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Note.- The enhanced uplink ELM protocol provides u higher dutu link cupucity by permitting parallel delivery of
uplink ELM messages by up to sixteen interrogators, one for each II code. Operation without the need for multisite uplink
ELM reservations is possible in regions of overlapping coverage for interrogators equipped for the enhanced uplink ELM
protocol. The protocol is fully conformant to the standard multisite protocol and thus is compatible with interrogators that
ure not equipped for the enhunced protocol.
3.1.2.7.6.1 General
3.1.2.7.6.1.1 The interrogator shall determine from the data link capability report whether the transponder supports
the enhanced protocols. If the enhanced protocols are supported, uplink ELMs delivered using the multisite protocol may
be delivered without a prior reservation. If the enhanced protocols are not supported by both the interrogator and thc
transponder, the multisite reservation protocols specified in 3.1.2.7.4.1 shall be used.
3.1.2.7.6.1.2 Recommendation.- I f the transponder and the interrogator are equipped for the enhanced protocol,
the interrogator should use the enhanced uplink protocol.
3.1.2.7.6.1.3 The transponder shall be capable of storing a sixteen segment message for each of the sixteen 11 codes.
3.1.2.7.6.2 Reservation processing. The transponder shall support reservation processing for each II code as specified
in 3.1.2.7.4.1
Note I.- Reservation processing is required for interrogators that do not support the enhanced protocol.
Note 2.- Since the transponder can process simultaneous uplink ELMs for all sixteen II codes, a reservation will
always be granted.
3.1.2.7.6.3 Enhanced uplink ELM delivery and closeout. The transponder shall process received segments separately
by I1 code. For each value of II code, uplink ELM delivery and closeout shall be performed as specified in 3.1.2.7.4.2
except that the MD field used to transmit the technical acknowledgment shall also contain the 4-bit (33-36) IIS subfield.
Note.- The interrogator may use the II code contained in the technical acknowledgement in order to verifi that it has
received the correct technical acknowledgement.
3.1.2.7.7 MULTISITED OWNLINK ELM PROTOCOL
3.1.2.7.7.1 Initialization. The transponder shall announce the presence of a downlink ELM of n segments by making
the binary code corresponding to thc decimal value 15 + n available for insertion in the DR field of a surveillance or
Comm-B reply, DF equals 4, 5, 20, 21. This announcement shall remain active until the ELM is closed out (3.1.2.7.7.3,
3.1.2.7.8.1).
Chapter 3 Annex I0 - Aeronautical Telecommunications
3.1.2.7.7.1.1 Multisite downlink ELM reservation. An interrogator shall requesl a reservation for extraction of a
downlink ELM by transmitting a surveilla~lceo r Cornrn-A interrogation containing:
DI= I
US = assigned interrogator identifier
MES = 3 or 6 (downlink 'ELM reservation request)
Note.- A multisite downlink ELM reservation request is n o m l l y accompanied by a downlink ELM reservation status
request (RSS = 3). This causes the interrogator identifier of the reserved interrogator to be inserted in the UM field of
the reply.
3.1.2.7.7.1.1.1 Protocol procedure in response to this interrogation shall depend upon the state of the D-timer which
indicates if a downlink ELM reservation is active. This timer shall run for TR seconds.
Note 1.- The value of TR is given in 3.1.2.10.3.9.
a) if the D-timer is not running, the transponder shall grant a reservation to the requesting interrogator by:
1) storing the IIS of the interrogation as the Comm-D 11; and
2) starting the D-timer.
A multisite downlink ELM reservation shall not be granted by the transponder unless a downlink ELM is waiting
to be transmitted.
b) if the D-timer is running and the IIS of the interrogation equals the Comm-D 11, the transponder shall restart the Dtimer;
and
c) if the D-timer is running and the IIS of the interrogation does not equal the Comm-D 11, there shall be no change
to the Comm-D II or D-timcr.
Note 2. In case c) the reservation request has been denied.
3.1.2.7.7.1.1.2 An interrogator shall determine if it is the reserved site through coding in the UM field and, if SO, it
may proceed to request delivery of the downlink ELM. Otherwise, ELM activity shall not be started during this scan and
a new reservation request may be made during the next scan.
3.1.2.7.7.1.1.3 If downlink ELM activity is not completed on the current scan, the interrogator shall ensure that it still
has a reservation before requesting additional segments on a subsequent scan.
3.1.2.7.7.1.2 Multiuite-directed Ju wnlink ELM ~rutrsmiss.ions. To direct a downlink ELM message to a spccific
inkrrogator, the multisite downlillk ELM protocol shall be used. When the D-timer is not running, the interrogator
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