PCM30 Working Principle What is PCM30/32
PCM30 Equipment Working Principle
A 30-channel PCM device refers to equipment providing 30 user interfaces, capable of directly offering 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 passes through the input interface circuit for channel coding, being converted into a non-return-to-zero (NRZ) code composed of "0" and "1" bits. Then, in the code conversion circuit, the code type is converted into an mBnB code or an inserted code suitable for optical line transmission, and subsequently sent to the optical transmission circuit, where the electrical signal is converted into an optical signal and launched into the optical fiber for transmission.
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 subjected to timing regeneration 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, 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 called a timeslot, with a timeslot interval of 125/32, i.e., 3.91μs. Each timeslot carries 8-bit encoded data, and each frame carries 32×8=256 bits of encoded data. In each frame, the TS0 timeslot carries the frame synchronization code, and the TS16 timeslot carries signaling. Synchronization and signaling are the foundation of communication technology and will be described later. The remaining 30 timeslots carry 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 the commonly referred to PDH primary rate signal, also known as the E1 signal.