ETSI TR 103 552 V1.2.1 (2019-01)

Study item on VHF Data Link Mode 2 ground-based equipment standardization optimization; Evolution of Data Link European Norms

RTR/ERM-TGAERO-64

General Information

• Status: Published
• Publication Date: 28-Jan-2019
• Technical Committee: ERM TGAERO - Aeronautics
• Current Stage: 12 - Completion
• Due Date: 05-Feb-2019
• Completion Date: 29-Jan-2019

Standards Content (Sample)

ETSI TR 103 552 V1.2.1 (2019-01)






TECHNICAL REPORT
Study item on VHF Data Link Mode 2 ground-based equipment
standardization optimization;
Evolution of Data Link European Norms

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2 ETSI TR 103 552 V1.2.1 (2019-01)



Reference
RTR/ERM-TGAERO-64
Keywords
aeronautical, air traffic management, radio,
testing, VHF

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3 ETSI TR 103 552 V1.2.1 (2019-01)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
1 Scope . 5
2 References . 5
2.1 Normative references . 5
2.2 Informative references . 5
3 Definition of terms, symbols and abbreviations . 7
3.1 Terms . 7
3.2 Symbols . 7
3.3 Abbreviations . 7
4 Evolution of VHF Data Link Mode 2 European Norms . 8
4.1 Overview . 8
4.1.1 Relationships between the ETSI ENs . 8
4.2 Update of References . 10
4.3 Impact of ELSA Report and other relevant Projects/Frameworks . 10
4.4 EUROCAE findings having an impact on ETSI DL ENs . 17
4.5 Additional improvements (Phase 1+) . 18
4.6 Summary . 18
History . 22


ETSI

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4 ETSI TR 103 552 V1.2.1 (2019-01)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (https://ipr.etsi.org/).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Trademarks
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ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Electromagnetic compatibility and Radio
spectrum Matters (ERM).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.

ETSI

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5 ETSI TR 103 552 V1.2.1 (2019-01)
1 Scope
The present document intends to investigate the improvements needed in the VHF Data Link Mode 2 European Norms
(ETSI EN 303 214 [i.7], ETSI EN 301 841 parts 1 [i.4] to 3 [i.6]) in order to determine real-world compliance to
relevant regulations, and if there is a need to align them with the relevant ELSA reports [i.12], [i.13], [i.14], [i.15]
findings and further inputs from PATH1, PATH2 and other SDM/EASA mandates (e.g. CEF2017/IP1), as well as the
relevant regulation.
The present document considers also some specific aspects addressing ATN Baseline 1 and Baseline 2 compatibility.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] LINK 2000+/LIT/ED120/PERF Interpretation of EUROCAE ED-120/RTCA DO-290
Performance Requirements.
[i.2] Eurocontrol CFC/Datalink/ACSPGEN Generic Requirements for an ATN/VDL Mode 2
Air/Ground Communications Service Provider.
[i.3] ETSI EN 300 113-1: "Electromagnetic compatibility and Radio spectrum Matters (ERM); Land
mobile service; Radio equipment intended for the transmission of data (and/or speech) using
constant or non-constant envelope modulation and having an antenna connector; Part 1: Technical
characteristics and methods of measurement".
[i.4] ETSI EN 301 841-1 (V1.4.1): "VHF air-ground Digital Link (VDL) Mode 2; Technical
characteristics and methods of measurement for ground-based equipment; Part 1: Physical layer
and MAC sub-layer".
[i.5] ETSI EN 301 841-2 (V1.1.1): "Electromagnetic compatibility and Radio spectrum Matters (ERM);
VHF air-ground Digital Link (VDL) Mode 2; Technical characteristics and methods of
measurement for ground-based equipment; Part 2: Upper layers".
[i.6] ETSI EN 301 841-3 (V2.1.1): "VHF air-ground Digital Link (VDL) Mode 2; Technical
characteristics and methods of measurement for ground-based equipment; Part 3: Harmonised
Standard covering the essential requirements of article 3.2 of the Directive 2014/53/EU".
[i.7] ETSI EN 303 214 (V1.2.1): "Data Link Services (DLS) System; Community Specification for
application under the Single European Sky Interoperability Regulation EC 552/2004;
Requirements for ground constituents and system testing".
[i.8] EUROCAE ED-92C: "MOPS for an Airborne VDL Mode-2 System operating in the frequency
range 118-136,975 MHz".
ETSI

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6 ETSI TR 103 552 V1.2.1 (2019-01)
[i.9] EUROCAE ED-120: "Safety and Performance Requirements Standard For Initial Air Traffic Data
Link Services In Continental Airspace" (SPR IC)".
[i.10] ICAO Doc 9705: "Manual of Technical Provisions for the Aeronautical Telecommunication
Network (ATN)".
[i.11] ICAO Doc 9880: "Manual on Detailed Technical Specifications for the Aeronautical
Telecommunication Network (ATN) using ISO/OSI Standards and Protocols".
[i.12] ELSA D8, Edition 01.03.00: "Work Area 1 Final Report - VDL2 Performance analysis".
[i.13] ELSA D9, Edition 00.03.04: "ELSA WA2 Final Report - Implementation Options for VDL M2
Multi-Frequency".
[i.14] ELSA D10, Edition 01.12.00: "VDL Mode 2 RF Analysis & Avionics Interoperability Tests -
Final Report".
[i.15] ELSA D11, Edition 00.01.03: "VDL Mode 2 Measurement, Analysis and Simulation Campaign -
Final Report".
[i.16] ETSI EN 301 489-22: "Electromagnetic compatibility and Radio spectrum Matters (ERM);
ElectroMagnetic Compatibility (EMC) standard for radio equipment and services; Part 22:
Specific conditions for ground based VHF aeronautical mobile and fixed radio equipment".
[i.17] Directive 2014/53/EU of the European Parliament and of the Council of 16 April 2014 on the
harmonisation of the laws of the Member States relating to the making available on the market of
radio equipment and repealing Directive 1999/5/EC.
[i.18] Commission Regulation (EC) No 29/2009 of 16 January 2009 laying down requirements on data
link services for the single European sky.
[i.19] Regulation (EC) No 552/2004 of the European Parliament and of the Council of 10 March 2004 on
the interoperability of the European Air Traffic Management network (interoperability
Regulation), OJ L 96, 31.03.2004, p. 26 as amended by Regulation (EC) No 1070/2009, OJ L 300,
14.11.2009, p. 34.
[i.20] ITU Radio Regulations 2016.
[i.21] ICAO Convention on International Civil Aviation: "Annex 10 - Aeronautical
Telecommunications, Volume III - Communication Systems, Part I - Digital Data Communication
Systems, Second Edition, July 2007, incorporating Amendments up to 88A (applicable 14/11/13).
Chapter 6 - VHF Air-ground Digital Link (VDL)".
[i.22] ICAO Document 9776/AN970 (first edition, 2001): "Manual on VHF Digital Link (VDL)
mode 2".
[i.23] ARINC 631-6: "VHF Digital Link (VDL) Mode 2 Implementation Provisions".
[i.24] ARINC 631-7: "VHF Digital Link (VDL) Mode 2 Implementation Provisions Standards".
[i.25] EUROCAE ED-110B: "Interoperability requirements standard for aeronautical telecommunication
network baseline 1 (ATN B1 Interop standard)".
[i.26] EUROCAE ED-23C: "MOPS for airborne VHF receiver - transmitter operating in the frequency
range 117.975-137.000 MHz".
[i.27] ISO/IEC 8208: "Information technology - Data communications - X.25 Packet Layer Protocol for
Data Terminal Equipment".
[i.28] EUROCAE ED-92C: "Minimum Operational Performance Standard (MOPS) for an Airborne
VDL Mode-2 System Operating in the Frequency Range 118-136.975 MHz".
ETSI

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7 ETSI TR 103 552 V1.2.1 (2019-01)
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the terms given in ETSI EN 301 841-1 [i.4], ETSI EN 301 841-2 [i.5], ETSI
EN 301 841-3 [i.6], ETSI EN 303 214 [i.7] and Commission Regulation (EC) No 29/2009 [i.18] apply.
3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
ACK ACKnowledgement
ANSP Air National Service Provider
AOA Acars Over AVLC
AOC Airline Operational Control
ATN Air Traffic Network
ATS Air Traffic Service
AVLC Aviation VHF Link Control
CM Context Management
CPDLC Controller Pilot Data Link Communication
CSC Common Signaling Channel
CSP Communications Service Provider
CVME Centralised VME
DISC DISConnect (frame)
DL Data Link
DLS Data Link Service(s)
DM Disconnected Mode (frame)
DP Deployment Programme
DPMG Data link Performance Monitoring Group
EASA European Aviation Safety Agency
EC European Commission
ED EUROCAE Document
ELSA Enhanced Large Scale ATN deployment
EMC ElectroMagnetic Compatibility
EN European Norm
ENR EN-Route
FSL Frequency Support List
GIHO Ground Initiated HandOver
GRAIHO Ground Requested Air Initiated HO
GSIF Ground Station Identification Frame
ICAO International Civil Aviation Organisation
ICS Internet Communications Service
IDRP Inter Domain Routing Protocol
LACK Logical ACK
MF Multi Frequency
MOPS Minimum Operational Performance Specification
PAE Park Air Electronics
PICS Protocol Implementation Conformance Specification
RF Radio Frequency
SDM SESAR Deployment Manager
TRM TeRMinal
TX Transmitter
VCRM Verification Cross-Reference Matrix
VDL VHF Data Link
ETSI

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8 ETSI TR 103 552 V1.2.1 (2019-01)
VDLm2 VHF Data Link mode 2
VGS VDLm2 Ground Station
VHF Very High Frequency
VME VDLm2 Management Entity
XID eXchange IDentification
4 Evolution of VHF Data Link Mode 2 European Norms
4.1 Overview
4.1.1 Relationships between the ETSI ENs
The ETSI specifications related to VDLm2 datalink are the following:
• ETSI EN 301 841-1 [i.4]: "VHF air-ground Digital Link (VDL) Mode 2; Technical characteristics and
methods of measurement for ground-based equipment; Part 1: Physical layer and MAC sub-layer".
This European Norm provides functional specifications and test procedures for physical layer and MAC
sub-layer.
• ETSI EN 301 841-2 [i.5]: "VHF air-ground Digital Link (VDL) Mode 2; Technical characteristics and
methods of measurement for ground-based equipment; Part 2: Upper layers".
This European Norm provides functional specifications and test procedures for link and sub-network access
layers.
• ETSI EN 301 841-3 [i.6]: "VHF air-ground Digital Link (VDL) Mode 2; Technical characteristics and
methods of measurement for ground-based equipment; Part 3: Harmonised Standard covering the essential
requirements of article 3.2 of the Directive 2014/53/EU".
This European Norm contains requirements to demonstrate that ". Radio equipment shall be so constructed
that it both effectively uses and supports the efficient use of radio spectrum in order to avoid harmful
interference" according to the Directive 2014/53/EU [i.17].
• ETSI EN 303 214 [i.7]: "Data Link Services (DLS) System; Community Specification for application under
the Single European Sky Interoperability Regulation EC 552/2004; Requirements for ground constituents and
system testing".
This European Norm is applicable to the ground implementation of data link services, derived from the ICAO
standard Context Management (CM) and Controller Pilot Data Link Communication (CPDLC) applications
and provides presumption of conformity against the essential requirements of the interoperability
Regulation EC 552/2004 [i.19].
• ETSI EN 301 489-22 [i.16]: "ElectroMagnetic Compatibility (EMC) standard for radio equipment and
services; Specific conditions for ground based aeronautical mobile and fixed radio equipment;
Part 22: Specific conditions for ground based aeronautical mobile and fixed radio equipment".
This European Norm contains requirements to demonstrate that ". Radio equipment shall be constructed so as
to ensure an adequate level of electromagnetic compatibility as set out in Directive 2014/30/EU" according to
the Directive 2014/53/EU [i.17].
• The ETSI Data Link ENs refer also to ETSI EN 300 113-1 [i.3].
The links between these ETSI European Norms and other relevant specifications is shown in figure 1.

ETSI

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9 ETSI TR 103 552 V1.2.1 (2019-01)

Figure 1: Data Link specifications and their relationship
As a result of ELSA and SDM reports and the mandatory implementation of the i4D services based on DataLink
foreseen for year 2022, the above mentioned European Norms will be likely impacted.
The following will be taken into account in this investigation:
• the ELSA report recommendations for improvements to ground systems;
• the SDM Deployment Plan 2016 requirements (2017 is under definition);
• the results from PATH1, PATH2 Projects and operational experience, also considering the DPMG activities;
• end-to-end testing as specifically required by all the previously listed reports and projects considering the
traffic load foreseen for AOC and ATS;
• updated references and recent developments;
• ensuring appropriate coverage of (and traceability to) the legislative requirements, and that the standard
includes associated conformity assessment material which is necessary and sufficient for an ANSP to
demonstrate compliance;
• the need for continual in-service monitoring of datalink performance to ensure ongoing compliance with
legislative and operational requirements;
• other improvements and corrections as appropriate.
ETSI

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10 ETSI TR 103 552 V1.2.1 (2019-01)
4.2 Update of References
Specifications normatively referenced might have been revised or even superseded.
In particular ICAO Doc 9705 Edition 2 [i.10] plus the identified defect resolutions contains the baseline technical
provisions for the LINK 2000+ Programme [i.1] and remains the primary reference in the EUROCONTROL DLS
Specification [i.2] and ETSI EN 303 214 [i.7]. However, it is not available in the current ICAO catalogue.
ICAO Doc 9705 Edition 3 [i.10] does incorporate the identified defect resolutions, but also includes many other
additions and amendments. It is available in the current ICAO catalogue.
ICAO Doc 9880 [i.11] was intended to replace Doc 9705 [i.10]. It is available in the current ICAO catalogue as an
"Advance Release". One of the identified activities when updating ETSI EN 303 214 [i.7] is to determine precisely
which requirements from each document need to be met.
When updating the references to the applicable documents (e.g. ICAO, EUROCAE, etc.) it will be necessary to assess
the applicable requirements and verify if they are unconditionally mandatory, or if they are
conditional/optional/recommended.
EUROCAE ED-92C [i.28] revises/supersedes ED-92B [i.8].
4.3 Impact of ELSA Report and other relevant
Projects/Frameworks
The ELSA final report [i.15] states: "This (ELSA) comprehensive report indicates several interlinked problems, and
provides a preliminary plan in form of concrete actions". The preliminary plan should now be updated in the light of
ongoing datalink deployments and incremental performance improvements.
For each recommendation in section 6 of the ELSA Final Report [i.15], the potential impact on ETSI European Norms
is hereafter assessed, with recommendations for revisions where appropriate.
In addition, other relevant projects/frameworks are also assessed with recommendations for revisions of the ETSI Data
Link European Norms as appropriate.
For each EN, the clauses that are expected to be impacted are also indicated. However it is understood that
modifications could also have an impact to other clauses of the same specification.
NOTE: No impact on the EMC EN 301 489-22 [i.16] is expected.
ELSA Ground-01 Use a dedicated channel for transmissions at the airport in areas with high traffic levels
in en-route.
Assessment:
ETSI standards should cope with any future foreseen traffic level allowing the necessary equipment requirements
and related tests. Tests need to demonstrate the capability of the equipment/systems to operate with the
maximum number of frequencies.
Appropriate end to end test need to be defined.

Impact on ETSI EN 303 214 [i.7]: Possibly add text to clause 4.1.6 and test case to clause 5.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-02 Progressively implement additional VDL2 frequencies in accordance with the traffic
level.
Assessment:
See comment on ELSA Ground-01 above.
Appropriate end to end test need to be defined.

Impact on ETSI EN 303 214 [i.7]: Possibly add text to clause 4.1.6 and test case to clause 5.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ETSI

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11 ETSI TR 103 552 V1.2.1 (2019-01)
ELSA Ground-03 Optimize the en-route VGS network coverage.
Assessment:
See comment on ELSA Ground-01 above.
Appropriate end to end test need to be defined.

Impact on ETSI EN 303 214 [i.7]: Possibly add text to clause 4.1.6 and test case to clause 5.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-04 Use the CSC as common control channel only, unless traffic level is very low.
Assessment:
See comment on ELSA Ground-01 above.
Appropriate end to end test need to be defined.

Impact on ETSI EN 303 214 [i.7]: Possibly add text to clause 4.1.6 and test case to clause 5.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-05 Implement ELSA recommended protocol optimization: limit the AVLC frame size.
Assessment:
The ELSA proposal is to reduce the maximum AVLC frame size for AOA to match the average ATN frame size, in
order to reduce channel occupancy induced in case of retransmission, decreasing the probability of uplinks
collisions. The proposed value of AVLC N1 parameter is 251 bytes.
Therefore, the limitation of AVLC N1 parameter to 251 bytes should be evaluated, as recommended by ELSA.

Impact on ETSI EN 303 214 [i.7]: Possibly add AVLC N1 parameter value in clause 4.1.6.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: Possibly update clause 5 (e.g. clauses 5.2.6 and 5.3.5) and 8 in accordance
with updates in ETSI EN 303 214 [i.7].
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-06 Ensure the availability of a fifth VDL2 frequency (at a minimum).
Assessment:
This is needed in order to achieve a sufficient level of RF capacity.

Impact on ETSI EN 303 214 [i.7]: None.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-07 Favour alternative communications means for AOC, with a priority to the airport domain.
Assessment:
ELSA advocates advancing the implementation of alternative (non-VDL/2) AOC communication means. This is not
within the scope of ETSI EN 303 214 [i.7].

Impact on ETSI EN 303 214 [i.7]: None.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

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12 ETSI TR 103 552 V1.2.1 (2019-01)
ELSA Ground-08/SDM Implement the MF VDL2 target technical solution: in each Service area, one single RF
DP Action Plan network that operates reserved VDL frequencies supporting two -GSIF channels (see
definitions in [i.13]).

Figure 2: DLS Model Description
Assessment:
This ELSA recommendation addresses the implementation of the MF VDL2 target technical solution. The target
"model D" is also addressed by the SDM Deployment Program Action Plan (2. Strategic View - paragraph 4). See
figure 2.
Provision for the implementation of multiple GSIF announced on each frequency.
Consider Definition of Class D Station, together with a distinction between Model B and Model D Ground Stations,
and appropriate test needs to be specified, in line with Regulation EC 29/2009 [i.18].

Impact on ETSI EN 303 214 [i.7]: Add text and test cases in clauses 4 and 5.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: Possibly add text, requirements and test cases in clauses 5.2, 5.3 and 8.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-09 Fix the unbounded retry issue in certain VGSs.
Assessment:
ELSA noted that in some cases the VGS N2 retry value was unbounded. N2 is specified in ETSI
EN 301 841-2 [i.5], so no additional specification is needed. ETSI EN 303 214 [i.7] does not specify any test cases
corresponding to requirements in clause 4.1.6 for VDL/2 ground communications equipment. ETSI
EN 303 214 [i.7] requires VDL/2 PICS to be completed; CSP declaration should be sufficient.

Impact on ETSI EN 303 214 [i.7]: None.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-10 Fix the Clear Request issue.
Assessment:
ELSA notes that this issue is already addressed.

Impact on ETSI EN 303 214 [i.7]: None.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Ground-11 Optimize the Disconnect Mode management.
Assessment:
ELSA observed excessive downlink VDL/2 DM frames. The CSP indicated that an optimization was being
implemented. This is out of scope of ETSI EN 303 214 [i.7].

Impact on ETSI EN 303 214 [i.7]: None.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

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13 ETSI TR 103 552 V1.2.1 (2019-01)
ELSA Standards-01/SDM Define and implement an effective datalink end-to-end system certification process
(including both ground and air components) and reference material for the ground
network infrastructure (MOPS-like).
Assessment:
Currently ETSI EN 303 214 [i.7] does not specify end-to-end performance tests, as the specified System Testing
requirements in clause 5 do not currently cover real effects testing. All tests can be passed in the lab environment,
or with a single grounded aircraft, but the real-world performance may still fall below requirements.
For this reason, tests should take into account the real-world VDL2 deployment environment, e.g. simulating
realistic channel loads to represent a real environment, taking in account the foreseen traffic increase.

Impact on ETSI EN 303 214 [i.7]: Possibly add requirements and test cases in clauses 4.1, 4.2, 4.3 and 5.
Impact on ETSI EN 301 841-1 [i.4]: None.
Impact on ETSI EN 301 841-2 [i.5]: None.
Impact on ETSI EN 301 841-3 [i.6]: None.

ELSA Standards-02 Include the selected interoperability improvements and clarifications in the relevant
standards, and implement the resulting changes.
Assessment:
ELSA proposed some possible areas of enhancement to existing base standards, mainly the VDL/2 standards:
A) Proposal to merge CPDLC LACK with TP4 ACK would appear to break protocol layering. In practice,
CPDLC LACK seems to have been found not useful for ground systems, and appears to be redundant
given that TP4 has the same function.
B) Removal of IDRP from X25 traffic has been hypothesized, in order to get benefits in terms of ca
...