Is Synchronous Ethernet a Must Have or just a Gimmick for the Broadcasting Industry? 

Over the last few years, the Precision Time Protocol (PTP) has evolved to become the preferred method of choice for accurate time transfer over Ethernet networks for every application domain. PTP being an IEEE standard (IEEE1588) has helped but was by no means the only reason for this development. Semiconductor and device manufacturers alike have been adding PTP hardware support to their network products – a mandatory requirement to reach sub-µs accuracies. Most importantly, PTP can be tailored to the specific requirements of an application domain via PTP Profiles – a feature many industries made extensive use of. The All-IP Studio, for example, uses the PTP broadcasting profile (SMPTE ST 2059-2) for accurate time transfer.

As a physical transport medium, Ethernet has superseded legacy solutions which were commonly used for many applications in the past. Ethernet is inherently asynchronous with only two adjacent nodes being synchronized with each other. This feature greatly simplifies deployment and maintenance and was possibly the driving factor of its success. When it comes to time and frequency transfer there is an obvious drawback. Accurate time must be transferred via a constant stream of packets, while frequency transfer cannot. Every end node must regenerate the frequency derived from the time information. This method has proven to be sufficiently accurate for many applications and is widely deployed, yet it has its limits concerning overall accuracy. If the quality of the time information deteriorates, the quality of the re-generated frequency will suffer as well. Specifically-optimized digital phase-locked loops can mitigate that effect but only to a certain extent. If end devices require accurate as well as highly stable frequencies for their operation, this limitation must be carefully considered.

To circumvent this problem, the local synchronicity of Ethernet can be extended to provide a common frequency for the complete network. How can this be accomplished? Whenever two devices establish a communication channel via a physical medium, a transport frequency must be provided by either of the two nodes to which the other must synchronize to. In standard Ethernet, the selection of the respective devices taking over that role is arbitrary. If, however, the selection process is made user-definable, a common frequency can be propagated through the complete network.

In this paper, we will describe synchronous Ethernet’s (SyncE’s) basic principles as specified by ITU. We will highlight the prerequisites of network devices to comply with SyncE requirements. Furthermore, we will focus on the software and system aspects of deploying and maintaining a SyncE network. Special consideration will be taken on how to best combine SyncE and PTP to improve both the accuracy and the resiliency of time and frequency transfer. Although SyncE was primarily designed to provide highly accurate time and frequency for modern telecom applications, we will analyze whether and to which extent the broadcasting industry can benefit from this technology. The paper concludes with real-world measurement in networks with SyncE and PTP support. We will highlight its performance under different operating conditions and demonstrate the impact of different failure modes. We will compare the performance of PTP with SyncE-assisted PTP.

Nikolaus Kerö | Oregano Systems; Nvidia; European Broadcasting Union | Vienna, Austria; Geneve, Switzerland
Thomas Kernen| Oregano Systems; Nvidia; European Broadcasting Union | Vienna, Austria; Geneve, Switzerland
Ievgen Kostiukevych | Oregano Systems; Nvidia; European Broadcasting Union | Vienna, Austria; Geneve, Switzerland