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identifier of the desired destination shall be stored as the Comm-D II. Simultaneously, the D-timer shall be started and
the DR code (3.1.2.7.7.1) shall be set. For a multisite-directed downlink ELM, thc D-timer shall not automatically timc
out but shall continue to run until:
a) the message is read and closed out by the reserved site; or
b) the message is cancelled (3.1.2.10.5.4) by the data link avionics.
N o t e . The protocols of 3.1.2.7.7.1 will then result in the delivery of the messuge to the reserved site. TIze dutu link
avionics may cancel the rnessage 8 delive y to the reserved site cannot be uccomplished.
3.1.2.7.7.2 Delivery of downlink ELMS. The interrogator shall extract a downlink ELM by transmitting a Cornm-
C interrogation with RC equals 3. This interrogation shall carry the SRS subfield which specifies the segments to bc
transmitted. On receipt of this request, he transponder shall transfer the requested segments by means of Comm-D
replies with KE equals 0 and ND corresponding to the number of the segment in MD. The first segment shall be
transmitted 128 microseconds plus or minus 0.25 rnicros~cond following the sync phase reversal of thc izlterrogation
Annex 10 - Aeronautical Telecommunications Volume IV
requesting delivery and subsequent segments shall be transmitted at a rate of one every 136 microseconds plus or minus
1 microsecond. If a request is received to transmit downlink ELM segments and no message is waiting, each reply
segment shall contain all ZEROS in the MD field.
Note I.- The requested segrnents may be transmitted in any order
Note 2.- Segments lost in downlink transmissions will be requested again by the interrogator on a subsequent
interrogation carqing the SRS sul?field. This process is repeated until all segments have been transferred.
3.1.2.7.7.2.1 SRS, segment request subfield in MC. This 16-bit (9-24) uplink sublield in MC shall request the
transponder to transfer downlink ELM segments. Starting with bit 9, which denotes segment number 0, each of the
following bits shall be set to ONE if the transmission of the corresponding segment is requested. SRS shall appear in
MC if RC equals 3 in the same interrogation.
3.1.2.7.7.2.2 D-timer restart. The D-timer shall be restarted each time that a request for Comm-D segments is
received if the Comm-D I1 is non-zero.
Note.- The requirement for the Comm-D 11 to be non-zero prevents the D-timer from being restarted durin,g a nonselective
downlink ELM transaction.
3.1.2.7.7.3 Multisite downlink ELM closeout. The interrogator shall close out a multisite downlink ELM by
transmitting either a surveillance or a Comm-A interrogation containing:
either DI = 1
US = assigned interrogator identifier
MES = 4, 5 or 7 (downlink ELM closeout)
or DI = 0, I or 7
IIS = assigned interrogator identifier
PC = 6 (downlink ELM closeout).
The transponder shall compare the IIS of the interrogation to the Comm-D 11 and if the interrogator identifiers do not
match, the state of the downlink process shall not be changed.
If the interrogator identifiers match, and if a request for transmission has been complied with at least once, the transponder
shall set the Comm-D 11 to 0, reset the D-timer, clear the DR code for this message and clear the messagc itself.
If another downlink ELM is waiting to be transmitted, the transponder shall set thc DR code (if no Comm-B message is
waiting to be delivered) so that the reply contains the announcement of the next message.
3.1.2.7.7.4 Automatic expiration of downlink ELM reservation. If the D-timer period expires before a multisite
closeout has becn accomplished, the Comm-D 11 shall be set to 0, and the D-timer reset. The message and DR code shall
not be cleared.
Note.- This makes it possible for another site to read and clear this message.
3.1.2.7.8 NON-SELLCTIVE DOWNLINK ELM
Note.- In cases where the multisite protocols are not required (Le. no overlapping coverage or sensor coordination
via ground-to-ground communicutian), the non-selective downlink ELM protocol may be used.
Non-selective downlink ELM delivery shall take place as described in 3.1.2.7.7.2.
3.1.2.7.8.1 Non-selective downlink ELM closeout. The interrogator shall close out a non-selective downlink ELM by
transmitting PC equals 6 (downlink ELM closeout) in a surveillance or Cornrn-A interrogation. On receipt of this
command, and if a request for transmission has been complied with at least once, the transponder shall perform closeout
unless the D-timer is running. If the D-timer is running, indicating that a multisitc reservation is in effect, the closeout
shall be accomplished as per 3.1.2.7.7.3.
Chapter 3 Annex 10 - AerunuuticaZ Td~ccommufcications
3.1.2.7.9 ENHANCED DOWNLINK ELM PROTOCOL
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