Flight trial demonstration of seamless aeronautical networking

This article presents the in-flight demonstration of a new integrated aircraft communications system combining legacy and future radio technologies. This system, developed and validated under real environmental conditions during flight trials, integrates all the aeronautical service domains within a common IPv6-based aeronautical network. The flight trials were held within the framework of the European SANDRA project at Oberpfaffenhofen, Germany, in June 2013. The presented outcomes emphasize the flexibility and scalability of the developed network and demonstrate the seamless service coverage of the given architecture across different airspace domains.


Introduction
Aeronautical communications are currently facing a continuous increase in capacity demand.
This ceaseless request for more communication capacity is because of the constant growth in the number of passengers and, thus, aircrafts, which are expected to double by 2035 [1] and the introduction of new aeronautical communication services with high data volume demand. The latter comprise, among others, new operational safety critical services, such as 4D-trajectory and nonsafety critical services like wireless in-cabin connectivity for passengers. To cope with this high demand in communications capacity, part of the ongoing research aims at developing new concepts and technologies for future aeronautical communications (like the European SESAR Joint Undertaking program [2] and the FAA Next Generation Air Transportation System (NextGen) [3]) with a strong emphasis on the development of new link technologies, such as the terrestrial L-band Digital Aeronautical Communications System (L-DACS) link [4] and the European Space Agency (ESA) Iris program [5].
The introduction of new digital communication links is of paramount importance in the aeronautical sector as the existing Air Traffic Management (ATM) communication infrastructure already operates close to its maximum capacity [6]. Although the new systems will eventually replace the legacy communication systems, there will be a lengthy period in which an aircraft will be fitted with all of the systems for global interoperability. Hence, there is a need to integrate legacy and future data links into one large seamless aeronautical network to serve future communication demand.
The design, development, and validation of such a seamless network correspond to the focus of the European-funded research project SANDRA (Seamless Aeronautical Networking through integration of Data links Radios and Antennas) [7], which integrates different communication links (legacy and future data links) and networks (such as ATN/OSI or ATN/IPS) with all the aeronautical service domains (ATS, AOC/AAC, and APC) in a safe, high-performance, and cost effective way through IPv6 as the unification point. The development of the entire corresponding ground network infrastructure is also part of the SANDRA architecture. The validation of the latter was realized by performing flight trials on the airport of Oberpfaffenhofen, close to Munich, Germany [8]. This paper gives an overview of the outcomes of the first SANDRA flight trials with a strong emphasis on the seamless handovers that were carried out between legacy and future data links, namely, VDL2, BGAN, and the newly developed AeroMACS [9], thus proving the flexibility and scalability of the SANDRA network. The seamless service coverage aspect of the SANDRA architecture was demonstrated by the successful test of various applications in all aeronautical service domains.
The rest of the article is organized as follows. The SANDRA concept is introduced, followed by the details of the overall system setup and the most relevant components.

The SANDRA Concept
The vision of SANDRA is the integration of aeronautical communications systems by using well-proved industry standards to enable a cost-efficient global provision of distributed services.
SANDRA system is considered as a "system of systems" addressing four levels of integration: service, network, radio, and antenna.

System Setup
The system setup of the SANDRA flight trials is composed of two major segments: the airborne segment and the ground infrastructure.

Airborne Segment
The SANDRA airborne system was integrated in an Airbus A320 as displayed in Figure 2. To be integrated in the aircraft, the SANDRA airborne system was divided into four separate racks containing different pieces of equipment as illustrated in Figure 2. The distribution of equipment within the racks was based on the different functionalities, whereas the locations of the racks within the cabin were defined based on the positions of the antennas on the aircraft's fuselage.
The racks were organized as follows. The first rack contained the integrated router and the connectivity to the different end-user systems. The second rack was equipped with the two integrated modular radio processing platforms, thus representing the link between the IR and the different RF equipment (one IMR used as redundancy backup, cf. IMR-PC2 in Figure A.). The third rack was fitted with the RF units for the VDL2 and AeroMACS data links. Finally, the RF components that handle the BGAN satellite link were located in the fourth rack at the rear of the cabin. To maximally reduce the antenna cable losses, the third and fourth racks were placed in the cabin right below the respective antennas.

Ground Infrastructure
The core part of the SANDRA ground infrastructure was located at Oberpfaffenhofen, Germany. It is composed of all the IP-based networking components, such as the access router and the home agent. The home agent includes functionalities like IPsec (IPv6) to provide authentication and integrity and the NEMO protocol [10] to guarantee mobility to the airborne terminal.
Furthermore, the home agent provides global reachability accessing the mobile node via the home address during handover from/to AeroMACS to/from BGAN when attachment point changes from one access network to another. The IPsec integrates an IPv6-over-IPv4 transition mechanism, entitled NeXT [11]. The access router also provides the router advertisement messages Finally, for the ATN/OSI ground infrastructure, a VGS for VDL2 was installed on the roof of the SANDRA laboratory close to the airfield, although the ATN/OSI ground-end system was located at Montreal, Canada, and connected to the SANDRA laboratory via a wide area network (WAN). The satellite connection was made over the BGAN satellite network. Further insights on the SANDRA ground infrastructure and the overall SANDRA test bed can be found in [12].

Description of Flight Sorties
Six sorties were made in 3 days with the D-ATRA aircraft at a rate of two flights per day. The focus of the first day was mainly to evaluate the correctness of data transmission over the air for each of the three data links. Once the links were operational, the flight trials of the second and third days aimed at validating the SANDRA concept by performing a set of scenarios that were previously identified. To do so, various applications ranging from ATS over AOC, AAC, and APC services were tested onboard the aircraft.
On average, each sortie lasted roughly 90 minutes including taxiing, take-off, and landing phases. The scenarios were performed onboard during the 45 minutes of cruise. For each sortie, the aircraft was flew over the Oberpfaffenhofen Airport and continued its route until the VHF connection was lost and turned around to fly back over the airport. Once out of the VHF coverage, the system seamlessly switched over to BGAN / L-Band with no loss of traffic in the scenario under test. This allowed testing the seamless functionality of the SANDRA concept.

Seamless Layer 3 Handover
Whenever a change of traffic routing policy involving two different data links occurred, a handover was performed. During the flight trials, handovers were performed between all the three link technologies in both directions (e.g., BGAN to VDL2 and VDL2 to BGAN) and also between some combinations of different quality of service contexts within the same technology (BGAN background to BGAN streaming). Additionally, the handovers were classified depending on the triggering condition. One type of handover was the "IMR triggered handover," initiated by the integrated modular radio when the aircraft was moving (or was already) out of coverage of one of the available links. The other type, the "IR triggered handover," was a handover caused by an automatic or manual change of the routing on the integrated router based on a policy (e.g., changing to a newly available and preferred link).
To test the "IMR triggered handover," an AeroMACS context was open while the aircraft was in the parking position. Once set, traffic was generated from the end systems to put some load on the link. Then, the IMR was told that the aircraft was changing from a "standing" position to "en route." Because AeroMACS is not available while the aircraft is cruising, the IMR initiated the procedure to open a new BGAN context and notified the IR of the upcoming change. Figure 3(a) shows the handover and how traffic is sent over the BGAN again after the handover is completed.
An "IR triggered handover" can be observed in Figure 3(b). Initially, all traffic is sent over a BGAN background context. Although this best-effort type of service is good enough for applications like browsing or e-mailing, it is not suitable for jitter-sensitive applications like voice-over-IP (VoIP).
Therefore, a manual change of policy routing was performed. Instead of interrupting the traffic on the request, the traffic is routed through the new context only after this was completely established, thereby avoiding an interruption of the communication. The VoIP call members did not notice any loss of communication, and in fact, no packets were lost during the handover, and only one packet suffered reordering.
It should be noted that the "IMR triggered handover" is performed when a link-in-use unexpectedly goes down. The communication is temporarily interrupted while the connection with the new link is established. "IR triggered handovers" are carried out while the link is still active, opening a context with a different link and modifying the routing only when new link is operational.
After disabling AeroMACS data link (step (1) in Figure 3(a)) the content request by the IMR from the BGAN network takes around 30sec. Now, the care-of-address can be provided towards the IR. Then, IR can initiate establishment of the IP connectivity with the ground (NeXT, NEMO, and policy routing update). Improvements in all these handover procedures can be seen here.
(a) Integrated Modular Radio triggered handover from AeroMACS to BGAN (b) Integrated Router triggered handover for VoIP

Seamless Layer 2 Handover
The IMR, which represents the data link and physical layer of the OSI stack, consists of the different radio protocol stacks (AeroMACS, VDL2, and BGAN), includes an adaptation layer called joint radio resource manager (JRRM) that is responsible for managing and controlling the underlying radios in a uniform and consistent manner, and provides a single interface to the network layer (cf. Figure A).

ATN/OSI over IP-SNDCF
The use of IP subnetwork dependent convergence function (SNDCF) enables the ATN/OSI upper layers and network (CLNP-IDRP) protocols to be conveyed over the IP protocol. This allows IPbased networks to be used for providing the underlying ATN subnetwork links between the ATNrouting entities. It was decided to experiment on the use of IP-SNDCF on the aircraft (whereas today it is only used on ground) and thus on a mobile system. The objective was to assess if the ATN/OSI CLNP packets could be conveyed over VDL2 (as done today) and over the SANDRA broadband radio-IP links in a seamless way. The advantage of a mobile IP-SNDCF is that avionics and ground stations can implement a single (or multiple but standardized) SNDCF for all mobile communication technologies instead of having different interfaces for each technology, which is the case at present.
A prototype mobile IP-SNDCF module was developed and integrated on existing ground architecture and on the aircraft, cf. Figure B. This allowed demonstrating end-to-end ATN/OSI communications over VDL2 and SANDRA mobile IP implementation over BGAN and AeroMACS [13].

AMBEATC10B VoIP
The AMBEATC10B VoIP is an experimental hardware voice-over-IP appliance based on the AMBE ATC 10B vocoder circuit board. This is currently the only digital vocoder certified for air traffic control. The circuit board is integrated with a micro-controller and installed in a rack-mountable case with a push-to-talk button. The micro controller board runs a customized version of the Linux operating system that reads/writes voice samples from the vocoder board and sends/receives them over the SANDRA network using the user datagram protocol (UDP)/IPv6. Both the airborne and the ground appliances were equipped with commercially available ATC headsets.
The quality of service delivered by the SANDRA network for VoIP applications was evaluated using the AMBEATC10B VoIP appliance according to ITU recommendation P. The subjective rating of the voice quality should therefore be understood as relative to the established ATC voice systems.
Four of the 65 conversations were interrupted by reconfigurations of the data links.
Handovers from the AeroMACS link to the satellite link were seamless and generally not noticed by the conversing subjects. However, the smaller round-trip delay of the AeroMACS system when compared with the satellite link was perceived. The users noticed occasional packet loss on the satellite link by missing the syllables in the conversations, but it was not perceived as a great problem.
The mean score over all the conversations was 4.33 (excellent = 5, good = 4) on the airborne side and 3.93 (fair = 3) on the groundside. The perceived lower audio quality of the ground users can be explained by the background noise in the aircraft that was included in the transmission. On the aircraft, the background noise was attenuated by the headsets, providing the airborne user with a clear reproduction of the ground signal recorded in a quiet room.

CPDLC ATN/OSI Application
For the controller-pilot data link communications (CPDLC) ATN/OSI application, the ground-  Context management (CM)/CPDLC messages were routed seamlessly over one medium or the other, without any impact on the upper layers. During the flight tests, the IP path (BGAN or AeroMACS) was given priority, and whenever both the VDL2 and the IP paths were available at the same time, traffic was automatically routed over the IP path. When the IP path became unavailable, traffic fell back to using the VDL2 path.

Airline Operational Services
Two applications that pertain to the operational service domain of an airliner are described in this section. These applications were integrated and tested during the flight trials.

Performance of Applications over Future Data Link
Finally, through a closer look of the new integrated AeroMACS nonlegacy data link, it was observed that the end-to-end connectivity was affected not only by AeroMACS but also by all other networking systems (integrated router, integrated modular radio, access router, home agent, etc.).
This was verified using the internet control message protocol (ICMP) pings that measured the

Conclusions
In this paper, the outcomes of the flight trial of a new integrated aircraft communications system were presented. Developed within the framework of the SANDRA project, this system was integrated in an Airbus A320 and tested in real flight conditions in June 2013 at the Oberpfaffenhofen Airport, Germany.
During these flight trials, the two key features of the SANDRA concept were demonstrated.
On the one hand, the seamless service coverage of the SANDRA architecture across different airspace domains was shown. By keeping IPv6 as the unification point, it was proven that this system integrates a full range of aeronautical applications (ATS, AOC/AAC, and APC).
The second key feature of the SANDRA concept that was demonstrated during the flight trials was its global interoperability between legacy (VDL2 and BGAN) and future data links (AeroMACS).
This was realized by performing, first, a handover on the ground between VDL2 and AeroMACS data links and, second, a handover while flying between the VDL2 and the BGAN satellite links (for both cases, handovers were performed in both directions). Transparent to the end-user, these handovers have proven the interoperable and scalable aspects of the SANDRA network, which can switch reciprocally between legacy (non-IP) and future (IP) data links.  BGAN), looking into the wireless and network control, system architecture design, protocol development and design of security architectures. He also works on various image and video processing techniques, motion detection and capturing methods, vision based robotics, eye/gaze tracking and gesture recognition systems. He also has extensive experience in designing and developing advanced embedded systems and intelligent robots. Dr Pillai is a member of IET and a fellow of the HEA.