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242 DIGITAL SUBSCRIBER SIGNALING SYSTEM NO. 1Suppose that the most recently accepted I frame had N(S) ¼ 17, and that the Iframe with N(S) ¼ 18 incurred a transmission error and was discarded byLAPD-E. The next I frame [with N(S) ¼ 19] passes LAPD-E error-checking butarrives out-of-sequence and is discarded by LAPD-E sequence-checking. LAPD-Enow sends a Reject (REJ) frame with N(R) ¼ 18. This requests LAPD-T to retransmitthe I frames in its retransmission buffer, starting with the frame with N(S) ¼ 18.At LAPD-E, the sequence-checking continues to discard I frames until it receivesthe (retransmitted) frame with N(S) ¼ 18.The two message streams (terminal to network, and vice versa) in a point-to-pointdata link connection are independent of each other and independent of the messagestreams in the other point-to-point connections in the D-channel. On a D-channelwith n point-to-point connections, there are 2n independent N(S)/N(R) sequences.Unacknowledged Message Transfer. Supervision (S) and unnumbered (U)frames do not include a N(S) field. They are accepted when received withouterrors and are not acknowledged. Supervision frames include a N(R) field toacknowledge received information frames.Unnumbered Information (UI) frames do not include N(S) and N(R) fields,because they are always sent with the “group” TEI (127), and it is not possible tocoordinate send and receive sequence numbers for the “group” functions inthe terminals on a DSL.Primitives. A network-layer function requests the acknowledged mode for an outgoingmessage by passing it in a DL-data request primitive to its LAPD, which thentransfers the message in an I frame. Unacknowledged transfer is requested bypassing the message in a DL-unitdata request, and the message is then carried ina UI frame.10.2.6 Supervision of Data Link ConnectionsThe LAPDs at the ends of a point-to-point data link connection can send and receivethe following supervision frames:RR (Receive Ready) is sent by a LAPD to indicate that it is ready to receive Iframes.RNR (Receive Not Ready) is sent by LAPD to indicate that it is not able to receiveI frames but will process received supervision frames.REJ (Reject) indicates that the sending LAPD has rejected a received I frame.A supervision action can originate at either end of the connection. Figure 10.2-4shows a few examples and illustrates the use of the C/R, P, and F bits. LAPD-T andLAPD-E denote the data link layer functions at the terminal and the network side ofthe connection.