Enhancement for a clock synchronization protocol in heterogeneous networks

Communication networks for industrial automation are usually heterogeneous, i.e. a combination of different wired and wireless technologies. A possible setup could be a Real-time Ethernet (RTE) extended by a wireless local area network (WLAN) for all mobile system components. However, the required clock synchronization with IEEE 1588v2 for such a system can only be accomplished for the wired RTE part. Currently, incorporating standard network components without IEEE 1588v2 support (e.g. switches, wireless links, etc.) into a certain synchronization domain leads to a decreased synchronization accuracy which is unacceptable for most applications. Hence, this paper introduces a novel approach for a protocol enhancement to enable clock synchronization when having devices without IEEE 1588v2 support inside a synchronization domain. The proposed approach compensates non-deterministic forwarding delays of synchronization frames by using different send rates for Sync messages. Basically, the Sync message send rate on a connection with a high forwarding jitter has to be high, links with a smaller jitter can use a lower Sync send rate. In order to achieve this, synchronization devices with the ability to scale the aforementioned send rate are necessary and have been realized. They are evaluated in a real setup consisting of a RTE with a wireless extension.