Definition and Principle of Telephone Exchange

Analog Subscriber Interface is the interface circuit through which a key telephone system or a program-controlled exchange connects to ordinary analog terminals, also commonly referred to as the subscriber port. In a switching system, since switching processing is digitized and integrated, electrical signals with relatively high current and voltage cannot be processed directly, so many functions must be handled by the subscriber interface circuit. This circuit unit typically accounts for about 60% of the total cost of the exchange system, which is what we refer to in key telephone systems and program-controlled exchanges as: analog subscriber boards, dedicated extension boards, digital extension boards, etc. The unit is generally calculated by the number of lines or ports, for example: 8 ports per board, 16 ports, 24 ports.

Functions of the Subscriber Circuit:

1. Signal Conversion

The subscriber circuit is actually the interface between analog circuits and digital circuits. The signals received by the subscriber circuit from the terminal side are all analog electrical signals, so they are converted into digital signals when submitted to the processing system; this part is implemented by the encoder. Similarly, when the processing result received is a digital signal that needs to be converted into an analog signal, this part is implemented by the decoder.

2. Status Monitoring

The following states can be fed back through the on/off status of the line:

(1) Subscriber off-hook, on-hook, and conversation states.

(2) Telephone dialing and code transmission.

3. Hybrid Circuit

The signals sent by ordinary telephones are all analog signals, transmitted bidirectionally over two wires, while dedicated telephones use PCM digital signals, requiring encoding in the outgoing direction and decoding in the incoming direction, which necessitates four-wire transmission. This is why dedicated telephones on some key telephone systems and program-controlled exchanges require four-wire cabling. The so-called digital dedicated telephones are not necessarily fully digital telephones; strictly speaking, they should be called: hybrid dedicated telephones.

4. Feeding

There are two feeding methods: voltage feeding and current feeding. All terminals connected to the switching system are powered by the exchange. Generally, the feeding voltage of a program-controlled exchange is 48V, and the conversation feeding current is between 20 mA and 50 mA. The feeding voltage of small key telephone systems and some imported key telephone systems is around 30V, with a feeding current between 20 mA and 60 mA. This is one of the reasons why program-controlled exchanges have a longer transmission distance than key telephone systems.

5. Overvoltage Protection

Even if a protector is installed in the external wiring section and grounded, the residual terminal voltage can still exceed 100V during transient high voltages such as lightning strikes, which is a fatal blow to the exchange. Therefore, overvoltage protection measures must be adopted. Correspondingly, overcurrent and overvoltage protection are also provided.

6. Ringing

The ringing voltage is relatively high, generally around 90V domestically. Relays are typically used to control the ringing current contacts. Ringing is controlled by the exchange system; when ringing is required, a control signal is issued to close the relay, and the ringing circuit sends the ringing current signal to the subscriber. When the subscriber goes off-hook, the off-hook signal is detected by the ringing monitoring circuit, which immediately cuts off the ringing. On some key telephone systems, this function is implemented by high-voltage electronic devices, eliminating the relay.