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6.5.4.4.12.3 Exceptional cases. If the responding LME
can not support the expedited subnetwork connection establishment
or maintenance but can support the link establishment or
handoff, it shall respond with XID-RSP with the Connection
Management "v" bit set to 0 and shall not include the Expedited
Subnetwork Connection pararncters in the XID-RSP. If
T3min expires, the responding LME shall include all responses
(i.e. CALL CONflRMATION or CLEAR REQUEST) that it
has received up to that point in the XZD-RSP. Any late
responses lrom respective DTE(s) shall be sent to the initiating
LME in INFO frames.
Note.- All XID_CMD retransmissions will cause the
responding LME to respond with the same XID-RSP without
fr.~rther processing. All late subnetwork connection responses
from ground DTEs will not be included in the retransmitted
XID-RSP.
6.6 SUBNETWORK LAYER
PROTOCOLS AND SERVICES
6.6.1 Architecture
Thc subnetwork layer protocol used across the VHF air-ground
subncrwork is referred to formally as a subnetwork access
protocol (SNAcP) and shall conform to IS0 8208, except as
noted below. The SNAcP is referred to within this document
as the subnetwork protocol. If there are any differences
between this document and the cited specifications, this
document shall have precedence. On the air-ground interface,
the aircraft subnetwork entity shall act as a DTE and the
ground subnetwork entity shall act as a DCE.
6.6.1.1 Access points. The subnetwork service access
point (SNSAP) shall be uniquely identified by the subnetwork
data terminal equipment (DTE) address. SNSAPs shall define
the subnetwork point of attachment (SNPA) used by the
service primitives that define 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 implementations. 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 Subnerwork 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 maximized.
6.6.2.2 Packet fragmentation and reassembly. This
subnetwork capability shall allow the fragmenting of large data
units passed from the subnetwork user for transmission across
the air-ground portion of the subnetwork. Reassembly sliall be
performed at the receiving end of the subnetwork.
6.6.2.3 Error recovery. REJECT packct types shall be
used for subnetwork-level error recovery. These packets shall
be sent between subnetwork entities to causc 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 error that can be handled by REJECT.
Aircraft DTEs shall accept REJECT packets and should
retransmit the specified packets.
6.6.2.3.1 Recommendation.- The ground DCE with
wlzich an aircraft has a VDL link should not clear subnetwork
connections on receipt of REJECT packets but should
retransmit the specified packet.
6.6.2.4 Connectionjow control. DATA packet sequence
numbering combined with the use of a sliding window shall be
used for passive flow control.
6.6.2.4.1 Recommendation.- Receive not ready (RNR)
packets should not be used for explicit flow control.
Note.- The use of e.xplicit RNRs requires a subsequeitt
packet to clear the@ (DXE RECEIVE NOT READY) state (see
IS0 8208). The RNRs and subsequent RR frames will cause
more RF utilization than would be caused by merely delaying
the acknowledgment.
6.6.3 Packet format
Except as qualified below, the packet format shall be as
specified in IS0 8208, Section 12. During call setup, VDL
shall use the extended format in conjunction with the fast
select facility.
6 . 3 1 General format identifier. The Qualifier bit
(Q-bit) in DATA packets shall be set to 0 in VDL. Modulo 8
sequencing shall be used in the VDL.
-611 it97
No. 72
Annex 10 - Aeronautical Telecommunications Volume 111
Note.- A subnetwork entity may receive a CLEAR RECEIVE NOT READY) shall be used in the VDL. Packet
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