Abstract:
In prior art, the provision of back-up circuits for ATM cells in ring-type structures is efficiently controlled using linear structures. According to the invention, this function is extended to ring-type MPLS network architectures as follows: linear MPLS structures are formed into a ring-type MPLS structure and two unidirectional MPLS connections which respectively connect the same switching devices and run in opposite directions are logically associated to one another, whereby the operating link and the back-up link are routed via different physical paths.

Description:
CLAIM FOR PRIORITY 
     This application claims priority to International Application No. PCT/EP01/00434 which was published in the German language on Sep. 7, 2001. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a circuit arrangement for the protection switching of transmission devices in ring architectures carrying MPLS packets. 
     BACKGROUND OF THE INVENTION 
     A circuit arrangement for the protection switching of transmission devices in ring architectures is known from German patent application DE 197 039 93.6. 
     This circuit arrangement relates to transmission devices via which information is conducted in accordance with an asynchronous transfer mode (ATM). In this arrangement, a transmission device for the bidirectional transmission of digital signals is provided in which two switching devices acting as terminal stations are connected to one another via a multiplicity of operating links and one protection link. The two terminal stations in each case contain monitoring devices for detecting transmission disturbances. A switching system, which can be controlled by a monitoring device, connects a receiving device to the operating link in a first switching state and to the protection link in a second switching state. 
     The disadvantageous factor of this circuit arrangement is that it relates exclusively to ATM transmission devices. In the Internet, information is supplied to the receiving subscriber via a multiplicity of network nodes which can be constructed as routers. Between the routers, MPLS networks can be arranged. However, there is no mention whatsoever of MPLS networks in the known circuit arrangement. 
     SUMMARY OF THE INVENTION 
     The invention relates to a circuit arrangement in such a manner that information which is transmitted in accordance with an Internet protocol can be transmitted with great reliability over a multiplicity of network nodes. 
     One advantageous embodiment in the invention is that a multiplicity of linear transmission sections formed from MPLS switching devices are joined to form a ring system. In this arrangement, a transmission section is formed by an operating link and/or a protection link. Furthermore, two oppositely directed unidirectional MPLS connections which in each case connect the same MPLS switching devices are logically associated with one another, and operating link and protection link are conducted via different physical routes. The MPLS switching devices are constructed as label switched routers. This is associated with the advantage that in the case of a fault on the operating link, MPLS connections can be maintained efficiently in such evolved ring systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the text which follows, the invention will be explained in more detail with reference to an exemplary embodiments. 
     In the figures: 
     FIG. 1 shows an MPLS network linked into the Internet. 
     FIG. 2 shows a configuration for transmitting MPLS packets in a linear 1+1 structure. 
     FIG. 3 shows a configuration for transmitting MPLS packets in a linear 1:1 structure. 
     FIG. 4 shows the circuit arrangement according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows by way of example how information coming from a subscriber TLN 1  is supplied to a subscriber TLN 2 . The transmitting subscriber TLN 1  is connected to the Internet network IP through which the information is conducted in accordance with an Internet protocol such as, e.g., the IP protocol. This protocol is not a connection-oriented protocol. The Internet network IP exhibits a multiplicity of routers R which can be intermeshed with one another. The receiving subscriber TLN 2  is connected to a further Internet network IP. Between the two Internet networks IP, an MPLS (Multiprotocol Packet Label Switching) network is inserted through which information is switched through in a connection-oriented manner in the form of MPLS packets. This network exhibits a multiplicity of mutually intermeshed routers. In an MPLS network, these can be so-called label switched routers (LSR). One of the routers is designated as transmitting device W and another one is designated as receiving device E. 
     MPLS packets in each case have a header and an information section. The header is used for accommodating connection information whereas the information section is used for accommodating user information. The user information used is IP packets. The connection information included in the header is arranged as MPLS connection number. However, this has validity in the MPLS network. When thus an IP packet from the Internet network IP penetrates into the MPLS network, the header valid in the MPLS network is appended to it. This includes connection information which predetermines the path of the MPLS packet in the MPLS network. If the MPLS packet leaves the MPLS network, the header is removed again and the IP packet is routed further as determined by the IP protocol in the Internet network IP following it. 
     FIG. 2 shows by way of example two nodes of an MPLS network in a linear configuration which are in each case arranged as switching device W, E. In the present exemplary embodiment, it is assumed that these switching devices are MPLS cross-connect switching devices. Using switching devices of such a construction, however, does not signify a restriction of the invention and other switching devices such as, e.g., ATM. switching devices can similarly be used. In FIG. 2, MPLS (Multiprotocol Label Switched) packets are then to be transmitted from the switching device constructed as label switched router W to the switching device constructed as label switched router E. 
     FIG. 2 shows a linear 1:1 structure. It also shows the case of bidirectional transmission. The transmission of MPLS packets in the MPLS network is defined as being unidirectional. Accordingly, a total of two “connections” (one for the forward direction and one for the reverse direction) should be set up for the forward and reverse transmission of MPLS packets, belonging to a connection WT, between the label switched router W and the label switched router E in the case of bidirectional transmission. A “connection” in the MPLS network is called a Label Switched Path (LSP). 
     The label switched routers W, E are connected to one another via an operating link (WORKING ENTITY) and one protection link PE (PROTECTION ENTITY). The operating link WE may also optionally be constructed from a plurality of operating links. Furthermore, switching systems S (BRIDGE) are shown via which the incoming MPLS packets are received and the operating links WE are transmitted toward the label switched router E. Furthermore, FIG. 2 shows selection devices SN, the task of which is to supply the MPLS packets transmitted via the operating link WE to the output of the label switched router E. According to the present exemplary embodiment, the selection devices SN are constructed as switching network. The switching network SN is arranged both in the label switched router W and in the label switched router E. 
     Furthermore, monitoring devices ÜE 0 , ÜE 1 , (PROTECTION DOMAIN SINK, PROTECTION DOMAIN SOURCE) which monitor the state or the quality of the MPLS packets transmitted via the operating links WE are shown in the two label switched routers W, E. For example, the MPLS packets of the connection with the number  1  WT 1 , before they are transmitted via the operating link WE toward the label switched router E, are provided with control information in the monitoring device ÜE 1  of the label switched router W, which control information is extracted and checked by the monitoring device ÜE 1  of the receiving label switched router E. Using this control information, it is then possible to determine whether the transmission of the MPLS packet has been correct or not. In particular, a total failure (SIGNAL FAIL FOR WORKING ENTITY) of the operating link WE can be determined here. Similarly, degradations in the transmission quality (SIGNAL DEGRADE) however can also be determined by using known methods. 
     The monitoring devices ÜE 1  terminate the operating link WE at both ends. Other monitoring devices ÜE 0  are arranged at both ends of the protection link PE. In the case of a fault, this is to be used as transmission link for the operating link WE taken out of operation. Furthermore, protection switching protocols ES are transmitted via this link so that the integrity of the protection link has top priority. 
     In each of the label switched routers W, E, central controllers ZST are also arranged. These contain in each case local and global priority tables in which the status and priority of the local label switched router W (local priority table) and the status and priority of the local and of the remaining label switched router E (global priority table) are conducted. The introduction of the priorities has the result that when a number of protection switching requests occur at the same time, that operating link which is to be protection-switched when a number of operating links are present is specified. Similarly, the protection switching requests are prioritized in the priority tables. Thus, for example, there is a high-priority request from a user. Since this protection switching request is assigned a high priority, it is thus controlled with preference. A protection switching request which is controlled by one of the operating links and is assigned a lower priority will thus be rejected. 
     The central controllers ZST of the label switched routers W, E exchange information in a protection switching protocol ES. This protocol is transmitted via the protection link PE and extracted by the associated monitoring device ÜE 0  from the respective receiving label switched router, and supplied to the relevant central controller ZST. Furthermore, the central controller ZST ensures that the switching devices S are appropriately driven in the case of a fault. 
     In the protocol ES, information K 1 , K 2  is stored. The former is information relating to the protection switching request generated, while the latter is information relating to the current states of the switching systems. The protocol ES is in each case exchanged between the two label switched routers W,E when a protection switching request is generated. In a special embodiment of the invention, there is provision for the protocol ES to be transmitted cyclically between the two label switched routers W, E. 
     FIG. 3 shows a further linear structure by means of which MPLS packets can be protection-switched. This is a1: 1  structure. The difference compared with the 1+1 structure shown in FIG. 2 is that the bridge devices S shown there are constructed as switching devices S 0 , S 1 . The selection device SN is also constructed as switching network here, as in FIG.  2 . The MPLS packets conducted via the protection link PE are supplied to this switching network. Here, the logical MPLS connection number is taken from the packet header, evaluated and switched through the switching network. In this case, there is thus no driving of switching systems in the receiving switching device. The two label switched routers W, E according to FIG. 3 also contain central controllers (not shown) with local and global priority tables. 
     The protection link PE can remain unused during this time. If necessary, however, it is also possible to supply special data (EXTRA TRAFFIC) to the label switched router E during this time. 
     The protocol ES is arranged differently from the first case. Thus, further information is stored here in addition to the information regarding the protection switching request generated, already discussed in the first case. This is information with respect to the current states of the switching devices S 0 , S 1 . In the case of a fault, the switching devices S 0 , Smust be correspondingly controlled. The protocol is in each case exchanged between the two label switched routers W, E when the protection switching request is generated. In a special embodiment of the invention, it is provided to transmit the protocol ES cyclically between the two label switched routers W, E. 
     FIG. 4 shows the circuit arrangement according to the invention (dedicated protection). Dedicated protection means that the bandwidth per MPLS switched path is permanently reserved both on the operating link WE and on the protection link PE. The label switched routers are connected in such a manner that a closed ring is produced. According to the present exemplary embodiment, this ring is to be configured from linear connection elements, as shown according to FIG. 2 or FIG.  3 . According to the present exemplary embodiment described in FIG. 4, the ring is to be formed from the linear 1+1 structures shown in FIG. 2 without this being intended to indicate a preference for this structure. 
     FIG. 4 shows label switched routers N A , N B  , N C  and N D  Two of these label switched routers in each case terminate transmission sections. Using the example of the label switched routers N A  and N D , these are the connection elements WE A-D  and WE D-A . Using the example of the label switched routers N A  and N B , N C  or N C , N D , these are the connection elements PE A-D  and PE D-A . 
     According to FIG. 4, each of these label switched routers exhibits a bridge device S and a selection device SN. The label switched routers are intended to be connected in such a manner that the respectively active operating links WE A-D  and WE D-A  are arranged between the label switched routers N A  and N D . The MPLS packets arriving via the connection WT A-D  and WT D-A  are then conducted via these operating links. In contrast, the protection links are conducted from the label switched router N A  via further label switched routers N B , N C  to the label switched router N D , where the MPLS packets leave the ring in order to be supplied to other devices. 
     Furthermore, the ring formed by the label switched routers is arranged to be bidirectional in FIG.  4 . 
     The reverse direction of the connection WT A-D  is formed by the connection WT D-A . To obtain a better understanding, it should be noted that both connections are dealt with separately even though this is a bidirectional connection. An essential factor is that in each case exactly one protection link is allocated to the respective operating link. Thus, the protection link PE A-D  is allocated to the operating link WE A-D  and the protection link PE D-A  is allocated to the operating link WE D-A  acting as reverse direction. Furthermore, a case of unidirectional transmission is also possible as an embodiment of the configuration disclosed in FIG.  4 . 
     Furthermore, monitoring devices—not shown in FIG.  4 —are arranged in the individual label switched routers. These in each case terminate the operating links WE A-D  and WE D-A  and the protection links PE A-D  and PE D-A . Furthermore, MPLS “connections” (label switched paths) conducted via the same physical path are logically combined to form a group and two protection switching connections are created for this group. The first of these protection switching connections is conducted via the operating link WE (MPLS protection switching LSP (label switched path)) as a result of which it is conducted via the same physical path between the label switched routers W and E as all associated individual connections. The second of these protection switching connections is set up via the protection link PE. 
     In the group protection switching method, the two protection switching connections are now monitored for failures and disturbances in the monitoring devices ÜE 1 , ÜE 0 . The individual connections are no longer monitored. In the case of a protection switching request, the priority-controlled protection switching decision is made in the local priority logic, as before. In the protection switching case, however, individual connections belonging to a group are jointly switched over by the switching device SN. During this process, a single protection switching protocol needs to be run over the protection link PE. 
     An advantageous factor is that a multiplicity of individual connections can be monitored and protection-switched by a single protection switching connection and a single protection switching protocol in order to thus be able to respond appropriately to the fault cases occurring most frequently in practical operation. Furthermore, one protection switching protocol is entered in the local priority table. This has the advantage that the protection switching protocol is transmitted once in the case of a fault in an operating link. This assumes that otherwise it would be necessary to transmit one protection switching protocol per MPLS path number. However, this would lead to a dynamic loading of the ring due to the multiplicity of MPLS connections. Since, however, a plurality of connections having the same MPLS connection number use the same transmission section and, as a rule, a possible disturbance affects the complete transmission section, logical grouping of the MPLS connection number to form a logical group number is advantageous. 
     In the text which follows, it will be assumed that the connection WT A-D  requires that the associated MPLS packets are supplied to the ring via the label switched router N A  and leave it again via the label switched router N D . In this case, the MPLS packets belonging to the connection WT A-D  thus supplied to the bridge device S arranged in the label switched router N A . Since this is permanently set, the MPLS packets are supplied to the label switched router N D  both via the operating link WE A-D  and via the protection link PE A-D  and leave the ring there. 
     In the trouble-free case, i.e. when there is no operating failure on the active operating link, the MPLS packets are routed directly to the label switched router N D  from the label switched router N A . If, however, an operating failure occurs here, this is determined by the monitoring device arranged in the receiving label switched router. In the present case, this is to be the label switched router N D . The latter thereupon immediately supplies the protection switching protocol via the associated protection link PE A-D  to the transmitting label switched router, that is to say label switched router N A . At the same time, the selection device SN is controlled into the operating state which accepts MPLS packets via the protection link PE A-D . 
     The monitoring devices also check the operating state on the protection links. If, for example, MPLS packets are transmitted via the operating link-WE A-D  and the receiving label switched router, that is to say label switched router N D , determines a case of a fault on the associated protection link PE A-D , the transmitting label switched router N A  is informed of this via information stored in the protection switching protocol. This prevents a changeover to a faulty protection link PE A-D  in the case of an additional fault on the active operating link WE A-D . 
     According to the invention, the protection switching protocol ES is exchanged when faults occur, but a cyclic exchange can also be controlled. Similarly, the protection switching protocol can be exchanged in a connection-oriented manner per MPLS connection number. In this case, however, it should be considered that the ring is additionally loaded dynamically in these cases. The advantageous factor is, however, that it is possible to deal with connection-oriented faults with respect to the MPLS connection number by means of such a procedure. Using the preferred embodiment of group protection switching, it is possible to deal with the case of a fault on the route which occurs most frequently. 
     Finally, it should be noted that, although the ring is configured from linear 1+1 structures in the present exemplary embodiment, using a 1:1 structure according to FIG. 3 brings further advantages. Although the protection switching protocol is more complex in this case, it is possible here to transmit special data via the protection link during the trouble-free time on the operating link. The special data used can be control data of a general type. According to the invention, the special data can also be special traffic data. 
     The special data transmitted via the protection link can also be low-priority traffic which is transmitted in the network when there are sufficient resources. In this case, the low-priority traffic is then automatically displaced by protection switching of the high-priority traffic. In this case, in the protection switching case, the special data are not displaced by changing over the switching device S 0  in FIG. 2, but rather by prioritizing the high-priority traffic with respect to the low-priority special data in each transmission device. 
     The operating and protection links WE and PE must be set up before start-up. For this purpose, connections must be set up (configured) between the label switched routers W and E and, if necessary, at intermediate transmission devices. 
     These connections are usually set up by TMN (telecommunication management network) but can also be set up by means of an MPLS signaling protocol. For this purpose, the path of the operating and protection link is established by signaling. In addition, bandwidth is reserved in the transmission devices via the signaling protocol so that the transmission of the information via the operating link and protection link is ensured.