Abstract:
A method and apparatus for initializing an end-to-end link in a fiber optic communications system in which a pair of nodes interconnect a pair of end devices. A first node, upon initializing a device link segment with an end device to which the node is coupled, sends a signal to the other node over a network link segment indicating that the sending node has initialized its device link segment. The first node completes initialization of the end-to-end link upon receiving a signal from the other node over the network link segment indicating that the other node has initialized its device link segment. In an alternative initialization scheme, a node momentarily operates its data channel in a loopback mode to allow its end device to initialize the device link segment in accordance with a predetermined protocol before returning to a transparent mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to the commonly owned, concurrently filed application of the same inventors, Ser. No. 11/314,261, entitled “Open Fiber Control and Loss of Light Propagation in Time Division Multiplexed Inter-System Channel Link”. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention applies to fiber optic networks that use a combination of time and wavelength division multiplexing and transport communication protocols that require either loss of light (LOL), open fiber control (OFC), or a combination of the two states to be transported across the network. 
   2. Description of the Related Art 
   Recent advances in fiber optic dense wavelength division multiplexing (DWDM) equipment have made more efficient use of the fiber&#39;s available bandwidth by using a combination of wavelength division multiplexing (WDM) and time division multiplexing (TDM). Typically, in a TDM/WDM system, multiple input signals with data rates up to 1-2 gigabits/second (Gbit/s) are time multiplexed into a single, high-speed data stream. This is then modulated onto one of the optical wavelengths in a wavelength division multiplexing (WDM) network, which may be operating at 10 Gbit/s or faster. This approach provides a cost-effective way to scale the capacity of an optical network and is currently being applied to industry-standard protocols such as Gigabit Ethernet, Fibre Channel, Asynchronous Transfer Mode (ATM), and others. 
   Some data communication protocols require special accommodations to operate in this environment. For example, IBM has developed a set of protocols known as Inter-System Channel (ISC) links, which are used for clustering of mainframe computers in a Geographically Dispersed Parallel Sysplex (GDPS) architecture. (Geographically Dispersed Parallel Sysplex and GDPS are trademarks of IBM Corporation.) This approach is used for high availability and disaster recovery at larger companies worldwide and requires the extension of ISC links over DWDM networks to distances of 50-100 kilometers (km) or more. Until recently, there was no need to time multiplex the ISC channels, as the maximum data rate per wavelength in a WDM network was about 2.5 Gbit/s, approximately the same as an ISC channel. (These channels can operate in either peer mode at 2.125 Gbit/s or compatibility mode at 1.0625 Gbit/s; the compatibility mode links also use a version of open fiber control (OFC) protocols.) With the recent increase in WDM per wavelength data rates to 10 Gbit/s and beyond, it is necessary to find a method for time multiplexing several ISC channels over a common wavelength so that the GDPS architecture remains cost competitive. 
   This operation requires two essential steps. The first is a method for speed matching the FIFO buffers between the ISC channel and the WDM network. This requires knowledge of IBM data frame structures, and algorithms to accomplish this are described in U.S. Patent Application Publication 2005/0100337 (DeCusatis et al.), incorporated herein by reference. The second essential step is to accommodate time multiplexing of the channel initialization and control information, including open fiber control (OFC) and loss of light (LOL) propagation. 
   OFC is described in U.S. Pat. Nos. 6,356,367, 6,359,709 and 6,359,713 (DeCusatis et al.), as well as in U.S. Pat. No. 6,438,285 (DeCusatis et al) and U.S. Patent Application Publication 2003/0072516 (DeCusatis et al.), all of which are incorporated herein by reference. As explained in the referenced patent application publication, OFC is a laser eye safety interlock implemented in the transceiver hardware; a pair of transceivers connected by a point-to-point link must perform a handshake sequence in order to initialize the link before data transmission occurs. Only after this handshake is complete will the lasers turn on at full optical power. If the link is opened for any reason (such as a broken fiber or unplugged connector), the link detects this and automatically deactivates the lasers on both ends to prevent exposure to hazardous optical power levels. When the link is closed again, the hardware automatically detects this condition and reestablishes the link. OFC is defined for various laser wavelengths and data rates in the ANSI Fibre Channel Standard; the OFC timing and state machine are also defined in this standard. OFC is still required to interoperate with other devices attached to the fiber links, even where it no longer serves a laser safety function. 
   LOL is described in the above-identified U.S. Pat. Nos. 6,356,367, 6,359,709 and 6,359,713 and U.S. Patent Application Publication 2003/0072516, as well as in such patents as U.S. Pat. No. 5,504,611 (Carbone et al.) and U.S. Pat. No. 5,136,410 (Heiling et al.), incorporated herein by reference. Even links that do not implement OFC protocols must sometimes propagate a loss of light (LOL) condition along the length of the fiber. As explained in the referenced patent application publication 2003/0072516, propagating loss of light is not the same as sending a long string of zero data on the link; the attached computer equipment must be able to determine the difference between an open optical connection and a long run of zeros (potentially corrupted data) since the error recovery is different in each case. 
   The present invention addresses these two critical link states, LOL and OFC, which must be propagated through a TDM/WDM network to insure proper functionality of an ISC channel. 
   Previously, this control information was passed across WDM networks on a per-wavelength optical supervisory channel (OSC). This is illustrated in  FIG. 1 , which shows a prior art system  100  containing a WDM transmitting node  102  and a WDM receiving node  104  coupled via a network  106 . (This example has been simplified somewhat, since each of the node  102  and  104  contains both transmitting and receiving functions.) 
   Transmitting node  102  contains a plurality of input channels, each of which drives a common WDM multiplexer  120 . In each of these input channels, an optical signal  108  on a link from a client (not shown) drives an optical-to-electrical (OE) transducer or optical receiver (RX)  110  to produce an electrical output signal  112 . This electrical signal  112  is combined with an electrical overhead control signal  114  and the result fed to an electrical-to-optical (EO) transducer or optical transmitter (TX)  116 . Transducer  116  has an internal laser (not separately shown) that provides an optical signal  118  of a particular wavelength to WDM multiplexer  120 . WDM multiplexer  120  combines the optical signals  118  from all of these input channels (which have different wavelengths) to provide a single multiple-wavelength optical output signal  122  to the network  106 . 
   Correspondingly, at the receiving node  104 , a WDM demultiplexer  126  takes a multiple-wavelength optical input signal  124  from the network  106  and separates it into multiple optical signals  128  of different wavelengths that are processed in respective output channels. In each of these output channels, an optical receiver  130  converts the optical signal  128  to an electrical signal  132 , from which an overhead control signal  134  is extracted using well-known techniques. Finally, an optical transmitter  136  takes the electrical signal  132  from which the control signal was  134  extracted and, using another internal laser, converts it to an optical output signal  138  corresponding to the original input signal  108 . 
   In the system  100  illustrated in  FIG. 1 , an input optical data stream  108  is converted into electrical form  112 , then remodulated onto another laser signal  118  whose wavelength is compatible with the WDM network  106 . In the process, overhead bits  114  that carry network management information for this wavelength are added to the data flow. This overhead channel does not occupy a significant fraction of the available bandwidth (perhaps a few percent), and is confined within the WDM network  106 ; it is stripped off by the receiver function at the destination WDM node  104 . In this manner, if there is a fiber or component failure anywhere in the link, the WDM equipment can deactivate both its network laser connection and client laser connection. Similarly, if the link is equipped with OFC protocols, the entire optical link can be deactivated until the failure is corrected; at that time, OFC automatically reinitializes the end-to-end link. Otherwise, the WDM interface transparently passes along any input data to the output node. 
   This approach cannot be used, however, if one plans to time multiplex several channels of ISC traffic. In this latter scenario, let us consider, for example, LOL propagation. If there is an equipment failure in the TDM stage that affects only one ISC channel, it is no longer possible to disable the lasers throughout the link, since they are still carrying ISC traffic for other input channels. It is also not possible to simply transmit all zeros. This is true for many reasons, including the fact that such a transmission would violate disparity on the ISC link. It would be misinterpreted as a data error and inhibit proper channel error recovery, and the clock recovery circuits in the receiver would drift out of lock under these conditions. Similar considerations apply to OFC propagation when time multiplexing several channels of ISC traffic. 
   Previously, there have been various efforts to multiplex various communication protocols in optical networks. The following patents are representative. 
   U.S. Pat. No. 6,587,615 (Paiam) describes an optical wavelength demultiplexer with a substantially flat output response within its passband. This is accomplished by using a two-stage optical wavelength multiplexing process, in which the first WDM has a free spectral range approximately equal to the second WDM. Various embodiments are described, including resonant optical cavities, array waveguide gratings, and others. This patent only addresses the optical spectral properties of a WDM system. It does not incorporate time division multiplexing technology and does not address LOL or OFC state propagation across a network. 
   U.S. Pat. No. 5,814,557 (Otsuka et al.) describes a method and apparatus for scrambling the polarization of optical signals in a WDM system to suppress nonlinear effects and improve transmission fidelity. Various embodiments are proposed, including per wavelength polarization scramblers and a two-stage wavelength combination scheme with a scrambling stage in between. This patent only addresses the nonlinear effects in a long-haul WDM system that can arise from variations in the optical polarization. It does not incorporate time division multiplexing technology and does not address LOL or OFC state propagation across a network. 
   U.S. Patent Publication 2003/0081294 (Lee et al.) describes a free-space WDM system which couples the received channels into an optical fiber to facilitate the use of optical amplifiers. A light beam emitting and focusing unit is described to facilitate this coupling, which includes an optical circulator, WDM coupler, and amplified spontaneous emission fibers. This patent does not incorporate time division multiplexing technology and does not address LOL or OFC state propagation across a network. 
   The above-mentioned family of U.S. Pat. Nos. 6,359,709, 6,359,713 and 6,356,367 describe a method, apparatus, and computer program product for a fiber optic network that allow OFC conditions to propagate across a WDM network. This is accomplished by using an outband signal which carries the OFC state; an alternative embodiment using an electrical wrap mode is also presented. The technology described in these patents only applies to a WDM system, without TDM, and is not extendable to include TDM systems. In fact, the approach described in these patents will not work in a TDM environment. Thus, the patents describe using a per wavelength control channel to propagate OFC state information, which means that only one data channel per wavelength can be supported. These patents also do not address LOL propagation across a WDM or TDM network. 
   SUMMARY OF THE INVENTION 
   The shortcomings of the prior art are overcome and additional advantages are provided through the present invention, which relates to a method and apparatus for propagating link state conditions across an optical network. Some of the more significant aspects of our invention include the following: 
   One aspect of our invention, which is the subject of the above-identified concurrently filed application, relates to the transport of an Inter-System Channel (ISC) or similar protocol across an optical network which uses time division multiplexing (TDM) stages, typically in combination with wavelength division multiplexing (WDM) stages. As noted above, it is necessary to correctly propagate link conditions such as a loss of light (LOL) condition across each TDM subchannel within a single WDM wavelength where WDM is used. This aspect of the invention, therefore, contemplates a control channel for each TDM subchannel to carry this information, with the control channels being time multiplexed in the same fashion as the data. 
   More formally, this aspect of the invention contemplates a method and apparatus for transmitting signals from a plurality of input channels over a TDM optical network, where each of the input channels contains an optical data signal and an electrical control signal containing control information relating to the optical data signal. In accordance with the invention, respective optical receivers convert the optical data signals to respective electrical data signals, which a TDM data multiplexer time-multiplexes to generate a multiplexed data signal. A TDM control signal multiplexer time-multiplexes the electrical control signals to generate a multiplexed control signal that is combined with said multiplexed data signal to generate a composite electrical signal. An optical transmitter generates a composite optical signal from the composite electrical signal that is transmitted over the network. 
   At the receiving end, an optical receiver receives the composite optical signal over the network and generates a composite electrical signal from the composite optical signal that is separable into a multiplexed data signal and a multiplexed control signal. A TDM data demultiplexer demultiplexes the multiplexed data signal to generate respective electrical data signals for output channels corresponding to the input channels, while a TDM control signal demultiplexer demultiplexes the multiplexed control signal to generate respective electrical control signals for the output channels. Finally, respective optical transmitters convert the electrical data signals to optical data signals for the output channels. 
   In the system described, in response to detecting a loss of an optical data signal in one of the input channels, an electrical control signal is generated for that input channel indicating the loss of the optical data signal. That control signal will be propagated to the appropriate output channel by virtue of the signal handling described above. 
   If desired, the invention may be used in a TDM/WDM system in which a WDM multiplexer combines the composite optical signal with one or more other composite optical signals of different wavelengths before being transmitted over the network. In such case, a WDM demultiplexer separates the received composite optical signal into composite optical signals of different wavelengths, each of which is fed to an optical receiver to generate a composite electrical signal from which individual data signals and control signals are generated for output channels in the manner described above. 
   More particularly, in accordance with this aspect of the present invention, each input channel (e.g., an ISC input channel) has its own dedicated overhead control channel that is distinguished (e.g., by an identifier) from control channels for other input channels in the same TDM block. These control channels are time multiplexed to yield a single control channel for each optical wavelength, compatible with the previously established approach. These control channels are operational regardless of whether any data is actually being transmitted across the links. Any equipment or link failures that produce loss of light will now be recognized at the next downstream element, and the TDM (including TDM/WDM) equipment can insert a control character for the affected channel that is recognized by the other WDM nodes. 
   Another aspect of our invention, which is a subject of the present application, relates to a method and apparatus for propagating an open fiber control (OFC) condition across a hybrid TDM/WDM network. Disclosed are a peer-to-peer embodiment and a master-slave embodiment for propagating OFC across a hybrid TDM/WDM network. 
   More formally, this aspect of the invention contemplates a method and apparatus for initializing an end-to-end link in a fiber optic communications system in which a pair of nodes interconnect a pair of end devices. The nodes are coupled to the end devices via respective device link segments and are coupled to each other via a network link segment, the device link segments and the network link segment together forming an end-to-end link. In accordance with the invention, a first node initializes the device link segment with the end device to which the node is coupled. Upon initializing that device link segment, the first node sends a signal to the other node over the network link segment indicating that the sending node has initialized its device link segment. The first node completes initialization of the end-to-end link upon receiving a signal from the other node over the network link segment indicating that the other node has initialized its device link segment. 
   The first node may disable the device link segment with the end device to which the node is coupled and return to the initializing step upon a failure of the other node to initialize its device link segment. This disablement may be performed upon a failure to receive a signal from the other node within a predetermined time period indicating that the other node has initialized its device link segment. Alternatively, this disablement may be performed upon receiving a signal from the other node indicating that the other node has not initialized its device link segment. 
   The initialization steps may be performed by each of the nodes as a peer of the other node. Alternatively, the steps may be performed by one of said nodes as a master node which, upon initializing its device link segment, signals the other node to perform its own initialization procedure as a slave node. Such slave node, upon being signaled by the master node, initializes the device link segment with the end device to which the slave node is coupled and, upon initializing that device link segment, sends a signal to the master node over the network link segment indicating that the slave node has initialized its device link segment. 
   Yet another aspect of our invention, which is also a subject of the present application, relates to a method and apparatus for propagating an OFC state across a hybrid TDM/WDM network that does not require the end nodes in the network to generate OFC-compliant signals. Rather, this aspect of the invention uses an algorithm for handling the OFC signals generated by the end devices attached to the network, as described further below. 
   More formally, this aspect of the invention contemplates a method for initializing an end-to-end link in accordance with a predetermined protocol in a fiber optic communications system in which a pair of nodes interconnect a pair of end devices. Each of the end devices is capable of initializing a link segment between it and a similar device according to a predetermined protocol. The nodes have respective device ports coupled to the end devices via respective device link segments, respective network ports coupled to each other via a network link segment, and respective data channels extending between the device ports and the network ports, the device link segments and the network link segment together forming an end-to-end link. 
   In accordance with the invention, as performed by one of said nodes, the data channel is normally operated in a transparent mode in which an optical signal received at the device port is retransmitted from the network port, while an optical signal received at the network port is retransmitted from the device port. In response to detecting a predetermined link state at said device port, however, the data channel is momentarily operated in a loopback mode, in which an optical signal received at the device port is also looped back to the device link segment from the device port while being retransmitted from the network port, thereby allowing the end device to initialize the device link segment in accordance with the predetermined protocol, then returning to the transparent mode. More particularly, if the device port comprises an optical transmitter and an optical receiver, the predetermined link state comprises the absence of a signal from said optical receiver together with the presence of a signal to the optical transmitter. 
   Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  shows the prior art in which a dedicated control channel is used for each optical wavelength in a WDM network. 
       FIG. 2  shows the network environment in which multiple TDM and WDM channels are propagated across a network. 
       FIG. 3  shows an embodiment of this invention in which the control channels for multiple TDM data channels are combined into a single TDM channel for transport across the network shown in  FIG. 2 . 
       FIG. 4  shows a peer-to-peer procedure for propagating OFC states across a combined TDM/WDM network. 
       FIG. 5  shows a master-slave procedure for propagating OFC states across a combined TDM/WDM network. 
       FIGS. 6-10  show an alternative embodiment for propagating OFC states across a network. 
   

   The detailed description below explains the preferred embodiments of the invention, with advantages and features, by way of example with reference to the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2 and 3  show one embodiment of the present invention, in which control channels for multiple TDM data channels are combined into a single TDM channel for transport across a network. More particularly,  FIG. 2  shows an end-to-end system containing a pair of such nodes coupled over a network, while  FIG. 3  shows the transmitter and receiver portions of those nodes. 
   Referring first to  FIG. 2 , that figure shows a system  200  containing a first TDM/WDM node  202  (node A) interconnecting a first host system  206  (host A) and the network  106  and a second TDM/WDM node  204  (node B) interconnecting the network and a second host system  208  (host B). In the discussion that follows, reference is made to transmitter functions in node  202  and to receiver functions in node  204 . Each node, of course, contains both functions, which from the two halves of a duplex link. 
   Referring now to  FIG. 3 , the transmitter portion of the first node  202  contains a plurality of TDM channels  209 , each of which operates at a different wavelength and one of which is shown. Each TDM channel  209  typically resides on a separate card and services one or more input channels. Each input channel serviced by a TDM channel  209  of the transmitter has an input optical data signal  108  driving an optical receiver  210  to produce an electrical data signal  212 , as was shown for system  100  above. Here, however, each of these electrical data signals  212  is then fed to a first TDM multiplexer (MPX)  214 , which combines the data signals  212  into a single multiplexed data signal  216 . Additionally, an overhead control signal  218  (OC 1 -OC 3 ) for each of the input channels serviced by the TDM channel  209  is fed to a second TDM multiplexer  220 , which combines the control signals  218  into a single multiplexed control signal  222 . Multiplexed data signal  216  and control signal  222  are then combined to produce a single composite electrical signal  223 . This composite electrical signal is then fed to an optical transmitter  224 , which provides (via an internal laser) a composite optical signal  226  of a particular wavelength to WDM multiplexer  228 . WDM multiplexer  228  combines this optical signal  226  together with optical signals  226  of different wavelengths from other TDM channels (not shown) to provide a single multiple-wavelength optical output signal  230  to the network  106 . 
     FIG. 3  also shows the receiver portion of node  204 , which performs a corresponding reverse sequence of operations to generate the optical signals  138  corresponding to the original optical signals  108 . More particularly, a WDM demultiplexer  252  separates a multiple-wavelength optical signal  250  from the network  106  into plural single-wavelength optical signals  254  corresponding to the TDM channels  209 . Each of these optical signals  254  drives an optical receiver  256 , which produces a corresponding composite electrical signal  258  for that TDM channel. Each such composite electrical signal  258  is separated into a multiplexed data signal  260  and a multiplexed control signal  262 . A TDM data demultiplexer  264  separates the multiplexed data signal  260  into individual data signals  266  corresponding to the output channels. Respective optical transmitters  268  convert these demultiplexed signals into the desired optical signals  138 . Finally, a TDM control signal demultiplexer  270  separates the multiplexed control signal  262  into individual control signals  272  (OC 1 -OC 3 ) corresponding to the data signals  266 . 
   To see how the present invention works in the system  200  shown in  FIGS. 2-3 , assume that the client-side input  108  is disconnected from ISC link  1 . At the transmitting node  202 , the optical receiver  210  forming the WDM client interface detects this, but does not disable the WDM network-side laser in optical transmitter  224 . Instead, that optical receiver  210  inserts a control character indicating loss of light (LOL) onto the overhead subchannel  218  for ISC link  1 . TDM multiplexer  220  time multiplexes this control character with similar control information for the other input channels, and this control information is passed through the WDM network to the receiving node  204 . At the receiving node  204 , the control character is stripped off the corresponding overhead control signal  272  and the WDM equipment acts to disable the client-side output laser  268  corresponding to ISC link  1 , leaving the rest of the links intact and unaffected. Thus, removing a cable at the network input  108  for an ISC link results in a loss of light at the corresponding remote port  138 , just as if the WDM network provided a long virtual connection between these two points; however there is no loss of light on the WDM network. A similar process propagates LOL from a failure on the output client  208  side to the input client  206  side, or from a failure in the network  106  to both client sides. 
   OFC Propagation 
   Next, consider the problem of OFC propagation in a TDM/WDM hybrid network such as the one shown in  FIGS. 2 and 3 . 
   Direct propagation of OFC signals would require very fast lasers in the TDM network to comply with OFC timing requirements and is likely not practical. While OFC is defined by the ANSI Fibre Channel standard, it is only defined for point-to-point links and not for repeated or WDM networks; it is also not defined for TDM networks. Further, a vendor may use a proprietary, nonstandard version of OFC on an ISC channel implementation to achieve longer distances. While the propagation of OFC over WDM has been addressed previously, the propagation of OFC over a combined TDM/WDM network is a different problem, which is addressed by this aspect of the invention. 
   Either a peer-to-peer approach or a master-slave approach may be used. Although the peer-to-peer approach is considered a preferred approach, either may be used, and both are described. 
   The peer-to-peer approach is illustrated by the algorithm and flow chart in  FIG. 4 . Each of a pair of interconnected nodes, acting as a peer, follows the same procedure, which will be described as it is performed by node A. Each of nodes  202  and  204  contains suitable logic for performing this procedure (and that of  FIG. 5 ), such as the logic  630  shown in  FIG. 6 . 
   Referring now to  FIGS. 2-4 , when an end-to-end ISC link (e.g., ISC link  1 ) is first created, hosts  206 (A) and  208 (B) on both sides of the network link  106  independently attempt to initiate an OFC handshake with their respective TDM/WDM nodes  202 (A) and  204 (B). First, consider the handshake on side A. As shown in  FIG. 4 , node  202 (A) first attempts to initiate a handshake with host  206 (A) (step  402   a ). If the handshake attempt fails (step  404   a ), node  202  disables link segment  1  (connecting host  206  and node  202 ) and retries the handshake (step  406   a ), while at the same time continuing to send an “OFC incomplete” message it has been sending to the other node  204 (B) (step  408   a ). 
   If, on a first or subsequent try, the handshake attempt is successful (step  410   a ), then link segment  1  (connecting host  206  and node  202 ) initializes (step  412   a ). Node  202 (A) then sends an “OFC complete” message to the other node  204 (B) and starts a timer for receiving an acknowledgment (ACK) signal from that other node, indicating that it has established a link (link segment  3 ) with host  208 (B) (step  414   a ). Node  202  sends the “OFC complete” message by inserting a control character into the subchannel  218  for ISC link  1  to reflect this state. This control character is time multiplexed into the per-wavelength control channel  222 , passed through the network link  106  (constituting link segment  2 ), and detected at node  204 (B). Node  204  responds to node  202  with another control character, indicating whether it has completed a handshake on link segment  3  (connecting node  204  and host  208 ). 
   When the time for receiving an ACK signal has expired, node  202 (A) checks to determine whether it has received an ACK signal from node  204 (B) (step  416   a ). If node  202  has not received a timely ACK signal (step  418   a ), then node  202  forces a disconnect on link  1  by dropping its client-side laser signal (from a transmitter similar to the transmitter  268  shown in  FIG. 3 ) (step  420   a ), then returns to step  402   a . If node  202  does receive a timely ACK signal from node  204 (B) (step  422   a ), it checks to determine whether that signal indicates that the latter has established a link over link segment  3  with host  208 (B) (step  424   a ). If the ACK signal does not indicate initialization of link segment  3  (step  426   a ), then node  202  likewise forces a disconnect on link  1  (step  420   a ) and returns to step  402   a.    
   If node  202 (A) does receive a timely ACK signal from node  204 (B) and that signal indicates initialization of link segment  3  (step  428   a ), then node  202  keeps link segment  1  online, since link initialization is now complete (step  430 ). 
   Recall that the same process is taking place on side B. More particularly, node  204 (B) executes a series of steps  402   b - 428   b  that are identical to steps  402   a - 428   a  and will therefore not be exhaustively described. Basically, node  204 (B) attempts to handshake with host  208 (B) (step  402   b ) and, if successful ( 410   b ), link segment  3  is initialized (step  412   b ) and a control signal is propagated upstream to node  202 (A) (step  414   b ). As with node  202 (A) and link  1 , node  204 (B) will only maintain link  3  if it receives a favorable acknowledgment from node  202  (step  428   b ); otherwise it drops link segment  3  ( 420   b ). This process continues until both node  202  and  204  have established a client-side connections and have receive acknowledgments from the opposite node; the link is then initialized end-to-end (step  430 ). 
   As a practical matter, node  202  or  204  may be configured to drop its client signal if it fails to receive any form of acknowledgment from the other node within a predetermined interval, which will depend on the latency of the WDM network. This can either be configured in software or preset to the maximum supported network latency. This step also prevents the client sides (link segments  1  and  3 ) from initializing in the event of a network fiber break on link segment  2  or failure in the TDM/WDM equipment. 
   The peer-to-peer approach is preferred because of its symmetry and ease of implementation. An alternative approach is a master-slave approach, shown in  FIG. 5 . The master-slave procedure is similar to the peer-to-peer procedure shown in  FIG. 4 , with appropriate modifications for the master-slave relationship of the two nodes. The two nodes first establish, during configuration of the TDM/WDM equipment, which node will act as the master (step  501 ). This can be either provisioned manually or negotiated by default settings in the equipment. 
   Once the master-slave relationship is established and confirmed at both ends of the network, the master controls all subsequent OFC handshaking by performing a series of steps  502 - 530 . These steps are generally similar to the like-numbered steps in  FIG. 4  performed by the separate nodes  202  and  204  (with the exceptions indicated), and are therefore not all individually described. The master node first attempts to initiate a handshake with the attached host  206  or  208 , ordering the slave node (through a suitable signal over the network link segment) to remain disabled (step  502 ). If the handshake attempt fails (step  504 ), the master node disables the client link and retries the handshake (step  506 ), while at the same time continuing its previous action of sending an “inhibit” message to the slave node (step  508 ). When the master successfully completes a handshake (step  510 ), the master node initiates the client link (step  512 ), sends an “enable” message to the slave node, releasing the slave to attempt a handshake on the opposite side of the link, and starts a timer for receiving an acknowledgment (ACK) signal from the slave node (step  514 ). 
   When the slave handshake completes, it sends an acknowledgment back to the master within a predetermined timeout interval to complete the link initialization process. If the ACK signal is not received in time (steps  516  and  518 ), the master node disables the client (step  520 ) and returns to step  502  to attempt another handshake with the client. If the ACK signal is received in time (step  522 ) and is in proper form (steps  524  and  528 ), then link initiation is complete (step  530 ). If the ACK is not in proper form (step  526 ), then the master node likewise disables the client (step  520 ) and returns to step  502  to attempt another handshake with the client. 
   Note there is no need for additional handshakes to take place across the network link  106 ; the slave assumes that it will not be allowed to handshake until the master&#39;s handshake is completed first. There is also no need for a timeout interval on the slave side (steps  514 - 528 ) for the same reason. However, there is a tradeoff between these features and the additional complexity involved with establishing the initial master-slave relationship. 
   Alternative Embodiment for OFC Propagation 
   In addition to the embodiments described above, we propose an alternative embodiment for OFC propagation across a TDM/WDM network with duplex links. This alternative approach does not require the TDM/WDM network nodes to generate OFC handshake pulses. Rather, it operates by echoing the OFC pulses from the ISC channels back to their original source if the link conditions are suitable for link initialization, as described in more detail below. 
     FIG. 6  shows a system  600  comprising a pair of interconnected TDM nodes  602   a  (node A) and  602   b  (node B) coupling a first ISC channel  604   a  (channel A) and a second ISC channel  604   b  (channel B). A duplex optical link  606   a  interconnects TDM node  602   a  and ISC channel  604   a , while a similar duplex optical link  606   b  interconnects TDM node  602   b  and ISC channel  604   b . An additional duplex optical link  608 , in particular a network link, interconnects TDM nodes  602   a  and  602   b  to each other. Each of ISC channels  604   a  and  604   b  contains an optical transmitter (TX)  610  and receiver (RX)  612  and uses a predetermined protocol, such as the one defined in the ANSI standard referenced above, for exchanging OFC signals with an attached device. 
   For simplicity,  FIG. 6  shows only one ISC channel  604  attached to each TDM node  602 . However, the approach can easily be replicated for multiple channels  604  on the same TDM node  602 . Further, while the approach is described for TDM signals only, it can obviously be extended to include TDM signals running over a WDM network as well. In such a TDM/WDM network, the components shown for a node  602  in  FIG. 6  would correspond to a single TDM channel, with multiple TDM channels interfacing with a single WDM multiplexer and demultiplexer as shown in other embodiments above. 
   Each of TDM nodes  602   a  and  602   b  contains a plurality of input ports collectively indicated by the reference numeral  614 , two of which, port  614 - 1  (port  1 ) and  614 - 2  (port  2 ), are shown. Each input port  614  has transmitter (TX)  616  and receiver (RX)  618  that interface with a corresponding receiver (RX)  612  and transmitter (TX)  610 , respectively, of an ISC channel  604   a  or  604   b . Each receiver  618  supplies one input to a TDM multiplexer (MPX)  620 , which time multiplexes the input with inputs from the other receivers of the same node  602   a  or  602   b . The TDM multiplexer  620  of each node  602   a  or  602   b  in turn drives a transmitter  622  coupled via link  608  to a corresponding receiver  624  in the other node, the transmitter  622  and receiver  624  of each node  602  constituting an output port  626 . Each receiver  624  drives a TDM demultiplexer (DMPX)  628 , which supplies demultiplexed signals to the port transmitters  616  of that node. 
   In operation of the system  600  described above, an optical signal originating from transmitter  610  of ISC channel  604   a  reaches receiver  612  of ISC channel  604   b  by way of link  606   a , receiver  618  of input port  614 - 1  (of node  602   a ), multiplexer  620 , transmitter  622 , link  608 , receiver  624  of node  602   b , demultiplexer  628 , transmitter  616  of input port  614 - 1  and link  606   b . An optical signal originating from transmitter  610  of ISC channel  604   b  reaches receiver  612  of ISC channel  604   a  via a similar path in the other direction, and similarly for optical signals originating from other ISC channels and traversing other duplex input ports. 
   Most of the time, each TDM node  602  operates transparently: any input signal from an ISC channel  604  is time multiplexed and transmitted along the network link  608  to the other node, while any received TDM signal from the network link is demultiplexed and routed via an input port  614  to the appropriate ISC channel  604 . In order to initialize the overall link (comprising all of the components between a pair of intercommunicating ISC channels  604 ) using OFC, this aspect of the present invention contemplates that a TDM node  602  operate in a non-transparent way under certain conditions. More particularly, this aspect of the present invention contemplates that logic  630  incorporated into each TDM node  602  monitor signals present at the TDM input ports  614  and the TDM output port  626 , then implement a state machine that routes the handshake signals as desired. Logic  630  may be implemented in any suitable manner, such as by a special-purpose digital circuit—e.g., an application-specific integrated circuit (ASIC)—firmware (i.e., microcode) or a combination of the two. 
   To illustrate how this would work, first assume that ISC channel  604   a  is connected to TDM node  602   a  and initiates an OFC handshake pulse from its transmitter  610  to the port  1  receiver  618  of node  602   a . Since the corresponding link  606   b  from ISC channel  604   b  is not yet active, there is no receive signal on the line from the network-side receiver  624  to the port  1  transmitter  616 . (Note that there may still be signals running on other ports  614  of the TDM channel serviced by the multiplexer  620  and demultiplexer  628 , and so other signals may be flowing on the transmitter and receiver links to node  602   a .) 
   Referring now to  FIG. 7  and to the flowchart of  FIG. 10 , this aspect of the invention comes into play when (1) the signal from an ISC channel  604  to the receiver  618  of a input port  614  of a node  602  contains an OFC pulse  702  (step  1002 ) and (2) the signal from the transmitter  616  of the same input port  614  is low (as indicated in the figure for input port  614 - 1  of node  602   a ) (step  1004 ). When these two events co-occur, the digital logic  630  at that node  602  forces the OFC signal  702  to both (1) loop back to the receiver  612  of the ISC channel  604 , using transmitter  616  (as shown at  704 ), and (2) be transmitted from the transmitter  622  of the node  602  across the TDM link  608  (as shown as  706 ) (step  1006 ). This state is maintained at the originating node  602  for a predetermined fixed amount of time, after which the node  602  returns to a transparent state and propagates any signals it receives (step  1008 ). Looping the signal back to the ISC channel  604  causes the OFC handshake to complete on this channel and initializes the first link segment, in this case the segment comprising the link  606   a  between ICS channel  604   a  and node  602   a.    
   Referring now to  FIG. 8 , the same forwardly transmitted OFC pulse  706  ( FIG. 7 ) is subsequently received at the other end of the network link  608  at the receiver  624  of TDM node  602   b  (as shown at  802 ). However, the receiver  618  of port  614 - 1  of TDM node  602   b  still does not have a signal. When these two signals are detected in these states, the digital logic  630  at the receiving TDM node  602   b  passes the OFC pulse  802  along to the receiver  612  of ISC channel  604   b  (as shown at  804 ). At the same time, it transparently passes through any signal it receives from ISC channel  604   b . Upon receiving this signal  804 , ISC channel  604   b  responds with an OFC handshake pulse  806  from its transmitter  610 . 
   Referring now to  FIG. 9 , the OFC handshake pulse  806  from ISC channel  604   b  propagates through TDM node  602   b , as shown at  902 , across the network link  608  back to TDM node  602   a . By this time, the time delay at node  602   a  has expired, so the received signal is simply passed transparently through. ISC channel  604   a  has in the meantime been sending optical signals, which pass transparently through TDM node  602   a  across the network link  608  to TDM node  602   b ; this is sufficient to complete the handshake at TDM node  602   b . Both nodes  602   a  and  602   b  are now in transparent mode, and the link spanning ISC channels  604   a  and  604   b  has been fully initialized. 
   It should be apparent that the same procedure could have been followed if ISC channel  602   b  had been the first to initiate OFC handshaking instead of ISC channel  602   a . As an additional feature, the digital logic  630  ensures that if there is an open link condition at any point in the network, nodes  602   a  and  602   b  will detect this and shut down their corresponding transmitters, interrupting data transmission. Once the link segments are deactivated, ISC channels  604   a  and  604   b  will attempt to initiate an OFC handshake every 10 seconds, in accordance with the existing OFC handshaking protocol referenced above. When the link is repaired, the procedure described above will initialize the link once again. Deadlock conditions are prevented by having each TDM node  602  check whether it is already receiving a signal from the other end of the network link  608  before using its own state machine  630  to begin the handshake process. 
   The embodiment described above has several potential advantages. It does not require the TDM nodes  602  to generate an OFC signal pulse, which can save on hardware if multiple ISC channels  604  are to be accommodated on a single card. Also, it does not require an optical supervisory channel to verify that the links are connected. The maximum link length, or the longest allowed round-trip delay time from node  602   a  to node  602   b  and back to node  602   a  again, is limited by the fixed delay built into the digital logic  630 ; the link has to establish during this time. The fixed delay must also be short enough to avoid causing error conditions at the ISC channels  604  due to the looped back signals. By selecting a fixed delay of a few milliseconds, it should be possible to extend the links beyond 100 kilometers; this delay could also be made programmable and adjusted depending on link length or other conditions. In this approach, the ISC channel  604  which first initiates a handshake becomes the master for the handshake between itself and the TDM node  602 ; while at the other end of the link, the other TDM node  602  acts as the master for the handshake between itself and the other ISC channel  604 . 
   While certain features of the present invention (such as the optical transmitters and receivers) necessarily incorporate hardware, other features (such as the digital logic described above) may be implemented in firmware or some combination of hardware and firmware. 
   As one example, one or more aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media may have embodied therein, for instance, computer readable program code means for providing and facilitating the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately. 
   Additionally, at least one program storage device readable by a machine, tangibly embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided. 
   The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
   While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.