Abstract:
A communication network using a ring structure incorporates shared protection channels ( 22   ab   , 22   bc   , 22   bd ) to reduce costs in implementing protection spans. The shared protection network elements ( 12   a   , 12   b   , 12   c   , 12   d ) use a protocol of conventional messaging to integrate with traditional fully redundant network elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates in general to telecommunications and, more particularly, to shared protection architectures. 
     2. Description of the Related Art 
     Over the last decade, the importance of telecommunications has increased dramatically. In order to accommodate the vast amount of information passed over telecommunications systems, such as the Public Switched Telephone Network (PSTN) and data networks, copper wires are being replaced with optical fibers, which are capable of carrying significantly more information. 
     A single fiber may transmit information over many different channels using DWDM (dense wavelength division multiplexing) techniques. Improvements in fiber technology and electronics are increasing the number of channels that may be distinguished over a fiber and, thus, the amount of information that may be passed by a single fiber. 
     Increases in information bandwidth over a fiber, however, increase the importance of providing mechanisms to bypass failures in the network, until the failure can be corrected. Common failures include, for example, fiber breakages (typically caused by construction activities inadvertently cutting a fiber), fiber disconnects caused by accidents in the central office, and network element failures, such as laser failures. 
     In order to maintain communications in spite of a failure, ring architectures are often used. In a ring architecture, a series of network elements are connected in a ring, such as shown in  FIG. 1 . Each ring  10  has multiple network elements  12  coupled to one another to form a closed loop. Typically, there are four fibers  14  connecting adjacent network elements  10 —two working fibers and two protection fibers, although other configurations are possible. The working fibers (W) carry traffic between adjacent nodes. Protection fibers (P) are available to carry traffic in the event of a working fiber failure. The protection fibers also convey control information between network elements; when not being used for traffic, the protection fibers may carry low-priority interruptible traffic. As shown in  FIG. 1 , network elements  12  may be shared between different rings. 
     The ring architecture shown in  FIG. 1   a  is a very simple architecture. In many circumstances, multiple rings  10  may connect various network elements  12  as shown in  FIG. 1   b . Failures of a working fiber in any of the rings  10  may cause protect lines in multiple rings to be used. 
       FIG. 2   a  illustrates one prior art method of circumventing a failure of a working fiber W. In this embodiment, a ring  10  having five network elements  12  (referenced individually as network elements  12   a – 12   e ) has a broken working fiber W between network elements  12   c  and  12   d . For purposes of illustration, only one working fiber W and one protection fiber P is shown, it being understood that a similar pair of working and protection fibers are used for traffic in the opposite direction. To pass traffic between network elements  12   c  and  12   d , network element  12   d  connects the working lines  16   de  to protect lines  18   cd  and network element  12   c  connects working lines  16   bc  to protect lines  18   cd . In other words, traffic that would normally be routed over working lines  16   cd  is switched to the associated protect lines  18   cd . This is referred to as a “span” switch. 
       FIG. 2   b  illustrates a situation where both the working and protection lines have failed between network elements  12   c  and  12   d . In this case, a “ring” switch is implemented where working line  16   de  is rerouted to protect line  18   de  and working line  16   bc  is rerouted to protect line  18   bc . Accordingly, the remaining viable protect lines all carry traffic. Every network element can still communicate with all the other network elements  12  on the ring. 
       FIG. 3  illustrates an architecture wherein two rings  10   a  and  10   b  share a protection path between network elements  12   a  and  12   b . In WO 99/23773 (PCT/IB98/01955) to Elahmadi et al, the use of a single physical span between these two network elements is proposed. This single span provides protection for two rings  10   a  and  10   b . A failure on either ring can be remedied by using the shared protect line  18   ab  to carry traffic. This architecture reduces costs, which can be significant if the distance between the shared network elements is long (or there are other infrastructure costs involved), but increases the chance of a traffic outage on one ring if a failure occurs while there is another failure on another ring. 
     Another problem with shared protection spans is the lack of an established protocol. To realize the full cost savings inherent in one or more shared protection spans, it is desirable that traditional, fully redundant network elements be used in portions of the rings. Preferably, the operation of the shared protection network elements can be transparent to the traditional network elements, eliminating costs involved in replacing or modifying the traditional network elements. Further, it is important to maximize the use of shared spans to correct failures, so that communications traffic is maintained as much as possible. 
     Therefore, a need has arisen for a method and apparatus for using shared protect lines along with traditional protection architectures as efficiently as possible. 
     BRIEF SUMMARY OF THE INVENTION 
     In the present invention, a method and apparatus for controlling communications in a shared protection architecture is provided, where first and second network elements support communications over a plurality of working channels of respective rings using a shared protection channel common to all of the rings. Responsive to an indicated span switch on a first ring, control information for the first ring is passed over the shared protection channel while the network elements indicate the availability of the shared protection channel to rings other than the first ring. Responsive to an indication that the shared protection channel is needed to pass communications traffic for a second ring, the network element cease to pass the control information for the first ring over the shared protection channel and indicate the non-availability of the shared protection channel to rings other than the second ring. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1   a  and  1   b  illustrate prior art ring architectures; 
         FIGS. 2   a  and  2   b  illustrate prior art span switches and ring switches, respectively; 
         FIG. 3  illustrates a prior art shared protection scheme; 
         FIG. 4  illustrates a shared protection span with no communications errors on any associated ring; 
         FIG. 5  illustrates a shared protection span with a working channel failure between shared protection network elements; 
         FIG. 6   a  illustrates a shared protection span with an indicated span switch on one ring; 
         FIG. 6   b  illustrates a shared protection span with an indicated span switch on two rings; 
         FIG. 7  illustrates a shared protection span with an indicated ring switch on one ring; 
         FIG. 8  illustrates a shared protection span with an indicated span switch on one ring followed by an indicated ring switch on another ring; 
         FIG. 9  illustrates an exemplary three ring architecture with no communications errors; 
         FIG. 10  illustrates the architecture of  FIG. 9  with an indicated ring switch on one ring; 
         FIG. 11  illustrates the architecture of  FIG. 9  with an indicated ring switch on a different ring; 
         FIG. 12  illustrates the architecture of  FIG. 9  with an indicated span switch on one ring; 
         FIG. 13  illustrates the architecture of  FIG. 9  with an indicated span switch on one ring followed by a ring switch on another ring; 
         FIG. 14  illustrates the architecture of  FIG. 9  with two ring switches formed pursuant to a failure of working and shared protection spans; 
         FIG. 15  illustrates the architecture of  FIG. 9  with an indicated span switch on one ring followed by an indicated span switch on another ring; 
         FIGS. 16   a  through  16   b  illustrate the architecture of  FIG. 9  with two spans switches followed by a ring switch; and 
         FIG. 17  illustrates a state diagram showing operation of the shared protection network elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is best understood in relation to  FIGS. 4–17  of the drawings, like numerals being used for like elements of the various drawings. 
       FIGS. 4–8  illustrate a pair of network elements  12   a  and  12   b  using a shared protection channel  22   ab  communicating as different switches are implemented to maintain communication in various rings. In the illustrated embodiment, three rings, Ring 1 , Ring 2  and Ring 3 , are coupled to the each network element  12   a  and  12   b . For purposes of illustration, only one channel pair, a working channel pair  20  and a protection channel pair  22 , are shown from each ring; these channel pairs are supported by a single shared protection channel pair  22   ab . In normal operation, as shown in  FIG. 4 , working channels  20   ab   1  are coupled to working channels  20  of Ring 1 , working channels  20   ab   2  are coupled to working channels  20  of Ring 2  and working channels  20   ab   3  are coupled to working channels  20  of Ring 3 . Working channels  20  would typically be from separate fibers coupled to the network elements, although they could be channels from separate rings carried on a single fiber. In an actual implementation, each network element would be coupled to multiple fibers, each fiber providing communication over many channels. Similarly, the protection line between network elements  12   a  and  12   b  would support multiple channels as well. The illustrated embodiments of  FIGS. 4–8  are used to show the operations that occur between sets of channels of the various fibers and rings coupled to the network elements that are protected using a shared protection channel. 
     During normal operation the shared protection channels may be used to transfer control information between network elements  12   a  and  12   b . A NR (no requests) signal is output from both network elements  12   a  and  12   b  on their outgoing protection channels  22  during normal operation. 
       FIG. 5  illustrates the operation of the network elements  12   a  and  12   b  where a working channel between the network elements fails. In  FIG. 5 , working channel  20   ab   3  fails. This causes a span switch to occur at both network elements  12   a  and  12   b , and the shared protection channel pair  22   ab  is used to pass traffic from the working channels  20  of Ring 3 . The network elements  12   a  and  12   b  output a SPAN-SW (span switch signal) on their respective outgoing, non-shared protection channels  22  for Ring 3  and output a LOP (lockout of protection) signal on the outgoing protection channels for Ring 1  and Ring 2 . The LOP signal indicates that the shared protection channel pair  22   ab  between network elements  12   a  and  12   b  is no longer available. The LOP signal includes a field indicating the source of the signal. The LOP is output as long as the span switch is in place. 
       FIG. 6   a  illustrates the operation of the network elements  12   a  and  12   b  where a span switch occurs elsewhere on a ring coupled to network elements  12   a  and  12   b . In  FIG. 6 , a span switch has occurred on Ring 3 , thus causing a SPAN-SW signal to be received on the incoming protection channels  22  of Ring 3  at the network elements  12   a  and  12   b . In order to propagate the SPAN-SW signal, the shared protection channel pair  22   ab  is coupled between the protection channels  22  of Ring 3 . NR signals are output on outgoing, non-shared protection channels for Ring 1  and Ring 2 , since the shared protection channel pair  22   ab  is still available, if necessary, to pass communications traffic, which would be considered a higher priority, as will be shown below. 
     In  FIG. 6   b , a second span switch on Ring 2  occurs in addition to the span switch on Ring 3 , as indicated by the SPAN-SW signal on the incoming protection channels  22  of Ring  2 . Since the shared protection channel pair  22   ab  is being used to passthrough the signals from protection channels  22  of Ring 3 , it is not available for a second passthrough operation. Hence, a LOP is signal is output on outgoing, non-shared protection channels  22  for Ring 2 . A NR signal is still placed on the outgoing, non-shared protection channels  22  of Ring 1 , since the shared protection channels  18   ab  are still available to correct a communications failure. If the failure necessitating the span switch is corrected, the LOP signal is dropped and Ring 2  could use the shared protection channel pair for control information passthrough. Also, if the failure necessitating the span switch on Ring 2  is corrected, the LOP is dropped as well. 
     In  FIG. 7 , a ring switch is indicated on Ring 3 , as indicated by the RS (ring switch) signal received at the incoming protection channels  22  of Ring 3 . In this case, the protection channels  22  will be used to pass communications traffic once the ring switch is set up. The RS signals are passed over the shared protection channels to the outgoing protection channels  22 , so that every network element  12  in the ring receives an indication that a ring switch is needed. An ACK (acknowledge) signal (not shown) is returned to complete the setup protocol. The protection channels  22  of Ring 3  are then coupled to the shared protection channel pair  22   ab  to pass communications traffic. A LOP signal is placed on the outgoing, non-shared protection channels of Ring 1  and Ring 2 , since the shared protection channels are no longer available. When the ring switch is dropped, the LOP signal is dropped as well. 
     In  FIG. 8 , a span switch on Ring 3  is superceded by a ring switch on Ring 1 . The initial condition for the span switch on Ring 2  is shown in  FIG. 6   a . When the RS signal is received at the incoming protection channels of Ring 1 , the shared protection channel pair  22   ab  is decoupled from passing control information from the protection channels  22  of Ring 3  and is coupled to the protection channels  22  of Ring 1 , which will be passing communications traffic once the ring switch is set up at all of the associated network elements on the ring. To indicate that the shared protection channel pair  22   ab  is no longer available, a LOP signal is placed on the outgoing, non-shared protection channels  22  of Ring 2  and Ring 3 . It should be noted that this does not affect the passing of traffic over the span switch on Ring 3 , which remains operable, it only stops the passthrough of the control information through network elements  12   a–b.    
     The passthrough of control information could also be superceded by a span switch between network elements  12   a  and  12   b , as shown in  FIG. 5 . 
       FIGS. 9–16   b  illustrate the operation of the network elements of  FIGS. 4–8  in a multiple ring configuration.  FIG. 9  illustrates a network comprising eight network elements  12 , individually referenced as network elements  12   a – 12   h . The spans between network elements  12   a  and  12   b , between network elements  12   b  and  12   c , and between network elements  12   b  and  12   d  use a shared protection scheme, as shown in  FIGS. 4–8 . The spans between the remaining network elements use a conventional fully redundant protection scheme. 
     The network configuration  30  of  FIG. 9  provides three ring structures: RingA, RingB and RingC. Network elements  12   a ,  12   b ,  12   d ,  12   g  and  12   h  form RingA. Network elements  12   b ,  12   d ,  12   f  and  12   c  form RingB. Network elements  12   a ,  12   b ,  12   c  and  12   e  form RingC. As above, working channels  20  are shown in solid line and protection channels  22  are shown in dashed lines. Network elements  12   a ,  12   b ,  12   c  and  12   d  support shared protection lines as described above. The remaining network elements can be of conventional design. 
     During normal operation (no working channel or protection channel failures), each of the shared protection network elements  12   a – 12   d  output NR signals as described in connection with  FIG. 4 . 
       FIGS. 10–16   b  illustrate different failure scenarios. In  FIG. 10 , a ring switch is indicated away from the shared protection due to a failure of working and protection channels between network elements  12   g  and  12   h . Network elements  12   g  and  12   h  will issue RS signals to network elements  12   d  and  12   a , respectively, to set up a ring switch. The RS signals will pass through the remaining network elements in the ring on the shared and non-shared channels (for example, the RS signal from network element  12   h  will be sent to network element  12   a  on non-shared protection channel, where it will be passed to network elements  12   b  and  12   d  through their shared protection channels, and finally to network element  12   g  via the non-shared protection channel). An ACK signal will pass in the opposite direction via the same shared and non-shared protection channels. The shared protection network elements that are part of the ring switch, i.e., network elements  12   a  and  12   d , will issue LOP signals on their outgoing, non-shared protection channels, as described in connection with  FIG. 7 . Network element  12   b  does not issue a LOP signal, since it is not connected to any outgoing, non-shared protection lines. 
       FIG. 11  illustrates the effect of a loss of working and protection channels between network elements  12   c  and  12   f . In this case, a ring switch is set up around RingB, with network elements  12   c  and  12   f  performing the coupling of the working and protection channels. Each network element in RingB that is coupled to a non-shared protection channel outside of the RingB issues an LOP signal. Hence, network element  12   c  issues a LOP signal to network element  12   e  and network element  12   d  issues a LOP signal to network element  12   g.    
     In  FIG. 12 , a span switch is implemented to circumvent a working channel failure between network elements  12   a  and  12   e . Network element  12   e  couples the working channel from network element  12   c  with the protection channel from network element  12   a ; network element  12   a  couples the working channel from network element  12   b  with the protection channel from network element  12   c . The shared protection channels  22   ab  and  22   bc  are placed in passthrough mode to pass control information around RingC. 
       FIG. 13  illustrates a span switch in RingC followed by a ring switch in RingA caused by a loss of working and protection channels between network elements  12   d  and  12   g . The initial condition caused by the span switch in RingA will be the same as shown in  FIG. 12 . Specifically, the shared protection channels  22   ab  and  22   bc  will be used to pass control information for RingC. The ring switch in RingA, however, supercedes the control passthrough in RingC (the span switch continues to function without passing the control information). Consequently, shared protection channel  22   ab  stops passing control information for RingC and is used to setup the ring switch in RingA by passing RS and ACK signals. Subsequently, protection channel  22   ab  is used to pass traffic for the ring switch. An LOP signal is output to network element  12   a  to network element  12   e  and from network element  12   d  to network element  12   f.    
       FIG. 14  illustrates a span switch in RingC followed by a span switch in RingA, caused by a working channel failure between network elements  12   g  and  12   h . The initial condition for the span switch in RingC is shown in  FIG. 12 . The span switch in RingC causes a SPAN-SW signal to be sent from network element  12   h  to network element  12   a  and from network element  12   g  to network element  12   d . Since the shared protection channel pair  22   ab  is used by the span switch in RingC, network element  12   a  returns a LOP signal to network element  12   h , indicating that the shared protection channel  22   ab  is not available to pass control information; nonetheless, the span switch can be implemented to pass communications traffic over the protection channel between network elements  12   g  and  12   h.    
     In  FIG. 15 , ring switches in RingC and RingA are necessitated by a failure of the working and shared protection channels between network elements  12   a  and  12   c . Shared protection channel  22   bd  is used for the ring switch for RingA; the remaining spans of the ring switches are made using non-shared channels. Network element  12   c  issues a LOP signal to network element  12   f  and network element  12   d  issues a LOP to network element  12   f . The remaining network elements are involved in the ring switch and do not receive LOP signals. 
       FIGS. 16   a–b  illustrate a situation which could involve an isolated network element, but for the prioritization of failure conditions. In  FIG. 16   a , a working channel failure between network elements  12   e  and  12   a  causes a span switch. As shown in connection with  FIG. 12 , network elements  12   a ,  12   b  and  12   c  provide a passthrough path for control information over shared protection channels  22   ab  and  22   bc . A second span switch is necessitated by a working channel failure between network elements  12   d  and  12   f . In response to receiving the SPAN-SW signal from network elements  12   d  and  12   f , network element  12   c  issues an LOP signal to network element  12   f . At this point, shared protection channel  22   bc  continues to be used for passing control information for RingC. RingB does not have a passthrough for control information, however, the span switch is still implemented such that communications traffic can pass between network elements  12   f  and  12   d.    
     As shown in  FIG. 16   b , an ensuing failure of working and protection channels between network elements  12   g  and  12   h  causes a ring switch on RingA. When network element  12  receives a RS signal, it drops the passthrough of control information for RingC and passes the RS to network element  12   b  via shared protection channel  22   ab . Similarly, network element  12   b  drops the passthrough of control information through protection channel  22   bc  and couples shared protection channel  22   ab  to protection channel  22   bd . After the RS and ACK signals have promulgated through each network element in RingA, the ring switch is implemented and shared protection channels  22   ab  and  22   bc  are used to pass traffic. LOP signals are generated by network element  12   d  to network element  12   f  and by network element  12   a  to network element  12   e.    
     With reference to  FIG. 4 ,  FIG. 17  illustrates a state diagram showing the signals generated by shared protection network elements  12   a  and  12   b  according to various states of the shared protection channel  22   ab . In state  40 , the SP (shared protection channel)  22   ab  is not being used by any ring coupled to the network elements  12   a–b , either for control information passthrough or for communications traffic. As shown in  FIG. 4 , an NR signal is generated on all outgoing, non-shared protection (NSP) channels  22 . 
     If the network elements  12   a–b  receive a SPAN-SW signal, indicating a span-switch on a RingR (where RingR is an arbitrary ring supported by the shared protection network elements), the state transitions to state  42 , where the shared protection channel  22   ab  is used for control information passthrough (as shown in connection with  FIG. 6   a ). NR signal are output on the non-shared protection channels  22  for all rings other than RingR, except for rings that subsequently have a SPAN-SW indication. For rings with a subsequent SPAN-SW indication, an LOP signal is generated on the non-shared protection channels  22 . 
     Once the failure is remedied, the state transitions to state  40 . If there were subsequent SPAN-SW signals that were still enabled, the state would transition back to state  42 . 
     If a ring switch is indicated on RingM (an arbitrary ring) or the RingM working channel  20   ab  between the network elements  12   a–b  fails, while in either state  40  or state  42 , the state transitions to state  44 , where the shared protection channel is used to carry traffic. In the case of a ring switch, the shared protection channel  22   ab  is coupled between other protection channels  22  to effect the ring switch ( FIG. 7  and  FIG. 8 ). In the case of a working channel failure, the shared protection channel  22   ab  is coupled between working channels  20  in a span switch ( FIG. 5 ). In either case, LOP signals are generated on the non-shared protection channels of all rings other than RingM. In the case of the span switch, the SPAN-SW signal is generated on the non-shared protection channels associated with RingM. 
     Again, once the failure is remedied, the state returns to state  40 . 
     If there is a failure of the shared protection channel  22   ab , the shared protection channel will be unable to pass either traffic or control information (state  46 ). In this case, ring switches can be formed for any ring where the associated working channel  20   ab  has failed as well ( FIG. 15 ). Otherwise, a LOP signal is generated on the non-shared protection channels  22  for each ring. When the failure is corrected, the state returns to state  40 . The state could also transition to state  46  from either state  42  or state  44 . 
     The present invention provides significant advantages over the prior art. First, the LOP, NR, RS, and SPAN-SW signals are compatible with fully redundant network elements. The actions taken in response to these signals allow integration of traditional network elements with the shared protection network elements. The prioritization of action based on various conditions described herein provides maximization of the shared protection channels for maintaining communications traffic, while allowing use of the shared protection channels for passing control information where appropriate. Situations where a node could be isolated from status signals from other network elements due to multiple line failures, such as shown in  FIGS. 16   a–b , are eliminated; therefore, all network elements may participate restorations regardless of the sequence of failures. 
     Although the Detailed Description of the invention has been directed to certain exemplary embodiments, various modifications of these embodiments, as well as alternative embodiments, will be suggested to those skilled in the art. The invention encompasses any modifications or alternative embodiments that fall within the scope of the Claims.