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later in this section.
Note.- In this section, the specific functions of the DLS
are described with no reference to service primitives used for
these functions. The link layer service primitives and protocol
state machine are described in the VDL Guidance Material.
6.5.3.2.1 Frame sequencing. The receiving DLS sublayer
shall ensure that duplicated frames are discarded and all
frames are delivered exactly once over a point-to-point
connection.
Note.- Sequence numbers are included in the frame
format to facilitate this service.
6.5.3.2.2 Error detection. The DLS sub-layer shall
ensure that all. frames corrupted during transmission are
detected and discarded.
Note.- FCS is included in the frame format to facilitate
this service.
6.5.3.2.3 Station identification. The DLS sub-layer shall
accept over a point-to-point connection only frames that are
addressed to it.
Note.- Unique source and destination addresses are
irlcluded in the frame format to facilitate this service.
6.5.3.2.4 Broadcast addressing. The VDL shall support
broadcast addresses that shall be recognized and acted upon by
all appropriate receivers.
6.5.3.2.5 Data transfez Data shall be transferred in the
information fields of VDL INFO, UI and XID frames, per IS0
7809. The link layer shall process the largest packet size,
specified in 6.6 of this document, without segmenting. Only
one data link user packet shall be contained in an INFO or UI.
6.5.3.3 AVLC DATA LINK SERVICE
PROTOCOL SPECIFICATION
6.5.3.3.1 Frame format. AVLC frames shall conform to
IS0 3309 framc structure except as specified in Figure 6-2.
6.5.3.3.2 Address structure. The address field shall
consist of eight octets. As described in IS0 3309, option 7, thc
least significant (first transmitted) bit of each octet shall be
reserved for address exlension. When set to binary 0 it shall
indicate that the rcst of the following octet is an extension of
6/11/97
No. 72
Part 1 Annex 10 - Aeronautical Telecommunications
the address field. The presence of binary 1 in the first transmitted
bit of the address octet shall indicate that the octet is
the final octet of the address field.
6.5.3.3.3 Address fields. The address field shall contain
a destination address field and a source address field. The
destination address field shall contain a destination DLS
address or a broadcast address. The source address field shall
contain a DLS address. There is a status bit in the source
address and a status bit in the destination address field, which
shall be set by the transmitting station to reflect status
information. The status bits and address details are defined in
6.5.3.3.3.1 to 6.5.3.3.3.7.
Note.- See the guidance material for information on
address field.
6.5.3.3.3.1 Air-ground status bit. The status bit in the
destination address field (bit 2, octet 1) shall be the air-ground
bit. The air-ground bit shall be set to 0 to indicate that the
transmitting stalion is airborne. It shall be set to 1 to indicate
that the transmitting station, either fixed or mobile, is on the
ground. The default value for the airlground bit shall be 0 for
aircraft that do not provide this information at the link level;
the value shall be 1 for ground stations.
6.5.3.3.3.2 Comrnand'response status bit. The status bit
in the source address field (bit 2, octet 5) shall be the
command/response (C/Rj hit. The C/R bit shall be set to 0 to
indicate a command frame, and set to 1 to indicate a response
frame.
6.5.3.3.4.1 Encoding. The broadcast addresses shall be
encoded as in Table 6-8.
6.5.3.3.5 Link control jield. The basic repertoire of
commands and responses for AVLC shall be as detailed in
Table 6-9 and shall be encoded as per IS0 4335.
6.5.3.3.6 Information field. The information field of an
SREJ shall be as defined in 6.5.3.11.2, an XID shall be as
defined in 6.5.4.2, and all other frames shall be as defined in
IS0 4335.
6.5.3.4 DATA LINK SERVICE
SYSTEM PARAMETERS
These parameters needed by the DLS sub-layer shall be as
listed in Table 6-10 and detailed in 6.5.3.4.1 through 6.5.3.4.7.
DLS parameters shall be set using XID frames.
6.5.3.4-1 Timer TI (delay bqfore retransmission). Timer
TI shall be set to the time that a DLE will wait for an
acknowledgement before retransmitting an INFO, RR (P=l),
SREJ (P=l) or a FRMR frame.
The value of Timer T1 shall be computed by the following
formula:
Timer T1 = Tlmin + 2TD9, + min(U(x),Tl max)
where:
6.5.3.3.3.3 Data link service addresses. The DLS U(x) is a uniform random number generated between 0
　
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