Advanced Network Tap Application for Flight Test Instrumentation Systems

Abstract

Digital data distribution systems are widely used in Aerospace and Defense products to allow devices to communicate with one another. In many cases it is desirable to monitor the data traffic flowing between two points in a copper or fiber based Operational or Onboard Network System (ONS) for Flight Test Instrumentation (FTI) purposes because these ONS systems may carry important data which can be used without duplicating/installing a specific FTI data acquisition system to receive this data. The two types of network taps that can be used are Inline Network Taps and network end-point taps. This paper examines the usage of Inline Network Taps for FTI applications and how they can support network access strategies and objectives. An Inline Network Tap is a hardware device which allows access to data flowing across a network. These devices are typically active/powered and have a number of ports: a first tap port, a second tap port, and one or more mirror ports. An in-line network tap inserted between the first and second tap port passes all data traffic through unimpeded but also copies that same data to one or more mirror ports. Some Inline Network Tap devices may also pass packets when the tap is not powered or a malfunction is detected on the device via an integrated by-pass function. If the Inline Network Tap device goes offline the unit automatically bypasses the tap connection and data traffic is directed through the bypass directly to network devices. This capability is crucial for inline usage on mission critical network segments that cannot afford the risk of losing the network connection. An in-line network tap can either be based on copper or fiber technology and as a "filterable" network tap can also provide advanced packet filtering capabilities. These filterable network taps can selectively pass data, e.g., based on VLAN ID or other parameters, to a mirror port for deep analysis, monitoring and recording. Another advanced tap function that is presented in this paper is the support for inserting time stamps at the tap level in monitored packets which provides a reference time when the data content of a given packet was generated at a data source. This capability is a significant feature for FTI applications as most ONS systems do not provide time stamped data.