BACKGROUND People can communicate with each other through a myriad of networks. For example, people can use the Public Switched Telephone Network (PSTN), mobile radiotelephone networks, satellite links, paging networks, two-way radio, the Internet, and other public networks. Many of these networks are interconnected to each other and often, however, people have a need to establish a private network for enabling communication. For example, a hospital may have announcements made over a public address system, communication links between patients and nursing stations, and communication links between hospital staff. In an airplane or ship, a captain may need to communicate directly with certain crew members, with certain groups of crew members, and over a public address system. Other examples include warehouses or factories where workers may need to communicate with each other in order to ensure the optimal operation of the business. Even some private residences include communication systems to allow residents to communicate with each other and possibly with hired help.

Many of these communication systems, some of which are commonly called intercoms, are hard-wired to each other with electrical connectors. For example, the communication network on the C130J aircraft is hard-wired to have sixteen channels. One drawback with such a network is that it is difficult, if not impossible, to expand the network to include additional communication stations. As mentioned above, the C130J communication network has sixteen channels and because it is hard-wired the hardware simply cannot support any additional channels. Such a communication network would therefore be unsuitable in environments where additional stations may need to be added or where a larger number of stations may need to communicate with each other.

Another drawback with these electrical communication networks is that they may not provide optimal performance. One way in which performance suffers with electrical communication networks is with their susceptibility to electromagnetic noise. For example, in helicopters, airplanes, and factory settings motors and other electrical components generate electromagnetic fields that can be picked up by and carried by electrical communication networks. This electromagnetic noise can result in static and other interference on the communication channels and can increase the cost of the network due to the need for filters, signals processing, and electromagnetic shielding. Another way in which performance suffers in electrical communication networks is in the cabling and weight associated with the network. Weight in aircraft is of paramount concern and efforts are made to minimize the weight of the communication network.

Because electrical communication networks are susceptible to electromagnetic noise and present an added weight, many communication networks are being implemented through optical networks. Optical signals are immune to electromagnetic radiation and optical fibers present a considerably lighter weight than conventional electrical cables. For example, the NASA website http ://eol. gsfc. nasa. gov/Teclmology/FibOpt. html and in particular the sublink therein to the Spaceborne Fiber Optic Bus Data (SFODB) discloses an optical communication network that enables communication between multiple stations. Each station is interconnected to the next station through an optical fiber and thus presents a lighter weight than a comparable electrical network and is also immune to electromagnetic radiation. As described in the SFODB tutorial document, each station receives optical signals from a neighboring station, converts the optical signals into electrical signals, and then regenerates optical signals and forwards them on to the next station. Thus, optical signals pass successively from station to station around a closed ring.

Some communication networks must be highly reliable and cannot have any single point of failure. This requirement is certainly true for many military applications where damage that might occur at one station must not bring down the entire communication network. The basic optical communication network described in the NASA document offers improvements with regard to electromagnetic noise and weight but suffers from the disadvantage in that, without considerable redundancy and complexity, it presents single

points of failure. As mentioned above, each station in that network receives optical signals from a previous station, converts the optical signals into electrical signals, and then regenerates optical signals and forwards them to the next station. If one of these stations went off-line, that station would form a break in the ring interconnecting the stations. As a result, optical signals could not pass by that station and reach subsequent stations unless extraordinary steps are taken to provide redundancy. Additionally and perhaps more importantly, if the Control Host station ceases to, function for any reason the entire network ceases to function. In addition to presenting a single point failure, optical networks such as the one shown in the NASA document also suffer from a disadvantage in that it is difficult to add or drop stations. Every time a station is added or dropped from the network, connections between the stations must be added and rerouted in order to complete a serial ring of stations.

Expanding such an optical network is therefore difficult.

Other optical networks have been designed that do not convert the signals into electrical signals and recreate the optical signals at each point or station. Some of these optical networks involve splitting the optical signals off of a bus and directing the optical signals toward each station. While those types of optical networks do not impose the same problems with a single point of failure at a non-Control Host station, many of those networks are unidirectional and are limited in the number of stations that may be deployed in a network. Because the optical signals are diverted to each station, splitting losses may limit the number of stations to only 8 or 10 stations. Such optical networks are therefore unsuitable for the C130J communication network which has sixteen channels.

Another requirement in many communication networks is that the new or upgraded network must maintain the same functionality and look and feel of an existing communication network. In military communication networks, military personnel have already been trained to operate communication networks installed on ships or aircraft. If a new or upgraded communication network differed substantially from the existing communication network, all of the personnel who may have a need to operate the communication network must be retrained at a considerable cost to the military.

A need therefore exists for communication networks that are not susceptible to electromagnetic radiation, can be easily expanded to include more stations, has the capacity

for a greater number of stations, is easily modified to remove stations, does not present a single point of failure, yet does not differ substantially in the look and feel or functionality provided by existing communication networks.

SUMMARY The invention addresses the problems above by providing systems and methods for communication through optical signals. Networks according to the invention comprise a bi- directional optical bus for carrying optical signals between a plurality of stations. Each station is coupled to the optical bus through a passive optical coupler which directs optical signals traveling in either direction along the bus toward the station and which impresses optical signals from the station onto the optical bus in both directions simultaneously. The networks also include amplifiers that perform bi-directional amplification of the optical signals. The network provides time division multiplexed access and the stations transmit and receive optical signals in given time slots within a frame.

The stations can be equipped to have varying degrees of communication functionality.

Some of the stations can be receive-only stations and only receive data from other stations.

Other stations may be transmit-only stations and permit personnel only to send data to other stations. Yet other stations may provide both transmit and receive functionality to personnel and may optionally include a panel and display. In a preferred embodiment, the panel and display serves as a user interface for receiving inputs from a user and for displaying the status of the station to the user. From this panel and display, a user can select a desired communication channel or channels over which the station transmits or receives signals.

Networks according to the invention are suitable for use in aircraft and other facilities that require personnel to communicate with each other. The networks are immune to electromagnetic radiation since the stations transmit and receive optical signals. The networks also present a low weight to an aircraft or facility due to the use of a bi-directional optical bus. Furthermore, unlike some conventional optical networks, the networks according to the invention can accommodate large numbers of stations and thus can be expanded well beyond 16 stations, such as to 256 or more stations.

Other advantages and features of the invention will be apparent from the description

below, and from the accompanying papers forming this application.

BRIEF DESCRIPTION OF DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention and, together with the description, disclose the principles of the invention. In the drawings: Figure 1 is a diagram of a communication network according to a preferred embodiment of the invention; Figure 2 is a schematic diagram of a station according to one embodiment of the invention; Figure 3 is a schematic diagram of a station according to second embodiment of the invention; Figure 4 is a schematic diagram of a station according to third embodiment of the invention; Figure 5 is a schematic diagram of a station according to fourth embodiment of the invention ; Figure 6 is an example of a first interface available at the station of Figure 5 for an Interphone Control Panel; Figure 7 is an example of a first interface available at the station of Figure 5 for a Monitor Panel; Figure 8 is an example of a first interface available at the station of Figure 5 for an Auxiliary Monitor Panel; Figure 9 is an example of a first interface available at the station of Figure 5 for a UHF 2 panel; and Figure 10 (A) is an exemplary time slot allocation and Figure 10 (B) is a corresponding network diagram within an aircraft for that time slot allocation.

DETAILED DESCRIPTION Reference will now be made in detail to preferred embodiments of the invention, non- limiting examples of which are illustrated in the accompanying drawings.

I. Overview Communication networks according to preferred embodiments of the invention provide highly reliable central intercommunications systems (ICS). The networks include a bi-directional fiber optic bus that broadcasts optical signals simultaneously to all stations connected to the bus. The stations communicate with each other through time domain multiplexing (TDM) on a single wavelength, although multiple wavelengths may be employed. The networks are easily expandable permitting the installation on the fiber optic bus of additional stations.

The networks are intended to operate in an environment with other lightwave-based systems, co-existing on a single optical fiber carrying the highest level systems through the use of wavelength division multiplexing. In order to minimize the number of wavelengths used on the fiber, the networks employ only one wavelength. The stations are connected to the optical bus in any suitable manner, such as through simple add-drop couplers, tunable filters, wavelength division multiplexers, or couplers described in U. S. Patent Nos. 5,898, 801 and 5,901, 260, which are incorporated herein by reference.

Communications networks according to the invention can carry any type of audio, video, and/or data being communicated over the bus and multiplexed onto the bus at or near the physical location of each station. Each station comprises an optical-to-electrical converter and may also include an electrical-to-optical converter. The electrical-to-optical and optical-to-electrical converters may be provided as part of an electro-optical interface circuit (EOIC) as described in U. S. Patent Nos. 5,898, 801 and 5,901, 260.

The networks may employ"distributed multiplexing"so as to eliminate the need to route all audio, video, and data signals to a central location for handling by a central processor. Each station can then perform the demultiplexing of the audio, video, and data from the bus. To implement distributed multiplexing, a receiver is placed at each station and optionally an optical source is provided for transmitting signals onto the bus. As will be described in more detail below, the stations may be receive only, transmit only, receive and transmit, or transmit and receive with display capabilities. The optical source is preferably modulated in a return-to-zero fashion by the digitized audio, video, and data at each station.

A time division multiplexing (TDM) protocol is preferably selected so that each station places their transmission on a given time slot or set of time slots. The protocol for the stations is preferably one described in co-pending provisional application serial no.

60/252,253 entitled"Systems and Methods Employing an Optical Network Transport Protocol"filed on November 21,2000, and co-pending patent application entitled"Physical Layer Transparent Transport Information Encapsulation Methods and Systems"filed on August 7,2001. Another suitable protocol is a masterless protocol such as the Jam-TDMA protocol described by Koopman and Upender in"Time Division Multiple Access Without a Bus Master, "United Technologies Research Center Report RR-9500470. In a preferred embodiment, the commutator rate for the TDM bus is on the order of 1 Gbps and provides a maximum of 256 audio channels with 20 kHz bandwidth and 90 db dynamic range in 18-bit words. In this embodiment, the network also provides a maximum of 256 data channels and contact closures and includes overhead bits to accommodate framing and built-in test information.

A network 10 according to a preferred embodiment of the invention is shown in Figure 1. The network 10 includes a bi-directional optical bus 14 and a plurality of stations 12. The optical bus 14 is preferably a single-mode fiber that carries optical signals in both directions simultaneously to all stations 12 connected to the bus 14. The optical bus 14 also preferably provides bi-directional optical amplification of the signals traveling along the bus, such as described in U. S. Patent Nos. 5,898, 801 and 5,901, 260. Thus, the amplification of the optical signals may occur along a section 14a of the bus 14 with section 14a forming at least part of the interconnection between two of the stations 12. The optical amplification need not occur along the interconnection sections 14a but alternatively may be provided along paths 18 which interconnect the stations 12 to couplers 16. Furthermore, the optical amplification may occur within the stations 12 or within the couplers 16. The optical amplification may be performed through fiber amplifiers, such as erbium-doped fibers or other rare-earth doped fibers, as described in U. S. Patent Nos. 5, 898, 801 and 5,901, 260. The amplification may also be performed by devices separate from the fiber, such as any of the various discrete laser amplifiers.

The couplers 16 provide bi-directional coupling of signals from stations 12 onto the

bus 14 and also direct signals traveling in both directions along the optical bus 14 toward the stations 12. As mentioned above, suitable couplers 16 include those described in U. S. Patent Nos. 5, 898, 801 and 5,901, 260. Other examples of couplers 16 include add-drop couplers, tunable filters, and wavelength division multiplexers.

The stations 12 can provide varying levels of communication functionality. The stations may include only a receiver for detecting communications from the other stations 12 and may have a transmitter for sending communications to the other stations 12. The stations 12 may also include additional functionality, such as a display interface. The stations 12 are depicted with four different types of stations 12a, 12b, 12c, and 12d. While only four stations have been shown, networks 10 according to the invention may include other numbers of stations, may include additional or fewer types of stations, and may include only one type of station. Additional details of the stations 12 will become apparent from the description below.

II. Stations A. Station Si A first type of station S1 12a is a receive-only station and does not have any transmit capability. Preferably, the station 12a has a pre-set selection of a time slot to which it listens.

For example, in a C-130 aircraft, the station 12a may comprise a cockpit recorder preset to listen to TDM slots assigned to the pilot, copilot, flight engineer, and wide-area microphone.

The stations 12a can listen to one or more of the TDM slots and the output can be provided to any suitable device, such as but not limited to, a speaker, recorder, repeater, or to another network.

A preferred diagram of the station 12a is shown in Figure 2. The station 12a includes a receiver module 22 for receiving optical signals from the bus 14. The receiver module 22 converts the optical signals into electrical signals and provides those signals to a demultiplexer 24 for performing demultiplexing of the TDM signals. The demultiplexer 24 preferably comprises an Application Specific Integrated Circuit (ASIC) which may additionally perform analog-to-digital and digital-to-analog conversions and other signal processing. The output of the multiplexer 24 is provided to a processor 26, which preferably

comprises a digital signal processor (DSP). The DSP 26 provides an analog audio output.

The time slot or slots associated with station 12a can be programmed into the DSP 26 and ASIC 24 through a system load port 28. The demultiplexer 24 also provides a data output, which preferably comprises RS232 simplex data although other formats of data may be provided.

B. Station S2 Station S2 12b is a transmit-only station and performs the functions of processing and formatting audio and data for placement on the bus 14. The station 12b has the receiver module 22 to receive timing information from the bus 14. The station 12b also includes a DSP 36 for receiving input audio and an ASIC 34 which functions as a multiplexer/demultiplexer for controlling an optical source 30 to place optical signals onto the bus 14 during a given time slot or time slots. The optical source 30 preferably comprises a laser module although it may comprise other sources of optical signals. Further, the optical source 30 may be directly modulated as shown in Figure 3 or may be externally modulated by a separate modulator. The ASIC 34 preferably includes memory for storing digitized audio messages, such as 360 seconds of audio messages. The ASIC 34 preferably samples the messages at a 11 kHz or greater sampling rate and compresses the messages to reduce memory requirements. The messages may be placed on the bus 14 as a result of data input to the ASIC 34 or due to contact closure prompting. Among other purposes, the stations 12b are useful in providing warnings, alarms, and audible alerts to crew members.

C. Station S3 Station S3 12c provides both transmit and receive functionality for audio, video, and/or data. The station 12c transmits its data onto preset time slots and receives signals over other preset time slots. Thus, any station 12 on the bus 14 that wishes to transmit to a Station S3 12c need only place its data onto that station's receive time slot. Also, any station 12 that desires to receive signals from a Station S3 12c need only select that station's corresponding transmit TDM slot for monitoring. An example of a station 12c is a UHF radio interface station that is pre-programmed to place its UHF receiver audio on one time

slot and to accept its UHF transmit audio on another time slot. The station 12c also includes the DSP 26 for outputting audio and a port 48 for interfacing with the DSPs 26 and 36, ASIC 34, optical source 30, and receiver 22.

D. Station S4 Station 12d is a transmit and receive station 12 and also includes additional communication functionality which, in this example, is a panel and display 40. The station 12d includes the optical source 30 for transmitting optical signals during preset and/or panel selected time slots and a receiver 22 for receiving optical signals in preset and/or panel selected time slots. The station 12d also includes an ASIC 44 that provides multiplexing/demultiplexing functionality and a DSP 46 that performs the functions of DSPs 26 and 36. The DSP 46 also provides the necessary interface to the panel and display 40.

The station 12d also includes a port 58 for interfacing with the optical source 30, receiver 22, ASIC 44 and DSP 46.

The panel and display 40 provides a human interface to the station 12d and permits selection of time slots to monitor and time slots within which to transmit. The panel and display 40 may indicate specific time slots on the display or, more preferably, indicates the significance of given time slots. For example, one time slot may correspond to a broadcast to all stations and the panel and display 40 indicates a selection for a"Broadcast"function, rather than any numerical time slot within a transmitting frame. As should be apparent to those skilled in the art, the time slots may be allocated in a number of different ways according to a desired configuration of stations 12 within a network.

In an exemplary aircraft configuration, the station 12d is suitable for use by crew members and provides a control panel and display 40 for human interface. The panel preferably provides the capability to talk simultaneously on any combination of interphone, loudspeaker systems, VHF, UHF, HF, and SATCOM radios. The station 12d through the optical source 30 sends the same digitized audio into multiple time slots according to the crew member's selections in order to talk simultaneously to one or more destinations. The panel 40 provides the capability to monitor any combination of warnings, interphone, VHF, UHF, HF, and SATCOM radios. The station 12d can listen simultaneously to multiple

stations 12 by extracting digitized audio from multiple time slots according to the crew member's selections.

E. Panel and Display The panel and display 40 may comprise any suitable input/output device that is capable of providing indications of different outputs to a person and of receiving input. The panel and display 40 may include, but is not limited to, any combination of keyboards, mouse, touch-screens, voice-activated units, head-mounted displays, switches, buttons, touch pads, or dials.

In an aircraft environment, the panel and display 40 preferably simulates panels and displays currently in use by crew members. As a result, the panel and display 40 does not require the crew members to go through much, if any, training on operating the network 10.

Some examples of panels and displays 40 will now be described with reference to Figures 6 to 9. Each panel preferably includes a combination of real and virtual switches and real and virtual volume controls. These displays are preferably Night Vision Imaging System (NVIS) compatible LCD, plasma, or other display type. Major groupings of interfaces 40 can be selected through software-generated switches through the displays 40. For example, a screen button of"INTERPHONE"is selected to display the Interphone Control Panel 42 shown in Figure 6, a"MONITOR"screen button is selected to display a Monitor Panel display 44 shown in Figure 7, and an"AUXILIARY"screen button is selected to display an Auxiliary Monitor Panel shown in Figure 8. As shown by the screen buttons for scrolling, which are marked with"+"and"-"symbols, panels 40 may include additional buttons for other groupings of screens and also may include screen buttons to scroll through the different groupings.

The panels 40 also include a second set of screen buttons to select individual controls on the screens. For example, with the Interphone Control Panel 42 shown in Figure 6, a crew member can select the"UHF-2"screen button to receive a"UHF 2"panel 48 shown in Figure 9. Through the UHF 2 panel 48, a crew member can set a transmit functionality to either"ON"or"OFF"and can also set a volume level of the UHF receiver in the headset.

By setting the transmit function to"ON"the crew member causes the audio from the

station's microphone input to be placed in the appropriate transmitter's audio time slot and to cause the station's push-to-talk switch to control the corresponding transmitter.

An item to be selected is preferably highlighted as cursor keys are used to move around any screen or sub-screen. After an item has been set to"ON, "that item is preferably permanently highlighted in a manner that visually confirms its"ON"status without resorting to any sub-screen. In that manner, a crewmember obtains a visual summary of the"ON"or "OFF"status of all functions on a particular screen. A physical"CALL"switch connects a microphone output to a CALL timeslot, a physical"HOTMIKE"switch connects the microphone output to the HOTMIKE timeslot, and a physical master"VOLUME"control controls the level of volume in the headset.

The screen buttons or other input devices will vary with the desired configuration of the network 10 and with the type of stations 12. The examples given above with reference to Figures 6 to 9 reflect just one way out of a plurality of different ways in which interfaces may be provided to crew members in an aircraft. Other embodiments of the invention may associate buttons with different functions, employ additional buttons or input devices, may group panels and displays differently, or may otherwise vary from these examples.

F. Modularity As should be apparent from Figures 2 to 5, the stations 12a to 12d share some common structure. For example, all of the stations 12 have the receiver module 22, an ASIC, a DSP, and a port. Some of the stations additionally have the optical source 30 and a panel and display 40. As mentioned above, the configuration of the stations 12 is preferably programmable, such as the slots during which the stations transmit or receive messages.

Consequently, the stations 12 may be configured identically, such as with the DSP 46, ASIC 44, optical source 30, and port 58 and be programmed to operate as station 12a, 12b, 12c, or 12d. Stations 12b, 12c, and 12d would additionally be equipped with the optical source 30 and station 12d would also have the panel and display 40.

This modular design greatly simplifies the manufacturing of the stations 12 and also the installation of the stations 12. Instead of needing three different types of DSPs 26,36, and 46, with some stations 12 being equipped with DSP 26, others with DSP 36, some with

both DSPs 26 and 36, and yet others with only DSP 46, all of the stations 12 can be equipped with DSP 46. Not only does this simplify manufacturing, but the stations 12 can easily be upgraded with new communication functionality. For example, a station 12a with DSP 46 can later be modified to have the transmit capability and to have the panel and display 40 with no need to replace the DSP 46.

As another example of the simplification of manufacturing and installation, all of the stations 12 may be equipped with the ASIC 44. The stations 12 discussed with reference to Figures 2 to 5 are illustrated as having ASIC 24, ASIC 34, or ASIC 44. By providing all of the stations 12 with the same ASIC 44, the manufacturing of the stations 12 is greatly simplified. Additionally, as with the DSP 46, stations 12a, 12b, and 12c can be easily upgraded to have additional communication functionality without any need to replace ASICs 24 or 34 with a new ASIC. Similarly, the stations 12 may all be equipped with just the port 58 and not one of ports 28, 38, or 48.

III. Sample TDM Configuration A preferred implementation of the network 10 is within aircraft, such as the C-5 or C- 130 aircraft. The network 10 is airframe independent, compatible with active noise reduction (ANR) headsets, and expandable while maintaining existing ICS functions. The network 10 allows for monitoring of and transmission on all radios from all locations as selected by individual crewmembers. The network 10 supports the capability to talk simultaneously on any combination of available radios from all cockpit crew positions with no compromise of emissions security. The network 10, when implemented as an upgrade, is capable of supporting, as a minimum, the same number of stations 12 as the existing aircraft ICS with no degradation as more crewmembers utilize the system. The network 10 provides audio warnings, EMI immunity, high quality sound, and EW threat audio. The network 10 includes the ability to address passengers and crew through a speaker system throughout the aircraft. The network 10 provides audio transmissions that are intelligible at all operational ambient noise levels. In the event of a main aircraft power failure, the network 10 remains operable and, at a minimum, in an emergency situation all aircrew members are able to talk with each other. The network 10 also provides an emergency radio useable at all time by the

aircrew.

Figure 10 (A) illustrates a possible allocation of time slots in a frame 50 for an ICS network 60 installed on C5 aircraft. As shown in Figure 10 (A), each frame 50 includes slots for starting and ending sync messages and then includes a plurality of slots for pilot, copilot, navigator, engineer, etc. The precise positioning of the time slots and the number of time slots may vary from that shown.

Figure 10 (B) shows a diagram of the network 60 with a bus 62, couplers 64, and a plurality of stations 66. The stations 66 may comprise any combination of stations 12a, 12b, 12c, and 12d. For example, station 12a is used to extract voice data for the cockpit recorder.

The stations 12b launch the IFF, ADF1, ADF2, TACAN1, TACAN2, and Marker Beacon audio onto the fiber optic bus. Another station 12b launches Stallimeter Warning information. Six stations 12c are shown providing bus connectivity for HF1, HF2, VHF1, VHF2, UHF1, and UHF2 radios. Three additional stations 12c provide two way ICS functions for Vertical Stabilizer Top, Vertical Stabilizer Bottom, and ADS & Paratroop Jump locations. Twelve stations 12d provide the Pilot, Co-pilot, Navigator, Flight Engineer, Observer station, Relief Crew Compartment, Avionics Equipment Rack, Aft Troop Compartment, LH Aft Cargo, RH Forward Cargo, and LH Center Cargo locations with access to the ICS, communications, warnings, alerts, and any other audio source or destination on the fiber bus.

The Interphone function is implemented between the stations 12d on the bus 14 and each station 12d has a dedicated (ICS) time slot for its outgoing audio. Any station 12 with the ICS function listens to the ICS audio traffic by monitoring all of the other ICS time slots.

In the example of Figure 10, time slots 1 and 2 are ICS slots for the pilot and co-pilot. At any one station 12, the audio from all of the other stations 12 appear to occur simultaneously to a crewmember because the audio from each is extracted at the audio sampling rate. A communications radio system such as HF, VHF, UHF, or SATCOM requires one dedicated time slot for incoming audio or data and one for outgoing audio or data.

The foregoing description of the preferred embodiments of the invention has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and

variations are possible in light of the above teaching.

For example, the stations 12 may be allocated to certain time slots in a variety of ways. According to one aspect, the stations 12 may be hard-wired to operate in certain time slots and would need to be physically altered to operate in other time slots. As another option, the stations 12 may be programmed to operate in certain time slots, such as during manufacturing and/or through ports 28, 38, 48, or 58. As yet another option, the stations 12 may be allocated to time slots dynamically during operation, such as during set-up of the network. This dynamic allocation may form part of the network protocol.

Also, preferred embodiments of the network have been described as operating on a single wavelength. The invention is not limited to a single wavelength but instead the stations 12 may operate on two or more wavelengths. Thus, the optical signals on the networks may be wavelength division multiplexed as well as time division multiplexed.

While the optical signals are described as being return-to-zero modulated, the optical signals may be modulated in other ways.

The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated.