• Analysis and Application Scenarios for Telemetry Data Transmission and Synchronisation over Wireless LAN

      Cranley, Nikki; Corry, Diarmuid; ACRA Control Inc. (International Foundation for Telemetering, 2008-10)
      The use of IEEE 802.11 Wireless LAN (WLAN) technology offers numerous advantages over wired Ethernet including high bandwidth, device mobility, and the elimination of network wiring within the aircraft. With such benefits, there are certain caveats associated with the ability and performance of WLAN technology to carry time-sensitive and critical telemetry data using current IEEE 802.11 WLAN standards. In this paper, the limitations of WLAN for real-time data transmission are experimentally investigated. In particular, it will be shown how the fundamental wireless access mechanism and contention impact on the WLANs ability to carry real-time data. Although telemetry data is constant, the wireless access mechanism causes the WLAN throughput and per-packet delays to vary over time. Moreover, with the increased popularity of the IEEE 1588 Precision Time Protocol (PTP), the ability of the WLAN to provide time synchronisation is investigated. It is shown that asymmetric data loads on the uplink and downlink introduce synchronization errors. To mitigate some of these issues, this paper will discuss how the Quality of Service (QoS) Enabling WLAN standard, IEEE 802.11e, can be used to provide differentiated services and prioritised transmission for critical data.
    • Key Components in a Networked Data Acquisition System

      Corry, Diarmuid; ACRA Control Inc. (International Foundation for Telemetering, 2008-10)
      With the growing interest in networked data acquisition there has been a lot of focus on networked data acquisition systems. However, the requirements of a flight test instrumentation system go beyond networked DAU's. For example, a FTI network fabric has particular requirements for switches, time grandmasters, recorders, data servers and network terminals to the ground. This paper discusses these components and how they inter-operate in a single, fully networked system and discusses some FTI oriented requirements for same. Where relevant, we discuss the results of some experiments with network latencies; packet losses etc. and discuss some enhancements that can contribute to improved efficiency for flight test programs.
    • Real-Time Transport Protocols for Telemetry Data and Signaling

      Cranley, Nikki; Corry, Diarmuid; ACRA Control Inc. (International Foundation for Telemetering, 2008-10)
      There are many network transport and application layer protocols that have been developed and designed for the delivery of time-sensitive and critical data over best-effort Ethernet networks. Choosing the appropriate protocols is crucial to ensuring the performance of the network for data transmission. Network layer protocols, such as TCP/IP or UDP/IP, determine the reliability of the data transmission while application layer protocols are designed to enhance the services of the underlying transport protocol. This paper will compare the Quality of Service (QoS) tradeoffs for both UDP and TCP. In this paper it is shown that UDP or TCP alone is insufficient to meet the transmission requirements of telemetry data and to overcome these limitations, an application layer protocol is required to enhance the delivery of real-time sensitive data. This paper presents the Real-time Transport Protocol (RTP) and proposes a RTP telemetry profile payload definition that provides complete payload self-description and up to 98% packetization efficiency.