Abstract:
A dual-output Mach-Zehnder modulator (MZM2) and configurations of optical transmitters based on the MZM2 which obviate the optical splitter that is typically used to provide keep-alive functionality in redundantly-connected, protected optical networks, SONET add-drop multiplexers, and optical IP routers. The configurations provide equivalent keep-alive utility at a reduced system cost relative to the prior art. The configurations also support enhanced Operations, Administration, Maintenance, and Provisioning (OAM&amp;P) functionality at little to no additional cost relative to the prior art. Instead of being a direct copy of the service signal, the keep-alive of this configuration is an inverted version of the service signal. This inverted version of the service signal is supported at a client by utilization of means for detecting and righting the inverted signal. The inverted state of the signal on the protection path can be used as an inband indicator to notify the client in the event of a failure on the service path. This inband indicator requires little or no additional circuitry to accommodate. Another aspect of this invention allows the provisionable substitution of the inverted signal with a preemptive signal to support low-priority traffic when there is no fault on the service path S.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical networks and more specifically to techniques for protecting optical physical links using redundant protection channels. 
     2. Description of the Related Art 
     Optical transmission systems, such as those using Dense Wavelength Division Multiplexing (DWDM), provide extremely wide bandwidth for communications. Each DWDM transmission system carries a plurality of optical channels (wavelengths) on each optical fiber and through each optical repeater. However, there is a trade off between the lower cost of transport provided by wider bandwidth communications channels and their vulnerability to a large-scale disruption of communications services because of a transmission equipment and/or medium failure. It is, therefore, important that DWDM optical transmission systems have the capability to quickly recover from such transmission failures. 
     Protection of optical networks in the event of failures (e.g., fiber cuts, transmitter failure, and amplifier instabilities) typically involves redirecting the service traffic from a channel on the optical fiber within which it was originally carried (i.e., the service channel, denoted by S) that has been affected by the failure to another unaffected source of bandwidth (i.e., the protection channel, denoted by P) whereby the service traffic may ultimately reach its intended destination. 
     Typically, optical switches located within a node are used to accomplish this redirection. For example, it is typical to direct optical signals transmitted from edge equipment along one direction on the network (e.g., East) to another (e.g., West). In a ring, mesh, hypercube, or other redundantly connected optical network topology, performance monitoring that analyzes and monitors the traffic on S and P at the various destination and intermediate nodes can be used by a microcontroller to autonomously switch over to a protection channel or path P by sensing a failure on the primary service path S. Note that the protection channel P can be the same or different optical wavelength (i.e., wavelength diversity), but it is typically on a different fiber, and that fiber is typically carried in a different bundle along a unique path from the first (i.e., path diversity). 
     There are a number of different optical protection schemes in use today that build upon this basic principle. These include 1+1 protection, span protection, 1:1 protection, and shared protection. These schemes are described in detail in Al-Salameh, D. Y., Korotky, S. K., Levy, D. S., et al.,  Optical Fiber Telecommunication—Volume IVA , Elsevier Science, USA, Ch. 7, pp. 318–327, incorporated herein by reference. Additional shared optical protection schemes denoted 1:N are discussed in detail in U.S. patent application Ser. No. 09/675,733 filed on Sep. 30, 2000 as attorney docket no. Al-Salameh, D. Y., 10-1-2-5-35, also incorporated herein by reference. 
     It is a generally accepted practice to provide a continuous or “keep-alive” signal to the protection channel P to allow the system to determine that P is alive and alarm free (i.e., kept alive) prior to a given failure event. Keep-alive signals can be provided in numerous ways; however, it is typical to use a fairly accurate copy of the service signal as the keep-alive source, and it is typical of all of the schemes referenced above to derive this copy via an optical splitting function of some nature. 
     There are two basic schemes in use today for modulating a light signal with data. The first scheme, termed “direct modulation” involves the application of the data or modulation signal directly to the laser source, essentially switching the laser on and off corresponding to a modulating data stream of logical “1”s and “0”s. This scheme suffers from instability in the transmission wavelength of the laser referred to as “chirp” and related transient effects that result from the direct amplitude modulation of the laser. The second and generally preferred scheme for optical modulation is termed “external modulation.” In this preferred scheme, the laser is driven at a constant power level and the resulting continuous wave (CW) output of the laser is fed to an “external modulator” such as a Mach-Zehnder (MZ) device. 
     Thus a typical optical transmitter configuration is a CW laser followed by an MZ external modulator and, in protected optical networks, it is typical to follow this configuration with an optical splitter to generate the signals that will supply light to the service S and protection P channels. 
     Use of an optical splitter to generate the keep-alive signal has the inherent disadvantage of introducing a splitter loss (e.g., ˜3.5 dB) into the signal path that may result in higher system costs to overcome (e.g., additional optical amplifiers in the path, higher-cost transmitter lasers, or more-expensive low-loss components in the transmitter or optical pathways to save power budget). As an alternative to an optical splitter, a network&#39;s transmission equipment (e.g., an optical translation unit (OTU)) can be designed to have an extra transmitter that serves the keep-alive function. However, such a device is expensive due to the cost of the high-speed optoelectronics needed in the extra transmitter. Optionally, a single-channel OTU in the line system can be designed (i.e., programmed) to transmit a keep-alive signal when it is not being fed by an input signal. This approach is still burdened with the cost of the additional OTU hardware and requires intelligence in the OTU and complex control algorithms to distinguish between transients on the line system and actual failures. 
     SUMMARY OF THE INVENTION 
     The present invention involves a new use of a dual-output Mach-Zehnder modulator (MZM2) and new configurations of optical transmitters based on the MZM2 that obviate the optical splitter typically used to provide keep-alive functionality in redundantly connected, protected optical networks, synchronous optical network (SONET) add-drop multiplexers, and optical internet-protocol (IP) routers. The new configurations provide equivalent keep-alive utility at a reduced system cost relative to the prior art. The new configurations also support enhanced Operations, Administration, Maintenance, and Provisioning (OAM&amp;P) functionality at little to no additional cost relative to the prior art. Instead of being a direct copy of the service signal, the keep-alive of this new configuration is an inverted version of the service signal. This inverted version of the service signal is supported at a client by utilization of means for detecting and righting the inverted signal. The inversion of the signal on the protection path relative to the service path may be used as an inband indicator to notify the client in the event of a failure on the service path. This inband indicator requires little or no additional circuitry to accommodate. Another aspect of this invention allows the provisionable substitution of the inverted signal, or more generally, the keep-alive signal, with a preemptive signal to support low-priority traffic when there is acceptable signal quality on the service path S. This involves minor modifications to the client to realize simplified support for preemptive traffic. 
     In one embodiment, the present invention is an apparatus for transmitting optical signals over an optical communications network. An input port is configured to receive an input signal, and a modulator is configured to generate first and second modulated optical signals based on the input signal, wherein the first and second modulated optical signals are substantially inverted versions of each other. First and second output ports are configured to provide the first and second modulated optical signals. 
     In another embodiment, the present invention is a method for transmitting optical signals over an optical communications network. The method includes the steps of receiving an input signal, generating first and second modulated optical signals based on the input signal (where the first and second modulated optical signals are substantially inverted versions of each other), and then outputting the first and second modulated optical signals. 
     In another embodiment, the present invention is a method for receiving optical signals over an optical communications network, which includes the steps of receiving a first optical signal (e.g., S) associated with a service channel in the network, receiving a second optical signal (e.g., P) associated with a protection channel in the network, detecting whether the service channel is acceptable, and selecting the second optical signal if the service channel is not acceptable, where the second optical signal is a substantially inverted version of the first optical signal. 
     In another embodiment, the present invention is a method for receiving an optical signal over an optical communications network, which includes the steps of receiving the optical signal, determining whether the optical signal is a protection channel signal by detecting that the optical signal is a substantially inverted version of a corresponding service channel signal; and inverting the optical signal upon determining that the optical signal is the protection channel signal. 
     In another embodiment, the present invention is an optical communications network including a first transceiver redundantly connected to a second transceiver via first and second optical paths. The first transceiver is configured to convert an input signal from a first client in the network into first and second modulated optical signals that are substantially inverted versions of each other. The first optical path is configured to convey the first modulated optical signal from the first transceiver to the second transceiver. The second optical path is configured to convey the second modulated optical signal from the first transceiver to the second transceiver. Finally, the second transceiver is configured to receive the first and second modulated optical signals and select one of them for transmission as an output signal to a second client in the network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
         FIG. 1  is a block diagram of an optical network of the prior art. 
         FIG. 2  illustrates the internal architecture of transceiver (XCVR)  108  of  FIG. 1 . 
         FIG. 3  illustrates the internal architecture of XCVR block  128  of  FIG. 1 . 
         FIG. 4  illustrates a transceiver according to one embodiment of the current invention. 
         FIG. 5  illustrates the internal architecture of a modulator-splitter of the prior art. 
         FIG. 6  illustrates the internal architecture of a dual-output Mach-Zehnder modulator as utilized in one implementation of the present invention. 
         FIG. 7  is a block diagram of an exemplary transceiver of the present invention illustrating support for the insertion of preemptive traffic. 
         FIG. 8  illustrates an exemplary client architecture according to the present invention. 
         FIG. 9  illustrates an alternative implementation of XCVR block  128  of  FIG. 1  according to this invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
     Prior Art Optical Protected Network 
       FIG. 1  is a top-level view  100  of a typical redundantly connected, protected optical network of the prior art showing detail for two clients (Client A and Client B) that are participating in a two-way communication. Transmitter  104  of Client A  102  sends data to receiver  118  of Client B  116  through transceiver (XCVR)  108 , optical network  110 , 2×1 optical switch  112 , and XCVR  114 . Transmitter  120  of Client B  116  sends data to receiver  106  of Client A  102  through XCVR  122 , optical network  110 , 2×1 optical switch  124 , and XCVR  126 . 
     Prior Art Transceiver 
     A simplified block diagram of XCVR  108  of  FIG. 1 , including the components of a conventional Optical Translation Unit (OTU), is illustrated by  FIG. 2 .  FIG. 2  shows XCVR  108  receiving optical input signal  202  on a given wavelength wl. Optical-to-electrical (OE) unit  204  converts optical input signal  202  to the electrical domain for further processing. The resulting electrical signal  208  feeds external modulator  210  (typically implemented using a single- or dual-output Mach-Zehnder modulator) which then modulates the output of local CW laser source  206  of wavelength w2, where w2 is typically but not necessarily a different wavelength from the input wavelength w1. The output of modulator  210  then feeds optical splitter  212  which optically splits its input signal into two roughly identical output signals. These output signals then drive two separate fibers representing the service channel S  214  and the protection channel P  218 , respectively. As illustrated by the representative waveform  216  for the service channel S and the representative waveform  220  for the protection channel P, the two outputs of splitter  212  are in phase (i.e., of the same polarity). 
     Suitable alternative devices may be substituted for external modulator  210  of this illustration including any devices from the classes known as electro-absorption and electro-optic modulators, the former class including devices composed of materials used in semiconductor lasers, and the latter class composed of materials whose refractive index can be altered by an applied electric field. The waveforms are provided for the purpose of illustration of polarity and represent only approximately the amplitude vs. time plot for an optical signal driven by a bit pattern of “1, 0, 1, 0, 1, 0, 1”. Details of framing and protocol encapsulation are not illustrated. Additionally, in this simplified illustration, for clarity, details of optical amplification, wavelength selection, demodulation, error detection/correction, and other standard processes that typically occur within a transceiver are not explicitly shown in  FIG. 2 . 
     Failure Protection in the Prior Art 
     Referring back to  FIG. 1 , it should be understood that the signal carried on the protection channel P is typically an in-phase (i.e., same polarity) approximate copy of the signal that is driving the service channel S. The protection channel is typically on a different fiber, and that fiber is typically carried in a different bundle along a unique path from the service channel (i.e., path diversity). In the event of a failure on the service channel S, for example, in a transmission from Client A  102  to Client B  116 , the redundant copy of the service signal carried on the protection channel P can optionally be selected (via appropriate control of 2×1 switch  112 ) until such time as the service channel is recovered. Typically a microcontroller or microcomputer will be used in combination with a Performance Monitor (PM) to assess the relative signal quality of S and P. This arrangement is illustrated by the  FIG. 3 , which represents a simplified view of XCVR block  128  of  FIG. 1 . As shown in  FIG. 3 , PM  310  will typically sample both service channel S  302  and protection channel P  304  at a receiving node prior to, or in common with input to 2×1 switch  112 . Samples  306  of S and samples  308  of P will feed PM  310 , which will report the relative signal quality of S and P to microcontroller  326  via reporting interface  324 . Microcontroller  326  will respond to the information received from PM  310  and in accordance with its software programming and in consideration of other information available to it (for example, via interface  328  with Management Interface Unit (MIU)  330 ), control 2×1 switch  112  to select the signal carried on either  5   302  or P  304  to drive GE unit  312 . Output  314  of GE unit  312  will control the modulation by modulator  322  of output  320  of local CW laser  318 . This selection ultimately determines signal  316 , which exits XCVR  114 . Similarly, referring back to  FIG. 1 , for transmission from Client B  116  to Client A  102 ,  5  and P are sampled just prior to input to 2×1 switch  124  by a performance monitor (detail not shown) associated with XCVR  126  and an appropriate selection made by a microcontroller (detail not shown) via control of 2×1 switch  124 . 
     Optical network cloud  110  of  FIG. 1  redundantly connects the nodes in the network via wavelength or physically distinct paths, either in a ring, mesh, or other optical network topology, whereby the protection channel, which provides protective coverage for one or more service signals, and the service channel itself, may reach a destination node directly or via multiple hops through intermediate nodes. 
     Transmitter Utilizing Dual-Output Mach-Zehnder (MZM2) 
     In one embodiment of this invention, the combination of external modulator  210  and optical splitter  212  of  FIG. 2  is replaced with a dual-output Mach-Zehnder modulator (MZM2). This is illustrated by  FIG. 4  where MZM2  410  is shown driving both service channel S  412  and protection channel P  416  fibers directly, without the need for an intervening splitter in the path. MZM2  410  serves as both modulator and splitter with the exception that the two signals generated by MZM2  410  are inverted versions of each other as illustrated by waveform  414  representing the polarity of the service signal that is carried on the service channel S and waveform  418  which represents the opposite polarity that is carried on the protection channel P. Depending on the implementation, the amplitudes of the service signal, and its substantially inverted representation that is carried on the protection channel, may be the same or different. Analogous to XCVR  108  of  FIG. 2 , in  FIG. 4 , optical input  402  to XCVR  400  feeds OE  404 , which then outputs modulator control  406 . Also analogous to XCVR  108  of  FIG. 2 , in  FIG. 4 , CW laser  408  provides an output for modulation. However, in XCVR  400 , the output of laser  408  is modulated by MZM2 device  410  under the control of OE output  406 . Accommodation of the protection signal as a substantially inverted copy of the service signal can be made with minor additional system cost. The result of the invention is thus the replacement of two components, i.e., a modulator and a splitter with one MZM2, yielding a reduced component cost in a typical transceiver. Additionally, because the power loss of a single MZM2 can be much less than that of the modulator-splitter solution of the prior art, the overall system cost of the invention might also be reduced relative to the prior art since less amplification may be required in the end-to-end optical pathways. 
     Accommodating the Inverted Protection Channel 
     To accommodate the signal on the protection channel P of a network incorporating the XCVR configuration of this invention as illustrated by  FIG. 4 , an inversion or “righting” of the signal carried on P might need to be done prior to delivery to a final destination at a client. Note that, in the hybrid electro-optical networks of today, optical-to-electrical and electrical-to-optical conversion of a signal is done repeatedly in client-to-client communications. Thus, the signal on P will find itself represented electrically at many points in the network. Inversion in the electrical domain is a low-cost operation. Additionally, because differential transport is commonly encountered at various points along a hybrid electro-optical network for purposes of noise immunity and signal recovery, it is typical for both physical-layer protocols and the devices that interface to these protocols to include inversion detection and correction mechanisms. Thus, to a certain extent, the circuitry and protocols defined for these communications systems intrinsically tolerate or correct inversion, and for those cases where they do not, a means to detect and correct the inversion is inexpensive to implement. 
     As an example, in SONET networks, a basic STS-1 frame repeats every 810 bytes and begins with the start-of-frame delimiter 0xF628 or 1111,0110,0010,0100b. The inverted version of this delimiter is 0000,1001,1101,1011b or 0x09DB. A circuit that detects the pattern 0x09DB at a recurring 810-byte interval can determine that the signal is inverted and can right the signal before transmitting it further or dropping it to a local destination. This circuit can be incorporated in a client as illustrated by inversion detection block  806  and inversion correction block  808  of  FIG. 8 , or alternatively, the inversion can be handled at any point within a XCVR where the inverted service signal is represented in the electrical domain (not illustrated). 
     Inversion as OAM&amp;P Indicator 
     It is an aspect of many of the protection schemes in use in networks today (e.g., 1+1) that switching to the protection channel can be accomplished autonomously via local performance monitoring and microcomputer control. For such networks, carrying of an inverted copy of the service signal on the protection channel P, according to this invention, can be used as part of an Operations, Administration, Maintenance, and Provisioning (OAM&amp;P) scheme whereby the failure of S is indicated by a XCVR to a client implicitly by the presence of the inverted signal. For example, as discussed previously, if there is a failure on S, a 2×1 switch and XCVR combination as illustrated previously by  FIG. 3  will select the signal on P to pass along to the client along interface  316 . For systems that use the XCVR configuration of this invention as illustrated by  FIG. 4 , this signal will be inverted with respect to the service signal. The presence of an inverted signal at a client can thus be used as an indicator of the presence of a failure on the primary or service path S. This indicator can be communicated to the client without the need for additional circuitry or bandwidth associated with a separate management interface and could in some cases eliminate the need for a management interface at the XCVR. 
     Modulation-Splitting using MZM2 
       FIG. 5  provides additional detail on one implementation of modulator  210  and splitter  212  of  FIG. 2  where modulator  210  is a single-output Mach-Zehnder modulator (MZM1). In  FIG. 5 , CW laser input  502  is first split  504  by MZM1  210  into two legs. Following modulation and relative phase shifting, the two legs are recombined  506  before being output from the MZM 1. Finally single output  508  of MZM 1 is then input to splitter  212  where it is again split  510  into two legs. 
     In contrast,  FIG. 6  illustrates one view of the internal structure of a modified or dual-output Mach-Zehnder modulator (MZM2)  410  of  FIG. 4 . In  FIG. 6 , CW laser input signal  602  is split  604  as in the prior art and the two legs subjected to modulation and phase shifting as before. However, in MZM2  410 , the two legs are each individually output  606  from the device. Note that the losses intrinsic to MZM1  210  of  FIG. 5  and MZM2  410  of  FIG. 6  are nearly identical and that, although MZM2  410  of  FIG. 5  exhibits roughly the same loss as MZM1  210  of  FIG. 6 , the prior art implementation depicted by  FIG. 5  is further burdened by the additional loss component, splitter  212 . The end result, as stated previously, is that two devices associated with the prior art implementation are replaced with a single device with a resulting decrease in power loss. Although  FIG. 5  illustrates an implementation of the external modulator  210  of  FIG. 2  in terms of an MZM1, as discussed earlier, any of the classes of electro-absorption and electro-optic modulators could instead have been substituted in accordance with the present invention. 
     Preemptive Traffic 
     Another embodiment of this invention makes use of the inverted nature of one output of a MZM2 to provide for the simplified detection of the insertion of preemptive or low-priority traffic into the network and simplifies the architecture for carrying either keep-alive or preemptive traffic.  FIG. 7  illustrates a transceiver  700  that can be used instead of XCVR  108  and/or XCVR  122  in network  100  of  FIG. 1  according to this embodiment of the present invention. As in the prior embodiment, an optical signal  704  of wavelength w1 is input to XCVR  700 . The signal is converted to the electrical domain by OE device  712  and the result  714  is used to modulate the output of CW laser  716  using MZM2  718  to produce a service path signal  726  of wavelength w1 which can be different from w2 and an inverted approximation thereof  722 . However, in this embodiment, in contrast to the embodiment of this invention illustrated by  FIG. 4 , in  FIG. 7 , the inverted output  722  of MZM2  718  does not drive the protection path directly. Instead, it feeds one input of 2×1 switch  724  where the other input is fed with Preemptive Traffic (PT) input  720 . As illustrated by  FIG. 7 , PT is the result of the modulation  708  of the output of local CW laser  706  under control of the output of OE device  710  with external input  702  of wavelength w3, where w3 can be different from w1 and/or w2. Alternatively, the modulator for the signal PT may be external to transceiver  700 . Microcontroller  736  is used at the XCVR (alternatively remote computational/control resources (not shown in  FIG. 7 ) may be utilized), optionally in combination with information received via management interface unit  734 , to select whether the inverted version of service signal  722  or signal PT  720  is used to drive protection channel P  730 . The waveform  732  of the signal carried by P  730  tracks either PT  720  or the inverted version  722  of the service signal S carried by  726 . Waveform  728  represents the polarity of the service signal S. PT  720  may contain bursty or low-bandwidth traffic that is opportunistic and whose quality of service (QOS) is not necessarily guaranteed. In the event that the service channel S is fault free, the protection path P can be used to carry preemptive traffic PT at little additional cost to the network provider by utilizing otherwise unutilized bandwidth. This reduces the steady state 100% overhead burden on the network of carrying a duplicate (albeit in this application inverted) version of the service signal on the protection channel P. 
     Interpreting Inversion in the Context of Preemptive Traffic 
       FIG. 8  illustrates a simplified block diagram of the receive-side of an exemplary client according to this invention analogous to receiver (Rx)  118  of  FIG. 1 . In the previous embodiment discussed in the context of the XCVR of  FIG. 4 , where the keep-alive signal carried on the protection path was simply an inverted version of the service signal and there was no facility for carriage of preemptive traffic on P, the presence of an inverted signal at the input to a client would unambiguously indicate a failure on S. As illustrated by  FIG. 8 , and as discussed earlier, inversion on the input signal  802  could be detected by inversion detection circuitry  806  after conversion of the optical signal to the electrical domain by OE  804 . The presence of the inversion would be communicated via signal  820  to microcontroller  814  which in turn would command inversion circuitry  808  via signal  818  to correct the polarity of the signal prior to passing it on to be further processed (e.g., reassembly, etc., in hardware or software) byprocessing block  810  and ultimatelypassed on to the client&#39;s local intelligent agent  812  (e.g., the microcontroller). The presence of the inverted signal might also be communicated to microcomputer  812  and/or Management Interface Unit (MIU)  816  for reporting purposes as part of an OAM&amp;P scheme. This reporting might be via separate LAN, WAN, Internet, etc., interface  822  or “in-wavelength”inserted as part of the outbound traffic to the network on the transmit side of the client (not shown). In the current embodiment, however, where the simplified XCVR of the prior embodiment of this invention (as illustrated by  FIG. 4 ) is replaced with the XCVR of  FIG. 7 , the interpretation of the input to a client becomes more complex. Input  802  to the client of  FIG. 8  is the output of a performance-monitoring combination of a 2×1 switch and XCVR such as that illustrated by  FIG. 3  where the signal carried on P can be either an inverted version of the service signal S or preemptive traffic (PT). In this case, the presence of an inverted signal at the input of a client still indicates a failure on S but the presence of a non-inverted signal at the input to a client might indicate either service traffic or preemptive traffic. This last ambiguity can be sorted out by the client during the reassembly process or via appropriate communication of the state of the 2×1 switch state in the local XCVR serving the client, as communicated to the client from MIU  330  over management interface  332  in  FIG. 3 . In the event of a failure on  5 , XCVR  128  of  FIG. 3  will select P at 2×1 switch  112  to be ultimately driven to the client via output  316 . In this case, it should also report the failure on S via management interface  332  so that the inverted version of the service signal can be switched back into P at the sourcing XCVR, illustrated in this example by  FIG. 7 . Alternatively, 2×1 switch  724  in sourcing XCVR  700  of  FIG. 7  can be cycled back and forth under control of microcontroller  736  via control signal  738  between driving P  730  with preemptive traffic  720  and driving P  730  with an inverted version  722  of the service signal. The duty cycle and frequency of this cycling or toggling can be varied to compromise between providing bandwidth for preemptive traffic and providing an inverted signal that the client at the receiver can unambiguously interpret as associated with a failure on S. Again, in this scheme, the detection of a failure on S (potentially at device  128  of  FIG. 3  or by virtue of an inverted signal detection at the client of  FIG. 8 ) in this embodiment should be generally followed by an OAM&amp;P communication to the XCVR that is sourcing this toggled signal (in this example, the XCVR of  FIG. 7 ) that there is a failure on the service channel and that preemptive traffic insertion must cease in order to accommodate the use of P exclusively for carriage of the (inverted) service signal. This can be done via in-band or out-of-band signaling around the network. After recovery from the failure on  5 , the transmission on P of preemptive traffic or a combination of preemptive traffic and inverted service signal can resume. 
     Generally, preemptive traffic is received from the protection channel P by an additional preemptive traffic receiver (not shown) that serves a preemptive signal destination client (also not shown). Another embodiment of this invention involves an enhancement to the transceiver of  FIG. 3  that supports concurrent reception of the service signal and the preemptive signal. Assuming there is no failure on the service channel S, this enhanced transceiver can serve both a service signal destination client and a preemptive signal destination client.  FIG. 9  illustrates transceiver  900 . In XCVR  900 , 2×2 switch  902  replaces 2×1 switch  112  of  FIG. 3 . In addition, XCVR  900  includes a second OE conversion and modulation block  908 . 2×2 switch  902  in XCVR  900  is set via control  914  by microcontroller  912  in consideration of various factors including the relative quality of the signals on the service channel S and the protection channel P. The settings of 2×2 switch  902  are summarized in Table A below. In Table A, the switch setting under normal circumstances (e.g., where the service channel is of sufficient quality for normal operation relative to the protection channel) will be designated by “Normal.” If the service channel degrades sufficiently either in absolute terms, or relative to the quality of the protection channel, the switch setting will be changed. This new setting is designated by “Failure” in Table A. If the switch setting is “Normal,” the protection channel P is connected via 2×2 switch  902  to OE-modulator  908  input port  904  and ultimately output via interface  910  to the preemptive signal destination client. At the same time, the service channel S is connected via 2×2 switch  902  to OE-modulator  114  input port  906  and ultimately output via interface  316  to the service signal destination client. If the switch setting is “Failure,” the protection channel P is again connected via 2×2 switch  902  to OE-modulator  908  input port  904  and ultimately output via interface  910  to the preemptive signal client. At this same time, the protection channel P is additionally connected via 2×2 switch  902  to OE-modulator  114  input port  906  and ultimately output via interface  316  to the service signal destination client. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE A 
               
               
                   
                   
               
               
                   
                   
                 To Preemptive Signal 
                 To Service Signal 
               
               
                   
                 Switch Setting 
                 Destination Client 
                 Destination Client 
               
               
                   
                   
               
             
             
               
                   
                 Normal 
                 P 
                 S 
               
               
                   
                 Failure 
                 P 
                 P 
               
               
                   
                   
               
             
          
         
       
     
     In an alternative implementation (not illustrated), since the protection channel P signal is fed to the preemptive signal destination client independent of the setting of 2×2 switch  902 , P could be optically split into two legs at the input to XCVR  900  with one leg hardwired to input  904  of OE-modulator  908 . The other leg of P and the service signal S could be input to a 2×1 optical switch that could select under the control of microcontroller  912  which of those inputs would drive input  906  of OE-modulator  114  to supply signal to the service signal destination client. 
     In another alternative implementation (not illustrated), the signals carried by the service channel S and the protection channel P can be converted from the optical domain to the electrical domain at the front-end of XCVR  900  producing electrical service and protection signals Se and Pe, respectively. Pe could then be used directly to control the modulator of OE modulator  908 . Additionally, Se and an electronic copy of Pe could feed an electronic 2×1 switch under the control of microcontroller  912  whose output would control the modulator of OE-modulator  114 . 
     Depending on the application, the signals processed in accordance with the present invention may be analog or digital. 
     Note that throughout this document the terms copy, version, and approximation have been used with regard to the service signal to denote a reasonable approximation to the service signal or to an inverted copy thereof. These signals should be understood to be sufficiently similar to or substantially the same as the service signal or the inverted service signal, as the case may be, such that recovery of these signals is reasonably achievable using electrical and optical components of the current state of the art or reasonable extensions thereof. 
     Also note that, throughout this document, the laser and modulator are depicted in separate boxes. Depending on the implementation, different parts of those components may be implemented in the same or different housings, circuit packs, circuit cards, multi-chip modules, substrates, or mixed-mode ASICs, potentially along with other circuitry. In one possible implementation, the laser and the modulator are integrated together onto the same substrate. 
     The present invention may be implemented using Mach-Zehnder modulators of the lithium niobate type, although other suitable types of MZ modulators and suitable modulators other than MZ modulators may also be used. 
     Although this invention has been described broadly with respect to optical networks, it should be understood by one skilled in the art that it is equally applicable to related optical subsystems, including Synchronous Optical Network (SONET) add-drop multiplexers and optical internet-protocol (IP) routers. As used in the claims, the term “network” should be interpreted to cover any of these different subsystems. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims. 
     Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.