XLi IEEE 1588 Grandmaster

Why IEEE 1588?
IEEE 1588 protocol overcomes Ethernet time degradation caused by network and application latency and jitter through hardware time stamping at the physical layer of the network. The results are two profound benefits.

1. With time transfer accuracy in the 10-100 nanosecond range, IEEE 1588 provides a significant improvement in time synchronization accuracy over network time protocol (NTP) and IRIG time code.

2. IEEE 1588 offers major cost savings in time distribution cable since it is deployed using hardware clocks with standard Ethernet LAN interconnect components between them. Regular hubs, switches and category 5 cable comprise the network infrastructure.

The Symmetricom Grandmaster is IEEE 1588-2002 compliant. It time stamps PTP packets with a time stamp accuracy of 30 RMS nanoseconds to UTC. (The XLi GPS synchronized clock is accurate to 30 nanoseconds RMS to UTC while tracking GPS satellites). Measured synchronization accuracy at a PTP client has been shown to be as good as a 17 nanosecond offset from the XLi Grandmaster.

The XLi Grandmaster contains a dedicated high-speed 1588 time stamp processor. Operating at 100Base-T line speed with deep time stamp packet buffers, the XLi Grandmaster can support thousands of 1588 clients. This is made possible in part by sending 1588 Sync & Follow_Up messages periodically using multicast addressing, and in part by being able to quickly and accurately process 1588 client initiated unicast Delay_Req and Delay_Resp messages.

The IEEE 1588 Grandmaster capability is incorporated on an XLi option module which facilitates field upgrades to Symmetricom's XLi GPS Time and Frequency System.

Time Interval Measurement Capabilities
The Xli Grandmaster takes into account those network infrastructure elements such as hubs and switches that have an effect on time transfer accuracy when using 1588. Hubs and particularly switches add nondeterministic latencies and jitter to the packet transit times from a 1588 master to 1588 slaves. As a result, the 1588 slave synchronization accuracy is degraded somewhat from that of the master. Hubs interfere the least whereas an Ethernet switch can easily add 100’s of nanoseconds of nondeterministic delay to packets transiting the network. (Delay from routers renders IEEE 1588 protocol no better than NTP protocol in terms of synchronization accuracy).

To aid in characterizing the delay and jitter of network elements such as a switch, the XLi Grandmaster comes standard with Time Interval measurement capabilities. 1588 slave clocks typically output a 1PPS on-time signal. By synchronizing the slave via 1588 with and without the switch in-between, and analyzing the slave 1PPS with the XLi Grandmaster Time Interval function, the latency of the switch can be characterized. This is very useful in estimating the time degradation effects of the switch (or any network element) before deployment in the network.

Master and Slave
The XLi Grandmaster also operates as a 1588 slave on the network. Time derived from another 1588 master clock can be the primary or secondary time reference in the XLi Grandmaster. This is a means by which accurate time can be transferred over Ethernet to the XLi and, for example, then output as IRIG B time code.

The XLi Grandmaster is also useful in measuring the network accuracy of existing 1588 infrastructure where a GPS master clock is already deployed. Simply use the GPS referenced XLi to measure the 1PPS output from the 1588 slave using the Time Interval function.