Abstract:
A network node for use in a WDM communications network, the network node comprising a first network interface unit arranged, in use, to demultiplex an incoming WDM optical signal and to convert the incoming WDM optical signal into a plurality of electrical channel signals, a regeneration unit for regenerating the electrical channel signals utilizing at least 2R regeneration, a second network interface unit arranged, in use, to convert and multiplex at least one of the electrical channel signals into an outgoing WDM optical signal, and a secondary interface unit arranged, in use, to convert at least one of the electrical channel signals into an optical signal and to drop the optical signal at the network node.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates broadly to a network node for use in a Wavelength Division Multiplexing (WDM) communications network, to a WDM communications network incorporating such a network node and to a method of transmitting communication data in a WDM communications network.  
         BACKGROUND OF THE INVENTION  
         [0002]    In optical WDM communications networks it is known to utilize non-regenerative reconfigurable optical add/drop multiplexers (OADMs) at the network nodes of the communications network. However, such network designs have the disadvantage that they suffer from high optical losses at the network nodes due to the passive optical multiplexing and switching devices utilized in the reconfigurable OADMs. The high optical losses result in design constraints in as much as either fewer network nodes can be implemented in a network, or expensive optical amplifiers must be incorporated in the network so that more network nodes can be added in series.  
           [0003]    On the other hand, in communications networks which are based on regenerative synchronized optical network (SONET) multiplexers, optical losses are reduced as a result of utilizing electrical time domain multiplexing (TDM) multiplexer components and electrical cross connect components instead of OADMs. However, such communications networks have the disadvantage of a lower capacity and scalability when compared with WDM communications networks.  
           [0004]    There is thus a need to provide a communications network which preferably provides the regenerative benefits of SONET/TDM based communications networks as well as the capacity, scalability, and flexibility of a WDM based communications network.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with a first aspect of the present invention there is provided a network node for use in a WDM communications network, the network node comprising a first network interface unit arranged, in use, to demultiplex an incoming WDM optical signal and to convert the incoming WDM optical signal into a plurality of electrical channel signals, a regeneration unit for regenerating the electrical channel signals utilizing at least 2R regeneration, a second network interface unit arranged, in use, to convert and multiplex at least one of the electrical channel signals into an outgoing WDM optical signal, and a secondary interface unit arranged, in use, to convert at least one of the electrical channel signals into an optical signal and to drop the optical signal at the network node.  
           [0006]    In one embodiment, the secondary interface unit is further arranged, in use, to receive at least one single wavelength optical signal and to convert it into a corresponding electrical signal, and the second network interface unit is further arranged to convert and multiplex the corresponding electrical signal into the outgoing WDM optical signal. The secondary interface unit may preferably be arranged, in use, to regenerate the corresponding electrical signal utilizing at least 2R regeneration.  
           [0007]    The network node may further comprise an electrical switching unit arranged, in use, to facilitate that any electrical channel signal can selectively be converted and dropped at the network node via the secondary interface unit or converted and multiplexed into the outgoing WDM signal via the second network interface unit.  
           [0008]    The first network interface unit may comprise at least one trunk interface card for converting respective channels of the incoming WDM optical signal into the electrical channel signals.  
           [0009]    The second network interface unit may comprise at least one trunk interface card for converting the respective electrical channel signals into optical channel signals for multiplexing into the outgoing WDM optical signal.  
           [0010]    The secondary interface unit may comprise at least one secondary interface card for converting respective electrical channel signals into optical signals for dropping at the network node and for converting respective received single wavelength signals for multiplexing into the outgoing optical WDM signal.  
           [0011]    The switching unit may be arranged in a manner such that, in use, any one of the trunk or secondary interface cards can be selectively connected to any one of the trunk or secondary interface cards. Accordingly, wavelengths may be switched at the network node.  
           [0012]    The regeneration unit may be arranged to regenerate the electrical channel signals utilizing 3R regeneration. The regenerating unit in such an embodiment may comprise a programmable Clock Data Recovery (CDR) component for each electrical channel signal. Accordingly, different communication protocols can be utilized on different channels.  
           [0013]    The first and second network interface units are each preferably capable, in use, of functioning as the other network interface unit, whereby the network node is west-east/east-west traffic transparent. The first and second network interface units may each comprise a passive WDM multiplexing/demultiplexing component. The WDM components are preferably arranged as coarse WDM (CWDM) components.  
           [0014]    The switching unit may be incorporated in the first or second network interface units. Preferably, a redundant switching unit is incorporated in the other network interface unit for failure protection.  
           [0015]    In a preferred embodiment, the regeneration unit is implemented as a very large scale integration (VLSI) structure.  
           [0016]    In accordance with a second aspect of the present invention there is provided a network node for use in a WDM communications network, the network node comprising a first network interface unit arranged, in use, to demultiplex an incoming WDM optical signal and to convert the incoming WDM optical signal into a plurality of electrical channel signals, a regeneration unit for regenerating the electrical channel signals utilizing at least 2R regeneration, a second network interface unit arranged, in use, to convert and multiplex at least one of the electrical channel signals into an outgoing WDM optical signal, and a secondary interface unit arranged, in use, to receive at least one single wavelength optical signal and to convert it into a corresponding electrical signal, and the second network interface unit is further arranged to convert and multiplex the corresponding electrical signal into the outgoing WDM optical signal.  
           [0017]    In accordance with a third aspect of the present invention there is provided a WDM network incorporating at least one network node as defined in the first or second aspects of the present invention.  
           [0018]    In accordance with a fourth aspect of the present invention there is provided a method of transmitting communication data in a WDM communications network, the method comprising the steps of, at a network node of the WDM communications network, demultiplexing an incoming WDM optical signal and converting the incoming WDM optical signal into a plurality of electrical channel signals, regenerating the electrical channel signals utilizing at least 2R regeneration, converting and multiplexing at least one of the electrical channel signals into an outgoing WDM optical signal, and converting at least one of the electrical channel signals into an optical signal and dropping the optical signal at the network node.  
           [0019]    In accordance with a fifth aspect of the present invention there is provided a method of transmitting communication data in a WDM communications network, the method comprising the steps of, at a network node of the WDM communications network, demultiplexing an incoming WDM optical signal and converting the incoming WDM optical signal into a plurality of electrical channel signals, regenerating the electrical channel signals utilizing at least 2R regeneration, converting and multiplexing at least one of the electrical channel signals into an outgoing WDM optical signal, and receiving at least one single wavelength optical signal, converting it into a corresponding electrical signal, and converting and multiplexing the corresponding electrical signal into the outgoing WDM optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Preferred forms of the present invention will now be described, by way of example only, with reference to the accompanying drawings.  
         [0021]    [0021]FIG. 1 shows a schematic diagram of a network node structure embodying the present invention.  
         [0022]    [0022]FIG. 2 shows a schematic diagram of a detail of the network node structure of FIG. 1.  
         [0023]    [0023]FIGS. 3 and 4 show an example circuit diagram of the detail shown in FIG. 2.  
         [0024]    [0024]FIG. 5 is a schematic drawing illustrating an optical communications network embodying the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0025]    The preferred embodiments described provide a network node structure which enables the utilization of the regenerative benefits of SONET/TDM based communications networks as well as the capacity, scalability, and flexibility of a WDM based communications network.  
         [0026]    [0026]FIG. 1 shows a schematic diagram of a network node structure  10  embodying the present invention. The node structure comprises two network interface modules  12 ,  14 , an electrical connection motherboard  16  and a plurality of tributary interface modules e.g.  18 .  
         [0027]    The network interface modules  12 ,  14  are connected to an optical network east trunk  20  and an optical network west trunk  22  respectively, of an optical network (not shown) to which the network node structure  10  is connected in-line.  
         [0028]    Each of the network interface modules  12 ,  14  comprises the following components:  
         [0029]    a passive CWDM component  24 , in the exemplary embodiment a 8 wavelength component;  
         [0030]    an electrical switch component, in the exemplary embodiment a 16×16 switch  26 ;  
         [0031]    a microprocessor  28 ;  
         [0032]    a plurality of receiver trunk interface cards e.g.  30 ; and  
         [0033]    a plurality of transmitter trunk interface cards e.g.  32 , and  
         [0034]    a plurality of electrical regeneration unit e.g.  40  associated with each receiver trunk interface card e.g.  30 .  
         [0035]    In the exemplary embodiment, each regeneration unit e.g.  40  performs 3R regeneration on the electrical channels signal converted from a corresponding optical WDM channel signal received at the respective receiver trunk interface card e.g.  30 . Accordingly, the network node structure  10  can provide signal regeneration capability for each channel signal combined with an electrical switching capability for add/drop functionality, i.e. avoiding high optical losses incurred in OADMs.  
         [0036]    Details of the receiver trunk interface cards e.g.  30  and regeneration unit e.g.  40  of the exemplary embodiment will now be described with reference to FIG. 2.  
         [0037]    In FIG. 2, the regeneration component  40  comprises a linear optical receiver  41  of the receiver trunk interface card  30 . The linear optical receiver comprises a transimpendence amplifier (not shown) i.e. 1R regeneration is performed on the electrical receiver signal within the linear optical receiver  41 .  
         [0038]    The regeneration unit  40  further comprises an AC coupler  56  and a binary detector component  58  formed on the receiver trunk interface card  30 . Together the AC coupler  56  and the binary detector  58  form a 2R regeneration section  60  of the regeneration unit  40 .  
         [0039]    The regeneration unit  40  further comprises a programmable phase lock loop (PLL)  50  tapped to an electrical input line  52  and connected to a flip flop  54 . The programmable PLL  50  and the flip flop  54  form a programmable clock data recovery (CDR) section  55  of the regeneration unit  40 .  
         [0040]    It will be appreciated by a person skilled in the art that at the output  62  of the CDR section  55  the electrical receiver signal (converted from the received optical CWDM channel signal over optical fibre input  64 ) is 3R regenerated at the network node structure. It is noted that in the exemplary embodiment shown in FIG. 2, a 2R bypass connection  66  is provided, to bypass the CDR section  55  if desired.  
         [0041]    Returning now to FIG. 1, each of the tributary interface modules e.g.  18  comprises a tributary transceiver interface card  34  and an electrical performance monitoring unit  36 . In the exemplary embodiment, a 3R regeneration unit (not shown) similar to the one described in relation to the receiver trunk interface cards e.g.  30  with reference to FIG. 2 is provided. Accordingly, 3R regeneration is conducted on each received electrical signal converted from received optical input signals prior to the 16×16 switch  26 .  
         [0042]    As can be seen from the connectivity provided through the electrical motherboard  16 , each of the electrical switches  26  facilitates that any trunk interface card e.g.  30 ,  32  or tributary interface card e.g.  18  can be connected to any trunk interface card e.g.  30 ,  32 , or tributary interface card e.g.  18 . Accordingly, e.g. each wavelength channel signal received at the western network interface module  14 , e.g. at receiver trunk interface card  38  can either be dropped at the network node associated with the network node structure  10  via any one of the tributary interface modules e.g.  18 , or alternatively can be through connected into the optical network trunk east  20  via the east network interface module  12 .  
         [0043]    Furthermore, it will also be appreciated by the person skilled in the art that the network node structure  10  is west-east/east-west traffic transparent. Also, due to the utilization of network interface modules  12 ,  14  which each incorporate a 16×16 switch  26 , a redundant switch is readily provided for the purpose of protecting the tributary interface cards e.g.  18  from a single point of failure. The tributary interface cards e.g.  18  are capable of selecting to transmit a signal to either (or both) network interface modules  12 ,  14  and the associated switches e.g.  26 . The function of the switches e.g.  26  is to select the wavelength that the optical signal received from the tributary interface cards e.g.  18  will be transmitted on into the optical network.  
         [0044]    [0044]FIGS. 3 and 4 show a circuit diagram for the example embodiment.  
         [0045]    [0045]FIG. 4 shows an exemplary optical communications network  70  comprising an access ring network  72  and a sub-ring network  74 . The access ring network  72  comprises a plurality of network nodes  76 , each incorporating a network node structure of the type of network node structure  10  described above with reference to FIGS.  1  to  3 . The sub-network  72  can comprise a single wavelength SONET based network, with one of the 8 available wavelengths in the example embodiment being dropped and re-added at the network node  76 A. In the example embodiment, the wavelength utilized in the sub-ring network  74  is denoted λ A . Importantly, this wavelength may be different to any one of the wavelength λ 1 -λ 8  and the associated tributary interface card (not shown) is configured accordingly. An example wavelength utilized in the sub-ring network  74  may be 1310 nm, whereas the wavelength chosen in the access ring  72  may be:  
         λ 1 =1470 nm  
         λ 2 =1490 nm  
         λ 3 =1510 nm  
         λ 4 =1530 nm  
         λ 5 =1550 nm  
         λ 6 =1570 nm  
         λ 7 =1590 nm  
         λ 8 =1610 nm  
         [0046]    At the other network nodes  76 , e.g. at network node  76 B, other wavelengths are dropped and added to individual subscriber connections, e.g. at network node  76 B. Again, the tributary interface cards (not shown) may add/drop the signals a different wavelengths than the ones used within the access ring network  72 , in the exemplary embodiment denoted λ A  and λ B .  
         [0047]    In the exemplary embodiment shown in FIG. 4, the access ring network  72  is configured as a CWDM network having eight channels i.e. relatively widely spaced wavelength signals which reduces the likelihood of cross talk between channels, thus enabling less stringent design parameters in the implementation of the network. Furthermore, this also reduces the possibility of adjacent channel cross talk due to temperature related wavelength drift, thus permitting the application of the invention to outside enclosures that are subjected to wide temperature variations.  
         [0048]    As a result of utilizing electrical regeneration of the CWDM channel signals at each network node  76 , no costly optical amplification units need to be provided in a typical access network environment, i.e. typical transmission distance between network nodes of the order of 20 km. Thus, the exemplary embodiment can be implemented as a cost effective upgrade of an existing SONET based and/or SONET/TDM based optical network to form the access ring network  72 .  
         [0049]    At each of the network nodes  76 , any of the eight wavelength channels can be dropped or added into the access ring network  72 . Due to the west-east/east-west transparency of each of the network nodes  76 , communications between individual network nodes may be transmitted along different directions around the access ring network  72  to effect path protection. The wavelength allocation scheme must merely account for the fact that each wavelength can only be utilized once in each direction between the individual network nodes  76  should a single fibre bi-directional connection be used between the nodes  76  as in the example embodiment shown in FIG. 4. However, it is noted that due to the selective switching configuration of the network nodes  76  wavelengths may be switched at individual network nodes  76  to maximize the overall wavelength usage between the individual network elements  76  and ultimately in the overall access ring network  72 .  
         [0050]    It is noted that the present invention may also be implemented with two or more fibre connections between network nodes, in which case the wavelength resources between the network nodes in increased. The wavelength allocation scheme in such embodiments can be expanded accordingly.  
         [0051]    One of the applications/advantages of embodiments of the present invention is that the electronic switches support broadcast and multicast transmissions of the same signal over multiple wavelengths. This can have useful applications in entertainment video or data casting implementation. Many optical add/drop solutions do not support this feature, instead, they only support logical point-point connections since the signal is dropped at the destination node and does not continue to the next node.  
         [0052]    It will be appreciated by the person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.  
         [0053]    In the claims that follow and the in the summary of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprising” is used in the sense of “including”, i.e. the features specified may be associated with further features in various embodiments of the invention.