Begin of Message
(BoM) is a control sequence of a data block with which the start of the message is marked. The Begin of Message sequence is used, for example, in character-oriented backup protocols and in Distributed Queue Dual Bus (DQDB). In ATM, the BoM signal indicates whether the start of a service data unit (SDU) is located in the corresponding protocol data unit (PDU). The BoM field is an identification field of the Physical Data Unit (PDU) of the Segmentation and Reassembly Sublayer (SAR-PDU) of the ATM adaptation layer of AAL-3/4.