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received from the subnetwork layer only. The transmission of
all other LIDUs 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 AES
may be processing the transmission of several LSDUs
(reference number is assigned and an SU set is generated from
each LIDU to transmit the LSDU). However, for RLS, at each
Q number only one LSDU is permitted to be in process for
transmission to the GES at one time. Other RLS LSDUs with
the same Q number will be stored in the AES until the
transmission of the current RLS LSDU at that Q number is
over, either successfully or unsuccessfully. Thus, at the GES,
only one RLS SU set per Q number per AES could be in the
reassembly process at a given time and the GES will discard
an incomplete RLS SU set if it starts receiving another RLS
SU set with the same Q number from the same AES. After
either a successful or an unsuccessful transmission of an SU
set, the AES link layer releases all link layer resources
associated with that SU set, i.e. the assigned reference number,
activated timers, memory allocation, etc.
5.5.4 Far RLS, the R channel protocol implements' a
selective-repeat-ARQ scheme for error control. Accordingly,
upon transmitting an SU set, the AES expects to receive an R
channel acknowledgement (RACK) SU from the GES. The
WCK SU may indicate either complete or incomplete
reception using its acknowledgement control field. In the latter
case, the RACK SU also indicates the missing SUs of the SU
set by identifying their sequence indicators. The AES.transrnits
the' indicated missing SUs without ally alterations to their
contents. The largest SU set on the R channel contains.three
SUs, thus, the GES may identify at the most two missing SUs
in a RACK SU, one SU of the set must be correctly received
at the GES in order. to detect an attempt of an SU set
transmission.
5.5.5 The R channel is a multiple access channel.
therefore. the GES concurrently receives SUs from all AES
assigned to one set of R-ch annel frequencies. Each- received
SU contains the source AES ID. Thus, the received SUs are
first sorted out according to their AES IDS and then
independently processed. Annex 10, Volume III, Part I, 4.5.8.3
contains the R channel protocol requirements cornspanding to
the processing of SUs from -a given AES.
6. LINK LAYER T CHANNEL AND
SUB-BAND C CHANNEL PRCYIUCOLS
6.1 General
The link layer T c h m l protocols specified in Annex. 10,
Volume III, Part I, Chapter 4, 4.6 applies to link sewice data
units (LSDUs) in the from-aircraft direction which cannot be
transmitted via the R channel (elg. kcause its length exceeds
33 octets). There are two T channel protocols: one forquesting
the T channel capacity and the other for the
transmission of SUs in assigned capacity. The, Link layer subband
C channel protocol specified in Annex 10, Volume 111,
Part I, Chapter 4, 4.6 applies to the transmission of signalling
SUs corresponding to the circuit-mode services in the toaircraft
and from-aircraft directions.
6.2 SU set generation
An SU set cannot be generated from the received LIDU as
described in Annex 10, Volume IJJ, Part I, Chapter 4, 4.6.4.2.1
until a reference number is assigned to it by the T channel
reservation protocol. The LSDU (used data) is encoded in the
user data field of the JSU/SSU of a T channel SU set. The
number of SUs needed in the SU set depends on the LSDU
Attochmcrnt A to Part I Annex 10 - Aeronautical Telecommunications
length. Each SU, except the last one, will contain a maximum the length of the reservation actually assigned for
permitted number of user data octets. The number of user data retransmission by the GES if, for example, some TACK SUs
octets of the last SU which belongs to the set may have less of the TACK SU set are not received by the M S .
than the maximum length; unused octets are then padded with
dummy bits. The number of octets in the final SSU is encoded 6.3.2.4 The AES saves a copy of an SU set for
in the "No. of octets in the final SSU" field of the ISU. retransmission purposes. After passing the transmission status
indication LIDU to the subnetwork layer indicating success or
failure, the AES destroys the saved copy.
6.3 T channel transmission prot~~ol
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