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the LICI and the information generated by the internal link
layer protocol processes.
4.5.3.2.3.3 For LIDU comprised of
LZCI only
For an LIDU containing LICI only, the SU set shall be
generated by mapping the information contained in the LICI
and generated by the internal link layer protocol processes into
the fields of the control information of the appropriate SC'.
4.5.3.2.4.1 At the receiving end, an SU set shall be
reassembled into an LIDU before being passed to the link
service user.
4.5.3.2.4.2 For an SU set with SU(s) containing user
data. the resulting LIDU shall contain LSDU and LICI. The
LSDU shall be reassembled by combining the user data field
of each SU of the received complete SU set. The LJCI shall
be generated from the control information in the SUs.
4.5.3.2.4.3 For an SU set with SU(s) containing no user
data. the resulting LIDU shall contain only LICI. The LICI
shall be generated from the control information in the received
SUs.
4.5.3.3.2 SUs not recognized by the receiving link layer
shall be ignored.
Note.- SUs nor specified in Appendix 2 to Chapter 4 nay
be in use under mutual agreement between an AMSS service
provider and an AMSS user for non-safety services.
4.5.4 LIDU routing
The receiving link layer (in either the GES or the AES) shall
use the message type parameter in the LICI to route the LIDU
to the appropriate link service user. The transmitting link layer
(in the GES and the AES) shall use the routing parameter in
the LICI: received from the circuit-mode services (Table 4-38)
to route the SU set corresponding to an LIDU, for
transmission on the sub-band C channel or the IUP channels.
4.5.5 Precedence (Q number)
Each SU of the SU set generated from an LIDU shall be
assigned a precedence (Q number) by the link layer according . .
to the precedence parameter passed to it iii the LICI by the
link service user. Each link layer signalling SU shall be
assigned a Q number according to its message type. A
Q number shall be in the range from 0 (lowest precedence) to
15 (highest precedence).
4.5.6 DLS and RLS services
4.5.6.1 The link layer shall provide two distinct types of
services, designated as direct link service (DLS) and reliable
link service (RLS).
4.5.6.2 DLS shall be a link-layer service in which the
SUs shall be transmitted to the peer link layer without making
any provision for identification and retransmission of any lost
sus.
Note.- DLS is only available to AEYGES management
functions and circuit-mode services.
4.5.6.3 RLS shall be a link-layer service that provides
for the identification and retransmission of any lost SUs.
4.5.3.3 SIGNAL UNIT FORMAT
4.5.3.3.1 The formats of all SUs transmitted on the P
and the R channels shall be as shown in Appendix 2 to
Chapter 4. The signal unit field mapping and the transmission
order of the bits shall be as shown in Figure A2- 1. The signal
unit field coding and definitions shalI be as described in
Appendix 3 to Chapter 4.
4.5.7 P channel protocol
Note.- In this subsecrion, the use of the terms "AES" and
"GES" refer to the link layer j%nctians for the P channel
protocol within the AES and the GES.
Annex 10 - Aertrnautical Te2ecurnmunications Volume ZZZ
The link layer functions for the P channel shall be as defined
in 4.5.7.2 and 4.5.7.3.
4.5.7.2 GROUNDE ARTH STATION (GES)
4.5.7.2.1 Upon receipt of an LIDU, the GES shall assign
an available (not currently assigned to an LIDU with the same
Q number) reference number to it whenever this can be done
in accordance with 4.5.7.2.2; however, the GES shall not
assign a reference number to LIDUs received from the circuitmode
services and to system table broadcast LIDUs received
from the GES management. If a reference number cannot be
assigned to an LIDU, the LIDU shall be stored until one is
assigned to it.
Note.- The circuit-mode LIDUs are assigned application
reference numbers by the circuit-mode services.
4.57.2.2 At any Q precedence level, each assignment of
a reference number to an LIDLJ destined to a particular AES
shall be to the oldest LDU among all the LIDUs without a
reference number at that Q level destined to this AES. The GES
shall not assign a reference number to an RLS LIDU if there is
a reference number currently assigned to another LlDU with the
same Q number and addressed to the same AES. Two consecutive
reference number assignments for LDUs of the same Q
number shall not be made with the same reference number
except for two DLS LIDUs which are mapped into LSUs. In the
 
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