backward sequence number (SS7) (BSN)

Structure of the Message Signaling Unit (MSU)

Backward Sequence Number(BSN) is a 7-bit long data field in the Message Transfer Part( MTP) of the Message Signal Unit( MSU). The Backward Sequence Number is used to acknowledge the received character units from the other side, therefore no acknowledgement characters are required. The BSN data field contains the sequence number of the acknowledging signal unit. The Backward Sequence Number together with the 1-bit Backward Indicator Bit( BIB) data field forms an octet.