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same II code, the II code in each segment shall be deleted and
the remaining message bits retained as user data for further
processing. If all of the segments do not contain the same II
code, the entire uplink ELM shall be discarded.
Note.- An uplink ELM frame consists of two to si-wen
associated Comm-C segments, each of which contains the 4-bit
II code. Therefore, the capacity for packet transfer is 19 to
152 bytes per uplink ELM frume.
5.2.2.2.1.2 Linking protocol
5.2.2.2.1.2.1 In the Comm-B protocol, the initial segment
shall be transmitted using the air-initiated or multisitedirected
protocols. The LBS field of the initial segment shall
indicate to the ground the number of additional segments to be
transferred (if any). Before the transmission of the initial
segment to the transponder, the remaining segments of the
SLM frame (if any) shall be transferred to the transponder for
transmission to the interrogator using the ground-initiated
Comm-B protocol. These segments shall be accompanied by
control codes that cause the segments to be inserted in groundinitiated
Comm-B registers 2, 3 or 4, associated respectively
with the second, third, or fourth segment of the frame.
5.2.2.2.1 -2.2 Close-out of the air-initiated segment that
initiated the protocol shall not be performed until all segments
have been successfully transferred.
Note.- The linking procedure including the use of the
ground-initiated Comm-B pmtocol is petformed by the ADU
5.2.2.2.1.3 Directing SLM frames. If the SLM frame is
to be multisite-directed, the ADLP shall determine the I1 code
of the Mode S interrogator or cluster of interrogators
(5.2.8.1.3) that shall receive the SLM frame.
5.2.2.2.2 ELM FRAME
Note.- A downlink ELM consists of one to sixteen
associated Comrn-D segments.
5.2.2.2 DOWNLINKFR AMES
5.2.2.2.2-1 Procedure. A downlink ELM frame shall
5.2-2.2.1 SLMframe- A SLM frame be consist of from 10 to 160 bytes and shall be formed using the
composed of UP Comm-B segments. The ME the protocol defined in 3.1.2.7 of Annex 10, Volume IV,
first Comm-B segment of the frame shall contain a Zbit linked
Cornm-B subfield (LBS, bits 1 and 2 of the MI3 field). This 5.2.2.2.2.2 Directing ELM frames. If the ELM frame is
subfield shall be used to control linking of up to four Comrn-B to be multisite-directed, the ADLP shall determine the 11 code
segments. of the Mode S interrogator or cluster of interro~ators
Note.- The LBS uses the first 2-bit positions in the first
segment of a multi or single segment downlink SLM frame.
Hence, 54 bits are available for Mode S packet data in the
first segment of a downlirik SLM frame. The remaining
segments of the downlink SLM frame, if any, have 56 bits
available.
5.2.2.2. I. 1 LBS coding. Linking shall be indicated by the
coding of the LBS subfield of the MB field of the initial
Comm-B segment of the SLM frame.
The coding of LBS shall be as follows:
LBS MEANING
0 single segment
1 initial segment of a two-sepmcnt SLM frame
2 initial segment of a three-segment SLM frarne
3 initial segrncnt of a four-segment SLM frame
(5.2.8.1.3) that shall receiie the ELM frame.
5.2.2.3 XDLPframe processing. Frame processing shall
be performed on all Mode S packets (except for the MSP
packet) as specified in 5.2.2.3 to 5.2.2.5. Frame processing for
Mode S specific services shall be performed as specified in
5.2.7.
5.2.2.3.1 Packet length. The user data field shall be an
integral multiple of bytes in length. A 4-hit parameter (LV)
shall be provided in the Mode S DATA, CALL REQUEST,
CALL ACCEFT, CLEAR REQUEST and XNTERRUlT
packet headers so that during unpacking no additional bytes
are added to the user data field. The LV field shall. define the
number of full bytes used in the last segment of a frame.
During LV calculations, the 4-bit II code in the last segment
of an uplink ELM message shall be (1) ignored for uplink
ELM frames with an odd number of Comm-C segments md
7/11/96
No. 71
Part I Annex 10 - Aeronautical Xelecommunications
(2) counted for uplink ELM frames with an even number of a) a length field of all zeros; or
Comm-C segments. The value contained in the LV field shall
be ignored if the packet is multiplexed. b) less than eight bits left in the frame.
Note.- A specific ZengthJield is used to deJine the length
of each element of a multiplexed packet. Therefore the LV$eld 5.2.2.3.3 MODE S CHANNEL SEQUENCE PRESERVATION
value is not used.
5.2.2.3.3.1 Application. In the event that multiple Mode
5.2.2.3.2 Multiplexing. When multiplexing multiple S frames from the same SVC are awaiting transfer to the same
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