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restarted on each subsequent receipt of a valid transmission below to provide a reliable connection between thc aircraft and
from that station. It shall never be cancelled. If Timer TG2 ground-based system. Frame collision processing (see 6.5.3.8)
Part I Annex 10 - Aeronautical Telecommunications
shaH be applied before determining if a framc is illegal or
unexpected (see 6.5.4.2.4.7). If an LME rcceives any valid
XID-HO or XID-LPM frame from a systcrn with which it
does not have a link, it shall respond with an XID-LCR with
the "d bit set to 1 in the Protocol Violation Cause Code.
6.5.4.4.1 Frequen,~? management procedures. The
aircraft LME shall use the following procedures to acquire a
frequency on which reliable VDL services are available.
6.5.4.4.1.1 Frequency search. The aircraft LME shall
initiate the frequency search procedurc on system initialization
or after link disconnection, if it can no longer detcct uplink
VDL frames on the current frequency. It shall attempt to
identify a frequency on which W L service is available by
tuning the radio to the CSC and to other frequencies on which
it knows a-priori that VDL service is available. It shall scan
until it detects a valid uplink VDL frame with an acceptable
source address or until Timer TG1 expires, in which case it
shall tune the radio to another frequency and continue to scan.
6.5.4.4.1.2 Frequency recovery. The aircraft LLME shall
initiate the frequency rccovery procedure if it can no longer
establish a link on the current frequency or if the MAC entity
indicates that the current frequency is congested. It shall tune
the radio to an alternate frequency using the data in the
Frequency Support List previously received on the current
link.
6.5.4.4.2 Link connecfiviiy procedures. The aircraft and
ground LMEs shall use the following procedures to maintain
connectivity across the VHF link:
a) ground station identification;
b) initial link establishment;
c) link parameter modification;
d) aircraft-initiated handofk
e) aircraft-requested ground-based initiated handoft
f) ground-based initiated handoff;
g) ground-based requested aircraft-initiated handoff;
h) ground-based requested broadcast handoff; and
i) autotune.
6.5.4.4.3 Ground station ident$cation. A ground station
shall scnd a GSIF by broadcasting a XID-CMD (P = 0) with
parameters as per Table 6-51 if its Timcr TG3 expires,
(meaning that it has not transmitted any hame in TGS),
seconds, or if its Timer TG4 expires, (meaning that it has not
sent a GSIF in TG4 seconds). If a ground station offers Mode
2 service, the operator of that ground station shall ensure that,
besides transmitting GSIFs on the service frequcncy, GSIFs
are transmitted on the CSC. Aircraft LMEs receiving a GSIF
shall process its content to identify the functionality of the
ground station as well as the correct operational parameters to
be used when communicating with it. Aircraft LMEs which
have a connection wilh thc transmitting ground station shall
process only informational parameters and those parameters
specified for an XID-CMD-LPM as per Table 6-51.
6.5.4.4.4 Link cstablishmenl: The aircraft LME shall
initiate the link establishment procedure with a ground station
only to establish an initial link with the ground-based system.
An aircraft transmitting or rccciving a DM frame shall initiate
link establishment if no links remain.
6.5.4.4.4.1 Aircraft initiation. The aircraft LME shall
choose a ground station with which it wishes to establish a
link based on the signal quality of all received uplink frames
and on information in any received GSIFs. It shall then
attempt to establish a link with the chosen ground station by
sending an XID-CMI-LE (P=l) hame. This frame shall
include the mandatoxy parameters as per Table 6-5 1 and also
any optional parameters for which the aircraft LME does not
wish to use the default value. If the aircraft LME has received
a GSlF from the ground station to which it is transmitting the
XID-CMD-LE (P=l), then it shall use the parameters as
declared; otherwise, it shall use the default parameters.
6.5.4.4.4.2 General ground response. If the ground LMH
receives the XU-CMI-LE, it shall confirm link establishment
by sending an XID-RSP-LE frame containing the parameters
as per Table 6-51. The ground LME shall include in the
XTDRSPLE any optional parameters for which it is not
using the default values. If the XIDRSP-LE includes the
Autotune parameter then the Replacement Ground Station List
parameter shall be included indicating the ground stations on
the new frequency that the aircraft LME can establish a new
　
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