Abstract:
A method is provided for seamless migration from static to agile optical networking at a network switching node in an optical transport network. The seamless method includes: providing an optical signal splitter at the input of thenetwork switching node, the signal splitter being adapted to receive an optical multiplexed signal having a plurality of data signals and at least one data signal being agile; providing an optical signal combiner at the output of the network switching node; and introducing a photonic cross-connect switch between the signal splitter and the signal combiner, where the photonic switch is operable to switch the agile data signals.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to photonic switching in optical transport networks and, more particularly, to a method of seamless migration from static to agile optical networking. 
   BACKGROUND OF THE INVENTION 
   Connections through current optical networks are either manually provisioned and remain static, and/or use electrical cross-connect switches for more automated provisioning and flexible connectivity. 
   Static connections are appropriate for services that are unlikely to change, and include the advantage of lowest possible loss. For high capacity networks, static connections can be rapidly provisioned into pre-planned end-to-end bands of wavelengths. For example, a wavelength division multiplexing (WDM) system may support the photonic routing of wavelengths in a group rather than individually, the group being called a waveband. An example size for a waveband is eight wavelengths. Once a waveband has been set up across the network, new wavelengths can be quickly added at the two endpoints of the previously established waveband without having to modify the network core. In this case, connections are agile at the network edge, while still static in the network core. There is also a need for connections not only edge agile, but core agile as well. Core network agility can be provided through the use of electrical cross-connect switches. However, this approach has the disadvantage of introducing numerous optical-electrical-optical conversion devices and related costs into the network. Photonic switching enables an agile optical layer, providing remote re-configuration and automated restoration. 
   Therefore, it is desirable to provide agility by means of photonic switching, and a seamless technique for supporting static and agile services in optical network. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a method is provided for seamless migration from static to agile optical networking at a network switching node in an optical transport network. The seamless method includes: providing an optical signal splitter at the input of the network switching node, the signal splitter being adapted to receive an optical multiplexed signal having a plurality of data signals and at least one data signal being agile; providing an optical signal combiner at the output of the network switching node; and introducing a photonic cross-connect switch between the signal splitter and the signal combiner, where the photonic switch is operable to switch the agile data signals. 
   For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1C  are block diagrams illustrating a first preferred technique for in-service migration from static optical networking to static plus agile optical networking in accordance with the present invention; 
       FIG. 2  is a block diagram illustrating how the in-service migration technique may be applied to a switching node that supports four fiber pairs which carry a mix of static and agile connections; 
       FIGS. 3 and 4  are block diagrams illustrating how the in-service migration technique may be applied to a switching node that supports the addition of at least one fiber pair that carries all static and/or all agile connections; 
       FIG. 5  is a block diagram that illustrates a technique for improving isolation in the switching node in accordance with the present invention; 
       FIGS. 6 and 7  are block diagrams illustrating how unused static bandwidth can be recovered, by either VOAs or switches, for use by the agile connections of the switching node in accordance with the present invention; 
       FIG. 8  is a block diagram illustrating a second preferred technique for in-service migration from static optical networking to static plus agile optical networking in accordance with the present invention; 
       FIG. 9  is a diagram of how network traffic may be statically pre-selected within a demultiplexer and multiplexer of the switching node; 
       FIG. 10  is a diagram of how network traffic may be flexibly selected within a demultiplexer and multiplexer of the switching node; 
       FIG. 11  is a diagram depicting an exemplary selector for a degree of flexibility selecting network traffic in a demultiplexer and multiplexer of the switching node; 
       FIGS. 12A and 12B  are block diagrams illustrating a third preferred technique for migrating from static optical networking to static plus agile optical networking in accordance with the present invention; and 
       FIG. 13  is a block diagram illustrating how simple open/closed switches may be employed to better isolate static connections through the photonic switch of the switching node in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A seamless technique for in-service migration from static optical networking to static plus agile optical networking is depicted in  FIGS. 1A-1C . Agile optical networking is generally achieved through the introduction of photonic switching at a network switching node  10 , where the switching node  10  interconnects at least two optical transport line systems. The optical transport line systems may employ a pair of unidirectional optical fibers (also referred to as fiber pairs) or a single bidirectional optical fiber. Referring to  FIG. 1A , the exemplary network switching node  10  is shown as a fixed optical add/drop multiplexer  12 . However, it is envisioned that this technique may be applied to other initial network arrangements residing in a core optical network. 
   In a WDM optical transport network, numerous optical data signals are multiplexed together to form a single optical system signal. The optical system signal may be constituted in an optical line hierarchy as is known in the art. For example, the optical system signal may be constructed from a plurality of optical band signals, where each of the optical band signals is constructed from a plurality of optical waveband signals and each of the optical waveband signals are constructed from a plurality of optical wavelength signals. Although the fixed optical add/drop multiplexer  12  preferably operates to add, drop, manually route, or otherwise manipulate optical wavelength signals, it is readily understood that the multiplexer may support optical data signals at any one of the hierarchical layers that form an optical system signal. Optical band signals and optical waveband signals are herein referred to as optical multiplexed signals. 
   In-service migration is enabled by a properly terminated optical splitter  14  located at the node input and a properly terminated optical combiner  16  located at the node output as shown in FIG.  1 B. The optical splitter  14  receives an optical multiplexed signal from a first optical transport line  22 . The optical splitter  14  in turn splits the optical multiplexed signal into two (or more) optical multiplexed signals as is well known in the art. 
   The fixed optical add/drop multiplexer  12  receives one of the optical multiplexed signals  17  from the signal splitter  14 . The optical multiplexed signal  17  embodies a plurality of data signals. In accordance with the present invention, the optical multiplexed signal includes (or will include) at least one agile data signal (also referred to as an agile connection). The remaining data signals (or connections) are configured statically within the fixed optical add/drop multiplexer  12 . The fixed optical add/drop multiplexer  12  enables manual connection of static data signals. 
   A photonic cross-connect switch  30  may be subsequently introduced between the signal splitter  14  and the signal combiner  16  as shown in FIG.  1 C. Specifically, the photonic switch  30  receives a second optical multiplexed signal  19  from the signal splitter  14 . The photonic switch  30  can then switch or otherwise process the agile data signals. At introduction, the photonic switch  30  initially blocks (or disables) all of the data signals received. The photonic switch  30  then enables agile data signals as they materialize. 
   A signal combiner  16  receives optical multiplexed signals from both the optical multiplexer  12  and the photonic switch  30 . The signal combiner  16  in turn combines the two optical multiplexed signals to form a single optical multiplexed signal. The optical multiplexed signal may then be launched into a second optical transport line  24 . In this way, a seamless technique is provided for in-service migration from static optical networking to static plus agile optical networking. For simplicity, only one direction of transmission has been described. However, it is readily understood that the switching node is ordinarily configured to support bidirectional traffic, meaning another mirror image system for the other direction. 
   New agile service connections are introduced through the add/drop side of the photonic switch  30 . At switching nodes with no agile add/drop service connections, the photonic switch  30  is not essential, but can still be deployed to enable more flexible network reconfiguration and restoration of agile service connections that pass through the switching node. Thus, agile pass through traffic growth is inherent, and agile add/drop traffic growth is ‘pay-as-you-go’ in terms of as required additional local agile service interfaces. 
   Implementation of this in-service migration requires adequate isolation between the static and agile network traffic. It is envisioned that isolation may be increased by variable optical attenuators (VOAs) that further suppress static connections at the output of the photonic switch  30 . Additional isolation techniques are described below. In any case, the optical transport system must be able to tolerate any limitations on isolation of blocked static connections through the photonic switch which will combine with static connections at the signal combiner. Similarly, the optical transport system must be able to tolerate any noise in unused static connections which will combine with agile connections at the signal combiner. Lastly, optical losses introduced by the optical splitter and combiner are nominally 3 dB per branch, but may differ depending on loss tolerance of static and agile paths. These losses may be cancelled by common equipment amplifiers with negligible optical signal-to-noise ratio (OSNR) impairments. 
     FIG. 2  illustrates in-service migration for a switching node  40  that supports four fiber pairs, where the additional fiber pairs may carry a mix of static and agile connections. In this case, the switching node, including the photonic switch, is initially configured to support up to four fiber pairs. When less than four fiber pairs are connected to the switching node, additional fiber pairs can be subsequently added in a non-disruptive manner. Depending on the scalability of the photonic switch, one skilled in the art will readily recognize that this arrangement is further extendable to switching nodes that support more or less than four fiber pairs. 
   When the additional fiber pairs  42  carry all agile connections, there is no need for corresponding multiplexers and demultiplexers within the context of the fixed optical add/drop multiplexer as shown in FIG.  3 . However, multiplexers and/or demultiplexers may be non-disruptively added later if static traffic materializes. Similarly, when the additional fiber pair  44  carries all static connections, there is no need for a connection to the photonic switch as shown in FIG.  4 . Again, multiplexers, demultiplexers and/or switch connections may be non-disruptively added later if previously unexpected static and/or agile traffic materializes. 
     FIG. 5  illustrates an additional technique for improving isolation in the switching node. This technique introduces a pre-switch filter  52  to improve isolation of blocked static connections through the photonic switch. The filter is located between the signal splitter  14  and the photonic switch  30 . The filter  52  rejects static data signals and passes agile data signals to the photonic switch  30 . The switching node otherwise operates as described above. 
   In the case of an optical waveband architecture, it is further envisioned that unused static bandwidth can be recovered for use by the agile connections as shown in FIG.  6 . In general, selected pass-through wavebands are ‘rolled’ to the photonic switch  30  for higher fill. Preferably, one waveband is rolled at a time with subsequent verification testing. After the ‘roll’, the pass-through patch cords for the corresponding waveband can be removed from the multiplexer  12 . This prevents interference between static and agile pass-through connections as well as prevents any noise in unused static connections from combining with corresponding agile connections at the signal combiner  16 . 
   More specifically, a plurality of variable optical attenuators (VOAs)  62  are inserted into the static connections of the fixed optical add/drop multiplexer  12 . The photonic switch  30  initially blocks all static connections and enables all agile connections. To recover unused static bandwidth in a waveband, the preferred approach employs local control as described below. First, the corresponding VOA ramps down the selected waveband power to as low as possible and at a slow rate that is non-disruptive to any other connections. The photonic switch  30  then enables all static connections in this waveband to pass through the switch. A photonic switch equipped with VOAs would ramp-up all static connections in the waveband to the correct power level and at a slow rate that is non-disruptive to any other connections. Unused bandwidth in this waveband can then be used for agile connections. As will be apparent to one skilled in the art, this approach causes a brief disruption to the static connections being rolled, but does not affect the other connections. The slow power ramp down and power ramp up is optional, and depends on the requirements of the downstream optical network. It is not required if the downstream network can handle the transients resulting from a fast roll-over. For example, certain semiconductor-based “linear optical amplifiers” may be able to handle transients, e.g. dropping some channels, while causing no effect on remaining channels. 
   In an alternative embodiment, a plurality of open/closed switches  72  are inserted into the static connections of the fixed optical add/drop multiplexer  12  as shown in FIG.  7 . In this embodiment, the corresponding switches open the waveband path, thereby enabling all static connections in the waveband to pass through the photonic switch  30 . Unused bandwidth in this waveband can then be used for agile connections. Although simpler than the approach described above, this approach causes a brief disruption to all of the connections, not just those being rolled. This approach does not support the option of slowly ramping down the power in the static waveband that is to be rolled to the photonic switch  30 . Again, the severity depends on the behavior of the downstream optical network. However, the downstream optical network may be able to handle the resulting transients without disrupting the other connections. 
   In an alternative approach, static and agile traffic is selected within the demultiplexer as generally shown in FIG.  8 . 
   In a first embodiment, static traffic is pre-selected. Referring to  FIG. 9 , static traffic is passed through to the multiplexer; whereas agile traffic is routed from the demultiplexer to the photonic switch. Pre-selection assumes traffic will not change over time or requires considerable disruption to subsequently alter the nature of the connections. 
   In a second embodiment, the allocation of static traffic may be flexibly changed within the demultiplexer as shown in FIG.  10 . For instance, a selector is used to flexibly allocate static traffic. Again, static traffic is passed through to the multiplexer; whereas agile traffic is routed from the demultiplexer to the photonic switch. An exemplary selector  90  is depicted in  FIG. 11 , for a degree of flexible selectivity. 
     FIGS. 12A and 12B  illustrates a service affecting technique for migrating from static optical networking to static plus agile optical networking. In this alternative embodiment, 2×2 switches  102  are located at the input and output of the fixed optical add/drop multiplexer  104 . The switches  102  are initially configured to pass through the optical multiplexed signal as shown in FIG.  12 A. The fixed optical add/drop multiplexer  104  enables manual connection of static data signals. 
   A photonic cross-connect switch  106  may be subsequently located between the two switches  102 . At introduction, the photonic switch  106  initially blocks all of the data signals and operates the 2×2 switches  102  to a “cross” configuration which routes the optical multiplexed signal towards the photonic switch  106  as shown in FIG.  12 B. If required, the photonic switch  106  would also then increase initially low optical amplifier  118  gains to the correct levels, or would enable the amplifier to start amplifying. 
   On the input side of the node, a signal splitter  114  is located between the 2×2 switch  102  and the photonic switch  106 . The signal splitter  114  receives an optical multiplexed signal from the switch  102  and splits it into two optical multiplexed signals. One of the optical multiplexed signals is directed to the photonic switch  106 ; whereas the other optical multiplexed signal is routed back through the 2×2 switch  102 . The photonic switch  106  can switch the agile data signals, thereby enabling agile optical networking. The 2×2 switch  102  also provides a return path for the static signal channels to the fixed optical add/drop multiplexer  104 . 
   On the output side of the node, a signal combiner  116  is located between the 2×2 switch  102  and the photonic switch  106 . The signal combiner  116  receives an optical multiplexed signal from the 2×2 switch  102  and the photonic switch  106 . The signal combiner  116  in turn combines the two optical multiplexed signals and launches the combined signal into an outgoing optical transport line system. 
   In the initial static arrangement, the 2×2 switches have less optical loss than the splitter/combiner of the first preferred embodiment. However, existing network traffic is briefly disrupted when the 2×2 switches are operated and the photonic switch is introduced at the node. In addition, when traffic is routed through the photonic switch, the cumulative optical loss of the 2×2 switches  102  in conjunction with the signal splitter  114  and the signal combiner  116  is greater than for the first preferred embodiment. Again, these losses may be cancelled by common equipment amplifiers with negligible optical signal-to-noise ratio (OSNR) impairments. 
   Furthermore, optical amplifiers  118  may be optionally located between the 2×2 switches and the signal splitters/combiners to compensate for these additional losses. When the 2×2 switches  102  are initially configured in a pass through state, the optical amplifiers may be reduced in gain or disabled to suppress any oscillation in the feedback loop formed between the switch  102  and the signal splitter  114 . Lastly, note that static pass-through connections being routed through the photonic switch enables recovery of stranded waveband bandwidth, and recovery of guard bands between adjacent wavebands. The static add and drop wavelengths or wavebands are still maintained. 
   A variation of this service affecting technique is shown in  FIG. 13. A  plurality of open/close switches  122  are inserted into the static connections of the fixed optical add/drop multiplexer. In an initial closed state, the switches  122  pass through the static data signals. At introduction, the photonic switch  106  initially blocks all of the data signals and operates the 2×2 switches  102  as described above. The photonic switch  106  may also open certain of the switches  122  residing in the fixed optical add/drop multiplexer. This enables corresponding static connections to be enabled through the photonic switch  106 . 
   After the photonic switch has been introduced, the switches and pass-through patch cords for the operated switches  122  can be removed from the node. As a result, there is no possibility of interference between static and agile connections and any noise in unused static channels is prevented from combining with corresponding agile connections at the signal combiner  116 . Lastly, note again that static pass-through connections being routed through the photonic switch enables recovery of stranded waveband bandwidth, and recovery of guard bands between adjacent wavebands. The static add and drop wavelengths or wavebands are still maintained. 
   While the invention has been described in its presently preferred form, it will be understood that the invention is capable of modification without departing from the spirit of the invention as set forth in the appended claims.