Encapsulation Data usually transferred in blocks Protocol data units(PDUs) Each Pdu contains data and control information Some PdUs only control Three categories of control(三类控制字段) Address Of sender and/or receiver Error-detecting code E.g. frame check sequence Protocol control Additional information to implement protocol functions Addition of control information to data is encapsulation(在数据外添加控制信息称之为 封装) Data accepted or generated by entity and encapsulated into PDU Containing data plus control information e.g. TFTP, HDLC, frame relay, ATM, AAL5 Figure 11.15), LLC, IEEE 802.3, EEE802.113/76 Encapsulation • Data usually transferred in blocks - Protocol data units (PDUs) - Each PDU contains data and control information - Some PDUs only control • Three categories of control (三类控制字段) • Address - Of sender and/or receiver • Error-detecting code - E.g. frame check sequence • Protocol control - Additional information to implement protocol functions • Addition of control information to data is encapsulation(在数据外添加控制信息称之为 封装) • Data accepted or generated by entity and encapsulated into PDU - Containing data plus control information - e.g. TFTP, HDLC, frame relay, ATM, AAL5 (Figure 11.15), LLC, IEEE 802.3, IEEE 802.11