Who doesn't need Ethernet timestamps?
Integrated MAC- or PHY-layer hardware timestamp
Timestamp accuracy directly influences the resulting clock synchronization accuracy. Software-only timestamps based on a UDP/IP socket interface are considerably less accurate than hardware timestamps directly mapped into the interrupt service of an Ethernet controller.
Figure 2: MAC or PHY-based hardware timestamp significantly improves clock synchronization accuracy
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Generating timestamps closer to the wire reduces delay and software stack jitter to increase clock synchronization accuracy (Figure 2). Hardware timestamps are typically generated in a timestamp unit (TSU) included in the Media Independent Interface (MII) which operates together with the real-time clock hardware (Figure 3).
For example, laboratory tests of an evaluation board for the Freescale MPC8360 PowerQUICC processor running a Linux operating system and version 2 of the IXXAT IEEE 1588 application software gave a timestamp resolution of 8 nanoseconds (ns) with standard deviation +/- 7 ns and maximum deviation +/- 30 ns.
Figure 3: IEEE 1588 implementation with IEEE 1588 hardware timestamp in Ethernet MAC
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Hardware timestamps with accuracies better than 1 µs may be considered for applications such as:
• Telecommunication remote switches, digital loop carriers, mobile telephone switching stations, base stations and enterprise PBXs/routers, which need an accurate and stable timing source to support clear communication over long distances
• Digital video broadcasting, which requires all transmitters in a Single Frequency Network to be time synchronized to give identical emitted signals
• Data network switches which must be time synchronized to reduce jitter and delay
• Geospatial intelligence for imagery and analysis of geospatial information requires accurate time synchronization of video feeds
• Aerospace and defense applications which require accurate time synchronization for network timing, radar systems, traffic control, test ranges, satellites, monitoring and ground based instrumentation
• Building automation for synchronizing video, alarms, access, records and reporting
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