1. VDL Mode 4 Performance Simulator (DLS enhancements) presented by EUROCONTROL
Montreal, 26 October 2004

2. 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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3. 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
Brief recap on the validation work done in phase 1.
- say report available on Eurocontrol web pages.
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4. 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?
With this new tool now in hand…
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5. 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
Keep this simple a few words to introduce each WP, mention the main aim of each one
Work plan lays out the tasks in progressive steps to address the main questions.
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6. 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%)
SCENARIO DESCRIPTION
This is a baseline scenario. Single GS, WP2 will consider more than one GS. Static aircraft, WP4 will consider dynamic aircraft.
All logon within 10 minutes. Gives possiblity to analyse only the pure data rate (all CTRL bursts during the first 10 minutes only)
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7. 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)
AVERAGE DELAY
Mention at the onset that these are a sample of the results showing the trends. The full results are under analysis.
The underlying result is that by tuning the protocol parameters we can reduce the latency
(reduced p persistance to 0.1 for both curves)
reducing the candidate range for selection of the ACK slot
SHORT
Demand (kbps)
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8. WP1: Preliminary Results (2)
Demand vs. Throughput
Lower persistence ensure capacity is matched for higher demand
P=0.1
P=0.25
Throughput (kbps)
Demand (kbps)
Throughput vs. Retransmissions
The unicast/info transfer protocols can be tuned to reduce retransmissions
Demand vs capacity: Reducing the persistence value gives more throughput for a given demand.
- Throughput vs Retransmissions: After a certain breaking point, the number of retransmissions goes exponential. This breaking point cannot be avoided because sooner or later the slot selection process runs out of slots. However reducing the persistence and candidate slot range for unicast/info transfer protocols results in less retransmissions for a given demand (to a certain point).
No. Retransmissions
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Throughput (kbps)

9. 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
2 RESULTS:
Reducing p-persistence to 0.1 adds to stability - delays point at which there is a rapid increase in re-transmissions
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10. 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
ADVANTAGES
Random access is inefficient and requires several unused or unsuccessful access attempts for each successful one
In scenarios where there are more than one ground station, ground transmissions can easily interfere with each other. This is an important cause of inefficiency in uncoordinated systems
CHALLENGE
Work on defining efficient ground coordination algorithms is ongoing. Results to be available soon.
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11. 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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12. 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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13. 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
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