1 LAPB has entered a disconnected -------------------------------- 0 phase -------> (i.e. Conrol , see Note 3) 3 LAPB has entered a disconnected -------------------------------- 0 phase -------> (i.e. Conrol , see Note 3) Any Physical level has no circuit to -------------------------------- 0 a remote DTE -------> Note 1 - Status indicates to LAPB that the sending of frames is enabled. Note 2 - Status indicates to LAPB that the sending of frames is inhibited. Note 3 - Control indicates that LPAB has entered the disconnected phase (equivalent to ADM of HDLC). Note 4 - HDTM uses the idle data link channel state indication (at least 15 contiguous 1's) for requesting that the remote DTE enter the half-duplex sending state. Fascicle VII.5 - Rec. T.71 PAGE1 TABLE 2/T.71 Description of state transitions in terms of the V.26 bis modem interface Present Event Action New state state 0 Calling DTE: Circuit 107 ON Turn circuit 105 0N 4 Connect LAPB to circuit 103 0 Called DTE: Circuit 107 ON Start timer T 2 4 Circuit 106 ON Enable sending of LAPB frames 1 (see Note 1) 1 Transmission concluded (see Note Inhibit sending of LAPB frames 2) Disconnect LAPB from circuit 103 Hold circuit 103 in the binary 1 2 condition Turn circuit 105 OFF (see Note 3) Start timer T 2 Reception of a flag Stop timer T 3 2 Expiry of timer T Turn circuit 105 ON Release circuit 103 from binary 4 1 condition Connect LAPB to circuit 103 3 Reception of 15 contiguous 1 Turn circuit 105 ON bits (see Notes 4 and 5) Release circuit 103 from binary 1 condition Connect LAPB to circuit 103 PAGE6 Fascicle VII.5 - Rec. T.71 4 1 LAPB has entered a disconnected Turn circuit 105 0FF 0 phase 3 LAPB has entered a disconnected -------------------------------- 0 phase -------> Any Circuit 107 OFF Turn circuit 105 OFF 0 Note 1 - It is necessary to ensure that at least one full flag is transmitted after circuit 106 comes ON. This flag may be the opening flag of the first frame. co concluded by either of the following: V counting a sequence of contiguous flags on circuit 103 while in state 1, V a timeVout, T, V a signal from another source, e.g., from a higher level. However, if no frame is transmitted while in state 1, not less than five contiguous flags shall be sent in state 1 before entry into state 2. Note 3 V It is recommended that circuit 105 not be turned OFF until 15 bit times after the binary 1 condition is established on circuit 103. This will assure transmission of an idle sequence to the remote DTE. Note 4 V It is recognized that whether or not an idle sequence is sent by the remote DTE, the DTE will detect an idle sequence after circuit 109 goes OFF, since according to Recommendation V.26 bis, this will hold circuit 104 in the binary 1 condition. Note 5 V It is understood that circuit 109 will go OFF. Entry into state 4 may be made dependent on this OFF condition, as an implementation option. Fascicle VII.5 - Rec. T.71 PAGE1 ANNEX A (to Recommendation T.71) Additional rules making for greater efficiency in half-duplex transmission A.1 General considerations - Greater efficiency is obtained in recovery situations. - The application of these rules is optional. - The application of these rules does not imply any incompatibility or entail any amendment of DTEs (or DCEs) which observe the procedures described in Recommendation T.71. A.2 Rules of operation 1) Before the DTE (or DCE) gives the turn b that it has acknowledged all the frames received and accepted before it received the turn. 2) If the DTE (or DCE) receives or takes the turn, it will always first retransmit all the I-frames which have not been acknowledged. 3) The DTE (or DCE) must replace the last RR frame in each turn, if any, by an REJ frame carrying the appropriate N(R). PAGE6 Fascicle VII.5 - Rec. T.71