Steven C. Bretmersky

Performance of VDL mode 2 for the Aeronautical Telecommunications Network

VHF Digital Link (VDL) has been identified as a method of communication between aircraft and ground stations in the Aeronautical Telecommunications Network (ATN). VDL Mode 2, the first of three modes to be implemented, has been modeled in software for simulation. The simulations investigate the performance of a single ground station communicating with up to 150 aircraft in several scenarios. The number of aircraft that can be supported by a single ground station is determined based on the FAA specified limit of the subnetwork delay.

Comparison of VDL modes in the aeronautical telecommunications network

VHF digital link (VDL) has been identified as a method of communication between aircraft and ground stations in the aeronautical telecommunications network (ATN). Three different modes of VDL have been suggested for implementation. Simulations were conducted to compare the data transfer capabilities of VDL modes 2, 3, and 4. These simulations focus on up to 50 aircraft communicating with a single VDL ground station. The data traffic is generated by the standard file transfer protocol (FTP) and hyper text transfer protocol (HTTP) applications in the aircraft. Comparisons of the modes are based on the number of files and pages transferred and the response time.

Evaluation of VDL modes in the en-route domain

In this paper we investigate the performance of VDL modes 2,3, and 4 in the en-route domain by simulating mobile aircraft that dynamically establish and disconnect links to the ground station. In simulation one, the aircraft modeled are within a single sector of airspace which is the current boundary for en-route aircraft communication. The second simulation models all aircraft within range of the VDL network, a 200 nautical miles radius. In both simulations, the aircraft trajectories simulate real traffic patterns of aircraft controlled by the Cleveland Air Route Traffic Control Center (ARTCC). The primary statistic used to evaluate the performance of the VDL modes is the subnetwork delay. Based on the simulation scenarios investigated in this research, the subnetwork delays for all three modes are substantially less than the corresponding maximums specified in the standards.

Estimation of VDL mode 2 with hidden transmitters

The VDL Mode 2 aeronautical data link uses a p-persistent Carrier Sense Multiple Access (CSMA) channel access protocol that, like all CSMA systems, is susceptible to the hidden transmitter condition. Simulations have shown that the impact of hidden transmitters in the aeronautical environment can be significant. In this paper, a method of estimating channel performance with hidden transmitters is derived on a radio by radio basis. This method proves to be faster than the VDL Mode 2 simulations, allowing scenarios to be evaluated quickly. The process is applied to a large dynamic data set that demonstrates the usefulness of this approach.

Characteristics and Capacity of VDL Mode 2, 3, and 4 Subnetworks

In the current National Airspace System (NAS), voice communications for Air Traffic Services (ATS) are transmitted by analog VHF radios. Aeronautical Operational Communications (AOC) use a character-oriented system operating in the VHF band known as Aircraft CommunicationsAddressing and Reporting System (ACARS). These radios are neither capable of supporting the growth in air traffic expected in the near future, nor are they capable of enabling new services under development. Three bit-oriented digital VHF radios, VDL Modes 2, 3, and 4, have been proposed as solutions to frequency congestion in the aeronautical VHF band. In this paper each mode is investigated through simulations which explore their individual characteristics under variable loads. The three modes are also tested in realistic terminal and en route domain scenarios. The capacity of each mode is estimated based on the subnetwork delays specified in the standards. The results indicate that VDL Mode 2 can support the highest traffic load, but cannot provide deterministic delays for critical applications. Mode 3 supports the least load due to large downlink delays, but provides the required mechanisms for critical information transfer. Mode 4 has a marginally higher capacity than Mode 3 with similar prioritization capabilities.

Comparison of VDL modes in the terminal domain

In This work we investigate the comparative performance of VDL modes 2, 3 and 4 by simulating mobile aircraft that dynamically establish and disconnect links to the ground station. The principal consideration was to model and simulate realistic data traffic and aircraft traffic patterns in the terminal domain, which in these simulations is the area within a 70 nautical mile radius from the terminal. The simulated aircraft traffic patterns are based on actual data from the Detroit Metropolitan Airport. We consider two aircraft traffic patterns, and the data traffic profile used for the simulations is defined in the relevant standard. The performance criteria used to compare the performance of the three modes is the subnetwork delay. For the simulation scenarios investigated in this research, we recommend VDL Mode 3 in the terminal domain.

A collisi on avoidance concept for VDL mode 2

The FAA is currently investigating the migration path to data-enabled digital air traffic services using VDL mode 2. Under certain loading conditions, simulations have shown that VDL mode 2 cannot meet future delay requirements with more than a few dozen aircraft per channel. Since the VDL radios must share limited spectrum with the current analog voice radios, this poses a problem during the transition period. Analysis has show that collisions of the transmitted data frames lead to retransmissions and increased delays. This paper investigates a method to reduce theses collisions through modification of the MAC protocol in the hope of increasing the capacity of a VDL mode 2 channel. Several possible future traffic profiles are used to compare the performance of the modified VDL mode 2 protocol against the standard. A discussion on the modifications necessary to implement these changes, along with interoperability issues with standard VDL mode 2 radios, is presented.

Performance of a regional aeronautical telecommunications network

This paper reports the findings of the simulation of the ATN (Aeronautical Telecommunications Network) for three typical average-sized US airports and their associated air traffic patterns. The models of the protocols were designed to achieve the same functionality and meet the ATN specifications. The focus of this project is on the subnetwork and routing aspects of the simulation. To maintain continuous communication between the aircrafts and the ground facilities, a model based on mobile IP is used. The results indicate that continuous communication is indeed possible. The network can support two applications of significance in the immediate future-FTP and HTTP traffic. Results from this simulation prove the feasibility of development of the ATN concept for AC/ATM (Advanced Communications for Air Traffic Management).