Patent Publication Number: US-2004052530-A1

Title: Optical network with distributed sub-band rejections

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention relates generally to optical transport systems, and more particularly to an optical network with distributed sub-band rejections.  
       BACKGROUND OF THE INVENTION  
       [0002] Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting the signals over long distances with very low loss.  
       [0003] Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is based on the number of wavelengths, or channels, in each fiber and the bandwidth, or size of the channels.  
       SUMMARY OF THE INVENTION  
       [0004] A node for an optical network includes a first transport element operable to be coupled to an optical ring and to transport traffic in a first direction and a second transport element operable to be coupled to the optical ring and to transport traffic in a second, disparate direction. The first and second transport elements each include an optical splitter element operable to split an ingress signal into an intermediate signal and a drop signal. A filter in each node is operable to reject at least a first sub-band of the network from the intermediate signal to generate a passthrough signal including a plurality of disparate sub-bands of the network. Each node further includes an add element operable to add local traffic in at least the first sub-band to the passthrough signal for transport in the network.  
       [0005] Technical advantages of the present invention include includes providing an optical ring network with distributed sub-band rejections. In a particular embodiment, a disparate sub-band of the network is open at each node. As a result, an open ring network with flexible channel spacing within the sub-bands is provided. The network need not be physically opened at any one point and Unidirectional Path-Switched Ring (UPSR) protection switching is thus supported.  
       [0006] Other technical advantages of particular embodiments may include optical cross-connect capability with tunable band-pass filters. The provisioning of a simple, low-loss, and low-cost optical network may provide flexible channel spacing within sub-bands. Node configurations may allow for broadcasting of traffic, and negligible pass-band narrowing occurs within a sub-band. Ring-interference may be avoided, with low node loss (&lt;4 dB) and low loss variations. Also, no channel power equalization may be necessary.  
       [0007] It will be understood that the various embodiments of the present invention may include some, all, or none of the enumerated technical advantages. In addition, other technical advantages of the present invention may be readily apparent to one skilled in the art from the following figures, description and claims.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like numerals represent like parts, in which:  
     [0009]FIG. 1 is a block diagram illustrating an optical ring network in accordance with one embodiment of the present invention;  
     [0010]FIG. 2 is a block diagram illustrating details of an add/drop node of FIG. 1 in accordance with one embodiment of the present invention;  
     [0011]FIG. 3A is a block diagram illustrating operation of the band pass filter of the node of FIG. 2 in accordance with one embodiment of the present invention;  
     [0012]FIG. 3B is a diagram illustrating the add, drop, and pass-through sub-bands of FIG. 3A in accordance with one embodiment of the present invention;  
     [0013]FIG. 4 is a block diagram illustrating exemplary travel paths of sub-bands of the network of FIG. 1 in accordance with one embodiment of the present invention;  
     [0014]FIG. 5 is a block diagram illustrating exemplary bandwidth travel paths on the optical ring of FIG. 1 and showing high-level details of the add/drop nodes in accordance with one embodiment of the present invention;  
     [0015]FIG. 6 is a block diagram illustrating protection of the travel paths of FIG. 5 in accordance with one embodiment of the present invention;  
     [0016]FIG. 7A is a block diagram illustrating details of an add/drop node in accordance with another embodiment of the present invention;  
     [0017]FIG. 7B is a block diagram illustrating details of an add/drop node in accordance with yet another embodiment of the present invention;  
     [0018]FIG. 8A is a block diagram illustrating exemplary travel paths of sub-bands on the network of FIG. 1 provisioned with the nodes of FIG. 7A or  7 B in accordance with another embodiment of the present invention;  
     [0019]FIG. 8B is a block diagram illustrating redundancy features in an add drop note in accordance with yet another embodiment for the present invention;  
     [0020]FIG. 9 is a block diagram illustrating exemplary travel paths of sub-bands on the network of FIG. 1 in accordance with yet another embodiment of the present invention;  
     [0021] FIGS.  10 A-C illustrate details and operation of an amplified spontaneous emission (ASE) filter in accordance with one embodiment of the present invention;  
     [0022]FIG. 11 is a flow diagram illustrating a method of managing traffic on an optical network accordance with one embodiment of the present invention; and  
     [0023]FIG. 12 is a flow diagram illustrating a method of inserting a new node into an optical network in accordance with one embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0024]FIG. 1 illustrates an optical network  10  in accordance with one embodiment of the present invention. In this embodiment, the network  10  is an optical ring network in which a number of optical channels are carried over a common path at disparate wavelengths. The network  10  may be a wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM), or other suitable multi-channel network. The network  10  may be used in a short-haul metropolitan network, and long-haul inter-city network or any other suitable network or combination of networks.  
     [0025] As described in more detail below, network  10  is a ring network with sub-band rejections distributed around the ring. A sub-band, as used herein, means a portion of the bandwidth of the network comprising a subset of channels of the network. In particular embodiments, the entire bandwidth of a network may be divided into sub-bands of equal bandwidth, or, alternatively, of differing bandwidth. Sub-bands may be of In one embodiment, each node is assigned a sub-band in which to add its local traffic. The node also filters out or otherwise rejects ingress traffic in this band that has already circulated around the ring. Thus, each node controls interference of channels in the network  10  by both adding and removing traffic in its sub-band.  
     [0026] Referring to FIG. 1, the network  10  includes a plurality of nodes  12  and an optical ring  26  comprising a first optical fiber  14  and a second optical fiber  16 . Optical information signals are transmitted in different directions on the fibers  14  and  16  to provide fault tolerance. Thus each node both transmits traffic to and receives traffic from each neighboring node. As used herein, the term “each” means every one of at least a subset of the identified items. It will be understood that optical ring  26  may comprise a two unidirectional optical fibers, as illustrated, or may comprise a single, bi-directional optical fiber. The optical signals have at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time and/or other suitable data. Modulation may be based on phase shift keying (PSK), intensity modulation (IM) and other suitable methodologies.  
     [0027] In the illustrated embodiment, traffic in the first fiber  14  travels in a clockwise direction. Traffic in the second fiber  16  travels in a counterclockwise direction. The nodes  12  are operable to add and drop traffic to and from ring  26 . At each node  12 , traffic received from local clients is added to ring  26  while traffic destined for local clients is dropped. Traffic may be added to ring  26  by inserting the traffic channels or otherwise combining signals of the channels into a transport signal of which at least a portion is transmitted on one or both fibers  14  and  16 . Traffic may be dropped from the ring  26  by making the traffic available for transmission to the local clients. Thus, traffic may be dropped and yet continue to circulate on a fiber  14  and/or  16 .  
     [0028] In one embodiment, the nodes  12  are further operable to multiplex data from clients for adding to the ring  26  and to demultiplex channels of data from the ring  26 . The nodes  12  may also perform optical-to-electrical or electrical-to-optical conversion of the signals received from and sent to the clients.  
     [0029] Signal information such as wavelengths, power and quality parameters may be monitored in the nodes  12  and/or by a centralized control system. Thus, the nodes  12  may provide for circuit protection in the event of a line cut in one or both of the fibers  14  and  16 . In one embodiment, an optical supervisory channel (OSC) may be used by the nodes to communicate with each other and with the control system. In other embodiments, as described further below in reference to FIG. 2, network  10  may be a Unidirectional Path-Switched Ring (UPSR) network in which a switch is toggled so as to forward to a local client traffic from a direction (clockwise or counterclockwise) corresponding to the lower bit error rate (BER) and/or higher power level.  
     [0030]FIG. 2 illustrates details of the node  12  in accordance with one embodiment of the present invention. In the illustrated embodiment, at the node  12 , traffic is passively dropped from ring  26  with a passive splitter. “Passive” in this context means without power, electricity, and/or moving parts. An active device would thus use power, electricity or moving parts to perform work. In a particular embodiment, traffic may be passively or otherwise dropped from ring  26  by splitting, which is without multiplexing/demultiplexing, in the transport rings and/or separating parts of a signal in the ring. A filter is operable to reject an assigned sub-band of the network, with the remaining sub-bands passing through. Local traffic may be added to ring  26  in the assigned sub-band. The traffic may be passively or otherwise added.  
     [0031] Referring to FIG. 2, the node  12  comprises a first, or counterclockwise transport element  30 , a second, or clockwise transport element  32 , a combining element  36  and a distributing element  34 . The transport elements  30  and  32  add and drop traffic to and from the ring  26 , remove previously transmitted traffic, and/or provide other interaction of the node  12  with the ring. The combining element  36  generates the local add signal passively or otherwise. The distributing element  34  distributes the drop signals into discrete signals for recovery of local drop traffic passively or otherwise. In a particular embodiment, the transport, combining and distributing elements  30 ,  32 ,  36  and  34  may each be implemented as a discrete card and interconnected through a backplane of a card shelf of the node  12 . In addition, functionality of an element itself may be distributed across a plurality of discrete cards. In this way, the node  12  is modular, upgradeable, and provides a pay-as-you-grow architecture.  
     [0032] Each transport element  30  and  32  is connected or otherwise coupled to the corresponding fiber  14  or  16  to add and drop traffic to and from the ring  26 . Each transport element  30  and  32  comprises an optical splitter element  42  operable to split an ingress signal into an intermediate signal and a drop signal, a filter  44  operable to reject an assigned sub-band of the network from the intermediate signal to generate a passthrough signal including a plurality of disparate sub-bands of the network, and an add element operable to add local traffic in the assigned sub-band to the passthrough signal for transport in the network. In the illustrated embodiment, filter  44  also acts as the add element. In other embodiments (for example, the embodiment illustrated in FIGS. 7A and 7B), the add element is a separate element. An add element may comprise a filter, coupler, or other suitable device for adding traffic to the optical network. Components may be coupled by direct, indirect or other suitable connection or association. In the illustrated embodiment, the elements of the node  12  and devices in the elements are connected with optical fiber connections, however, other embodiments may be implemented in part or otherwise with planar wave guide circuits and/or free space optics.  
     [0033] Optical splitter elements (“splitters”)  42  may each comprise an optical fiber coupler or other optical splitter operable to combine and/or split an optical signal. Splitters  42  provide flexible channel-spacing, herein meaning with no restrictions concerning channel-spacing in the main streamline. As used herein, an optical splitter or an optical coupler is any device operable to combine or otherwise generate a combined optical signal based on two or more optical signals without multiplexing and/or to split or divide an optical signal into discrete optical signals or otherwise passively discrete optical signals based on the optical signal without demultiplexing. The discrete signals may be similar or identical in frequency, form, and/or content. For example, the discrete signals may be identical in content and identical or substantially similar in power, may be identical in content and differ substantially in power, or may differ slightly or otherwise in content. In one embodiment, the splitter  42  may split the signal into two copies with substantially equal power. The coupler may have a directivity of over 55 dB. Wavelength dependence on the insertion loss may be less than about 0.5 dB over 100 nm. The insertion loss for a 50/50 coupler may be less than about 3.5 dB.  
     [0034] Filter  44 , as described in further detail below in reference to FIGS. 3A and 3B, is operable to reject traffic in an assigned sub-band, and to pass the remaining traffic. Reject, as used herein, may mean terminate or otherwise remove from the traffic streamline. Filter  44  may also add local traffic in assigned sub-band. Filter  44  may be optically passive in that traffic multiplexing and/or demultiplexing is not required.  
     [0035] In one embodiment, the transport elements  30  and  32  each include an amplifier  40 . Amplifiers  40  may be erbium-doped fiber amplifier (EDFAs) or other suitable amplifiers capable of receiving and amplifying an optical signal. The output of the amplifier may be, for example, 17 dBm. As the span loss of clockwise fiber  14  may differ from the span loss of counterclockwise fiber  16 , amplifiers  40  may use an automatic level control (ALC) function with wide input dynamic-range. Hence amplifiers  40  may deploy automatic gain control (AGC) to realize gain-flatness against input power variation as well as variable optical attenuators (VOAs) to realize ALC function. In a particular embodiment, one or a plurality of nodes  12  in network  10  may include an amplified spontaneous emission (ASE) filter (not illustrated) coupled to amplifiers  40  to prevent the buildup of unwanted spontaneous emission or noise from the amplifiers of the network  10 . ASE filters are described further below in reference to FIGS. 7 and 9.  
     [0036] In operation of the transport elements, amplifier  40  receives an ingress transport signal from the connected fiber  14  or  16  and amplifies the signal. The amplified signal is passed to optical coupler  42 . Optical coupler  42  splits the amplified signal into an intermediate signal and a local drop signal from the fiber  14  or  16 . Filter  44  rejects an assigned sub-band of the network from the intermediate signal to generate a passthrough signal, and adds local traffic in the assigned sub-band to the passthrough signal for transport on fibers  14  and  16 . The local drop signal is passed to the distributing element  36  for processing. In this way, for example, traffic is passively dropped from the ring  26  in the node  12 .  
     [0037] Distributing element  34  may comprise drop splitters  50  receiving dropped signals from fibers  14  or  16 . Splitters  50  may comprise splitters with one optical fiber ingress lead and a plurality of optical fiber drop leads. The drop leads may be connected to a switch  52  which allows for UPSR protection switching and one or more filters  54  which in turn may be connected to one or more optical receivers  56 .  
     [0038] In a particular embodiment, switch  52  is initially set-up so as to forward to the local client traffic from a direction (clockwise or counterclockwise) corresponding to a lower bit error rate (BER). A threshold value is established such that the switch remains in its initial set-up state as long as the BER does not exceed the threshold. Another threshold level may be established for power levels. If the BER exceeds the BER threshold or the power becomes less than the power threshold, the switch selects the other signal. Commands for switching may be transmitted via connection  57 . This results in local control of and simple and fast protection.  
     [0039] The combining element  36  may comprise couplers  60  which receive traffic from a plurality of optical fiber add leads which may be connected to one or more add optical senders  62  associated with a local client or other source. Combining element  36  further comprises two optical fiber egress leads which feed into amplifiers  40 . In other embodiments, amplifiers  40  may be omitted. Amplifiers  40  may comprise EDFAs or other suitable amplifiers. Thus, copies of the same traffic are forwarded to each of transport elements  30  and  32  via band-pass filters  44  to be added to ring  26  in both the clockwise and counterclockwise directions.  
     [0040]FIG. 3A is a block diagram illustrating operation of filter  44  of node  12  of FIG. 2 in accordance with one embodiment of the present invention. Filters  44  may comprise thin-film, fixed filters, tunable filters, or other suitable filters, and each filter  44  may comprise a single filter or a plurality of filters connected serially, in parallel, or otherwise. In the illustrated embodiment, filter  44  is a single band-pass filter.  
     [0041] As illustrated in FIG. 3A, band-pass filter  44  is operable to receive an optical signal  80  carrying traffic in a plurality of sub-bands. A sub-band is a portion of the bandwidth of the network. Each sub-band may carry none, one or a plurality of traffic channels. The traffic channels may be flexibly spaced within the sub-band. Band-pass filter  44  rejects an assigned sub-band  86  from the signal  80  and passes the remaining sub-bands  82  of the network. The rejected traffic is previously transmitted traffic which is removed to prevent re-circulation and channel interference. The passed traffic may be rejected at another node in the network  10 . Local traffic in the assigned sub-band  86  may also be added to signal  80 .  
     [0042]FIG. 3B is a diagram illustrating the sub-bands passed and added/dropped at filter  44  as illustrated in FIG. 3A in accordance with one embodiment of the present invention. As described above in reference to FIG. 3A, band-pass filter  44  may pass through selected sub-bands  82 , and reject one or more selected sub-bands  86  from the signal  80 . In the illustrated embodiment, the pass-through sub-bands  82  comprise sub-bands A and B, which comprise a plurality of channels in the lower end of the C-band spectrum. In the illustrated embodiment, sub-band A comprises four 2.5 Gb/s channels, one 10 Gb/s channel, and one 40 Gb/s channel (represented respectively by the small, medium, and large arrows), and sub-band B comprises one 10 Gb/s channel and seven 2.5 Gb/s channels. Pass-through sub-bands  82  also comprise sub-band D which is at the upper end of the C-band spectrum and comprises four 2.5 Gb/s channels and four 10 Gb/s channels. Rejected sub-band C comprises two 10 Gb/s channels and two 40 Gb/s channels in the same mid-range of the C-Band spectrum. Exemplary channel spacing is illustrated in FIG. 3B; however, channel spacing may be flexible, i.e., there is no restriction on the channel spacing, within the sub-bands. It will be understood that the bandwidth of the network may comprise other suitable bands, that the bandwidth may be otherwise subdivided into sub-bands of different sub-bandwidths, and that the rejected sub-bands may comprise different sub-bands than the added sub-bands.  
     [0043] In particular embodiments, some non-traffic carrying bandwidth is provided between adjacent sub-bands to avoid interference. In the illustrated embodiment, spacing  90  comprises a 200 GHz guard-band between adjacent sub-bands. Traffic signals are not allocated in the guard-bands so as to minimize signal loss and/or interference.  
     [0044]FIG. 4 is a block diagram illustrating exemplary bandwidth travel paths on the optical ring of FIG. 1 in accordance with one embodiment of the present invention. In the embodiment shown in FIG. 4, each of the nodes  12  rejects traffic from ring  26  from an assigned sub-band and adds new traffic to ring  26  in the assigned sub-band, with each node rejecting a different assigned sub-band. For ease of illustration, only fiber  14  of ring  26  is illustrated. It will be understood that the paths shown in FIG. 4 have corresponding paths in the counterclockwise direction on fiber  16 .  
     [0045] Referring to FIG. 4, traffic is added at node  22  in sub-band A and travels the circumference of fiber  14  to be rejected from fiber  14  at node  22 . In this way, channel interference is avoided. Likewise, sub-band B is rejected and added at node  24 , sub-band C is rejected and added at node  18 , and sub-band D is rejected and added at node  20 . In a particular embodiment, sub-bands A, B, C, and D comprise sub-bands spanning the C-band spectrum, with each sub-band within the C-band is assigned to one of nodes  18 ,  20 ,  22 , and  24 .  
     [0046]FIG. 5 is a block diagram illustrating exemplary bandwidth travel paths on the optical ring of FIG. 1 in accordance with one embodiment of the present invention. For ease of reference, only high-level details of the add/drop nodes  12  are shown.  
     [0047] Referring to FIG. 5, lightpaths  200  and  202  represent a stream of the same traffic added to the network from an origination node  18  in a selected band (the “node  18  band”) in the counterclockwise and clockwise directions, respectively. In the illustrated embodiment, the intended destination node of the node  18  band is node  22 . During normal operations, each of lightpaths  200  and  202  begin and are terminated at node  18 , thus avoiding channel interference. As previously described, Each node adds and removes traffic in an assigned sub-band, and the lightpaths may be terminated by rejection by filter  44  which rejects all of the traffic in the assigned sub-band. It will be noted that, although FIG. 5 shows node  22  as the destination node, the node  18  band also reaches the drop ports of nodes  20 ,  24 , and  18 . Thus, the network has a broadcasting function. As described below in reference to FIG. 6, broadcasting of the node  18  band in both the clockwise and counterclockwise directions also provides protection in the event of a line cut or other interruption.  
     [0048]FIG. 6 is a block diagram illustrating protection of the travel paths of FIG. 5 during a line cut or other interruption in accordance with one embodiment of the present invention. In the example shown in FIG. 6, as described above, lightpaths  200  and  202  represent a stream of the same traffic added to the network from an origination node  18  in the counterclockwise and clockwise directions, respectively.  
     [0049] In the illustrated embodiment, line cut  250  prevents the node  18  band from reaching its destination node  22  via lightpath  202 . Pursuant to the protection switching protocol, node  22  may, in response to sensing a BER exceeding the BER threshold for clockwise traffic, while still remaining below within the BER threshold for counterclockwise traffic due to the line cut, toggle switch  54  to switch from receiving clockwise (fiber  14 ) traffic to receiving counterclockwise (fiber  16 ) traffic. After repair of the line cut, the network may be reverted to its pre-protection switching state shown in FIG. 5 or, alternatively, may remain in the switched state.  
     [0050]FIG. 7A is a block diagram illustrating details of an add/drop node in accordance with another embodiment of the present invention. In particular embodiments, one or all of the elements shown in node  300  of FIG. 7A may be used in place of elements shown in nodes  12  of FIG. 2.  
     [0051] Node  300  comprises combining element  36  and distributing element  34 , as described above in reference to FIG. 2. However, node  300  comprises, in place of transport elements  30  and  32 , transport elements  330  and  332  which each comprise a filter  304  between drop coupler  42  and an add element comprising add coupler  302 . Like drop coupler  42 , add coupler  302  is passive and allows for flexible channel spacing. Filter  304  rejects one or more bands from the connected fibers  14  or  16 , thus preventing channel interference. Filter  304  may comprise a tunable band-pass filter or another suitable filter. Filter  304 , as described above in reference to filter  44 , rejects traffic in an assigned sub-band; however, in the embodiment illustrated in FIG. 8, filter  304  may not add traffic to the network. Instead, local traffic is added via add coupler  302 . The configuration of transport elements  330  and  332  allows for traffic outside the assigned sub-band to be added by add coupler  302  and thus, in a non-UPSR mode, for path sharing in the network, which increases overall network capacity, as described further below in reference to FIG. 8.  
     [0052] Amplifiers  344  may be erbium-doped fiber amplifier (EDFAs) or other suitable amplifiers capable of receiving and amplifying an optical signal. Node  300  also includes an amplified spontaneous emission (ASE) rejection filter  346  coupled to amplifiers  344  to prevent the buildup of unwanted spontaneous emission due to ASE circulation along the ring or noise from the amplifiers of the network  10 . For example, a conventional EDFA has a gain bandwidth of 35 nm between 1530 nm and 1565 nm. The network may prevent the ASE circulation for any part of the entire gain bandwidth (1530-1565 nm) even if the node count in the ring is relatively small (for example, 3 nodes.) Therefore, in a particular embodiment, each ring has one ASE rejection filter  346  in at least one node on the ring. In a particular embodiment, ASE rejection filter  346 s may be included in the transport elements of one node of a multiple-node network. In a particular embodiment, ASE rejection filter  346  may filter out or reject noise in unused sub-bands of the band of the network. As additional nodes are added to the network, additional sub-bands may be used for carrying traffic, and ASE rejection filter  346  may selectively reduce the sub-bands it filters so as to accommodate such additional sub-bands of traffic. As described below in reference to FIG. 9, ASE rejection filter  346  may comprise a multiple band-pass filter set to allow for expandability of the network as additional nodes are added.  
     [0053]FIG. 7B is a block diagram illustrating details of an add/drop node in accordance with yet another embodiment of the present invention. Add/drop mode  350  comprises distributing element  334  and combining element  336 , and transport elements  352  and  354 . Transport elements  352  and  354 , like transport elements  330  and  332  of FIG. 7A, each comprise a filter  304  between drop coupler  42  and an add element comprising add coupler  302 . 2×2 switches  356  are disposed between amplifiers  344  and drop couplers  42 , and are operable to open the transport elements and thus the optical ring at node  350 . In a particular embodiment, a 2×2 switch  356  may be opened in the event of a failure of an ASE rejection filter  346  such that the ASE rejection filter  346  cannot prevent ASE circulation for unused sub-bands. For example, if ASE rejection filter  346  in transport element  352  fails, 2×2 switches in transport element  352  and  354  are opened so as to effectively create a fibber cut in this segment. Under a UPSR protection regime, light paths would be protected under such an effective fiber cut situation.  
     [0054] Distributing element  334  may comprise drop splitters  50  receiving dropped signals from fibers  14  or  16 . As with node  12 , splitters  50  may comprise splitters with one optical fiber ingress lead and a plurality of optical fiber drop leads. However, one splitter  50  in node  300  is coupled to filter  308  which in turn is coupled to optical receivers  310 , and one splitter is coupled to filter  312  which in turn is coupled to filter  314 . Similarly, combining element  336  comprises coupler  316  coupled to sender  320  and coupler  318  coupled to sender  322 . In this way, 1+1 protection and network redundancy is provided for in both the distributing and combining elements.  
     [0055] UPSR protection schemes may be supported through redundancy of receivers  62 . In a particular embodiment, a receiver  62  may receive the same sub-band traffic from both the clockwise and counter-clockwise directions, thus allowing for simultaneous BER monitoring. In this embodiment, even if the BER of the working traffic slightly exceeds the BER threshold, the receiver corresponding to the lower BER may continue to receive traffic.  
     [0056]FIG. 8A is a block diagram illustrating exemplary bandwidth travel paths on an optical ring accordance with another embodiment of the present invention. In the embodiment shown in FIG. 8, path sharing allows for increased overall network capacity.  
     [0057] In FIG. 8A, nodes  18 ,  20 ,  22 , and  24  comprise nodes  300  as described in reference to FIG. 7. As described above in reference to FIG. 4, sub-band B is rejected and any sub-band may be added at node  24 , sub-band C is rejected and added at node  18 , and sub-band D is rejected and added at node  20 . However, for clarity, only the sub-band A lightpath is shown in FIG. 8.  
     [0058] Working traffic is added at node  22  in sub-band A in only the clockwise direction and travels the circumference of fiber  14  to be rejected from fiber  14  at node  22 , as described above in reference to FIG. 4. However, the node configuration of FIG. 8 also allows for path sharing by allowing additional traffic in sub-band A to be added to fiber  16  at node  20 . Such additional traffic may be referenced to as protection channel access (PCA) traffic. Both working and PCA sub-band A traffic is rejected at node  22  for both fibers  14  and  16 , thus avoiding channel interference.  
     [0059]FIG. 8B is a block diagram illustrating transmitter and receiver redundancy features of an add drop note in accordance with another embodiment of the present invention. The transmitter redundancy elements shown in FIG. 8B may be added to the combining element  34  of FIGS. 2, 7A, or otherwise suitably employed in the present invention. Similarly, the receiver redundancy elements shown in FIG. 8B may be added to the distributor element  36  of FIGS. 2, 7A, or otherwise suitably employed in the present invention.  
     [0060] Redundant 1×2 switches  362  and redundant transmitters  366  and  368  provide for redundancy of traffic being added to the clockwise and counter-clockwise rings. Likewise, redundant filters  370 , redundant receivers  372  and  374 , and 1×2 switches  362  provide redundant avenues for receipt of traffic from the clockwise or counter-clockwise rings. In particular embodiments, redundancy may be provided for 1+1 protection or for N:1 protection.  
     [0061]FIG. 9 is a block diagram illustrating exemplary bandwidth travel paths on an optical ring in accordance with another embodiment of the present invention. Similar to the ring described in reference to FIGS. 1 and 4, network  380  comprises a plurality of nodes  382 ,  384 ,  386 , and  388  in an optical ring comprising a clockwise optical fiber  390  and a counterclockwise optical fiber. The counterclockwise fiber is not shown for purposes of clarity. Similar to the embodiment shown in FIG. 4, each of the nodes  382 ,  384 ,  386 , and  388  rejects traffic from the ring from an assigned sub-band and adds new traffic to the ring in the assigned sub-band with each node rejecting a different assigned sub-band. Traffic is added at sub-band  382  in sub-band G and travels the circumference of fiber  390  to be rejected from fiber  390  at node  382 . Likewise, sub-band H is rejected and added at node  384 , sub-band E is rejected and added at node  386 , and sub-band F is rejected and added at node  388 .  
     [0062] In contrast to the nodes described above, nodes  382 ,  384 ,  386 , and  388  comprise an additional sub-band filter operable to reject and add an additional sub-band, sub-band Z. In the illustrated embodiment, sub-band Z is rejected and added at each of nodes  382 ,  384 ,  386 , and  388 . Thus, channels within common sub-band Z are added and dropped at each node. The dropped channels within sub-band Z can be reinserted into the ring or terminated at every node. If terminated, these drop channels in sub-band Z can be shared by different traffic in other nodes. In this way, the overall capacity of the network may be increased.  
     [0063] FIGS.  10 A-C illustrate details and operation of an ASE rejection filter in accordance with one embodiment of the present invention. FIG. 10A is a block diagram illustrating a configurable ASE rejection filter  400  in accordance with one embodiment of the present invention. In a particular embodiment, ASE rejection filter  346  may comprise multiple filter set  400  to allow for expandability of the network as additional nodes and additional sub-bands are used for carrying traffic. It will be understood that ASE rejection filter  346  may in other embodiments comprise one or more filters connected serially, in parallel, or otherwise.  
     [0064] Filter set  400  may comprise a plurality of individual band-pass filters  404 . Individual filters  404  and  406  may be provisioned to pass a selected sub-band, which may comprise one or more frequencies, and to reject other sub-bands. Switches  402  may be disposed so as to terminate traffic corresponding to particular filters  404  and  406 . Filters  404  are operable to demultiplex the sub-bands, and filters  406  are operable to mulitplex the sub-bands in the illustrated embodiment band pass filters  404  and  406  correspond to sub-bands A-H.  
     [0065] In the cascaded filter set  400 , both transmission and reflection of each sub-band are utilized. For example, if the input of ASE consists of all sub-bands (A, B, . . . H), sub-bands B through H are filtered at the filter  404  corresponding to sub-band A, and sub-band A is passed through. In a particular embodiment, the spectral power (mW/Hz) of the sub-band A light in the reflected light is {fraction (1/10000)} of the spectral power of the passed-through sub-bands (B, C, D, . . . H), and the spectral power of the rejected sub-bands (B, C, D, . . . H) in the transmitted light is {fraction (1/100)} of the spectral power of sub-band A. When switch  202  corresponding to sub-band A is in the “on” or “through” position, the spectral power of rejected sub-bands (B, C, D, . . . H) is {fraction (1/10000)} of the spectral power of the passed through sub-band A.  
     [0066] The reflected sub-bands (B, C, D, . . . H) from sub-band A filter  404  enter the filter  404  corresponding to sub-band B. Then reflected sub-bands at the sub-band B filter  404  contain only sub-band C, D, E, F, G, and H. At the last filter  404 , sub-band H light enters sub-band filter H  404  and passes through sub-band filter H  406 . As power-loss at reflection is quite small, loss of each sub-band is substantially the same, resulting from loss from the two sub-band filters ( 404  and  406 ) and from switch  402 . Therefore, wavelength (or sub-band) dependent loss of multiplexed light at the output is small.  
     [0067] Second filters  406  are provisioned to further filter the passed-through light. For example, sub-band B light (if the corresponding switch  202  is on; “through”) passes sub-band B filter  406  and then is mixed with the passed-through and reflected sub-band A light, thereby multiplexing sub-bands A and B. As described above, by controlling switches  202 , ASE rejection filter varies its bandwidth on sub-band basis.  
     [0068] As additional nodes and/or sub-bands are added to the network, additional switches  402  may be closed to allow additional sub-bands to pass. For example, as shown in FIG. 10B, a four-node network may carry four sub-bands A, B, C and D. The filter set  400  may be provisioned to reject all but sub-bands A, B, C and D, thus reducing or eliminating noise in the other, unused sub-bands. As an additional sub-band E is added, as illustrated in FIG. 10C, additional switches  402  corresponding to the additional sub-bands may be closed, thus allowing the additional band-pass filters  404  and  406  corresponding to the additional nodes to pass traffic corresponding to those bands.  
     [0069]FIG. 11 is a flow diagram illustrating a method of transporting traffic on an optical network accordance with one embodiment of the present invention. As described above, traffic is transported in an optical ring network, with each node assigned a sub-band of the network to add channels. The sub-bands may include any suitable number of traffic channels. The traffic may be transported in a first direction and a second direction on the optical ring.  
     [0070] Beginning with step  500 , at each node coupled to the ring, a transport signal comprising ingress traffic is passively split into a drop signal and an intermediate signal. At step  502 , a band-pass or other suitable filter rejects one or more sub-bands of channels from the intermediate signal to create a passthrough signal.  
     [0071] Proceeding to step  504 , traffic is added to the passthrough signal. The traffic may be added in sub-bands via the band-pass filter, or may be added via an optical coupler.  
     [0072]FIG. 12 is a flow diagram illustrating a method of inserting an additional node into an optical network in accordance with one embodiment of the present invention. The method of FIG. 12 may be utilized in an embodiment such as that shown in the FIG. 8 wherein path sharing is utilized for protection channel access (PCA) traffic.  
     [0073] Beginning with step  1000 , PCA traffic is removed from the network by ceasing PCA traffic transmission or otherwise. Proceeding to step  1002 , all working channels are switched to the counter-clockwise ring. At step  1004 , the clockwise fiber is disconnected where the new node is to be inserted, and the new node is inserted into the network and connected to the clockwise fiber. Proceeding to step  1006 , the clockwise ASE rejection filter corresponding to the new node is switched to the “on” or through position.  
     [0074] Proceeding to step  1008 , all working channels are switched to the clockwise direction. At step  1010 , the counter clockwise fiber is disconnected where the new node is to be inserted, and the new node is connected to the counter-clockwise fiber. At step  1012 , the counter-clockwise ASE rejection filter corresponding to the new node is switched to the on position. Finally, at step  1014 , the network is provisioned as shown in FIG. 8 or otherwise suitably provisioned for path sharing such that PCA traffic may be transmitted on the network.  
     [0075] In an embodiment of the present invention wherein UPSR protection switching is utilized, the method of FIG. 12 would not be utilized. Instead, insertion of a new node would involve disconnecting the optical ring at the point on the ring where the new node is to be inserted, and connecting the new node to the clockwise and counter-clockwise optical fibers. The switches  52  will automatically protect any traffic interrupted by the temporary opening of the ring by switching to the signal corresponding to the lowest BER. ASE rejection filter  344  may be provisioned to allow transmittal of the new sub-band corresponding to the new node, by, in a particular embodiment, switching the sub-band filter corresponding to the new node to the on position, as described above in reference to FIGS.  10 A- 10 C.  
     [0076] Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.