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

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 WGM/9 26/10/2004

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. WGM/9 26/10/2004

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… WGM/9 26/10/2004

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. WGM/9 26/10/2004

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) WGM/9 26/10/2004

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) WGM/9 26/10/2004

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 WGM/9 26/10/2004 Throughput (kbps)

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 WGM/9 26/10/2004

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. WGM/9 26/10/2004

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 WGM/9 26/10/2004

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 WGM/9 26/10/2004

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 WGM/9 26/10/2004