This is the size, in bytes, of the data block that the channel's CRC error-detection check covers, determining the granularity at which this VDSL channel can detect transmission errors. A smaller block length means errors are detected more precisely but with more CRC overhead per byte of actual data carried; a larger block length is more efficient but groups more data under each single CRC check. If error counters seem disproportionately high relative to actual line quality, checking whether this block length is unusually large for the channel's configured rate can reveal that CRC granularity, not line quality, is driving the apparent error rate.