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a VDL mobile SKDCF Call User Data Field indicating a
request to maintain SNDCF contexl. If the ground DTE can
accept the call, it shall respond wilh a CALL ACCEPTED
packet with a fast select facility containing a VDL mobile
SNDCF Call User Data field indicating whether the SNDCF
context was maintained. If the DTE or a DCE is unable to
accept the call, it shall send a CLEAR REQUEST packet to
the aircraft DTE including a diagnostic specifying the cause of
failure. The ground DTE shall use the Called Line Address
Modified Notification facility to inform the aircraft DTE of
theground DTE's X. 121 address.
6.6-6.3.3.2 Expedited subnetwork connection maintenance.
An LME initiating expedited subnetwork connection
maintenance shall implement this section. If both the aircraft
and ground LMEs support expedited subnetwork procedures,
then the procedures described in 6.5.4.4.12 shall be invoked.
The initiating DTE shall reissue CALL REQUESTS for those
logical channels for wluch responses (i-e. a CALL
CONFKMATION or a CLEAR REQUEST) were not included
in the XID-RSP-HO. A ground DTE shall set the Calling
Address field to its X.121 address. The ground DCE shall use
the Called Line Address Modified Notification facility lo
inform the aircraft DTE of the ground DTE's X.121 address.
Note.- The CLEAR CONFIRMATION, if required, will be
transferred in an INFO &me. How the ground LME obtains
the CALL REQUEST packet(s) (in ground-initiated handoffs)
is outside the scope of this document.
6.6.6.5 Acknowledgments. An RR packet shall be
generated only when a valid DATA packet is received with a
P(s), wluch closes the acknowledgment window. The aircraft
DTE or ground DCE shall transmit an RR packet
acknowledging the outstanding packets as soon as it is able.
6.7 THE VDL MOBILE
SUBNETWORK DEPENDENT
CONVERGENCE FUNCTION (SNDCF)
6.7.1 Introduction
The VDL mobile SNDCF shall be the standard mobile
SNDCF.
6.7.2 New function
The VDL mobile SNDCF shall support maintaining context
(e.g. compression tables) across subnetwork calls. The SNDCF
shall use the same context (e.g. compression lables) across all
SVCs negotiated to a DTE, when negotiated with the same
parameters. The SNDCF shall support at least 2 SVCs sharing
a context.
Note.- Because handoffs can be expected to reorder
packets, certain compression algorithms do not lend
themselves to use over the VDL. Furrherr implementors of
dictionary-based compression algorithms must be sensitive to
the problem of updates arriving on either the old or mwly
established call,
6.6.6.3.3.3 Broadcast subnetwork connection
muinrenunce. In order to set the 'b," bit in the XID AVLC
Specific Options parameter lo 1, an LME shall support this 6.7.3 Cali user data encoding
section. The procedures described in 6.5.4.4.10 shall be
invoked for each aircraft chat indicates supwfi for broadcast The Call User Data field shall be as detailed in the ATN
subnetwork procedures. The ground DTE and DCE and as modified
aircraft ~ m - s h a lals sume those subnetwork connection have
been maintained per 6.5.4.4.10. If an aircraft DTE cannot 6.7.3.1 ISH PDU. The ISH PDU shall be included in both
accept a call, it shall send a CLEAR REQUEST. If the ground the CALL REQUEST and CALL CONEllRMAlTON packets.
DTE indicated that it maintained the SNDCF context but the 6.7.3.2 Muintain.ed/initiulized status bit. The fifth bit of
aircraft DTE cannot maintain the SNDCF context, it shall send the compression technique octet (i.e- [he actet of the Call
a CALL REQUEST indicating that the SNDCF context is not user Data field) shall be the maintined/initi&zed status
to be maintained. bit which is used to indicate whether the SNDCF context (e.g.
the compression state) was maintained from an old SVC to a
Note.- The CLEAR CONFIRMATION, if required, will be new SVC.
transferred in an INFO frame. How the ground and aircrufi
M E k~tow how to create the calls with their associated
negotiated facilities is outside the scope of this document.
6.6.6.4 Error handling. An aircraft DTE or ground DCE
shall send a CLEAR REQUEST, RESET REQUEST, or
RESTART REQUEST packet only for recovery from a DTE
failure. When an aircraft DTE or ground DCE receives a
packet with a bad sequence number, it shall transmit a
REIECT, as specified in IS0 8208, Section 13.4.
6.7.3.3 CALL REQUEST If the calling SNDCF is
requesting that the SNDCF context be maintained from an
existing call to the new call being established, it shall set the
MA bit to 1; olherwise, the MII bit shall be sel to 0.
6.7.3.4 CALL CONFIRMATION. I f the called SNDCF has
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