4.1 Example 1: CRC-6 in the 1544 kbit/s Frame Structure

The CRC-6 procedure is a foundational method for ensuring link quality at the 1544 kbit/s (T1/DS1) level.

• Purpose: Its primary function is performance monitoring.
• Structure: The error check is not performed on every single frame. Instead, it is implemented over a larger structure called a 24-frame multiframe.
• Location: The six individual CRC check bits, labeled c₁ through c₆, are not sent together. They are distributed across the multiframe, transmitted in a special bit position known as the F-bit (a bit used for frame alignment, performance monitoring, and data link functions), specifically in frames 2, 6, 10, 14, 18, and 22.
• Message Block: The CRC-6 value is calculated over the entire data content of one multiframe. This block of 4632 bits is formally called the CRC-6 Message Block (CMB).

4.2 Example 2: CRC-4 in the 2048 kbit/s Frame Structure

At the 2048 kbit/s (E1) level, a different procedure called CRC-4 is used to provide even more robust checking.

• Purpose: This procedure provides an “enhanced error monitoring capability.”
• Structure: It relies on a specific CRC-4 multiframe, which consists of 16 frames. These 16 frames are further divided into two 8-frame blocks known as Sub-Multiframes (SMF I and SMF II).
• Location: The four CRC-4 check bits, C₁, C₂, C₃, and C₄, are transmitted in bit 1 of specific frames. They are sent in frames 0, 2, 4, and 6 within Sub-Multiframe I, and this pattern is repeated in frames 8, 10, 12, and 14 of Sub-Multiframe II.
• Error Indication: A key feature of this implementation is the use of CRC-4 error indication bits (E-bits). These bits are used by the receiving equipment to signal back to the transmitting equipment that it has detected an error in a received sub-multiframe.

These examples show how CRC is carefully integrated into the framing rules. This leads to the final, critical question: what does the system do with this information?