Abstract:
Methods and apparatuses of open ring optical networks are disclosed, as well as optical nodes, for example open intercepts and closed intercepts, supporting open ring optical networks. Open ring optical networks can prevent optical wavelengths from traversing an optical node in an optically transparent network. Optical feedback can be prevented in open ring optical networks, for example optical networks containing optical amplifiers. Coherent crosstalk can be prevented in open ring optical networks. Some open ring networks further allow standard communication protection techniques that are typically associated with rings which are topologically closed. These network architectures can enable low cost-of-maintenance bandwidth upgrades to the entire network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the U.S. Provisional Patent Application No. 60/280,347 filed Mar. 29, 2001, which is incorporated by reference herein its entirety. Additionally this application is related to co-pending U.S. patent application No. ______, filed Mar. 29, 2002, concurrently herewith entitled “Methods and Apparatus for Reconfigurable WDM Lightpath Rings” claiming the benefit of the U.S. Provisional Patent Application No. 60/280,550, filed Mar. 29, 2001 which are incorporated by reference herein in their entirety. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The invention relates generally to optical networking, and more particularly, to metro area optical networks.  
           [0004]    2. Description of Related Art  
           [0005]    Reconfigurable WDM networks will require the ability to change the optical path of connections, which includes changing the optical length. Optical length can include the length of fiber and the number of nodes passed through. Optical length can be proportional to optical loss. The larger the network, the larger the range of optical losses to be accommodated. Scaling this kind of network requires optical amplifiers to make the network “transparent”, which effectively decouples the length from the loss, for example with optical amplifiers to make up for the losses.  
           [0006]    One of the simplest and most robust kinds of network employs a ring topology at the physical layer. Rings are one logical choice for reconfigurable WDM networks. However, in a closed ring, optical signals are free to propagate completely around the ring, over and over. Closed rings mean a closed optical path, or a path in which photons have a route which they repeatedly traverse in some type of a “loop”. Closed rings lead to several types of transmission impairments. Difficulties arise in the presence of optical amplification.  
           [0007]    The amplified spontaneous emission (ASE) from the optical gain media undergoes feedback via the closed ring into the gain media. This leads to several phenomena that place very severe constraints on the system.  
           [0008]    First, the net gain of the system has a firm limit of 1 (0 dB or effectively no loss), the threshold for lasing to occur in the ring. Attempting to increase gain beyond this only increases the power in the lasing line that develops. The closed optical loop effectively becomes a ring laser. This lasing can be extremely unstable leading to further sources of transmission impairment. WDM signals may not increase in power.  
           [0009]    Second, if the net gain approaches very close (within a few dB) of the lasing condition (net gain of 1) the ASE noise floor increases dramatically. This noise creates severe WDM signal degradation. In the presence of optical amplification, the accumulation of ASE noise is a significant impairment. Effectively, closing the loop is equivalent to a large, or infinite, number of amplifiers in series. This increased ASE, leads to significant signal impairment through the mechanism of ASE-signal, and ASE-ASE mixing. This impairment is greatly enhanced in the presence of a closed optical loop.  
           [0010]    These effects serve to greatly limit the available dynamic range for connections within the network. This is compounded by the fact that optical amplifiers can have non-uniform gain as a function of wavelength. The result is that the closed ring with amplification does not scale to a very large number of wavelengths and/or nodes.  
           [0011]    With many present optical networks, the above constraints are less problematic or a nonissue, because many present optical networks lack optical transparency to some degree. Nontransparent networks, for example a 2.5 Gb/s SONET network, require one or more optical-to-electrical-to-optical conversions of signals, even when the signals are merely transiting an intermediate network node along the way from a source node to a destination node.  
           [0012]    Closed rings with optical transparency also encounter coherent crosstalk and gain transients.  
           [0013]    The impairment of coherent crosstalk arises when a particular wavelength is intended to be completely dropped at an optical add drop multiplexer (OADM) in a network, but is incompletely dropped instead. Incomplete dropping of a wavelength is not unusual, insofar as wavelength drop elements can be imperfect. If the signal at the wavelength that was incompletely dropped is allowed to return to the site of the signal&#39;s intended drop, through the “loop” of a closed ring, a delayed version of the signal interferes with the original signal, producing the impairment of coherent crosstalk. This can easily lead to a significant signal power penalty. This can become especially severe if the closed loop is nearly completely transparent, as could be the case in many practical implementations that incorporate optical amplification. One solution to coherent crosstalk minimizes the impairment of coherent crosstalk in the presence of closed rings, but relies exclusively on extremely high-performance wavelength drop elements, which can be very expensive.  
           [0014]    Wavelength reconfiguration of the type used in the optical networks can lead to abrupt changes in the optical signal which is input to the optical amplifiers in the system. This leads to transients in the gain of these amplifiers. These transients can be significantly enhanced or amplified, with amplifiers placed in series. When these enhanced transients feedback on themselves in a closed ring, the instabilities in gain can be further exacerbated, for example through control instability phenomena known as limit-cycles.  
         SUMMARY OF INVENTION  
         [0015]    Some embodiments of the invention take advantage of an open ring architecture for on optically transparent optical network. The open ring architecture can prevent an optical signal from topologically looping and thereby prevent the optical signal from returning to the origin of the optical signal.  
           [0016]    Some embodiments of an optical network include at least two optical fiber segments, one or more optical nodes, and one or more decoupling nodes. The first optical fiber segment has a first end and a second end. The second optical fiber segment has a first end and a second end. The one or more optical nodes are coupled to the second end of the first optical fiber segment and the second end of the second optical fiber segment. The one or more optical decoupling nodes are coupled to the first end of the first optical fiber segment and the first end of the second optical fiber segment. The one or more optical nodes include one or more optically transparent nodes optically transparent in at least one direction for one or more optical wavelengths at least optically traversing the one or more optically transparent nodes. One or more optical wavelengths carry traffic of the optical network. The one or more optical decoupling nodes substantially prevent at least one of the one or more optical wavelengths from at least optically traversing the one or more optical decoupling nodes.  
           [0017]    Some embodiments of an optical network include at least two optical fiber segments, one or more optical nodes, and one or more optical anti-feedback nodes. The first optical fiber segment has a first end and a second end. The second optical fiber segment has a first end and a second end. The one or more optical nodes are coupled to the second end of the first optical fiber segment and the second end of the second optical fiber segment. The one or more optical anti-feedback nodes are coupled to the first end of the first optical fiber segment and the first end of the second optical fiber segment. The one or more optical anti-feedback nodes substantially prevent optical feedback in the optical network. The one or more optical nodes include one or more optically transparent nodes optically transparent in at least one direction for one or more optical wavelengths at least optically traversing the one or more optically transparent nodes.  
           [0018]    Some embodiments of an optical network include at least two optical fiber segments, one or more optical nodes, and one or more optical anti-crosstalk nodes. The first optical fiber segment has a first end and a second end. The second optical fiber segment has a first end and a second end. The one or more optical nodes are coupled to the second end of the first optical fiber segment and the second end of the second optical fiber segment. The one or more optical anti-crosstalk nodes are coupled to the first end of the first optical fiber segment and the first end of the second optical fiber segment. The one or more optical anti-crosstalk nodes substantially prevent coherent crosstalk in the optical network. The one or more optical nodes include one or more nodes optically transparent in at least one direction.  
           [0019]    Some embodiments of an optical node include one or more housings. The one or more housings at least substantially enclose a wavelength add, a wavelength drop, and at least two optical ports. The first optical port is adapted to at least optically couple to a first node of one or more optical nodes. The first optical port is at least optically coupled to the wavelength add. The second optical port is adapted to at least optically couple to a second node of the one or more optical nodes. The second optical port is at least optically coupled to the wavelength drop. The one or more optical nodes include one or more optically transparent nodes optically transparent in at least one direction for one or more optical wavelengths at least optically traversing the one or more optically transparent nodes. The first optical port and the second optical port remain at least optically substantially decoupled within the optical node at one or more optical wavelengths that carry traffic in at least part of the one or more optical nodes.  
           [0020]    Some embodiments of an optical node include one or more housings. The one or more housings at least substantially enclose a wavelength add, a wavelength drop, and at least two optical ports. The first optical port is adapted to at least optically couple to a first node of one or more optical nodes. The first optical port is at least optically coupled to the wavelength add. The second optical port is adapted to at least optically couple to a second node of the one or more optical nodes. The second optical port is at least optically coupled to the wavelength drop. The one or more optical nodes include one or more optically transparent nodes optically transparent in at least one direction for one or more optical wavelengths at least optically traversing the one or more optically transparent nodes. The optical node substantially prevents optical feedback in the one or more optical nodes.  
           [0021]    Some embodiments of an optical node include one or more housings. The one or more housings at least substantially enclose a wavelength add, a wavelength drop, and at least two optical ports. The first optical port is adapted to at least optically couple to a first node of one or more optical nodes. The first optical port is at least optically coupled to the wavelength add. The second optical port is adapted to at least optically couple to a second node of the one or more optical nodes. The second optical port is at least optically coupled to the wavelength drop. The one or more optical nodes include one or more optically transparent nodes optically transparent in at least one direction for one or more optical wavelengths at least optically traversing the one or more optically transparent nodes. The optical node substantially prevents coherent crosstalk in the one or more optical nodes.  
           [0022]    Some embodiments are methods of optical networking. A first group of one or more optical signals is added to an optical ring carrying wavelength division multiplexed signals. A second group of one or more optical signals is dropped from the optical ring. The optical ring is broken.  
           [0023]    Some embodiments are methods of optical networking. At least one of: 1) adding one or more optical signals to an optically transparent optical ring, and 2) dropping one or more optical signals from the optically transparent optical ring is performed. Optical feedback in the optically transparent optical ring is substantially prevented.  
           [0024]    Some embodiments are methods of optical networking. At least one of: 1) adding one or more optical signals to an optically transparent optical ring, and 2) dropping one or more optical signals from the optically transparent optical ring is performed. Coherent crosstalk in the optically transparent optical ring is substantially prevented.  
           [0025]    In some embodiments, an add drop multiplexer is included in an optical node, an optical decoupling node, an optical anti-feedback node, and/or an optical anti-feedback node.  
           [0026]    In some embodiments, at least two optical nodes and one or more optical fiber segments coupling the at least two optical nodes is included in the one or more optical nodes.  
           [0027]    In some embodiments, at least two optical decoupling nodes, and one or more optical waveguides coupling the at least two optical decoupling nodes, is included in the one or more optical decoupling nodes. The one or more optical waveguides can include one or more optical fiber segments.  
           [0028]    In some embodiments, at least two optical anti-feedback nodes, and one or more optical waveguides coupling the at least two optical anti-feedback nodes, is included in the one or more anti-feedback optical nodes. The one or more optical waveguides can include one or more optical fiber segments.  
           [0029]    In some embodiments, at least two optical anti-crosstalk nodes, and one or more optical waveguides coupling the at least two optical anti-crosstalk nodes, is included in the one or more anti-crosstalk optical nodes. The one or more optical waveguides can include one or more optical fiber segments.  
           [0030]    In some embodiments, at least one node of the one or more optical nodes, one or more optical decoupling nodes, one or more optical anti-feedback nodes, and/or one or more optical anti-crosstalk nodes includes a wavelength add, a wavelength drop, and at least two optical ports. The first optical port is at least optically coupled to the wavelength add. The second optical port is at least optically coupled to the wavelength drop. In multiple embodiments, at least four optical ports may be included. The third optical port is at least optically coupled to the wavelength add. The fourth optical port is at least optically coupled to the wavelength drop. In several embodiments, the third optical port and the fourth optical port are at least optically coupled with at least an optical waveguide. The optical waveguide can include one or more optical fiber segments.  
           [0031]    In some embodiments, each optical decoupling node, each optical anti-feedback node, and/or each optical anti-crosstalk node, is associated with a logical ring of the optical network.  
           [0032]    In some embodiments, the optical network is adapted to carry out protection switching.  
           [0033]    In some embodiments, optical feedback in the optical network is prevented by breaking an optical ring. In some embodiments, coherent crosstalk in the optical network is prevented by breaking an optical ring. The optical ring includes the first optical fiber segment, the second optical fiber segment, and the plurality of one or more optical nodes. The optical ring can be broken at least somewhere between the first end of the first optical fiber segment and the first end of the second optical fiber segment.  
           [0034]    In some embodiments, the wavelength add includes a directional coupler. In some embodiments, the wavelength drop includes a drop filter.  
           [0035]    In some embodiments, the one or more housings at least substantially further enclose a third optical port at least optically coupled to the wavelength add and a fourth optical port at least optically coupled to the wavelength drop.  
           [0036]    In some embodiments, a wavelength add adds a single wavelength and/or a wavelength drop drops a single wavelength. In other embodiments, a wavelength add adds multiple wavelengths and/or a wavelength drop drops multiple wavelengths.  
           [0037]    Some embodiments employ protection switching responsive to a communication impairment, for example a fiber break.  
           [0038]    In some embodiments, available bandwidth of the optical network is increased by adding one or more nodes to the plurality of one or more nodes. In some embodiments, the plurality of one or more nodes and the added nodes are located together. This may not disrupt traffic in the optical network in some embodiments. Manual modification of the plurality of one or more optical nodes may be unnecessary to increasing the available bandwidth.  
           [0039]    Some embodiments practice at least in part technology disclosed in “Optical Networks” by Ramaswami, “Fiber-Optic Communication Systems” by Agrawal, “Erbium-Doped Fiber Amplifiers” by Becker, and/or “Scaling Limitations in Transparent Optical Networks Due to Low-Level Crosstalk” by Goldstein in  IEEE Photonics Technology Letters  of January 1995, all incorporated herein by reference.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0040]    [0040]FIG. 1 shows an embodiment of an optical network.  
         [0041]    [0041]FIG. 2 shows an embodiment of open intercepts.  
         [0042]    [0042]FIG. 3 shows an embodiment of closed intercepts.  
         [0043]    [0043]FIG. 4 shows a further embodiment of a closed intercept.  
         [0044]    [0044]FIG. 5 shows yet another embodiment of a closed intercept.  
         [0045]    [0045]FIG. 6 shows an embodiment of an open intercept.  
         [0046]    [0046]FIG. 7 shows a further embodiment of an open intercept.  
         [0047]    [0047]FIG. 8 shows another embodiment of a closed intercept that adapts an open intercept.  
         [0048]    [0048]FIG. 9 shows an embodiment of a wavelength add.  
         [0049]    [0049]FIG. 10 shows an embodiment of a wavelength drop.  
         [0050]    [0050]FIGS. 11 and 12 show an embodiment of protection switching in response to a communication impairment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0051]    [0051]FIG. 1 shows an embodiment of an optical network  100 . The optical network  100  has two physical rings, a working ring  110  and a protection ring  120 . Other embodiments can have one physical ring or more physical rings. The physical rings are “broken” by the open intercepts  130 . The open intercepts  130  can prevent each physical ring from forming a toplogical loop. Open intercepts  130  can substantially prevent one or more optical wavelengths from optically traversing the open intercepts  130 . For example, one or more wavelengths  142  travel into the open intercepts  130  on the working ring  110 . One or more wavelengths  144  travel from the open intercepts  130  on the working ring  110 . When one or more wavelengths  142  are compared with one or more wavelengths  144 , no wavelengths have optically traversed the open intercepts  130 . One or more wavelengths  142  and one or more wavelengths  144  may have no wavelengths in common. One or more wavelengths  142  and one or more wavelengths  144  may have one or more wavelengths in common, but which have undergone at least one optical-to-electrical-to-optical conversion in the process of traversing the open intercepts  130 . In this case, the one or more wavelengths in common between one or more wavelengths  142  and one or more wavelengths  144  have not optically traversed the open intercepts  130 , because of the intervening optical-to-electrical-to-optical conversion. One or more wavelengths  142  and one or more wavelengths  144  may have one or more wavelengths in common, but the optical signal or content carried by the one or more wavelengths in common may be totally different. For example, the one or more wavelengths in common of the one or more wavelengths  142  may be dropped by the open intercepts  130 , and then the one or more wavelengths in common of the one or more wavelengths  144  may be added by the open intercepts  130 . In this case as well, the one or more wavelengths in common between one or more wavelengths  142  and one or more wavelengths  144  have not optically traversed the open intercepts  130 , because of the intervening drops and adds.  
         [0052]    Open intercepts  130  can serve as optical decoupling nodes, optical anti-feedback nodes, and/or optical anti-crosstalk nodes. If there are two or more open intercepts  130 , the open intercepts  130  include optical waveguides, for example optical fiber, coupling together the open intercepts  130 . The open intercepts  130  are coupled by optical fiber to closed intercepts  150 , which can be optical nodes. If there are two or more closed intercepts  150 , the closed intercepts  150  include optical fiber coupling together the closed intercepts  150 . Open intercepts  130  have the ability to support up to as many logical rings as the number of open intercepts  130 , in each physical ring. There should be at least as many closed intercepts  150  as there are logical rings.  
         [0053]    In one embodiment with the open intercepts  130  being serially connected, incremental and modular upgrades to open intercepts  130  can be accomplished to add available bandwidth to the optical network. One or more open intercepts can be added to, for example, the last of the open intercepts  130  having free ports, and which can be the last of the open intercepts  130  in the series of open intercepts  130 . In some embodiments, such upgrades do not disturb traffic in the optical network.  
         [0054]    [0054]FIG. 2 shows an embodiment of open intercepts  132  and  134 . Also shown are optical fiber segments  136  leading, for example, to other open intercepts or to closed intercepts. The open intercepts  132  and  134  work with the protection ring  120  and the working ring  110  respectively. One embodiment of open intercepts  132  and  134  employs four port nodes.  
         [0055]    [0055]FIG. 3 shows an embodiment of closed intercepts  152  and  154 . Also shown are optical fiber segments  156  leading, for example, to other closed intercepts or to open intercepts. The closed intercepts  152  and  154  work with the working ring  110  and the protection ring  120  respectively. One embodiment of closed intercepts  152  and  154  employs two port nodes.  
         [0056]    [0056]FIG. 4 shows an embodiment of a closed intercept  400 . Closed intercepts can include an optical add drop multiplexer.  
         [0057]    [0057]FIG. 5 shows an embodiment of a closed intercept  500 . Closed intercept  500  includes a wavelength drop  510 , a wavelength add  520 , an optical receiver  530 , an optical transmitter  540 , processing electronics  550 , and electronic connections to other network processes  555 . Closed intercept  500  also includes Port A  560  and port B  570 . Port A  560  receives one or more wavelengths  502 . Wavelength drop  510  drops a wavelength from one or more wavelengths  502 , producing one or more wavelengths  504 . Wavelength add  520  adds a wavelength to one or more wavelengths  504 , producing one or more wavelengths  506  for port B  570 . The wavelength dropped by wavelength drop  510  goes to the optical receiver  530 , and becomes processed by processing electronics  550 . The wavelength added by wavelength add  520  come from the optical transmitter  540 , and before that is processed by processing electronics  550 , which communicates with electronic connections to other network processes  555 . In other embodiments, the wavelength add  520  precedes the wavelength drop  510 .  
         [0058]    [0058]FIG. 6 shows an embodiment of an open intercept  600 . Open intercepts  600  can include an optical add drop multiplexer.  
         [0059]    [0059]FIG. 7 shows an embodiment of an open intercept  700 . Open intercept  700  includes a wavelength drop  710 , a wavelength add  720 , an optical receiver  730 , an optical transmitter  740 , processing electronics  750 , and electronic connections to other network processes  755 . Open intercept  700  also includes Port A  790 , port B  770 , port C  760 , and port D  780 . Port C  760  receives one or more wavelengths  702 . Wavelength drop  710  drops a wavelength from one or more wavelengths  702 , producing one or more wavelengths  704  for port D  780 . Wavelength add  720  adds a wavelength to one or more wavelengths  706  from port A  790 , producing one or more wavelengths  708  for port B  770 . The wavelength dropped by wavelength drop  710  goes to the optical receiver  730 , and becomes processed by processing electronics  750 . The wavelength added by wavelength add  720  come from the optical transmitter  740 , and before that is processed by processing electronics  750 , which communicates with electronic connections to other network processes  755 .  
         [0060]    [0060]FIG. 8 shows another embodiment of a closed intercept  800 . Closed intercept  800  is similar to open intercept  700 , and includes an optical waveguide  895 , such as an optical fiber. The optical waveguide  895  couples port D  880  and port A  890 . Closed intercept  800  includes a wavelength drop  810 , a wavelength add  820 , an optical receiver  830 , an optical transmitter  840 , processing electronics  850 , and electronic connections to other network processes  855 . Open intercept  800  also includes Port A  890 , port B  870 , port C  860 , and port D  880 . Port C  860  receives one or more wavelengths  802 . Wavelength drop  710  drops a wavelength from one or more wavelengths  802 , producing one or more wavelengths  805  for port D  880 . Wavelength add  720  adds a wavelength to one or more wavelengths  805  from port A  890 , producing one or more wavelengths  808  for port B  870 . The wavelength dropped by wavelength drop  810  goes to the optical receiver  830 , and becomes processed by processing electronics  850 . The wavelength added by wavelength add  820  come from the optical transmitter  840 , and before that is processed by processing electronics  850 , which communicates with electronic connections to other network processes  855 .  
         [0061]    Thus, an embodiment with more ports than necessary can replace an embodiment with fewer ports.  
         [0062]    [0062]FIG. 9 shows an embodiment of a wavelength add  900 . Wavelength add  900  receives one or more wavelengths  910 , adds a wavelength  920 , and produces one or more wavelengths  930 . One or more wavelengths  930  include one or more wavelengths  910  and the wavelength  920 . One embodiment of the wavelength add  900  adds only a wavelength to a stream of wavelengths.  
         [0063]    Another embodiment of the wavelength add adds multiple wavelengths to a stream of wavelengths, along with corresponding multiple optical transmitters.  
         [0064]    In some embodiments, multiple open intercepts, multiple closed intercepts, and/or one or more open intercepts and one or more closed intercepts can be in one housing, for example, where each intercept includes one or more parts of FIGS. 5, 7, and/or  8 .  
         [0065]    [0065]FIG. 10 shows an embodiment of a wavelength drop  1000 . Wavelength drop  1000  receives one or more wavelengths  1010 , drops a wavelength  1020 , and produces one or more wavelengths  1030 . One or more wavelengths  1030  include one or more wavelengths  1010  except for the wavelength  1020 . One embodiment of the wavelength drop  900  drops only a wavelength from a stream of wavelengths.  
         [0066]    Another embodiment of the wavelength drop drops multiple wavelengths from a stream of wavelengths, along with corresponding multiple optical receivers.  
         [0067]    [0067]FIGS. 11 and 12 demonstrate an embodiment adapted to perform one method of protection switching in response to a communication impairment.  
         [0068]    [0068]FIG. 11 shows an optical network  1100  with a working ring  1110 , a protection ring  1120 , open intercepts  1130 , and closed intercepts  1150 . A communication impairment occurs, for example a fiber cut  1160  between closed intercepts  1155 . Prior to the fiber cut, two open loops exist, the working ring  1110  and the protection ring  1120 . Both open loops are terminated by the open intercepts  1130 .  
         [0069]    [0069]FIG. 12 shows an embodiment of one response to a communication impairment. FIG. 12 shows an optical network  1200  with a working ring  1210 , a protection ring  1220 , open intercepts  1230 , closed intercepts  1250 , and closed intercepts by the fiber cut  1253  and  1257 . Responsive to the communication impairment, protection switching occurs. The dashed line shows a data path  1270 , such as for a new logical ring. After the fiber cut, one topological loop exists for the data path  1270 , which may no longer be transparent. One end of the data path  1270  begins at the open intercepts  1230  of the working ring  1210 , for example. The data path  1270  continues through the working ring  1210  until reaching one of the closed intercepts  1253  by the fiber cut on the working ring  1210 . The data path  1270  moves from the closed intercept  1253  by the fiber cut on the working ring  1210  to the closed intercept  1253  by the fiber cut on the protection ring  1220 , perhaps undergoing optical-to-electrical-to-optical conversions. The data path  1270  continues through the protection ring  1220  and loops through the open intercepts  1230  on the protection ring, perhaps undergoing optical-to-electrical-to-optical conversions. Then, the data path  1270  continues through the protection ring  1220  until reaching one of the closed intercepts  1257  by the fiber cut on the protection ring  1220 . The data path  1270  moves from the closed intercept  1257  by the fiber cut on the protection ring  1220  to the closed intercept  1257  by the fiber cut on the working ring  1210 , perhaps undergoing optical-to-electrical-to-optical conversions. The data path  1270  then ends with the open intercepts  1230  on the working ring  1210 . This can be implemented, for example with standard SONET UPSR (unidirectional path switching ring) and others. In some embodiments, protection is enabled for single-fiber cuts.  
         [0070]    When a claim or a claim limitation or part of a claim limitation “comprises A and B” or “includes A and B”, the claim or the claim limitation or the part of a claim limitation is open ended, allowing further inclusion of, for example, C, or C and D, etc.