Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 1872

h) the transition to d1 shall also include a transition to i1, j1, f 1 and g1. 5.2.6.7.2 The reset procedure shall apply to the DATA TRANSFER state (p4). The error procedure in Table 5-16 shall be followed. In any other state the reset procedure shall be abandoned. 5.2.6.8 REJECT PROCEDURE 5.2.6.8.1 When the XDCE receives a Mode S DATA packet from the peer XDCE with incorrect format or whose packet sequence number (PS) is not within the defined window (Table 5-19) or is out of sequence, it shall discard the received packet and send a Mode S REJECT packet to the peer XDCE via frame processing. The Mode S REJECT packet shall indicate a value of PR for which retransmission of the Mode S DATA packets is to begin. The XDCE shall discard subsequent out-of-sequence Mode S DATA packets whose receipt occurs while the Mode S REJECT packet response is still outstanding. 5.2.6.8.2 When the XDCE receives a Mode S REJECT packet from the peer XDCE, it shall update its lower window value with the new value of PR and begin to (re)transmit packets with a sequence number of PR. 5.2.6.8.3 Reject indications shall not be transferred to the DCE. If the ISO 8208 interface supports the reject procedures, the reject indications occurring on the ISO 8208 interface shall not be transferred between the DCE and the XDCE. 5.2.6.9 PACKET RESEQUENCING AND DUPLICATE SUPPRESSION Note 1.— If the frames for an SVC include both types (SLM and ELM), the sequence of packets may be lost due to the different delivery times. The order may also be lost if multiple interrogators are used to deliver frames for the same SVC to a given XDLP. The following procedure will correct for a limited amount of desequencing. Note 2.— This process serves as an interface between frame processing and the XDCE function. 5.2.6.9.1 Resequencing. Resequencing shall be performed independently for the uplink and downlink transfers of each Mode S SVC. The following variables and parameters shall be used: SNR A 6-bit variable indicating the sequence number of a received packet on a specific SVC. It is contained in the SN field of the packet (5.2.5.2.1.1.7). NESN The next expected sequence number following a series of consecutive sequence numbers. HSNR The highest value of SNR in the resequencing window. Tq Resequencing timers (see Tables 5-1 and 5-13) associated with a specific SVC. Annex 10 — Aeronautical Communications Volume III 22/11/07 I-5-36 All operations involving the sequence number (SN) shall be performed modulo 64. 5.2.6.9.2 Duplication window. The range of SNR values between NESN – 32 and NESN – 1 inclusive shall be denoted the duplication window. 5.2.6.9.3 Resequencing window. The range of SNR values between NESN + 1 and NESN + 31 inclusive shall be denoted the resequencing window. Received packets with a sequence number value in this range shall be stored in the resequencing window in sequence number order. 5.2.6.9.4 TRANSMISSION FUNCTIONS 5.2.6.9.4.1 For each SVC, the first packet sent to establish a connection (the first Mode S CALL REQUEST or first Mode S CALL ACCEPT packet) shall cause the value of the SN field to be initialized to zero. The value of the SN field shall be incremented after the transmission (or retransmission) of each packet. 5.2.6.9.4.2 The maximum number of unacknowledged sequence numbers shall be 32 consecutive SN numbers. Should this condition be reached, then it shall be treated as an error and the channel cleared. Note.— A limit on the number of unacknowledged packets is required since the SN field is six bits long and therefore has a maximum of 64 different values before the values repeat. 5.2.6.9.5 RECEIVE FUNCTIONS 5.2.6.9.5.1 Resequencing. The resequencing algorithm shall maintain the variables HSNR and NESN for each SVC. NESN shall be initialized to 0 for all SVCs and shall be reset to 0 when the SVC re-enters the channel number pool (5.2.5.1.2). 5.2.6.9.5.2 Processing of packets within the duplication window. If a packet is received with a sequence number value within the duplication window, the packet shall be discarded. 5.2.6.9.5.3 Processing of packets within the resequencing window. If a packet is received with a sequence number within the resequencing window, it shall be discarded as a duplicate if a packet with the same sequence number has already been received and stored in the resequencing window. Otherwise, the packet shall be stored in the resequencing window. Then, if no Tq timers are running, HSNR shall be set to the value of SNR for this packet and a Tq timer shall be started with its initial value (Tables 5-1 and 5-13). If at least one Tq timer is running, and SNR is not in the window between NESN and HSNR + 1 inclusive, a new Tq timer shall be started and the value of HSNR shall be updated. If at least one Tq timer is running, and SNR for this packet is equal to HSNR + 1, the value of HSNR shall be updated. 5.2.6.9.5.4 Release of packets to the XDCE. If a packet is received with a sequence number equal to NESN, the following procedure shall be applied:

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