Abstract:
A system and method of notifying a first communication network of a fault related to a second communication network is provided. The first communication network is connected to the second communication network. The first communication network has a first OAM protocol adapted to monitor integrity of the first communication network and the second communication network has a second OAM protocol adapted to monitor integrity of the second communication network. The method includes detecting the fault related to the second communication network and generating a first OAM cell of the first OAM protocol indicating detection of the fault. The method further includes transmitting the first OAM cell to the first communication network to indicate the fault to the first communication network.

Description:
[0001]     This application is a continuation of U.S. patent application Ser. No. 10/015,575, filed Dec. 17, 2001, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to digital communication systems and more specifically to a network mode capable of providing asynchronous transfer mode traffic to a multi-protocol label switching (MPLS) link.  
       BACKGROUND OF INVENTION  
       [0003]     MPLS is quickly gaining support in the communication industry as a high-speed core of many communication networks. Networks are being developed and deployed which interface ATM networks with MPLS networks. New network systems are being deployed which incorporate both MPLS and ATM network topologies.  
         [0004]     For each topology, there is a separate, operations, administration and maintenance (OAM) protocol for generating and transmitting OAM messages whether in cells, packets, frames or another format.  
         [0005]     There is a need for a system and method for transmission of information relating to OAM packets between ATM and switching networks, such as MPLS networks, as they are generated and transmitted.  
       SUMMARY OF INVENTION  
       [0006]     In a first aspect, a method of notifying a first communication network of a fault related to a second communication network is provided. The first communication network is connected to the second communication network. The first communication network has a first OAM protocol adapted to monitor integrity of the first communication network and the second communication network has a second OAM protocol adapted to monitor integrity of the second communication network. The method includes detecting the fault related to the second communication network and generating a first OAM cell of the first OAM protocol indicating detection of the fault. The method further includes transmitting the first OAM cell to the first communication network to indicate the fault to the first communication network.  
         [0007]     The first communication network may be connected with the second communication network at a network element.  
         [0008]     The first communication network may be an ATM network, the first OAM protocol may be an ATM OAM protocol, the second communication network may be a MPLS network and the second OAM protocol may be a MPLS OAM protocol.  
         [0009]     Detecting the fault may occur at the network element.  
         [0010]     The network element may be adapted to receive connectivity verification cells from the second communication network.  
         [0011]     The fault related to the second communication network may be detected at the network element by recognition of expiry of a time interval in which a number of connectivity verification cells have not been received.  
         [0012]     The number of connectivity verification cells may be at least three.  
         [0013]     The first OAM cell may be an ATM AIS cell.  
         [0014]     In a second aspect, a network element for connecting a first communication network to a second communication network is provided. The first communication network has a first communication protocol and a first OAM protocol for use in monitoring integrity of the first communication network. The second communication network has a second communication protocol and a second OAM protocol for use in monitoring integrity of the second communication network. The network element includes a first section adapted to provide communications for the network element with the first communication network and notify the network element of the fault in the first communication network. The first section detects the faults related to the first communication network. The network element also includes a second section adapted to provide communications for the network element with the second communication network, detect that the first section has notified the network element of the fault and notify the second communication network of the fault.  
         [0015]     In other aspects of the invention, various combinations arid subsets of the above aspects are provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The foregoing and other aspects of the invention will become more apparent from the following description of specific embodiments thereof and the accompanying drawings which illustrate, by way of example only, the principles of the invention. In the drawings, where like elements feature like reference numerals (and wherein individual elements bear unique alphabetical suffixes):  
         [0017]      FIG. 1  is a block diagram of a communication network comprising two edge switches according to an embodiment of the invention which each link an ATM network to a Multi-Protocol Label Switching (MPLS) network;  
         [0018]      FIG. 2  is a block diagram of the tunnel in the MPLS network connecting the ATM/MPLS edge switches of  FIG. 1 ;  
         [0019]      FIG. 3  is a block diagram of an ATM cell and an ATM frame and their equivalent MPLS frame generated and manipulated by one edge switch of  FIG. 1 ;  
         [0020]      FIG. 4  is a block diagram illustrating points of failure in the ATM network, the ATM/MPLS edge switch and the MPLS network of  FIG. 1 ;  
         [0021]      FIG. 5  is a block diagram illustrating the relationship between ATM OAM cells and MPLS OAM frames in the network of  FIG. 1 ;  
         [0022]      FIG. 6  is a block diagram showing the interworking of MPLS fault indications with ATM connection control in the ATM/MPLS edge switch of  FIG. 1 ; and  
         [0023]      FIG. 7  is a state diagram of a MPLS OAM state machine present in the ATM/MPLS edge switch of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0024]     The description which follows, and the embodiments described therein, are provided by way of illustration of an example, or examples, of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not limitation, of those principles and of the invention. In the description which follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals.  
         [0000]     1.0 Basic Features of System  
         [0025]     Briefly, the embodiment provides a system and method for transmission of information in OAM messages from a non-ATM network.  
         [0026]     In the embodiment, an ATM/MPLS edge switch, provides a connection point for an ATM network to a MPLS network, monitors the integrity of a routing path in the MPLS network connected to the edge switch and transmits fault information received in MPLS OAM frames associated with the MPLS routing path to the connected ATM networks.  
         [0027]     First a description of a network configuration which a network element associated with the embodiment is provided, followed by a description of components of the elements. Next, error OAM protocols received and processed by the network element for two different network. protocols are described, and finally an interface system for the different OAM schemes for the two different networks is described.  
         [0000]     2.0 System Configuration  
         [0028]     Following is a description of a network having a switch associated with the embodiment. Referring to  FIG. 1 , communications network  100  is shown, comprising a first ATM network  102 ( 1 ), a second ATM network  102 ( 2 ) and a multi-protocol label switched (MPLS) network  104 . ATM networks  102  comprise a number of interconnected ATM switches  106  connected by communications links  108  which can each carry ATM traffic thereon.  
         [0029]     At the edge of ATM network  102 ( 1 ), ATM edge switch  110 ( 1 ) provides a connection for Customer Premise Equipment (CPE)  112 ( 1 ) to ATM network  102 ( 1 ) via link  116 ( 1 ). Similarly, at an edge of ATM network  102 ( 2 ), ATM edge switch  110 ( 2 ) provides a link for CPE  112 ( 2 ) to ATM network  102 ( 2 ) via link  116 ( 2 ). ATM edge switch  110 ( 1 ) is shown having a connection to another ATM network  114 . ATM edge switch  110 ( 1 ) is connected to elements in ATM network  102 ( 1 ). Similarly, ATM edge switch  110 ( 2 ) is connected to elements in ATM network  102 ( 2 ).  
         [0030]     MPLS network  104  comprises MPLS switches  118  which are connected together via communications links  120 . At one edge of MPLS network  104 , ATM/MPLS edge switch  122 ( 1 ) provides an interface between ATM network  102 ( 1 ) and MPLS network  104 . At another edge of MPLS network  104 , ATM/MPLS edge switch  122 ( 2 ) provides an interface between ATM network  102 ( 2 ) and MPLS network  104 .  
         [0031]     Referring to  FIG. 2 , ATM/MPLS edge switches  122  communicate with MPLS switches  118  in MPLS network  104  via communications links  124 . Communications links  124  are two fibre optic cables carrying uni-directional data between an MPLS switch  118  and an ATM/MPLS edge switch  122 . Communications links  120  are two fibre optic cables carrying uni-directional data between two MPLS switches  118 . In communications link  124 ( 1 ), communications carried from ATM/MPLS edge switch  122 ( 1 ) to MPLS switch  118 A, in the direction of ATM/MPLS edge switch  122 ( 2 ), are carried on a cable  210 ; communications in the other direction are carried on a cable  212 . Similarly communications links  120  and  124 ( 2 ) have cables  210  in the direction of ATM/MPLS edge switch  122 ( 2 ) and cables  212  in the direction of ATM/MPLS edge switch  122 ( 1 ).  
         [0032]     MPLS switches  118  utilize dedicated and pre-arranged MPLS routing paths to carry traffic within MPLS network  104 . These dedicated and pre-arranged MPLS routing paths or tunnels are composed of two uni-directional label switched paths (LSPs)  200  and  202  between ATM/MPLS edge switches  122 ( 1 ) and  122 ( 2 ). It will be appreciated that the term “tunnel” is interchangeable with the term “MPLS routing path”. In tunnel  204 , LSP  200  carries traffic from ATM/MPLS edge switch  122 ( 1 ) to ATM/MPLS edge switch  122 ( 2 ). Similarly, in tunnel  204 , LSP  202  carries traffic from ATM/MPLS edge switch  122 ( 2 ) to ATM/MPLS edge switch  122 ( 1 ).  
         [0000]     2.1 ATM and MPLS Format Conversion  
         [0033]     As ATM/MPLS edge switches  122  are at the edge of an ATM network and a MPLS network, each ATM/MPLS edge switch  122 ( 1 ) must translate ATM cells and frames to MPLS frames and vice versa for traffic sent between ATM network  102  and MPLS network  104 . Referring to  FIG. 3 , aspects of the conversion of ATM cells and frames received from CPE  110 ( 1 ) by ATM/MPLS edge switch  122 ( 1 ) to MPLS frames, and vice versa are shown.  
         [0034]     As is known in the art ATM data may be either encapsulated into cells or frames. Cell  300  comprises  48  bytes of data in data field  302  and five bytes of header data in header field  304 . The header field  304  includes data relating to error checks and destination information. Frequently, ATM cells  300  are used to encode voice calls in AAL 1/2/5 signalling parameters. ATM frames are used to carry larger sizes of data. Exemplary ATM frame  306  comprises data field  308  which may have 65 Kbytes of data. Header field  310  is comparable to header field  304  used in ATM cell  300 . When ATM traffic leaves ATM network  102  and is provided to MPLS  104 , for each of the ATM cells  300  and the ATM frames  306 , the data and header information must be transposed into a MPLS frame  312 .  
         [0035]     MPLS frame  312  comprises data field  314 , header field  316 , first label field  318  and second label field  320 . When transposing data of an ATM cell  300  or frame  306  to a MPLS frame  312 , the respective ATM data field (either data field  302  or  308 ) is inserted into MPLS data field  314 . Similarly, the contents of the respective ATM cell or frame header fields (either header field  304  or header field  310 ) is inserted into MPLS header field  316 . Header field  316  may be a component of data field  314 . The contents of first label  318  and second label  320  provide routing information for MPLS frame  312  through MPLS network  104 . First label field  318  contains identification information relating to the MPLS routing path for the MPLS frame  306 . Second label field  320  contains connection information relating to the particular internal ATM connection which is to be used by the ATM/MPLS edge switch  122  at the end of the routing path for a connection to the downstream ATM network  102 .  
         [0000]     2.2 Switch Description  
         [0036]     Referring to  FIG. 4 , ATM/MPLS edge switch  122  comprises an ATM card  400 , a fabric card  402 , an MLPS card  404 , at least one ATM port  406 , at least one MPLS port  408 , two fabric ports  410  and  412 , a control card (not shown in this figure) and two I/O cards (not shown).  
         [0037]     ATM/MPLS edge switch  122  interfaces ATM network  102  with MPLS network  104 . Therefore, it has an ATM (line) card  400  for interfacing with a connection to ATM network  102  and a MPLS (line) card  404  for interfacing with a connection to MPLS network  104 . ATM network  102  connects to ATM card  400  of ATM/MPLS edge switch  122  via link  108 ( 1 ) through ATM port  406  and an input/output (I/O) card (not shown). MPLS network  104  connects to MPLS card  404  via link  124 ( 1 ) through MPLS port  408  and an I/O card (not shown). Inside ATM/MPLS edge switch  122 , ATM card  400  can provide traffic to fabric card  402  through fabric port  410  and fabric card  402  can connect to MPLS card  404  through fabric port  412 .  
         [0038]     In direction  440 , ATM card  400  receives cells  300  and frames  306  transmitted from ATM network  102  and converts them into an internal format for processing by fabric card  402 . For traffic destined for MPLS network  104 , fabric card  402  forwards the traffic to the appropriate MPLS card  404 . MPLS card  404  converts its received traffic into MPLS frames  312  as described earlier and transmits the MPLS frames  312  to MPLS network via link  124 ( 1 ).  
         [0039]     In direction  442 , MPLS card  404  receives MPLS frames  312  transmitted from MPLS network  104  over link  124 ( 1 ) and converts them into an internal format for processing by fabric card  402 . MPLS card  404  forwards the internal data to fabric card  402 . For traffic destined for ATM network  102 , fabric card  402  forwards its output to the appropriate ATM card  400 . ATM card  400  converts the traffic into cells  300  or frames  306  as shown in  FIG. 3  and transmits them into ATM network  102  over link  108 ( 1 ).  
         [0040]     In addition to providing a traffic interface for ATM/MPLS traffic ATM/MPLS edge switch  122  is capable of indicating a failure in MPLS network  104  to ATM network  102  by monitoring tunnel  204  and by messaging to ATM network  102  the arrival of MPLS OAM frames.  
         [0000]     3.0 ATM and MPLS OAM  
         [0041]     Referring again to  FIG. 1 , in an ATM network  102 ( 1 ), when communications are sent from CPE  112 ( 1 ), through an ATM network  102 ( 1 ) to CPE  112 ( 3 ), a routing path is established from CPE  112 ( 1 ), through ATM edge switch  110 ( 1 ), switches  106  in ATM network  102 ( 1 ), ATM edge switch  110 ( 3 ) and is received at CPE  112 ( 3 ). Upon a failure of an element in this routing path, for example communications link  108 , an ATM OAM cell is generated and transmitted over ATM network  102 ( 1 ) to inform ATM edge switches  110 ( 1 ) and  110 ( 3 ) of the failure. In particular, an ATM OAM Alarm Indication Signal (AIS) cell is generated and transmitted in the downstream direction towards CPE  112 ( 3 ). An ATM AIS cell is transmitted from ATM switch  106 A in lieu of the data cells to maintain transmission continuity and to indicate to ATM edge switch  110 ( 3 ) that there is a transmission interruption located either at the equipment originating the ATM AIS cell or upstream of that equipment. When ATM edge switch  110 ( 3 ) receives the ATM AIS cell, it generates and transmits an ATM OAM Remote Defect Indicator (RDI) cell over the same routing path towards CPE  112 ( 1 ) which arrives at ATM edge switch  110 ( 1 ). This informs both ATM edge switches  110 ( 1 ) and  110 ( 3 ) about the failure in the routing path. ATM edge switches  110 ( 1 ) and  110 ( 3 ) can then inform CPEs  112 ( 1 ) and  112 ( 3 ) of the failure.  
         [0042]     Referring to  FIG. 2 , in MPLS network  104  LSPs  200  and  202  may each carry specialized operation, administration and maintenance (OAM) MPLS frames. The MPLS OAM frames follow ITU Y.17 MPLS standards, which are incorporated herein by reference. There are three types of MPLS OAM frames used by the embodiment:  
         [0043]     1) Connectivity verification (CV) frames;  
         [0044]     2) Backward defect indicator (BDI) frames; and  
         [0045]     3) Forward defect indicator (FDI) frames.  
         [0046]     The type of MPLS OAM frame transmitted within an LSP  200  or  202  is identified via the header information and the second label field  320  in a MPLS frame  312 . The first label field  318  contains the tunnel identification information related to the OAM destination. Presently, in the embodiment, a MPLS OAM frame is identified with a value defined by the MPLS standards bodies. Currently, the value is “5”. This value is placed in second label field  320 . The contents of the data field  314  identify the type of MPLS OAM frame.  
         [0047]     In the embodiment for a given MPLS tunnel  204 , MPLS OAM frames are sent therein to monitor the status of the tunnel  204 .  
         [0048]     To monitor and test the integrity of tunnel  204 , dedicated CV frames are transmitted from ATM/MPLS edge switch  122  at one end of tunnel  204  to the switch at the other end. At the downstream switch MPLS OAM frame(s) are received and analysed. Depending on the results of the analysis, the downstream switch may generate a type of response MPLS OAM frame which is transmitted upstream to the originating switch along its the associated LSP. At the originating switch, MPLS OAM frame(s) are received and analysed. The MPLS OAM frame(s) received at the originating switch  122  will indicate the status of the entire tunnel  204 . For the purposes of the example, upstream switch is ATM/MPLS edge switch  122 ( 1 ) and downstream switch is ATM/MPLS edge switch  122 ( 2 ) for tunnel  204 .  
         [0049]     There are four cases of conditions in the LSPs  200 ,  202  or the receiving ATM/MPLS edge switch  122 ( 2 ) and other downstream components beyond ATM/MPLS edge switch  122 ( 2 ). In Case A, tunnel  204  has no transmission problems therein. ATM/MPLS edge switch  122 ( 1 ) generates and transmits CV frames on LSP  200 . The CV frames are received by ATM/MPLS edge switch  122 ( 2 ) which generates other CV frames which are transmitted over LSP  202 . These other CV frames are received at ATM/MPLS edge switch  122 ( 1 ) and ATM/MPLS edge switch  122 ( 1 ) can determine that tunnel  204  is fully operational by the receipt of the CV frames with no other MPLS OAM frames.  
         [0050]     CV frames are generated by ATM/MPLS edge switches  122  every one second according to ITU Y.17 standards. Accordingly, after a certain transmission and frame processing delay, when tunnel  204  and its downstream components which affect tunnel  204  are fully operational, the CV frames received by ATM/MPLS edge switch  122  should arrive approximately once every second. It will be appreciated that other time intervals could be used for transmitting CV frames.  
         [0051]     In Case B, it is presumed that there is a failure in LSP  200 . Accordingly, when ATM/MPLS edge switch  122 ( 1 ) generates and transmits a CV frame onto LSP  200 , ATM/MPLS edge switch  122 ( 2 ) does not receive it. ATM/MPLS edge switch  122 ( 2 ) generates a response MPLS BDI frame which indicates that a failure has occurred backwards in LSP  200 . The MPLS BDI frame is transmitted on LSP  202  and is received at ATM/MPLS edge switch  122 ( 1 ). ATM/MPLS edge switch  122 ( 1 ) then can determine that tunnel  204  is not fully operational.  
         [0052]     In Case C, it is presumed that there is a failure in both LSP  200  and LSP  202 . As with Case B, CV frame from ATM/MPLS edge switch  122 ( 1 ) is not received by ATM/MPLS edge switch  122 ( 2 ). In response ATM/MPLS edge switch  122 ( 2 ) generates and transmits a MPLS BDI frame to ATM/MPLS edge switch  122 ( 1 ). Further, ATM/MPLS edge switch  122 ( 1 ) does not receive the MPLS BDI frame. Accordingly, ATM/MPLS edge switch  122 ( 1 ) recognizes the absence of any response to the originally transmitted CV frame and determines that tunnel  204  is not fully operational.  
         [0053]     Case D is a variation of Case A. In a normal situation if tunnel  204  is fully operational, ATM/MPLS edge switch  122 ( 1 ) and ATM/MPLS edge switch  122 ( 2 ) will be able to transmit and receive CV frames therebetween. However, if ATM/MPLS edge switch  122 ( 2 ) has an indication that downstream to it, there is a further failure which affects tunnel  204 , ATM/MPLS edge switch  122 ( 1 ) generates a MPLS FDI frame, which indicates that forward of ATM/MPLS edge switch  122 ( 2 ) there is a further integrity problem associated with tunnel  204 . The MPLS FDI frame is transmitted from ATM/MPLS edge switch  122 ( 2 ) to ATM/MPLS edge switch  122 ( 1 ) via LSP  202 . The MPLS FDI frame is received by ATM/MPLS edge switch  122 ( 1 ). ATM/MPLS edge switch  122 ( 1 ) can then recognize the fault downstream of tunnel  204 . This case may also be an extension of Case B if a fault in tunnel  204  causes ATM/MPLS edge switch  122 ( 2 ) to not receive CV frames from ATM/MPLS edge switch  122 ( 1 ). In this case, both a MPLS FDI frame and a BDI frame are transmitted from ATM/MPLS edge switch  122 ( 2 ) to ATM/MPLS edge switch  122 ( 1 ) via LSP  202 .  
         [0054]     The Cases are summerized in the following Table A:  
                                               Frames received at                   ATM/IP edge       Case   switch 122   LSP 200 Status   LSP 202 States                   A   CV   Up   Up       B   CV, BDI   Down   Up       C   None   Unknown   Down       D1   FDI   Up   Down       D2   BDI, FDI   Down   Down                  
 
         [0055]     It will be appreciated that MPLS switches  118  may be configured to generate and transmit MPLS OAM frames such that the use of CV frames is not necessary to detect failures in tunnel  204 . If MPLS switches  118  can detect a failure in LSP  200 , an MPLS switch  118  may generate and transmit a MPLS FDI frame downstream along LSP  200 , toward ATM/MPLS edge switch  122 ( 2 ). Upon receiving the MPLS FDI frame, ATM/MPLS edge switch  122 ( 2 ) generates and transmits a MPLS BDI frame over LSP  202  to ATM/MPLS edge switch  122 ( 1 ).  
         [0000]     3.1 Failures Resulting in Generation and Transmission of OAM Cells or Frames  
         [0056]     Referring again to  FIG. 4 , propagation of error information in a tunnel in MPLS network  104  to ATM network  102  by switch  122  is shown.  
         [0057]     If tunnel  204  fails per Cases C, Case D 1  and Case D 2  indicating a fault in tunnel  204  in direction  442  (for example, due to a failure at point  436  in a cable leading towards ATM/MPLS edge switch  122 ), MPLS card  404  receives and interprets the corresponding error state. ATM/MPLS edge switch  122  processes the error state information and further generates ATM AIS cell which is transmitted from ATM card  400  (point  424 ) towards ATM network  102  in direction  442  on connections that were associated with tunnel  204 .  
         [0058]     If ATM/MPLS edge switch  122  receives MPLS BDI frames per Cases C and Case D 2  indicating a fault in tunnel  204  in direction  440  (for example, due to a failure at point  436  in a cable leading away from ATM/MPLS edge switch  122 ), no ATM AIS cells are generated and transmitted to ATM network  102 . Instead, ATM RDI cells will pass through LSP  202  to ATM network  102 , as described below.  
         [0059]     If there is a fault in MPLS port  408  or its I/O card (i.e. a failure at point  434 ), ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell which is transmitted from ATM card  400  (point  424 ) towards ATM network  102  in direction  442  on ATM connections linked to MPLS card  404  and MPLS port  408 .  
         [0060]     If MPLS card  404  fails and there is no redundant MPLS card  404  (i.e. a failure at point  432 ), ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell which is transmitted from ATM card  400  (point  424 ) towards ATM network  102  in direction  442  on ATM those connections linked to MPLS card  404 .  
         [0061]     If fabric port  412  fails (i.e. a failure at point  430 ), ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell which is transmitted from ATM card  400  (at point  424 ) towards ATM network  102  in direction  442  on ATM connections linked to fabric port  412 . Also, a MPLS FDI frame is generated and transmitted from the MPLS card  404  having a connection to the fabric port  412  in direction  440  to all connected tunnels  204 .  
         [0062]     If fabric card  402  fails or is removed and there is no redundant fabric card  402  (i.e. a failure at point  428 ), ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell which is transmitted from ATM card  400  towards ATM network  102  in direction  442  on all connections associated with fabric card  402 . Also, a MPLS FDI frame is generated and transmitted from MPLS card  404  (point  432 ) in direction  440  on all tunnels  204  that have tunnel monitoring enabled.  
         [0063]     If fabric port  410  fails (i.e. a failure at point  426 ), ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell which is transmitted from ATM card  400  towards ATM network  102  in direction  442  on ATM connections associated with fabric port  410 . An ATM AIS cell is generated and transmitted from the MPLS card  404  (point  432 ) towards the MPLS network  104  in direction  440  on those connections from the ATM card  400  with the failed fabric port  410 . ATM OAM cells are supported in MPLS network  104  using cell mode encapsulation by transparently passing ATM OAM cells through MPLS network  104 . MPLS card  404  simply passes ATM OAM cells from ATM network  102  with data cells through MPLS tunnel  204  if the connection is configured to use cell mode encapsulation. No ATM AIS cells are transmitted for connections configured using frame mode encapsulation. These ATM AIS cells are then carried transparently across their corresponding MPLS tunnel  204  to the far-end ATM network  102 .  
         [0064]     If ATM card  400  of ATM/MPLS edge switch  122  fails and there is no redundant ATM card  400  (i.e. a failure at point  424 ), ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell which is transmitted from MPLS card  404  (point  432 ) towards MPLS network  104  in direction  440 . This is done for all connections in direction  440  using cell mode encapsulation connected to the faulted ATM card  400 . These ATM AIS cells are then carried transparently across their corresponding MPLS tunnel  204  to the far-end ATM network  102 .  
         [0065]     If there is a fault in ATM port  406  or its I/O card (a failure at point  422 ), then ATM/MPLS edge switch  122  recognizes the fault and generates an ATM AIS cell from ATM card  400 . ATM/MPLS edge switch  122  transmits the ATM AIS cell in direction  440 , into ATM/MPLS edge switch  122  for connections associated with the failed ATM port  406  configured to use cell mode encapsulation. ATM/MPLS edge switch  122  passes these ATM OAM cells through MPLS tunnel  204 . If the connection is configured to use frame mode encapsulation, then no ATM AIS cells are generated and transmitted.  
         [0066]     In the event of a failure in ATM network  102  (for example, a failure in communications link  108  at point  420 ), then an ATM AIS OAM cell is generated by a switch in ATM network  102  and transmitted in direction  440 , towards ATM/MPLS edge switch  122 , on all ATM connections. ATM/MPLS edge switch  122  passes this ATM OAM cell transparently through MPLS tunnel  204  as a typical cell if the connection uses cell mode encapsulation. If the connection is configured to use frame mode encapsulation, then the ATM OAM cell is terminated at ATM card  400  of ATM/MPLS edge switch  122 .  
         [0000]     4.0 MPLS and ATM OAM Interworking  
         [0067]     Referring to  FIG. 5 , the correlation of ATM OAM cells and MPLS OAM frames is illustrated in more detail. In network  100 , CPE  112 ( 1 ) transmits cells or frames to CPE  112 ( 2 ), as described previously. If a fault  500  occurs in MPLS network  104 , ATM/MPLS edge switch  122 ( 2 ) detects the fault by either receiving MPLS FDI frames from MPLS switch  118  at point  502  or by failing to receive CV frames. The transmission of MPLS FDI frames, if any, is indicated by arrow  510  extending between point  502  and point  504 .  
         [0068]     ATM/MPLS edge switch  122 ( 2 ) transmits MPLS BDI frames from MPLS card  404 ( 2 ) (point  504 ) towards ATM/MPLS edge switch  122 ( 1 ). ATM/MPLS edge switch  122 ( 2 ) also transmits ATM AIS cells from ATM card  400 ( 2 ) (point  506 ) towards CPE  112 ( 2 ). The transmission of ATM AIS cells is indicated by arrow  512  extending between point  506  and point  520 .  
         [0069]     When the ATM AIS cells arrive at ATM edge switch  110 ( 2 ), ATM edge switch  110 ( 2 ) notifies CPE  112 ( 2 ) and transmits ATM RDI cells towards CPE  112 ( 1 ). These ATM RDI cells transmitted towards CPE  112 ( 1 ) are transparently passed through MPLS network  104  over tunnel  204 . The ATM RDI cells arrive at ATM edge switch  110 ( 1 ) which informs CPE  112 ( 1 ) of the fault. The transmission of ATM RDI cells is indicated by arrow  514  extending between point  520  and point  522 .  
         [0070]     When the MPLS BDI frames arrive at ATM/MPLS edge switch  122 ( 1 ), MPLS card  404 ( 1 ) removes the MPLS BDI frames from the datastream at point  508 . MPLS BDI frames do not continue to ATM edge switch  110 ( 2 ) since ATM RDI cells are transmitted to ATM edge switch  110 ( 2 ). The transmission of MPLS BDI frames is indicated by arrow  516  extending between point  504  and point  508 . Therefore, MPLS OAM frames are transmitted only on MPLS network  104 .  
         [0000]     4.1 Tunnel Status and ATM Connection Control  
         [0071]     Referring to  FIG. 6 , further detail of ATM/MPLS edge switch  122  is provided. ATM/MPLS edge switch  122  has an LSP management system  600  residing on a control card  602 . ATM card  400  is illustrated having ATM connection control  614 . MPLS card  404  of ATM/MPLS edge switch  122  is also illustrated having MPLS OAM state machine  604 , OAM task  624  and Change FIFO  620 . Change FIFO  620  is comprised of Change of State entries  630 .  
         [0072]     Briefly, OAM task  624  communicates with LSP management system  600  to inform it of changes of state of MPLS OAM state machine  604 . From this information, LSP management system  600  determines whether tunnel  204  is available for ATM connections and whether ATM AIS cells should be transmitted into ATM network  102 , communicating this information to ATM connection control  614 . Accordingly, the embodiment provides an interface allowing MPLS tunnel status information to be provided to ATM nodes.  
         [0073]     As noted earlier, MPLS network  104  transmits MPLS OAM frames, either CV, FDI or BDI frames, to ATM/MPLS edge switch  122 . MPLS OAM frames arrive at OAM state machine  604  in MPLS card  404 , shown by arrow  606 . There is one MPLS OAM state machine  604  for each tunnel  204  and each failure state, either CV, FDI or BDI failure.  
         [0074]     Referring to  FIG. 7 , MPLS OAM state machine  604  is shown for one tunnel  204  and one failure state. A MPLS OAM state machine  604  for CV failures will transition from Unknown state  702  to OK state  704  if connectivity verification of the tunnel is successful. Connectivity verification may be successful upon receipt of a consecutive number of CV frames. MPLS OAM state machine  604  for CV failures will transition from Unknown state  702  to Defect state  706  if connectivity verification fails.  
         [0075]     In performing connectivity verification, CV frames should be received by MPLS OAM state machine  604  for CV failures periodically, about once every second. However, after a period of time has elapsed without receiving a CV frame, MPLS OAM state machine  604  for CV failures moves to Defect state  706 . This time period may be configurable. A debounce mechanism is provided for CV frames by setting the period of time after which a CV failure is declared to a time longer than one second. In the embodiment, the time period is three seconds.  
         [0076]     For MPLS OAM state machine  604  for BDI failures, when in Unknown state  702  or OK state  704  and it receives a BDI frame, MPLS OAM state machine  604  moves to Defect state  706 .  
         [0077]     The same is true for MPLS OAM state machine  604  for FDI failures, moving from Unknown state  702  to Defect state  706  when it receives a FDI frame.  
         [0078]     While in Defect state  706 , the defect can be cleared. For MPLS OAM state machine  604  for CV failures, the defect is cleared when the MPLS OAM state machine  604  receives a series of consecutive CV frames. The number of CV frames may be configurable. In the embodiment, the default number of CV frames needed to clear a CV failure is 3. For MPLS OAM state machine  604  for BDI failures, the defect is cleared when the MPLS OAM state machine  604  does not receive a further BDI frame within a defined period of time. The defined period of time may be varied by the MPLS OAM state machine  604 . In the embodiment, the default defined period is three seconds. A defect in MPLS OAM state machine  604  for FDI failures is similarly cleared. Upon the clearing of a defect, MPLS OAM state machine  604  moves to OK state  704 .  
         [0079]     In OK state  704 , transitions are made to Defect state  706  upon the absence of receipt of a number of CV frames or the receipt of either a BDI or FDI frame, as described above.  
         [0080]     Referring back to  FIG. 6 , when a MPLS OAM state machine  604  for tunnel  204  indicates a change of state to Defect state  706 , it generates and enqueues a Change of State entry  630  on Change FIFO  620 , indicated by arrow  622 . The entry  630  contains information about the destination LSP and MPLS OAM status information. OAM task  624  periodically monitors FIFO  620  for new entries. Upon detection of a new entry therein, OAM task  624  transmits an indication of the failure state of either or both of LSPs  200  and  202  in tunnel  204  to LSP management system  600 , indicated by arrow  608 .  
         [0081]     If tunnel  204  is in a state indicating that LSP  200  is “down” (see table A), LSP management system  600  broadcasts this to ATM connection control  614  in ATM cards  400  and LSP  200  cannot be used. This broadcast is indicated by arrows  610  and  612 . ATM connection control  614  in each ATM card  400  receives this message and stops transmitting data over a failed LSP  200 .  
         [0082]     When a MPLS OAM state machine  604  is informed by arriving MPLS OAM frames or lack thereof that LSP  200  which was “down” is now “up”, it transitions to OK state  704  and writes a change of state entry  630  to Change FIFO  620 , indicated by arrow  622 . OAM task  624  reads this change of state from Change FIFO  620 , indicated by arrow  626 , and indicates the change to LSP management system  600 , shown by arrow  608 . LSP management system  600  broadcasts this to ATM connection control  614  in ATM cards  400  and LSP  200  can now be used to transmit data. This broadcast is indicated by arrows  610  and  612 . ATM connection control  614  in each ATM card  400  receives this message and can again transmit data over LSP  200 .  
         [0083]     If the information from Change of State entry  630  provided to LSP management system  600  indicates that LSP  202  is “down”, LSP management system  600  broadcasts this to ATM connection control  614  in ATM cards  400 . This broadcast is indicated by arrows  610  and  612 . For each ATM connection connected over a failed LSP  202 , ATM connection control  614  starts inserting ATM AIS cells into ATM network  102 , indicated by arrow  616 . The ATM AIS cell contains a code identifying it as an ATM AIS cell. The ATM cell header contains the virtual path identifier (VPI) and virtual connection identifier (VCI) that uniquely specifies the individual ATM connection.  
         [0084]     When a MPLS OAM state machine  604  is informed by arriving MPLS OAM frames or lack thereof that LSP  202  which was “down” is now “up”, it transitions to OK state  704  and writes a change of state entry  630  to Change FIFO  620 , indicated by arrow  622 . OAM task  624  reads this change of state from Change FIFO  620 , indicated by arrow  626 , and indicates the change to LSP management system  600 , shown by arrow  608 . LSP management system  600  broadcasts this to ATM connection control  614  in ATM cards  400 . This broadcast is indicated by arrows  610  and  612 . For each ATM connection connected over a recovered LSP  202 , ATM connection control  614  stops inserting ATM AIS cells into ATM network  102  and transmits the normal data cells received, indicated by arrow  616 .  
         [0085]     The system and method of interworking MPLS OAM frames with ATM OAM cells informs ATM networks  102  of failures in MPLS network  104  providing information that can be used to route around failures in network  100 . ATM networks  102  can be instructed by LSP management system  600  to avoid transmitting data on failed tunnels  204 , as is described above, providing a more reliable network  100 .  
         [0086]     It is noted that those skilled in the art will appreciate that various modifications of detail may be made to the present embodiment, all of which would come within the scope of the invention.