A performance comparison between VDL mode 2 and VHF ACARS by protocol simulator

VHF Aircraft Communications Addressing and Reporting System (ACARS) is the most popular VHF aeronautical datalink. It is used for such applications as airline operation and air traffic control. VHF ACARS has only a 2400 bit/s (bps) transmission rate air-ground link. The VHF Digital Link mode 2 (VDL2) system, which has a thirteen times higher transmission rate air-ground link than VHF ACARS, is very similar to VHF ACARS. Both systems can deal with messages in ACARS format. Even though the systems are well used for various operations, their effective link capacities are not so clear. Finding out effective communication performance, including real link capacity and acceptable maximum delay, is useful for the design layout of ground facilities and rebuild datalink operation guidance in the near future. We built both VHF ACARS and VDL2 protocol models on an OPNET protocol simulator to evaluate their effective communication performance. This paper gives an outline of the protocol models and comparison results of these performances by simulation. Statistics such as transmission delay and throughput have been obtained under various load conditions for up to 200 aircraft. Before starting the simulation, we analyzed message data length and the message generation interval of VHF ACARS from real communication logs in Japan. The message data length was mostly distributed randomly, below 660 bytes, and messages exceeding 660 bytes rarely appeared. The message generation interval mostly followed Pareto distribution. We assumed that the message data length and data generation interval followed uniform distribution and Pareto distribution respectively. We found the following by analyzing simulation results. i) When data traffic load generated from an aircraft equaled the load directed to the aircraft, the VDL2 system was able to process 4.6 times more congested load than the VHF ACARS. ii) When data traffic load generated from an aircraft was five times higher than the load directed to the aircraft, the VDL2 system was able to process 8.8 times more congested load than the VHF ACARS. The load condition approximated real VHF ACARS data generation ratio between forward link and reverse link.