ITU-T T.71 PDF

General

The half-duplex transmission module (HDTM) for extending the use of LAPB for operation of Teletex terminals connected to the PSTN where use of half-duplex 2400 bit/s modems is planned. This is referred to in Recommendation T.70 as LAPX.

Before the HDTM begins operation the physical circuit must be established by the appropriate PSTN call control procedures. The operation of the HDTM is such that the calling DTE will initially have the right to transmit. For the link addressing conventions refer to Recommendation T.70.