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428 AIR INTERFACE SIGNALING IN CDMA NETWORKSFigure 13.3-11. LAC PDU structure before SAR segmentation.the layer 3 interface. In the receive direction, the MCSB is generated by the SARsublayer when it receives a SDU from MAC.When transmitting, the LAC sublayer builds LAC PDUs, or PDU fragments, andthen passes them (as data blocks) to the MAC sublayer. The process is visualized inFig. 13.3-13. When receiving, the process works in reverse.Automatic Repeat Request (ARQ) Sublayer. The ARQ sublayer addsacknowledgment fields to the PDU to provide different levels of reliability inmessage transfer. Reliability is managed by automatic retransmission of messagesuntil an acknowledgment is received.Two types of ARQ operations are supported: assured mode and unassured mode.With assured mode a message is resent periodically until an acknowledgment isreceived. With unassured mode no acknowledgment is sent or expected.Authentication, Message Integrity, Addressing, and UtilitySublayers. These sublayers add fields to the PDU for authentication, messageintegrity, MS identification, message type, and padding (the number of octets in aPDU must be an integer). The message type is communicated by layer 3 in theMCSB and is inserted in the header by the utility sublayer as the MSG_TYPE field.Figure 13.3-12. SAR segmentation of LAC PDUs into encapsulated PDU fragments.