The main task of dispatching switch is to realize the connection of calls 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; Control devices mainly include markers and recorders in crossbar switches, while in digital dispatching switches, control devices are electronic computers, including central processing unit (CPU), memory and input/output devices.
In essence, digital dispatching switch is a kind of switch that uses computer to "store program control". It compiles various control functions and methods, stores them in memory, and controls and manages the whole switching system by scanning external state data and storing 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 directly feeds 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.
Monitoring: monitoring the on-off state of the subscriber line with the help of the scanning point, detecting the subscriber line signals such as off-hook, on-hook and dialing pulse of the telephone, and transmitting them to the control equipment to indicate the busy and idle state of the user and the 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 dispatching switches, coding and decoding functions are not needed; In digital dispatching switches, except for some small switches with specific applications that use incremental modulation, most others 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. As an interface between analog relay and switching network, the basic functions of Analog Relay Interface Unit (ATU) generally include:
1. Send and receive line signals indicating trunk status (such as idle, occupied, response, released, etc.). ).
2. Forward and receive the transmitter signal representing the called number.
3. Provide call power and signal tone.
4. Provide the received line signal to the control equipment.
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 digital dispatching 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 switching and management functions.
The main control equipment of digital dispatching exchange is microprocessor, 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.