PCM technology

PCM equipment has two systems, PCM30/32 and PCM24. Our country uses the former E1 system, and the latter is called the T1 system. The signal processing process of the PCM equipment is like this. The upper limit frequency of the voice signal is 4kHz, the sampling frequency is 8kHz, and each sampled value is non-uniformly quantized to 8bit. After digital processing of the voice signal, the information rate is 8kHz×8bit=64kbit/ s, called a time slot. The equipment adopts TDM (time-division multiplexing) technology. PCM30/32 has a total of 32 time slots, and the export rate is 64kbit/s×32=2.048Mbit/s, namely E1 interface, commonly known as 2M port.

  1. PCM device interface type
    There are many types of PCM equipment interfaces. The more commonly used ones are loop relay interface FXO (foreign exchange office, peripheral exchange office side interface, that is, voice relay interface), subscriber line interface FXS (foreign exchange subscriber, directly connected to the telephone, namely Voice terminal interface), two-wire audio interface, four-wire audio interface, two-wire E&M interface [ear and mouth, E wire is used for signal reception control, M wire is used for signal transmission control, E&M is used for PBX (private branch exchange, program-controlled exchange) Interconnection], four-wire E&M interface, asynchronous RS-232/V.24 interface, synchronous RS-232/RS-422/RS-485 interface, V.35 interface (1~30×64k bandwidth), 10Base_T interface ( 1~30×64k bandwidth), G.703 same direction data interface.

1) Dispatching telephone and administrative telephone interface.
The voice relay interface (FXO) and the voice terminal interface (FXS) are widely used in dispatching telephones and administrative telephones. Both FXO and FXS use a pair of audio lines. There is a DC voltage of about 48V between the two outgoing lines of FXS. , Can be directly measured with a multimeter.

2) Four-wire analog relay interface (4w E/M).
In the power communication network, the four-wire analog relay interface (4w E/M) usually uses six audio lines, two for receiving (Rx), two for transmitting (Tx), and one for E/M. The signal line and There is only a weak AC voltage between the ground wires (signal ground wire, non-public ground), which is difficult to measure with a multimeter. Common EM interfaces include Bell Type I EM interface, Bell Type II EM interface, Bell Type III EM interface, Bell Type IV EM interface, Bell Type V EM interface. The Bell type ⅤEM interface is common in our country. In the power grid, many user machines use the E&M analog relay interface to interconnect, and the analog channel of telecontrol data is accessed through this interface.

3) Analog remote control channel interface (VFB).
PCM equipment can provide audio analog dedicated line interface for transmitting audio analog signals, including remote analog signals, similar to the above four-wire analog relay interface, but without E and M signaling.

4) G.703 same direction data interface (RS-232 serial port).
Use three audio lines, which are receiving (Rx), sending (Tx), and ground (GND). There is a DC voltage below 12V between the signal wire and the ground wire (signal ground wire, non-common ground), which can be directly measured with a multimeter . The 64kbit/s data stream conforming to the G.703 protocol can be connected to the PCM through this interface.

5) G.703 same direction data interface (RS-232 serial port).
When using PCM equipment to transmit 64K protection channel signals, it generally does not go through the audio matching jumper, but uses the direct connection method of the equipment interface.

  1. PCM device type
    PCM equipment is divided into terminal type and convergent type. The terminal-type PCM device has only one relay direction; the convergent-type PCM device uses n×64kbit/s (n≤32) as the rate for multi-directional transmission, generally not more than 16 directions. Convergent PCM equipment can cross the automation data of different substations to different dispatch centers.

PCM equipment can be maintained by a unified network management platform or by terminal network management.