Patent Publication Number: US-7590353-B2

Title: System and method for bandwidth allocation in an optical light-trail

Description:
TECHNICAL FIELD 
     The present invention relates generally to optical networks and, more particularly, to a system and method for bandwidth allocation in an optical light-trail established in an optical communication network. 
     BACKGROUND 
     Telecommunication systems, cable television systems, and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting optical signals over long distances with very low loss of signal strength. 
     Recent years have seen an explosion in the use of telecommunication services. As the demand for telecommunication services continues to grow, optical networks are quickly becoming overburdened by the increasing amount of information communicated over such networks. The addition of new networks or the expansion of existing networks may however be too costly to be practical solutions to this problem. Thus, efficient use of network resources has become an important goal in developing and operating optical networks. 
     Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is based on the number of wavelengths, or channels, in each fiber and the bandwidth, or size of the channels. By using WDM add/drop equipment at network nodes, the entire composite signal can be fully demultiplexed into its constituent channels and switched (added/dropped or passed through). In such networks, traffic from one network node to another network node are often assigned to a particular wavelength on which the traffic is communicated over the network. By assigning different traffic streams to different wavelengths, interference between different traffic streams is prevented. However, in certain situations, this creates inefficiency in the network. For example, if the traffic from a node that is assigned a particular wavelength does not typically use much of the bandwidth (capacity) associated with the wavelength, then inefficiencies are created. 
     SUMMARY 
     The present invention relates to bandwidth allocation in an optical light-trail. Optical light-trails enable a plurality of nodes included in a light-trail to share the use of an optical wavelength to transmit traffic between the nodes included in the light-trail. A method for allocating the use of an optical light-trail includes calculating a bid at each of one or more nodes included in the light-trail. Each of the nodes calculates a bid with consideration for the criticality of the node&#39;s need to transmit particular traffic on the light-trail. The method also includes transmitting the calculated bids from one or more of the nodes to an arbiter node. The arbiter node determines the maximum received bid, determines whether to allocate the use of the light-trail to the node associated with the maximum bid, and then communicates one or more control messages to the nodes that transmitted the bids indicating to which node use of the light-trail is allocated. 
     Technical advantages of certain embodiments of the present invention may include efficient techniques for using transmission resources on optical networks. More specifically, in particular embodiments of the present invention, nodes of an optical network are capable of establishing an optical “light-trail” that includes one or more other nodes for the transmission of optical traffic. Such a light-trail may be shared by the nodes included in the light-trail to transmit traffic to other nodes included in the light-trail. The use of such light-trails may result in more efficient communication of information in the optical network since a number of nodes can share the bandwidth provided by a wavelength at which the light-trail is established. 
     In order for nodes to share a light-trail, a number of different techniques may be used to allocate use of the light-trail for a particular amount of time to a particular node in the light-trail. Embodiments of the present invention use an “auction” algoritm that allows various nodes to submit bids for use of the light-trail and for a decision to be made regarding allocation of the light-trail based on these bids. This technique is an efficient and fair way to allocate the use of the light-trail amongst the nodes in the light-trail. 
     It will be understood that the various embodiments of the present invention may include some, all, or none of the enumerated technical advantages. In addition other technical advantages of the present invention may be readily apparent to one skilled in the art from the figures, description, and claims included herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an optical ring network in which light-trails may be implemented in accordance with one embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a particular embodiment of a node that may be utilized in an optical network implementing light-trails; 
         FIG. 3A-3C  illustrate example operation of nodes of an optical network in establishing a light-trail; and 
         FIG. 4  is a flowchart illustrating an example auction technique for sharing the use of a light-trail established in the optical network. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an optical network  10  in accordance with one embodiment of the present invention. Optical network  10  includes a plurality of nodes  14  coupled to an optical ring  20 . During operation, nodes  14  transmit and receive traffic on optical ring  20  on one of a plurality of wavelengths. In particular, a light-trail, such as light-trail  30  in  FIG. 1 , may be established over which nodes  14  may transmit optical traffic to other nodes  14  located on that light-trail. Nodes included in a light-trail share the light-trail, as appropriate, to transmit information to other nodes included in the light-trail on a wavelength associated with the light-trail. Thus, a light-trail is a generalization of a light path (an optical wavelength circuit) such that multiple nodes along the path can take part in communication along the path. Therefore, the use of these light-trails addresses the inefficiency discussed above associated with assigning a wavelength for traffic communicated from a single node to another node. In addition, light-trail communications allow optical multicasting and dynamic provisioning. 
     Nodes  14  that allow light-trail communication have specific characteristics that enable the nodes  14  to implement light-trails. For example, these characteristics include a drop and continue function (where traffic received by an element of the node is both dropped and forwarded, so as to allow the traffic to continue along the light-trail), passive adding of traffic by the node (“passive” in this context means the adding of traffic without using optical switches that use power, electricity, and/or moving parts), and the use of an out-of-band control channel (as opposed to control signals that are in-band with the data being communicated on the network  10 ). As described below,  FIG. 2  illustrates a particular embodiment of a node  14  including these characteristics. 
     Referring again to  FIG. 1 , although a single light-trail  30  is illustrated, nodes  14  may establish light-trails on one or more wavelengths utilized by optical network  10  and multiple non-overlapping light-trails may exist at a particular time on a particular wavelength. To prevent optical interference caused by multiple nodes  14  transmitting simultaneously on a particular light-trail in optical network  10 , nodes  14  may utilize particular techniques for sharing the light-trail, as described below. Therefore, there are two levels of “arbitration” associated with light-trails. The first level is the establishment and termination of light-trails to meet particular demands, as well as the “dimensioning” of light-trails (growing or shrinking the trails to meet particular demands). The second level of arbitration is the allocation of the use of the light-trail to nodes in the light-trail. Nodes may be allocated bandwidth according to defined rules or heuristics, predefined bandwidth allocation algorithms, “round robin” techniques (as discussed below), on a dynamic basis, and/or using any other suitable techniques. 
     Although  FIG. 1  illustrates a particular embodiment and configuration of ring network  10 , mesh, linear, or other suitable types of optical networks may be used in accordance with the present invention. In the illustrated embodiment, network  10  is an optical network in which a number of optical channels are carried over a common transmission media at different wavelengths. For example, network  10  may be a wavelength division multiplexed (WDM) network, a dense wavelength division multiplexed (DWDM) network, or any other suitable multi-channel network. Network  10  may represent all or a portion of a short-haul metropolitan network, a long-haul intercity network, or any other suitable network or combination of networks. Network  10  may include, as appropriate, a single uni-directional fiber, a single bi-directional fiber, or a plurality of uni- or bi-directional fibers. 
     Optical ring  20 , in the illustrated embodiment, comprises a pair of uni-directional fibers, first fiber  16  and second fiber  18 , transporting traffic in a counterclockwise and clockwise direction, respectively. Optical ring  20  optically couples the plurality of nodes  14   a - 14   f , and optical traffic propagates between nodes  14  over optical ring  20 . As used herein, “traffic” means information transmitted, stored, or sorted in the network. Such traffic may comprise optical signals having at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time and/or other suitable data. Modulation may be based on phase shift keying (PSK), intensity modulation (IM), and other suitable methodologies. Additionally, the information carried by this traffic may be structured in any suitable manner. Although the description below focuses on an embodiment of network  10  that communicates traffic on optical ring  20  in the form of optical frames, network  10  may be configured to communicate traffic structured in the form of frames, as packets, or in any other appropriate manner. 
     Using established light-trails, nodes  14  facilitate communication between a plurality of client devices (not shown) coupled to each node  14  through a plurality of client ports. As described in greater detail below, each node  14  may receive traffic from client devices coupled to that node  14  and add this traffic to optical ring  20  to the optical traffic propagating on optical ring  20 . Each node  14  may also receive traffic from optical ring  20  and drop traffic destined for client devices of that node  14 , such as personal computers (PCs), telephones, fax machines, hard drives, web servers, and/or any other appropriate communication device. Although  FIG. 1 , illustrates one embodiment of network  10  that includes a particular number of nodes  14 , network  10  may include any appropriate number of nodes  14  configured in any appropriate manner. 
     In operation, nodes  14  generate optical traffic at one or more wavelengths based on electrical signals received by nodes  14  from client devices coupled to nodes  14  and add this optical traffic to optical traffic propagating on optical ring  20 . Nodes  14  also receive and drop traffic propagating on optical ring  20  that is destined for one or more of its clients. For the purposes of this description, nodes  14  may “drop” traffic by transmitting a copy of the traffic to any appropriate components that are a part of or coupled to the relevant node  14 . As a result, nodes  14  may drop traffic from optical ring  20  by transmitting the traffic to these components while allowing the traffic to continue to downstream components on optical ring  20 . Each node  14  drops and electrically converts traffic received on particular wavelengths at which that node  14  is configured to receive traffic and either does not drop or discards traffic transmitted at other wavelengths. Once traffic is dropped from the optical ring  20 , nodes  14  may provide optical-to-electrical conversion of the dropped traffic. Nodes  14  then extract, based on addressing information in the traffic, portions of this traffic destined for client devices coupled to that node  14 . In certain embodiments, each node  14  includes, or has associated with it, a switching element which may forward the traffic, or a portion thereof, to one or more of a plurality of client devices based on addressing information. 
     Since nodes  14  time-share a wavelength associated with a particular light-trail, the data flow patterns through a light-trail dominant network may be somewhat “bursty” in nature due to the interleaving of data streams from multiple nodes  14 . However, client devices (typically, Layer-2 devices) associated with a node  14  expect that the optical layer will provide uninterrupted communication to the devices. Therefore, to facilitate an interface between the bursty optical layer (due to time sharing of the bandwidth of light-trails) and the continuous client layer, nodes  14  include a device called a burstponder. A burstponder is a device that allows a node  14  to time share a wavelength while creating an impression to client device of the node  14  that the wavelength is available on a seamless and continuous basis. Such a burstponder is described in further detail in conjunction with  FIG. 2 . 
     Additionally, nodes  14  may be configured to establish light-trail  30  and transmit or receive some or all optical traffic on light-trail  30 . Light-trail  30  represents an optical path on a portion of fiber connecting any two or more components in optical network  10 . Light-trail  30  is illustrated in  FIG. 1  as a shaded portion of fiber  16 . Once light-trail  30  is established, any of the nodes  14  connected to light-trail  30  may transmit optical traffic on light-trail  30  to nodes  14  located downstream from the transmitting node  14  in the direction traffic is propagating along light-trail  30 . A particular node  14  may terminate or reconfigure light-trail  30  at any appropriate time. Additionally, as noted above, in particular embodiments, multiple light-trails may be established in optical ring  20 , with each light-trail associated with a particular wavelength. Furthermore, multiple, non-overlapping light-trails may be associated with a common wavelength. The operation of a particular embodiment of optical network  10  in establishing a light-trail is illustrated in  FIGS. 3A-3C . 
     As mentioned above, to coordinate the establishment and sharing of light-trails, optical network  10  supports an optical supervisory channel (OSC) or other out-of-band control channel on which control signals are exchanged between nodes  14  and/or other components of optical network  10 . Nodes  14  may exchange control messages on the OSC to initiate and terminate light-trails and to manage use of established light-trails. In a particular embodiment, the OSC represents one or more wavelengths, among a plurality of wavelengths utilized by optical network  10 , that are dedicated to control signals. Alternatively, the OSC may represent a separate fiber in optical ring  20  on which nodes  14  may exchange control signals. According to particular embodiments, control signals associated with a particular light-trail may be transmitted on the OSC in the direction of traffic on that light-trail, in a direction opposite to the direction of traffic on that light-trail, or in both directions on the OSC. 
     Use of light-trails may result in more efficient transmission of traffic between nodes  14 . In particular embodiments, nodes  14  may be configured to use light-trails to transmit all traffic and may establish additional light-trails if the amount of traffic flowing on a particular light-trail exceeds a particular threshold, or if a particular node  14  is unable to transmit traffic (due to use of the light-trail by other nodes  14 ) that cannot be delayed. In general, however, nodes  14  may be configured to establish light-trails based on any appropriate criteria, factors, or considerations. 
     In particular embodiments, as described below in conjunction with  FIG. 4 , nodes  14  may use an auction technique to bid on the use of a light-trail (for example, based on the criticality of the node&#39;s need to transmit particular data). In other embodiments of optical network  10 , nodes  14  may share use of a light-trail through a “round robin” or “weighted round robin” system. In yet other embodiments, a particular node  14  is granted use of an existing light-trail to transmit optical traffic to other nodes  14  based on a priority associated with that node  14 . Thus, when more than one node  14  is attempting to transmit optical traffic on the same light-trail at the same time, an element of optical network  10  may determine which node  14  will be granted use of that light-trail based on a comparison of the priorities of the competing nodes  14 . These techniques, or other suitable techniques for sharing a light-trail, may result in more efficient communication of information as transmission by certain nodes  14  or the transmission of certain information may be given priority over other transmissions, allowing, for example, particular nodes  14  to satisfy minimum quality of service (QoS) requirements for their transmissions. 
       FIG. 2  is a block diagram illustrating a particular embodiment of a node  14  for use in implementing light-trails. As shown, node  14  includes transport elements  50   a  and  50   b , distributing/combining elements  80   a  and  80   b , a managing element  120 , a drop element  130 , an add element  140 , a burstponder  150 , and a switching element  160 . Transport elements  50  add traffic to and drop traffic from fibers  16  and  18 . More specifically, transport elements  50  may generate one or more copies of optical signals propagating on fibers  16  and  18  for communication of particular portions of the traffic carried in these optical signals to devices coupled to node  14 . Additionally, transport elements  50  may include components appropriate to add traffic generated by node  14  or received from client devices of node  14  to fibers  16  and  18 . For example, in the illustrated embodiment, each transport element  50  includes a coupler  60   a  which splits traffic received by transport elements  50  into two copies and forwards one copy of the traffic to drop element  130 , while forwarding the other copy along the relevant fiber. Furthermore, each transport element  50  includes a coupler  60   b  which adds traffic received from add element  140  to traffic already propagating on the associated fiber. Although two couplers  60   a  and  60   b  are illustrated in each transport element  50 , particular embodiments may include a single coupler that both adds and drops traffic. Such a single coupler may be used, as an example, in particular embodiments which do not include a wavelength blocking unit  54  (as is described below). 
     Each transport element  50  also includes, in the illustrated embodiment, a wavelength blocking unit (WBU)  54  configured to terminate particular wavelengths of traffic propagating on fibers  16  and  18 . As a result, traffic that has already been received by its intended destination or destinations may be terminated at a subsequent node  14 . Furthermore, WBU  54  may be used to isolate a light-trail, as described below. Although shown as a functional block in  FIG. 2 , WBU  54  may represent and/or include suitable components configured in any appropriate manner to provide the functionality of dynamically blocking certain wavelengths and passing other wavelengths. As one example, WBU  54  may represent a wavelength-selective switch (WSS) operable to output any particular wavelength, or set of wavelengths, received at the input of WBU  54  on the output of WBU  54 . 
     As another example, WBU  54  may represent a structure that includes an optical demultiplexer and an optical multiplexer connected by a series of switches. In such an embodiment, the demultiplexer may demultiplex the signal into its constituent channels. The switches may then be dynamically configured to selectively terminate or forward each channel to the multiplexer based on control signals received by each switch The channels that are forwarded by the switches are received by the multiplexer, multiplexed into a WDM optical signal, and forwarded to downstream elements. 
     As another example, WBU  54  may represent a collection of tunable filters tuned to allow only traffic on appropriate wavelengths to be forwarded on fibers  16  or  18 . In such an embodiment, a coupler of WBU  54  may receive optical signals input to WBU  54  and split the optical signals into a plurality of copies, transmitting each of these copies to a particular tunable filter. Each tunable filter may then selectively pass traffic propagating at a particular wavelength or within a particular range of wavelengths and block traffic propagating at all other wavelengths. Each tunable filter then forwards the passed traffic propagating at the associated wavelength or wavelengths to an output coupler of WBU  54 . The output coupler then combines the output of the various tunable filters to produce an output WDM optical signal and forwards the output optical signal to components downstream from WBU  54 . 
     Transport elements  50  may also include appropriate components to allow node  14  to transmit and receive information pertaining to the status and operation of fibers  16  and  18 , other nodes, any light-trails established in network  10 , or any other appropriate elements or functionality of optical network  10 . In particular, each node  14  may include elements to allow node  14  to receive and transmit messages on an optical supervisory channel (OSC). In the illustrated embodiment, each transport element  50  includes an OSC ingress filter  66   a  that processes an ingress optical signal from its respective fiber  16  or  18 . Each OSC filter  66   a  filters the OSC signal from the optical signal and forwards the OSC signal to a respective OSC receiver  112 . Each OSC filter  66   a  also forwards the remaining optical signal to other components of transport element  50 . Each transport element  50  also includes an OSC egress filter  66   b  that adds an OSC signal from an associated OSC transmitter  116  to the optical signal propagating on the associated fiber  16  or  18  and forwards the combined signal to elements located downstream on fiber  16  or  18 . The added OSC signal may be locally-generated data or may be OSC data received by node  14  and passed through managing element  120 . 
     Distributing/combining elements  80  may each comprise a drop signal splitter  82  and an add signal combiner  84 . Splitters  82  may each comprise a coupler connected to one optical fiber ingress lead and a plurality of optical fiber egress leads which serve as drop leads  86 . Each drop lead  86  may be connected to a drop element  130  associated with a particular local port of node  14 . Although the illustrated embodiment shows a splitter  82  coupled to one drop lead  86 , splitter  82  may be coupled to any appropriate number of drop leads  86 . 
     Splitter  82  may, in general, represent any appropriate component or collection of components capable of splitting the optical signal received by splitter  82  into a plurality of copies each to be propagated on a particular drop lead  86 . In particular embodiments in which four drop leads  86  are implemented, splitters  82  may each specifically comprise a 2×4 optical coupler, where one ingress lead is terminated, the other ingress lead is coupled to a coupler  60  via a fiber segment, and the four egress leads are used as drop leads  86 . 
     Combiners  84  similarly may each comprise a coupler with multiple optical fiber ingress leads, which serve as add leads  88 , and one optical fiber egress lead. Each add lead  88  may be connected to an add element  140  associated with a particular port of node  14 . In particular embodiments in which combiner  84  is coupled to four ingress leads, combiner  84  may comprise a 2×4 optical coupler, where one egress lead is terminated, the other egress lead is coupled to a coupler via a fiber segment, and the four ingress leads comprise add leads  88 . As with splitter  82 , the described components of combiner  84  may be replaced by any suitable component or collection of components for combining a plurality of optical signal into a single output signal. Although the illustrated embodiment shows a combiner  84  coupled to one add lead  88 , combiner  84  may be coupled to any appropriate number of add leads  88 . 
     Drop elements  130  selectively couple ports of burstponder  150  to outputs of distributing/combining elements  80  through filters  100 , which are each capable of isolating traffic in a different wavelength from each copy of the optical signal created by splitter  82 . As a result, drop elements  130  may output particular wavelengths of traffic from fibers  16  and  18  to particular ports of burstponder  150 . Add elements  140  also couple particular ports of burstponder  150  to combining/distributing elements  80 . Drop element  130  and add element  140  may include, respectively, a drop switch  132  and an add switch  142 , or other suitable components, to selectively connect associated ports of burstponder  150  to fiber  16  or  18 . Alternatively, add switch  142  may be replaced by a coupler which can split a signal from the associated transmitter  104  and by a pair of shutters (one for each branch of the split signal) that can control whether the signal is added to fiber  16 , fiber  18 , or both fibers  16  and  18 . As a result, drop element  130  and add element  140  may be utilized to support protection switching for node  14 . Alternatively, particular embodiments of drop element  130  and add element  140  may omit drop switch  132  and add switch  142 , respectively, and couple different ports of burstponder  150  to each fiber  16  and  18 . Moreover, in particular embodiments, node  14  may include multiple drop elements  130  and/or add elements  140 , each associated with a particular wavelength supported by optical network  10 . 
     Burstponder  150  converts bursty or time-interleaved optical traffic received from drop elements  130  to seamless and continuous data traffic for delivery to client devices of node  14  and converts data traffic received from client devices to optical traffic for transmission on fiber  16  or  18  in bursts when the node  14  has use of the light-trail. As described above, burstponder  150  allows node  14  to time share a light-trail while creating an impression to client devices of the node  14  that the wavelength is available on a seamless and continuous basis. Burstponder  150  may include any appropriate number of receivers  102  operable to receive optical signals and generate electrical signals based on these optical signals and transmitters  104  operable to receive electrical signals and to transmit optical signals based on these electrical signals. Depending on the configuration of node  14 , each of these receivers  102  and transmitters  104  may be fixed or tunable. Each of these receivers  102  and transmitters  104  may be a burst-mode receiver or transmitter. Such burst-mode receivers may have burst mode clock and data recovery operation. As described below, switching element  160  may represent any appropriate component or components for transmitting data traffic output by burstponder  150  to appropriate client devices of node  14  and for transmitting data traffic received from client devices of node  14  to burstponder  150 . Although shown as part of node  14  in  FIG. 2 , switching element  160  may be physically separate from node  14 . 
     Managing element  120  may comprise OSC receivers  112 , OSC interfaces  114 , OSC transmitters  116 , and an element management system (EMS)  124 . Each OSC receiver  112 , OSC interface  114 , and OSC transmitter  116  set forms an OSC unit for one of the fibers  16  or  18  in the node  14 . The OSC units receive and transmit OSC signals for the EMS  124 . EMS  124  may be communicably coupled to a network management system (NMS)  126 . NMS  126  may reside within node  14 , in a different node, or external to all nodes  14 . 
     EMS  124  and/or NMS  126  may comprise logic encoded in media for performing network and/or node monitoring, failure detection, protection switching and loop back or localized testing functionality of the optical network  10 . In a particular embodiment, EMS  124  and/or NMS  126  generate, transmit, receive, and/or process control messages associated with the establishment, operation, and termination of light-trails. Any logic included in EMS  124  or NMS  126  may comprise software encoded in a disk or other computer-readable medium, such as memory, and/or instructions encoded in an application-specific integrated circuit (ASIC), field programmable gate array (FPGA), or other processor or hardware. It will be understood that functionality of EMS  124  and/or NMS  126  may be performed by other components of the network and/or be otherwise distributed or centralized. For example, operation of NMS  126  may be distributed to the EMS  124  of nodes  14 , and the NMS  126  may thus be omitted as a separate, discrete element. Similarly, the OSC units may communicate directly with NMS  126  and EMS  124  omitted. 
     EMS  124  monitors and/or controls elements within node  14 . For example, EMS  124  may control operation of transmitters  104 , receivers  102 , and WBU  54  to facilitate the establishment and use of light-trails. In the illustrated embodiment, EMS  124  receives an OSC signal from each of fiber  16  and  18  in an electrical format via an OSC receiver  112  associated with that fiber (the OSC receiver  112  obtains the signal via an OSC filter  66   a ). This OSC signal may include one or more of multiple types of control messages, as described above. EMS  124  may process the signal, forward the signal and/or loop-back the signal. EMS  124  may be operable to receive the electrical signal and resend the OSC signal via OSC transmitter  116  and OSC filter  66   b  to the next node on fiber  16  or  18 , adding, if appropriate, locally-generated control messages or other suitable information to the OSC. 
     NMS  126  collects information from all nodes  14  in optical network  10  and is operable to process control messages transmitted by nodes  14  to manage particular aspects of the use of light-trails. For example, in a particular embodiment, NMS  126  may be operable to select a particular node  14  for transmission on a light-trail when multiple nodes  14  request use of the light-trail. As noted above, NMS  126  may represent a portion or all of EMSs  124  of all nodes  14  in optical network  10 . Moreover, although the description below describes particular embodiments of optical network  10  in which functionality is divided between NMS  126  and EMSs  124  in a particular manner, in alternative embodiments the described functionality may be distributed between NMS  126  and EMSs  124  in any appropriate manner. Additionally, although NMS  126  and EMS  124 , as shown in  FIG. 2 , represent, at least in part, components located within node  14 , some or all of NMS  126  and/or EMS  124  may be located external to nodes  14 . 
     Although not shown in  FIG. 2 , node  14  may also include a memory operable to store code associated with EMS  124 , NMS  126 , and/or other components of optical network  10 , information specifying a wavelength assignment scheme utilized for protection traffic on optical network  10 , and/or any other suitable information used during operation of optical network  10 . Memory may represent one or more memory devices that are located within node  14  or that are physically separate from node  14 . Additionally, memory may be shared with other components of optical network  10  including other nodes  14 . Memory may represent computer disks, a hard disk memory, random access memory (RAM), read-only memory (ROM), or any other suitable storage media. 
     In operation, transport elements  50  receive traffic from fibers  16  and  18 . In the illustrated embodiment, traffic received from fibers  16  and  18  includes an OSC signal, and transport elements  50  are operable to add and drop the OSC signal to and from fibers  16  and  18 . More specifically, each OSC ingress filter  66   a  processes an ingress optical signal from its respective fiber  16  or  18 . OSC ingress filter  66   a  filters the OSC signal from the optical signal and forwards the OSC signal to its respective OSC receiver  112 . Each OSC ingress filter  66   a  also forwards the remaining transport optical signal to the associated amplifier  64 . Amplifier  64  amplifies the signal and forwards the signal to its associated coupler  60   a . In particular embodiments, amplifier  64  may be omitted, depending on the circumstances. 
     EMS  124  may process control messages transmitted by other nodes  14  or other components of optical network  10  and adjust operation of node  14  in response. In particular, EMS  124  may reconfigure WBU  54 , transmitters  104 , filters  100 , receivers  102 , and/or any other appropriate element of node  14  in response to control messages received by EMS  124 . As one example, EMS  124  may, in response to receiving a setup message, configure a WBU  54  of node  14  to allow traffic propagating at a particular wavelength to pass through WBU  54 . As another example, EMS  124  may, in response to receiving an intimation message from another node  14 , tune a particular filter  100  and/or a particular receiver  102  to allow node  14  to receive optical traffic on a particular wavelength associated with a light-trail. 
     Furthermore, EMS  124  may also generate control messages for transmission to other nodes  14  or other components of optical network  10 . For example, EMS  124  may generate electronic signals associated with setup messages, intimation messages, request messages, and/or any other appropriate type of control messages and communicate these electronic signals to OSC transmitter  116  to transmit optical signals representing the appropriate control message to the associated transport element  50 . These control messages may then be added to the optical traffic on fiber  16  or  18 , as appropriate. 
     Meanwhile, coupler  60   a  splits the signal from the amplifier  64  into two copies: a through signal that is forwarded to WBU  54  and a drop signal that is forwarded to distributing/combining element  80 . Distributing/combining element  80  may then split the drop signal into one or more copies and forward the copies of the drop signal to one or more drop elements  130 . In a particular embodiment, each drop element  130  includes a drop switch  132  that allows drop element  130  to selectively couple a drop signal from either fiber  16  or fiber  18  to a filter  100  included in that drop element  130 . Additionally, filter  100  may be tuned to a particular wavelength. As a result, in such an embodiment, traffic propagating at a particular wavelength on the selected fiber is output to burstponder  150 . 
     Burstponder  150  receives the output of a plurality of drop elements  130 . A receiver  102  in burstponder  150  that is associated with each drop element  130  converts the optical signal received from that drop element  130  into data traffic. The data traffic generated by each receiver  102  is then output to switching element  160 . In particular embodiments of node  14 , burstponder  150  may include buffers (not shown) and the output of receivers  102  may be stored in one or more buffers to be transmitted to switching element  160  at an appropriate time. 
     Switching element  160  receives seamless and continuous data traffic output by burstponder  150  and switches this data traffic in any appropriate manner to facilitate transmission of this data traffic to an appropriate client device of node  14 . The data traffic received by switching element  160  from burstponder  150  may include information in the form of packets, frames, and/or datagrams, and/or information structured in any other appropriate form. For example, in a particular embodiment, switching element  160  may represent an L2 switch and may receive electrical signals from burstponder  150  in the form of packets. 
     Switching element  160  also receives data traffic from client devices coupled to switching element  160  and switches this data traffic to communicate the data traffic to an appropriate port of burstponder  150 . The data traffic received by switching element  160  from the client devices may include information in the form of packets, frames, and/or datagrams, and/or information structured in any other appropriate form. As noted above, switching element  160  may represent an L2 switch and may receive data traffic from the client devices in the form of packets. In such an embodiment, the L2 switch may switch each packet, based on a header included in that packet, to deliver the packet to a port of the L2 switch coupled to an appropriate port of burstponder  150 . 
     Burstponder  150  receives data traffic from switching element  160  on one or more ports of burstponder  150 . Certain ports of burstponder  150  are configured to receive data traffic from switching element  160 , and each of these ports may pass the received data traffic to a particular transmitter  104  in burstponder  150  associated with that port. Each transmitter  104  may then generate a burst of optical traffic from the data traffic received from switching element  160  and transmit that optical traffic to a particular add element  140  associated with that transmitter  104 . In particular embodiments, EMS  124  may tune transmitters  104  of burstponder  150 , and transmitters  104  may generate optical traffic at a particular wavelength determined by EMS  124 . In other embodiments, transmitters  104  transmit at a fixed wavelength. Additionally, burstponder  150  may include one or more buffers that store data traffic from switching element  160  to be input to transmitter  104  at an appropriate time (such as when the node is granted use of a light-trail). Such buffering is useful since a node  14  may not be able to transmit traffic when it is received because another node  14  is using a shared light-trail. 
     Optical traffic output by transmitters  104  of burstponder  150  is then received by an appropriate add element  140  associated with the transmitter  104  that generated the optical traffic. Each add element  140  may include an add switch  142  capable of selectively coupling that add element to a combiner  84  in a distributing/combining element  80  associated with either fiber  16  or  18 . As a result, optical traffic generated by transmitters  104  of burstponder  150  may be added to an appropriate fiber  16  or  18  based on the circumstances. For example, particular embodiments of node  14  may support protection switching and add switch  142  may be reconfigured in response to the detection of a fault on one fiber to transmit optical traffic on the other fiber. The appropriate distributing/combining element  80  then forwards the optical traffic received from burstponder  150  to the coupler  60   b  of the associated fiber. 
     Returning to the operation of couplers  60   a , in addition to forwarding the drop signal as described above, each coupler  60   a  forwards the through signal to its respective WBU  54 . WBUs  54  receive the optical signal and selectively terminate or forward channels of the through signal. In a particular embodiment of node  14 , EMS  124  may control operation of WBU  54  to establish a light-trail on a specified wavelength on a particular fiber  16  or  18  in response to a setup message received from a convener node  14   a . In particular, if node  14  represents a node on the interior of the requested light-trail, EMS  124  may configure WBU  54  to allow optical signals propagating at the specified wavelength on the relevant fiber to pass through WBU  54 . If node  14  represents a node  14  at the beginning or end of a light-trail, EMS  124  may configure WBU  54  to block optical signals propagating at the specified wavelength on the relevant fiber. In this way, traffic transmitted by a node in a light-trail does not leave the light-trail. Because of this, multiple non-overlapping light-trails may be formed using the same wavelength in the same fiber. 
     In particular embodiments, however, WBUs  54  may be omitted from the node. In such embodiments, the node will be unable to block the transmission of traffic through the node (since there would be nothing to terminate any of the wavelengths of the copy of the optical signal forwarded from couplers  60   a ). Therefore, in such embodiments, multiple light-trails may not be formed in the same wavelength. However, in many network topologies, such as ring networks, at least one such node (or some other device in the network) must be able to stop the propagation of optical signals added from the nodes around the network to prevent interference. As an example, otherwise traffic being added in a particular wavelength at a node will propagate around the network and return to the adding node, where it will interfere with new traffic being added in that wavelength. Therefore, particular embodiments may include one or more nodes that include a WBU (such as nodes  14 ) and one or more other nodes that do not include a WBU. If multiple nodes that include a WBU are used in such embodiments, it may be possible to create multiple light-trails in a single wavelength; however, the locations of these light-trails would be limited according to the number and placement of the nodes including the WBUs. 
     Returning to the operation of the illustrated node  14 , each coupler  60   b  may subsequently combine the output of the associated WBU  54  with the traffic received from an associated combiner  84 . After coupler  60   b  adds locally-derived traffic to the output of WBU  54 , coupler  60   b  forwards the combined signal to the associated amplifier  64  and OSC egress filter  66   b . Each OSC egress filter  66   b  adds an OSC signal from the associated OSC transmitter  116  to the combined optical signal and forwards the new combined signal as an egress transport signal to the associated fiber  16  or  18  of optical network  10 . 
       FIGS. 3A-3C  illustrate example operation of nodes of an optical network in establishing a light-trail  330  (shown in  FIG. 3C ). In particular,  FIGS. 3A-3C  illustrate an example operation of a particular embodiment of an optical network as a particular node  314  attempts to establish a light-trail  330  in response to receiving data traffic from a client device of that node  314 . Nodes  314  and fibers  316  and  318  shown in  FIGS. 3A-3C  may represent a complete optical network or may represent a portion of a larger optical network, such as optical network  10  shown in  FIG. 1 . Furthermore, although shown as being coupled in a linear manner, nodes  314  may be coupled in a ring, a mesh, or in any other suitable fashion. For example, nodes  314   a - f  may represent nodes  14   a - f  of network  10  of  FIG. 1 . Moreover, nodes  314  may have any suitable design. As an example only, nodes  314  may be implemented using the configuration illustrated in  FIG. 2  or any other appropriate configuration. 
       FIG. 3A  illustrates an example operation of an optical network as node  314   a  (referred to below as “convener node  314   a ”) receives data traffic  310  from a client device coupled to convener node  314   a . To transmit optical traffic based on the data traffic, convener node  314   a  determines that a light-trail  330  should be established between convener node  314   a  and node  314   e  (referred to below as “end node  314   e ”) along fiber  16 . As indicated above, convener node  314   a  may decide to establish light-trail  330  in response to determining that the amount of optical traffic flowing on other light-trails that couple convener node  314   a  and end node  314   e  exceeds a predetermined threshold. Alternatively, any other node or device may initiate the establishment of light-trail  330  for any suitable purpose. 
     Convener node  314   a  may establish light-trail  330  by sending one or more control messages to end node  314   e  and/or other nodes  314  on the OSC or other control channel. As used herein, a “message” may represent one or more signal pulses, packets, or frames, or information structured in any other suitable format. For example, in a particular embodiment, convener node  314   a  transmits a setup message  340  to end node  314   e  and to all nodes  314   b - d  between this particular convener node  314   a  and end node  314   e  in the direction of traffic. These nodes between the convener node and end node that are to be included in the light-trail may be referred to as “intervening nodes” (it should be noted, however, that not every node between the convener node and end node need be included in a light-trail). Depending on the configuration of the optical network, convener node  314   a  may transmit setup message  340  on the OSC in the same direction as optical traffic is flowing on fiber  316 , in the opposite direction (for example, the OSC on fiber  318 ), or in both directions (for example, the OSC on both fibers  16  and  18 ). In the illustrated example, the OSC is assumed to represent a separate wavelength from the wavelengths used to transmit data on fiber  316 , and convener node  314   a  transmits setup message  340  on fiber  316  in the direction traffic is propagating on fiber  316 . 
     Setup message  340  may identify convener node  314   a  and end node  314   e , specify the direction and wavelength to be used for transmissions on light-trail  330 , and/or include any other appropriate information to be used by intervening nodes  314   b - d  and end node  314   e  to establish light-trail  330 . Intervening nodes  314   b - d  may store setup message  340  until receiving an appropriate indication from end node  314   e , such as an acknowledgement message, that end node  314   e  is prepared to establish light-trail  330 . 
       FIG. 3B  illustrates an example operation of the optical network after end node  314   e  receives setup message  340 . End node  314   e , in response to receiving setup message  340 , may reconfigure a wavelength blocking unit of end node  314   e  to prevent traffic propagating at the wavelength associated with the requested light-trail  330  from continuing past end node  314   e  on fiber  316 . End node  314   e  transmits an acknowledgement message  350  to convener node  314   a  and/or intervening nodes  314   b - d  once end node  314   e  has configured the wavelength blocking unit or at any other appropriate time after receiving setup message  340 . Acknowledgement message  350  indicates to nodes  314  receiving the acknowledgment message that end node  314   e  is ready to establish light-trail  330 . Convener node  314   a  and/or intervening nodes  314   b - d  may configure themselves in any appropriate manner to facilitate establishment of the light-trail, in response to receiving the acknowledgement message  350  or another appropriate form of indication from end node  314   e . For example, intervening nodes  314   b - d  may each reconfigure a wavelength blocking unit of each node  314  to allow the wavelength associated with light-trail  330  to pass through that particular node  314 . Additionally, convener node  314   a  may configure a wavelength blocking unit of convener node  314   a  to block traffic propagating on fiber  316  at the wavelength, as described above with respect to  FIG. 2 . By blocking traffic propagating on fiber  316  at the wavelength associated with light-trail  330 , convener node  314   a  may allow other light-trails that do not overlap with light-trail  330  to utilize the same wavelength as light-trail  330  without interfering with traffic transmitted on light-trail  330 . 
     Additionally, each node  314  may maintain a light-trail table or matrix that maintains information regarding light-trails established on optical network  10  or light-trails to which that node  314  is coupled. These light-trail tables may include any appropriate information for the relevant light-trails. For example, light-trail tables may include information specifying the convener node and end node of each light-trail, the wavelength associated with each light-trail, whether each light-trail is currently being used, and/or any other suitable information about each light-trail. 
       FIG. 3C  illustrates a state of optical network  10  after node  314   a  receives acknowledgement message  350  and performs any appropriate reconfiguration. As a result of the reconfiguration of convener node  314   a , intervening nodes  314   b - d  and end node  314   e , light-trail  330  is formed which couples convener node  314   a  to each intervening node  314   b - d  and to end node  314   e . Once light-trail  330  is established, convener node  314   a  and/or intervening nodes  314   b - d  may utilize light-trail  330  for transmissions to downstream intervening nodes  314   b - d  or to end node  314   e . Example operation of nodes in transmitting optical traffic on an established light-trail is described below with respect to  FIG. 4 . 
     As mentioned above in conjunction with  FIG. 1 , one technique that may be used to allocate use of a light-trail between the nodes included in that light-trail is an auction technique. Auction algorithms provide a real-time method for arbitrating between users and objects. In this case, the nodes are users and light-trails (more particularly, data slots in a light-trail) are the objects. Such an auction mechanism provides an efficient, fair, and stable technique to converge to a solution to the parallel problem of arbitrating between a number of nodes requesting the use of an established light-trail. In this technique, each node requiring use of a light-trail places a bid for use of the light-trail during a specified period (such as for a particular time slot). Such a bid will be based on the node&#39;s network parameters and its requirement for the light-trail, as described below. 
     In particular embodiments, bids are placed contiguously with fixed periodicity using a slotted system. Two levels of time-slotted hierarchy exist in such embodiments: at the control layer and at the data layer. At the control layer, the control channel (for example, the OSC) may synchronized between the nodes since the control channel is optically dropped and electronically processed at each node. The time slots may be of a small duration/length (for example, measured in microseconds) that is just large enough to carry bids from nodes and/or to carry any other signaling information. The time slots at the data layer are larger (for example, a few milliseconds) than the slots of the control channel. At the data layer, the slots of each data channel are synchronized with respect to the control channel. In other words, the data transmission timings are controlled by the control channel, and since the control channel itself is synchronized, the data layer is also synchronized. Since the slots of the control channel are smaller than the data slots, a number of control slots will occur in the control channel during the duration of a single data slot in each data channel. 
     The upper bound of the duration of each data slot may depend on the minimum latency requirement of the node traffic having the highest degree of stringency. As an example only, if d is the maximum permissible latency of the traffic with highest stringency, and p is the maximum propagation delay, then the maximum allowable control packet size may be (d−p)/2 WN seconds in duration. Based on this calculation, the control channel line rate (C control ) may be defined as:
 
 C   control =2 WNb   control /( d−p )
 
where, b control  is the number of bits in a control packet.
 
     Furthermore, the lower bound on data slot duration may be restricted in particular embodiments by the number of control messages that must be transmitted in the control channel (in other words, the number of required control slots) during the duration of a data slot, as well as the line rates for the control and data channels. This lower bound ensures that the control channel provides enough capacity for all potentially-able nodes to bid on light-paths established at every wavelength of the network. For example, if a network includes six nodes (such as network  10 ) and has twenty data channels, then one hundred twenty control slots will occur in the control channel during the time that one data slot occurs in each of the twenty data channels. Each of these control slots may be associated with a particular node and with a particular light-trail. 
     In embodiments implementing the auction technique, nodes may bid on the use of each individual data slot in each light-trail. Each node may bid on a particular data slot of a particular light-trail by inserting a control message into one of the control slots corresponding with the data slot that is just before the data slot being bid on. In other words, in such embodiments, the control slots that are synchronized with a particular data slot will include bids from one or more nodes for use of the next data slot. As is described in greater detail below, in particular embodiments these bids may be based on two parameters: criticality (generally, the service needed by the traffic from a node and the amount of traffic buffered at the node) and staticity (generally, the quality and quantity of the node&#39;s previous use of the light-trail). Both of these parameters help translate buffer and service statistics into auction bids. Each node may include software stored in a computer-readable medium or firmware that is used to calculate the bids for that node. As an example only, this software or firmware may be associated with the element management system. 
     These bids are then sent to one of the nodes in the light-trail (or some other suitable node or device) that acts as a controller for the light-trail. In particular embodiments, this controller is the end node of a light-trail. Upon receiving bids from the nodes in a light-trail, the end node of each light-trail computes the highest bid that it received from nodes in the light-trail and decides (based on an algorithm described below) which node should have transmission rights to the next data slot in the light-trail. The end node send control messages in the control channel to the other nodes in the light-trail indicating the node that was assigned the next data slot. These control messages may be sent on a different control channel that is transmitted in the opposite direction of the control channel in which the bids were transmitted (such as the OSC on the opposite fiber), they may be sent in the same control channel, or some may be sent in one direction and some in the other (for example, if the controller is one of the intervening nodes). This process, which is described in further detail below, repeats for each data slot. 
       FIG. 4  is a flowchart illustrating an example method for the auctioning of data slots in a light-trail. The example method describes the process for bidding on and using a single data slot in a particular light-trail. This method may be repeated for each data slot in the light-trail, as well as be performed for data slots of other light-trails in a network. Furthermore, although this example method describes bidding for each data slot in a light-trail, any other suitable “objects” may be bid on (as an example, use of the light-trail for a longer period of time (multiple data slots)). Moreover, although the bids are calculated based on criticality and staticity parameters in the manner described below, other embodiments of the present invention may calculate these parameters differently than described below and/or may use alternative or additional parameters to calculate bids. 
     For the calculations of bids in the example auction method, the following variables are defined:
         N: The number of nodes in the network, where N i  is a particular node   B(t): an N×N matrix that denotes the occupancy levels of the buffers of all nodes   B ik  (t): the occupancy level of a buffer at a particular node i at time t with data intended for transmission in light-trail k   Service types of the traffic buffered at a node: S={1,2,3,4, . . . h, . . . , s}, where S h  is the h th  service type, and Δ 1 &gt;Δ 2 &gt;Δ 3 &gt; . . . Δ s , where Δ s     h    is the maximum allowable delay for service S h      C: the light-trail bit-rate   t c : The connection provisioning time in a light-trail   w: The time required to set up a light-trail   T data : The duration of data slot       

     Nodes implementing the example auction method use these variables to determine the criticality and staticity parameters, which are then used to calculate a bid. The example method, as illustrated in  FIG. 4 , begins at step  500  where each node contending for a particular data slot in a particular light-trail calculates it criticality. Criticality is a representation of the urgency of a node to provision a connection in a particular light-trail. In the example embodiment, criticality depends on two factors: service delay and buffer occupancy. The former represents the need to provision connections in a light-trail assuming the end-to-end service delay, while the latter represents the occupancy of a buffer with respect to the total available memory in the buffer. For computing criticality, the following variables are used:
         σ ik (t): The number of data slots that have occurred since the last successful bid by node i for light-trail k.   Ψ ik  (t): The critical allowable limit of buffer at node i for data intended for transmission in the k th  light-trail at time t.       

     The critical allowable limit Ψ ik  (t) depends on the time at which the first packet (or other suitable form of traffic) with service type S h  arrived in the buffer as compared to any packet of other service types having less stringent latency requirements than service type h. Ψ ik  (t) is calculated as follows:
 
Ψ ik ( t )=min[Δ s     i     −x   i ,Δ S     k   ], i=1,2, . . . , s
 
where x i  is the time since the first packet of service i entered the buffer B ik . Note that the critical allowable limit is independent of flow statistics and is a time-dependent value (meaning it can change for each data slot).
 
     Using the critical allowable limit, the criticality parameter for a buffer at node i at time t for light-trail k can be calculated. The criticality parameter, α ik (t), is defined as the maximum of the two ratios of service criticality and buffer criticality. Thus, the criticality parameter is calculated as follows: 
                 α   ik     ⁡     (   t   )       =     max   ⁡     [           σ   ik     ⁡     (   t   )           Ψ   ik     ⁡     (   t   )         ,         B   ik     ⁡     (   t   )         B   max         ]             
where α ik (t) is the criticality parameter of node i with reference to light-trail k at time t, and B max  is the buffer size in bits. For normal operations, 0≦α ik (t)≦1. Thus, in general, the criticality parameter is determined to be higher of either a ratio reflecting the criticality caused by the service requirements for the traffic having the most stringent latency requirements or a ratio reflecting the criticality caused by the node&#39;s buffer reaching its capacity. Although this calculation of criticality may indicate the node that has the most critical need for a light-trail at any given instant, “ringing” will typically occur if criticality is the only parameter considered in placing a bid. “Ringing” is rapid switching between nodes having the right to transmit traffic on the light-trail, which creates inefficiencies.
 
     Therefore, at step  502  of the example method, each node also calculates a staticity parameter. As described above, staticity is a parameter that gives a measure of the quality and longevity of a connection (use of a light-trail by a node). For a given connection, staticity is defined according to the following ratio: 
                 ST   ik     ⁡     (   t   )       =       T   DC       1   -     [       1       T   DC     ⁢   C       ⁢       ∑     i   =     t   -     T   DC         t     ⁢           ⁢       B   ik     ⁡     (   t   )           ]               
For a given connection in the k th  light-trail between node i and some other node, if the connection duration is T DC  (which is self-explanatory) then staticity for this connection is calculated as follows (indicating how connection quality is determined):
 
                 ST   ik     ⁡     (   t   )       =       connection   ⁢           ⁢   duration       connection   ⁢           ⁢   quality             
For normal operations, 0≦ST ik  (t)≦1.
 
     Based on the criticality and staticity calculations, each node contending for a particular data slot of light-trail k calculates a bid at step  504 . The bid is calculated as the smaller of two quantities: (i) the maximum successful bid for the previous data slot, or (ii) the sum of the criticality and staticity parameters for the node at the beginning of the current data slot. In particular embodiments, an amount ε is also added to the smaller of these two quantities. This amount ε enables complementary slackness—a condition defined in Bertsekas, D. P., “Auction Algorithms,” Lab. for Information and Decision Systems Working Paper, M.I.T., Cambridge, Mass., pp. 6-8—to help achieve orthogonality between nodes. The value of ε is node-specific and is given a value of 1/(h-1) of the maximum bid for the previous data slot, where h is the average number of nodes in a light-trail (typically between 3N/8 and 5N/8 for an N-node ring network with symmetric traffic demands). Therefore, a node i computes a bid for light-trail k at time t as follows:
 
bid ik ( t )=min[α ik ( t )+ ST   ik ( t ),  mbid   k ( t −1)]+ε
 
     After calculating a bid at step  504 , each node bidding on a data slot determines whether to place the bid at step  506 . For a given bidding cycle, every node computes a bid for every light-trail that is of interest to the node (for example, light-trails that include the destination node for the traffic that a node desires to transmit). However, in particular embodiments, a node will not necessarily place all of the bids that it calculates. In determining whether to place a bid, each node i determines a quantity, α ik (t), that represents the benefit of light-trail k to that node at time t. This benefit depends on the previous history of the node with the light-trail, and is calculated as follows:
 
α ik ( t )=α ik ( t )− mbid   k ( t −1)
 
where α ik (t) is the criticality parameter of node i with respect to light-trail k and where mbid k (t−1) is the maximum bid for the previous data slot in light-trail k (this maximum bid for the previous data slot may communicated by the end nodes of the light-trail to the other nodes in the light-trail). Each node determines this benefit at step  506  and decides to bid on a light-trail k if the net benefit of the light-trail denoted by α ik (t) for that node is maximum amongst all light-trails available to that node. If a node decides not to bid on particular light-trails, the example method (for those light-trails) returns to the start of the method and the method begins again for subsequent data slots in the light-trails. If a node decides to bid on a light-trail, the method (for that light-trail) proceeds to step  508  where the node transmits the bid in an appropriate control slot in the control channel (for example, a control slot associated with the node and with the light-trail being bid on.
 
     At step  510 , the end node for each light-trail in the network (or other node designated as the arbiter node to receive bids and allocate the light-trail) receives bids from nodes in that light-trail for a particular data slot. The end node (or other arbiter node) may receive bids by retrieving and processing the contents of particular control slots that are associated with its light-trail. After receiving the bids for a particular data slot, at step  512  the end node determines the maximum bid that was received and from which node it was transmitted. However, the node placing the highest bid does not necessarily receive permission to use the light-trail. Instead, at step  514  the end node decides whether to re-assign the light-trail to the node placing the highest bid or to permit the node that was most recently assigned the light-trail to continue transmitting. In the example embodiment, the end node determines a “prevailing price,” p k (t), of a light-trail k at time t as follows:
 
 p   k ( t )= mbid   k ( t− 1)− CT   data  
 
The end node then uses this prevailing price for the light-trail to determine at step  514  whether to re-assign the light-trail. If the maximum bid received for a data slot is greater than or equal to the calculated prevailing price (mbid k (t)≧p k (t)), then the light-trail is re-assigned to the node that sent the maximum bid. If the maximum bid received for a data slot is less than the calculated prevailing price (mbid k (t)&lt;p k (t)), then the status quo is maintained and the light-trail is not reassigned from the node that was previously assigned the light-trail (even though that node did not submit the highest bid).
 
     In certain cases, although a node may not be assigned a light-trail, the conditions associated with that node (the criticality and/or the staticity) may require that a new light-trail be established so that the node may transmit its traffic. Therefore, the end node determines at step  516  whether new light-trails are needed. For example, if (mbid k (t)&lt;p k (t)) and α k (t)&gt;1−w/Δ s  for the node transmitting the maximum bid, then this signifies the need to set up a new light-trail for that node. Similarly, if a node bidding on a light-trail k is not assigned the light-trail and if Ψ ik (t)−w≦B ik (t)≧Ψ ik (t)−w−t c  for that node (node i), then a new light-trail needs to be set up for that node. 
     At step  518 , the end node transmits a control message to each of the nodes in the light-trail indicating which node has been assigned the light-trail (either the node placing the highest bid or the node that was previously assigned the light-trail). In particular embodiments in which the network allows traffic to be transmitted in opposite directions (such as a bi-directional ring), the control messages from the end node to the other nodes in the light-trail may be communicated in the OSC that is transmitted in the opposite direction of the OSC that the other nodes used to submit the bids. This ensures that the response to the bids from the end nodes reaches the other nodes in the most time-efficient manner. Furthermore, if a new light-trail needs to be established, the end node may initiate its establishment and may send a control signal to the node for which the new light-trail was established indicating that it has been assigned the new light-trail. Alternatively, the node needing the new light-trail may establish the light-trail. In either case, the node needing the new light-trail may be the convener node in the new light-trail. 
     At step  520 , the nodes in the light-trail receive the control messages and the node that has been assigned the light-trail transmits traffic in the data slot that was being bid on. Furthermore, if a new light-trail was established, the node assigned to that light-trail also transmits traffic on that new light-trail. The example method then returns to its beginning where the process is repeated for the next data slot in each light-trail in the network. It should be understood that some of the steps illustrated in this flowchart may be combined, modified or deleted where appropriate, and additional steps may also be added to the flowchart. Additionally, the steps may be performed in any suitable order. 
     Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.