Abstract:
A facility is provided to allow a supervisory message to quickly propagate through a transmission network without delay. Specifically, a supervisory message is quickly routed from one node to a next node by (a) splitting the control channel signal carrying the supervisory message at a receiving node, (b) sending one of the split control channel signals to an output via switchable apparatus for immediate transmission the next node and (c) sending the other split signal to a controller for analysis. If the controller invokes a predetermined procedure as a result of the content of the message, e.g., invokes protection switching, then the controller forms a supervisory message identifying the invoked procedure, operates the switchable apparatus so that the message identifying the invoked procedure may be routed to the output in place of the split channel signal message.

Description:
TECHNICAL FIELD 
     The invention relates to optical communications networks, and more particularly relates to a protection ring architecture for such networks. 
     BACKGROUND OF THE INVENTION 
     Optical transmission systems and, especially systems using Dense Wavelength Division Multiplexing (DWDM), are desirable since they provide extremely wide bandwidths for communications channels. Each communications channel in a DWDM transmission system carries a plurality of, for example, 16, 40 even 80, optical channels (wavelengths) on a single optical fiber and single optical repeater. However, there is a trade off between providing wider bandwidth communications channels, with their corresponding lower cost of transport, and their vulnerability to a large-scale disruption of communications services due to a transmission medium failure. Therefore, it is important that an optical transmission system, for example, those employing DWDM, have the capability to recover quickly from a transmission medium failure because of the impact that such a failure has on communications services. 
     When a node detects a transmission failure, i.e., a loss of signal (LOS), it enters a series of restoration logic steps to determine if it ought to invoke protection switching procedures, e.g., so called 1+1 or 0×1 protection switching, to quickly recover from the failure. If not, then the node sends a message identifying the failure in a control (supervisory) channel to a next node. The next node similarly determines if it ought to invoke protection switching to deal with the failure. If not, then that node sends the message identifying the failure in the control channel to a next node, an so on. When a node receiving the failure message determines that it ought to invoke protection switching and does so, it then notifies the other nodes via the control channel that such protection switching has been invoked. 
     It can be appreciated that the above procedure is time consuming, and is especially so for those transmission systems that have to invoke protection switching as quickly as possible to meet customer expectations relating to system “down time”. 
     SUMMARY OF THE INVENTION 
     We have recognized that the best way in which to route a supervisory message from one node to a next node of a transmission system is to split the control channel signal carrying the supervisory message, send one of the split control channel signals to an output via switchable apparatus for immediate transmission to the next node and send the other split control channel signal to a controller for analysis. If the controller, responsive to the supervisory message, invokes a predetermined procedure, e.g., protection switching, it then forms a supervisory message identifying the invoked procedure, operates the switchable apparatus to route the latter message to the output in place of the split control channel signal message. 
     These and other aspects of the invention will be appreciated from the following detailed description, accompanying drawings and ensuing claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is broad block diagram of a optical ring transmission system in which the principles of the invention may be practiced; and 
     FIG. 2 is a broad block diagram of an optical node of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     The claimed invention will be discussed in the context of an optical transmission system having a ring architecture. It is understood of course that that should not be construed as a limitation, since the art will appreciate from the following discussion that the claimed invention may be practiced in other than ring architectures. 
     With that in mind, a bi-directional optical transmission system  100  shown in simplified form in FIG. 1 comprises a plurality of optical nodes connected in a ring configuration. For the sake of simplicity and clarity only four of the optical nodes,  200 - 1  through  200 - 4 , are shown, in which each of the nodes incorporates an embodiment of the invention. Optical nodes  200 - 1  through  200 - 4  are interconnected in a ring configuration by optical transmission media  110  which transports, e.g., active (primary) service transmission capacity. Nodes  200 - 1  thorough  200 - 4  are also interconnected in a ring configuration by optical transmission media  120  which transports, e.g., standby (protection) transmission capacity. The latter capacity may be used to transport what we call pre-emptory (secondary) service traffic, which a node sheds in favor of the active traffic when a transmission failure occurs. In an illustrative embodiment of the invention, optical transmission medium  110  and  120  are formed from optical fibers, and each may be comprised of a single optical fiber or two optical fibers. To say it another way, optical transmission system  100  could be either a two optical fiber system or a four optical fiber system, in which each of the fibers includes one-half of the service bandwidth and one-half of the protection bandwidth. In an alternative embodiment of the invention, one of the two optical fibers can carry active service transmission capacity and the other one of the two fibers can carry protection transmission capacity. In a four optical fiber system, separate optical fibers are used to transport active service transmission capacity in both directions of transmission and separate optical fibers are used to transport standby protection transmission capacity in both direction of transmission. Optical transmission system  100  may transport  8 ,  16 ,  32 ,  40 ,  80 , etc. communications channels, i.e., wavelengths. Note that, in addition to the communications channels, an extra telemetry control (supervisory or control) channel may be used as a maintenance channel in the two optical fiber arrangement or the four optical fiber arrangement. Thus, in an eight channel system, nine channels are transported, in a sixteen channel system, 17 channels are transported, and so on. The maintenance channel is used, among other things, to transport protection switching information for configuring nodes  200 - 1  through  200 - 4  in system  100  when a transmission failure occurs and is detected by a node. 
     Assume that a transmission failure occurs in the path between nodes  200 - 1  and  200 - 4  as represented by the “X” in path  110 . The transmission failure may be manifested by one of a number of different failures, including, for example, (a) a decrease in the power level of the signal below a predetermined threshold; (b) the error rate of the received signal exceeds a predetermined error rate; or (c) the signal-to-noise ratio for the received signal falls below a predetermined signal-to-noise ratio. When a node detects one of these failures, it then declares a loss of signal (LOS) and determines if it ought to invoke protection switching to recover from the failure. If not, then, as mentioned above, it sends a message identifying the failure to a next node. For example, if node  200 - 4  detects a loss of signal for the inbound path  110  from node  200 - 1 , then node  200 - 4  enters the aforementioned logic steps to determine if it ought to invoke protection switching, and sends a supervisory message upstream in the control channel to node  200 - 3  if it concludes otherwise. Node  200 - 3  upon receipt of the control channel message immediately outputs the message to the next node, e.g., node  200 - 2 , and in parallel therewith, determines if it ought invoke protection switching to deal with the loss of signal. Thus, in accord with an aspect of the invention, the supervisory message is not delayed at a node until a determination is made by the node, but is immediately outputted to a next node in parallel with making such a determination. 
     A broad block diagram of an optical node  200 -i arranged in accordance with the principles of the invention is shown in FIG. 2. A discussion of optical node  200 -i equally pertains to the other nodes shown in FIG.  1 . Moreover, a discussion of the way in which the control channel, λ c , carried in the easterly bound service signal, λ SE , arriving at optical amplifier  210 - 1  is processed equally pertains to the way in which the control channel, λ c , carried in the westerly bound service signal, λ SW , arriving at optical amplifier  310 - 1  is processed. Further, such processing includes similar equipment associated with a common master controller  250 . Optical node  200 -i includes in the easterly direction, inter alia, optical amplifier  210 - 1  that amplifies an inbound optical signal, λ SE , received via service path  110 . The amplified signal, in turn, is demultiplexed at demultiplexer  215 - 1  into a plurality of component signals (also referred to herein as “channels”) λ 1 , through λ n . Information carried by individual ones of the channels is processed at node  200 -i and routed to respective customers as represented by the dashed lines for signals λ 1  and λ 3 . The demultiplexed signals also include the control channel λ c . In an illustrative embodiment of the invention, the control channel is processed at a particular wavelength, e.g., 1310 nanometers, but is transmitted at another particular wavelength, e.g., a wavelength in the 1550 nanometer range. This is done to take advantage of so-called off-the-shelf, short-reach optical equipment, which generally operates in the 1310 nanometer range. It is understood of course that this should not be taken as a limitation since short-reach optical equipment that would operate in 1550 nanometer range could be readily designed to eliminate the need to convert an incoming channel, e.g., the control channel, from the 1550 nanometer range to 1310 nanometers to process the contents of the channel. 
     The demultiplexed control channel is supplied to Optical Translation Unit (OTU)  220 , which, in a conventional manner, amplifies and then converts (translates) the control channel from a signal in the 1550 nm range to a signal in the 1310 nm range. The converted  1310  nm optical signal is supplied to a conventional optical splitter  225 , which supplies a portion of the converted signal to the “A” terminal of switch (SW)  240  and a portion to receiver  230 - 1  of transceiver  230 . (Note that switch  240  is a fast optical switch, for example, a Mach Zender optical switch, but is represented in the FIG. as a conventional electrical switch for the sake of simplicity and clarity.) In a default mode, terminals A and C of switch  240  are connected to one another to supply the converted signal to OTU  245 , which amplifies and converts (translates) 1310 nm wavelength signal back to a signal having a wavelength in the 1550 nm range. The latter optical signal is then supplied to multiplexer  215 - 2 , which multiplexes the control channel signal with the service signals/channels λ 1  through λ n  to a multiplexed output. The outputted multiplexed signals are then amplified by conventional optical amplifier  210 - 2  and forwarded to the next node, without delay, all in accordance with an aspect of the invention. 
     Receiver  230 - 1 , on the other hand, converts, in a conventional manner, the optical signal/channel that it receives via splitter  225  into an electrical digital signal, and supplies the latter signal to OC 48  controller  235 . Controller  235  demultiplexes the OC 48  signal to recover the supervisory message carried by the OC 48  signal, and supplies the recovered message to master controller  250 . Master controller  250  then analyzes the message to determine what action it needs to take if any. If master controller  250  concludes that it does not need to take any action, then it disregards the message. If the message happens to pertain to a loss of signal, and master controller determines that it ought to invoke protection switching, then master controller  250  causes the service traffic to be switched to the protection channels of transmission path  120  to recover from the loss of signal. In addition, master controller  250  forms a supervisory message indicating that such protection switching has been invoked at node  200 -i, and supplies the message to OC 48  controller  235 . In addition, master controller  250  send a control signal via lead  251  to controllably (switchably) operate switch  240 , causing terminal C to switch to terminal B. Controller  235  maps the message into a message having an OC 48  signal format and outputs the message to optical signal transmitter  230 - 2 . Transmitter  230 - 2  converts the digital signal that it receives from controller  235  into its optical equivalent having a wavelength of 1310 nm and supplies the converted optical signal to terminal B of switch  240 . When so operated, switch  240  routes the signal from terminal B to the input of OTU  245 , which, in a conventional manner, converts the wavelength of the optical signal from 1310 nm to an optical signal in the 1550 nm range. The supervisory message originated by master controller  250  is then multiplexed to the output of MUX  215 - 2  and transported to the next node via OA  215 - 2 , all in accordance with the principles of the invention. 
     In the westerly direction, optical node  200 -i similarly includes, inter alia, optical amplifiers  310 - 1  and  310 - 2 , demultiplexer  315 - 1 , multiplexer  315 - 2 , splitter  325 , transceiver  330 , OC 48  controller  335 , optical switch  340 , OTUs  320  and  345  as well as commoner master controller  250 . As mentioned above, the foregoing discussion equally pertains to such equipment and to the service signal, λ SW , traveling in the westerly direction 
     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.