This read-write integer defines the Committed Burst size (bc) in bytes, defaulting to 5100, which cannot be 0 when cir is nonzero and must be 0 when cir is 0; its peak value is 2,097,151 bytes on FRSM-VHS cards (and FRSM12, per its own note) but only 65,535 bytes on other cards, and on FRSM-VHS the CIR/bc relationship must keep the resulting Tc under 512 seconds. An administrator raises bc to allow larger committed bursts of traffic without exceeding cir over time, while respecting the card's ceiling and the Tc timing constraint. For example, on a non-VHS card bc could be set no higher than 65,535 bytes, whereas the same connection moved to an FRSM-VHS card could use the full 2,097,151-byte range as long as Tc stays below 512 seconds.