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cancelled. On expiration, if the ground station is operational,
then it shall transmit a GSF, the timer shall be restarted. The
value to set the TG4 timer to shall consist of a fixed value
equal to the minimum value plus a random value uniformly
chosen between 0 and 20 seconds.
shall he applied before determining if a frame is illegal or
unexpected (scc 6.5.4.2.4.7). If an LME receives any valid
XJD-HO or XU-LPM frame from a system with which it
does not have a link, it shall respond with an XLD_LCR with
the "d bit set to 1 in the Protocol Violation Cause Code.
6.5.4.4.1 Frequency management procedums. 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 frcqucncy search procedure on system initialization
or after link disconnection, if it can m longer detect uplink
VDL frames on the current frequency. It shall attempt to
identify a frequency on which VDL 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 Txmer TGl 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 L'ME shall
initiate the frequency recovery 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 connectivity procedums. The aircraft and
ground LMEs shall use the following procedures to maintain
connectivity across the VHF link:
a) ground station identification;
Note.- There is one Timer TG4 per ground station.
b) initial link establishment;
6.5.4.3.5 Timer TG5 (maximum link overlap time). Timer
TG5 shall be set to the maximum time that initiating and
responding LMEs shall maintain the old link during hmdoffs.
The LME initiating the handoff shall start its Timer TG5 when
it receives an XTD-RSP-HO. The LME responding to the
handoff shall start its Timer TG5 when it transmits its
XlD-RSPHO. The initiating LME shall never restart its
Timcr TG5; the responding LME shall restdart its Timer TG5 if
it retransmits an XXD-RSPHO. Timer TG5 shall be cancelled
if either the old or new link is prematurely disconnccted. After
TG5 expires, each LME shaU silently disconnect its half of the
old link.
Note.- There is one Timer TG5 per LME,
6.5.4.4 DESCRIRIONO F LME
PROCEDURES
The aircraft and ground LMEs shall use the XID frame types
listed in Table 6-51 and the procedures described in the tex~
below to provide a reli.able connection between the aircraft and
ground-based system. Frame collision processing (see 6.5.3.8)
c) link parameter modification;
d) aircraft-initiated handoft
e) aircraft-requested ground-based initiated handoe
i) ground-based initiated handoff;
g) ground-based requcstcd aircraft-initiated handoff;
h) ground-based requested broadcast handoff; and
j) autotune.
6.5.4.4.3 Ground station identl$cation. A ground station
shall send a GSIF by broadcasling a XD-CMD (P = 0) with
parameters as per Table 6-51 if its Timer TG3 expires,
(meaning that it has not transmitted any frame in TG3),
seconds, or if its Timer TG4 expires, (meaning that it has not
sent a GSF in TG4 seconds). If a ground station offers
Mode 2 service, the operator of that ground station shall ensure
that, besides transmitting GSTFs on the service frequency,
2/11/00
No. 75
Annex 10 - Aeronautical Telec~mmunic~~ons Volume 111
GSIFs are transmitted on the CSC. Aircraft LMEs receiving a
GSIF shall proccss its content to identify the functionality of
the ground station as well. as the correct operational. parameters
lo be used when communicating with it. Aircraft LMEs which
have a connection with the transmitting ground station shall
process only informational parameters and those parameters
specified for an XTD-CMD-LPM as per Table 6-51.
6.5.4.4.4 Link establishment. 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 receiving a DM frame shall initiate
link establishment if no links remain.
6.5.4.4.5 Link parameter modification
6.5.4.4.5.1 Ground-based initiation. The ground LME
　
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