PCM30 Working Principle: What Is PCM30/32

PCM30 Equipment Working Principle

The 30-channel PCM equipment refers to equipment that provides 30 user interfaces and can directly provide 30 terminal service interfaces over a standard PCM30 primary group, i.e., a 2M transmission channel.

Optical Transmitter Composition

The electrical signal sent from the PCM equipment is a code type suitable for PCM transmission, either HDB3 code or CMI code. After the signal enters the optical transmitter, it first enters the input interface circuit for channel coding, converting it into a non-return-to-zero (NRZ) code composed of "0" and "1" bits. It then undergoes code conversion in the code conversion circuit, transforming it into an mBnB code or an insertion code suitable for optical line transmission. The signal is then sent to the optical transmission circuit, where the electrical signal is converted into an optical signal and transmitted over the optical fiber.

Optical Repeater

Traditional optical repeaters adopt the optical-electrical-optical (O-E-O) mode. The photodetector first converts the very weak and distorted optical signal received from the fiber into an electrical signal, which is then amplified, reshaped, and retimed to restore it to an electrical pulse signal identical to the original signal. This electrical pulse signal is then used to drive the laser to emit light, converting the electrical signal back into an optical signal and sending optical pulses to the next section of fiber. Repeaters with the three functions of re-amplifying, re-shaping, and re-timing are commonly referred to as "3R" repeaters.

Optical Receiver

The optical signal transmitted from the fiber enters the optical receiving circuit, where it is converted into an electrical signal and amplified, then retimed and regenerated to restore it to a digital signal. Since code conversion is performed at the transmitting end, reverse code conversion must be performed at the receiving end. The signal is then sent to the output interface circuit, where it is converted into HDB3 code or CMI code suitable for PCM equipment transmission and delivered to the PCM equipment.

What Is PCM30/32

The frame period of PCM30/32 is 1/8000 second, i.e., 125μs. One frame is evenly divided into 32 parts, each of which is called a timeslot, with each timeslot interval being 125/32, i.e., 3.91μs. Each timeslot transmits 8-bit encoding, and each frame transmits 32×8=256 bits of encoding. In each frame, timeslot TS0 transmits the frame synchronization code, and timeslot TS16 transmits signaling. Synchronization and signaling are the foundation of communication technology and will be described later. The remaining 30 timeslots transmit voice. The interval between the first arrival, second arrival, and each subsequent arrival of the first voice channel is one frame period. Since 8000 frames are transmitted per second (with a sampling frequency of 8000), each voice channel is 8bit×8000=64kbit, and 32 channels equal 2.048Mbit, which is commonly referred to as the PDH primary rate signal, or the E1 signal.