1588 PTP Networks: Tutorial

How to achieve the higher accuracy of in real-time data transfer in automotive system? Applications such as audio-video, motion sensing , parking assistance and automated driving all need distributed real-time data transfer. A clear trend is emerging, with the migration towards full in-car Ethernet networks and away from point-to-point and ring topologies from CAN and MOST systems respectively. This is due to the benefits an Ethernet network affords in reduced weight and simplified connections, all whilst maintaining accurate data transferral and the potential for new capabilities.

IEEE 802.1 is a ‘living’ set of standards, continuing to develop to serve the needs of Time-Sensitive Networks by providing techniques to deliver accurate synchronisation, deterministic latency and controlled bandwidth. Clearly, accurate synchronisation is one of the key factors for a network implementation. The higher the level of synchronization that can be guaranteed through networks and devices, the better the potential end performance, which is especially important in safety-critical aspects of automotive systems.

gPTP, developed from the Precision Time Protocol (PTP), is one of the solutions for achieving high-accuracy time synchronisation in ethernet environments. PTP provides benefits such as multiple clock paths for added redundancy to ensure accurate time is maintained in failover conditions. Specifically, gPTP also makes use of a logical syntonization technique that can lock and maintain accurate timing through a system within seconds; this facilitates the “fast turn-on” capability required in an automotive system. This mechanism is further defined in the IEEE 802.1AS/AS-REV standards.

Proving compliance of timing implementations
It is essential to test the system prior to deployment to ensure standards compliance. Testing can help ensure four key aspects of standards compliance and timing performance:

• Interoperability in the network
  Interoperability is one of the challenges in deploying gPTP networks. A network requires compatible applications of the PTP profile across all devices. A robust testing structure can save time and avoid costs associated with misconfigurations discovered during deployment.

• Device-Specific Timing Performance
  For any specific end application, every device in the network must have performance validated in order to comply with the specified requirements. In order to validate the network equipment’s performance and ensure that the ‘steady state’ timing accuracy is maintained, test equipment should have a measurement accuracy of at least an order of magnitude greater than the specified requirements.

• Testing ‘negative condition’ responses (e.g. error messages)
  It is important to ensure that the network can handle negative conditions such as protocol error, timing offsets, etc., gracefully. However, it can be hard to evaluate just how the equipment will respond under such conditions. Testing equipment can simulate negative conditions to validate the network’s performance under such scenarios.

• Real-time analysis of Time error>br> Test equipment can provide a real-time test environment to analyse device delay calculations, path delay calculations, neighbour rate ratio calculations simultaneously, together with any stress-testing stimuli.

How can Calnex help?
No matter whether you want to validate PTP fields for interoperability, optimise network design, test devices in negative conditions, or even recreate real-world conditions in your lab, Calnex Paragon family and Sentinel can help. Our equipment is designed to: help address the test challenges, reduce the complexity of testing, enhance test effectiveness and validate standards compliance of devices and networks to the highest levels of accuracy available.