Abstract:
An optical network has an optical line termination coupled to a backbone network, in particular to an optical long haul network and a local exchange coupled to an optical access network. The local exchange provides an optical connection between an optical network unit of a tree topology and the optical line termination, which is part of a ring topology. There is also described a method for processing data in such an optical network.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an optical network and to a method for processing data in an optical network. 
     A passive optical network (PON) is a promising approach regarding fiber-to-the-home (FTTH), fiber-to-the-business (FTTB) and fiber-to-the-curb (FTTC) scenarios, in particular as it overcomes the economic limitations of traditional point-to-point solutions. 
     Conventional PONS distribute downstream traffic from the optical line terminal (OLT) to optical network units (ONUs) in a broadcast manner while the ONUs send upstream data packets multiplexed in time to the OLT. Hence, communication among the ONUs needs to be conveyed through the OLT involving electronic processing such as buffering and/or scheduling, which results in latency and degrades the throughput of the network. 
     In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to enabling bidirectional communications over one strand of fiber. 
     WDM systems are divided into different wavelength patterns, conventional or coarse and dense WDM. WDM systems provide, e.g., up to 16 channels in the 3rd transmission window (C-band) of silica fibers of around 1550 nm. Dense WDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system may use 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. Some technologies are capable of 25 GHz spacing. Amplification options enable the extension of the usable wavelengths to the L-band, more or less doubling these numbers. 
     Optical access networks, e.g., a coherent Ultra-Dense Wavelength Division Multiplex (UDWDM) network, are deemed to be the future data access technology. 
     Within the UDWDM concept, potentially all wavelengths are routed to each ONU. The respective wavelength is selected by the tuning of the local oscillator (LO) laser at the ONU. The selected wavelength is unique to each ONU at any point in time, corresponding to the channel that is assigned to this ONU. Since the wavelength, which is used for communication from the ONU to the OLT (the upstream wavelength), is derived from this selected wavelength (the downstream wavelength), the upstream wavelength is also unique to the ONU and no interference at the OLT occurs between channels assigned to different ONUs. 
     Today&#39;s communication networks are separated into a long haul (LH) segment, typically using dense wavelength division multiplexing (DWDM), metro networks, often using coarse wavelength multiplexing (CWDM), and access networks, which rely on DSL or passive optical networks. 
     The LH network is a meshed network, the metro network is a ring network, and the access network has a tree topology. Each segment uses specific technology and components. Interfaces between these networks require optical-electrical-optical (OEO) conversion. 
       FIG. 1  shows such a commonly used network structure. A long haul network termination  101  is connected to a carrier Ethernet switch CET  102 , wherein such connection is used for conveying electrical signals. Hence, the long haul network termination  101  provides a conversion from optical to electrical signals and the CET  102  provides a conversion from electrical to optical signals, said optical signals being fed to a Gigabit Ethernet ring network  103  comprising 8 CWDM channels. For each such CWDM channel one DSLAM  104  to  111  is connected to the optical ring network  103 . Considering for example DSLAM  105 , several subscribers  112  to  114  are connected to this DSLAM  105 , which is equipped with a filter that allows the DSLAM  105  to receive and to transmit on one of 8 CWDM wavelengths only. The DSLAM  105  further comprises a light source that generates light on the matching wavelength. Data is modulated onto this light and it is transmitted to the CET  102 . Here, the light is converted into electrical data, conveyed to the long haul network termination  101  and then converted into light again. The uplink is redundant; the data is sent both clockwise and counterclockwise through the ring network  103 . 
     In downstream direction (towards the subscriber), the CET  102  determines the correct wavelength to use for the particular DSLAM to which the subscriber of the data is connected to. 
     This architecture provides eight protected Gigabit Ethernet channels per fiber pair, even though these fibers could transport several terabits of information. Each DSLAM is equipped with a dedicated filter card; hence, the network is rather inflexible and cannot easily be re-configured. It is also a disadvantage of such a solution that the network cannot easily be adjusted to emerging traffic, because the filters of the DSLAMs would have to be replaced in case additional wavelengths are introduced. Furthermore, only a limited number of DSLAMs can be connected to a single ring topology; hence several rings are needed in order to supply a higher number of DSLAMs. This results in a costly and complex structure. 
     BRIEF SUMMARY OF THE INVENTION 
     The problem to be solved is to overcome the disadvantages stated above and in particular to provide an efficient and flexible solution to supply subscribers at least partially via an optical network. 
     This problem is solved according to the features of the independent claims. Further embodiments result from the depending claims. 
     In order to overcome this problem, an optical network is provided
         comprising an optical line termination coupled to a backbone network, in particular to an optical long haul network;   comprising a local exchange coupled to an optical access network, wherein said local exchange provides an optical connection between an optical network unit of a tree topology and the optical line termination, which is part of a ring topology.       

     It is noted that the optical line termination may be any device to be coupled with the backbone network, wherein such coupling may be via an electrical and/or an optical connection. 
     It is further noted that the backbone network may be an optical or an electrical network, in particular an optical long haul network. 
     The local exchange may be regarded as “colorless”, i.e. it is not pre-defined to fit a particular wavelength or a selection of wavelengths (only). 
     The local exchange may be any means that couples said optical network with an access network in the optical domain. 
     Advantageously, this approach allows for only a single optical-electrical-optical conversion at the interface between the backbone network and the optical line termination. 
     As an option, the optical line termination may be deployed at or associated with a central office that is connected to the backbone network. 
     In an embodiment, the optical access network is a passive optical network to which several optical network units are connectable. 
     The optical access network may have a tree topology with a bi-directional single fiber that is attached to, e.g., a ring topology of the optical network. 
     In another embodiment, the optical network is an UDWDM network. 
     In a further embodiment, said local exchange comprises means to convert upstream and downstream traffic conveyed via separate fibers to upstream and downstream traffic conveyed via a single fiber and/or vice versa. 
     It is noted that upstream direction refers to signals or traffic conveyed from the subscribers or ONUs towards the OLT or backbone network and downstream refers to the opposite direction towards the subscribers or ONUs. 
     Hence, the local exchange may in particular comprise means for converting upstream and downstream traffic being conveyed via separate fibers to/from one single fiber to be connected to the optical access network, in particular the PON to which the subscribers, e.g., ONUs, are attached. 
     In a next embodiment, said means to convert upstream and downstream traffic comprises a circulator. 
     It is also an embodiment that the optical network comprises a ring topology that is connected via the optical line termination to the backbone network and via the local exchange to the access network. 
     The ring topology provides the advantage that two paths (directions of the ring) could be utilized in order to reach a component that is connected to the ring. These paths could be referred to as active path and as protection path. In an exemplary configuration of the ring topology, upstream and/or downstream traffic may—by default—be sent via both paths of the ring topology. Preferably, with regard to the downstream traffic, the local exchange may either convey the traffic of the active path or the traffic of the protection path towards the subscribers attached to the local exchange. This can be achieved, e.g., by controlled optical attenuators, which can be switched on or off. 
     Pursuant to another embodiment, the ring topology comprises an active path and a protection path. 
     According to an embodiment, in downstream direction, signals from the active or from the protection path are fed towards the optical network units. 
     According to another embodiment, the downstream signals are branched off (from the ring topology) via an adjustable coupler. 
     The adjustable couplers may be controlled such that they branch off the amount of light required for further processing in or toward the access network. The adjustable coupler may even consider the position of the local exchange within the ring topology and therefore adjust the amount of light branched off depending on the actual attenuation the light has experienced when reaching this particular local exchange. 
     In yet another embodiment, the local exchange comprises a controlled attenuator that selects signals in downstream direction from the active or from the protection path. 
     It is noted that such selecting could be switching signals, e.g., either conveying signals from the active or from the protection path. 
     Said controlled attenuator may be a variable optical attenuator. It may comprise a monitoring capability, e.g., monitoring light sensors (diodes), that could detect a loss of light (power) and may initiate a local switching to the respective other path even without intervention from a central ring component such as the optical network termination. 
     According to a next embodiment, in upstream direction, signals are fed into the ring topology in both directions of the ring topology. 
     The optical network termination may hence obtain redundant signals fed to the ring. 
     Optionally, the upstream signals may be fed into the ring topology utilizing adjustable couplers in order to (at least partially) compensate an attenuation the light experiences when travelling along the ring topology in different directions. Hence, the upstream signals may be fed into the ring topology using different directions and different levels of light thereby adjusting the energy split depending on the way the light has to travel in the ring before it reaches the optical line termination. 
     Pursuant to yet an embodiment, the optical network replaces or is provided to replace a metro network. 
     The problem mentioned above is also solved by a method for processing data in the optical network according to any of the preceding claims. 
     In addition, the problem is solved by a method for processing data in an optical network,
         comprising an optical line termination coupled to a backbone network, in particular to an optical long haul network and a local exchange to be coupled to an optical access network,   wherein said local exchange converts upstream and downstream traffic conveyed via separate fibers to upstream and downstream traffic conveyed via a single fiber and/or vice versa.       

     According to an embodiment, the local exchange
         selects signals in downstream direction from the active or from the protection path and/or   branches off downstream signals via an adjustable coupler thereby adjusting the power of these signals and/or   conveys upstream signals towards the optical line termination via an adjustable coupler thereby adjusting the power of these signals.       

     Embodiments of the invention are shown and illustrated in the following figures: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagram of a prior art network structure; 
         FIG. 2  shows a schematic network architecture comprising an UDWDM PON that is attached via an OLT to a long haul network termination; 
         FIG. 3  shows the local exchange of  FIG. 2  in more detail in particular comprising a configurable splitter and a two-fiber to one-fiber conversion. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The solution presented in particular suggests providing a network using UDWDM instead of the CWDM-metro network and the DSL access networks. 
     This UDWDM network can in particular handle distances up to 100 km and hundreds of Gigabit Ethernet (GE) channels, it does also not require a separated metro network to get connected to the long haul (LH) network. Since wavelength selection is achieved by means of a local oscillator laser at the ONU, the LE need not be wavelength selective. 
     An optical line termination (OLT) can be deployed or associated with the LH node. This efficiently requires only a single optical-electrical-optical conversion thereby significantly reducing the amount of equipment required in the network. 
     As an option, a tree topology of a network with a bidirectional single fiber can be attached to a ring network comprising two unidirectional fibers. 
       FIG. 2  shows a schematic network architecture comprising an UDWDM PON  203  that is attached via an OLT  202  to a long haul network termination  201 . The UDWDM PON  203  combines the metro network (ring topology) and the access network (tree topology). 
     The OLT  202  can be deployed with a central office, which is connected to the long haul network via the long haul network termination  201 . 
     At a local exchange LE  204 , traffic is distributed towards subscribers  205  to  207 . For redundancy purposes, light that is transmitted (uplink or upstream) from the subscribers can be fed into the ring topology in both directions. In downstream direction, only the working path may be connected in order to avoid interference between signals of the working path and the protection path. 
     This can be achieved by a controlled or monitored attenuator. Hence, one path can be completely blocked while the other path is open. In case of a fiber cut, monitoring diodes, which could be co-located with the attenuator, detect a loss of power and initiate a local switching to the protection path. This can be achieved without intervention from the OLT. 
     Such variable attenuators may provide a minimum attenuation at 0 voltage or a maximum attenuation at  0  voltage. In the downstream splitting device, one of each such variable attenuators can be used, so that in the case of a power failure at the LE, only one path can be connected. 
       FIG. 3  shows the local exchange  204  of  FIG. 2  in more detail in particular comprising a configurable splitter and a two-fiber to one-fiber conversion. 
     According to  FIG. 3 , a single fiber topology  302  is converted to a two-fiber topology  303   a ,  303   b  of the ring network via a circulator  301 . 
     The ring network comprises two fibers  303   a ,  303   b  conveying traffic in opposite directions (fiber  303   a  conveys traffic from the left hand side to the right hand side). 
     A coupler  304  (e.g., a passive coupler, a 90/10 splitter or an adjustable coupler) conveys traffic from the fiber  303   a  of the ring network  203  to a variable optical attenuator VOA  306 ; a coupler  305  (e.g., a passive coupler, a 90/10 splitter or an adjustable coupler) conveys traffic from the fiber  303   b  of the ring network  203  to a variable optical attenuator VOA  307 . The output signals of the VOAs  306  and  307  are fed to a coupler  308  (e.g., a passive 3 dB coupler or an adjustable coupler) and the output of said coupler is conveyed via the circulator  301  and thus via the single fiber topology  302  to a splitter  310  and further to the subscribers (not shown) attacked to this splitter  310 . Uplink traffic from such subscribers is conveyed via the splitter  310  and the single fiber topology  302  to the circulator  301  and further to a coupler  309  (e.g., a passive 3 dB coupler or an adjustable coupler), from where it is split and forwarded towards the fibers  303   a  and  303   b.    
     The LE  204  may use passive broadband splitters to branch off some of the power for its local PON. Therefore the channel power may depend on the location of the LE  204  along the ring network as well as on the direction of the used channel. Hence, channel powers for the working and protection paths may not be equal. If the dynamic range is too high for the receiver, the passive splitters may be replaced by voltage controlled splitters with an adjustable splitting ratio. Hence, the LE  204  can branch off less power from a channel that has experienced little attenuation, and more power from a channel that arrives already significantly weakened at the LE  204 . This way, the LE  204  provides a comparable level of light towards its subscribers. Accordingly, the adjustable splitter may feed more power into a channel of longer distance and less power into a channel which will suffer little attenuation. In the upstream direction, this can also be achieved by adjusting the energy-split between the two directions (clockwise or counter-clockwise in the ring network) before the signal is fed into the respective fiber  303   a ,  303   b  of the ring network. 
     The splitting ratio could be configured during installation of the LE  204 , e.g., a manually adjustable splitter could be used, or remotely via the OLT  102 . 
     FURTHER ADVANTAGES 
     The bandwidth usable via the metro ring topology can be significantly increased (e.g., hundreds of GE instead of 8 GE). 
     A power consumption of the local exchange can be reduced; only VOAs and low-power signal processors for switching purposes are required instead of a DSLAM. 
     The local exchange does not have to distinguish between frequencies as does the DSLAM with fixed filter cards. The local exchange in this regard is colorless and could thus be used flexibly everywhere in the ring network for every subscriber. 
     An existing fiber network can be efficiently adapted utilizing the approach presented; for example, an additional fiber may be deployed from the local exchange to the subscriber without any complex adjustments of the network. Hence, also a migration of existing structures to the UDWDM PON approach is simplified. 
     LIST OF ABBREVIATIONS 
     CWDM Coarse Wavelength Division Multiplexing 
     DSL Digital Subscriber Line 
     DSLAM Digital Subscriber Line Access Multiplexer 
     DWDM Dense Wavelength Division Multiplexing 
     GE Gigabit Ethernet 
     LE Local Exchange 
     LH Long Haul 
     NGOA Next Generation Optical Access 
     OLT Optical Line Termination 
     ONU Optical Network Unit 
     PON Passive Optical Network 
     UDWDM Ultra Dense Wavelength Division Multiplexing 
     VOA Variable Optical Attenuator 
     WDM Wavelength Division Multiplexing