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ISU, or RTX SU for a retransmission set, first followed by the
SSUs according to the descending order of the SSUs sequence
numbers. At the instance of transmission, an SU would be
selected from the highest priority queue with one or more SUs
in it.
5.4 P channel protocol
5.4.1 The P channel link layer protocol is used in the toaircraft
direction for processing the transmission of: 1) the
LIDUs received from the GES management, 2) the LIDUs
received from the circuit-mode services with routing
parameters indicating transmission on the P channel, and 3) dl
LIDUs received from the subnetwork layer. The P channel
protocol reliable link sewice (RLS) is used for processing the
transmission of the LWUs received from the subnetwork layer
only. The transmission of all other WDUs is processed
according to the P channel protocol direct link service (DLS).
The P channel is also used for the transmission of the
following link layer signalling SUs:
a ) T channel reservation (RES) and reservation
forthcoming (RFC) SUs associated with the T channel
reservation protocol (Annex 10, Volume IU. Part I,
Chapter 4,4.6);
b) T channel acknowledgement (TACK) SU; and
C) R channel acknowledgement (RACK) SU.
The above SUs are transmitted in accordance with the P
channel protocol DLS service.
5.4.2 At a given time, the P channel protocol may be
processing the transmission of several LSDUs (a reference
number has been assigned and an SU set has been generated
from each WDU to transmit the LSDU). However, fur RLS, at
each Q number only one LSDU of the LSDUs destined to the
same AES is permitted to be in process for transmission at one
time. The other RLS LSDUs with the same Q number and
destined to the same AES will be stored until the transmission
of the current LSDU at that Q number is over, either
successfully or unsuccessfully. Thus, at the AES, only one
RLS SU set per Q number could be in the reassembly process
at a given time and the AES will discard an incomplete RLS
SU set if it starts receiving another RLS SU set with the same
Q number and a different reference number. After either a
successful or an unsuccessful transmission of an SU set, the
GES link layer releases all link layer resources associated with
that SU set, i.e. the assigned reference number, activated
timers, memory allocation, etc.
5.4.3 For RLS, the P channel protocol implements a
selective-repeat-ARQ scheme for emr control. Accordingly,
upon transmitting an SU set the GES expects to receive a P
channel acknowledgement (PACK) SU from the AES. The
PACK SU may indicate either complete or incomplete
reception of the SU set using its acknowledgement control
field. In the latter case, this field also indicates whether a
complete or a partial retransmission of the SU set is required.
For a partial retransmission request, this field, which is present
in every PACK SU, further indicates the number of PACK SUs
identifying the remainder of the missing SSUs of the SU set to
follow; each PACK SU can identify, at most, three missing
SSUs. A partial retransmission from the GES is assembled by
Yllloo
No. 75
Annex 10 - Aeronautical Telecommunications Volume 111
the AES as any other SU set; however, a retransmission SU set
is headed by an RTX SU rather than an ISU and the
retransmitted SSUs' Sequence Numbers are as prescribed in
Annex 10, Volume III, Part I, Appendix 3 to Chapter 4, Item
59. Failure to receive an acknowledgement from the AES
would cause the GES to solicit such acknowledgement by
sending a request for acknowledgement (RQA) SU to that
AES. The AES keeps track of the status of the RLS SU set that
the AES is currently processing at a given Q number in order
to correctly respond to an RQA.
5.4.4 In an SU set, original or retransmitted,.only the ISU,
or the RTX SU, identifies the destination AES Dl. In order to
avoid erroneous processing due to the broadcast nature of the
P channel, the AES discards an SSU if received without
previously receiving the corresponding (same reference
number and Q-number) ISU or RTX SU.
55 R charnel protocol
5.5.1 The R channel link layer protocol is used in the
from-aircraft direction for processing the transmission of: 1)
the LIDUs received from the AES management, 2) the LIDUs
received from the circuit-mode services with routing parameter
indicating transmission on the R channel, and 3) some of the
LIDUs carrying LSDUs of 33 or less octets received from the
subnetwork layer. The R channel protocol reliable link service
(RLS) is used for processing the transmission of the LIDUs
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