This counts the total octets carried in transmitted SIP3 frames, the byte-level companion to lportSmdsCntOutFrSip3s, giving actual data volume rather than just frame count. An admin divides bytes by frames from these two counters to get average SIP3 frame size, useful for capacity planning and for spotting unusually small or large average frame sizes that might indicate inefficient application traffic patterns. For example, a customer whose lportSmdsCntOutByteSip3s grows much faster relative to lportSmdsCntOutFrSip3s than typical is sending unusually large SMDS frames, worth knowing when planning trunk capacity.