Abstract:
A connectionless packet network and an optical WDM network are interconnected by one or more optical interface modules (gateways) that include both optical to electrical interfaces, as well as a connection management module, or control element, that is arranged to control the OADM&#39;s and the configuration of lasers and port assignments within the gateways, such that a route through the optical network to a desired endpoint is selected. The OADM&#39;s can be programmed, (i.e., locally or remotely controlled by the control element) such that the wavelengths that can be added or dropped by an OADM can be changed, thereby allowing routes to be established through the optical network, from an originating gateway to a destination gateway. In addition, the optical interface modules can include a plurality tunable lasers that can be controlled such that routes can be established through the optical network without requiring changes to the routing table that associates particular endpoints with particular ports. The system can be operated in a “provisioned” mode, where connections are set up a priori (i.e., before actual traffic flow starts), or in a “switched” mode, where connections are set up on a session by session (call by call) basis. The interface modules can be integrated with the components otherwise present in conventional packet routers, or housed separately in intelligent gateways that interconnect conventional packet routers with OADM&#39;s on an optical WDM network.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical networks that use wavelength division multiplexing (WDM), and, more specifically, to internetworking of such networks with connectionless (CL) packet networks such as internet protocol (IP) networks. 
     BACKGROUND OF THE INVENTION 
     Known optical networks use wavelength division multiplexing (WDM) for point-to-point communication between nodes disposed on an optical transmission medium such as an optical fiber. Data, i.e. information bearing packets associated with a plurality of individual calls, is used to modulate a laser having a wavelength corresponding to a particular WDM channel, and the optical signal is inserted onto the transmission medium in an optical add/drop module (OADM) at one node. The optical signal is transported on the medium to a destination node, where another OADM extracts the optical signal, whereupon demodulation is performed to recover the data. 
     An arrangement for internetworking of optical and connectionless packet networks such as Internet Protocol (IP) networks, is described in a copending application Ser. No. 09/333406 and filed Jun. 15, 1999 and entitled “Wideband Optical Packet Ring Network”, assigned to the same assignee as the present invention, which is incorporated herein by reference. In the aforementioned application, specially equipped routers in the packet networks are arranged to have optical interfaces to OADM&#39;s in the optical network. These routers, sometimes referred to as “optical gateways” or simply “gateways”, include hardware and software that performs several functions. First, each gateway includes a packet framer and an optical transceiver that converts an “electrical” stream of packets into an optical signal that modules a laser having a particular wavelength. Second, each gateway is functionally arranged to control the OADM&#39;s in order to implement routing tables that associate specific destinations on the optical network (i.e., remote gateways connected to other OADM&#39;s) with a particular “port” on the router. The laser output, which is available at the above-mentioned particular port on the gateway, is then combined with (added to) other wavelengths on the optical transmission medium to form the WDM signal. At the destination gateway, the portion of the optical signal at the particular wavelength is then extracted (dropped) from the other wavelengths on the optical transmission medium, and the optical signal is reconverted to a stream of packets in another optical transceiver, which can then be transmitted on toward a desired destination. The path which an IP packet takes through WDM network is determined by the wavelength on which it enters the WDM network and the state of the particular ones of the OADMs through which the packet travels. 
     A prior art arrangement of the type just described is illustrated in FIG. 1. A packet network  110  includes a plurality of interconnected routers, such as conventional routers  111  and  112 , and gateways  121  and  122 , which are routers that interface both with conventional routers and also with particular OADM&#39;s in an optical WDM network designated generally at  150 . Thus, gateway  121  has a connection to OADM  151 , while gateway  122  has a connection to OADM  152 . An optical WDM transmission medium  160  with counter-clockwise optical flow, interconnects OADM  151  to OADM  153 , OADM  153  to OADM  154 , and OADM  154  to OADM  152 . OADMs  153  and  154  are, in turn, connected to gateways  133  and  134 , respectively, which may be part of packet network  110  or may be part of a different packet network. 
     For simplicity of description, assume that gateways  121 ,  122 ,  133  and  134  each have two ports, called port  1  and port  2 , each arranged as part of an electrical to optical interface at a specific wavelength λ 1  and λ 2 , respectively. In a real implementation (such as an implementation using AllWave™ fiber technology available from Lucent Technologies), each gateway could be arranged to simultaneously support many more WDM channels; (e.g., as many as 2000 channels) on transmission medium  160 . Each router in packet network  110 , including gateways  121 ,  122 ,  133 ,  134  and  135 , has a routing table, which specifies which port an incoming packet (i.e., a packet received from another router in the packet network) should be applied to in order to be transported on the optical network to a particular destination gateway. Thus, for example, as shown in FIG. 1, the routing tables for gateways  121  and  134  may be as set forth in tables 1 and 2 below, respectively: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Gateway 121 Routing Table 
               
             
          
           
               
                   
                 Destination 
                 Port Assignment 
               
               
                   
                   
               
               
                   
                 Gateway 133 
                 Port 1 
               
               
                   
                 Gateway 122 
                 Port 2 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Gateway 134 Routing Table 
               
             
          
           
               
                   
                 Destination 
                 Port Assignment 
               
               
                   
                   
               
               
                   
                 Gateway 133 
                 Port 1 
               
               
                   
                 Gateway 122 
                 Port 2 
               
               
                   
                   
               
             
          
         
       
     
     In the example of FIG. 1, assume that a first “connection” from gateway  121  to gateway  133  is desired. This connection can be established through the optical WDM network by applying packets received at gateway  121  (from other routers in the packet network  110 ) to port  1 , which is associated in Table 1 with the desired destination (gateway  133 ). The packets are used to modulate a laser having a wavelength λ 1  associated with port  1 , and are inserted via OADM  151  onto transmission medium  160 , which in this case is arranged to “add” the laser output to the signals already travelling in a counterclockwise direction on transmission medium  160 . In this example, OADM  153  is arranged to extract (drop) the optical signal on transmission medium  160  at wavelength λ 1  from the other *WDM signals on the transmission medium, so that the information bearing packets can be recovered by demodulation in a transceiver in gateway  133 . Also assume that a second connection from gateway  134  to gateway  122  is desired. This connection can be established through the optical WDM network by applying packets received at gateway  134  (from other routers in another packet network not shown in FIG. 1) to port  2 , which is -associated in Table 2 with the desired destination (gateway  122 ). The packets are used to modulate a laser having wavelength λ 2  associated with port  2 , and are inserted via OADM  154  onto transmission medium  160 , which in this case is arranged to “add” the laser output to the signals already travelling in a counterclockwise direction on transmission medium  160 . In this example, OADM  152  is arranged to extract (drop) the optical signal on transmission medium  160  at wavelength λ 2  from the other WDM signals on the transmission medium, so that the information bearing packets can be recovered by demodulation in a transceiver in gateway  122 . 
     While the first and second connections just described are ongoing, it will be observed that if a connection from gateway  121  to gateway  122  is concurrently requested, the request would have to be denied. This would be true even though gateway  121  has an idle port, namely port  2 . This is because if port  2  were to be used, the incoming packets would be used to modulate a laser at wavelength λ 2 . This signal would be added at OADM  151  and dropped at OADM  152 . While no interference would occur in the portion of transmission medium  160  between OADM  151  and OADM  154 , it is noted that the same wavelength, λ 2 , would be used in the portion of transmission medium  160  between OADM  154  and OADM  152 , causing impermissible interference. Accordingly, the general object of the present invention is to enable efficient allocation of network resources (e.g., bandwidth) in an optical WDM network, and provide the ability to internetwork optical and packet networks so as to provide truly guaranteed connections to satisfy service requirements. A specific object is to enable intermetworking of optical WDM and packet networks in a manner in which the previously described interference is avoided. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a connectionless packet network and an optical WDM network are interconnected by one or more optical interface modules (gateways) that include both optical to electrical interfaces, as well as a connection management module, or control element, that is arranged to control the OADM&#39;s and the configuration of lasers and port assignments within the gateways, such that a route through the optical network to a desired endpoint is selected. 
     In one embodiment of the present invention, the OADM&#39;s can be programmed, (i.e., locally or remotely controlled by the control element) such that the wavelengths that can be added or dropped by an OADM can be changed. This flexibility allows routes to be established through the optical network, from an originating gateway to a destination gateway, under circumstances such as those described above in conjunction with FIG. 1, where a route would otherwise be unavailable. In this embodiment, changes to the routing tables of at least some of the gateways are generally required. The programmable OADM&#39;s can use fiber Bragg grating technology in which the gratings are tuned using temperature or magnetic-strain. Alternatively, thin-film technology can be used in which tuning is realized by mechanically translating the filter. 
     In another embodiment of the present invention, the OADM&#39;s are programmable, and, in addition, the optical interface modules include a plurality of N tunable lasers, where N is an integer equal to the number of WDM channels present on the optical transmission medium. With this capability to change the wavelength of the laser associated with a particular port in the gateway, routes can be established through the optical network, from an originating gateway to a destination gateway, under circumstances such as those described above in conjunction with FIG. 1, where a route would otherwise be unavailable, and, in addition, changes to the routing table that associates particular endpoints with particular ports is advantageously not required. If the system is operated in a “provisioned” mode, where connections are set up a priori (i.e., before actual traffic flow starts), based upon resource requirements that are computed using some estimate of the expected traffic, the lasers can be re-tuned on a fairly infrequent basis On the other hand, when the system is operated in a “switched” mode, where connections are set up on a session by session (call by call) basis, the lasers can be re-tuned much more frequently. In this embodiment, transceiver within the interface modules are arranged to insert “fillers” into the optical outputs when no packet data is being inserted onto the WDM channels. This advantageously avoids the need for burst-mode receivers in the interface modules. 
     In still another embodiment of the present invention, the number of tunable lasers can be fewer than the number of WDM channels available on the optical transmission medium, in which case burst-mode receivers are required in the interface modules. Advantageously, in this arrangement, as in the one just described, the port assignments (routing table) in the gateways do not have to be changed. 
     The interface modules contemplated by the present invention can be physically integrated with the components otherwise present in conventional packet routers. Alternatively, these modules can be housed separately in intelligent gateways that interconnect conventional packet routers with OADM&#39;s on an optical WDM network. The WDM network can use dense wavelength division multiplexing, or, if desired, coarse WDM of the type described in an article entitled “Optical Networking”, by Daniel Y. Al-Salameh et al., published in the  Bell Labs Technical Journal , January-March 1998, pps. 39-61. 
     The connection management module is arranged to determine, in response to receipt of a stream of packets intended to be routed to a remote destination, the “shortest path” to that destination, which may be through the packet network or through the optical WDM network. If routing through to the optical network is preferred, the management module is arranged to (a) analyze the existing traffic on the optical network to determine if a route is available without changing existing light paths, and (b) if a route is available, to “set up” that route, and (c) if a route is not available, to appropriately control various elements in the optical network so as to both shift existing traffic to alternate routes as well as to set up the desired route. The “set up” just described may involve tuning the wavelength of a tunable laser, rearranging the port assignment of a fixed laser, and/or reconfiguring of the programmable OADM&#39;s. In some cases, the management module also needs to change routing tables in the optical interface modules. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be more fully appreciated by consideration of the following detailed description, which should be read in light of the accompanying drawing in which: 
     FIG. 1 discussed above, is a diagram illustrating a prior art arrangement for internetworking between a packet network, such as an IP network, and an optical WDM network; 
     FIG. 2 is a diagram similar to FIG. 1, illustrating how a connection management module that can “program” the OADM&#39;s can establish a path through the optical network that could not be established in the arrangement of FIG. 1; 
     FIG. 3 is a diagram similar to FIG. 1, illustrating how a connection management module that can “program” the OADM&#39;s and control the wavelength of the lasers in the gateways, can establish a path through the optical network that could not be established in the arrangement of FIG. 1, without the need to change the routing tables in any of the gateways; 
     FIGS. 4-6 are diagrams that show the “before” and “after” status of gateway  134  in the arrangements depicted in FIGS. 1-3 respectively, wherein the before status represents the case where a first lightpath exists between gateway  121  and gateway  133  with wavelength λ 1  and a second lightpath exists between gateway  134  and gateway  122  with wavelength λ 2  and wherein the “after” status represents the case where an additional lightpath is requested to connect gateway  121  to gateway  122 ; 
     FIG. 7 is a diagram illustrating the process by which the arrangement of FIG. 2, in which the OADM&#39;s are programmable and the gateways include multiple fixed lasers, can be operated to enable a requested connection between gateways; 
     FIG. 8 is a diagram illustrating the process by which the arrangement of FIG. 3, in which the OADM&#39;s are programmable and the gateways include as many multiple tunable lasers as there are wavelengths supported in the optical transmission medium, can be operated to enable a requested connection between gateways; and 
     FIG. 9 is a diagram similar to FIG. 8, in which the OADM&#39;s are programmable and the gateways include fewer tunable lasers as compared to the number of wavelengths supported in the optical transmission medium. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 2, there is shown a diagram similar to FIG. 1, illustrating how a connection management module  201  connected to the OADM&#39;s can “program” one or more OADM&#39;s by changing the wavelength that is added and/or dropped, and thereby establish a path through the optical network that could not be established in the arrangement of FIG.  1 . This is because the path which a packet takes through WDM network  150  is determined by the wavelength on which it enters the WDM network and the state of the particular ones of the OADMs  151 - 155  through which the packet travels. In FIG. 2, the same reference designations as used in FIG. 1 are retained. As in FIG. 1, it will be assumed that optical fiber transmission medium  160  is a unidirectional loop, with light travelling in a counter-clockwise direction. However, in most real implementations, two separate unidirectional loops carrying traffic in opposite directions are often used, such that a fault in an OADM or repeater can be handled by reconfiguring the two loops as a single loop that bypasses the fault. 
     In FIG. 2, consider an example in which router  111  has packet traffic to send to router  112 . Advantageously, router  111  will be arranged to send data to router  112  through WDM network  150  rather than through IP network  110 , if the path through network  150  is “shorter” than the path through network  110 . (Each link is assigned a weight, and route “lengths” may be calculated using an algorithm such as, for example, the one described by Dijkstra in “Data Networks”, by Bertsekas and Gallagher, Prentice Hall, 1991. If a route through WDM network  150  is used, GW  121  must set up a lightpath from OADM  151  to OADM  152  and thence to GW  122  in order to send the data received from router  111  to router  112 . 
     Assume that communications between gateway  121  and gateway  133  are ongoing using λ 1  and that communications between gateway  134  and gateway  122  are ongoing using λ 2 . Thus, when gateway  121  seeks to establish a path to gateway  122 , an initial determination by connection management module  201  determines that a path is not available. However, if OADM&#39;s  154  and  152  are respectively arranged to add and drop wavelength λ 1  rather than wavelength λ 2 , the same wavelength, λ 1 , would then be used on two segments, namely, to connect gateway  121  to gateway  133 , and also to connect gateway  134  to gateway  122 . In this event, wavelength λ 2  is now available on which to establish the additional desired connection between gateway  121  and gateway  122 . T his is accomplished by connection management module  201  also controlling OADM&#39;s  151  and  152  to respectively add and drop that wavelength. 
     Note, in the embodiment of FIG. 2, after the OADM&#39;s are reprogrammed, the routing table in gateway  134  must be changed. Instead of port  2  being used for the connection to gateway  122 , port  1  must be used, since that is the port associated with the laser at wavelength λ 1 . Note also that the arrangement illustrated in FIG. 2 is exemplary, in that the gateways are shown as distinct elements located between IP network  110  and WDM network  150 . Alternatively, the functionality of the gateways can be provided either (a) within the routers to which the gateways are connected (i.e., gateway  121  could be part of router  111 , gateway  122  could be part of router  112 , etc.) and the routers would then be equipped with appropriate WDM line cards including the laser modulators, or (b) within the OADMs (i.e., gateway  121  could be part of OADM  151 , gateway  122  could be part of OADM  152 , etc.) and the OADM&#39;s would then be equipped with appropriate packet routers. As yet another alternative, some other combination of the foregoing could be arranged. 
     While the embodiment of FIG. 2 describes gateways and OADM&#39;s having the ability to use only two optical channels at wavelengths λ 1  and λ 2 , it is to be understood that in a real implementation, a large number of WDM channels with different wavelengths will exist simultaneously on fiber transmission loop  160 , and that the OADM&#39;s and gateways would have the capability of processing information on these channels in a manner similar to that just described. 
     Turning now to FIG. 3, in which components like those in FIGS. 1 and 2 retain like designations, another embodiment of the present invention is illustrated in which, like the arrangement of FIG. 2, OADMs  151 - 154  are arranged to be reconfigured or programmable, but unlike the arrangement of FIG. 2, gateways  121 ,  122 ,  133  and  134  are arranged to include multiple tunable lasers. As will be seen, this advantageously enables a larger number of network connections to be supported than with fixed-wavelength lasers, while, at the same time, not requiring the routing tables in the gateways to be changed. In this embodiment, we assume that OADMs  151 - 154  on fiber transmission loop  160  are each able to add and drop only 2 wavelengths, λ 1  and λ 2 . Furthermore, each gateway  121 ,  133 ,  134  and  122  includes two ports, called Port  1  and Port  2 , as well as multiple tunable lasers, in this example, capable of being controllably tuned to wavelengths λ 1  or λ 2 , depending upon control inputs received from connection management module  201 . 
     Let us suppose that an initial condition exists in which the routing tables for gateways  121  and  134  are as shown in FIG. 1, and that the laser in port  1  of GW  121  is tuned to λ 1 , which is used to support a connection from GW  121  to GW  133 . At the,same time, the laser in port  2  of GW  134  is tuned to λ 2 , which is used to support a connection from GW  134  to GW  122 . 
     Now, suppose that we wish to set up a new path (i.e., to make a new connection) from GW  121  to GW  122 . At GW  121 , λ 1  is already in use, and adding  2  would interfere with the GW  134  to GW  122  connection. If the lasers in GW  121  are fixed-wavelength, as would be found conventionally in known arrangements, then the connection request must be denied. However, if, in accordance with this embodiment of the present invention, multiple tunable lasers are available in GW  134 , then the GW  121  to GW  122  connection request may be accommodated, as shown in FIG.  3 . First, the laser in port  2  of GW  134  would be retuned to λ 1 , to continue supporting the GW  134  to GW  122  connection. Now, the laser in port  2  of GW  121  may be tuned to λ 2 , to accommodate the desired new GW  121  to GW  122  connection. Note that certain of the OADMs, namely OADMs  152  and  154 , also needed to be reprogrammed. However, it was not necessary to change the routing table associated with gateway  134 , as was the case in the embodiment described in connection with FIG.  2 . In an actual implementation of this embodiment of the present invention, additional electronics and signaling capability would be built into the gateways to (a) identify which port, and hence which tunable laser, an IP packet will be passed to, and (b) to control the tuning of the lasers. 
     For the embodiment of FIG. 3 to work in the manner described above, in which the additional connection can be accommodated without a change in the routing tables, it is necessary that the number of tunable lasers available each gateway be equal to the number of wavelengths supported in the transmission medium. In this type of embodiment of the present invention, transceiver within the interface modules are arranged to insert “filters”into the optical outputs when no packet data is being inserted onto the WDM channels. This advantageously avoids the need for burst-mode receivers in the interface modules. 
     An alternative arrangement is possible, in which the number of tunable lasers available at each gateway is fewer than the number of wavelengths supported in the transmission medium. Reducing the number of tunable lasers is advantageous, because it reduces the overall cost of the system. However, that arrangement has the associated penalty of requiring the use of burst mode receivers, because with fewer tunable lasers than wavelengths, data will not be sent constantly on all wavelengths. Thus receivers that are capable of synchronizing quickly are needed. Such receivers are called “burst-mode receivers.” Also, in this arrangement, routing table updates would also be required, because when a connection is set up using some wavelength, the wavelength could be associated with a port different from the port indicated to reach the destination in the routing table. 
     The differences between the arrangements of FIGS. 1,  2  and  3  is further illustrated by reference to FIGS. 4-6, which focus on and show the status of gateway  134  “before” (when a first lightpath exists between gateway  121  and gateway  133  with wavelength λ 1  and a second lightpath exists between gateway  134  and gateway  122  with wavelength λ 2 ) and “after”, when an additional lightpath is requested to connect gateway  121  to gateway  122 , in the situations depicted in FIGS. 1-3, respectively. With respect to FIG. 4, which corresponds to the situation of FIG. 1 in which the OADM&#39;s are not programmable and the lasers are not tunable, in the “before” status, gateway  134  supports the connection to gateway  122  via a lightpath with wavelength λ 2  using port  2  of that gateway. In this figure, the “after” status has not changed, since the requested connection could not be supported. With respect to FIG. 5, which corresponds to the situation of FIG. 2 in which the OADM&#39;s are programmable but the lasers are not tunable, the “after” status has changed, such that the connection from gateway  134  to gateway  122  is now supported by a lightpath of wavelength λ 1  via port  1 . As noted previously, the routing table in gateway  134  must be altered accordingly. With respect to FIG. 6, which corresponds to the situation of FIG.,  3  in which the OADM&#39;s are programmable and the lasers are tunable, the “after” status has changed, such that the connection from gateway  134  to gateway  122  is now supported by a lightpath of wavelength λ 1  via port  2 . The routing table in gateway  134  need not be altered in this embodiment. 
     Before proceeding with a description of the control process of the various embodiments of the present invention, it is important to point out that the invention is applicable both in “switched” and “provisioned” modes of operation. In the switched mode of operation, it is contemplated that the lightpaths that enable the connections between gateways are established and maintained only for the duration of a single session or “call” (i.e., the transmission of a series of packets between a specific source and destination that continues during a limited, relatively short, time period), and that the available bandwidth on the fiber transmission loop is then released and reused for subsequent connections. The connection management control elements that enable the switched mode of operation can advantageously be distributed in the interconnected individual routers and gateways that together form the communication system. On the other hand, in the provisioned mode of operation, the OADMs and lasers that are involved in a connection or lightpath are reconfigured or retuned infrequently. This arrangement is well suited for long-term “lease-a-wavelength” applications. Here, the connection management control elements that enable the provisioned mode of operation can advantageously be centralized in a single network management system and executed a priori, i.e., when the communication system is initially set up. 
     The benefit afforded by the present invention is greater in the switched mode than in the provisioned mode, since it is here that frequent updates to the routing tables would otherwise be required. Our invention advantageously reduces the processing load placed on the elements in the system that control packet routing, and this reduction is especially dramatic for switched mode operation. Said differently, our invention is a solution for internetworking WDM and packet networks in a manner that enables highly efficient resource utilization with reduced layer  3  processing, to be realized by combining signaling/routing protocol software with tunable laser/OADM WDM technology. 
     Referring now to FIG. 7, there is shown a diagram illustrating the process by which the arrangement of FIG. 2, in which the OADM&#39;s are programmable and the gateways include multiple fixed lasers, can be operated to enable a requested connection between gateways. The process begins in step  701 , in which a request is received in connection management module  201  for a lightpath from a first (source) gateway or router Ri to a second (destination) gateway or router Rj. In step  703 , connection management module  201  looks to see if a route can be found for the requested path without changing any existing lightpaths. This involves consulting a status table or querying the various network elements. If a route can be found, a positive or “yes” result occurs in step  705 , and the process proceeds to step  707 . Otherwise, if a route cannot be found, a negative or “no” result occurs in step  705 , and the process proceeds to step  715 . 
     In step  707 , the OADM&#39;s involved in the new lightpath are appropriately programmed to add and drop wavelength λk, in order to realize the desired new lightpath. In step  709 , the identity of the port p in Ri whose laser is tuned to wavelength λk is determined. Then, in step  711 , the IP routing table entry in router Ri is updated to indicate that Rj can be reached via port p. The process then terminates. If a no result occurs in step  705 , connection management module  201  searches for a route through the network that could be set up after rerouting one or more existing. lightpaths. Clearly, such rerouting should be minimized, if possible. If a route is found, a positive or “yes” result occurs in step  717 . Then, in step  719 , the affected OADM&#39;s are reprogrammed so as to appropriately change the existing lightpaths, after which the process proceeds to step  707  in order to setup the new lightpath in the manner previously described. If a route is not found in step  715 , a negative or “no” result occurs in step  717 , and; the requested route must be rejected in step  721 , whereupon the process is terminated. 
     Referring now to FIG. 8, there is shown a diagram illustrating the process by which the arrangement of FIG. 3, in which the OADM&#39;s are programmable and the gateways include as many multiple tunable lasers as there are wavelengths supported in the optical transmission medium, can be operated to enable a requested connection between gateways. The process begins in step  801 , in which a request is received in connection management module  201  for a lightpath from a first (source) gateway or router Ri to a second (destination) gateway or router Rj. Because this arrangement is achieved without changing routing tables, the request identifies the port k on Ri that is to be used. In step  803 , connection management module  201  looks to see if a route can be found for the requested path without changing any existing lightpaths. This involves consulting a status table or querying the various network elements. If a route can be found, a positive or “yes” result occurs in step  805 , and the process proceeds to step  807 . Otherwise, if a route cannot be found, a negative or “no” result occurs in step  805 , and the process proceeds to step  815 . 
     In step  807 , the OADM&#39;s involved in the new lightpath are appropriately programmed to add and drop wavelength λk, in order to realize the desired new lightpath. In step  809 , a determination is made as to whether λk is the wavelength to which port k&#39;s laser is tuned. If so, a positive or “yes” result occurs in step  809 , and the process is terminated in step  811 . If not, a negative or “no” result occurs in step  809 , and the process proceeds to step  813 , in which the tunable laser at port k is retuned to wavelength λk. 
     If a no result occurs in step  805 , connection management module  201  searches for a route through the network that could be set up after rerouting one or more existing lightpaths. Clearly, such rerouting should be minimized, if possible. If a route is found, a positive or “yes” result occurs in step  817 . Then, in step  819 , the affected OADM&#39;s are reprogrammed so as to appropriately change the existing lightpaths, after which the process proceeds to step  807  in order to setup the new lightpath in the manner previously described. If a route is not found in step  815 , a negative or “no” result occurs in step  817 , and the requested route must be rejected in step  821 , whereupon the process is terminated. 
     Referring now to FIG. 9, there is shown a diagram similar to FIG. 8, in which the OADM&#39;s are programmable and the gateways include fewer tunable lasers as compared to the number of wavelengths supported in the optical transmission medium. The process begins in step  901 , in which a request is received in connection management module  201  for a lightpath from a first (source) gateway or router Ri to a second (destination) gateway or router Rj. Because this arrangement cannot be achieved without changing routing tables, the request does not identify in advance a particular port on Ri that is to be used. Rather, that port is determined later in the process. 
     In step  903 , connection management module  201  looks to see if a route can be found for the requested path without changing any existing lightpaths. This involves consulting a status table or querying the various network elements. If a route can be found, a positive or “yes” result occurs in step  905 , and the process proceeds to step  907 . Otherwise, if a route cannot be found, a negative or “no” result occurs in step  905 , and the process proceeds to step  915 . 
     In step  907 , the OADM&#39;s involved in the new lightpath are appropriately programmed to add and drop wavelength λk, in order to realize the desired new lightpath. In step  909 , a determination is made as to a port number pk which can be associated with wavelength λk. The process then proceeds to step  911 , in which the entry in the routing table at Ri is changed to indicate that Rj can be reached via port pk. 
     If a no result occurs in step  905 , connection management module  201  searches, in step  915 , for a route through the network that could be set up after rerouting one or more existing lightpaths. Clearly, such rerouting should be minimized, if possible. If a route is found, a positive or “yes” result occurs in step  917 . Then, in step  919 , the affected OADM&#39;s are reprogrammed so as to appropriately change the existing lightpaths, after which the process proceeds to step  907  in order to setup the new lightpath in the manner previously described. If a route is not found in step  915 , a negative or “no” result occurs in step  917 , and the requested route must be rejected in step  921 , whereupon the process is terminated. 
     There are several options for remote programming of the OADMs. For example, if fiber Bragg grating technology is used, the gratings may be tuned using temperature or magnetic-strain. For thin-film technology, tuning may be realized by mechanically translating the filter. Programmable OADMs in the provisioned mode may be reconfigured on a slow timescale. In this case, temperature-tuned fiber Bragg gratings (FBG), for example, would be used. In the switched mode, the OADMs would be reprogrammed on a session-by-session basis, during connection setup. Here, technology such as magnetically-strained FBGs, which offers programmability on a millisecond timescale, would be used. 
     Various modifications and adaptations of the present invention are possible. Accordingly, the present invention is to be limited only by the appended claims.