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access attempt. The counter shall be incremcnted after every included in the frame format to facilitate this service.
unsuccessful access attempt. When the counter reaches the
maximum numbcr of attempts (Ml), autllorization to transmit 6.5.3.2.4 Broadcast addressing. The VDL shall support
shall be granted as soon as the channel is idle. broadcast addresses that shall be recognized and acted upon by
all appropriate receivers.
6.5.2.4 DESCRI~OOFN P ROCEDURES 6.5.3.2.5 Duta transfez Data shall be transferred in the
information fields of VDL INFO, UI and XID frames. per IS0
6.5.2.4.1 Chan.r~els ensing. Before attempting to access 7809. The link layer shall Process the largest packet size,
the channel, the MAC sub-layer shall verify that the channel is specified in 6.6 of this document, without segmenting. Only
idle. one data link user packet shall be contained in an INFO or UI.
6.5.2.4.2 Access attempt. An access attempt shall be made
when Xmer TMl expires, the channel transitions from busy to 6.5.3.3 AVLC DATA LINK SERVICE
idle or a transmission request amves from the DLS while the PROTOCOL SPECIFICATION
channel is idle.
6.5.3.3.1 Frame fonrtat- AVLC frames shall conform to
IS0 3309 frame structure except as specified in Figure 6-2.
6.5.3 Data Enk service sub-layer 6.5.3.3.2 Address structure. The address field shall consist
6.5.3.1 The DLS shall support bit-oriented simplex
air-ground communications using the aviation VHF link
control (AVLC) protocol specified in this section.
Note.- The DLS is derived from HDLC, as specl;fied by
IS0 3309, IS0 4335. IS0 7809, and lS0 8885. Any definitions
of service are derived from the OSI Data Link Service
Definition IS0 8886.3. AVLC is a variant of HDLC and
derived from, but is not fully speclj7ed by, options 1, 3.2, 4, 7,
and 12 of IS0 7809. Explicit references to these documents are
made 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
fitnctions. The link layer service primitives and protocol state
machine are described in the VDL Gzcidunce Material.
6.5.3.2.1 Frame seque~iciiig. The receiving DLS sub-layer
shall ensure that duplicated frames are discarded and all
frames are delivered exactly once over a point-to-point
connection.
Note.- Sequence numberr urc included in the frame format
to facilitate this sen-ice.
6.5.3.2.2 Error derection. The DLS sub-layer shall ensure
that all frames corrupted during transmission are detected and
discarded.
Note.- FCS is in,clrdded in the frame format to .facilitate
this service.
of eight octets. As described in IS0 3309, option 7, the least
significant (first transmitted) bit of each octet shall be reserved
for address extension. When set to binary 0 it shall indicate
that .the rest of the following octet is an extension of 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 gtiidance 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 station is airborne. It shall be set to I to indicate
that the hansmitting station, either Rxcd or mobile, is on rhc
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 Commmzdresponse statlis bit. The status bit in
the source address field (bit 2, octet 5) shall bc the
comrnand/response (C/R) bit. The C/R bit shall be set to 0 lo
indicate a command frame, and set to Z to indicate a response
frame.
6.5.3.2.3 Slurion identij7cution. The DLS sub-layer shall 6.5.3.3.3.3 Data link sewice addresses. The DLS address
accept over a point-to-point comection only frames that are shall be 27 bits, divided into a 3-bit type fieId and a 24-bit
addressed to it. specific address field.
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