Abstract:
The present invention comprises a method and apparatus for optimizing redundant link usage so as to allow a portion of the wasted bandwidth in a redundant link system to be utilized for additional data traffic without compromising the ability of the system to respond to and correct for a failure of a link. In one embodiment, two independent, individually addressable links are established between to endpoints. One of the links is selected as a nominal communication path, and the other as a standby communication path. Data traffic between the endpoints is divided into path independent traffic and path dependent traffic. The path independent traffic is sent via the nominal communication path, while the path dependent data is sent via the nominal communication path or the standby path, in accordance with the dependence of the traffic. In another embodiment, the nominal communication path and standby communication path each comprise a time sensitive path and a normal path. Critical time sensitive traffic is sent via the nominal and standby time sensitive paths, while normal traffic is sent via the nominal normal path, and non-critical traffic is sent via the standby normal path.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of network elements, and more particularly to a method and apparatus for optimization of redundant link usage in a multi-shelf network element.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Multi-Shelf Network Elements  
           [0003]    An example of a multi-shelf network element with which the present invention may be used is multi-shelf switch system  100  shown in schematic block form in FIG. 1. Switch system  100  comprises peripheral shelves  110   a - n , switching shelves  120   a - b , and control complex  130 . Peripheral shelves  110   a - n  provide input/output (I/O) interfaces for connection to data paths  115   a - n  for which switching services are provided by switch system  100 . Switching shelves  120   a - b  provide the switching services for the system. Control complex  130  provides central management for the switch system. The various shelves of switch system  100  are interconnected by intershelf links  140   a - h  and  150   a - f . An example embodiment of the switch system  100  of FIG. 1 is the Alcatel  7670  Router Switch Platform (RSP) Multi-Shelf system (Alcatel  7670 ). The Alcatel  7670  is a multiprotocol backbone system designed to switch ATM cells and route IP traffic through the same switching fabric. Depending on its configuration, the Alcatel  7670  provides from 14.4 Gbps up to 450 Gbps of switching capacity.  
           [0004]    In one embodiment of switch system  100  of FIG. 1, each of switching shelves  120   a - b  contains a switching fabric core and up to 32 switch access cards (SAC), each providing 14.4 Gbps of cell throughput to and from the core. Switching shelves  120   a - b  are configured as a redundant pair and are referred to as switching shelf X and switching shelf Y.  
           [0005]    In one embodiment, two types of peripheral shelves  110   a - n  may be used in switching system  100 . The first type, called a high speed peripheral shelf (HSPS), comprises high speed line processing cards (HLPC) each providing 10 Gbps throughput, high speed I/O cards (HIOC), high speed fabric interface cards (HFIC) and two high speed shelf controllers (HSC). The second type, called simply a peripheral shelf (PS), comprises lower speed line processing cards (LPC) each providing 2.5 Gbps throughput, I/O Cards (IOC) and dual or quad port fabric interface cards (DFIC/QFIC). Each of peripheral shelves  110   a - n  also typically includes a shelf control system which may comprise control complex  130  (in the case of peripheral shelf  110   a ) or one or preferably two redundant peripheral shelf controllers (PSC).  
           [0006]    In one embodiment, switching shelves  120   a - b  connect to the peripheral shelves  110   a - n  in the system via fabric interface cards (FIC) to provide cell switching to the line processing cards (also referred to as “line cards”) in the peripheral shelves. Examples of these connections are illustrated in FIG. 2.  
           [0007]    [0007]FIG. 2 includes two switching shelves  120   a  (designated “switching shelf X”) and  120   b  (designated “switching shelf Y”) and three peripheral shelves: two normal speed peripheral shelves  110   a  (designated “PS1”) and  110   b  (designated “PS2”) and one high speed peripheral shelf  110   c  (designated “HSPS3”). In one embodiment, each of the peripheral shelves may comprise up to sixteen (16) line processing cards. In FIG. 2, two line cards  225   a - b  are shown for peripheral shelf  110   a , four line cards  235   a - d  for peripheral shelf  110   b , and four high speed line cards  245   a - d  for high speed peripheral Shelf  110   c . Each pair of line cards in each peripheral shelf communicate with an associated pair of I/O cards, which connect to the data stream for which switching services are being provided. Thus, in the embodiment of FIG. 2, line cards  225   a - b  communicate with associated I/O cards  220   a - b , line cards  235   a - b  communicate with I/O cards  230   a - b , line cards  235   c - d  communicate with I/O cards  230   c - d , high speed line cards  245   a - b  communicate with I/O cards  240   a - b , and high speed line cards  245   c - d  communicate with I/O cards  240   c - d . Each line card is also connected to two redundant fabric interface cards (FIC) which provide high speed connections between each line card and each switching shelf  120   a  and  120   b . One of each pair of FIC&#39;s is designated FIC “X” and the other is designated FIC “Y”. Three types of FIC&#39;s are shown in FIG. 2. Peripheral shelf  110   a  contains two dual FIC&#39;s (DFIC)  270   a  and  270   b . Peripheral shelf  110   b  contains two quad FIC&#39;s (QFIC)  275   a - b , and high speed peripheral shelf  110   c  contains four high speed FIC&#39;s (HFIC)  280   a - d.    
           [0008]    In one embodiment, each switching shelf  120   a - b  comprises a switch core and up to 32 switching access cards (SAC). In the embodiment of FIG. 2, switching shelf X  120   a  comprises switch core  250   a  and SAC&#39;s  255   a - h , and switching shelf Y  120   b  comprises switch core  250   b  and SAC&#39;s  265   a - h.    
           [0009]    In one embodiment, the connections between the various components within a shelf are provided via one or more circuit boards in each shelf, referred to as “midplanes,” to which the respective components are mounted.  
           [0010]    In the embodiment of FIG. 2, each “X”-designated FIC on a peripheral shelf is connected to one or more SAC&#39;s on switching shelf X  120   a , and each “Y”-designated FIC on a peripheral shelf is connected to one or more SAC&#39;s on switching shelf Y  120   b . A DFIC connects to up to two SAC&#39;s, a QFIC to up to four SAC&#39;s, and a HFIC to a single SAC. In the embodiment of FIG. 2, the connection between the FIC&#39;s on a peripheral shelves  110   a - c  and the SAC&#39;s on switching shelves  120   a - b  is via high speed intershelf links (HISL)  290   a - p , which carry the cell-switched data path traffic. Thus DFIC X  270   a  of peripheral shelf  110   a  is connected to SAC&#39;s  255   a - b  on switching shelf X  120   a  via HISL&#39;s  290   a - b , and DFIC Y  270   b  of peripheral shelf  110   a  is connected to SAC&#39;s  265   a - b  on switching shelf Y  120   b  via HISL&#39;s  290   i - j . Similarly, on peripheral shelf  110   b , QFIC X  275   a  is connected to SAC&#39;s  255   c - f  on switching shelf X  120   a  via HISL&#39;s  290   c - f , and QFIC Y  275   b  is connected to SAC&#39;s  265   c - f  on switching shelf Y  120   b  via HISL&#39;s  290   k - n . Finally, on high speed peripheral shelf  110   c , HFIC&#39;s X  280   a  and  280   c  are connected via HISL&#39;s  290   g - h  to SAC&#39;s  255   g - h  on switching shelf X  120   a , and HFIC&#39;s Y  280   b  and  280   d  are connected via HISL&#39;s  290   o - p  to SAC&#39;s  265   g - h  on switching shelf Y  120   b , respectively.  
           [0011]    The switch cores  250   a - b , SAC&#39;s  255   a - h  and  265   a - h , HISL&#39;s  290   a - p , and FIC&#39;s  270   a - b ,  275   a - b  and  280   a - d  comprise the system&#39;s “switching fabric.” 
           [0012]    In the switch system of FIGS. 1 and 2, peripheral shelf  110   a , designated “Peripheral Shelf 1,” contains control complex  130 . In one embodiment, control complex  130  comprises a logical grouping of cards that provide the central management for all features of switch system  100 . The local and remote user interfaces to the system are provided by control complex  130  via user terminals  160  and  170 . In addition, control complex  130  maintains a database for all cards on the system. One embodiment of control complex  130  is illustrated in FIG. 3.  
           [0013]    In the embodiment of FIG. 3, control complex  130  comprises two control cards  300   a - b  (designated “Control Card A” and “Control Card B”, respectively), two intershelf connection (ICON) cards  310   a - b , two control interconnect (CIC) cards  315   a - b , two intershelf connection I/O (ICON I/O) cards  320   a - b , and two intershelf connection I/O expansion cards (ICON I/O Exp)  330   a - b . Control complex  130  also comprises a facility card (FAC) that provides an RS232 management port and two timing ports that can be attached to a timing source for system synchronization. In addition to control complex  130 , peripheral shelf  110   a  also comprises its own set of line processing cards  340  that communicate with control complex  130  via CIC cards  315   a - b.    
           [0014]    Control complex  130  communicates with each shelf in the switch system via shelf controllers on the peripheral and switching shelves. In the embodiment of FIG. 3, peripheral shelf  110   b  comprises a redundant pair of shelf controllers  340   a - b , high speed peripheral shelf  110   c  comprises a redundant pair of high speed shelf controllers  350   a - b , and switching shelves  120   a  and  120   b  each comprise a switching shelf controller  360   a  and  360   b , respectively.  
           [0015]    ICON I/O cards  320   a - b  and ICON I/O Exp cards  330   a - b  provide interfaces for connecting control complex  130  to other shelves (such as peripheral I/O and switching shelves) in the switch system via physical connections. In one embodiment, these connections are referred to as Control Services Links (CSL)  370   a - h . In the embodiment of FIG. 3, each ICON I/O and ICON I/O Exp card contains eight CSL ports. ICON I/O A  320   a  and ICON I/O B  320   b  provide ports for CSL connections to the two switching shelves X and Y  120   a - b  and to the first six peripheral I/O shelves (designated peripheral shelves  2  to  7 ), while ICON I/O Exp A  330   a  and ICON I/O Exp B  330   b  provide ports for CSL connections to eight additional peripheral I/O shelves (designated peripheral shelves  8  to  15 ). ICON cards  310 , ICON I/O cards  320 , ICON I/O Exp cards  330 , CSL&#39;s  370 , and shelf controllers  340 ,  350  and  360  provide an intershelf connection (ICON) infrastructure that allows the transfer of control traffic between control complex  130  and the controllers and cards in peripheral shelves  110  and switching shelves  120  without using any bandwidth on the main data path. Data path traffic goes through the switching fabric shown in FIG. 2 whereas the control traffic uses the out of band ICON infrastructure (also sometimes referred to as the “control infrastructure”) of FIG. 3 to transfer data. Operation of the control infrastructure is not affected by the switching fabric, and the switching fabric is not affected by the control infrastructure.  
           [0016]    In one embodiment, a CSL link is physically embodied in a twelve-conductor cable comprising three separate, internal 4-conductor Cat-5-type cables. Each 4-conductor internal cable provides one of three different types of communications channels: a time division multiplexed (TDM) channel (providing E1-type capabilities), a full duplex (Ethernet) messaging channel, and a simplex differential channel.  
           [0017]    The TDM channel (also sometimes referred to as the “E1 channel”) is used by the ICON infrastructure to transport time sensitive transport activity, shelf numbering control and system timing information throughout the system. In one embodiment, the TDM channel operates at a frequency of 8000 Hz, providing 32 time slots with 8-bits per slot at a 125 microsecond refresh rate resulting in a data rate of 2.048 Mbps. In this embodiment, the TDM channel provides a guaranteed point-to-point channel between the ICON cards on peripheral shelf  1  and any shelf connected to one of peripheral shelf  1 &#39;s CSL ports.  
           [0018]    The full duplex messaging channel (also sometimes referred to as the “Ethernet channel”) is used for general communications with any shelf in the system. In one embodiment, the Ethernet channel operates at 100 Mbps. Each Ethernet link to a shelf is shared among all components (cards) within the shelf. As a result, every element in the system is capable of communicating with the control complex via the Ethernet channel.  
           [0019]    The Ethernet channel may be used for a large variety of communications, including connection information, software downloading to the individual components, debugging, alarm management, and configuration transfers. Communications between the control complex and the shelf controllers that does not travel over the TDM channel in general travels over the Ethernet channel.  
           [0020]    The simplex differential channel (also sometimes referred to as the “RTS channel”) is used to distribute a real time stamp (RTS) from the controller to each of the shelves in the system. The simplex differential channel is used instead of the TDM channel or the Ethernet channel because the RTS is sensitive to time delays that result from the applications running on the TDM and Ethernet channels. The RTS is used to align all elements of the system to the same time stamp for purposes such as debugging and billing. The RTS signal is generated by the control complex and is provided to the ICON cards via a direct circuit board (mid-plane) connection. The ICON cards are responsible for extracting the signal and transmitting it via the differential channel to all shelves in the system.  
           [0021]    The use of redundant pairs of control cards and icon cards in peripheral shelf  1  and redundant shelf controllers in other peripheral I/O shelves creates redundant pathways over which control traffic can flow between each control card of the control complex in peripheral shelf  1  and each line card of each peripheral I/O shelf. Examples of redundant pathways between control card  300   a  and a line card  550  of high speed peripheral shelf  110   c  are shown in FIGS. 5 a - b.  For simplicity, only the main relevant components are shown in FIGS. 5 a - b . For the same reason, the ICON I/O and ICON I/O Exp cards are not shown separately but are included as part of the respective ICON cards.  
           [0022]    A first pathway, shown in FIG. 5 a , comprises midplane link  505  between control card A  330   a  and ICON card A  310   c  in peripheral shelf  1   110   a , CSL link  370   c  between ICON card A  310   c  on peripheral shelf  1   110   a  and shelf controller A  350   a  on peripheral shelf  2   110   c , and midplane link  515  between shelf controller A  350   a  and line card  550  on peripheral shelf  2   110   c . A second pathway, shown in FIG. 5 b , comprises midplane link  530  between control card A  330   a  and ICON card B  310   b  on peripheral shelf  1   110   a , CSL link  370   d  between ICON card B  310   b  on peripheral shelf  1   110   a  and shelf controller B  350   b  on peripheral shelf  2   110   c , and midplane link  525  between shelf controller B  350   b  and line card  550  on peripheral shelf  2   110   c.    
           [0023]    [0023]FIG. 4 illustrates the control traffic flow of the pathway of FIG. 5 a . In the embodiment of FIG. 4, controller card a  300   a  comprises a microprocessor  405 , a real time stamp source  415 , and an Ethernet port  410 .  
           [0024]    Microprocessor  405  exchanges time sensitive control data and normal control data with line card  420  on peripheral shelf  2   110   c . Time sensitive control data travels over a set of “n” direct links  420  to a field programmable gate array (FPGA)  450  on ICON card A  310   a . FPGA  450  assembles the data for transmission via TDM, and transmits the time sensitive data via E1 channel  485  of CSL  370   c  to FPGA  470  of shelf controller A  350   a  on peripheral shelf  2   110   c . FPGA  470  extracts the time sensitive data for line card  420  from the TDM stream, and passes it via direct midplane links  495  to line card  420 .  
           [0025]    Normal control data, on the other hand, travels from microprocessor  405  to Ethernet port  410  of control card A  300   a , and from there via midplane Ethernet link  425  to switch  445  of ICON card A  310   a . Switch  445  passes the data via CSL Ethernet channel  490  to switch  465  on shelf controller A  350   a  of peripheral shelf  2   110   c . Switch  465  in turn transmits the data to Ethernet port  482  on line card  420 . A microprocessor  466  of shelf controller A  350   a  on peripheral shelf  2   110   c  is coupled to FPGA  470  via connection  467 , switch  465  via connection  468 , and Ethernet port  482  via connection  469 , allowing microprocessor  466  access to the time sensitive data and the normal control data and/or allowing microprocessor  466  to exert control over the operations of FPGA  470 , switch  465 , and/or Ethernet port  482  (or, more generally, line card  420  via Ethernet port  482 ).  
           [0026]    Real time stamp (RTS) source  415  may generate its own time signal, or may receive a time signal from an external source. RTS source  415  communicates its time signal via midplane link  430  to RTS transponder  455  on ICON card A  310   a . RTS transponder  455  converts the time stamp into proper form for transmission over the differential channel of CLS  370   a  and sends it to RTS transponder  475  on shelf controller A  350   a  of peripheral shelf  2   110   c . RTS transponder  475  extracts the RTS signal from CSL  370   c &#39;s RTS channel and distributes it to line card  420  via midplane link  497 .  
           [0027]    Even though FIG. 4 only shows the control data pathways between one control card and one ICON card, it will be understood that the same pathways exist between each control card and both of the ICON cards in peripheral shelf  1   110   a .  
           [0028]    2. APS Automatic Protection System  
           [0029]    A common method used to prevent data communications interruptions is known as Automatic Protection Switching (APS) 1+1. In an APS 1+1 system, components are connected via redundant, mirrored links. Each transmitting component sends the identical data over both links, and each receiving component listens to both links. One of the links is the normal “work” link, while the other is the “protection” link. As long as the work link is operating properly, the receiving component processes the data received from that link and discards the data received via the protection link. However, if at any time the work link becomes inoperable or defective, the receiving component immediately begins processing the data received over the “protection” link. Both links must carry identical data so that no data loss occurs when such a switch is made.  
           [0030]    APS is useful because it prevents communications failures resulting from a single link failure. However, it does so at the expense of bandwidth: because the same data is sent simultaneously over two separate links, the total bandwidth needed is twice the what is required by the data stream itself, even though the data transferred over the protection link is used only during the infrequent times when there is a failure of the work link, and is otherwise discarded.  
         SUMMARY OF THE INVENTION  
         [0031]    The present invention comprises a method and apparatus for optimizing redundant link usage so as to allow a portion of the wasted bandwidth in a redundant link system to be utilized for additional data traffic without compromising the ability of the system to respond to and correct for a failure of a link. In one embodiment, two independent, individually addressable links are established between to endpoints. One of the links is selected as a nominal communication path, and the other as a standby communication path. Data traffic between the endpoints is divided into path independent traffic and path dependent traffic. The path independent traffic is sent via the nominal communication path, while the path dependent data is sent via the nominal communication path or the standby path, in accordance with the dependence of the traffic. In another embodiment, the nominal communication path and standby communication path each comprise a time sensitive path and a normal path. Critical time sensitive traffic is sent via the nominal and standby time sensitive paths, while normal traffic is sent via the nominal normal path, and non-critical traffic is sent via the standby normal path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 shows an embodiment of a multi-shelf network element.  
         [0033]    [0033]FIG. 2 shows a data path infrastructure of an embodiment of a multi-shelf network element.  
         [0034]    [0034]FIG. 3 shows a control data path infrastructure of an embodiment of a multi-shelf network element.  
         [0035]    [0035]FIG. 4 shows intershelf control data paths between two shelves of an embodiment of a multi-shelf network element.  
         [0036]    [0036]FIGS. 5A and 5B show alternative redundant paths between two shelves of an embodiment of a multi-shelf network element.  
         [0037]    [0037]FIG. 6 shows intershelf control data path connections between two shelves of a multi-shelf element in an embodiment of the invention.  
         [0038]    [0038]FIG. 7 is a flow chart showing a process used to optimize redundant link usage in an embodiment of the invention.  
         [0039]    [0039]FIG. 8 is a flow chart showing a process used to optimize redundant link usage in an embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0040]    A method and apparatus for optimizing redundant link usage in a multi-shelf network element is disclosed. In the following description, numerous specific details are set forth to provide a thorough description of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the invention.  
         [0041]    [0041]FIG. 6 shows intershelf control data path connections between two shelves of a multi-shelf element in an embodiment of the invention. The left side of FIG. 6 shows the control data path connections between control card A  300   a  in peripheral shelf  1   110   a  and shelf controller A  350   a  of peripheral I/O shelf  110   c . These control data path connections are the same as those of FIG. 4, in simplified form. In addition, the right side of FIG. 6 shows a similar control data path between control card A  300   a  and shelf controller B  350   b  in peripheral I/O shelf  110   c.    
         [0042]    As shown in FIG. 6, the control data path between control card A  300   a  and shelf controller A  350   a  includes an intrashelf control data path  670  between control card A  300   a  and ICON card A  310   a  and an intershelf CSL link  370   c  between ICON card A  310   a  and shelf controller A  350   a . Similarly, the control data path between control card A  300   a  and shelf controller B  350   b  includes intrashelf control data path  675  between control card A  300   a  and ICON card B  310   b  and intershelf CSL link  370   d  between ICON card B  310   b  and shelf controller B  350   b . Similar intrashelf control data paths (not shown in FIG. 6) exist between control card B and ICON cards A  310   a  and B  310   b . For clarity, they are not shown here. However, in the discussion below, it will be understood that the description of the operation of control card A applies to control card B as well.  
         [0043]    Each of intrashelf paths  670  and  675  and CSL links  370   c  and  370   d  comprise three separate data channels: an RTS channel, an Ethernet channel, and a time sensitive data channel. Intrashelf path  670  includes an RTS channel  425 , an Ethernet channel  425 , and a direct multi-line channel  420 . Intershelf CSL link  370   c  includes an RTS channel  480 , an Ethernet channel  490 , and a TDM/E1 channel  485 . RTS channel  425  is connected through ICON A  310   a  to RTS channel  480 , Ethernet channel  425  is connected to Ethernet channel  490 , and multi-line channel  420  is connected to E1 channel  485 .  
         [0044]    Similarly, intrashelf path  675  includes RTS channel  660 , Ethernet channel  655 , and multi-line channel  650 , while intershelf CSL link  370   d  includes RTS channel  630 , Ethernet channel  620 , and TDM/E1 channel  610 . RTS channel  660  is connected through ICON B  310   b  to RTS channel  630 , Ethernet channel  655  is connected to Ethernet channel  620 , and multi-line channel  650  is connected to E1 channel  610 .  
         [0045]    In other embodiments, time sensitive data channels, for example, indirect multi-line channels, may be provided between a dual or quad fabric interface card (DFIC/QFIC) and each of control card A  300   a , ICON card A  310   a , and ICON card B  310   b.  Such channels may be provided in addition to, or in lieu of, direct multi-line channel  420  and multi-line channel  650 .  
         [0046]    The combination of intrashelf link  670  and intershelf link  370   c  forms a first threechannel communication path between control card A  300   a  and peripheral  1 / 0  shelf  110   c . This first communication path will be referred to as communication path A. The combination of intrashelf link  675  and intershelf link  370   d  forms a second, independent three-channel communication path between control card A  300   a  and peripheral I/O shelf  110   c . This second communication path will be referred to as communication path B. Control card A  300   a  is configured to allow it to individually address each of communication paths A and B.  
         [0047]    A concept that is relevant to the embodiment of FIG. 6 is the “health” of a communication path or link. The health of a link refers to the ability of the link to carry traffic without data degradation or loss.  
         [0048]    In the embodiment of FIG. 6, the health of communication paths A and B are continually monitored both by shelf controllers  350   a  and  350   b  on peripheral I/O shelf  110   c  and by control card A  300   a  on peripheral shelf  1   110   a . The health of a link is determined by examining the data coming over each channel of the link to determine the amount of data loss or degradation. The relative health of a link is then determined according to preset criteria. Different weight may be assigned to the contribution by each channel to the overall health of a link. For example, in one embodiment, the state of the channel that carries the most critical and/or time-sensitive data (e.g., in the embodiment of FIG. 6, the TDM/E1 channel) carries the greatest weight, while the channel that carries the least critical data (e.g. in the embodiment of FIG. 6, the RTS channel), is given the least weight. In one embodiment that uses a three channel link like CSL links  370   c - d  of FIG. 6, the RTS channel is given no weight at all in calculating the overall health of the link—the health is determined solely based on the conditions of the TCM/E1 and Ethernet channels.  
         [0049]    The relative health of two available communication paths or links between two endpoints (such as, for example, control card A  300   a  and peripheral I/O shelf  110   c  in FIG. 6) can be used to choose one of the two communication paths as a “nominal path” and the other as a “standby path.” The nominal path may be used to transfer the normal data traffic between endpoints, while the “standby path” is kept available to carry the normal data traffic should the health of the “nominal path” deteriorate.  
         [0050]    Various paradigms can be used for selection of the “nominal” and “standby” paths. In the embodiment of FIG. 6, the responsibility for selection of the “nominal” path is normally assigned to shelf controllers  350   a - b  of peripheral I/O shelf  110   c , who communicate with each other over link  640 , and who, together, can be considered to comprise a shelf control complex for peripheral I/O shelf  110   c . Each of shelf controllers  350   a - b  monitors the health of the particular CSL link that it is connected to. That is, shelf controller A  350   a  monitors the health of CSL link  370   c  and shelf controller B  350   b  monitors the health of CSL link  370   d . The shelf controllers negotiate among themselves based on the relative health of their respective CSL links to determine which link at any time is selected as the “nominal” link and which as the “standby” link. In addition, if a CSL link to a shelf controller deteriorates below a minimum threshold, the shelf controller may designate the link as being inoperative. Furthermore, in certain circumstances, for example if one of the links is being shut down for service, control card A may send a request to the shelf controllers to designate a particular link as the “nominal” link. However, in the embodiment of FIG. 6, the final decision rests with the shelf controllers.  
         [0051]    The identity at any point in time of the “nominal” link constitutes critical, time sensitive information. For example, a control card such as control card A  330   a  uses the identity of the nominal link to determine which path to use for data traffic. In one embodiment, the data identifying the nominal link is simultaneously sent by both shelf controllers via the respective TDM/E1 channels of their respective CSL links to the control cards to ensure that the identity of the nominal link is known, preferably at all times, by the control cards. Sending data regarding the current nominal path via the TDM/E1 channels of both CSL links allows the control card to be instantly informed of a switch in the nominal link, even if the previous link, including its TDM/E1 channel is completely severed. The control card can instantly redirect the normal data traffic to the standby link with little to no loss of data. It should be noted that the normal data traffic is considered to be “path independent” between the endpoints (i.e. control card A  300   a  and peripheral I/O shelf  110   c ). That is, it does not matter whether communication path A or communication path B is used—all that matters is that the data is transferred between endpoints.  
         [0052]    In the embodiment of FIG. 6, because TDM/E1 channels of CSL links  370   c  and  370   d  allow control card A to be instantly informed of a switch in the nominal link, it is not necessary, as in traditional APS schemes, for the normal data traffic over the Ethernet channel of the nominal link to be mirrored over the Ethernet channel of the standby link. As a result, the Ethernet channel of the standby link is available to carry additional data traffic. However, because the Ethernet channel of the standby link may at any time be called into use should a switch in the nominal link be commanded by the shelf controllers, such additional data traffic should normally constitute supplemental, non-critical data. Examples of such non-critical data include diagnostic data concerning the various elements and components in the standby path, and non-time critical software upgrades for elements and components in the standby path. Such data can be considered “path dependent” because it comprises data intended not for the endpoints, but for elements along a particular path.  
         [0053]    [0053]FIG. 7 is a flow chart showing a process used to optimize redundant link usage in an embodiment of the invention. Two independent communication links are established between endpoints at step  700  (e.g. a control complex and a peripheral or switching shelf or a card on a peripheral or switching shelf), and the health of each link is monitored at step  710 . At step  720 , a determination is made as to whether at least one link has sufficient health to be currently usable. If not, an alarm is set at  725  and the process returns to monitoring the health of the links at step  710 .  
         [0054]    If it is found at step  720  that at least one of the links is currently usable, then one usable link is designated as the nominal link based on the relative health of the links at step  730 . At step  740 , path independent data, and data dependent on the nominal path (i.e. the path formed by the link that has been designated as the nominal link) is transferred between endpoints via the nominal link. At step  750 , a determination is made as to whether the second link is currently usable. If the second link is found to be not usable, an alarm is set at step  755  and the process returns to step  710 . If the second link is found to be usable, then data dependent on the second path (i.e. the path formed by the second link) is transferred between endpoints via the second link. Thereafter the process returns to step  710 .  
         [0055]    [0055]FIG. 8 is another flow chart showing a process used to optimize redundant link usage in an embodiment of the invention. Two independent links between endpoints are established at step  800 . At step  805 , each of the two channels are provided with a time sensitive channel (for example, the TDM/E1 channel of the embodiment of FIG. 6) and a second channel (for example the Ethernet channel of the embodiment of FIG. 6). The health of each link is monitored at step  810 , which may include separately monitoring the health of each channel of each link. At step  815  a determination is made as to whether the health of at least one link is sufficient to be currently usable. If it is determined that no link is currently usable, an alarm is set at step  820  and the process resumes monitoring the health of the links at step  810 .  
         [0056]    If it is determined at step  815  that at least one link is currently usable, then one link is designated as the nominal link based on the relative health of the links at step  825 . The designation of that link as the nominal link is communicated between endpoints via the time sensitive channel of the nominal link at step  830 , and normal data traffic is transferred between endpoints via the second channel of the nominal link at step  835 .  
         [0057]    At step  840 , a determination is made as to whether the health of the second link is sufficient to render it currently usable. If the second link is not found to be usable, an alarm is set at step  845  and the process returns to monitoring the health of the links at step  810 . If the second link is found to be usable, the designation of the nominal link (which is sent via the time sensitive channel of the nominal link at step  830 ) is also sent via the time sensitive channel of the second link at step  850 . In addition, supplemental, non-critical traffic is transferred between endpoints via the second channel of the second link at step  860 . Thereafter the process returns to step  810 .  
         [0058]    Thus, a method and apparatus of optimizing redundant link usage in a multi-shelf network element has been presented. Although the invention has been described using certain specific examples, it will be apparent to those skilled in the art that the invention is not limited to these few examples. For example, although the invention has been described with respect to use in a multi-shelf network element, the invention may be used for optimization of redundant link usage within and between other communicating devices. Other embodiments utilizing the inventive features of the invention will be apparent to those skilled in the art, and are encompassed herein.