Patent Publication Number: US-2004052519-A1

Title: Protected linear optical network

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates broadly to a linear or bus optical network, and to a method of conducting transmission in a linear or bus optical network.  
       [0002] The present invention will be described herein with reference to a wavelength division multiplexing (WDM) linear optical network. However, it will be appreciated that the present invention does have broader applications, including to any optical linear network using an transmission technology for providing bi-directional transmission, such as e.g. synchronous optical networks (SONET) or synchronous digital hierarchy (SDH).  
       BACKGROUND OF THE INVENTION  
       [0003] Linear or bus optical networks comprise a linear link of network nodes. Due to the linear nature of such networks, as opposed to e.g. ring-networks, a return or redundant transmission path is not typically provided. Although a return path could be provided via another fibre in the same cable and conduit as the outward path, this is often impossible because the return transmission distance, which extends the entire length of the linear network, is typically too long, i.e. the return path is beyond link limits for e.g. un-amplified optical connections. Accordingly, such linear optical networks are un-protected in terms of optical fibre break or cable break (i.e. break of all fibres contained in one physical cable, e.g. a standard pair of fibres), or failure of a network node.  
       [0004] The present invention seeks to provide a linear optical network in which protection for failure of a node or a fibre break can be provided.  
       SUMMARY OF THE INVENTION  
       [0005] In accordance with a first aspect of the present invention there is provided a linear or bus optical network comprising first and second end nodes and a plurality of primary nodes disposed, in use, between the end nodes, wherein each end node is connected to its nearest neighbouring primary node and its 2nd nearest neighbouring primary node, and wherein each primary node is connected to its 2 nd  nearest neighbouring primary or end node on either side, or, where one of its nearest neighbouring nodes is one of the end nodes, to said one end node and to its 2 nd  nearest neighbouring primary or end node on the other side.  
       [0006] Preferably, the optical connection between neighbouring nodes is effected through a pair of optical fibres, wherein each fibre of the pair is arranged, in use, to carry bi-directional transmission, and wherein each primary node is connected to only one fibre of the pair on each side, whereby the primary nodes are alternately connected via single fibre connections, and wherein each end node is connected to both fibres of the pair.  
       [0007] In another embodiment, the optical connection between neighbouring nodes is effected through at least two pairs of optical fibres, wherein each fibre of the pairs is arranged, in use, to carry uni-directional transmission, with the transmission directions of the two fibres of each pair being opposite to each other, and wherein each primary node is connected to one of the pairs on each side, whereby the primary nodes are alternately connected via a pair of uni-directional fibres for bi-directional transmission, and wherein each end node is connected to both fibre pairs.  
       [0008] The network may further comprise one or more secondary nodes, where each secondary node is connected in-line between two connected ones of the end or primary nodes.  
       [0009] Advantageously, each of the nodes is arranged, in use, to regenerate the transmission signal.  
       [0010] The network may be arranged as a WDM network, a SONET network, or a SDH network.  
       [0011] One of the end nodes may be connected to a core or metro optical network. The core or metro optical network may be a protected optical ring-network.  
       [0012] In accordance with a second aspect of the present invention there is provided a method of conducting transmission in a linear or bus optical network comprising two end nodes and a plurality of primary nodes disposed between the end nodes, the method comprising the steps of transmitting from each end node to its nearest neighbouring primary node and to its 2nd nearest neighbouring primary node, and transmitting from each primary node to its 2 nd  nearest neighbouring primary or end node on either side, or, where one of its nearest neighbouring nodes is one of the end nodes, to said one end node and to its 2 nd  nearest neighbouring primary or end node on the other side.  
       [0013] Preferably, the transmitting between neighbouring nodes is effected utilising a pair of optical fibres, wherein each fibre of the pair carries bi-directional transmission, and wherein each primary node is connected to only one fibre of the pair on each side, whereby the intermediate nodes are alternately connected via single fibre connections, and wherein each end node is connected to both fibres of the pair.  
       [0014] In another embodiment, the transmitting between neighbouring nodes is effected utilising at least two pairs of optical fibres, wherein each fibre of the pairs carries unidirectional transmission, with the transmission direction of the two fibres of each pair being opposite to each other, and wherein each primary node is connected to one of the pairs on each side, whereby the primary nodes are alternately connected via a pair of uni-directional fibres for bi-directional transmission, and wherein each end node is connected to both fibre pairs.  
       [0015] Advantageously, the method further comprises the step of regenerating the transmission signal at each node.  
       [0016] The step of transmitting between two connected ones of the end or primary nodes may comprise transmitting via one or more secondary nodes connected in-line between said two connected nodes. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.  
     [0018]FIG. 1 is a schematic drawing illustrating an un-protected linear network;  
     [0019]FIG. 2 is a schematic drawing illustrating a hardware-protected linear network embodying the present invention.  
     [0020]FIG. 3 is a schematic drawing illustrating another hardware-protected linear network embodying the present invention.  
     [0021]FIG. 4 is a schematic drawing illustrating an extended version of the linear network of FIG. 3.  
     [0022]FIG. 5 is a schematic drawing of a network node structure for use in a protected linear optical network embodying the present invention.  
     [0023]FIG. 6 is a schematic drawing of a detail of FIG. 4.  
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
     [0024] The preferred embodiments described provide a linear optical network with protection for failure of a network node or a fibre break.  
     [0025]FIG. 1 shows a conventional linear network  10  comprising two end nodes,  12 ,  14  and a plurality of in-line nodes  16 . One of the end nodes  12  is connected to a core/metro network ring network  19 . The linear network  10  could be protected by the provision of a return path  18  between the end nodes  12 ,  14 , to effectively complete a logical ring connection between the various nodes of the optical network  10 . However, since the return path  18  extends the entire “length” of the linear network  10 , the transmission distance in the return path  18  will typically be beyond link limits realisable in such linear networks. In the example linear network  10  shown in FIG. 1, a maximum transmission distance between nodes may be 20 km, thus the 40 km return path  18  is beyond the link limits and thus unrealisable.  
     [0026] Turning now to FIG. 2, in an optical linear network  20  embodying the present invention, there are again provided two end nodes  22 ,  24  and a plurality of intermediate nodes  26 ,  28 ,  30 . One of the end nodes  22  is connected to a core/metro network ring network  36 .  
     [0027] The end nodes  22 ,  24  connect to both their nearest neighbour and second nearest neighbour, i.e. end node  22  is connected to intermediate node  26  and intermediate node  28 , whereas end node  24  is connected to intermediate node  30  and intermediate node  28 .  
     [0028] The intermediate node  26  is connected to the end node  22  on one side, and to the second nearest neighbour on the other side, i.e. to intermediate node  30 . Similarly, the intermediate node  30  is connected to end node  24  on one side, and the second nearest neighbour on the other side, i.e. intermediate node  26 .  
     [0029] Intermediate node  28  is connected to its second nearest neighbours on both sides, i.e. to end nodes  22  and  24 .  
     [0030] It will be appreciated by a person skilled in the art that accordingly each of the intermediate nodes  26 ,  28 ,  30  is alternately connected on the bi-directional “outward” path  32 , and the bi-directional return path  34 . In other words, while the maximum transmission distance between two nodes has effectively been increased by a factor of 2 to 20 kms, the transmission length of the return path  34  has been halved when compared with the linear network described above with reference to FIG. 1. Accordingly, this embodiment is well suited for optical networks for which the distance between nodes is less than half the possible transmission distance, but for which the total transmission distance of the linear network is above the possible transmission distance and so a direct return path is not realisable.  
     [0031] The protected linear network  20  can be thought of as a logical ring network within a physical linear cable containing a pair of fibres. In the case of the failure of any node or fibre between two nodes, then the nodes can protect as if they were on a ring network. In the example embodiment shown in FIG. 2, the optical linear network  20  is configured as a duplex 10 Gb/s capacity network on each single fibre, for which four 2.5 Gb/s course WDM (CWDM) channels propagating in each direction on the single fibre (i.e. 8 wavelength total) provide the 10 Gb/s duplex capacity. However, it will be appreciated that the invention is equally suitable for any linear network using any transmission technology regardless of the number of fibres required for bi-directional transmission. For example, for a standard SONET linear link which requires two fibres between nodes, the invention can be implemented using 4 fibres or 2 fibre pairs between nodes.  
     [0032] Turning now to FIG. 3, there is shown another protected linear network  40  embodying the present invention. The optical network  40  comprises two end nodes  42 ,  44 , and a plurality of intermediate nodes  46 ,  48 ,  50 ,  52 ,  54  and  56 . One of the end nodes  42  is connected to a core/metro network ring network  62 .  
     [0033] In the optical network  40 , each end node  42 ,  44  is connected to its nearest neighbouring intermediate node and its second nearest neighbouring intermediate node. Accordingly, end node  42  is connected to intermediate nodes  46  and  48 , whereas end node  44  is connected to intermediate nodes  54 , and  56 .  
     [0034] On the other hand, each of the intermediate nodes  46 ,  48 ,  50 ,  52 ,  54  and  56  is either connected to its second nearest neighbouring nodes on either side, or, where one of its nearest neighbouring node is one of the end nodes  42 ,  44 , to that end node and to its second nearest neighbouring node on the other side.  
     [0035] Accordingly, the interconnection of the intermediate nodes  46 ,  48 ,  50 ,  52 ,  54  and  56  is as follows:  
     [0036] Intermediate node  46 , connected to: end node  42  and intermediate node  50 .  
     [0037] Intermediate node  48 , connected to: end node  42  and intermediate node  52 .  
     [0038] Intermediate node  50 , connected to: intermediate node  46  and intermediate node  54 .  
     [0039] Intermediate node  52 , connected to: intermediate node  48 , and intermediate node  56 .  
     [0040] Intermediate node  54 , connected to: intermediate node  50 , and end node  44 .  
     [0041] Intermediate node  56 , connected to: intermediate node  52 , and end node  44 .  
     [0042] In the example protected linear network  40 , the optical connections between nodes are effected through two pairs of optical fibres  58 ,  60 , wherein each fibre of pairs  58 ,  60  carries unidirectional transmission, with the transmission directions of the two fibres of each pair  58 ,  60  being opposite to each other for bi-directional transmission. Each intermediate node  46 ,  48 ,  50 ,  52 ,  54  and  56  is connected to one of the pairs  58 ,  60  on each side, whereby the intermediate nodes  46 ,  48 ,  50 ,  52 ,  54  and  56  are alternately connected via a pair of uni-directional fibres for bi-directional transmission. On the other hand, both end nodes,  42 ,  44  are connected to both fibre pairs,  58 ,  60 , to complete the protection path.  
     [0043] In case of a fibre break in one or both fibres of the pair  60  as indicated by the cross between end node  42  and intermediate node  48  in FIG. 3, transmission between the end node  42  and the intermediate node  48  is switched to the alternative path, i.e. via nodes  52 ,  56 ,  44 ,  54 ,  50 , and  46 .  
     [0044] Furthermore, in case of a network node failure, e.g. at network node  50  as indicated by the cross, transmission between node  54  and node  46  is switched from the “direct” path, via the (faulty) node  50 , to the protection path via end node  44 , node  56 ,  52 ,  48 , end node  42 , and to node  46 .  
     [0045] A possible extension of the linear protected optical network  40  shown in FIG. 3 will now be described with reference to FIG. 4. In FIG. 4, the extended linear protected optical network  40   b  comprises an additional network node  64  located in-line on the fibre-pair  58  between node  46  and node  50 .  
     [0046] Importantly, during adding of the additional node  64 , which involves breaking the fibre-pair connection  58  between nodes  46  and  50 , the linear network  40   b  remains operable because of its protected nature. In other words, similar to the fibre break scenario described above with reference to FIG. 3, any traffic on the fibre pair connection  58  between nodes  46  and  50  will be diverted to the alternative transmission path.  
     [0047] It is noted that the addition of the node  64  between nodes  46  and  50  (and, indeed, further nodes if desired) does not impose new maximum transmission link restrictions, as it involves only portions of the original transmission link between nodes  46  and  50 , which are equal to or below the relevant maximum link length.  
     [0048] Another way of looking at the extended linear network  40   b  is, that it contains a plurality of primary and end nodes  44 ,  46 ,  48 ,  50 ,  52 ,  54  and  56 , all of which are in one embodiment characterised by the feature that the distances between second neighbouring end or primary nodes is of the order of the relevant maximum link length. The extended portion consists of a secondary node in the form of node  64  in the example embodiment shown in FIG. 4, and which is characterised in transmission links to the two primary nodes  46 ,  50 , to which it is connected in-line, that are shorter than the relevant maximum link length.  
     [0049] Furthermore, it will be appreciated by the person skilled in the art that the addition of node  64  does not interfere with the protected nature of the linear network  40   b , as it occurs “in-line” with the effective ring connectivity of the original protected linear network  40  (see FIG. 3) embodying the present invention.  
     [0050]FIG. 5 shows a schematic diagram of a network node structure  100  for use in protected linear WDM networks embodying the present invention. The node structure  100  comprises two network interface modules  112 ,  114 , an electrical connection motherboard  116  and a plurality of tributary interface modules e.g.  118 .  
     [0051] The network interface modules  112 ,  114  are connected to an optical network east trunk  120  and an optical network west trunk  122  respectively, of a protected linear optical network (not shown) to which the network node structure  110  is connected in-line.  
     [0052] Each of the network interface modules  112 ,  114  comprises the following components:  
     [0053] a passive CWDM component  124 , in the exemplary embodiment a 8 wavelength component;  
     [0054] an electrical switch component, in the exemplary embodiment a 16×16 switch  126 ;  
     [0055] a microprocessor  128 ;  
     [0056] a plurality of receiver trunk interface cards e.g.  130 ; and  
     [0057] a plurality of transmitter trunk interface cards e.g.  132 , and  
     [0058] a plurality of electrical regeneration unit e.g.  140  associated with each receiver trunk interface card e.g.  130 .  
     [0059] Each regeneration unit e.g.  140  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.  130 . Accordingly, the network node structure  100  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 optical add/drop multiplexers (OADMs).  
     [0060] Details of the receiver trunk interface cards e.g.  130  and regeneration unit e.g.  140  of the exemplary embodiment will now be described with reference to FIG. 6.  
     [0061] In FIG. 6, the regeneration component  140  comprises a linear optical receiver  141  of the receiver trunk interface card  130 . The linear optical receiver  141  comprises a transimpendence amplifier (not shown) i.e. IR regeneration is performed on the electrical receiver signal within the linear optical receiver  141 .  
     [0062] The regeneration unit  140  further comprises an AC coupler  156  and a binary detector component  158  formed on the receiver trunk interface card  130 . Together the AC coupler  156  and the binary detector  158  form a 2R regeneration section  160  of the regeneration unit  140 .  
     [0063] The regeneration unit  140  further comprises a programmable phase lock loop (PLL)  150  tapped to an electrical input line  152  and connected to a flip flop  154 . The programmable PLL  150  and the flip flop  154  form a programmable clock data recovery (CDR) section  155  of the regeneration unit  140 .  
     [0064] It will be appreciated by a person skilled in the art that at the output  162  of the programmable CDR section  155  the electrical receiver signal (converted from the received optical CWDM channel signal over optical fibre input  164 ) is thus 3R regenerated. It is noted that in the example shown in FIG. 5, a 2R bypass connection  166  is provided, to bypass the programmable CDR section  155  if desired.  
     [0065] Returning now to FIG. 5, each of the tributary interface modules e.g.  118  comprises a tributary transceiver interface card  134  and an electrical performance monitoring unit  136 . A 3R regeneration unit (not shown) similar to the one described in relation to the receiver trunk interface cards e.g.  130  with reference to FIG. 6 is provided. Accordingly, 3R regeneration is conducted on each received electrical signal converted from received optical input signals prior to the 16×16 switch  126 .  
     [0066] As can be seen from the connectivity provided through the electrical motherboard  116 , each of the electrical switches  126  facilitates that any trunk interface card e.g.  130 ,  132  or tributary interface card e.g.  118  can be connected to any one or more trunk interface card e.g.  130 ,  132 , or tributary interface card e.g.  118 . Accordingly, e.g. each wavelength channel signal received at the western network interface module  114 , e.g. at receiver trunk interface card  138  can be dropped at the network node associated with the network node structure  100  via any one of the tributary interface modules e.g.  118 , and/or can be through connected into the optical network trunk east  120  via the east network interface module  112 .  
     [0067] Furthermore, it will also be appreciated by the person skilled in the art that the network node structure  100  is west-east/east-west traffic transparent. Also, due to the utilisation of network interface modules  112 ,  114  which each incorporate a 16×16 switch  126 , a redundant switch is readily provided for the purpose of protecting the tributary interface cards e.g.  118  from a single point of failure. The tributary interface cards e.g.  118  are capable of selecting to transmit a signal to either (or both) network interface modules  112 ,  114  and the associated switches e.g  126 . The function of the switches e.g.  126  is to select the wavelength and direction that the optical signal received from the tributary interface cards e.g.  118  will be transmitted on and into the optical network.  
     [0068] One of the advantages of the network structure  100  (FIG. 5) 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.  
     [0069] It will be appreciated by the person skilled in the art that numerous modifications and/or variations 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.  
     [0070] In the claims that follow and 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.