Performance and Disruption Tolerance of Transport Protocols for Airborne Telemetry Networks

The airborne telemetry environment presents unique challenges to end-to-end communications due to the highly dynamic topology and time-varying connectivity of high-velocity wireless nodes. The AeroTP transport protocol uses multiple reliability modes to trade off end-to-end reliability and efficiency as appropriate for different categories of telemetry data. Based on the architecture previously presented, we have further developed the design of this protocol, as well as performing preliminary simulations of AeroTP using the ns-3 simulation platform. In this paper we present the operational modes of AeroTP in greater detail, as well as comparing the performance of TCP with the AeroTP domain-specific transport protocol. INTRODUCTION AND MOTIVATION Telemetry for airborne test and evaluation is an application that poses unique challenges. Traditionally, telemetry communication has consisted primarily of point-to-point links from multiple sources to a single sink. More recently, with the increasing number of sources in the typical telemetry test scenario, there is a need to move to networked systems in order to meet the demands of bandwidth and connectivity. This need has been recognized by various groups, including the Integrated Network Enhanced Telemetry (iNET) program for Major Range and Test Facility Bases (MRTFB) across United States [1]. The current TCP/IP-based Internet architecture is not designed to address the needs of telemetry applications [2] and there remain a number of issues to be solved at the network and transport layers [3]. Given the constraints and requirements of the aeronautical environment, the current Internet protocols are not suitable in a number of respects. These constraints include the physical network characteristics such as topology and mobility that present severe challenges to reliable end-to-end communication. In order to build a resilient network infrastructure, we need cross-layer enabled protocols at the transport, network, and MAC layers that are particularly suited for the airborne telemetry networks. At the same time, there is a need to be compatible with both TCP/IP-based devices located on the airborne nodes as well as with the control applications. Therefore, a new protocol suite, while being specific to the aeronautical telemetry environment, must also be fully interoperable with TCP/UDP/IP via gateways at the telemetry network edges [4]. Due to the limited bandwidth in telemetry networks and a priori knowledge of communication International Telemetering Conference (ITC 2009) needs of a given test, the iNET community is developing a TDM (time division multiplex)-based MAC for this particular environment [5]. This paper extends the design and evaluation of a transport protocol for this environment: AeroTP – a TCP-friendly transport protocol introduced in [6], and further developed in [7] with multiple reliability and QoS modes. When finalized, the protocol is intended to operate cooperatively with AeroNP network and AeroRP routing protocols [8, 9]. Previous research has demonstrated that domain-specific information can dramatically improve protocol performance [10]. However, in order to achieve this, we need to facilitate cross-layering across the multiple layers. Strict layering in the network stack is not particularly suitable for wireless networks due to mobility, limited bandwidth, low energy, and QoS requirements. Therefore, it is commonly agreed upon that a tighter, more explicit, yet careful integration amongst the layers will improve the overall wireless network performance in general; and in the case of highly-dynamic, bandwidth-constrained networks may provide the only feasible solution that meets the requirements of telemetry applications. NETWORKING CHALLENGES IN AIRBORNE TELEMETRY NETWORKS A typical T&E (test and evaluation) telemetry network consists of three types of nodes: test articles (TA), ground stations (GS) and relay nodes (RN). The TAs are the airborne nodes involved in the test and contain several data collection devices that are primarily IP devices (e.g. cameras) called peripherals. TAs house omnidirectional antennas with relatively short transmission range. The GSs are located on the ground and typically have a much higher transmission range than that of a TA through the use of large steerable antennas. In point-to-point communication mode, the GS tracks a given TA across some geographical space in a test range. However, due to the narrow beam width of the antenna, a GS can only track one TA at any given time. The GS also houses a gateway (GW) that connects the telemetry network to the Internet and several terminals that may run control applications for the various devices on the TA. Furthermore, the GSs can be interconnected to do soft-handoffs from one to another while tracking a TA. The RNs are dedicated airborne nodes to improve the connectivity of the network. These nodes have enhanced communication resources needed to forward data from multiple TAs and can be arbitrarily placed in the network. The flow of T&E information is primarily from the TAs to the ground stations GSs, however command and control data flows in the reverse direction. There are a number of challenges to communication protocols in this environment: • Mobility: The test articles can travel at speeds as high as Mach 3.5; the extreme is then two TAs closing with a relative velocity of Mach 7. Because of high speeds, the network is highly dynamic with constantly changing topology. • Constrained bandwidth: Due to the limited spectrum allocated to T&E and the high volume of data that is sent from TA to GS, the network in general is severely bandwidth constrained. • Limited transmission range: The energy available for telemetry on a TA is limited due to power and weight constraints of TA telemetry modules, requiring multi-hop transmission from TA to GS. • Intermittent connectivity: Given the transmission range of the TA and high mobility, the contact duration between any two nodes may be extremely short leading to network partitioning. Furthermore, the wireless channels are subject to interference and jamming.