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further processing. All late subnetwork connection responses
from grohnd DTEs will not be included in the retransmitted
XID-RSP
6.6 SUBNETWORK LAYER
PROTQCQLS AND SERVICES
6.6.1 Architecture
6.5.4.4.11.3 Exceptional cases. If the aircraft LME cannot
perform the autotune, it: shall transmit an XID-CMD-LCR
The subnetwork layer protocol used across the VHF
air-ground subnetwork is referred to formally as a subiletwork
(P=O); the current link shall not be affected. access protocol (SNAcP) and shall confornl to IS0 8208,
except as noted below. The SNAcP is referred to within this
6.5.4.4+12 Expedited sr~bnetwork connection
document as the subnetwork protocol. If there are any
management. If a LME implements ths section, then il shall differences between this document and the cited specifications,
set the "v" bit in both the AVLC Specific Options and in the
is document shall have precedence. On air-ground Comection Management parameters to 1; otherwise it shall set
them to 0. This section shall only be applicable for the link interface, the aircraft subnetwork entity shall act as a DTE and
the ground subnetwork entity shall act as a DCE.
establishment, air initiated hmdoff, and ground initiated
handoff processes.
6.5.4.4.12.1 Initiating station of subnetwork connection
management. To perform an expeditcd subnetwork connection
establishment or maintenance, the initiating LME shall include
in the XID-CMD the Expedited Subnetwork Connection
parameter for each subnetwork connection that needs to be
established or mainlined. Thc procedures for an expedited
link establishment and maintenance shall be the same as
outlined in 6.5.4.4.4, 6.5.4.4.6 and 6.5.4.4.8.
6.5.4.4.12.2 Generul responder action. If the responding
LME receives a XID_CMD with one or more Expedited
Subnetwork Connection parameters, it shall confirm
subnelwork connection establishment or maintenance by
sending an XIDRSP containing the parameters as per Table
6-51. The responding LME shall attempt to establish or
maintain the specified subnetwork connections as outlined in
6.6.6.3. The responding LME shall include in the XU>SP
the CALL CONFIRMATION or CLEAR REQUEST
responses (i.e. in the Expedited Subnetwork Connection
parameter) and my optional parameters for which it is not
using the default values. The ground LME shall not process
the Expedited Subnetwork Connection parameters if it
includes the Autorune parameter in the XID-RSP.
6.6.1.1 Access points. The subnelwork service access
point (SNSAP) shall be u~quclyid entified by the subnetwork
data terminal equipmen1 (IITE) address. SNSAPs shall define
the subnetwork point of attachment (SNPA) used by the
service primitives that detine the subnetwork service to the
subnetwork dependence convergence protocol.
6.6.2 Services
This section specifies the services offered by the subnetwork
sub-layer. The services are described in an abstract manner and
do not imply any particular implemcncations. The services
provided by the subnetwork to the subnetwork service user
shall include the functions described in 6.6.2.1 through 6.6.2.4.
6.6.2.1 Subnetwork connection management. A variety of
IS0 8208 packet types, procedures, and facilities shall be used
to establish, terminate, and manage connections across the
subnetwork. Connection status information shall be
maintained at both ends of the connection. Connection status
information shall also be maximized to ensure that the
minimum amount of information is passed with each data
transfer phase transmission and that ground system operational
control, of the subnetwork is mxximized.
2/1lJOo
No. 75
Atanex 10 - Aeronautical Telecommunications Volume Ill
6.6.2.2 Packet fragmentation und reassembly This
subnetwork capability shall allow the fragmenting of large data
units passed from the subnetwork user for transmission across
he air-ground portion of the subnetwork. Reassembly shall be
performed at the receiving end of the subnetwork.
6.6.2.3 Error recovery. REJECT packet types shall be
used for subnetwork-level ermr recovery. These packets shall
be sent between subnetwork entities to cause retransmission of
invalid packets and to recover from error response time-out
states. Under no circumstances shall RESET or RESTART be
used to recover from an ermr that can be handled by REIECT.
Aircraft DTEs shall accept REJECT packets and should
retransmit the specified packets.
6.6.2.3.1 Recommendation.- The gmunri- DCE with
which an aircrafr has a VDL link should not clear subnetwork
6.6.3.2.1.1 Address field. This variable length field is
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