Abstract:
The invention relates to a method for protection switching in a data network network including at least two rings (ERA, ERB) with a shared span (SP 2,3 ) and a plurality of nodes ( 1 - 4 ). Each ring (ERA, ERB) has an associated ring manager (RMA, RMB) for checking its associated ring (ERA, ERB) and the ring managers (RMA, RMB) have dedicated different priorities (PR 1,  PR 2 ). When a failure (interruption) occurs in a shared span, a shared span node ( 2, 3 ) sends a failure message (FSP 2,3 ) to a ring manager (RMB) of higher priority (PR 1 ) of an associated ring (ERB), and the ring manager (ERB) unblocks its port (PB 2 ) while the ring manager (RMA) of lower priority (PR 1 ) maintains its port (PA 2 ) blocked.

Description:
CLAIM FOR PRIORITY 
       [0001]    This application is a national stage application of PCT/EP2006/062537 filed on May 23, 2006, which claims the benefit of prior to EP 05011736 filed on May 31, 2005, the contents of which are hereby incorporated by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for protection switching in a data network. 
       BACKGROUND OF THE INVENTION 
       [0003]    Data networks including a plurality of data rings share certain ring spans to enable protection switching and, therefore, prevent single points of failures. However, if Ethernet technology or a similar technology is chosen for data transmission such a topology creates additional problems, because it is necessary to prevent the formation of loops in the network. 
         [0004]    The principle of the Ethernet ring protection mechanism ERP is described in U.S. Pat. No. 6,430,151. A redundancy manager, which is connected to the line ends of a ring containing a plurality of nodes, checks the network by transmitting and receiving test packets. If there is an interruption in the ring, the redundancy manager connects the line ends. 
         [0005]    This method is described in detail for one or more rings having a common node, in U.S. Pat. No. 6,766,482 B1 assigned to Extreme Networks and is known as Ethernet Automatic Protection Switching” EAPS. 
         [0006]    An Exsteme networks, Santa Clara, Calif. (US) “ExtremeWare 7.1.0 User Guide”, pages 281-290 describes two data rings with shared nodes and a shared link. One ring includes a controller; the other ring includes a partner. If the common link fails the controller is responsible for blocking to prevent a “super loop”. Before the common link comes back the controller goes from a blocking state to a “preforwarding” state, where the ports are still temporarily blocked to prevent a super loop. This protection procedure requires additional exchanges of different control packets between the controller and the partner. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a method which leads to a faster and simpler protection switching. 
         [0008]    The present invention provides a simpler method by allocating different priorities to ring managers and by a simpler protocol including alarm packets which are sent to the ring managers having higher priority. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention is described by way of different examples and accompanying drawings. The invention can be extended to a more complex network and to a network similar to the Ethernet. The invention will became more apparent with reference to the following detailed description along with the accompanying drawings, in which: 
           [0010]      FIG. 1  shows an exemplary two ring network with two ring managers. 
           [0011]      FIG. 2  shows a failure in a common span and the new network structure after protection switching. 
           [0012]      FIG. 3  shows an exemplary structure of more complex network with a plurality of shared spans and multiple failures. 
           [0013]      FIG. 4  shows an exemplary structure of a more complex network with a plurality if shared spans and multiple failures. 
           [0014]      FIG. 5  shows an exemplary network structure after protection switching. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  shows a network having two rings. A first ring ERA with nodes  1 ,  2 ,  3  including an associated ring manager RMA, and a second ring ERB with nodes  2 ,  3 ,  4  including an associated ring manager RMB having a higher priority PR 1  than the ring manager RMA with priority PR 2 . Each port of a ring manager or a node can transmit and receive dates. Both rings ERA and ERB share the span SP 2 , 3  between the “common span nodes”  2  and  3  (a common span node is a node, which has at least a port P 33 , P 23  connected to the common span, or is exceptionally inserted between these “end nodes” of the common span). According to the Ethernet requirements each ring manager RMA, RMB blocks one of its ports, e.g. PA 2  and PB 2 , so there is no connection between the ports PA 1  and PA 2  or PB 1  and PB 2 , which are shown on different sides of the ring managers in the drawing, and therefore (data) loops are avoided. 
         [0016]    Each ring manager supervises its associated ring. Ring manager RMA supervises Ring ERA by sending “test packets” TPA at the first data port PA 1  and receiving these “test packets” at the second port PA 2  over a virtual control local area network known as VCLAN, e.g. realized by a different wavelength or time slot on the same fiber, where the data is transmitted. An interruption, e.g. between node  1  and node  2 , would prevent the ring manager RMA from receiving its own test packets TPA. The ring manager RMA would determine “loss of test packets” LOTP and unblock the second port PA 2 . After this protection switching node  1  is connected over ring manager RMA with node  3  and therefore with the network. The ring ERB is controlled in the same way by sending test packets TPB. 
         [0017]    When the common span SP 2 , 3  between its common span nodes  2  and  3  is interrupted according to  FIG. 2 , the common span nodes  2  and  3  will detect it (no signal is received) and send a “failure message” FSP 2 , 3  to the ring manager RMB of higher priority PR 1 , which unblocks its second port PB 2 . The common span nodes know from their own management data base or from a management system, which ring manager has the highest priority. The unblocking of ring manager RMB leads to the network shown in  FIG. 2 , where all nodes are connected according to this “protection switching” of ring manager RMB. A “super loop”, shown as a dotted line, is avoided, because the second port PA 2  of ring manager RMA remains blocked. The failure message is also transmitted over the virtual control local area network. 
         [0018]    The test packets periodically send by ring manager RMA have an interval of e.g.  50 ms between two consecutive test packets and at least two lost packets have to be monitored before the ring manager RMA determines an alarm signal. The protection switching, that is unblocking of the ports of ring manager RMB, occurs in a much shorter time and the test packets of ring manager RMA will be forwarded by ring manager RMB back to ring manager RMA. So the ring manager RMA sees a complete ring and keeps its second port RA 2  blocked. It is sufficient and in a more complex network favorable, when the test packets are only transmitted over rings of higher priority. The detection “loss of test packets” LOTP of ring manager RMB has no consequences, because this ring manager has already unblocked its ports. The transmitting of test packets could be stopped, when the ports are already unblocked. 
         [0019]    After the detection of an interruption of the shared span SP 2 , 3  the ports P 23 , P 33  (at least one port) of the common span nodes  2  and  3  facing the common span SP 2 , 3  are set to a “preforward” state PFW as shown in  FIG. 3 , where these ports are blocked. When the common span has recovered, e.g. is repaired, and the ring manager RMB blocks it second port PB 2 , the common span nodes  2  and  3  will exit this preforward state PFS and unblock their ports (e.g. controlled by the ring manager RMB), so that the original configuration shown in  FIG. 1  is re-established. 
         [0020]      FIG. 4  shows a more complex network with four rings and three common spans SP 2 , 3 ; SP 4 , 5 ; SP 3 ,C and four ring managers RMA, RMB, RMC, and RMD associated to four rings ERA, ERB, ERC, ERD, each ring manager has a different priority PR 3 , PR 1 , PR 2 , and PR 4 . This priority is allocated to the associated rings. Only a few nodes  1 - 7  are shown. The ring managers are inserted in the links of the rings. The common spans are between two nodes of adjacent rings, e.g. ERA, and ERB, or between a ring manager, e.g. RMC and a node  6 . One port of all ring managers is blocked to avoid a loop. The priority of the ring manager is higher in the centre of the network and becomes lower at the edges. 
         [0021]    To make this example more complicated all common spans SP 1 , SP 2  and SP 3  may be interrupted at the same time as shown in  FIG. 5 .
       1 The nodes  2 ,  3  (belonging also to ring ERA of priority PR 3 ) and the nodes  4 ,  5   3  (belonging also to ring ERC of priority PR 2 ) send their “failure (interruption) messages” FSP 2 , 3  and FSP 4 , 5  to the associated ring manager RMB of higher priority PR 1 . Ring manager RMB unblocks his open port PB 2  at time (1).   2 About the same time (2) ring manager RMC of detects the “link down” of common span SP 3  and unblocks its open first port PC 1 .   3 All test packets are forwarded to rings of higher priority. So the test packets of the ring manager RMA respectively ring ERA is forwarded to rings ERB and ERC, and the test message of ring ERD is forwarded to ring ERC, ERB, and ERA. The test packets of ERB are restricted to its own loop ERB. Only the test packets of the rings ERA and ERD are necessary for the protection function because the ports of RMB and RMC are already unblocked. Therefore, the transmission of the test packets of the other ring managers could be stopped. At this time the ring managers RMA of the ring ERA and RMD of the ring ERD are transmitting test packets TPA and TPD. The test intervals for transmitting consecutive test packets of ring managers with lower priority are chosen to be longer than the test intervals of test packets being transmitted by ring managers of higher priority. Therefore, the ring manager RMA with priority PR 3  determines “loop down” LOTP before ring manager RMD with priority PR 4  and unblocks its second port PA 2  at time (3). The test packets of ERD are transmitted over RMC, nodes  5  and  4 , RMA, nodes  1 , 2 , RMB, nodes  4 ,  6  and  7  and received at the blocked port.       
 
         [0025]    In a variation of the method test packets may send to all rings, but they are only transmitted over ring managers with higher priority. Also the failure message FSPXX may be sent to all ring managers, but only the associated ring manager of the broken shared span with higher priority will accept it. This is only a question of the virtual VCLAN and does not influence the function.
     1 ,  2 ,  3 , node   ERA ring A   RMA ring manager A of ring ERA   SP 2 , 3  shared span between nodes  2  and  3     FSP 2 , 3  failure message of shared span SP 2 , 3     TPA test packet of RMA   PFW preforward state