Digital switching refers todigital signalFor the exchange of exchange objects, that is, it does not have to be transformed intoanalog signal, and the process of establishing connection directly with digital signal.Switches that transmit and exchange digital signals are calledDigital switch。[1]
The digital program-controlled switching system developed on the basis of digital communication, the signals exchanged by its switching network aredigital signalThe connected line is a time division multiplexing PCM line.Digital switching is a new switching mode. Each session (user) for switching occupies a designatedTime slot, its information（Binary encodingDigital signals) are transmitted in this time slot, and the time slots of multiple sessions are arranged in a certain order and transmitted along this common wire.[2]
The technology of establishing digital information channel between communication users through switching network.It is the basis of digital communication.Most adoptTime division multiplex communicationMode.Divided intoCircuit switchingandpacket switching。For circuit switching, it is necessary to establish a fixed circuit connection between the calling terminal and the called terminal until the end of communication, which is suitable for real-time services.Packet switching does not establish a fixed circuit connection, and only occupies the circuit when transmitting packets, which can improve the network utilization.With digital switching technology, the communication equipment has small volume, high transmission quality, fast connection speed, easy information encryption and centralized maintenance.[3]
Related concepts
Announce
edit
Digital signal:The characteristic of digital signal is that its amplitude value is limited to a limited number of values, and it is not continuous but discrete.For example, the signals processed by computers, the signals transmitted by telegrams, the signals received and sent by mobile phones, and the data signals transmitted by networks are all digital signals.[4]
Digital switch：Digital switchIt is aimed at traditional analog switches.First, let's understand the concept of the switch: Private Branch Exchange, which is the telephone service network used inside the company. The extension users inside the system share a certain number of external lines.Traditional PBX utilizationCircuit switchingAnd IP PBX usesTCP/IP protocol, using the principle of packet switching, voice, image and other information are converted into binary bytes represented by 0 and 1EthernetIP PBX or PBX based on IP protocol is a digital switch.
Digital switching network: onDigital switchIs used inDigital switching network。The main feature of the digital switching network is that it can directly exchange the digital signals coming from the digital transmission equipment, without the need for D/A and A/D conversion as in the past.
Time division multiplex communication：Time division multiplex communicationFrequency division multiplexing and time division multiplexing are commonly used.They divide channels by frequency and time respectively.
The frequency division multiplexing method is to divide the available frequency bands of the channel into several frequency bands that do not overlap each other, and the spectrum of each signal occupies one of the frequency bands to realize multiplex transmission.
Time division multiplexing is to divide a physical channel into several communication channels (such as sessions) according to different times, and each channel uses the physical channel in turn according to a certain period and order.In this way, from a macro perspective, one physical path can "simultaneously" transmit information from multiple channels.[5]
Development history
Announce
edit
In terms of digital switching, as early as the late 1950s, Bell Laboratories in the United States published the experimental results of digital circuit switching, and pointed out the development prospects of digital switching and digital transmission integration. However, due to the component conditions at that time, it could only stay in the laboratory and could not be put into practical application.
In the early 1970s, on the basis of a large number of applications of digital PCM transmission, France successfully developed a switch for direct exchange of PCM digital signals. In terms of control, it uses the program-controlled mode, and the call connection uses the time-division exchange mode realized by electronic devices. Since the control part and the connection part both use electronic devices, it also realizes full electronic exchange,This all electronic time-division digital program-controlled switching technology has shown various advantages, prompting countries all over the world to race to develop this program-controlled digital switching technology. Its advantages have been continuously improved, its performance has become more superior, but its cost has continued to decline. By the mid-1980s, it had developed to ignore the integration of transmission and switching,It is also cheaper to use it in all simulation environments than other exchange systems;It has replaced the analog program-controlled exchange of air separation and is in its heyday of development.
In the mid-1980s, China introduced the program-controlled digital switching technology, which directly leaped over the stages of air separation simulation and user level air separation, such as first centralized and then time-division, and soon moved from electromechanical crossbar system to program-controlled digital system;By the mid-1990s, several thousands of program-controlled digital switches of various systems (such as 1240, F-150, NEIOX-61, AXE-10, DMSl00, EWSD, ESS5, etc.) had been used in telephone networks all over China, and domestic program-controlled digital switches such as C&C08 had also been in production and application on a considerable scale.[6]
principle
Announce
edit
The basic principle of digital switching can be explained as follows: the 8000Hz sample value of the user voice is encoded into an 8-bit codeword by the hub to occupy a time slot of the composite signal in a time-division multiplexing manner and transmitted to the central exchange. The operation of finding a free time slot for the user is called "time slot matching". The central exchange uses a pulse phase shifter to make a short delay for a time slot number in the user voice code,It is connected to the free time slot that can be occupied on the called user side and transmitted to the called hub along with the multiplexed digital signaldecodeIt becomes a voice signal, enabling the calling user to talk with the called user.The whole process of switching and connection is based on digital signals, which is called "time slot conversion".This gap switching operation is controlled by a computer.[7]
advantage
Announce
edit
Digital switching adopts advanced program control mode, which has the following advantages in addition to the advantages of program control mode.
① Small size, light weight, saving the area of the machine room.
② Because LSI or even VLSI is adopted, the wiring between machines is simplified, which improves the reliability.
③ The switching network has small blocking rate, large capacity and fast action speed.
The blocking rate is the ratio of the number of unsuccessful calls due to the failure to find an idle path to the total number of calls.Due to time division multiplexing, the capacity of digital switching network is large and the blocking rate is very low, even it can be completely non blocking, thus improving the quality of service.Because the blocking rate is very small, it can adapt to large traffic fluctuations, and the trunk line can have a high utilization rate.
④ Easy to use PCM transmission and flexible network.Digital switching can directly exchange digital signals. When it cooperates with PCM transmission, analog digital and digital analog conversion are not required. Digital switching and digital transmission are combined to formIntegrated digital network。
Digital switches facilitate the opening of various non voice services, such as data, telegraph, image, etc., thus forming an integrated services digital network.[8]
