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

26/10/2004WGM/9 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?

26/10/2004WGM/9 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

26/10/2004WGM/9 WP1: Description Simulations designed on following principles Technical Manual uses default Quality of Service parameters l Average exchange time is currently 6.5 seconds l Can this latency be improved? l 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%)

26/10/2004WGM/9 WP1: Preliminary Results (1) Results in terms of Delay Throughput Retransmissions Average Delay Demonstrates ability to exercise control on transaction time SHORT P = 0.1 Demand (kbps) Average delay (sec)

26/10/2004WGM/9 WP1: Preliminary Results (2) 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) Throughput (kbps) P=0.1P=0.25 No. Retransmissions Throughput (kbps)

26/10/2004WGM/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

26/10/2004WGM/9 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

26/10/2004WGM/9 WP2: Current status Simulation experiments ongoing The results likely to show that VDL Mode 4 protocols support good organisation of the slot resource l High success factor for receipt of bursts l Slot sharing l Maintenance of capacity per ground station over wide area l 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

26/10/2004WGM/9 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

26/10/2004WGM/9 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