Abstract:
The channel protection facility in a two-fiber bi-directional line switched ring network may be significantly enhanced by protecting the service channels in one of the fibers with corresponding protection channels in the other fiber and vice-versa. Thus, if a failure occurs such that the content of the service channels cannot be forwarded to an upstream point, then, without having to perform a wavelength conversion function, the service channels may be substituted for the protection channels in the other fiber and forwarded to the upstream point in the opposite direction via the ring network.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical networks and more particularly relates to a signal protection feature for an optical ring network. 
     BACKGROUND OF THE INVENTION 
     Optical networks require a reliable transport mechanism to transport information between optical nodes (switches) forming a bi-directional ring network. A reliable transport mechanism typically includes diverse transport paths to deliver information signals from one optical node to another optical node. The optical nodes forming the ring network transmit information over the shortest path to an intended recipient. However, if the transmission path fails, then the node will re-transmit the information over the other diverse path to the intended recipient. In this sense, one path provides protection for the other path when a failure occurs. 
     SUMMARY OF THE INVENTION 
     I have recognized that such a protection scheme is very expensive to implement in a so-called two fiber Bi-directional Line Switched Ring (BLSR) network arranged in accordance with the well-known Synchronous Optical NETwork (SONET) standard. 
     Specifically, for a particular optical transmission capacity, e.g., an OC48 optical system, channels 1 through 24 in each direction of a two fiber BLSR are reserved for service and channels 25 through 48 are reserved for protection. Thus, if the fiber carrying the in-service channels fails in one direction, then the transmitting node has to transfer the information from those service channels to the protection channels carried by the other fiber or path. In an optical transport system channels correspond to respective wavelengths, and the transfer would entail converting the wavelengths of the signals of the corresponding service channels to the wavelengths carried in the protection channels. For example, for a 48 channel system, the wavelengths of the signals being carried in channels 1 through 24 (i.e., λ 1  through λ 24 ) would have to be respectively converted to the wavelengths carried in channels 25 through 48 (i.e., λ 25  through λ 48  ) of the protection path. The equipment needed to perform such converting is indeed expensive, and would have to be provisioned at each node of a two fiber BLSR network. 
     I have further recognized that the foregoing problem may be dealt with at the optical level in an optical ring network by providing a one-to-one correspondence between the service channels that are transported over one fiber and the protection channels that are carried in the opposite direction over the other fiber of a two fiber BLSR network. 
     These and other aspects of the instant invention will become more apparent from the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 is a broad block diagram of a two fiber Bi-directional Line Switched Ring (BLSR) network in which the principles of the invention may be practiced; and 
     FIG. 2 is a broad block diagram of a node of FIG.  1   
    
    
     DETAILED DESCRIPTION 
     The illustrative ring network  200  shown in FIG. 1 is formed from a plurality of two-fiber BLSR optical nodes (also referred hereinafter as just nodes)  100 - 1  through  100 - 4  that are interconnected by two optical transmission paths  110  and  120 . Paths  110  and  120  transport information over the ring network in opposite directions. Assuming that network  200  operates at the 48 wavelength (channel) level, then, priorly, channels 1 though 24 of paths  110  and  120  would have been reserved as service channels and channels 25 through 48 would have been reserved as protection channels. Information is usually transported from one node to another node via the shortest path. For example, assume that node  100 - 4  receives information destined for node  100 - 3  via the service channels, e.g., channels λ 1  through λ 24 , over path  110  connected to port  22  of node  100 - 4 . Node  100 - 4 , in turn, forwards the information to node  100 - 3  via the shortest transmission path, namely, the path  110  connected to port  21  of node  100 - 4 . Node  100 - 4  similarly forwards information that is destined for node  100 - 3  and received via another input path, e.g., path  601 . (Note that the term “channel” is taken to mean herein an optical signal of a particular wavelength and that a group of channels is taken to mean a group of optical signals of respective wavelengths. In a broader sense, the term “channel” could also be taken to mean “time slot”.) 
     If the path  110  connected to port  21  fails, as represented by the X, then node  100 - 4 , in prior instances, would have to convert the service channels λ 1  through λ 24  to protection channels λ 25  through λ 48 , respectively, so that the information may be forwarded to node  100 - 3  via the protection channels λ 25  through λ 48  of path  120  connected to port  12  of node  100 - 4 . The information would then be transported via nodes  100 - 1  and  100 - 2  and the path  120  segments to port  11  of node  100 - 3 . Node  100 - 3  would then convert the information signals received via protection channels λ 25  through λ 48  to their expected wavelengths of λ 1  through λ 24  so that the signals may be forwarded to their ultimate destinations. 
     As mentioned above, the apparatus needed to convert an optical signal from one wavelength to another wavelength is indeed expensive. Moreover, such apparatus would need to be provisioned at each node in the two-fiber BLSR. 
     As also mentioned above, such conversion may be eliminated by matching the wavelengths of the protection channels in one transmission path with the wavelengths of the service channels in the other transmission path. 
     Thus, for example, channels λ 1  through λ 24  of path  110  and channels λ 1  through λ 24  of path  120  would respectively serve as service channels and protection channels. In this way, one group of channels in one path would provide protection for corresponding channels in the other path. Similarly, channels λ 24  through λ 48  of path  120  and channels λ 24  through λ 48  of path  110  would respectively serve as service channels and protection channels. 
     As another example, the odd numbered channels of path  110  (λ 1 , λ 3 , λ 5 , etc.) and the odd numbered channels of path  120  may respectively serve as service channels and protection channels. Similarly, the even numbered channels of those paths may respectively serve as the service and protection channels. 
     Thus, a two fiber BLSR node arranged in accordance with the principles of the invention would not have to convert the wavelengths of the path  110  service channels to the wavelengths of the path  120  protection channels, or vice-versa, since those channels would respectively correspond to one another. 
     A broad block diagram of a node  100 -i arranged in accordance with the principle of the invention is illustrated in FIG.  2 . Specifically, a node includes, inter alia, a plurality of channel switches  50 - 1  through  50 -N which are assigned to respective channels λ 1  through λ N . A node also includes, for each transmission path; an optional incoming optical amplifier ( 10 - 1 ,  10 - 4 ); a wavelength/channel demultiplexer ( 15 - 1 ,  15 - 2 ) that demultiplexes an optical signal that it receives from an incoming optical amplifier (or incoming transmission path) into component channels/wavelengths and presents the demultiplexed wavelengths to respective channel switches  50 -i; a wavelength/channel multiplexer ( 25 - 1 ,  25 - 2 ) that receives the component channels/wavelengths from the channel switches  50 -i multiplexes the channels to an output connected to an optional output optical amplifier ( 10 - 2 ,  10 - 3 ) for presentation to a respective one of the transmission paths  110 ,  120 . (Note that the need for one or both of the optional optical amplifiers in a path would be determined by the power requirements set for a particular system implementation.) 
     Each channel switch processes a respective one of the channels, e.g., channel λ 1 , that it receives from paths  110 ,  120  via a respective DEMUX  15  or that it receives from input  60 - 1  connected to 1:2 signal splitter  40 . (It is noted that since channels switches  50 - 1  through  50 -N have similar structures, a description of one of the channel switches equally pertains to the other channel switches. Thus, the following description is directed to just one of the channel switches  50 -i, e.g., channel switch  50 - 1 .) Channel switch  50 - 1 , more particular, includes a pair of 1:3 switches  30 - 1  and  30 - 2 ; a pair of 3:1 switches  35 - 1  and  35 - 2 ; and 2:1 switch  45  as well as signal splitter  40 . Channel switch  50 - 1  responds to instructions issued by a controller  20  which communicates with each of the channel switches  50 -i via control bus  21 . Such communications includes instructions which control the switching performed by the 1:3 switches, 3:1 switches and 2:1 switch in a channel switch. In particular, 1:3 switch  30 - 1 , responsive to instructions that it receives from controller  20  switches the λ 1  signal that it receives from path  110  via port  22  and DEMUX  15 - 2  to one of three output paths, namely output path  31 - 1  extending to 3:1 switch  35 - 2 , path  31 - 2  extending to 2:1 switch  45  or path  31 - 3  extending to 3:1 switch  351 . The latter path as well as path  32 - 3  is a so-called loop-back path and is used for protection purposes and for routine maintenance purposes. The following discusses the protection function only. 
     Switch  30 - 2  responds to instructions that it receives from controller  20 , and thus performs a similar switching function with respect to the λ 1  signal that it receives from path  120  via port  11  and DEMUX  15 - 1 . The 3:1 switch  35 - 1 , on the other hand, performs an opposite switching function in that it switches one of its three inputs respectively connecting to loop-back path  31 - 3 , 1:2 splitter  40 , path  40 - 2  and path  32 - 4  to output path  24 - 1  connecting to MUX  25 - 1 . MUX  25 - 1  then multiplexes the signal that it receives from switch  35 - 1  and other channel switches to OA  10 - 2  connecting to path  120  at port  12 . Switch  35 - 2  similarly switches, pursuant to controller  20  instructions, one of three inputs to an output connecting to MUX  25 - 2 . 
     Referring now to FIGS. 1 and 2 and assuming that the path  110  between nodes  100 - 4  and  100 - 3  is operable, then an optical signal that node  100 - 4  receives via port  22  passes through OA  10 - 4  to DEMUX  15 - 2 . For a  48  wavelength optical system, for example, DEMUX  15 - 2  demultiplexes the received optical signal into the various component signals having wavelengths of λ 1  through λ 48 , respectively. The demultiplexed λ 1  signal/channel is supplied to the input of switch  30 - 1  of channel switch  50 - 1 . Similarly, the λ 2  through λ 48  signals are respectively supplied to channel switches  2  through N, where N equals 48 in the instant illustrative example. If protection switching has not been invoked, and the λ 1  signal is not being outputted to 2:1 switch  40 , then switch  30 - 1  outputs the λ 1  signal to path  31 - 1  connecting to 3:1 switch  35 - 2 . Switch  35 - 2  then outputs the signal that it receives via path  31 - 1  to MUX  25 - 2  which then multiplexes the λ 1  signal and the λ 2  through λ 48  signals received from channel switches  50 - 2  through  50 - 48 , respectively, to port  21  via OA  10 - 3 . 
     The optical signal that is received at port  11  via path  120  is similarly handled and the resulting optical signal is multiplexed onto path segment  120  extending to node  100 - 1  via port  12 . 
     Assume at this point that the path  110  segment between nodes  100 - 4  and  100 - 3  fails as represented by the large X on path  110  between nodes  100 - 4  and  100 - 3 , as shown in FIG.  1 . In that event, then, an external mechanism, e.g., a craftsperson or an automated mechanism, would so notify each controller  20  in nodes  100 - 4  and  100 - 3  via communication path  21 . Each such controller  20  would then invoke protection switching by so instructing the appropriate ones of the switches  30 - 1  in nodes  100 - 4  and  100 - 3 . Assuming that the path  110  service channels are λ 1  through λ 24 , then controller  20  in node  100 - 4  would instruct switch  30 - 1  in each of the channels switches  50 - 1  through  50 - 24  to switch an incoming signal to the loop-back path  31 - 3  to switch  35 - 1 . Controller  20  would also instruct switch  35 - 1  in each of the channel switches  50 - 1  through  50 - 24  to switch the signal on the respective loop-back path  31 - 3  to multiplexer  25 - 1 . Switch  35 - 1  in each of the channel switches  50 - 25  through  50 - 48  would continue to switch the path  120  service channels λ 25  through λ 48  received via port  11 , OA  10 - 1 , DEMUX  15 - 1  and the respective switch  30 - 2  to multiplexer  25 - 1 . Thus, multiplexer  25 - 1  receives the sequence of the path  110  service channels along with the sequence of path  120  service and multiplexes each such channel in proper order onto outgoing path  120  via OA is  10 - 2  and port  12  for delivery to node  100 - 3  via path  120  and node  100 - 1  and  100 - 2 . 
     Similarly, controller  20  in node  100 - 3  would instruct, for example, each of the channel switches  50 - 1  through  50 - 48  to switch the respective channel that it receives from DEMUX  15 - 1  via path  120  and OA  10 - 1  to an appropriate output, e.g., the associated path  60 - 2 , so that the channel may be forwarded to its intended destination. 
     As another illustrative example, assume that controller  20  of node  100 - 3  has instructed a particular channel switch, e.g., channel switch  50 - 1  of node  100 - 3 , to switch (“drop”) channel λ 1  received via path  110  to output path  60 - 2 . Also assume that controller  20  of node  110 - 4  has instructed channel switch, e.g., channel switch  50 - 1  of node  100 - 4 , to add the channel λ 1  signal that is received via path  60 - 1  to the signal that is to be outputted to path  110  via MUX  25 - 1 , OA  10 - 3  and port  21 . In response to such instructions at node  100 - 3 , 1:3 switch  301  switches the λ 1  signal that it receives from DEMUX  15 - 2  to path  31 - 2  connected to one input of 2:1 output switch  45 , which then outputs the signal to path  60 - 2  in accordance with the controller  20  instructions. At node  100 - 4 , 1:2 splitter  40 , on the other hand, outputs equal portions of the λ 1  signal that it receives from input path  60 - 1  to paths  40 - 1  and path  40 - 2 . In a non-failure state, only switch  35 - 2  would output that signal pursuant to the node  100 - 4  controller  20  instructions. That output signal is presented to MUX  25 - 2 , which then “adds” that signal to the signal that will be outputted to path  110  via OA  10 - 3  and port  21 . However, if the path  110  segment between nodes  100 - 4  and  100 - 3  has failed, as assumed above, then the node  100 - 4  controller  20  would instruct 3:1 switch  35 - 1  to switch the signal that it receives from splitter  40  to its output connected to MUX  25 - 1 . MUX  25 - 1  would then output the signal via OA  10 - 2  and port  10 - 2  in the λ 1  channel reserved for protecting that signal on path  120 , all in accordance with the principles of the invention. 
     It will thus be appreciated that, although the invention illustrated herein is described in the context of a specific illustrative embodiment, those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly shown or described herein, nevertheless, embody the principles of the invention and are within its spirit and scope.