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ground-initiated Comm-B protocol is pelformed by the ADLP
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 Comm-D segments.
5.2.2.2.2.1 Procedure. Downlink ELM frames shall be
used to deliver messages greater than or equal to 28 bytes and
shall be formed using the protocol defined in 3.1.2.7 of
Annex 10, Volume IV.
5.2.2.2.2.2 Directing ELM frames. If the ELM 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 ELM frame.
5.2.2.3 XDLP frame 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 spccificd in
5.2.7.
5.2.2.3.1 Packet length. All packets (including a group of
packcts multiplexed into a single frame) shall be transferred in
a frame consisting of the smallest number of segments necded
to accommoda~c h e packet. The user data field shall be an
integral multiplc of bytes in length. A 4-bit parameter (LV)
shall be provided in the Mode S DATA, CALL REQUEST,
CALL ACCEPT, CLEAR REQUEST and INTERRUPT
packet headers so that during unpacking no additional bytes
are added to lhc user data field. The LV field shall define the
number of full bytes used in thc last segment of a frainc.
During LV calculations, the 6bit 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 and (2)
counted for uplink ELM frames with an even number of
Comm-C segments. Thc value contained in the LV held shall
be ignored if the packet is multiplexed.
Note.- A spvpecific length field is used to d@ne the length
of each element of a multiplexed packet. Therefore the LV$eld
value is not used. LV field error hundling is described iiz
Tables 5-16 und 5-19.
5.2.2.3.2 Multiplexing. When multiplexing multiple
Mode S packets into single SLM on ELM frame, the following
procedures shall be used. Multiplexing of the packets within
the ADLP shall not be applied to packets associated with
SVCs of different priorities.
Nore.- M~~ltiplexinigs nor performed on MSP packets.
5.2.2.3.2.1 Multiplexing optimization
Recommendation.- When multiple packets are awaiting
transfer to the same XDLe they should be multiplexed into a
single frame in order to optimize throughput, provided that
packets associated with SVCs of digeerent priorities are not
miiltiplexed together:
5.2.2.3.2.2 Structure. The structure of the multiplexed
packets shall be as follows:
Note.- A number in the field signifies the Beld length in
bits; "v" sigrzi$es that the field is of variable length.
5.2.2.3.2.2.1 Multiplexing header: The headcr for the
multiplexed packets shall be as follows:
Where,
Data packet ype (DP) = 0
MSP packet ype (MP) = I
Superviso~pa cket (SP) = 3
Supervisory type (ST) = 2
Note.- See Figure 5-23 ,for a definition for the ,field
stnncture used in the multiplexing header:
5.2.2.3.2.2.2 Length. This field shall contain the length of
thc following packet in bytes. Any error detected in a
28/11/02
No. 77
Part 1 Annex 10 - Aeronautical Telecornrnririications
multiplexed DATA packet, such as inconsistency between
length as indicated in the LENGTH field and the length of the
frame hosting that packct, shall result in the discarding of the
packet unless the error can be detennined to be limited to the
LENGTH held, in which case a REJECT packet with the
expected PS value can be sent.
Recommendation.- For rniiltiplex packers it is recommended
that, if the entire packet cannot be de-multiplexed,
then the first constituent packet be treated as a format err06
and the remainder be discarded.
5.2.2.3.2.3 Termination. The end of a frame containing a
sequence of multiplexed packets shall be determined by one of
the following events:
a) a length field of all zeros; or
b) less lhan eight bits left in the frame.
5.2.2.3.3 MODE S CHANNEL SEQUENCE PRESERVATION
5.2.2.3.3.1 Application. In the event that multiple Mode S
frames from the same SVC are awaiting transfer to the same
XDLP, the following procedure shall be used.
5.2.2.3.3.2 Procedure
5.2.2.4.3 Aircraft address. GDLP frame processing shall
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