VDL Mode 4 Performance Simulator (DLS enhancements) presented by EUROCONTROL

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VDL Mode 4 Performance Simulator(DLS
enhancements)presented by EUROCONTROL

• Montreal, 26 October 2004

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Background

• VDL Mode 4 Performance Simulator (VPS)
• Developed under contract to Helios Technology
• Initially developed to investigate broadcast
  scenarios
• DLS protocol reviewed and updated in 2003
• DLS Simulations activities organized in two
  phases
• Phase 1 Implementation and validation of point
  to point protocols in VPS
• Phase 2 Investigations and optimisation of point
  to point protocols performance using VPS

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Recap on Phase 1 Development and validation

• DLS implementation completed
• ICAO Manual (Doc 9816) compliant
• Validated with testing and comparison against
  simple quantitative models
• Capable of handling static and dynamic scenarios
• Demonstrates the key DLS features
• Duplicate suppression
• Fragmentation of long packets
• Priority management
• Deterministic re-transmission when ACK not
  received
• Link establishment (ground-air and air-air)
• Short and long transmission protocols
• Combination of DLPDUs

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Phase 2 Scope

• This study targets to answer the following
  questions
• How can the system be optimised for latency and
  capacity?
• What is optimum burst size for throughput?
• What impact for multiple ground stations?
• What are the benefits of ground coordination?
• How many aircraft can be supported on a single
  channel?
• What is the impact of a dynamic scenario?

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Phase 2 Work plan

• WP1
• relationship between latency and capacity as a
  function of quality of service parameters
• WP2
• Investigate a means of coordinating ground
  station transmissions will be designed and tested
• WP3
• develop a realistic traffic and data scenario
• WP4
• validate the results for a dynamic aircraft
  scenario
• WP5
• enhance VPS to include a more automated LME

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WP1 Description

• Simulations designed on following principles
• Technical Manual uses default Quality of Service
  parameters
• Average exchange time is currently 6.5 seconds
• Can this latency be improved?
• What is the impact on capacity?
• Scenario description
• A single ground station with 641 static aircraft
• All aircraft log on within the first 10 minutes
• A range of channel loading considered (12 -
  130)

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WP1 Preliminary Results (1)

• Results in terms of
• Delay
• Throughput
• Retransmissions
• Average Delay

• Demonstrates ability to exercise control on
  transaction time

P 0.1
Average delay (sec)
SHORT
Demand (kbps)
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WP1 Preliminary Results (2)

• Demand vs. Throughput
• Lower persistence ensure capacity is matched for
  higher demand

P0.1
P0.25
Throughput (kbps)
Demand (kbps)

• Throughput vs. Retransmissions
• The unicast/info transfer protocols can be tuned
  to reduce retransmissions

No. Retransmissions
Throughput (kbps)
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WP1 Summary of results

• Two main results
• Reducing p-persistence to 0.1 adds to stability
  of performance
• Reducing minimum and maximum range can produce
  greatly reduced latencies
• Can define multiple QoS sets to carry out an
  automatic trade off of capacity and latency
• Thus VDL Mode 4 can support a number of streams
  with different QOS
• Limiting factor is the number of retransmissions
• WP2 will focus on reducing the retransmissions
  through efficient use of ground reserved slots

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WP2 Description

• Use of ground reserved slots is expected to
  produce increase in capacity
• WP1 scenarios use random access to initiate each
  transaction
• WP2 considers placing some of these transactions
  into ground reserved blocks
• Focus of WP2
• Use of ground reserved slots allows coordination
  of transmissions to avoid garbling
• Design of an efficient ground coordination
  algorithm

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WP2 Current status

• Simulation experiments ongoing
• The results likely to show that VDL Mode 4
  protocols support good organisation of the slot
  resource
• High success factor for receipt of bursts
• Slot sharing
• Maintenance of capacity per ground station over
  wide area
• Wide ranging ability to tune QOS
• Potential issues to be addressed
• There is a poor use of the burst payload capacity
  (match of data to message length)
• Loss in throughput through RTS, CTS, ACK

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WP3 Realistic scenarios

• Builds on the previous work by simulating
  realistic scenarios
• Traffic based on Core European scenario
• Data link traffic developed from Link2000
  environment
• Both single and multiple ground station scenarios
  considered

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Next Steps

• Finish Core European scenario simulations
• Produce definitive results on ground coordination
• Look at other optimisation possibilities
• Optimization of Burst formats
• Encouraging greater combination of messages