Abstract:
A first node ( 205 ) in a network includes a non-optical transceiver ( 415 ) and an optical subsystem ( 410 ). The non-optical transceiver ( 415 ) sends a request message to establish an optical link from the first node ( 205 ) to a second node via electrical signals over an electrically transmissive medium. The optical subsystem ( 410 ) establishes an optical link between the first node and the second node based on the receipt of the request granted message and transmits data between the first node and the second node via optical signals over the optical link.

Description:
RELATED APPLICATIONS 
     The present application is related to commonly assigned U.S. patent application Ser. No. 10/716,270, entitled “Optical Ad-Hoc Networks,” and filed on Nov. 17, 2003; and commonly assigned U.S. patent application Ser. No. 10/715,738, entitled “Systems and Methods for Implementing Coordinated Optical Channel Access,” and filed on Nov. 17, 2003, the disclosures of which are hereby incorporated herein by reference in their respective entireties. 
     FIELD OF THE INVENTION 
     The present invention relates generally to ad-hoc networks and, more particularly, to systems and methods for implementing contention-based optical channel access in ad-hoc networks. 
     BACKGROUND OF THE INVENTION 
     Wireless data communication is often required in an environment where communications infrastructure, such as base stations or a wired backbone network, does not exist, or is not economical or is impractical to use. For example, in military or emergency environments, adequate infrastructure often does not exist in necessary locations and constructing such an infrastructure would be either impractical or not economical for the short-term use that is often required. Mobile multi-hop radio frequency (RF) wireless networks have, therefore, been developed to provide wireless data communications in such environments. 
     In a conventional mobile RF wireless multi-hop network, each wireless node acts as a packet router that relays packets to other nodes in the network over an air interface link without routing the packets through any portion of a conventional cellular network, such as the wired backbone network, base station controllers, or base stations. Each wireless node, however, is limited in the distance over which it can reliably transmit, with transmission ranges of between a few feet and hundreds of feet being typical. Therefore, in communication environments that span large areas or have significant radio interference, packets transmitted from a sending node must often be hopped over multiple nodes in the wireless network to reach a destination. For such a multi-hop RF wireless network to perform effectively, all nodes must, therefore, be prepared to route packets on behalf of other nodes. 
     One drawback back with conventional multi-hop RF wireless networks is that the RF channel that is used to transmit data, such as packet data, can be relatively slow (e.g., kilobits or megabits per second of data throughput). Therefore, it would be desirable to employ other transmission mediums for transmitting data that can transmit data at much higher throughputs, such as, for example, multiple gigabits per second of data throughput. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention implement mechanisms for employing ad-hoc optical links and channels for transmitting data in an ad-hoc network. Optical links, consistent with the invention, permit very high throughputs, such as multiple gigabits per second, in an ad-hoc environment, where nodes may be moving around and links may be created and terminated with a high degree of frequency. The ad-hoc optical links of the present invention may be created automatically between two nodes in response to the traffic that the two nodes must convey. In an ad-hoc network consistent with the invention, any given node may attempt to establish an optical link to any other node in range whenever it wishes. 
     Systems and methods consistent with the present invention may use a hybrid RF/optical channel access scheme, where nodes use RF messaging to request access to the optical channels and, in response to the RF messaging, the ad-hoc nodes may establish optical links for high-speed communication via optical channels. In some exemplary embodiments, establishment of the optical links may include steering of one or more optical apertures, such as, for example, an optical telescope, to point towards the node to which an optical link is going to be established. Steering of the optical aperture may, thus, permit optical link establishment with mobile optical nodes that may frequently change position. 
     In accordance with the purpose of the invention as embodied and broadly described herein, a method of implementing optical channel access in a network that includes multiple distributed nodes includes requesting the optical channel access via radio-frequency (RF) messaging from one or more of the multiple distributed nodes. The method further includes granting the optical channel access to at least one of the multiple distributed nodes based on the RF messaging. 
     In another implementation consistent with the present invention, a method of establishing an optical link between a first node and a second node in a network, wherein at least one of the first and second nodes comprises a mobile node, is provided. The method includes sending a request message to establish the optical link from the first node to the second node via electrical signals over an electrically transmissive medium and receiving a request denied or a request granted message from the second node via electrical signals over the electrically transmissive medium. The method further includes establishing the optical link between the first node and the second node based on the receipt of the request granted message and transmitting data between the first node and the second node via optical signals over the optical link. 
     In a further implementation consistent with the present invention, a method of communicating between first and second nodes in a network is provided. The method includes establishing an optical channel between the first and second nodes by transmitting electrical signals over a non-optical channel. The method further includes communicating via the established optical channel between the first and second nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with the description, explain the invention. In the drawings, 
         FIG. 1  illustrates an exemplary network in which systems and methods, consistent with the present invention, may be implemented; 
         FIG. 2  illustrates optical nodes of the exemplary optical ad-hoc network of  FIG. 1  consistent with the present invention; 
         FIG. 3  illustrates one exemplary implementation, consistent with the present invention, in which optical nodes of the optical ad-hoc network include aircraft and/or satellites; 
         FIG. 4  illustrates exemplary components of an optical node of the optical ad-hoc network of  FIG. 2  consistent with the present invention; 
         FIG. 5  illustrates an exemplary vehicle coordinate system, that may be associated with individual optical nodes of  FIG. 2 , consistent with the present invention; 
         FIG. 6  illustrates an exemplary “request to send” message, consistent with the present invention, for requesting the establishment of an optical link between optical nodes; 
         FIG. 7  illustrates an exemplary. “clear to send” message, consistent with the present invention, for agreeing to the establishment of an optical link between optical nodes; 
         FIG. 8  illustrates an exemplary “reject” message, consistent with the present invention, for rejecting the establishment of an optical link between optical nodes; 
         FIG. 9  is a flow chart that illustrates an exemplary optical link establishment and tear down process consistent with the present invention; 
         FIG. 10  is a diagram that graphically illustrates the exemplary optical link establishment and tear down process of  FIG. 9  consistent with the present invention; and 
         FIG. 11  is a diagram that graphically illustrates an exemplary optical link establishment and tear down process involving three optical nodes consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Systems and methods consistent with the present invention implement a hybrid RF/optical channel access scheme that may be employed in a multi-hop, ad-hoc network. The use of optical links for transmission of significant quantities of data, as opposed to conventional RF links, permits very high throughputs, on the order of multiple gigabits per second, in an ad-hoc environment where nodes may be moving around and links may be created and terminated with a high degree of frequency. 
     Exemplary Network 
       FIG. 1  illustrates an exemplary network  100  in which systems and methods may implement contention-based optical channel access consistent with the present invention. Network  100  may include an optical ad-hoc sub-network  105 , sub-networks  110  and  115  and host(s)/server(s)  120 . Optical ad-hoc sub-network  105  may include a multi-hop, ad-hoc, optical packet-switched network. In other implementations consistent with the invention, sub-network  105  may include other types of networks, such as, for example, a circuit-switched network. Optical ad-hoc sub-network  105  may interconnect with sub-networks  110  and  115  via wired, wireless or optical links. 
     Sub-networks  110  and  115  may include one or more networks of any type, including a Public Land Mobile Network (PLMN), Public Switched Telephone Network (PSTN), local area network (LAN), metropolitan area network (MAN), wide area network (WAN), Internet, or Intranet. The one or more PLMNs may further include packet-switched sub-networks, such as, for example, General Packet Radio Service (GPRS), Cellular Digital Packet Data (CDPD), and Mobile IP sub-networks. One or more hosts and/or servers  120  may interconnect with sub-networks  110  and  115 . 
     As shown in  FIG. 2 , optical ad-hoc sub-network  105  may include multiple optical nodes  205 - 1  through  205 -N (collectively referred to as optical nodes  205 ) that each may have the capability to communicate via, for example, both radio-frequency (RF) and optical links. The optical links may include one or more optical channels that may carry light throughout the electromagnetic spectrum, including light in the human visible spectrum and light beyond the human-visible spectrum, such as, for example, infrared or ultraviolet light. The optical links may include a free-space optical path, such as, for example, a path through the atmosphere or outer space, or even through water (e.g., below the sea) or other transparent media. The RF links may include one or more RF channels that consist of some time, frequency or code division multiplexed portion of an RF spectrum. 
     In a multi-hop, ad-hoc, optical packet-switched network, each optical node  205  of sub-network  105  may route packets on behalf of other optical nodes and, thus, serve as an intermediate node between a packet source optical node and a destination optical node in sub-network  105 . Each optical node  205  may include a mobile entity, such as, for example, an automobile, an airplane, a helicopter, a missile or a satellite. Each optical node  205  may further include a stationary, or semi-stationary entity, such as, for example, a ground station, a cellular base station or a stationary satellite. Each optical node  205  may communicate with another optical node via an optical link established using, for example, a steerable aperture (not shown). The number of optical nodes  205  shown in  FIG. 2  is for illustrative purposes only. Fewer or greater numbers of optical nodes  205  may be employed in optical ad-hoc sub-network  105  consistent with the present invention. 
       FIG. 3  illustrates one exemplary embodiment of the invention in which the optical nodes  205  of optical sub-network  105  may include ground stations  305 - 1  through  305 - 2  and aircraft/satellites  310 - 1  through  310 - 3 . Ground station  305 - 1  (node A) may establish an optical link with node B  310 - 1 . Node B  310 - 1  may further establish an optical link with nodes C  310 - 2  and D  310 - 3 . Node C  310 - 2  may establish an optical link with ground station  305 - 2  (node E) and node D  310 - 3  may also establish an optical link with node E  305 - 2 . In accordance with the exemplary embodiment of  FIG. 3 , data may be transmitted using ad-hoc optical links between ground stations  305 - 1  and  305 - 2  via nodes B  310 - 1 , C  310 - 2  and D  310 - 3 . 
     Exemplary Optical Node 
       FIG. 4  illustrates exemplary components of an optical node  205  consistent with the invention. An optical node  205  may include a switch/router  405 , an optical link subsystem  410 , a RF transceiver  415  and a node location determining device(s)  420 . Though only a single optical link subsystem  410  is shown in  FIG. 4 , multiple (i.e., at least two) optical link subsystems  410  may be included in each optical node  205 . 
     Switch/router  405  may include a network control computer  425 , a forwarding engine  430  and Ethernet interfaces  435 . Network control computer  425  may execute the ad-hoc routing protocols and manage the ad-hoc network topology. Forwarding engine  430  may forward data through the ad-hoc sub-network  105  in accordance with routing data accumulated by network control computer  425 . Network control computer  425  and forwarding engine  430  may be linked by any form of communication mechanism, such as, for example, an Ethernet or a PCI bus backplane. Ethernet interfaces  435  may interconnect optical node  205  with other networks, such as, for example, sub-networks  110  and  115 . 
     Optical link subsystem  410  may include an optical modem  440 , a laser (transmitter) and detector (receiver)  445 , a steerable aperture  450 , and an optical acquisition, pointing and tracking subsystem  455 . Optical link subsystem  410  may “plug in” to forwarding engine  430  by a standard interface such as, for example, an Ethernet interface. Optical modem  440  may include conventional mechanisms for controlling the transmission and reception of data via optical pulses, such as, for example, mechanisms for modulating/demodulating an optical beam, and for implementing error correction, interleaving, etc., as required for reliable communication. Laser and detector  445  may include conventional lasers for transmitting optical pulses and conventional detectors for detecting optical pulses received from another optical node  205  as directed by optical modem  440 . Steerable aperture  450  may include, for example, a telescope, for establishing an optical link in a specified direction from optical node  205 . Optical acquisition, pointing and tracking subsystem  455  may keep track of the location, velocity and/or acceleration of neighboring optical nodes such that it can provide instructions to steer steerable aperture  450  to establish optical links with the neighboring nodes. 
     RF transceiver  415  may include conventional circuitry for communicating via radio frequencies. In one implementation consistent with the invention, RF transceiver  415  may include a RF transmit/receive suite, its associated electronics, and an omni-directional antenna (not shown). RF transceiver  415  may operate, in some implementations, in accordance with IEEE standard 802.11. RF transceiver  415  may include any type of RF radio device that runs in any RF spectrum. For example, RF transceiver  415  may run in the VHF, UHF, L, either of the ISM bands, or any other RF band. 
     Node location determining device(s)  420  may include one or more devices that provide node geographic location data. Device(s)  420  may include one or more of a Global Positioning System (GPS) device, an inertial management unit, or a vehicle navigation unit that provide a location of optical node  205 . Location determining device(s)  420  may determine a current latitude, longitude and altitude of optical node  205 . Location determining device(s)  420  may further determine a three-dimensional velocity vector and, possibly, a three-dimensional acceleration vector, associated with optical node&#39;s  205  current motion relative to a fixed point, such as, for example, the earth. 
     In other implementations consistent with the invention, if device(s)  420  includes a GPS device, then device  420  may supply geographic positions in global coordinates, such as standard world models like the World Geodetic System (WGS  84 ) or the Military Grid Reference System (MGRS). The World Geodetic System designates coordinates in latitude and longitude in degrees, and height over the geoid (mean sea level) in meters. The MGRS is based on the Universal Transverse Mercator (UTM) projection from 84 degrees north to 80 degrees south. In MGRS, the earth&#39;s surface is sliced into sixty North-South “orange slices,” with each slice being six degrees wide and projected onto a flat plane with coordinates Easting (distance in meters from the local meridian, which is centered every 6 degrees), Northing (distance in meters from the equator), and altitude (meters above sea level). MGRS has the advantage of providing genuine “local flat earth” three-vectors aligned with East (E), North (N) and up (U), suitable for local ballistics, intervisibility and other computations. 
     Location determining device(s)  420  may further keep track of optical node&#39;s  205  current pitch, roll and yaw. Pitch, roll and yaw may be determined from conventional sensor technology. 
     Exemplary Vehicle Coordinate System 
       FIG. 5  illustrates an exemplary vehicle coordinate system  500 , that may be associated with one or more optical nodes  205  of optical ad-hoc sub-network  105 , consistent with the invention. As shown, a vehicle body  505  for each optical node  205  has a local coordinate system in which the positive x axis  510  may be in the vehicle forward direction, the positive y axis  515  may be to the right of the vehicle forward direction, and the positive z axis  520  may be down. As with conventional aerospace standards, a number of motions may be associated with each axis. For example, surge/roll motions  525  may be associated with x axis  510 , sway/pitch motions may be associated with y axis  515  and heave/yaw motions  535  may be associated with z axis  520 . As shown in  FIG. 5 , the vehicle coordinate system includes a right-handed coordinate system, where rotations about the axes are also right handed. “Strap-down” sensors, such as, for example, an acceleration sensor and a magnetic field sensor may measure components of external vectors (e.g., gravity, magnetic field) relative to the local vehicle coordinate system x  510 , y  515  and z  520  axes. 
     Exemplary Request-to-Send Message 
       FIG. 6  illustrates an exemplary request-to-send (RTS) message  600  that may be sent from an initiator optical node to a responder optical node for the purpose of requesting the establishment of an optical link between the two optical nodes. RTS message  600  may include a message type field  605 , an initiator node identifier (ID) field  610 , a responder node ID field  615 , an optional message tag field  620 , an initiator node location field  625  and an initiator node pitch, roll and yaw field  630 . 
     Message type field  605  may designate message  600  as a request-to-send message. Initiator node ID field  610  may include a unique identifier associated with the optical node that is initiating the optical link establishment request (e.g., the initiator node). Responder node ID field  615  may include a unique identifier associated with the optical node to which the optical link establishment request is being sent. Message tag field  620  may include any type of identifier for identifying a particular RTS/CTS interaction, such as, for example, a sequence number, a unique identifier, a challenge/response field, etc. Initiator node location field  625  may include data identifying a geographic location of the initiator optical node. Such geographic location data may include, for example, a latitude, longitude and altitude associated with the location of the initiator node. Node location field  625  may be derived from data from node location determine device(s)  420 . In some exemplary embodiments, node location data field  625  may additionally include a three-dimensional velocity vector and, possibly, a three-dimensional acceleration vector that indicates a current motion associated with the initiator node. Initiator node pitch, roll and yaw field  630  may include data identifying pitch, roll and/or yaw motions of a vehicle body associated with the initiator optical node. The pitch, roll and yaw data field  630  may be encoded in any number of conventional ways. 
     Exemplary Clear-to-Send Message 
       FIG. 7  illustrates an exemplary clear-to-send (CTS) message  700  that may be sent from a responder optical node back to an initiator optical node, in response to an RTS message  600  from the initiator optical node, agreeing to establish an optical link with the initiator optical node. CTS message  700  may include a message type field  705 , an initiator node ID field  610 , a responder node ID field  615 , an optional message tag field  620 , a responder node location field  710  and a responder node pitch, roll and yaw field  715 . 
     Message type field  705  may designate message  700  as a clear-to-send message. Responder node location field  710  may include data identifying a geographic location of the responder optical node. Such geographic location data may include, for example, a latitude, longitude and altitude associated with the location of the responder node. Node location field  710  may be derived from data from node location determining device(s)  420  of the responder optical node  205 . In some exemplary embodiments, node location data field  710  may additionally include a three-dimensional velocity vector and, possibly, a three-dimensional acceleration vector that indicates a current motion associated with the initiator node. Responder node pitch, roll and yaw field  715  may include data identifying pitch, roll and/or yaw motions of a vehicle associated with the responder optical node. The pitch, roll and yaw data field  715  may be encoded in any number of conventional ways. Fields  610 - 620  are similar to those discussed with respect to RTS message  600 . 
     Exemplary Reject Message 
       FIG. 8  illustrates an exemplary reject message  800  that may be sent from a responder optical node back to an initiator optical node, in response to an RTS message  600  from the initiator optical node, rejecting the establishment of an optical link with the initiator optical node. Reject message  800  may include a message type field  805 , an initiator node ID field  610 , a responder node ID field  615 , an optional message tag field  620  and an optional retry time field  810 . Message type field  805  may designate message  800  as a reject message. Retry time field  810  may include scheduling information that identifies when the initiator optical node should attempt to send another request-to-send message. The scheduling information may include, for example, a “wall clock time” (e.g., 09:35:17), or it may be a relative time, e.g., a number of milliseconds from a reference time when the RTS retry should be made. Fields  610 - 620  are similar to those discussed with respect to RTS message  600 . 
     Exemplary Optical Link Establishment and Tear Down Process 
       FIG. 9  is a flowchart that illustrates an exemplary process, consistent with the present invention, for establishing and tearing down an optical link between an initiator optical node and a responder optical node in optical ad-hoc sub-network  105 .  FIGS. 10 and 11  may also be referred to below for the purposes of graphically illustrating the exemplary process of  FIG. 9 . 
     The exemplary process may begin with an initiator optical node  1005  ( FIG. 10 ) sending, via, for example, a RF channel, a RTS message  1015  (e.g., RTS message  600  illustrated in  FIG. 6 ) to a designated responder node  1010  in ad-hoc sub-network  105  [act  905 ]. Initiator node  1005  may learn of the presence of responder node  1010  through, for example, notification messages broadcast by responder node  1010  notifying neighboring nodes of responder node&#39;s unique identifier, and its location and, possibly, its velocity and/or acceleration. The RTS message  1015  sent by the initiator node  1005  may include the node&#39;s unique identifier in the initiator node ID field  610  and the initiator node&#39;s  1005  location in the initiator node location field  625  and the initiator node&#39;s  1005  pitch, roll, and yaw motion in the initiator node pitch, roll, and yaw field  630 . 
     The responder node  1010  may receive the RTS message  1015  and determine whether it is willing or able to establish a link with initiator node  1005  [act  910 ]. Responder node  1010  may be unable to establish a link with initiator node  1005  if, for example, all of responder node&#39;s  1010  optical link subsystems  410  are already employed communicating via optical links/channels with other optical nodes  205 . In one optional exemplary implementation, if responder node  1010  is either unwilling or unable to establish an optical link with initiator node  1005 , then responder node  1010  may choose not to return a CTS message to the initiator node  1005  [act  915 ]. Initiator node  1005  may wait a specified period of time after sending the RTS message  1015  [act  920 ] and may then return to act  905  above to send another RTS message  1015 . 
     In another exemplary implementation, if responder node  1010  is either unwilling or unable to establish an optical link with initiator node  1005 , then responder node  1010  may send a reject message  800  (not shown in  FIG. 10 ) to initiator node  1005  [act  925 ]. Reject message  800  may specify the initiator node&#39;s  1005  unique identifier in initiator node ID field  610  and responder node&#39;s  1010  own unique identifier in responder node ID field  615 . Reject message  800  may additionally include scheduling information in retry time field  810  that specifies when initiator node  1005  should retry sending another RTS message. Subsequent to receiving the reject message  800  from responder node  1010 , initiator node  1005  may wait a specified period of time [act  930 ] before returning to act  905  above to send another RTS message  1015 . The specified period of time may be preset at initiator node  1005 , or may be retrieved from the optional retry time  810  included in reject message  800 . 
     Returning to act  910 , if responder node  1010  is willing and able to establish an optical link with initiator node  1005 , then responder node  1010  may send a CTS message  1020 , via a RF channel, to the initiator node  1005  agreeing to the establishment of the optical link [act  935 ]. CTS message  1020  may include the initiator node&#39;s  1005  unique identifier in the initiator node ID field  610 , the responder node&#39;s  1010  unique identifier in the responder node ID field  615 , the responder node&#39;s  1010  location in the responder node location field  710  and the responder node&#39;s  1010  pitch, roll, and yaw motion in the responder node pitch, roll, and yaw field  715 . 
     In accordance with the location, pitch, roll and yaw information retrieved from RTS  1015  or CTS  1020  messages received at either the initiator node  1005  or the responder node  1010 , the optical acquisition, pointing and tracking systems  455  of both nodes point their apertures towards one another and establish an optical link [act  940 ] (see  1025 ,  FIG. 10 ). The location information retrieved from RTS  1015  or CTS  1020  may also include a three-dimensional velocity vector and a three-dimensional acceleration vector that may permit the initiator node  1005  and/or the responder node  1010  to steer its aperture along a predicted node trajectory, thus, increasing the odds of actually acquiring and locking in on the other node&#39;s optical aperture. 
     Initiator node  1005  and responder node  1010  may then communicate via the established optical link [act  945 ] (see  1025 ,  FIG. 10 ). When either node wishes to terminate the optical link, the terminating node (e.g., initiator node  1005  shown in  FIG. 10 ) may send a teardown message  1030 , via the optical link, to the other node to notify the other node to tear down the established optical link [act  950 ]. Initiator node  1005  and responder node  1010  may terminate the optical link responsive to teardown message  1030 . Alternatively, an optical link may simply fail of its own accord (e.g., when a cloud comes between two nodes, when equipment fails, etc.). Initiator node  1005  and responder node  1010  may then determine optical link failure by the loss of signals and may remove the link from service without any explicit control message. 
     In some implementations consistent with the invention, as shown in  FIG. 11 , responder node  1010  may begin establishing an optical link with another initiator node  1105  before terminating the optical link with initiator node  1005 . Initiator node  1105  may send an RTS message  1110  to responder node  1010  while responder node  1010  is still communicating with initiator node  1005  via an optical link. Similar to the exemplary process illustrated in  FIG. 10 , responder node  1010  may respond with a CTS message  1115  accepting establishment of the optical link with initiator node  1105  during the same time period that responder node  1010  is terminating an optical link with initiator node  1005  using a teardown message  1030 . Responder node  1010  and initiator node  1105  may transmit data via the established optical link (see  1120 ,  FIG. 11 ). 
     CONCLUSION 
     Systems and methods consistent with the present invention may use a hybrid RF/optical channel access scheme, where nodes use RF messaging to request access to the optical channels and, in response to the RF messaging, the ad-hoc nodes may establish optical links for high-speed communication via optical channels. Optical links, consistent with the invention, permit very high data throughputs, such as multiple gigabits per second, in an ad-hoc environment, where nodes may be moving around and links may be created and terminated with a high degree of frequency. In some exemplary embodiments, establishment of the optical links may include steering of one or more optical apertures, such as, for example, an optical telescope, to point towards the node to which an optical link is going to be established. Steering of the optical aperture may, thus, permit optical link establishment with mobile optical nodes that may frequently change position. 
     The foregoing description of exemplary embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. For example, while certain components of the invention have been described as implemented in software and others in hardware, other configurations may be possible. Furthermore, though the present invention has been described in the context of an ad-hoc network, the present invention may be employed in any environment where optical nodes contend for access to a shared optical communications medium. For example, the present invention may be used in a network that employs only optical fiber, or a mix of optical fiber and free-space optical links. Additionally, other electrically transmissive mediums may be used as an alternative to, or in conjunction with the RF medium used for transmitting the RTS and CTS messages of the present invention. Such other electrically transmissive mediums may include, for example (but are not limited to), a wired medium that may employ Ethernet, Internet, ATM or any other type of wired medium protocol. 
     While series of acts have been described with regard to  FIG. 9 , the order of the acts may vary in other implementations consistent with the present invention. Also, non-dependent acts may be performed in parallel. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. The scope of the invention is defined by the following claims and their equivalents.