IEE1588v2’s Characteristics

The IEEE1588v2 protocol has been extensively tested in these scenarios and has proven viable. Yet it is very important for service providers and equipment vendors to understand IEEE1588v2 is a protocol only. The clock recovery algorithm is the synchronization solution.

1. IEEE1588v2 employs a two-way methodology, where packets are sent back and forth from the clock master to the clock slaves. This overcomes high-amplitude, ultra-low-frequency wander that defeats other methods such as adaptive clock recovery techniques. Also, the standard is virtually independent of the physical media and can flow over low-speed twisted-pair, high-speed optical fiber, wireless, or even satellite links without requiring equipment design modifications.
2. Additionally, it isn’t limited to TDM circuit emulation like the in-band solutions, but it can support CES better than adaptive clocking by distributing a precise network clock to every inter-working function node in the system. It also can be used for any pure, packet-based network, providing synchronization for future backhaul networks to be deployed by mobile operators.
3. The standard can distribute time/phase, frequency, or both. Telecom operators can use it to sell a synchronization service to customers (residential, wireless operators, etc.). It’s resilient because a failed network node can be routed around. It also is resilient because the synchronization can come from one or more grandmaster clock nodes.
4. IEEE1588v2 packets fully comply with Ethernet and IP standards and are backward compatible with all existing Ethernet and IP routing and switching equipment. There is no requirement for intermediate switches or routers to be IEEE1588v2 aware. They see these timing packets as normal packet data.

The protocol calls for synchronization packets with time stamps to be sent from master clocks to all slave clocks and for individual slave clocks to send time-stamped packets to the master (Fig. 1). The clock grandmaster maintains a time base locked to a primary reference clock and establishes a separate synchronization session with each of the slaves it serves. The master and slave exchange timing packets according to the syntax of the IEEE1588v2 protocol.

This process provides the timing clock recovery algorithm with the time stamps it needs to precisely recreate the master time base. From this time base, the synchronization signals used by the network equipment are synthesized. The timing clock recovery algorithm filters most noise, packet queuing delay, and propagation delay created by the transport network.

The semiconductor suits all wired and wireless service provider network applications. Typically, each synchronization session produces approximately 15 kbits/s of traffic (in each direction, uplink and downlink), which represents a small portion of the total capacity, usually between 100 Mbits/s and 1 Gbit/s, in a backhaul network.

In April 2005, the world’s first IEEE 1588 proof of concept field trial began using IEEE1588v2 slave and master products from Semtech. The company’s ToPSync slave and master devices are fully integrated 1588 devices with built-in protocol support, IP stack, Ethernet frame control, clock recovery, and output generation.

Figure 2 illustrates the test bed of the live trial. Two boards, acting as pre-standard IEEE1588v2 master and slave, were connected to a live metro Ethernet backbone operated by a major U.S. carrier. The master was locked to an atomic clock. The synchronization packets travelled through the metro Ethernet before reaching the slave board, which was connected to an Agilent Omniber 718 measuring the time interval error.

Figure 3 shows the results of the measured maximum time interval error (MTIE). The top picture shows the MTIE against the G.823 synchronization interface mask for an E1 circuit as used in a non-North American GSM basestation. A North American GSM basestation would have a G.824 or T-1 mask target. The measured MTIE is well below the mask, confirming the excellent clock recovery capabilities of the algorithm.

Many carriers have tested the TopSync technology in live field tests, including an extensive European test completed by Vodafone that was presented at the November 2007 International Telecoms Synchronization Forum in London. The testing suite Vodafone used followed the ITU G.8261 specification with some special implementations unique to the company. These results confirmed the performance of the device.

Designing 1588 Into a System

When designing a 1588 solution into a network node as a slave device, the most critical thing to keep in mind is the time stamp located in the media access control (MAC) layer as described in the IEEE 1588-2008 standard. This 64-bit value is the “instant in time value” of the transmitting node’s time base for a 1588 event packet.

As the master and slave devices exchange packets per the IEEE 1588 message syntax, the differences in flight time or packet delay variation period between time stamps will affect the slave clock recovery systems. The system’s architects and designers need to control local system propagation from ingress port to 1588 slave input.

Whatever local system delay does exist needs to be deterministic and as input/output time symmetrical as possible. These design rules minimize the propagation effect from local systems on packet delay variation seen at the ingress of the slave clock. The same architecture rules apply in designing in a master device, even though the master does not perform clock recovery. The outbound delay imposed on the sync messages will be part of the overall packet delay variation seen by the slave. The greater the determinism in this path, the less its impact on the slave’s clock recovery.

When the system is ready for system testing, the performance template is the ITU G.8261 recommendation titled “Timing and Synchronization aspects in Packet Networks.” This document provides a model of a potential service provider carrier Ethernet network with loading parameters as well as target performance masks.

This testing recommendation provides a suite of timed tests with a change in load on both paths, master to slave and slave to master. The test suites and performance masks to date only apply to delivery of frequency from the 1588 slave device. This form of testing is a reasonable means to determine basic system behaviour under closely controlled conditions. Figure 4 shows the overall testing template from the G.8261 document.

It is important to note that G.8261 is a work in progress and as yet does not contain the specific performance targets or the network testing models for IEEE 1588-2008. Figure 4 is the network model being contemplated by the ITU.

Conclusion

Synchronization is an important part of today’s wireless backhaul networks. With the IEEE1588v2 protocol, carriers can achieve synchronization with accuracy matching that of alternative solutions without the cost or need to build overlay networks required by those solutions. This standard provides an essential technology that allows carriers to efficiently deploy IP networks for their wireless backhaul driving down the infrastructure costs of deploying 3G and 4G networks. With field trials showing interoperability and performance, IEEE1588v2 is a proven solution for IP synchronization.

Patrick (Pat) Diamond has more than 30 years of experience in the high-tech field.

He was instrumental in the creation of the IEEE 1588-2008 Precision Time

Protocol working group in 2004. He represents Semtech to the ITU-T,

IEEE-USA and has been with Semtech for more than eight years. He can be reached at pdiamond@semtech.com.