ASM / CADU

A Channel Access Data Unit (CADU) consists of an Attached Sync Marker (ASM) followed by the coded transfer frame. The standard TM ASM is the 32-bit pattern 0x1ACFFC1D, chosen because it has a sharp autocorrelation peak: when the receiver compares this pattern against the incoming bitstream (sliding it one bit at a time), the match score jumps sharply at the correct position and is low everywhere else. This makes it easy to detect the exact frame boundary even in noisy conditions. Proximity-1 links use a shorter 24-bit ASM (0xFAF320).

The receiver's frame synchronizer searches for the ASM pattern, then knows that the next $N$ bytes are the coded frame. By checking for the ASM at the expected offset after the current frame, the receiver can achieve flywheel synchronization: once locked, it expects frames at regular intervals and can tolerate brief signal dropouts.