Transfer Modes

LAPD transfers frames in one of the following two modes.

Multiframe Acknowledged Message Transfer. This mode is used only on point-to-point data-link connections, for the transfer of I frames. It includes error correction by retransmission, and in-sequence delivery of error-free messages. This mode is similar to the basic error correction of message signal units (MSU) in SS7 (Section 8.4).

The control field of an I frame has a send-number [N(S)] field and a receive-number [N(R)] field—see Table 10.2-1. These fields are comparable to the forward and backward sequence number fields (FSN, BSN) in MSUs. A LAPD assigns increasing "send" sequence numbers N(S) to consecutive transmitted I frames: N(S) = 0,1,2,...127,0,1,... etc.). It also stores the transmitted frames in a retransmission buffer where they are kept, and are available for retransmission, until positively acknowledged by the distant LAPD.

We examine the transfer of I frames on a data-link connection, sent from terminal to exchange—see Fig. 10.2-3. The procedure in the other direction is the same. LAPD-E and LAPD-T denote the data-link functions at the exchange and terminal respectively.

LAPD-E checks all received frames for errors. In addition, error-free I frames are "sequence checked." If the value of the N(S) is one higher (modulo 128) than the N(S) of the most recently accepted I frame, the new frame is "in-sequence" and therefore accepted, and its information field is passed to the specified network-layer function.

LAPD-E acknowledges accepted I frames with the receive number [N(R)] in its outgoing I and supervision frames. The value of N(R) is one higher (modulo 128) than the value of N(S) in the latest accepted I frame.

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