Frame Structure of 30/32-Channel PCM Primary Group

A communication system consists of transmitting equipment, receiving equipment, and transmission equipment. The investment in transmission lines often accounts for a large proportion of the total investment in the entire communication system. Therefore, how to improve line utilization and achieve multiplexing of transmission lines has become a very important topic.

1 Methods of Multiplexing

Multiplexing generally has three basic methods: Frequency Division Multiplexing (FDM), Code Division Multiplexing (CDM), and Time Division Multiplexing (TDM).

1.1 Frequency Division Multiplexing
Frequency Division Multiplexing is a transmission method widely used in analog communications. Its basic principle is to use modulation techniques and filtering technologies so that multiple signals can be transmitted simultaneously on the same line without mutual interference by means of frequency division.

1.2 Code Division Multiplexing
Code Division Multiplexing refers to the transmission of multiple signals on the same channel without mutual interference using different coding formats. It has now become an advanced method used in mobile communications.

1.3 Time Division Multiplexing
Time Division Multiplexing is the primary transmission method adopted in modern digital communications. Time Division Multiplexing is a method in which pulse sequences of several discrete signals within one channel are grouped, compressed, and cyclically sorted to become multiple signals that do not overlap in time and are transmitted together. For example, if many users at two locations need to communicate—User 11 to User 12, User 21 to User 22... User n1 to User n2—but there is only one pair of lines, a pair of rapidly rotating electronic switches SA1 and SA2 is added at both the transmitting and receiving ends (these two switches are actually a set of sampling gates and demultiplexing gates, whose opening and closing are controlled by sampling pulses). SA1 and SA2 rotate at the same frequency with corresponding initial positions, which is called synchronous operation. Initially, SA1 and SA2 stay on User 11 and User 12, then rotate sequentially to 21 and 22, 31 and 32, n1 and n2, and finally return to 11 and 12, repeating this cycle. Currently, the world's digital Time Division Multiplexing systems mainly include the 24-channel PCM system used in North America and Japan, and the 30/32-channel PCM system used in Europe and China.

2 Frame Structure of 30/32-Channel PCM Primary Group

2.1 Frame Structure
The concept of frame structure is the combination of digital code streams in which multiple voice digital codes and various inserted markers are arranged in a certain time sequence. China adopts the 30/32-channel PCM primary group structure, which means that when transmitting data, the 1st channel signal is transmitted first, then the 2nd channel signal, the 3rd channel signal... until the 32nd channel is transmitted, then the 1st channel, the 2nd channel... and so on. Each channel signal occupies a different time position, called a timeslot, represented by TS0, TS1, TS2, ... TS31. Among them, TS0 is used to transmit synchronization codes, monitoring codes, and remote alarm code groups (abbreviated as remote alarm codes); TS16 is used to transmit signaling codes; TS1—TS15 transmit the voice digital codes of the first 15 voice channels, and TS17—TS31 transmit the voice digital codes of the latter 15 voice channels. Obviously, among the 32 timeslots, only 30 are used for transmitting voice digital codes, denoted as PCM30/32. The time required to sample all voice channels once is called the frame length, which is also the time interval between two samples of the same voice channel. Since the sampling frequency of each voice channel is 8000 Hz, i.e., 8000 samples per second, the time interval between two sample values is 1/8000, equal to 125 µs, which determines that the frame length is 125 µs. Since encoding takes time, each sample value must have a certain width. This time width is the timeslot, i.e., the time each voice channel occupies in one frame, equal to 3.91 (125/32) µs. Each timeslot's sample value is encoded into 8 bits; therefore, each bit occupies 0.448 µs (3.91 µs/8).

2.2 TS0
In even frames, TS0 is used to transmit the frame synchronization code, where bits 2—8 are fixed at 0011011. This 7-bit code group is the frame synchronization code. The receiving end achieves synchronization by detecting the frame synchronization code group. Bit 1 is reserved for international use and is set to 1 when not used. In odd frames, TS0 is used to transmit monitoring codes, remote alarm codes, etc. Bit 2 is fixed at 1, called the monitoring code, which assists in the synchronization process. Bit 3 is A1, used to transmit the remote alarm code: 0 during normal synchronization, and 1 when out of synchronization. The other bits—bit 1 and bits 4—8—can be used for low-rate data communication and are set to 1 when not used. The function of the remote alarm code is: for normal call operation, both directions must be unobstructed. If one direction has a fault, the remote alarm code must be able to notify the opposite end. Obviously, the periods of the synchronization, monitoring, and remote alarm codes are all 250 µs.

2.3 TS16
To establish a call process, the correct transmission of signaling information is essential. In previous analog transmission and analog switching, signaling mainly consisted of DC or DC pulse signals, such as off-hook/on-hook signals and dial pulses. In other words, signaling was transmitted in the form of analog signals. In PCM communications, however, signaling information is transmitted via digital channels. It can occupy the same timeslot as voice information for transmission—for example, both the 1st channel's voice information and signaling occupy TS1 for transmission, which is how signaling is transmitted in 24-channel PCM communications. Voice information can also be transmitted separately from signaling. In 32-channel PCM communications, all 30 signaling channels are transmitted in TS16. From the sampling theorem, it is known that for voice information, the sampling frequency is 8000 Hz, meaning voice sample values are extracted every 125 µs. Theory and practice show that for each signaling channel, the sampling frequency is 500 Hz, i.e., sampling once every 2 ms. In digital communications, each signaling channel is first converted into a 4-bit digital signal and placed in 4 bits of TS16. Thus, the 8 bits of TS16 can accommodate two channels of digital signaling, and 30 channels of signaling require a total of 15 TS16 frames. Adding one frame before these 15 frames as a marker forms a multiframe, called the signaling multiframe. The 16 frames it contains are called subframes, represented by F0—F15, arranged as follows:
In F0, bits 1—4 of TS16 transmit the multiframe synchronization code group "0000," whose function is to ensure correct signaling transmission, i.e., to ensure synchronization between transmitted and received signaling; bit 6 is A2, transmitting the multiframe remote alarm code. Bit 6 = 0 indicates multiframe synchronization, and bit 6 = 1 indicates multiframe out-of-synchronization. Bits 5, 7, and 8 are reserved and temporarily set to 1 when not used.
In F1, bits 1—4 of TS16 transmit the 1st channel's signaling, and bits 5—8 transmit the 16th channel's signaling. In F2, bits 1—4 of TS16 transmit the 2nd channel's signaling, and bits 5—8 transmit the 17th channel's signaling... In F15, bits 1—4 of TS16 transmit the 15th channel's signaling, and bits 5—8 transmit the 30th channel's signaling. One signaling multiframe transmits all 30 channels of signaling exactly once, with a period of 2 ms, i.e., a signaling sampling frequency of 500 Hz.