Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 2044

d) The disconnected mode response (DM) shall be used to report a non-operational status where the station is logically disconnected from the link. No information field shall be permitted with the DM response. Note.— The DM response shall be sent to request the remote combined station to issue a mode-setting command or, if sent in response to the reception of a mode-setting command, to inform the remote combined station that the transmitting station is still in ADM and cannot action the mode-setting command. 8.6.4.3 EXCEPTION CONDITION REPORTING AND RECOVERY This section specifies the procedures that shall be employed to effect recovery following the detection or occurrence of an exception condition at the link level. Exception conditions described are those situations that may occur as the result of transmission errors, station malfunction, or operational situations. 8.6.4.3.1 Busy condition. A busy condition occurs when a station temporarily cannot receive or continue to receive I-frames due to internal constraints, e.g. due to buffering limitations. The busy condition shall be reported to the remote combined station by the transmission of an RNR frame with the N(R) number of the next I-frame that is expected. It shall be permissible for traffic pending transmission at the busy station to be transmitted prior to or following the RNR. Note.— The continued existence of a busy condition must be reported by retransmission of RNR at each P/F frame exchange. 8.6.4.3.1.1 Upon receipt of an RNR, a combined station in ABM shall cease transmitting I-frames at the earliest possible time by completing or aborting the frame in process. The combined station receiving an RNR shall perform a time-out operation before resuming asynchronous transmission of I-frames unless the busy condition is reported as cleared by the remote combined station. If the RNR was received as a command with the P bit set to “1”, the receiving station shall respond with an S-frame with the F bit set to “1”. 8.6.4.3.1.2 The busy condition shall be cleared at the station which transmitted the RNR when the internal constraint ceases. Clearance of the busy condition shall be reported to the remote station by transmission of an RR, REJ, SABM, or UA frame (with or without the P/F bit set to “1”). 8.6.4.3.2 N(S) sequence error. An N(S) sequence exception shall be established in the receiving station when an I-frame that is received error free (no FCS error) contains an N(S) sequence number that is not equal to the receive variable V(R) at the receiving station. The receiving station shall not acknowledge (shall not increment its receive variable V(R)) the frame causing the sequence error, or any I-frames which may follow, until an I-frame with the correct N(S) number is received. A station that receives one or more I-frames having sequence errors, but which are otherwise error free, shall accept the control information contained in the N(R) field and the P/F bit to perform link control functions, e.g. to receive acknowledgement of previously transmitted I-frames (via the N(R)), to cause the station to respond (P bit set to “1”). 8.6.4.3.2.1 The means specified in 8.6.4.3.2.1.1 and 8.6.4.3.2.1.2 shall be available for initiating the retransmission of lost or errored I-frames following the occurrence of a sequence error. 8.6.4.3.2.1.1 Where the REJ command/response is used to initiate an exception recovery following the detection of a sequence error, only one “sent REJ” exception condition, from one station to another station, shall be established at a time. A “sent REJ” exception shall be cleared when the requested I-frame is received. A station receiving REJ shall initiate sequential (re)transmission of I-frames starting with the I-frame indicated by the N(R) contained in the REJ frame. Part I Annex 10 — Aeronautical Communications I-8-21 22/11/07 FRMR INFORMATION FIELD BITS FOR BASIC (SABM) OPERATION First bit transmitted 9 10 13 14 20 21 rejected basic control field V(S) v V(R ) w x y z set to zero where: rejected basic control field is the control field of the received frame which caused the frame reject; V(S) is the current value of the send state variable at the remote combined station reporting the error condition (bit 10 = low order bit); V(R) is the current value of the receive state variable at the remote combined station reporting the error condition (bit 14 = low order bit); v set to “1” indicates that the received frame which caused rejection was a response; w set to “1” indicates that the control field received and returned in bits 1 through 8 are invalid or not implemented; x set to “1” indicates that the control field received and returned in bits 1 through 8 was considered invalid because the frame contained an information field which is not permitted with this command. Bit w must be set to “1” in conjunction with this bit; y set to “1” indicates that the information field received exceeded the maximum information field length which can be accommodated by the station reporting the error condition. This bit is mutually exclusive with bits w and x above; z set to “1” indicates that the control field received and returned in bits 1 through 8 contained an invalid N(R) count. This bit is mutually exclusive with bit w. 8.6.4.3.2.1.2 In the event a receiving station, due to a transmission error, does not receive (or receives and discards) a single I-frame or the last I-frame(s) in a sequence of I-frames, it shall not detect an out-of-sequence exception and, therefore, shall not transmit REJ. The station which transmitted the unacknowledged I-frame(s) shall, following the completion of a system-specified time-out period, take appropriate recovery action to determine the sequence number at which retransmission must begin. 8.6.4.3.2.1.3 Recommendation.— A combined station which has timed out waiting for a response should not retransmit all unacknowledged frames immediately. The station may enquire about status with a supervisory frame. Note 1.— If a station does retransmit all unacknowledged I-frames after a time-out, it must be prepared to receive a subsequent REJ frame with an N(R) greater than its send variable V(S). Note 2.— Since contention may occur in the case of two-way alternate communications in ABM or ADM, the time-out interval employed by one combined station must be greater than that employed by the other combined station so as to permit contention to be resolved. 8.6.4.3.3 FCS error. Any frame with an FCS error shall not be accepted by the receiving station and will be discarded. No action shall be taken by the receiving station as the result of that frame. 8.6.4.3.4 Frame reject exception condition. A frame reject exception condition shall be established upon the receipt of an error-free frame which contains an invalid or unimplemented control field, an invalid N(R), or an information field which Annex 10 — Aeronautical Communications Volume III 22/11/07 I-8-22 has exceeded the maximum established storage capability. If a frame reject exception condition occurs in a combined station, the station shall either:

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