Abstract:
This disclosure relates to wavelength division multiplexed (WDM) passive optical networks (PON), and the transmission of point-to-point and broadcast or multicast channels from an optical line transmitter (OLT) to an optical network unit (ONU). There is provided a WDM PON ( 100 ) comprising: an OLT ( 105 ) coupled to a plurality of ONU ( 110   a,    110   b ) using an optical fiber network ( 160 ); the OLT ( 105 ) having one or more WDM transceivers ( 115   a,    115   b ) each arranged to receive multi-cast and point-to-point channels and wherein the or each WDM transceiver ( 115   a,    115   b ) is arranged to transmit a said channel to a respective said ONU ( 110   a,    110   b ) using a respective dedicated wavelength (λa, λb); an input channel switch ( 130   a,    130   b ) associated with the or each WDM transceiver ( 115   a,    115   b ) and arranged to selectively couple the input of the WDM transceiver to the multi-cast channels and the point-to-point channels ( 140,135 ); wherein the selected multi-cast or point-to-point channels are directly converted to the respective dedicated wavelength.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. §371 National Phase Entry Application from PCT/EP2011/056075, filed Apr. 18, 2011, and designating the United States, which claims priority to European Application No. 10195870.0, filed Dec. 20, 2010. The above-mentioned applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     This disclosure relates to wavelength division multiplexed (WDM) passive optical networks (PON), and the transmission of point-to-point and broadcast or multicast channels from an optical line transmitter (OLT) to an optical network unit (ONU). 
     BACKGROUND 
     WDM PON uses different wavelengths for each ONU which increases bandwidth per user or ONU when compared with other PON technologies using shared wavelengths such as GPON. Other WDM PON advantages include broadband user connections, symmetric downstream/upstream traffic, service transparency, possibility of stepwise system upgrade, long distance, channels segregation, and low latency. Because a dedicated wavelength is used for each ONU the data processing requirements are simplified at the OLT or central office and the ONU. This is because the individual users do not need to be individually addressed as they would in a shared wavelength PON, because the dedicated wavelength is only intended for one ONU. The need for addressing requires layer 2 and layer 3 processing equipment at the OLT and ONU which increases their complexity and cost. Thus for example traffic for a particular ONU must be packaged to be carried in a dedicated timeslot of the shared channel, or addressed using Ethernet packets for example. By comparison, in a WDM PON the traffic can be processed in the OLT at layer 1 and the incoming traffic directly converted to optical signals for transmission to the ONU, rather than requiring packing into addressed packets for example. This layer 1 processing considerably simplifies the central office equipment and reduces cost. 
     Whilst point-to-point traffic has been predominant in PON traffic to date, increasingly PON is also carrying multi-cast and broadcast traffic. Point-to-point traffic occurs when a user or ONU communicates with one other terminal point, for example a website or video-on-demand (VOD) server. Increasingly multicast/broadcast traffic such as television channels are being provided in which traffic from one source is transmitted to a number of end users such as ONU. In this situation the traffic is in one direction only (downstream) and is provided (logically) in parallel to each end user. In practice this can be achieved in PON using layer 2 processing in which the broadcast channels are packaged into Ethernet frames or into a dedicated downstream timeslot which all ONU can access. 
     Multicast or broadcast services in a WDM PON can be carried at Layer2 or Layer3, by allocating a portion the downstream frames sent to the final users. Alternatively these multicast services can be packaged into Layer2 packets and added to each or the respective ONU wavelength. However, this prevents the aforementioned advantages of only using Layer1 processing at the OLT from being utilized. Layer1 processing could be maintained by utilizing tunable filters at the ONU to receive broadcast or point-to-point channels on different wavelengths as required. However the use of tunable filters at ONU is expensive and the need for power splitting of the common broadcast channels reduces their power and hence robustness and range or distance. In a further alternative parallel fibers could be employed but this is very expensive. 
     SUMMARY 
     This disclosure provides a WDM PON comprising an OLT coupled to a plurality of ONU using an optical fibre network. The OLT has one or more WDM transceivers each arranged to receive multi-cast and point-to-point channels and each arranged to transmit the received channel to a respective ONU using a respective dedicated wavelength. An input channel switch associated with each WDM transceiver is arranged to selectively couple the input of the WDM transceiver to the multi-cast channels and the point-to-point channels. The selected multi-cast or point-to-point channels are directly converted to the respective dedicated wavelength. The input channel switch may be arranged to select between the multi-cast or the point-to-point channels in response to a control signal from the ONU corresponding to the WDM transceiver. 
     This arrangement allows Layer1 processing of both point-to-point and multicast/broadcast channels at the OLT and ONU. By using a switch to selectively couple the point-to-point and multicast channels to a WDM transceiver, and directly converting the selected channel to the respective wavelength, the selected channel is carried by a dedicated optical signal (wavelength) to the ONU. No re-packaging of the selected input channel (point-to-point or multicast) into Layer2 or Layer3 packets is required. Therefore the benefits of Layer1 processing at the OLT and ONU can be maintained, even when both point-to-point and multicast input channels are provided. 
     In an embodiment the input channel switch can be an electrical switch which couples the selected point-to-point or multicast channel to a WDM transceiver which directly converts this channel onto the dedicated wavelength using a tuned laser for example. Alternatively the point-to-point and multicast channels may be directly converted to optical wavelength signals before coupling to an optical input channel switch. 
     In other embodiments, there is also provided a method of operating a WDM PON, an OLT, and ONU and methods of operating these. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described with reference to the following drawings, by way of example only and without intending to be limiting, in which: 
         FIG. 1  is a schematic of a WDM PON according to an embodiment; and 
         FIG. 2  is a flow chart illustrating a method of operating a WDM PON according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. “a” or “an”, “the”, this includes a plural of that noun unless something else is specifically stated. Elements or parts of the described nodes or networks may comprise logic encoded in media for performing any kind of information processing. Logic may comprise software encoded in a disk or other computer-readable medium and/or instructions encoded in an application specific integrated circuit (ASIC), field programmable gate array (FPGA), or other processor or hardware. References to nodes can encompass any kind of node, not limited to the types described, not limited to any level of integration, or size or bandwidth or bit rate and so on. References to software can encompass any type of programs in any language executable directly or indirectly on processing hardware. References to hardware, processing hardware or circuitry can encompass any kind of logic or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, ASICs, FPGAs, discrete components or logic and so on. 
     A wavelength division multiplexed (WDM) passive optical network (PON) according to an embodiment is shown in  FIG. 1 . The WDM PON  100  comprises an optical line terminal (OLT)  105  coupled to a number of optical network units (ONU)  110   a ,  110   b  using an optical fibre network  160  including a remote node  125 . The OLT  105  comprises a number of WDM transceivers  115   a ,  115   b  which directly convert electrical signals carrying traffic to optical signals which are transmitted downstream to the ONU  110   a ,  110   b . Each transceiver  115   a ,  115   b  transmits a dedicated wavelength associated with a respective ONU  110   a ,  110   b . The transceivers  115   a ,  115   b  also receive optical signals from their respective ONU  110   a ,  110   b , and directly convert these optical signals to electrical signals. The downstream and upstream wavelengths allocated to each ONU may be the same or different depending on system configuration. However each ONU is associated with a respective dedicated wavelength, and two dedicated wavelengths where the upstream and downstream wavelengths are different. 
     The process of directly converting traffic from electrical to optical signals in a PON will be well known to those skilled in the art, but briefly, incoming electrical signals carrying traffic will be inserted into frames and input to a laser device for electro-optical conversion and transmission onto the optical fibre. The frames ensure the correct optical signal format and timing to enable reception by the ONU. However there is no requirement for encapsulation of the signal into addressed packets such as Ethernet packets or time slots within the frame because each wavelength is only received and processed by one ONU. This enables electro-optical conversion or signal routing toward different equipment to be made at the physical layer (Layer 1), in order to be compatible with several technologies (eg Ethernet, SDH, OTN, GEM). Therefore direct conversion does not require complex, expensive and power consuming layer 2 or higher circuits for packet processing, or dedicated circuitry for each upper layer technology. The traffic intended for each ONU does not need to be addressed to the respective ONU or inserted into an ONU specific time slot but merely inserted into a common frame format which is directed to the respective ONU by conversion to the appropriate wavelength. 
     The OLT  105  also comprises a multiplexer  120  which multiplexes together the wavelengths λa, λb from the different WDM transceivers  115   a ,  115   b . The combined optical signal is then transmitted over the fibre network  160  to a remote node comprising a demultiplexer  125 . The demultiplexer  125  separates the wavelengths onto respective fibre links to respective ONU  110   a ,  110   b  as shown. Therefore each ONU  110   a ,  110   b  is associated with a respective OLT transceiver  115   a ,  115   b  using a respective dedicated wavelength λa, λb. Whilst only the downstream path from the transmission output Tx from each transceiver  115   a ,  115   b  to the respective ONU  110   a ,  110   b  is shown for simplicity, it will be appreciated that a corresponding upstream path exists from the ONU to a receiving input Rx of a respective transceiver  115   a ,  115   b . The demultiplexer  125  in this upstream path may function as a multiplexer at the remote node and the multiplexer  120  may function as a demultiplexer, or a parallel demultiplexer (not shown) may be used for upstream wavelengths as will be appreciate by those skilled in the art. 
     The OLT  105  also comprises a number of input channel switches  130   a ,  130   b  coupled between the input of a respective transceiver  115   a ,  115   b  and multicast channels  145  and point-to-point channels  135 . The point-to-point channels are typically provided by a router  135  coupled to the Internet  140 . The router may be an IP or Ethernet router which routes incoming packets to a respective ONU depending on their user address as is known. The router  135  will comprise a number of output ports on the OLT side, each coupled to an input channel switch  130   a ,  130   b  corresponding to a respective ONU. Therefore a packet received for ONU-a for example can be forwarded to input channel switch  130   a . Although not explicitly shown, packets will also be received at input ports on the OLT-side of the router  135  from the ONU  110   a ,  110   b . These packets can then be forwarded onto the Internet  140 , or to another ONU depending on their destination address. 
     The multicast channels will typically be provided by a broadcast server transmitting a number of TV channels. An arbitrary subset of multicast channels can be made available to all the ONUs that request them, without constraints. In certain broadcasting implementations the same set of channels can be made available to all the ONU 
     The input channel switches  130   a ,  130   b  selectively coupled the input of their respective WDM transceiver  115   a ,  115   b  to the multi-cast channels  146  and the point-to-point channels  135 . Whether the point-to-point router  135  or the broadcast server  145  is coupled to the transceiver  115   a ,  115   b  and ultimately to the corresponding ONU  110   a ,  110   b , is controlled by a control signal  150   a ,  150   b  from the respective ONU  110   a ,  110   b . Thus for example, in response to a control signal  150   a  from ONU-a  110   a , input channel switch  130   a  switches from coupling the transceiver  115   a  from the point-to-point router  135  to the broadcast server  145 . 
     The input channel switches  130   a ,  130   b  are typically layer1 electrical switches, but may alternately be optical switches where the outputs from the broadcast server  145  and the point-to-point router are optical signals. In this alternative, WDM transceivers (or transmitters only) will be used to directly convert the point-to-point and multicast channel signals into the optical domain before the input channel switch selection. 
     The selected multi-cast or point-to-point channels are then directly converted to the respective dedicated wavelength by the respective WDM transceivers  115   a ,  115   b . Therefore the input channel processing at the OLT can be handled at layer 1 even with the addition of a broadcast server or other multicast input channels source. This allows the central office to retain the same low cost equipment, whilst adding channel services for the addition only of a simple layer1 switch for each wavelength of the WDM PON. 
     Furthermore, control of whether the ONU receives point-to-point channels or multicast channels can be simply controlled by a user. In an embodiment, the point-to-point router  135  and each ONU comprises a control signal interface  155 . The interface  155  may be implemented using suitable software in an Ethernet router at each device which uses an Ethernet packet ( 150   a ,  150   b ) from an ONU addressed to the router  135  to control the appropriate input channel switch  130   a ,  130   b . Alternatively a specific timeslot in the upstream wavelengths could be dedicated to the control signal  150   a ,  150   b  and which is received by the respective transceiver  115   a ,  115   b  which in turn controls the respective input channel switch  130   a ,  130   b . Other alternative signalling mechanisms could be readily determined by the skilled person and are therefore not further described here. 
     In this embodiment only the downstream signal (the signal transmitted toward the user) is switched while the OLT continues to receive the upstream signal which is coupled to the point-to-point router  135 . This enables this embodiment to switch back to transmitting the point-to-point channels when desired by the user as the control signal channel is still maintained. 
     A method of operation of the WDM PON is described in more detail with respect to  FIG. 2 . At the ONU  110   a ,  110   b , the method  200  monitors for a user indication to switch channels at step  205 . For example the user may which to switch from browsing the Internet to watching broadcast TV. The user indication may be provided by suitable software on a set-top box or router for example. Once such a user indication is determined, the method sends a control signal  150  to the OLT to switch input channels at step  210 . The control signal may be generated by a dedicated interface in the ONU or using signalling from the set-top box or user router for example. A simple Ethernet packet may be used which is addressed to the point-to-point router in the central office and which indicates which ONU and hence input channel switch  130   a ,  130   b  to control, and which input to use point-to-point router  135  or broadcast router  145 . At the OLT, the method receives multicast and point-to-point channels at step  215 . Multicast or point-to-point channels are coupled to the corresponding WDM transceiver depending on the control signal at step  220 . 
     This method enables a user at an ONU of a WDM PON to switch between two different sources of input channels at the OLT; in an embodiment multicast and point-to-point channels. 
     In an example application, a user is browsing the Internet, uploading photos or videos. The input channel switch  130   a  for that user is thus connected to the point-to-point channel equipment  135 . When the user wants instead to have access to multi-cast services (e.g. an HDTV bouquet)
         The user sends a request ( 150   a ) to the OLT through the upstream channel   The OLT (or PTP router  135 ) commutes the switch  130   a  using a control line The user now has access to a number of HDTV channels via the ONU  105   a.          

     In practice, the input channel switches could be electrical switches embedded in the respective transceivers  115   a ,  115   b  and connected to multi-cast and WDM PON equipment  145 ,  135  via backplane. The switches could also be optical switches, connected via optical patch-cords between the transceivers  115   a ,  115   b  and the input channel equipment  135 ,  145 . 
     The embodiments provide various advantages including: 
     1) Supporting multi-cast and broadcast in a WDM PON 
     2) Requiring no changes of the infrastructure (e.g. no additional fibers) 
     3) Cost effective (e.g. electrical switches are very cheap) 
     4) Saving opex and capex at the central office, since dedicated equipments, optimized on a service basis, are used in place of general purpose ones 
     Modifications and other embodiments may come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the claims are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation unless specifically noted.