Who doesn't need Ethernet timestamps?
IEEE 1588 has matured from its humble beginning in 2002 as a mechanism to synchronize Ethernet-connected test equipment into a protocol that is widely used on many of the circuit boards it was originally designed to test.
In fact, IEEE 1588 is the root of new timestamping for Industrial Ethernet protocols such as PROFINET, CIP SYNC, IEEE 802.1AS audio video bridging, the LXI consortium for the Test & Measurement industry, ITU G.8261 for telecommunications, IEC 81650 and numerous proprietary solutions which synchronize disparate Ethernet-networked clocks.
As defined by the IEEE 1588 committee, its precision time protocol (PTP) supports:
• High clock synchronization accuracy typically better than a microsecond
• Fast synchronization of networked clocks in typically fewer than 20 clock cycles
• Minimal compute and network footprint
• Synchronization of heterogeneous clocks with varying accuracy, resolution, drift and stability characteristics
• Easy configuration and operation by non-expert users for low cost and administrative setup
In July 2008, the committee released a second version (V2), which extends IEEE 1588 into larger high-performance networks. It is expected that most network systems designers will select or migrate to V2 now that it is available.
But many embedded applications already benefit from Ethernet timestamp protocols and can only be expected to proliferate further in the future with the availability of Version 2.0.
Factory and process automation applications use them it to synchronize sensors and actuators over single-wire distributed network to control automated assembly processes and time-based motion control.
Aerospace applications use them to synchronize vehicle controls. Power line management applications synchronize across large-scale distributed power grid for smooth power transfer.
Networking and telecommunications use them to lower the cost of high-precision time synchronization between communicating nodes. Professional and consumer multimedia applications are starting to use them to ensure customers don't hear or see effects of packet delay or loss from Ethernet-connected speakers and monitors.
Implementation options
Potential users can choose from several implementation options. Depending on their requirements, timestamps may be applied:
• As the packet reaches the application layer on any central processing unit (CPU). This "software timestamp" is always the least accurate option.
• As the packet enters the media access (MAC) or physical (PHY) layer of an integrated processor. This "hardware timestamp" removes inaccuracy caused by interrupts and other applications running in the processor.
• As the packet enters the MAC or PHY layer of a second device adjacent to the processor. This hardware timestamp also removes inaccuracy caused by interrupts and other applications running in the processor.
The cost to develop and deploy these systems can vary greatly depending on system architecture and the level of synchronization accuracy that is required. We will now discuss which types of applications can best use each option.
Software timestamp. The software-only implementation timestamps packets in the application layer which is furthest away from the physical layer. This implementation yields the least accuracy due to the largest amount of delay and jitter that occurs in passing the timestamp through the various layers (Figure 1).
Figure 1: CPU-based software timestamp gives less accurate clock synchronization
View the full-size image (image provided by authors).
View an enlargement of image provided by authors (note: image will be slightly blurry).
Software timestamps typically give errors on the order of microseconds to milliseconds depending on the operating system and platform. For example, laboratory tests of a generic personal computer running a Windows operating system and IXXAT IEEE 1588 application software gave a timestamp resolution of 1 millisecond (ms) with standard deviation +/- 1.5 ms and maximum deviation +/- 50 ms.
An evaluation board for the Freescale MPC8349 PowerQUICC processor running a Linux operating system and the same IXXAT application software gave a significantly better timestamp resolution of 1 microsecond (µs) with the same standard and maximum deviations.
Software timestamps with accuracies between 1 ms and 1 µs may be considered for applications such as:
• Financial services auditing which must accurately timestamp financial data to comply with Sarbanes Oxley and recent standards developed by credit card service organizations
• Healthcare applications which require accurate time authentication to improve patient safety and meet requirements of regulations such as HIPAA and FDA
• Universities and colleges which must comply with FERPA and HIPAA regulations to authenticate records, digital signatures and secure accurate data
• Transportation services such as air, rail and bus which use time synchronization to improve operational efficiency
• Manufacturing facilities which must synchronize display clocks, monitor worker attendance and calibrate instruments
• Legal entities which use time synchronization for accurate billing, auditing, litigation and authenticating client messages.
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