3. A Tour of the G.704 Hierarchical Levels
The G.704 standard doesn’t just define one type of frame; it specifies structures for several standard transmission speeds, known as hierarchical levels. Each level is designed to carry a different amount of traffic.
Here is a high-level overview of the main digital hierarchies covered in G.704:
| Hierarchical Level (Bit Rate) | Bits Per Frame | Key Characteristic (for a beginner) |
| 1544 kbit/s | 193 bits | Often used in North America and Japan, this structure uses a special single bit (the “F-bit”) for alignment, monitoring, and control functions. |
| 2048 kbit/s | 256 bits | A foundational rate used in Europe and many other parts of the world. It is a fundamental building block of the digital telephone network. |
| 6312 kbit/s | 789 bits | Uses five dedicated ‘F-bits’ per frame to manage frame alignment, performance monitoring, and a 4 kbit/s data link. |
| 8448 kbit/s | 1056 bits | Organizes data into 132 octets (8-bit bytes) per frame, with a fixed frame alignment signal in the first 8 bits. |
| 44 736 kbit/s | 4760 bits (in a multiframe) | A high-capacity level that groups data into a large ‘multiframe’ structure, which is then divided into seven smaller sub-frames for management. |
You might have noticed that the first four levels in the table specify a frame repetition rate of 8000 Hz. This number is significant because it’s directly tied to the core function of the traditional telephone network. To digitize a human voice, it must be sampled 8000 times per second (a process called PCM). G.704’s frame rate of 8000 frames per second is a perfect match, allowing one sample of voice data to be placed into each frame, making it incredibly efficient for telephony.
These frames are more than just containers for data; their overhead bits perform critical jobs that keep the network healthy.