5. Basic concept of1.1

1. "Exchange"

"Exchange" refers to the mutual transmission of voice, image, data and other information between a large number of user terminals in the communication network according to the communication needs of users. So as to realize different forms of information interaction between terminals such as point-to-point, point-to-multipoint, multipoint-to-point or multipoint-to-multipoint.

Obviously, there will be a considerable number of user terminals in the communication network. If all user terminals are interconnected one by one and controlled by switches, communication between any two users can be realized. This connection is called direct connection, as shown in Figure 5- 1.

Figure 5- 1 direct connection mode

In this connection mode, when there are n users, N*(N- 1) pairs of connection lines need to be set. A slight increase in the number of users will lead to a sharp increase in the number of connected lines, and the line utilization rate is not high, because the lines are dedicated to each user. At the same time, in order to realize the controllability of the communication process, it is necessary to set (N-l) switches at each user terminal for control, so this interconnection method is neither economical nor easy to operate, and it is only suitable for extremely simple and small-scale communication networks, and has no practical value.

In order to solve the above problems, a feasible way is to attract a public interconnection device-switch to a large number of users. All user terminals are connected to the switch through a pair of dedicated lines called subscriber lines or subscriber loops. The function of the switch is to realize the free connection of any two user terminals through its own control function, and the location where the switch is located is called the switching node. By setting switches, on the one hand, the number of subscriber lines is greatly reduced, and the network construction cost is reduced; On the other hand, because the call connection and routing functions are all realized by switches, the complexity of control is also reduced and the reliability of the network is improved. This method is shown in Figure 5-2.

Figure 5-2 Switching Connection Mode

2. Switch network

Obviously, when the number of users is large and the distribution area is wide, it is necessary to set up multiple switching nodes. The switches of each node are interconnected by transmission lines according to a certain topological structure (such as star network, ring network, tree network, hybrid network, etc.). ) to form a switching network, as shown in figure 5-3.

Figure 5-3 Switching Network

The connecting lines between switching devices in Figure 5-3 are called trunk lines. At this time, the position of the switching node is similar to the user terminal in the above article, and multiple switching nodes cannot be directly connected, so it is necessary to introduce a tandem switching node, and the switching equipment of this node is called a tandem switch. In a switched network, any switch directly connected to a user's telephone or terminal is called a local switch. In the voice communication network, the exchange corresponding to the local exchange is called the local exchange or the end exchange; The office equipped with tandem office is called tandem office, the tandem office with relatively long communication distance is also called long-distance office, and the corresponding exchange office is also called long-distance office. In packet-switched networks, such as common IP networks, the devices corresponding to local switches are edge routers (switches), and the devices corresponding to tandem switches are core routers (switches) or backbone routers (switches).

The telephone communication network generally adopts a hierarchical network structure, and each switching node in the network is assigned a hierarchy, and each switching node in other layers except the highest layer must be connected to the switching node in a higher layer. The more network levels, the more times the call needs to be transferred. Such a network not only occupies a lot of lines, but also increases the complexity of network management. Therefore, it is necessary to plan the structure and network topology of the switching network reasonably according to the geographical scope of communication network services and the number of users.

3. Basic functions of switching equipment

Taking the ordinary voice communication network as an example, the telephone exchange should be able to realize the following call connection methods:

(1) Local connection: the connection between two subscriber lines of the same switch;

(2) Outgoing connection: the connection between the exchange subscriber line and the outgoing trunk line;

(3) Incoming connection: the connection between the incoming trunk line and the subscriber line of the switch;

(4) Transfer connection: the connection between the incoming relay and the outgoing relay of the switch.

In order to realize the above connection control, the basic functions of telephone switching equipment must include:

(1) Timely and correctly receive and identify the call signal and destination address signal sent along the subscriber line or trunk line;

(2) Selecting the correct route according to the destination address to connect the terminal devices of both communication parties is called call establishment;

(3) Start the billing system, monitor the change of user status, and accurately count the communication duration;

(4) After the communication, the connection is released in time according to the received release signal, which is called connection release.

The example of telephone exchange is extended to the general telecommunication exchange system, which has four basic technical functions: interface function, interconnection function, signaling function and control function.

(1) Interface function: Interface is divided into user interface and trunk interface, which are used to connect subscriber lines and trunk lines to switching equipment respectively. Devices with different switching technologies have different interfaces. For example, program-controlled digital telephone switching equipment should have interface circuits suitable for analog subscriber line, analog trunk line and digital trunk line; N-ISDN switching equipment should have a basic rate interface adapted to 2 B+D and a basic group rate interface adapted to 30 B+D; ATM switching equipment should have various physical media interfaces adapted to different code rates and different services; IP switching equipment needs to provide various transmission medium interfaces that can carry IP frames, such as twisted-pair Ethernet interface and optical fiber Ethernet interface.

(2) Interconnection function: The switching system adopts interconnection network (also called switching network) to realize the connection of any incoming line and any outgoing line. For different switching modes, the connection can be physical (magnetic switching, digital program-controlled switching and optical switching) or virtual (packet switching and cell switching). The topological structure of the interconnection network and the routing principle within the network directly affect the service quality of the interconnection network. In addition to trying to design a non-blocking network topology, two sets of redundant structures should be configured to enhance the fault recovery ability of the interconnection network.

(3) Control function: Effective control function is the guarantee of automatic exchange of information in the exchange system. There are two basic control modes: centralized control and decentralized control. The difference lies in the configuration scheme of the microprocessor. Modern telecommunication switching systems mostly adopt decentralized control, and the control functions are mostly realized by software. For example: address signal identification and digital analysis program of program-controlled telephone exchange, call admission control and automatic routing control of ATM exchange, routing protocols BGP and OSPF in IP exchange, etc.

(4) Signaling function: Signaling is a kind of connection control instruction in telecommunication network, which enables different types of terminal equipment, switching node equipment and transmission equipment to work together through signaling. Signaling transmission needs to be realized through a series of standardized signaling protocols. Due to the continuous development of switching technology, signaling protocols and signaling methods are also different according to different applications.

5. 1.2 development of switching technology

Switching technology originated from telephone communication and is one of the most common and commonly used technologies in modern communication networks. Switching technology has been developing since it appeared at the beginning of last century. The development of switching technology largely reflects the development of modern communication technology from manual to automatic and from analog to digital.

1. Analog switching technology

The first person who researched and invented the exchange equipment was an American named almon B. Strowger, who was the owner of a funeral home in Kansas, USA. He found that the telephone operator, whether intentionally or unintentionally, often connected his business phone to his competitors, thus losing a lot of his business. To this end, he flew into a rage and vowed to invent an automatic wiring device that does not require the operator to wire. From 1889 to 189 1, he devoted himself to researching a switch that can automatically connect wires, and as a result, he succeeded. 189 10 March 10 was granted the invention patent of "step-by-step automatic telephone connector". 1892165438+1On October 3rd, the "stepped automatic telephone exchange" made of connectors invented by Strowger was put into use in laporte, Indiana, USA, which was the first automatic telephone exchange in the world. Since then, telephone communication has entered a new era. But the rapid development of automatic telephone was in the 20th century. By the 1920s, only 65,438+05% telephones in the world were automatic telephones. With the development and progress of automatic telephone technology, by 1950s, 77% of the telephones in the world were automatic telephones.

Why is the automatic telephone exchange system invented by Strowger called "step by step system"? This is because it relies on the dialing pulse of the telephone user to directly control the gradual action of the switch machine. For example, the user dials "1" and sends out a pulse (the so-called "pulse" is a short-term current). This pulse makes the electromagnet in the connector suck once, and the connector moves forward one step. When the user dials the number "2", two pulses are sent out, so that the electromagnet is attracted twice, the connector moves forward two steps, and so on. Therefore, this kind of exchange is called "step-by-step automatic telephone exchange"

19 19, Swedish telephone engineers Palmgren and Bertrand invented an automatic connector called "crossbar connector" and applied for a patent. 1929, the world's first large-scale vertical and horizontal telephone office was built in Sonzwar, Sweden, with 3500 users. The name "vertical and horizontal system" comes from the structure of vertical and horizontal connectors, which consists of some vertical bars, horizontal bars and electromagnetic devices. The control equipment can attract the actions of the related vertical bars and horizontal bars by controlling the current of the electromagnetic device, so that the vertical bars and horizontal bars contact at a certain intersection, thus realizing the wiring work.

Both "crossbar system" and "stepping system" are connected by electromagnetic mechanical action, so they belong to "electromechanical automatic telephone exchange". However, crossbar system has little mechanical action and uses precious metal contacts, so it has less noise, less wear and mechanical maintenance workload and longer working life than stepping switch.

In addition, the control methods of vertical and horizontal system and step system are also different. Stepping system is called direct control system, which is directly controlled by users. The crossbar system is called indirect control system because users need to indirectly control the actions of connectors through public control equipment when dialing. Indirect control mode has obvious advantages over direct control mode. For example, it works flexibly, which is convenient to realize flexible exchange in a telephone network composed of multiple telephone offices, realize long-distance telephone automation, cooperate with the use of new technologies, and open new services. Therefore, its appearance has raised the automatic telephone switching technology to a new level.

The crossbar and step-by-step switches adopt mechanical technology in the voice part and control part, which are called analog switches. With the development of electronic technology, especially semiconductor technology, people began to introduce electronic technology into switches. At first, electronic technology was introduced into the control part of the switch, but analog technology was still used in the voice part with high voice quality, so quasi-electronic switches such as space division electronic switch and time division electronic switch appeared. They generally use mechanical contacts in the voice path and electronic devices in the control part, which are generally classified as analog switches.

2. Circuit switching

Circuit switching is the earliest switching technology developed for telephone service transmission. The biggest feature of this exchange method is to establish a channel for both parties before the call, and maintain this channel during the call until it is dismantled after the call.

The main representative of circuit switching technology is program-controlled switching. In the early 1970s, on the basis of a large number of applications of digital PCM transmission, France successfully developed a switch for directly exchanging PCM digital signals. The control adopts program-controlled mode, and the call connection adopts time-division switching mode realized by electronic equipment. Because electronic devices are used in the control part and the connection part, all-digital exchange is realized. This all-digital time-division program-controlled switching technology shows various advantages, prompting countries all over the world to develop this program-controlled digital switching technology. The continuous improvement of its implementation technology has made its performance more superior, but its cost has been declining. By the mid-1980s, it had replaced the air separation analog program-controlled exchange and was in its heyday, and the program-controlled digital telephone exchange began to spread all over the world. Techniques such as packet switching and message switching developed after digital program-controlled switching also belong to the category of digital switching technology.

3. Packet switching technology

Circuit switching technology is mainly suitable for transmitting voice-related services, and this network switching method has great limitations for data services. First of all, the expect the unexpected of data communication has a great difference between the peak bit rate and the average bit rate. If circuit switching technology is used to allocate circuit bandwidth according to peak bit rate, it will cause great waste of resources. If the bandwidth is allocated according to the average bit rate, a lot of data will be lost. Secondly, compared with voice service, data service has no strict requirements on time delay, but it needs error-free transmission. Voice signals can be distorted to some extent, but the real-time performance must be high. The early X.25 technology, as well as the current Ethernet switching technology and IP switching technology are typical packet switching technologies.

Packet switching technology is a switching method proposed according to the characteristics of data communication services. Its basic feature is to divide the data to be transmitted into many small pieces of data according to a certain length, and add corresponding header fields for routing and checking data before the data as the basic unit of data transmission, that is, grouping. Using packet switching technology, there is no need to establish a connection before communication. Each node first receives the data packet sent by the previous node and stores it in the buffer, and then selects the appropriate link to send to the next node according to the address information in the packet header, so that the bandwidth can be dynamically allocated according to the user's requirements and the network capacity during communication. Packet switching has higher circuit utilization than circuit switching, but the delay is longer. Each packet sent from the sender will be transmitted to the switch connected to the receiver by the packet switching network according to the address and control information inside the packet, but the transmission paths of different packets belonging to the same data frame are not unique, that is, each packet switch can choose different transmission paths for each packet according to the current state of the switching network during communication, thus avoiding network congestion caused by line congestion. On the contrary, circuit switching can only choose the path in the initial stage of establishing communication. After the packets are transmitted to the switch at the receiving end through the packet switching network, the assembly function of the packet switch arranges the packets according to the packet serial number carried in each packet, and restores the arranged packets to the original data through the subscriber line and sends them to the corresponding receiving end.

4. Information exchange

Message switching technology is similar to packet switching technology, and it also adopts store-and-forward mechanism. However, in message exchange, messages are used as transmission units. Because the message length varies greatly, long messages may cause great delay, and it is difficult for each node to allocate buffers. In order to meet the needs of messages of various lengths and achieve high efficiency, nodes need to allocate buffers of different sizes, otherwise data transmission may fail. In practical application, message switching is mainly used for short message transmission and communication services with low real-time requirements, such as public telegraph network. Message exchange appeared earlier than packet exchange. Packet switching is based on message exchange, which divides messages into packets and balances transmission delay and transmission efficiency, so it has been widely used.

5.ATM switching

With the wide application and development of packet switching technology, two major networks have emerged, namely, the circuit-switched network for transmitting voice services and the packet-switched network for transmitting data services. The separate transmission of voice service and data service urges people to think about a new technology to provide the advantages of circuit switching and packet switching at the same time, and to provide users with unified services, including voice service, data service and image information. Then, in the late 1980s, CCITT put forward the concept of broadband integrated services digital network and a brand-new technology-asynchronous transfer mode (ATM). ATM technology combines connection-oriented mechanism and grouping mechanism. Before the communication starts, it is necessary to establish a connection with a certain bandwidth according to the requirements of users. However, this connection does not monopolize a physical channel, but statistically multiplexes a physical channel with other connections. At the same time, all media information, including voice, data and image information, is divided and encapsulated into fixed-length packets for transmission and exchange in the network.

Another outstanding feature of ATM is that it puts forward a complete mechanism to ensure QoS. At the same time, because optical fiber communication provides a transmission channel with low bit error rate, flow control and error control can be moved to the user terminal, and the network is only responsible for information exchange and transmission, thus reducing transmission delay. ATM is very suitable for transmitting high-speed data services. From the technical point of view, ATM is almost impeccable, but the complexity of ATM technology leads to the extremely high cost of ATM switches, and no new services are introduced on top of ATM technology to drive the ATM market, which restricts the development of ATM technology. At present, the backbone network mainly adopts ATM switches, which mainly uses the high-speed characteristics of ATM switching and the QoS guarantee mechanism of ATM transmission to provide semi-permanent connections.

6. Optical switch

Due to the continuous development of optical fiber transmission technology, optical transmission has dominated the transmission field at present. The optical transmission rate has been marching to the order of terabits per second, and its high-speed broadband transmission characteristics make it difficult to adapt to the switching mode based on electrical signal packet switching, and in this mode, photoelectric conversion is needed at the transit node, which can not make full use of the bandwidth resources provided by the bottom layer. In this case, a new switching technology-optical switching was born. Optical switching technology is also a kind of optical fiber communication technology, which refers to directly switching input optical signals to different output terminals in the optical domain without any optical/electrical conversion. The ultimate development trend of optical switching technology will be all-optical switching under optical control, which is perfectly combined with optical transmission technology, that is, the transmission process of data from source node to destination node is carried out in the optical domain.

5.2 Digital Program-Controlled Switching

5.2. 1 General flow of call processing

First of all, we take the calling party as an example to explain the general process of call connection processing of digital program-controlled exchange.

(1) When the user goes off-hook, due to the change of line voltage, the user circuit will detect this action, and the exchange will investigate the user's category to distinguish ordinary phones, pay phones, PBX, etc. , find an idle number receiver and send a dial tone to the user;

(2) When the user dials, stop sending dial tone, start the number receiver to receive the number, and store the received number bit by bit;

(3) Analyze the prefix in preprocessing, and determine the call category (local call, outgoing call, long distance, special service, etc.). ) and determine the quantity to receive; When a complete and valid number is received, the switch analyzes the number according to the number;

(4) According to the result of number analysis, find out whether there are idle lines and called status from the local exchange where the called party is located. If all conditions are met, resources are occupied, and a ring back tone is sent to the calling user to ring the called user;

(5) After the called user goes off-hook, the voice is transmitted on the allocated line, and at the same time, the charging equipment is started for charging, and the status of the calling and called users is monitored;

(6) When one party hangs up, disconnect, release resources, stop charging operation and send a busy tone to the other party. At this point, a complete normal call process is completed.

5.2.2 Working principle of digital switching network

The core component of digital program-controlled switch is switching network, which has the following characteristics.

(1) Direct exchange of digital signals: between the subscriber circuits of multiple called users, the voice of the users exists in the form of digital signals, and there is no need for multi-channel/analog and analog/digital conversion like an analog switch. Moreover, digital signals can be easily processed in integrated circuits, so switching networks with higher complexity and larger scale can be designed.

(2) exchange according to the calling and called numbers; After receiving the number, the control circuit will analyze the number and generate corresponding information according to the result of the number analysis to select the route of the call connection. The process of establishing the call route through each switch is the switching process. In the early step-by-step switching, the switching route was selected according to the user's dialing pulse.

(3) Time slot exchange: In fact, exchange means exchanging information on different lines and different time slots, and moving these signals in different spaces and at different times. For example, switch TS5 on the incoming trunk line 1 with TS 18 on the outgoing trunk line 4, as shown in Figure 5-4.

Figure 5-4 Time slot exchange

The switching network of SPC exchange can be divided into time-division switching network, space-division switching network and hybrid switching network according to the organization form of switching network.

1. Time division switching

(1) Time-division switch the corresponding T connector to complete the switching between different time slots on the same relay;

(2) Composition: the T-connector consists of a voice memory and a control memory, wherein the voice memory is used for storing 8-8 bit coded digital voice signals of each time slot on the input multiplexing line; The control memory is used for storing the reading or writing address of the voice memory and controlling the reading or writing sequence of the contents of each unit of the voice memory;

(3) According to the different reading and writing control modes of voice memory, it can be divided into two types: sequential writing control reading and controlled writing sequential reading.

(1) Sequential write-in control read-out: the contents in the voice memory are written in the order in which time slots arrive, but the read-out is controlled by the control memory, and the contents in the voice memory are read out in which time slot according to the exchange requirements;

(2) Control the writing sequence and reading: The writing of the voice memory is controlled by the control memory, that is, the position where the contents of each time slot of the incoming relay are written into the voice memory is determined according to the destination time slot of the outgoing relay, while the reading is sequentially read from the voice memory.

The principle of time division switching is shown in Figure 5-5.

Figure 5-5 Time Division Switching Mode

2. Space division exchange;

(1) Space division switching, also known as S-switch, is used to switch the contents of the same time slot of different relays.

(2) composition; The air separation exchanger consists of a cross node matrix and a control memory. The intersection node matrix provides the possibility that each input trunk crosses any output trunk, and the closing time of these intersections is controlled by the control memory. The air separation exchanger also includes an output control and an input control.

The principle of space division exchange is shown in Figure 5-6.

Figure 5-6 Space division switching mode

3. Compound switching network:

For large-scale switching networks, it is necessary to switch between different time slots of the same trunk and the same time slots of different trunks, so it is necessary to combine time division switching and space division switching to form a composite switching network.

(1)TST switching network: This is the most widely used form of large-scale switching network. Among them, the input T connector is used to complete the exchange between different time slots of the same incoming trunk; S connector is responsible for space division switching between different buses; The output T connector is responsible for switching between different time slots of the same output trunk. Which control mode each switch adopts can be selected at will, and the input/output T switch needs to use the idle time slot inside the switch to complete the exchange;

(2)STS switching network: firstly, input the S switch to switch the time slot signal to the internal idle link; Then the T switch switches the signal on the link to the required time slot; Finally, the output S connector switches this signal to the required link;

(3) Multi-level switching network: In addition to the above two three-level switching networks, there are also multi-level switching networks. For example, a four-level network composed of TSST and a five-level network composed of TSST.

(4) Switching network integration: With the development of digital switching technology, some chip manufacturers have introduced switching network integrated chips. At present, the exchange chips of 2048×2048 and 4096×4096 are very mature commercial chips.

5.2.3 Composition of program-controlled switches

1. Basic components

The telephone exchange is mainly composed of two parts: voice equipment and control equipment.

(1) Voice equipment: completes the call connection between the calling party and the called party, and specifically transmits voice signals between users. Subscriber circuit, switching network, outgoing relay circuit and incoming relay circuit all belong to voice channel equipment;

(2) Control system: The control system controls the above-mentioned call connection action, and the control of the program-controlled exchange is completed by the software running in the central processor. The function of the control system includes two aspects: on the one hand, it handles calls; On the other hand, it manages, monitors and maintains the operation of the whole switching system. The hardware of the control system consists of three parts: first, the central processing unit (CPU), which can be the central processing chip of the general digital computer or the special chip of the switching system; The second is the memory, which stores the common programs, the programs being executed and the execution data of the exchange system; Thirdly, I/O system, including keyboard, printer, external memory, etc. , you can print out system data according to instructions, store abnormal running programs, and transfer them to memory when the programs run.

2. Composition of user circuit

Subscriber circuit is the interface circuit between switching network and subscriber line. Its functions are: on the one hand, it transmits voice information (analog or digital) to the switching network; On the other hand, other signals on the subscriber line (such as ringing, etc. ) isolated from the switching network to avoid damaging the switching network. The functions of user circuits can be summarized by Luo Songtang, and the corresponding functions correspond to different functional modules, which are explained below.

(1) Feed B: supply power to the user's telephone. In China, the supply voltage is -48V or -60V. If the subscriber line distance is long, the feed voltage may increase.

(2) Overvoltage protection O: The subscriber line is an external line, which may be struck by lightning or collide with the high-voltage line, so an overvoltage protection circuit must be set to protect the inside of the switch. Usually, the gas discharge device has been installed in the wiring of the subscriber line, but the voltage passing through the gas discharge device may still be several hundred volts, and the overvoltage protection circuit is mainly aimed at this voltage;

(3) Ringing R: Because the ringing voltage is relatively high, which is 75V 15V in China, it is still realized by controlling the ringing relay by electronic components, and the ringing current is controlled by the on-off of relay contacts. There are also switches that use high-voltage electronic devices to realize ringing function;

(4) Monitoring S: determining the on-off state of the subscriber line loop by monitoring the DC current of the subscriber line, and then detecting the subscriber state, such as off-hook, on-hook, dialing and talking;

(5) Coding filtering c: Complete the conversion between analog voice signal and digital signal, including three steps of sampling, quantization and coding. In addition, it is also responsible for filtering out frequency components outside the voice band;

(6) Hybrid circuit H: Hybrid circuit completes the conversion function between two-wire system and four-wire system. The analog signal of subscriber line is two-wire bidirectional, but the signal of PCM trunk line is four-wire unidirectional. Therefore, two-wire/four-wire conversion should be completed before encoding or after decoding;

(7) Test T: Connect the subscriber line to the test equipment to test the subscriber line.

In addition to the above seven basic functions, the user circuit also has the functions of polarity switching, attenuation control, charging pulse transmission, special telephone control (such as coin-operated telephone) and so on.

5.2.4 Classification of program-controlled switches

(1) can be divided into local exchange and user exchange according to the different service scope. The former completes the exchange between multiple local exchange offices or tandem offices. Connect with other exchange offices through access trunk lines. The latter is directly connected with users through local subscriber lines, and after the calls of these users are tandem, they are connected with other exchange offices through trunk lines;

(2) According to different switching modes, it can be divided into space switching and time switching. This is actually how the switching network works. Practical large telephone exchanges often adopt mixed switching mode;

(3) According to the different voice signals exchanged, it can be divided into analog switches and private branch exchanges. The former includes electromechanical switch and air separation switch. The objects exchanged by the latter are all coded digital signals.