This read-write integer defines the Initial Burst Size (ibs) in bytes, defaulting to 100, which must be less than or equal to bc when cir is greater than 0 and must be 0 when cir is 0; its peak is 2,097,151 bytes on FRSM-VHS cards versus 65,535 bytes elsewhere, and it is not supported on FRSM12. An administrator uses ibs to size the burst credit available right at connection startup, always keeping it within the bc ceiling configured for the same channel. For example, if bc is set to 10,000 bytes on a non-VHS card, ibs must be configured at or below 10,000 bytes rather than pushed toward the card's separate 65,535-byte absolute maximum.