AMCP/WG-M/1/WP 12
AERONAUTICAL MOBILE COMMUNICATIONS PANEL (AMCP)
Working Group M Meeting
Malmo, Sweden
12-19 December, 2000
Proposed Updates to VDL Mode 3 Protocols to Accommodate ATNP
Communique Input
Presented by the U.S. Member
(Prepared by)
Rob Morgenstern
Center for Advanced Aviation System Development
The MITRE Corporation
SUMMARY
This paper discusses work in progress proposed changes to the VDL Mode 3
technical details to accommodate the proposed generic Frame Mode SNDCF
being proposed by ATNP, as well as to possibly improve efficiencies in the
VDL3 Frame Mode interface by adding the option to support DEFLATE
compression. The Working Group is invited to consider the proposed changes to
the Manual on VDL Mode 3 Technical Specifications.
1.0
Introduction
The ATNP has begun definition of a generic Frame Mode interface in an attempt to more
efficiently utilize limited air/ground communications bandwidth. This paper includes
modifications to the Manual on VDL Mode 3 Technical Specifications to support this
interface via a specific Network Type value.
Due to concerns with the overhead associated with the generic ATN Frame Mode
interface, the VDL Mode 3-specific Frame Mode service is maintained. It is proposed to
add the capability for stream-based DEFLATE compression to the existing VDL Mode 3
Frame Mode service. It is felt that the subnetwork is the more appropriate place to
perform compression, as each link is more familiar with its effects on the data stream and
can better optimize compression. Since DEFLATE requires a guaranteed sequencing of
packets, the VDL Mode 3 must maintain a separate compression state for each priority
level, as sequencing among different priorities is not guaranteed. Depending on the
gains, it might make sense to run compression on AOC priority traffic, yet leave already
optimized CPDLC messaging alone to reduce complexity, but that is a local
implementation issue.
It is desired for DEFLATE to support the following capabilities based on work performed
by ATNP regarding its utility:
 Allow for specification of an initializing dictionary to enhance performance
 Allow for the compressor to be reset to a specified point in time to
resynchronize the transmit and receive peers of the compressor, thereby
reducing performance loss during a reset
 Allow the specification of the compression window size to allow performance
tradeoffs with complexity
 Provide a means to support broadcast data flow.
To support these capabilities, the VDL Mode 3 system will need to be enhanced with a
few XID parameters to allow the specification of the compression method, as well as
means to determine the current state of the peer compressor for recovery. The XIDs will
comprise Compression Algorithm, Compression Window Size, Dictionary Identifier, and
Resynchronize Windows. To support stream compression over broadcast via the VDL
Mode 3 Frame Mode interface, the broadcast stream is assumed to reset to the default
dictionary whenever the CLNP header is sent uncompressed.
What is yet undetermined, which will require validation to ascertain, is the specific
manner to reset the compressor state. Should the system reset whenever the header
compressor is signalled to restart? Or should there be a separate mechanism to reset the
stream compressor state? One advantage of the separate reset is that such a reset would
allow rolling back the DEFLATE window instead of resetting to the default dictionary.
Also, if a separate mechanism is provided, is it a global reset, or implemented for each
compressor state machine? This is a tradeoff of compression efficiency for overhead, and
only simulation and testing will determine the correct answer.
The Deflate Compression procedures are specified in the ATN Technical Provisions,
ICAO Doc 9705, Section 5.7.6.5.
2.0
Proposed Changes to Manual on VDL Mode 3 Technical Specifications
The following is markup text from the Technical Manual to incorporate the changes.
5.7.3.3.4.1.3 Network initialization parameter. The network initialization parameter defines the
network support which the avionics VDL Mode 3 installation can perform. It is used to pass any necessary
information to initialize the subnetwork protocol or any compression. Table 5-66a defines the format of
this parameter set. The length field shall contain the number of networks present. For each network, a
length field shall indicate the number of subparameters. If no subparameters are needed for a network, then
that network’s length field shall indicate zero.
Note 1. As many networks as applicable can appear, as defined in Table 5-66b.
Note 2. It is assumed that the ground system will support all options and the aircraft system will
support at least one.
A Network Type value of zero indicates that the VDL Mode 3 system is carrying ISO8208 packets which
shall be transferred to an ATN ISO8208 data communication equipment (DCE) at the remote end. The
ISO8208 packets shall be compressed using the VDL Mode 3 PLP Compression as defined in Section
6.7.2.1. A Network Type value of one indicates that the VDL Mode 3 system is carrying ATN CLNP
packets which shall be transferred to an ATN-conformant CLNP service at the remote end. The CLNP
packets shall be compressed using the VDL Mode 3 compression as defined in Section 6.7.2.2. A Network
Type value of two indicates that: (1) The VDL Mode 3 system is carrying ISO8208 packets which shall be
transferred to an ATN ISO8208 DCE at the remote end; and (2) No compression is performed. Similarly, a
Network Type value of three indicates that the VDL Mode 3 system is carrying ATN CLNP packets which
shall be transferred to an ATN-conformant CLNP service at the remote end with no compression. A
Network Type Value of four indicates that no (sub)network protocol shall be used. Data shall be sent to the
remote end unmodified with no (sub)network operands. A Network Type Value of five indicates that the
VDL Mode 3 system is carrying ATN frame mode packets which shall be transferred to an ATN Frame
Mode SNDCF at the remote end without compression within the subnetwork.
Setting of the high order bit of the Network Type field indicates the desire to support stream compression
upon the data stream as defined in Section 6.7.
Note 3. The ATN designation tells the VDL Mode 3 system to support any ATN-specific requirements.
For example, ATN join and leave events will be generated when network types of 0, 1, 2, 3 or 5 are sent
from the aircraft to the ground.
Note 4.  An indication of ‘No Compression’ in this parameter only indicates that the VDL Mode 3
subnetwork performs no compression. This does not indicate or prohibit any compression by the network
or higher layers.
Note 5.  Aircraft will support the raw interface and one or more network interfaces (e.g., CLNP or
ISO8208 with and without compression). The ground station will support all Network Types
Note 6. —The use of the raw interface requires that the largest packet received with the addition of the
payload and DLS frame header be a maximum of 930 bytes as no segmentation of the data is possible.
5.7.3.3.4.1.7 Compression Algorithm parameter. The compression algorithm parameter shall define
which stream compression algorithm is to be supported for one or more levels of link priority. Table 5-67f
defines the format of this parameter set. Table 5-67g defines the compression algorithm identifiers.
5.7.3.3.4.1.8 Compression Window Size parameter. The compression window size parameter shall
define the number of octets of user data history over which the compressor operates. Table 5-67h defines
the format of this parameter set.
5.7.3.3.4.1.9 Dictionary Identifier parameter. The dictionary identifier parameter shall define the
compression dictionary which is to be used to initialize the stream compressor. Table 5-67i defines the
format of this parameter set. Table 5-67j defines the dictionary identifier values.
5.7.3.3.4.1.10 Resynchronize Compressor parameter. The resynchronize compressor parameter shall
define the size of the compression window for the indicated priority stream. Table 5-67k defines the
format of this parameter set. The first octet of the parameter value indicates the link priority stream to be
resynchronized. The next two octets indicate how many octets the compression window shall be moved
backwards in the data stream. A value of zero shall indicate a reset to the current initializing dictionary.
6.7.2.4
ATN Frame Mode Payload Type. The type-specific information field shall be set to all
zeros whenever a payload of 'ATN Frame Mode' is specified.
6.7.4 Stream-based Compression.
Stream-based compression is a means to reduce the bandwidth requirements of the user data passing across
the system. The compressor shall operate upon the User Data identified in Figure 5-5.
7.2.4
The ATN Frame Mode SNDCF
The VDL Mode 3 shall support the ATN Frame Mode SNDCF for mobile subnetworks as defined in the
ATN SARPs, Sub-Volume V.
Table 5-59a. XID Parameters
Source
Destination
XID parameters
Public parameters
(8885:1993:Mode 3)
Parameter set ID
N1-downlink
N1-uplink
Counter N2
Timer T1
T-ack
Private parameters (V)
Parameter set ID
SQP
LCR cause
Modulation support
Private parameters (W)
Parameter set ID
Algorithm version number
Network initialization
Connection check
Version
Authentication
Compression Algorithm
Compression Window Size
Dictionary Identifier
Resynchronize Compressor
NM1
RR
WR
RE
RL
Timer T3
NL1
Timer TL3
Timer TL4
WE
Operative_GNIp
Aircraft_TMbB
MbB_Operations_Permitted
CT1
Timer CT2
Timer CT3
GDTE
VER
M-BIT
Expedited recovery
t
f
Former_GNIp
ADSA
Counter NL2
Air initiated
Link establishment
Aircraft
New ground
Station
Proposed ground
Aircraft
station
CTRL_CMD_LE
CTRL_RSP_LE
Link connection
rejection
Any station
Any station
GI
PI
CTRL_RSP_LCR
CTRL_CMD_LCR
80h
80h
80h
80h
80h
80h
01h
05h
06h
0Ah
09h
08h
N/A
N/A
N/A
N/A
N/A
N/A
O
O
O
O
O
O
N/A
N/A
N/A
N/A
N/A
N/A
F0h
F0h
F0h
F0h
00h
02h
06h
81h
O
O
N/A
O
O
O
N/A
N/A
M
N/A
M
N/A
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
00h
01h
02h
03h
04h
05h
06h
07h
08h
09h
42h
43h
44h
46h
47h
48h
49h
4Ah
4Bh
4Ch
4Fh
50h
51h
52h
53h
54h
55h
56h
57h
80h
81h
82h
83h
84h
C1h
M
O
M
N/A
O
O
O
O
O
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
O
O
O
O
O
O
N/A
O
O
O
O
O
O
O
N/A
M
O
M
N/A
O
O
O
O
O
N/A
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N/A
N/A
N/A
O
N/A
O
O
O
N/A
N/A
O
O
O
O
O
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
O
O
O
N/A
O
O
N/A
N/A
N/A
N/A
N/A
N/A
Key for Table 5-59a above:
Abbreviations used:
GI
PI
M
O
N/A
h
= ISO 8885 Group Identifier
= ISO 8885 Parameter Identifier
= Mandatory
= Optional
= Not applicable
= hexadecimal
Table 5-59b. XID Parameters
Addressed link parameter modification
Source
Destination
XID parameters
Public parameters
(8885:1993:Mode3)
Parameter set ID
N1-downlink
N1-uplink
Counter N2
Timer T1
T-ack
Private parameters (V)
Parameter set ID
SQP
LCR cause
Modulation support
Private parameters (W)
Parameter set ID
Algorithm version number
Network initialization
Connection check
Version
Authentication
Compression Algorithm
Compression Window Size
Dictionary Identifier
Resynchronize Compressor
NM1
RR
WR
RE
RL
Timer T3
NL1
Timer TL3
Timer TL4
WE
Operative_GNIp
Aircraft_TMbB
MbB_Operations_Permitted
Current ground
station
Aircraft
Aircraft
Broadcast link
parameter
modification
Current ground
station
Aircraft
GI
PI
CTRL_CMD_LPM
Current ground
Station
CTRL_RSP_LPM
80h
80h
80h
80h
80h
80h
01h
05h
06h
0Ah
09h
08h
O
O
O
O
O
N/A
N/A
N/A
N/A
N/A
N/A
0
O
O
O
O
O
N/A
F0h
F0h
F0h
F0h
00h
02h
06h
81h
O
O
N/A
N/A
O
O
N/A
N/A
O
O
N/A
N/A
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
00h
01h
02h
03h
04h
05h
06h
07h
08h
09h
42h
43h
44h
46h
47h
48h
49h
4Ah
4Bh
4Ch
4Fh
50h
51h
O
O
O
O
N/A
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N/A
O
O
N/A
O
O
O
O
O
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
CTRL_CMD_LPM
(GSIF)
CT1
Timer CT2
Timer CT3
GDTE
VER
M-BIT
Expedited recovery
t
f
Former_GNIp
ADSA
Counter NL2
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
F0h
52h
53h
54h
55h
56h
57h
80h
81h
82h
83h
84h
C1h
O
O
O
O
N/A
O
N/A
O
O
O
N/A
O
O
O
O
N/A
N/A
O
N/A
N/A
N/A
O
O
N/A
O
O
O
O
O
O
N/A
O
O
O
N/A
O
Table 5-66b. Network Type Values
Network Type
0000 0000
0000 0001
Protocol
ATN/ISO8208/VDL Mode 3 PLP Compression
ATN/ISO8473/ISO9542/VDL Mode 3 CLNP
Compression
ATN/ISO8208 (No Subnetwork Compression)
ATN/ISO8473/ISO9542 (No Subnetwork Compression)
Raw/VDL
ATN Frame Mode
Reserved
ATN/ISO8208/VDL Mode 3 PLP Compression with
Stream Compression
ATN/ISO8473/ISO9542/VDL Mode 3 CLNP
Compression with Stream Compression
ATN/ISO8208 (Stream Compression only)
ATN/ISO8473/ISO9542 (Stream Compression only)
Raw/VDL with Stream Compression
Reserved
0000 0010
0000 0011
0000 0100
0000 0101
0000 0110 to 0111 1111
1000 0000
1000 0001
1000 0010
1000 0011
1000 0100
1000 0101 to 1111 1111
Table 5-67f. Compression Algorithm Parameter
Compression
Algorithm
Parameter ID
0
0
0
0
0
1
1
0
Parameter length
n8
n7
n6
n5
n4
n3
n2
n1
Parameter value
n8
n7
n6
n5
n4
n3
n2
n1
Link Priority
n8
n7
n6
n5
n4
n3
n2
n1
Algorithm ID
Table 5-67g. Compression Algorithm Encoding
Value
0
Compression Algorithm
None
1
2-255
DEFLATE, per ICAO Doc 9705?, Section 5.7.6.5
Not currently specified
Table 5-67h. Compression Window Size Parameter
Compression
Window Size.
Parameter ID
0
0
0
0
0
1
1
1
Parameter length
0
0
0
0
0
0
1
0
Parameter value
n16
n15
n14
n13
n12
n11
n10
n9
Value
n8
n7
n6
n5
n4
n3
n2
n1
Value
Dictionary Identifier
Table 5-67i. Dictionary Identifier Parameter
Parameter ID
0
0
0
0
1
0
0
0
Parameter length
n8
n7
n6
n5
n4
n3
n2
n1
Parameter value
n8
n7
n6
n5
n4
n3
n2
n1
Value
Table 5-67j. Dictionary Identifier Encoding
Value
Dictionary Identifier
0
None
1
[TBD]
2-255
Not currently specified
Table 5-67k. Resynchronize Compressor Parameter
Parameter ID
0
0
0
0
1
0
0
1
Parameter length
n8
n7
n6
n5
n4
n3
n2
n1
Parameter value
0
0
0
0
b4
b3
b2
b1
n16
n15
n14
n13
n12
n11
n10
n9
n8
n7
n6
n5
n4
n3
n2
n1
1
2
3
Type-specific Info
4
5
6
Resynch Compressor
Link Priority
Value
7
Type
8
BIT
0000
0001
0010
0011
0100 - 1111
PROTOCOL
ISO8208
ISO8473/ISO9542
Raw
ATN Frame Mode
Reserved
Figure 6-1. Payload octet
3.0
Conclusions
The Working Group is recommended to consider these proposed changes in response to
input from ATNP.