Internal and External Clock Synchronization in a Power Line Network

The nontrivial problem of distributing time in communication networks is not only limited to the Internet world, where this is nowadays solved by NTP. As the digital divide gap has not only to be closed between the last mile to the end user’s PC and the nearest service point, also other systems like substation automation or energy meter management can benefit from a global notion of time as a service. As the usual high-performance Ethernet is, for the simple cost reasons of the cabling, not the most promising candidate to reach such nodes, other technologies such as power lines have to be investigated. This paper proposes a hierarchical clock synchronization scheme for power line networks. INTRODUCTION One very promising approach for low-cost clock distribution in large-scale systems is the usage of power line communication (PLC) systems. For this type of communication medium, clock synchronization is a crucial issue, not only because the PLC network itself requires synchronized clocks for maintaining timesliced communication, but also for backbone networks and access points to establish a coordinated, faulttolerant system-wide time base. This is needed in order to ensure fast log-on and log-off of nodes travelling from one access point to another. The problem of nodes and node groups disappearing on one side of the network and reappearing at different points, most likely at a different hierarchy as well, is caused by the usual way of energy suppliers switch whole net-groups from one transformer station to another, thus changing the logical hierarchy of the PLC network significantly. This paper will present an approach to synchronize clocks in such a system using the emerging standard IEEE 1588 [1]. In the lower levels of the hierarchical system, attention has to be paid to the special properties of the PLC network with respect to varying network topologies, short message length, and the like. To tackle this, a methodology using the IEEE 1588 format and protocol stack is presented.