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ground earth station (GES), waits for T channel acknowledgement
(TACK) SU(s) from the GES. The TACK SUs are
received at the AES by the receiving end of the P channel
protocol, which then passes them to the T channel transmission
protocol. The AES may receive more than one TACK
SU indicating the missing SUs of an SU set at the GES. Each
TACK SU can identify up to tluee missing SUs. The receipt
of a TACK SU of the TACK SU set identifying the missing
SUs, initiates a timer tA5 in the AES and indicates the number
of adbtional TACK SUs that are expected. If the timer tA5
expires before another TACK SW of the TACK SU set is
received, the AES transmits the missing SUs identified in the
so far received TACK SUs. If TACK SU(s) are received after
the corresponding timer tA5 has expired and the AES has not
yet transmined the retransmission SU set identified in the
received TACK SU(s), the AES discards the received TACK
SU(s).
6.3.2.3 The AES computes the length of the expected
reservation for retransmission based upon the number of
missing SUs indicated in the received TACK SUs of the
TACK SU set. This length is associated with the timer tA8
initiated by the T channel reservation protocol to supervise the
receipt of reservation for retransmission. This length may be
less than the length of the reservation actually assigned for
retransmission by the GES if, for example, some TACK SUs
of the TACK SU set are not received by the AES.
6.3.2.4 The AES saves a copy of an SU set for retransmission
purposes. After passing the mnsmission status indication
LIDU to the subnetwork layer indicating success or
failure, the AE3 destroys the saved copy.
6.3.2.5 If after several retries, an expected reservation
from the GES is not received. the AES discards at least the
number of SUs for which the reservation was requested. The
Annex 10 - Aeronautical Telecommunications Volume III
SUs discarded may not be those which initiated the original
request for reservation because of the fact that the transmission
of SUs is based upon the precedence described in Annex 10,
Volume III, Part I, Chapter 4, 4.6.4.2.2, but will be of equal
or lower precedence. If an ISU is among the SUs discarded at
the AES, the complete SU set headed by the discarded ISU is
discarded. This may result in discarding more SUs than
required. If the SU discarded is a REQ SU, then the AES
retransmits the REQ SU on either the T or R channel. If the
SU discarded is a last SU of an SU set, then the AES starts a
timer for the supervision of TACK SU(s) for that SU set.
6.3.3 GROUND EARH
STATION (GES)
6.3.3.1 This is the receiving end of the T channel
transmission protocol. The GES, upon receipt of an SU set,
reassembles it into an LSDU. Before reassembling the SU set,
the GES checks whether or not all the SUs of the SU set have
been received. If the SU set is not complete after no more SUs
of the SU set are expected (determined by the criterion specified
in the AMSS SARPs), the GES requests for the retransmission
of the missing SUs by transmitting appropriate number
of TACK SUs, indicating error, on the P channel to the AES.
6.3.3.2 If while waiting for retransmissions from the
AES, the GES receives an ISU with the same AES ID and Q
number as the SU set awaiting completion and with a reference
number belonging to a pair whose other member is the
reference number of the SU set awaiting completion, the GES
discards the SU set awaiting completion and proceeds with the
new SU set. The receipt of the ISU with the parameters
described above indicates that the AES has given up on the
transmission of the previous SU set and has released the
reference number, thus making available the other member of
the pair for assignment.
6.3.3.3 The GES may receive an RQA SU from the
AES in the following circumstances:
a) initial SU set loss (i.e, loss of at least the ISU): the
RQA SU does nor refer to an SU set which has been
processed just before;
b) negative acknowledgement loss or retransmission loss:
the RQA SUrefers to an SU set under correction; and
c) positive acknowledgement loss: the refers to
an SU set which has been processed just before.
When the initial SU set has been lost, the GES determines the
initial SU set length from the message length parameter in the
RQA SU in order to reserve and send the carrect T channel
capacity along with the request for complete retransmission.
6.4 T channel reservation
protocol
The T channel reservation protocol is used to request
reservations for the transmission of data on the T channel.
6.4.2 AIRCRAFTE ARTH
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