3. CRC Implementations Across G.704 Hierarchies
The G.704 standard specifies different CRC procedures for the various digital transmission speeds, known as hierarchical levels. The type of CRC and its placement within the data stream are precisely defined for each level. The following table summarizes the key implementations.
| Hierarchical Level (Bit Rate) | CRC Procedure | Key Implementation Detail |
| 1544 kbit/s (T1/DS1) | CRC-6 | Used for performance monitoring. The 6 check bits (c₁ to c₆) are placed in the F-bit position of specific frames within a 24-frame multiframe. |
| 2048 kbit/s (E1) | CRC-4 | Provides enhanced error monitoring. It is part of a CRC-4 multiframe structure where 4 check bits (C₁ to C₄) are sent in bit 1 of the frame. |
| 6312 kbit/s | CRC-5 | The 5 check bits (e₁ to e₅) occupy the last five bits of the multiframe. |
| 8448 kbit/s | CRC-6 | Used for end-to-end quality monitoring. The 6 check bits are located in bit positions 1 to 6 of channel time slot 99. |
To make this clearer, let’s dive into the details of two of the most common implementations from this table.