sequence count field (SC)

The sequence number field (SC) is in the protocol data unit ( PDU) of the segmentation and reunification sublayer ( SAR) depending on the type of ATM Adaption Layer ( AAL). For AAL type 1 it is a 3 bits long data field, for AAL type 3/4 it is a 4 bits long data field, which is used for sequence integrity control. Each SAR PDU is assigned a consecutive sequence number, modulo 8 in AAL-1 and modulo 16 in AAL-3/4.