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indicating transmission on the P channel, and 3) all
LTDUs received from the subnetwork layer. The P channel
protocol reliable link service (RLS) is used for processing the
transmission of the LIDUs received from the subnetwork layer
only. The transmission of all other LIDUs 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
Iayer signalling SUs:
a) T channel reservation (RES) and reservation forthcoming
(RFC) SUs associated with the T channel
reservation protocol (Annex 10, Volume III, Part I,
Chapter 4, 4.6);
Annex 10 - Aeronauh'cal Telecommunications Volume ZZZ
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 LIDU to transmit the LSDU). However, for 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 error 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 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 ID. 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.
5.5 R channel 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 msmission 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 received from the subnetwork layer
only. The transmission of all other LDUs is processed according
to the R channel protocol direct link service (DLS).
5.5.2 The R channel is also used for the transmission of
the following link layer signalling SUs:
a) P channel acknowledgements (PACKS);
b) T channel request for acknowledgement (RQA); and
C) T channel request for reservation (REQ).
The above SUs are transmitted in accordance with the
R channel protocol DLS service.
5.5.3 At a given time, the R channel protocol at the
　
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