2. Core Concepts and Terminology
The recommendation is built upon several fundamental concepts that define the structure and operation of digital transmission frames.
| Concept | Description |
| Frame Structure | The fundamental, repeating unit of data in a digital transmission stream. Each frame is of a fixed length and contains both user data (payload) and overhead bits for management and control. All specified interfaces operate at a frame repetition rate of 8000 Hz. |
| Multiframe | A sequence of a specific number of consecutive frames, grouped together to create a larger structure. This allows for the transmission of low-rate overhead information, such as advanced error checking or signalling, that would not fit within a single frame. |
| Frame Alignment Signal (FAS) | A distinct, predefined bit pattern transmitted within the overhead bits of a frame or multiframe. Receiving equipment searches for this signal to synchronize with the incoming data stream and correctly identify the start of each frame. |
| Cyclic Redundancy Check (CRC) | An error-detecting code used to verify the integrity of transmitted data. A block of data is processed to generate a short checksum (the CRC value) that is transmitted with it. The receiver performs the same calculation; a mismatch indicates a transmission error. G.704 specifies different CRC variants (e.g., CRC-4, CRC-6) for different hierarchical levels. |
| Data Link (DL) | An embedded communications channel created using designated overhead bits within the frame structure. It is used to carry messages for OAM purposes, such as performance reports, alarm indications, and control commands (e.g., loopback activation). |
| Signalling | The process of exchanging control information within a telecommunications network. G.704 defines methods for carrying signalling for user channels, including Common Channel Signalling (using a dedicated time slot) and Channel-Associated Signalling (embedding signalling bits within the multiframe). |