In some cases more than one PRI connects between equipment. LAPD allows multiplexing of layer 2 connections on the same physical connection. Eight users can have simultaneous D channel sessions. LAPD protocol provides framing, sequence control, error detection, and recovery of multiple logical data links on the same D channel. The Layer 2 operation starts with a Layer 3 request to establish a call, which prompts Layer 2 to demand Layer 1 to initiate the start-up procedure.
Initialization guarantees that the correct sequence numbers are used in processes on both ends.
Layer 2 protocol maintain the correct sequence of information due to many different types of error. Firstly, if frames lost more than three consecutive times, the re-establishment of the LAP will be done under the assumption that the connection has failed. Secondly, a timer mechanism and preset values of time-out intervals detect lost frames in the transmit direction. Thirdly, frames received out of sequence denote a lost frame in the receive direction. Unacknowledged operation uses unnumbered information frame.