The present invention is related to a local network, particularly an Ethernet network, having redundancy properties and a coupling device for such a network.
A local Ethernet network with redundancy properties is known from WO 99/46908. In the network described therein, coupling devices and a redundancy manager, each having at least two ports and being configured as layer 2 components, are interconnected in a ring-shaped topology by connecting two ports of adjacent devices. The term “layer 2 components” means that at least the layers 1 and 2 of the ISO OSI 7-layer model are implemented in the devices. The devices thus perform an address evaluation and message routing. In normal operation, i.e., if no errors have occurred in the network, the redundancy manager opens the ring. This means that messages received by the redundancy manager at its one port located in the ring are not forwarded via its other port located in the ring, but are blocked. This status is symbolized by an open switch. Thus, logically, the network has a linear topology in which the two line ends of the network are connected to a redundancy manager, which in the error-free case separates the two line ends from one another. In the event of an error, i.e., if an interruption occurs at some point in the network outside the redundancy manager, the redundancy manager connects the two line ends to again form a linear topology.
Physically, however, this is a ring-shaped network topology in which the devices form a ring by connecting two ports of adjacent devices. The redundancy manager ensures that at any given time the ring is interrupted only at a single location. To check whether there is already an interruption of the ring outside the redundancy manager, i.e., to check whether the network is operating error free, the redundancy manager sends test messages into the ring at defined time intervals via the two ports with which it is connected to the ring, and it opens the ring to form a linear topology with respect to the transmission logic if the test messages are received at its other port within an additional predefined time interval. Otherwise, i.e., if at least one test message has been received at the other port within the additional time interval, the redundancy manager closes the connection, such that the network has again a logically linear topology consisting of a continuous line. With respect to further embodiments and advantages of such a local network and a redundancy manager, reference is made to the above-cited publication WO 99/46908.
Networks in which messages are distributed from node to node and in which the response to the failure of a communication link between two nodes is to add a redundant communication path, have the risk that messages are copied or circulate within the network. Such errors can be triggered when a redundant communication path is mistakenly added. Thus, as a result of a malfunction of the conventional redundancy manager, the two line ends could inadvertently be connected and a logically ring-shaped network topology could be created. As a result, messages can circulate within the ring. In the worst case, the maximum bandwidth of the network available for the data traffic is occupied, such that further payload traffic among the users connected to the network can no longer be transmitted. When such an error occurs, the network is not available, or at least not fully available, for the connected users.