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STATION (as)
6.4.2.1 The T channel reservation protocol in the AES
assigns reference numbers to the LIDUs received from the
subnetwork layer for transmission on the T channel. The
reference numbers are assigned in such a way that no two
consecutive LIDUs with the same Q number transmitted from
an AES to a GES have the same reference number. The reference
number assigned to an LSDU is used by the T channel
reservation protocol in the request for reservation (REQ) SU
and by all the associated signalling and by the T channel
transmission protocol in the data SUs and by the associated
acknowledgements and quest for acknowledgement SUs.
6.4.2.2 The REQ SU is transmitted to the GES on the
T channel or on the R channel. Every time the REQ SU is
transmitted on the T channel, the requested length in the REQ
SU is incremented by one in order to account for the T
channel capacity used to transmit the REQ SU.
6.4.2.3 The timer tA8 in the AES is used to supervise
the receipt of expected T channel reservation (RES) SUs
associated with either previously received reservation
forthcoming (RFC) SUs. or received TACK SUs. Due to the
priotity scheme for the transmission of SU sets, resulrjng in
higher priority SU sets potentially occupying reservation
assignments for lower priority SU sets, there will be instances
of time where multiple tA8 timers associated with the same
SU set might have been initiated at the AES. Upon receipt of
an awaited RES SU for that SU set, the AES must stop the
corresponding tA8 timer (Annex 10, Volume 111, Part I,
Chapter 4, 4.6.5.2.6 and 4.6.5.2.7). However, since a one-toone
correspondence between a tA8 timer.and a received RES
SU is not guaranteed, a selection of a most suitable tA8 timer
must be made. Such selection is implementation dependent.
For instance, the selection could be made dependent on, the
time-out values of all outstanding tA8 timers, such that the
tA8 timer with the. shortest remaining time to time-out is
selected regardless of the size of reservation in the received
RES SU. In this case, care must be taken in the implementation
to request reservations for any resulting deficit in the total
size of outstanding reservations associated with an SU set at
the AES when all tA8 timers have been stopped. Other criteria
for the selection may be used; however, the constraint that at
least the total size of all outstanding reservations associated
Attathment A to Part I Annex 10 - Aeronautical Telecommunicahbns
with any given SU set must be received prior to releasing the
reference number assigned to that SU set must always be
observed.
6.4.2.4 Reservations may not be received in chronological
order. The minimum inter-reservation gap between two
reservations on two different T channels is made adequate to
allow for frequency (T channel) switching at the AES. A
reservation (RES) SU defines one or more burst allocations on
one T channel. The starting frame number of the initial burst
is encoded within the RES SU and is known as belonging to
one of the sixteen frames following the reception of the RES
SU (the time window). Thereafter, further reservations (if any)
occur every 2BI frames until the number of reserved bursts is
reached. BI is the encoding of the burst interval information
element within the RES SU. The AES computes the starting
frame for each reserved burst using both the starting frame
number of the initial burst and the burst interval information.
Once a reserved burst has elapsed, the AES deletes it from its
time plan.
6.4.3 GROUND EARTH
STATION (GES)
6.4.3.1 The GES assigns reservations in response to
requests for reservations from its logged-on AESs. The GES
also assigns reservations for retransmissions of missing SUs of
an SU set transmitted by the AES. So, whenever the T channel
transmission protocol at the GES transmits a TACK SU set
comprised of one or more TACK SUs indicating errors, the T
channel reservation protocol at the GES makes reservations for
the retransmissions.
6.4.3.2 The GES assigns reservations to an AES for any
one of the four T channels that may have been assigned to the
AES at the time of lag-on. The GES indicates the T channel
on which the reservation has been made to the AES in the
RES SU. A procedure to select a T channel, out of the
possible four assigned to the AES at the time of log-on, is
implementation dependent. The following methods may be
utilized for selecting the T channel:
a) all T channels available to a particular AES are polled.
Reservations are assigned any T channel on which
the minimum allowable reservation delay is
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