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5.2.6.9 PACKETRE SEQUENCING AND
DUPLICATE SUPPRESSION
Note I.- If the frames far an SVC include both types
(SLM and ELM), the sequence of packets m y be lost due to
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 incrernented after the transmission (or retransmission)
of each packet.
5.2.6.9.4.2 The maximum number of unacknowledged
scquence numbers shall be 32 consecutive SN numbers. When
this condition is reached, ,all further SVC packets for that
channel shall be held by the XDLP until acknowledgment of
successful delivery is received from the connected interrogator(~)
o r transponder for the oldest packet in transit.
Note.-- A limit on the number of unacknowledgtd packets
is required since the SNJield is six bits long and therefore has
a maximum of 64 different values before the values repcat.
511 1/98
No. 73
Part I Annex I0 - Aeronautical Telecommunications
5.2.6.9.5 RECEIVEFU NCTIONS 5.2.7 Mode S specific services processing
5.2.6.9.5.1 Resequehcing. 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. lf a packet
is received with a sequence number equal to NESN, the
following procedure shall be applied:
Mode S specific services shall be processed by an entity in the
XDLP termed the Mode S specific services entity (SSE).
Note 1.- This section describes the processing of control
and message data received from the Mode S spec$c services
interface.
Note 2.-- Control data consists of infomation permitting
the determination oj for example, message length, GZCB
register number and aircraft address.
5.2.7.1 ADLP PROCESSING
5.2.7.1.1 DoWNWNK PROCESSING
5.2.7.1.1.1 Specific services capability. The ADLP shall
be capable of receiving control and message data from the
Mode S specific services interface(s) and sending delivery
notices to this interface. The control data shall be processed to
determine the protocol type and the length of the message
data. When the message or control data provided at this
interface are erroneous (i.e. incomplete, invalid or inconsistent),
the ADLP shall discard the message and deliver an
error report at the interface.
Note.- The diagnostic content' and error reporting
mechanism are a local issue.
5.2.7.1.1.2 Broadcast processing. The 7-by te broadcast
message content (5.2.7.5) shall be extracted from the received
message data. The broadcast message content shall be used to
a) the packet and any packets stored in the the MB field of a Comm-B broadcast message and
resequencing window UP to the next missing sequence wansferred to the wansponder. The assignment of downlink
number shall be passed to the XDCE; broadcast identifier numbers shall be as specified in
Table 5-23.
b) NESN shall be set to 1 + the value of the sequence
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