functions, and realizes resource sharing and information transmission in the network with well-functioning network software (network communication protocol, information exchange mode and network operating system, etc.). ).
Second, the computer network consists of resource subnet (host (providing resources) and terminal T (requesting resources)) and communication subnet (network nodes and communication links), and the communication subnet is the inner layer of the computer network.
3. The evolution of computer network can be summarized as: 1, terminal-oriented computer network (SAGE) in the early 1950s, computer-computer network (ARPANET) in the late 1960s, and open standardized network.
Examples of computer networks: Internet, public data network and Ethernet.
Verb (abbreviation of verb) The function of computer network: sharing hardware resources, sharing software resources and exchanging user information.
6. Computer network classification: 1. Geography: Wan, Lan and Man; 2. Switching mode: circuit-switched network, message switching network and packet-switched network; 3. Topological structure: star network, bus network, ring network and tree network; 4. Uses: scientific research, education, commerce and enterprises; According to the transmission medium, it is divided into twisted pair network, coaxial cable network, optical fiber network and wireless network; According to the channel bandwidth, it is divided into narrowband network and broadband network.
Seven, computer network is applied to office automation, distance education, electronic banking, securities and futures trading, campus network, enterprise network (distributed system and computer integrated manufacturing system are two typical enterprise network systems), intelligent building and structured integrated wiring system.
Eight, the computer network standard-setting institutions are: International Organization for Standardization (ISO), International Telecommunication Union (ITU), American National Bureau of Standards (NBS), American National Standards Association (ANSI), European Computer Manufacturers Association (ECMA), Internet Engineering Task Force and Internet Engineering Steering Group.
Chapter II Basic Knowledge of Computer Networks
1. data can be defined as meaningful entities, which can be divided into digital data and analog data. Digital data is a discrete value, and analog data is a value that changes continuously in a certain interval.
Second, signals are electronic or electromagnetic codes of data, which can be divided into analog signals and data signals.
Analog signals are current, voltage and electromagnetic waves that change continuously with time, and data signals are a series of discrete electric pulses, which can be used to represent the data to be transmitted.
Third, information is the content and interpretation of data;
Four, the information source is the equipment or computer that generates and sends information;
5. An information receiver is a device or computer that receives and processes information;
6. A channel is a communication line between a source and a destination.
7. Data communication is a communication technology that completes the transmission, transmission, storage and processing of data coded signals through data equipment such as computers and communication lines. It takes the computer as the center and connects the data terminal equipment distributed in different places with communication lines. A data transmission system.
Eight, analog data and digital data can be represented by analog signals and digital signals.
Analog data is a function of time and occupies a certain frequency range (frequency band). It can be transmitted by analog signals occupying the same frequency band.
When analog data is represented by digital signal, the facility to complete the conversion function between analog data and digital signal is codec, digital data is represented by analog signal, and the conversion device is modem.
Methods to overcome the attenuation of long-distance transmission signal in data communication: analog signal: amplifier; Digital signal: repeater.
Communication modes are divided into parallel mode and serial mode. Parallel mode is used for short-distance communication (inside the computer) and serial mode is used for long-distance communication.
Directional structure of serial communication: simplex, half duplex and full duplex.
The process of converting digital signals into audio signals is called modulation, and the process of converting audio signals into digital signals is called demodulation. A device that integrates modulation and demodulation functions into one device is called a modem.
X data transmission rate method: the number of bits of binary information that can be transmitted per second (unit: bits per second). S= 1/T*log2N
Signal transmission rate: the number of symbols transmitted through the channel per unit time, in baud. Baud rate, symbol rate, modulation rate.
The difference between them: signal transmission rate refers to the number of symbols passed in unit time, and data transmission rate refers to the number of binary information bits of symbols.
Their relationship is: S=B*log2NB=S/log2N.
Channel capacity: indicates the data transmission capacity of a channel, which is the limit of data transmission capacity, and the data transmission rate is the actual data transmission rate.
1, discrete channel capacity: C=2*H*log2N(H: bandwidth (Hz), n: number of possible discrete values)
2. Continuous channel capacity: C=H*log2N*( 1+S/N)(S: signal power, n: noise power, S/N: signal-to-noise ratio).
The bit error rate is an index of transmission reliability (Pe=Ne/N), which is generally required to be 10-9 in computer networks.
XI。 Analog signal coding of digital data
The basis of analog signal transmission is carrier wave, which has three elements: amplitude, frequency and phase.
There are three basic forms of digital modulation: amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK).
Amplitude shift keying (ASK): The efficiency is low, and the achieved rate is 1200bps (data transmission rate).
Frequency shift keying (FSK): Full duplex operation can be realized, 1200bps.
Phase shift keying (PSK): Using two or more phase shifts can double the transmission rate.
PAM solves the problem that the number of phases reaches the upper limit, which is actually a combination of PSK and ASK.
The frequency band range of analog channel is 300-3400Hz, so to use it to transmit digital signals, it is necessary to convert digital signals into 30-3400Hz allowed by telephone network.
Twelve, digital signal coding of digital data
Baseband transmission is an electric pulse that directly transmits digital signals in the line. The problems to be solved are: the digital signal representation of digital data and the signal synchronization between the sender and the receiver, the negative current and positive current of bipolar return-to-zero pulse. It is difficult to determine the start and end of bits in the transmission of non-return-to-zero codes, and other methods are needed to synchronize them. The pulse of return-to-zero code is narrow, so it occupies a wider frequency band on the channel (the pulse width is inversely proportional to the transmission frequency band width).
Once the unipolar code service accumulates the DC component, the alternating mark inversion will not. (As a result, AC coupling cannot be provided, and in addition, the electroplated layer of the connection point will be damaged. )
Synchronization mode Bit synchronization mode (synchronous transmission) External synchronization mode (synchronization signal at the receiving end is sent by the sending end in advance)
Self-synchronization method (extracting synchronization signal from digital signal) (Manchester coding)
Group synchronization method (asynchronous transmission) is generally used for low-speed data transmission, asynchronous timing of character sound and synchronous timing between bits in characters.
Manchester coding means "1" from high to low, and "0" from low to high, and its data transmission rate is only 1/2 of the modulation rate.
In the transmission of group synchronization, each character consists of the following four parts: 1, 1 start bit; 2, 5-8 data bits; 3, 1 parity bits; 4. The stop bit of 1-2 is indicated by "1".
Thirteen, the common method of digital signal coding of analog data is pulse code modulation (PCM). Pulse code modulation is based on sampling theorem.
Fourteen The conversion process of signal digitization includes three steps: sampling, quantization and coding.
The advantages of digital transmission are strong anti-interference and good confidentiality.
15. Multiplexing technology is the technology of transmitting multiple signals simultaneously on one channel. The two most commonly used multiplexing technologies are frequency division multiplexing FDM and time division multiplexing TDM.
The principle of frequency division multiplexing is to divide the total bandwidth of a physical channel into several subchannels, which are the same as (or slightly wider than) the transmission of a single signal. The principle of time division multiplexing is to divide a physical channel into several time slices to distribute multiple signals in turn, and transmit multiple digital signals by using the intersection of each signal in time. Time division multiplexing is not only limited to transmitting digital signals, but also can transmit analog signals at the same time.
For fiber channel, frequency division multiplexing (FDM) is a variant of WDM.
Sixteen. T 1 carrier adopts pulse code modulation PCM and time division multiplexing. This user's comments have violated community regulations. This user's comments have violated community regulations. The carrier with data transmission rate of 1.544Mbps.E 1 is PCM carrier standard, and its data transmission rate is 2.048Mbps.
17. Asynchronous transmission (group synchronous transmission) only transmits one character at a time (consisting of 5-8 bits of data), and each character is indicated by a start bit (0) and a stop bit (1);
In synchronous transmission, a frame header and a frame trailer are added at the beginning and end of each data block, and the data added with the frame header and the frame trailer is called a frame.
18. Switching networks can be divided into circuit switching networks, message switching networks and packet switching networks.
1. Circuit switching: there is a dedicated physical connection line between the source node and the destination node, which consists of intermediate nodes until the end of data transmission; It goes through three processes: circuit establishment, data transmission and circuit removal. The advantage of circuit switching is that data transmission is reliable and fast, but the disadvantage is that the circuit will be wasted when it is idle; Its characteristic is that a dedicated channel must be set before data transmission starts, and the channel is completely occupied by a pair of users before the line is released. Telephone switching networks and technical applications are typical examples of circuit switching.
2. Message exchange: the transmission unit of the message exchange mode is a message (a data block to be sent at one time), with unlimited and variable length; The message exchange mode adopts "store and forward" mode; When sending a message, he first attaches a destination address to the message, and the network node sends the message to the next node according to the destination address information, and transmits it to the destination node one by one. Therefore, this exchange method does not need to establish a connection in advance by calling, because it needs buffer storage, so message exchange can not meet the requirements of real-time communication.
Compared with circuit switching, message switching has the following characteristics: 1, high circuit utilization rate, time-sharing of two-node channels, and low requirements for circuit transmission capacity; 2. When the traffic is large, the message is still acceptable and the transmission delay will increase; 3. message switching can send a message to multiple destinations, but circuit switching is difficult; 4. message switching network can convert speed and code (stations with different speeds can also be connected. The disadvantage of message exchange is that it cannot meet the requirements of real-time and interactive communication.
3. Packet switching is to divide the message into several packets, each packet has an upper limit (to improve the switching speed), and the packets are stored in memory to improve the switching speed. It is suitable for interactive communication, such as communication between a terminal and a host.
Packet switching can be divided into virtual circuit packet switching and datagram packet switching, which is the most widely used switching technology in packet-switched computer networks.
Virtual circuit mode: first, a logical path is established between the source node and the destination node of the network, and the data packet contains a virtual circuit identifier in addition to data. Because this circuit is not dedicated, it is called a virtual circuit. The main feature of virtual circuit technology is that a virtual circuit must be established through virtual call before data transmission. It is suitable for long-term data exchange between two terminals. Advantages: reliable and orderly; Disadvantages: If a fault occurs, all data passing through the fault point will be lost.
Each packet in datagram mode is processed separately, and each packet is called a datagram, and each datagram carries address information. Because they are processed separately, the path taken by each packet is not necessarily the same, so there is no guarantee that each datagram will arrive in order, and some will even be lost. There is no virtual circuit in the whole process, but each datagram should be routed, which is suitable for small data volume. The characteristic of data arrival is that the message needs to be reassembled at the destination. Advantages: If there is a fault, the fault point can be bypassed; There is no guarantee to arrive in order, nor can we know the loss immediately.
19. Comparison of circuit switching, message switching and packet switching: circuit switching needs to set a complete path and monopolize it in the transmission process, which is inefficient; Messages are stored and forwarded from the source to the destination, which is not suitable for real-time communication; Packet switching is similar to message switching, but the length is specified. LAN not only uses circuit switching, but also uses packet switching, but does not use message switching. Because it can't meet the requirements of real-time communication.
Twenty, network topology refers to the shape of the network, or its physical connectivity. The main structures of network topology are: star topology, bus topology, ring topology, tree topology, mixed topology and network topology.
Factors to consider when choosing a network topology: reliability, cost, flexibility, response time and throughput.
The 2 1. star topology consists of a central node and sites connected to the central node through point-to-point communication links. Star networks often use circuit switching and message switching, especially circuit switching. Advantages: simple control, easy fault diagnosis and isolation, and convenient maintenance. Disadvantages: the cable length and workload are large, the central node is overloaded, and the distributed processing capacity of each station is low.
Twenty-two, the bus topology uses a channel as the transmission medium, and each station is connected to the transmission medium through an interface to send signals to the transmission medium, which can be received by all other stations. The central line highlights the distributed control strategy and decides which station can send, mainly using packet switching. Advantages: few cables, simple structure, passive operation, high reliability, easy expansion and few users. Disadvantages: limited transmission distance, difficult fault diagnosis and isolation, and no real-time function.
Twenty-three, the ring topology network consists of sites and links connecting sites, forming a closed loop. The ring topology is controlled by distributed control strategy. Advantages: short cable length, simple addition and removal of workstations, and use of optical fiber. Disadvantages: node failure will cause the whole network failure, which is difficult to detect. When the load is light, the utilization rate is low.
24. The tree topology is like an inverted tree, which evolved from the bus topology. The advantages of tree topology are: easy expansion and easy fault isolation. The disadvantage is that it relies too much on the root.
A mixed topology is a mixed single topology.
The characteristics of transmission media include: physical characteristics, transmission characteristics, geographical range, anti-interference and relative price.
Selection of transmission medium: topology, actual communication capacity, reliability requirements and affordable price.
27. Baseband coaxial cable is used to transmit digital signals, with an impedance of 50 Ω and a maximum distance of several kilometers. Broadband coaxial cable can transmit both digital signals and analog signals, with an impedance of 75 Ω, and the maximum distance of broadband cable can reach tens of kilometers.
Twenty-eight, error control refers to the technology and method to find and correct errors in the process of data communication, and limit the errors as much as possible within the allowable range.
29. The inherent and persistent random noise of the channel is thermal noise. The error caused by thermal noise is called random error. The error of a symbol caused by thermal noise is isolated and has nothing to do with the symbols before and after, which usually leads to less random error. The noise caused by external specific short-term reasons is called impulse noise, which is the main cause of errors in transmission and will not affect a string of symbols.
30. There are two methods for error control using error control coding: automatic retransmission ARQ and forward error correction FEC. In FEC, the receiver can not only find the error, but also determine the position where the binary symbol appears, so as to correct it. ARQ only uses error detection codes, and FEC must use error correction codes.
3 1. coding efficiency: R=h/n=k/(k+r). K: the number of information bits in the codeword, r: the number of additional redundant bits, and n: the length of the coded codeword. The greater the coding efficiency r, the higher the effective utilization rate of symbols used to transmit information in the channel.
32. Parity check code is a method to make the number of "1" in a codeword odd or even by adding redundant bits. This is an error detection code.
Vertical parity check, also called vertical parity check, can detect all odd-numbered dislocations in each column, but can't detect even-numbered errors. Its coding efficiency is: R=P/(P+ 1), and the missed detection rate is close to half.
Horizontal parity check, also called horizontal parity check, can not only detect odd dislocation on the same bit of each segment, but also detect burst length. Horizontal and vertical parity, also called vertical and horizontal parity, can detect: a, all errors below three digits; B, odd dislocation; C, burst length.
Thirty-three, cyclic redundancy check code is also called polynomial code. K bits to be sent and R bits to be sent form a whole, where K bits to be sent correspond to the polynomial of (K+ 1) and R redundant bits correspond to the polynomial of (R- 1). The characteristic of cyclic redundancy check code is that it can detect all odd-numbered dislocations, all double-bit errors and all burst errors less than or equal to the parity bit length. (simple application)
Hamming code is a code that can correct a bit error. (simple application)
1, twisted pair has long been used for analog signal transmission in telephone communication and also for digital signal transmission. For analog data, an amplifier is needed about every 5-6 kilometers, and for digital signals, a repeater is used every 2-3 kilometers. The bandwidth of twisted pair can reach 268KHz, so frequency division multiplexing technology can be used. The transmission distance can reach 1 km at 100Kbps, but the distance is less than 100M at 100 m and 100 m transmission rates.
2. Baseband coaxial cable in coaxial cable is used for direct transmission of digital signals. Broadband coaxial cable is used for analog signal transmission through frequency division multiplexing, and can also be used for high-speed digital signal transmission and analog signal transmission without frequency division multiplexing.
3. The computer network adopts a transmission system consisting of two optical fibers (one in and one out). Optical fiber transmission rate can reach Gbps level and transmission distance can reach tens of kilometers. At present, only one carrier can be transmitted on an optical fiber line. With the development of technology, multiplexed optical fibers will be used. Optical fiber transmission does not need a repeater within a distance of 6-8 kilometers. WDM wavelength division multiplexing technology.