Abstract:
A cross point switch is configured to select from one of a first stream of ATM cells and a second stream of ATM cells, the first stream and the second stream being identical, and is further configured to select and substitute the second stream for the first stream upon detection of an error condition in the working circuitry operation on the first stream. The working circuitry, which is located on an interface module, is configured to receive and operate on the first stream of ATM cells. The protection circuitry, which is located on a redundant interface module, is configured to receive and operate on the second stream of cells. The protection circuitry and the working circuitry are functionally identical and synchronized to each other so that switching occurs from one stream to the other without interruption of network service.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to communication networks, and more specifically, to ATM (Asynchronous Transfer Mode) equipment having module redundancy for protecting network connections from failure. 
   2. Description of Related Art 
   ATM (Asynchronous Transfer Mode) high-speed switched networks have been proposed as a technology capable of integrating present digital services with new multimedia services, e.g., video on demand, live television from many sources, CD-quality music, LAN interconnection, and high-speed data transport for science and industry. To implement these different services, ATM networks are designed to handle a variety of traffic characteristics, e.g., constant rate, variable rate, and bursty, required for various real time, such as video conferencing, and non-real time, such as E-mail and web browsing, applications. The basic principles of ATM networks are well known to those skilled in the art and are discussed in numerous published references (see, for example, Andrew S. Tanenbaum, Computer Networks, Prentice Hall, 1996, 3rd ed.). 
   ATM switches transfer data using a cell switching technology. All data in an ATM network is transmitted between the switches of the network in small, fixed size 53 byte long ATM cells having 5 bytes for a header and 48 bytes for the information or data payload. The header of each cell contains a virtual connection identifier used for routing the cells over the network. Each ATM cell contains a two-part connection identifier in the cell header: a Virtual Path Identifier (VPI) and a Virtual Circuit Identifier (VCI). This two-part connection identifier uniquely identifies an ATM virtual connection on a physical interface. 
   ATM is a connection-oriented technology. That is, a call is required by user A to user B, similar to a telephone call, to set up a connection between the two users. Once the connection path is established, all data cells are transferred over the same connection path. This guarantees that cells will be delivered in order. Although ATM establishes a circuit between hosts, it establishes this circuit internally using cell switching technology. When a virtual circuit is established between hosts, what really happens is that table entries storing routing information are made in each switch along the path between the hosts. 
   The following is an example of what happens when a user A connects to a user B. First, user A sends a call request to user B. The call request arrives at an ingress switch of the ATM network. Call control software located at the ingress switch uses NNI signaling procedures combined with NNI routing services to locate user B and establish a virtual connection over which user A and user B exchange data. 
   Once a connection is established, data can be exchanged between User A and User B. Typically, the flow of data proceeds as follows. User A opens an application on the computer of User A which sends data to User B. Network software at User A&#39;s side will receive the data from the application of user A, and arrange the data into packets. Each packet is arranged as a variable length sequence of 53 byte ATM cells carrying the data payload preceded by a packet header cell containing information related to the application. The packets are sent over the ATM network cell by cell from User A to User B. The cells are reassembled into packets by software at User B&#39;s end and presented to User B as an application packet. 
   ATM networks provide several mechanisms for assuring quality of service and regulating traffic flow and congestion. Traffic shaping and traffic congestion control have been proposed for ATM networks. In traffic shaping, a quality of services contract is formed before data transmission between the customer and the network where the required transmission parameters are specified. For example, a customer could specify a requirement that data be transferred with a delay of 10 microseconds. In turn, the customer is required to abide by conditions of the contract, e.g., to transmit below an agreed upon rate. Traffic policing functions, e.g., user parameter control (UPC) within the ATM network enforce the user&#39;s agreement to the terms in the contract. 
   The physical layer connection of ATM networks normally consists of optical fibers working on the SONET (Synchronous Optical Network) standard. A SONET system consists of transport network elements such as add/drop multiplexers (ADMs), multiplexers, and repeaters all connected by optical fiber. An ATM switch in a SONET network may have one or more SONET interface unit boards which extract ATM cells from the optical signals of the fiber pathways to allow the ATM processing and routing functions to be applied by the ATM switch. 
   In ATM networks, the reliability of the ATM switch is critical because malfunctioning ATM equipment may result in the failure of an individual permanent virtual circuit (PVC) and a switched virtual circuit (SVC) connections. Malfunctions of the ATM equipment are generally classified into two types of failures: facilities failure and equipment failure. A facility failure refers broadly to the failure of anything external to the equipment such as a failure of one of the cables coming into the equipment, e.g., a fiber cut, or even connector problems. Someone could unplug a connector on the front of the equipment and that would be classified as a facility failure. An equipment failure includes any failure occurring within the electronics of the equipment itself (e.g. electronic failure). 
   It would be highly desirable to provide protection for ATM equipment or other communication switching equipment in order to preserve network reliability despite the above stated facility and equipment failures. 
   SUMMARY OF THE INVENTION 
   In order to overcome the above-stated problems, the present invention includes working circuitry configured to receive a first stream of cells and protection circuitry configured to receive a second stream of cells. Typically, the cells are ATM cells. The second stream is identical to the first stream. The protection circuitry and the working circuitry are functionally identical and synchronized to each other. A cross point switch is configured to select a first stream and is further configured to select and substitute a second stream for the first stream upon detection of an error condition in the working circuitry of the first stream. 
   The error condition may be a facility error condition (e.g., a disconnected cable) or an equipment error condition. An equipment error condition is a failure which occurs internal to equipment (e.g., the SONET Framer, the multiplexer, the user parameter control unit, the router, and the failure of other circuitry associated with the working circuitry and protection circuitry). 
   In a further aspect of the present invention, the working circuitry receives the first stream from an optical signal and the protection circuitry receives the second stream from the same optical signal. 
   In another aspect of the present invention, the working circuitry is implemented on a first board and the protection circuitry is implemented on a second board, the first board being separate and distinct from the second board. 
   In yet another aspect of the present invention, the working circuitry includes a plurality of ATM units performing ATM functions on the first stream and the protection circuitry includes a plurality of identical ATM units performing the same ATM functions on the second stream. One of the ATM units may be a multiplexer, a SONET framer, a router, a user parameter control unit, and a signaling unit. 
   In yet another aspect of the present invention, a first module has a plurality of ports wherein one or more of the ports accepts their stream as input to the working circuitry and the remaining ports may accept their stream as input to the protection circuitry. 
   In still another aspect of the present invention, a second module includes the protection circuitry and has a plurality of ports wherein one of the ports accepts the second stream as input into the protection circuitry and the remaining ports may accept their stream as input to the working circuitry. 
   In a further aspect of the present invention, an ATM switch fabric has a plurality of input ports, a plurality of output ports, and is configured to transfer an ATM cell received from a cell stream of one of the input ports to any one of the output ports. 
   In still another aspect of the present invention, input ports of the ATM switch fabric receives the ATM cells from a cell stream, either the first stream or the second stream as selected by the cross point switch. 
   In another aspect of the present invention, a second cross point switch is configured to receive ATM cells from one of the output ports of the ATM switch fabric and directs the ATM cells to a third stream and further directs the ATM cells to a fourth stream. 
   In still another aspect of the present invention, a second working circuitry is configured to receive the third stream of cells from the second cross point switch. Furthermore, a second protection circuitry is configured to receive the fourth stream of cells from the second cross point switch. The third stream is identical to the fourth stream. The second protection circuitry and the second working circuitry are functionally identical and synchronized to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a system diagram showing an exemplary ATM switch architecture using cross point switches to provide protection against failures according to the present invention. 
       FIG. 2  is a block diagram of an exemplary SONET Interface unit for operating on input ATM cell streams. 
       FIG. 3  is a block diagram of an exemplary SONET Interface unit for operating on output ATM cell streams. 
       FIG. 4  is an exemplary system diagram of an ATM switch architecture according to the present invention. 
       FIG. 5  is an exemplary block diagram illustrating data flow directed by cross point switches according to the present invention. 
       FIG. 6  is an exemplary block diagram illustrating signaling flow directed by cross point switches according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a preferred embodiment of an architecture  100  of the present invention for protecting an ATM switching fabric  118  from facility and equipment malfunction. However, it should be understand that other embodiments of the present invention may be applied to switching fabrics other than ATM. The present invention provides protection on the order of 50 milliseconds for each individual permanent virtual circuit (PVC) and switched virtual circuit (SVC) of the ATM network. In brief, the manner in which this is accomplished is by using 1+1 APS (Automatic Protection Switching) to provide identical information to two separate modules. In the event of a failure, the protection module will become active and provide service. The protection module is made active to the switch fabric by using cross point switches. 
     FIG. 1  shows the architecture  100  having four SONET (Synchronous Optical Network) interface modules (SIU)  112 ,  114 ,  122 ,  124 . The cross point switches  116  and  120  interface these four SIU modules to switch fabric  118 . In the preferred embodiment, the circuitry of modules  112  and  122  are implemented on the same circuit board as transceiver pairs and similarly the circuitry of redundant modules  114  and  124  are implemented on the same circuit board as transceiver pairs. However,  FIG. 1  depicts the modules  112 ,  114 ,  122  and  124  functionally and thus the present invention is not limited to any particular correspondence between modules  112 ,  114 ,  122 ,  124  and circuit board hardware. Thus, other configurations are possible. In addition, the circuitry of the modules  112 ,  114 ,  122  and  124  are bidirectional. Thus, even though data will be described herein as being input into modules  112 , and  114  flowing though the system and output through modules  122  and  124 , it should be noted that data may also flow in the opposite direction by being input to modules  122  and  124 , and flowing through the system and output through modules  112  and  114 . 
     FIG. 2  shows the preferred embodiment of SIU module  112  receiving streams  131   a–d  of SONET frames on ports  1 – 4 . Redundant module  114  works identically to module  112 . In the preferred embodiment, streams  131   a–d  are composed of SONET frames. A SONET frame is made up of frame overhead information and a payload consisting of a series of ATM cells. This process of supporting an asynchronous ATM layer using a synchronous SONET carrier is well-known in the telecommunications art. 
   SONET deframers  133   a–d  extract the ATM payloads from the SONET frames of streams  131   a – 131   d . The payload extracted from each SONET frame is a stream of ATM cells. 
   On port  0  only, the SIU module  112  contains an AAL5 (ATM adaption layer 5) unit  131   f  receiving an ATM electrical signal (non optical)  131   e  on port  0  from the ATM network. AAL5 unit  131   f  performs the signaling function by setting up and breaking down calls for the ATM network. In the embodiment of the present invention, there is one signaling function unit to serve four ports of each module. Future revisions or variations of the SIU modules are not necessarily constrained to this configuration. Multiplexer unit  135   a  combines the ATM signal from the output of AAL 5   131   f  with the stream of ATM cells output from the SFM  133   a.    
   Multiplexer units  135   a – 135   d  pass the data flow received from the SONET deframers (SFM units)  133   a – 133   d  and AAL 5   131   f  to the Parametric Control (UPC) units  137   a–d  which monitor the ATM cells streams received from units  135   a – 135   d  to enforce customer contracts and perform other policing functions well-known in the ATM network art. 
   The routers  139   a–d  route the ATM cells received from UPC units  137   a – 137   d  from the input ports  119  of switch fabric  118  to the appropriate output ports  121  of switch fabric  118  (see  FIG. 1 ). Each ATM cell has a header and a payload carrying information bits. The header of the ATM cell indicates a virtual address from which the router determines where to route the ATM cell. For example, an ATM cell input to port  1  of the ATM switch fabric  118  will be routed to one of switch fabric output ports  1 – 8  depending on the header. The router will place an internal code in the ATM cell for routing the cell through the switch fabric  118 . 
     FIG. 3  shows SIU module  122  operating on ATM cell streams  141   a–d  received from crosspoint switch  120  output ports  1 – 4  (in the default case where ports  1 – 4  are the working ports). Routers  143   a – 143   d  may perform operations on ATM cells streams  141   a–d  to prepare them for routing to the external network. Then UPC units  145   a – 145   d  may perform traffic shaping or other operations on the cell stream output from routers  143   a – 143   d . The multiplexers  147   a – 147   d  pass the data through from UPC units  145   a – 145   d . In port  0 , demultiplexer  147   a  separates the call signal data from the ATM data stream to signaling unit  142 . SFM modules  149   a – 149   d  create SONET frames for transporting the ATM cells received from MUX  147   a – 147   d . In addition, it should be noted that modules  122  and  124  are bidirectional and therefore router units (RT)  143   a – 143   d , UPC  145   a – 145   d , MUX  147   a – 147   d , SFM  149   a – 149   d , and AAL5 unit  142  of modules  122 ,  124  perform the same functions described above for the units of modules  112 ,  114  when ATM signal streams  151   a – 151   d  are received on ports  1 – 4 . 
   SIU module  114  is a redundant module having identical circuitry to SIU  112 . Similarly, SIU module  124  is a redundant module containing identical circuitry to SIU module  122 . The interface modules  112  and  122  are designated as the active modules. The interface modules  114  and  124  are designated as the standby modules. At any given time, a port of one of the modules can be configured to be either a working port or a protected port. The working port is actively processing input data while the protected port remains inactive. To clarify this, for example, either port  1  of SIU module  112  or port  1  of module  114  will be the working port and the other port will be the protected port. Similarly port  2  of SIU module  112  can be the working port while port  2  of SIU module  114  is the protected port or vis-a-vis. In the present exemplary embodiment all the ports of the active SIU module  112  are initially chosen as the working ports and all the ports of the SIU module  114  are protected ports. Referring again to  FIG. 1 , the switching fabric  118  is capable of routing an ATM cell from any one of the 8 input ports  119  to any one of the 8 output ports  121  (8 ports are shown in  FIG. 1  for simplification). The ATM switch, in general, may have an arbitrary number of inputs (N), that is, it could be an N×N matrix. Preferably, there are 16 input and output ports. 
   There are two cross-point switches  116 ,  120 , the cross point switch  116  on the input stage of the switch fabric  118  and the cross-point switch  120  on the output stage of the switch fabric  118 . The cross point switch  116  having input ports and output ports connects any of its input ports  136  to any of its output ports  117 . The cross point switch  120  connects any of its input ports  123  to any of its output ports  141 . 
     FIG. 4  illustrates the basic architecture  100  of the present invention in more detail than  FIG. 1  by including the circuitry of the SIU modules shown in  FIGS. 2 and 3 . 
     FIG. 5  illustrates the data flow operation of cross point switches  116  and  120  according to an exemplary embodiment of the present invention. In this particular configuration, ports  1  and  2  of SIU module  112  and port  3  of redundant SIU module  114  are the current working ports. Port  4  is not being used in this example. In fact, it is possible to have port  4  for both SIU  122  and SIU  124  to be operated in an unprotected mode. 
   The data flows of  FIG. 5  will now be explained. First, ATM cell stream  131   a  received at port  1  of SIU  5  module  112  is processed by the SONET deframer  133   a , the MUX  135   a , UPC unit  137   a , and router  139   a  and output as ATM cell stream  136   a . Cross point switch  116  directs ATM cell stream  136   a  from input port  1  to output port  1  of cross point switch  116  along path  201  so that the cell stream connects to and is received by port  1  of switch fabric  118 . 
   ATM cell stream  131   b  is received at port  2  of SIU module  112  and processed by SONET deframer  133   b , MUX  135   b , UPC unit  137   b , and router  139   b  and output as ATM cell stream  136   b . Cross point switch  116  directs ATM cell stream  136   b  from its input port  2  to its output port  2  along path  207  so that the cell stream connects to and is received by port  2  of switch fabric  118 . 
   ATM cell stream  131   c  received at port  3  of SIU module  114  are processed by SONET deframer  133   c , MUX  135   c , UPC unit  137   c , and router  139   c  and output as ATM cell stream  136   c . Cross point switch  116  directs ATM cell stream  136   c  from input port  7  to output port  3  of the cross point switch  116  along path  213  so that the cell stream connects to and is received by input port  3  of switch fabric  118 . Thus, the cross point switch  116  connects redundant module  114  output port  7  to input port  3  of switch fabric  118  in substitution for the default path, which when operative, connects port  3  of SIU module  112  to input port  3  of switch fabric  118 . 
   The ATM cell stream  131   a  is also provided to input port  1  of SIU module  114 . If there is an equipment failure in port  1  path in SIU  112 , then the communication can be switched and is being processed by port  1  of SIU module  114  and routed via crosspoint switch  116  to input port  1  of the switch fabric  118  without dropping the call. In effect the call is resetup on the fly without interruption to the user. This is an advancement in ATM network switching because the crosspoint switch handles equipment failures without dropping a communication or call. 
   In essence, an exemplary embodiment of the present invention can detect an equipment failure, such as in an SIU module (the working circuitry) and then protect against the failure by rerouting the ATM cell data from another SIU module&#39;s input (the protected circuitry) by disconnecting ATM cell data from a failed, working SIU and connecting a working protection SIU to the switch fabric by using the crosspoint. The crosspoint switch allows the rerouted cell data to be placed back on its original path through the switch fabric. The user does not have his call dropped in the event of the equipment failure. Thus, a more reliable ATM switch network is established. 
   Switch fabric  118  routes ATM cells received at its input ports  119  to its output ports  121  in accordance with routing instructions. ATM cells destined for output port  1  of switch fabric  118  form ATM cell stream  141   a . ATM cells of cell stream  141   a  composed of ATM cells routed to output port  1  by ATM switch  118  are then directed by cross point switch  120  along path  203  to input port  1  of SIU  122  and concurrently are directed along path  205  to output port  5  of cross point switch  120  which connects directly to port  1  of SIU  124 . Consequently, identical ATM streams are transported over paths  203  and  205  by cross point switch  120  to port  1  of SIU modules  122  and  124 . Thus, this provides redundant paths to support one-plus-one (1+1) SONET protection switching. 
   ATM cells destined for output port  2  of switch  15  fabric  118  form ATM cell stream  141   b . ATM cells of cell stream  141   b  composed of ATM cells routed to output port  2  by ATM switch  118  are then directed by cross point switch  120  along path  209  to port  2  of SIU  122  and concurrently are directed along path  211  to output port  6  of cross point switch  120  which connects directly to port  2  of SIU  124 . ATM cells destined for output port  3  of switch fabric  118  form ATM cell stream  141   c . ATM cells of cell stream  141   c  composed of ATM cells routed to output port  3  by ATM switch  118  are then directed by cross point switch  120  along path  215  to port  3  of SIU  122  and concurrently are directed along path  217  to port  7  of cross point switch  120  which connects directly to port  3  of SIU  124 . 
     FIG. 6  demonstrates an example of signaling data flow in an exemplary embodiment of the present invention as would appear during a call setup. In this particular configuration, ports  1  and  2  of SIU module  112  and Port  3  of redundant SIU module  114  are the current active  15  ports. Note that Port  4  is not being used in this example. 
   ATM cell stream  131   a  received at port  1  of SIU module  112  is processed by SFM  133   a , MUX  135   a , UPC  137   a , and RT  139   a  and output as ATM cell stream  136   a . Cross point switch  116  directs ATM cell stream  136   a  from its port  1  to its port  1  along path  301  so that the cell stream connects to and is received by port  1  of switch fabric  118 . 
   The ATM cell stream  131   b  received at port  2  of SIU module  112  is processed by SFM  133   b , MUX  135   b , UPC  137   b , and RT  139   b  and output as ATM cell stream  136   b . Cross point switch  116  directs ATM cell stream  136   b  from its port  2  to its port  2  along path  303  so that the cell stream connects to and is received by port  2  of switch fabric  118 . 
   The ATM cell stream  131   c  received at the input port  3  of SIU module  114  is processed by SFM  133   c , MUX  135   c , UPC  137   c , and RT  139   c  and output as ATM cell stream  136   c . Cross point switch  116  directs ATM cell stream  136   c  from its input port  7  to its output port  3  along path  305  so that the cell stream connects to and is received by input port  3  of switch fabric  118 . 
   The switch fabric  118  routes the ATM signaling cells from paths  301 ,  303 ,  305 , along paths  307 ,  309 , and  311  to output port  1  on switch fabric  118 . Thus, all the call setup information for ports  1 ,  2  and  3  are being routed to port  1  along path  312  to the signaling unit AAL 5   142  of SIU module  122  which processes the calls. Note that it is possible to cause a failure of AAL 5   142  on SIU module  122  to use AAL 5   142  on SIU module  124 . 
   Although only certain embodiments of the apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of additional rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims. Accordingly, it should be understood that the scope of the present invention encompasses all such arrangements and is solely limited by the claims as follows.