L-PTP: a Novel Clock Synchronization Protocol for Powerline Networks L. Lo Bello, A. Raucea, G. Patti, O. Mirabella Department of Electrical, Electronic and Computer Engineering – DIEEI University of Catania - Italy {lucia.lobello, antonino.raucea,orazio.mirabella}@dieei.unict.it, gaetanopatti@pgprojects.it Abstract This paper proposes a Lightweight version of the Precision Time Protocol, called L-PTP, for implementation on Powerline communication systems. The aim of this novel protocol is to reduce the number of messages exchanged to achieve synchronization, without penalizing its quality. L-PTP introduces several modifications to the original PTP and can be implemented on COTS devices. The paper focuses on the protocol and on the benefits of its combination with a virtual clock computation performed by a dynamic clock synchronization algorithm. 1. Introduction Powerline networks (PLNs) [1] are natural candidates for smart-grids and energy management systems, as they allow for data transmission over already deployed power lines without the need to build new network infrastructures, thus reducing additional costs. As narrowband Powerline networks offer a low data rate, for the sake of efficiency, the packet size and the number of exchanged messages should be minimized. PLNs to be used in smart grids and energy management systems also call for customized clock synchronization protocols. A common view of time is required, for instance, for the synchronization of voltage and current phasors of the distributed energy resources of a micro-grid working in island mode (autonomous), as a timing reference to dynamically adjust the power line frequency [2] is needed. According to the European CENELEC EN 50160 regulations, voltage frequency must be 50Hz ±1% (i.e., from 49.5Hz to 50.5Hz) which corresponds to a clock accuracy of about 198 microseconds. Clock synchronization is also needed to support several applications as for example smart metering applications, which require that sensors values be acquired at the same time. A suitable candidate for synchronization in PLNs is the IEEE 1588 standard, that defines a protocol known as the Precision Time Protocol (PTP) which provides precision clock synchronization [3] and it is widely used for its accuracy and simplicity. Implementations of IEEE 1588 on Powerline networks usually [4] use special hardware to timestamp packets in order to minimize the delay introduced by the different clocks in read/store operations. This approach improves the synchronization accuracy, but it is not cost-effective as custom hardware is required. In this work we propose a lightweight version of the Precision Time Protocol, called L-PTP, for implementation on bus networks based on Powerline communication. The deployment costs has been reduced by using existing hardware, i.e. commercially available Off-The-Shelf (COTS) narrowband embedded devices. As COTS embedded devices feature low data rate (2.4Kbps/s) and bandwidth saving is a mandatory requirement, the L-PTP here proposed aims to reduce as much as possible the number of messages exchanged to achieve synchronization, while still achieving an accurate synchronization. Moreover, L-PTP has been combined with an approach based on Dynamic Continuous Clock Synchronization (DCCS) [5] that, through the calculation of a virtual clock, reduces the rate of synchronization cycles, thus allowing for an efficient use of the limited bandwidth the embedded devices are equipped with. The L-PTP protocol is well suited for small Powerline networks (e.g., a cluster of generators in a micro-grid) where all nodes are visible to each other, so that broadcast communication can occur. In the following, we describe the L-PTP and its implementation on COTS devices, and present a preliminary performance evaluation in two flavors, i.e., L-PTP alone or combined with a DCCS-based virtual clock computation. Finally, we outline future work. 2. The Lightweight Precision Time Protocol To understand the novel L-PTP here proposed, a brief overview on the original PTP as defined in the IEEE 1588 standard is needed. In PTP, the synchronization process is divided into two phases, as shown in Fig.1. In the first phase, the time difference (offset) between the master and a slave is calculated as follows. The master sends a sync message and takes the transmission timestamp t 1 . After reception of the sync message, the slave stores the timestamp of the message arrival time t 2 . Later, the master inserts the value t 1 into a follow_up