VDL Mode 4 Performance Simulator(DLS enhancements)

presented by EUROCONTROL

Montreal, 26 October 2004

26/10/2004WGM/9

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

26/10/2004WGM/9

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

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7

Short delay S4

Long delay S4

Short delay S3

Long delay S3

•Demonstrates ability to exercise control on transaction time

SHORT

P = 0.1

Demand (kbps)

Ave

rage

del

ay (

sec)

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WP1: Preliminary Results (2)

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8 9 10

Cap

acit

y (A

chie

ved

Th

rou

gh

pu

t/kb

ps)

S1 (p=0.25)

S2 (p=0.25)

S3 (p=0.1)

ideal

Demand vs. Throughput• Lower persistence ensure capacity is

matched for higher demand

Throughput vs. Retransmissions

• The unicast/info transfer protocols can be tuned to reduce retransmissions

Demand (kbps)

Thr

ough

put (

kbps

)

P=0.1 P=0.25

0

5000

10000

15000

20000

25000

30000

0 1 2 3 4 5 6 7

Throughput (kbps)

Calculated S1 (p=0.25)

Achieved S1 (p=0.25)

Calculated S6 (p=0.25)

Achieved S6 (p=0.25)N

o. R

etra

nsm

issi

ons

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