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PCM device interface

PCM device interface: It includes user interface and transmission link interface.

User interface: An interface used to connect a user terminal device and connect the PCM device to the user device through cable.


I. the user interface: In general the PCM device commonly used user interface as following: 

1.loop relay interface (FXO interface)

It is used in the central office PCM device to connect the analog subscriber line interface of the program-controlled switch or the telephone line coming from the telecom, transmit and extend it to the remote subscriber station.

2.analog subscriber line interface (FXS interface)

It is used for remote PCM devices and is paired with the loop relay interface (FXO interface) of the central office and directly connect to the telephone. 

3. hotline interface

It can be used as an emergency telephone for users. The hotline telephone interface of the PCM is directly connected to the telephone. One end of the telephone lifts the machine, and the other end will automatically ring, and the call can be free after the lifting. No need to dial, no need to occupy the telephone line resources of the program-controlled switch, can be placed in the line with very frequent telephone contact.

4.magnet phone interface

It connect the hand-operated magnet phone which is commonly used in military and railway. The PCM at both ends is directly connected to the magnet phone. One end is shaken, and the other end will automatically ring, and the phone can be called freely after lifting the machine.

5.two / four line audio E & M interface

The two line indicates that the 2-wire method is adopted, and the receiving and transmitting are performed in a pair of lines; the four-line means adopting the 4-wire method, in which the receiving and transmitting each occupy a pair of lines; E and M are control lines, and the E (Ear) line is used for the control signal is accepted, and the M (Mouth) line transmits a control signal. Often used in: a) 2-wire, 4-wire audio or carrier analog signal transmission; b) connection between switches with E&M signaling relay interfaces to achieve connection between two central office telephone relay transmissions; c) simulation Telecontrol signal transmission.

6.RS232/422/485 interface

Used to transfer 64K synchronous or asynchronous data from users.

7.n * 64k Ethernet interface

One channel narrowband Ethernet data is transmitted between the two ends of the PCM to construct a local area network for the computer users at both ends.

8.Other user interfaces of PCM equipment also include billing telephones, V.21/V.22, V.24, V.28, V.34, V.35, G.703 synchronize 64k etc

II. the transmission link interface

The transmission link interface is an E1/2M interface, which is used to transmit data accessed by the local user interface to the peer end, and connect the PCM device to the transmission device through a coaxial cable.

The E1 interface impedance of PCM equipment has 75 ohms and 120 ohms. It is commonly used  75 ohms in China. The physical connectors are mostly coaxial plugs such as BNC, CC4 and CC3. In some countries such Europe and the Middle East, 120 ohms are used. It is the RJ45 crystal head.

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