The application claims priority under 35 U.S.C. §§119 and/or 365 to  9802169-4  filed in Sweden on Jun. 17, 1998; the entire content of which is hereby incorporated by reference: 
     TECHNICAL FIELD 
     The invention relates to a communications network and also to an add and drop node and an error handling method in such a network. 
     BACKGROUND 
     There exist different types of communication networks, with different ways of taking care of errors. There also exist different types of multiplexing in communication networks, such as frequency division multiplexing (FDM), time division multiplexing (TDM) and wavelength division multiplexing (WDM). 
     Optical multi-channel systems employing wavelength multiplexing are used both in new networks and in order to enhance the transmission capability of existing optical fiber networks. Thus, information channels that previously had to be transmitted on a plurality of separate fiber pairs can be forwarded in a single fiber pair in WDM networks. 
     Using optical wavelength division multiplexed channels means that a plurality of serial information signals, i.e. a plurality of serial binary signals, are transmitted on the same optical fiber. Each serial signal is modulated on a carrier of its own, having a definite wavelength clearly separated from the other carriers. These modulated light signals are then combined in an optical coupler or optical multiplexer arrangement to a composite light signal on the considered optical fiber. The signal modulated on a carrier together with the carrier can be called a channel or traffic channel. 
     Optical wave length multiplexing can be used in different optical fiber network configurations or architectures. Such an architecture is the FlexBus™ concept as described in B. S. Johansson et al., “Flexible bus: A self-restoring optical ADM ring architecture”, Electronics Letters, Dec. 5, 1996, Vol. 32, No. 25, and U.S. Pat. No. 5,680,235, this architecture comprising a ring configuration of optical links connecting a plurality of nodes. The FlexBus™ concept has emanated from the need for protecting ring networks against fiber cuts and optical amplifier failures, and to solve the problem, often associated with optical ring network architectures, of circulating signals and noise. 
     In the FlexBus™ architecture one section or segment of the fiber ring is always made passive or inactive by means of optical switches or amplifiers. This intentionally introduced break effectively eliminates all problems associated with circulating signals and hence allows that less circuit components having lower performance can be used. In the case of a real failure of a section or link, the segment being intentionally made inactive is logically “moved” from its former position to the failed section, by making the inactive segment active and the faulty segment inactive, what also is called that “the bus flexes”, and thereby the traffic is restored. 
     Different developments of the FlexBus™ may be found in for example WO 96/31025 and WO 96/24998. 
     SUMMARY 
     One problem with FlexBus™ and many other error handling algorithms in communication networks is that they can only take care of one error at the time. In FlexBus™ this is due to the fact that when a deactivated segment contains a fault it cannot be logically moved as described above until the fault is repaired, because else the bus would not work. 
     An object with the invention is to solve this problem by using protection links in parallel with the ordinary links. If a fault occurs the deactivated segment will logically move to the fault as usual. A bypass connection is made over protecting links parallel to the first deactivated segment and a second deactivated segment is created instead of the now deconnected deactivated faulty segment. 
     Since the second deactivated segment does not contain any fault it is possible to logically move in the case of the occurrence of a second fault. When the second deactivated segment has logically moved to the second fault a third deactivated segment will, in the same way, be created in parallel to the second deactivated segment. Thus, the third deactivated segment is possible to logically move in the case of a third fault etc. This is of course not extendable forever. It depends on the look of the net of protection links and it could eventually happen that the new fault occurs in a place where it is not remediable. It rarely occurs more than one or two errors at the same time though, so in practise there will be no problem. 
     The advantages with this are that a simple and cheap error handling method is provided, which is capable of taking care of many occurring errors. 
     If the links are also physically parallel a break in a protection link will probably occur at the same time as a break in an ordinary link. This problem is solved in an embodiment of the invention in that multi direction cross points are used to connect different protecting links in a more flexible way, forming a sort of net. 
     The advantages with this embodiment are that it is safer, that already existing links may be used as protecting links and that a more flexible bus reconfiguration is possible in the case when a larger part of the ordinary ring is faulty. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of non-limiting embodiments with reference to the accompanying drawings, in which 
     FIG. 1 is a general schematic view of an optical fiber network of WDM type using the flexible bus architecture, 
     FIG. 2 is a block diagram of a prior art add and drop node of a simple configuration and intended to be used in the network of FIG. 1, 
     FIG. 3 a  is a general schematic view of a network according to the invention, with a working deactivated segment Si, 
     FIG. 3 b  shows the network in FIG. 3 a  with a non-working deactivated segment S 1  and a working deactivated segment S 2 , 
     FIG. 3 c  shows the network in FIG. 3 a, b  with a non-working working deactivated segment S 1 , a non-working deactivated segment S 2  and a working deactivated segment S 3 , 
     FIG. 3 d  shows the network in FIG. 3 a, b  with a non-working deactivated segment S 1  and a working deactivated segment S 2 , 
     FIG. 4 is a block diagram of a multi-direction cross point. 
     FIGS. 5 a  and  5   b  are block diagrams of add and drop nodes according to a first embodiment, 
     FIG. 6 is a block diagram of an add and drop node according to a second embodiment, 
     FIG. 7 is a block diagram of an add and drop node according to a third embodiment, 
     FIG. 8 is a block diagram of an add and drop node according to a fourth embodiment, 
     FIG. 9 is a block diagram of an add and drop node according to a fifth embodiment, 
     FIG. 10 is a block diagram of an add and drop node according to a sixth embodiment, and 
     FIG. 11 is a block diagram of an add and drop node according to a seventh embodiment. 
    
    
     DETAILED DESCRIPTION 
     The concept of the known FlexBus™ will be described below in more detail. It is to be understood that even though only Wavelength Division Multiplexing (WDM) will be described, the error handling method works also in other multiplexing techniques, using nodes with a corresponding design. 
     A basic flexible bus structure for WDM communication on optical fibers is illustrated in FIG. 1. A plurality of optical add and drop nodes N are connected to each other by links  1  to form a network or bus comprising a physical ring structure having as basic elements a pair of optical fibers  2 ,  3  connected to form two parallel fiber rings. 
     Each fiber ring carries light propagating in a definite direction, the propagation directions of the two rings being opposite to each other. In one of the fiber rings light always propagates in the counter-clockwise direction, in the embodiment of FIG. 1 the inner ring  2 , this direction being called the east direction hereinafter. In the other one  3  of the rings of the pair of fiber rings light always propagates in the opposite direction, i.e. in the clockwise direction, as seen in FIG. 1, this direction being called the west direction. These directions are taken as related to a considered node N of the network. In order to understand the concepts of “east” and “west” correctly it could be helpful to picture the ring as the Equator. 
     A node N in the bus structure is thus connected physically to two neighbouring nodes. The connections of a considered node N include a west physical link  1   w  comprising a west line cable  4   w  and an east physical link  1   e  comprising an east line cable  4   e , the other end of each link  1   w ,  1   e  being connected to a neighbouring node. 
     Each piece  4   w ,  4   e  of line cables comprises a pair of optical fibers  2   w ,  3   w  and  2   e ,  3   e  respectively. In one  2   w ,  2   e  of the fibers of a fiber pair in a link  4   w ,  4   e  light always propagates in one direction, in the anti-clockwise direction as seen in FIG.  1 . In the other one  3   w ,  3   e  of the fibers of the fiber pair in a link  4   w ,  4   e  light always propagates in the opposite direction, in the clockwise direction, as seen in FIG.  1 . 
     Furthermore, a node N may contain receivers  5  and transmitters  6  for converting optical signals to electrical signals and vice versa, the electrical signals being transmitted to or received from other devices, links or networks, not shown. 
     One segment of the ring structure is always deactivated, see the link  7  in FIG. 1, so that at least no light carrying traffic to be transferred in the network can pass through, in any direction. Pure signalling may be permitted to pass the deactivated segment, e.g. by using a special channel for signalling which bypasses the node in a special path. Thus in one way also the deactivated segment may be considered as a part of the bus. 
     The deactivated segment prevents that light signals and ASE noise circulate along the ring structure in several turns, ASE noise being amplified spontaneous emission particularly from in-line optical amplifiers which usually are included in the nodes N. 
     When there is a failure in a link between neighbouring nodes N, the network can be reconfigured so that this link/segment will then be the deactivated one, whereas the previously deactivated segment  7  now is activated and operates like the other active links  1  in the ring structure passing traffic in the two opposite directions. This will, in this application, be referred to as “moving” a deactivated segment, “moving” in this context meaning “logically moving”. A flag is set somewhere indicating that the present deactivated segment contains an error and thus cannot be “moved”. 
     A basic structure of a node N in the basic flexible bus structure of FIG. 1 is shown in the block diagram of FIG.  2 . The optical WDM traffic comprising a plurality of WDM channels having definite, separate wavelengths, each channel occupying a wavelength band around the wavelength of the channel, enters the node from the left or west and from the right or east directions on the fibers  2   w  and  3   e  respectively. 
     The incoming signals can be amplified in optional optical preamplifiers P w , P e  respectively. The incoming light is then split in drop couplers  11   w ,  11   e . These couplers are optical power splitters that feed a part of the total power of the light propagating in one direction in the bus, through an optical combining coupler or power combiner  12 , adding the deflected power portions from each direction to each other, to a bank  13  of filters, which can also be called an optical demultiplexer, having one or more band-blocking filters for wavelengths used in the transmission in the network. Thus the filter bank  13  filters out channels, each channel carrying information in a definite wavelength band, which are then forwarded to opto-electrical receivers  5 , one optical receiver being arranged for each received channel. 
     The remaining part of the light power split in the drop couplers  11   w ,  11   e  is forwarded through the node N, via optional band-blocking filters  17   e ,  17   w  and is in add couplers  14   e ,  14   w  mixed with new traffic to be added in the node. This new traffic is obtained from electro-optical transmitters  6 , where each transmitter transmits optical signals of a wavelength band or a channel different from that of the other transmitters. The output signals of the transmitters  6  are added to each other in an optical combining coupler or optical multiplexer  15 . The resulting signal is split in a splitting coupler  16  in two parts having equal power, each of the two parts being transmitted to one of the add couplers  14   e ,  14   w.    
     The light signals obtained from the add couplers  14   e ,  14   w  for each direction are fed to the fibers  3   w ,  2   e , which are contained in the links connected to the node and carrying light going out from the node, through optional optical booster amplifiers Be, Bw. 
     The west side or the east side amplifiers Pw, Bw, or Pe, Be respectively can be used to deactivate the respective links or segments connecting the node to the two neighbouring nodes in the case where this link is to be the deactivated one, such as in the case of a failure of this link, which can be caused e.g. by one of the fibers of the pair of the link being broken or by one of the optical amplifiers connected to this link being faulty. 
     Most of the bus structure and node design according to FIGS. 1,  2  are described in the above cited article by B. S. Johansson et al. and in the cited U.S. Pat. No. 08/421,734. 
     In FIG. 3 a  is shown according to the invention a communication network with, as an example, eight nodes N 1 -N 8  connected in a ring as a FlexBus™ network as described above. Somewhere a deactivated segment S 1  is situated. Extra bi-directional links L 1 -L 11  for protection are connected between the different nodes N 1 -N 8 , via multi direction cross points C 1 -C 3  or directly between the nodes. 
     If an error occurs in the ordinary communication network, traffic is instead directed via one or more of the protection links L 1 -L 11  so that the error is by-passed. 
     The connections between the nodes N 1 -N 8  may look in different ways, as may be seen in FIG. 3 a . The important thing is that there is an extra way to get from one node to another node, without using the ordinary links. 
     Of course, it is possible to have, a bi-directional spare ring parallel to the ordinary ring, like the ninth protecting link L 9  between the fifth and sixth nodes N 5 , N 6 . However, if you accidentally cut the ordinary ring, then you probably will cut the spare ring too. Using multi direction cross points will not only make it safer, but allows already existing links to be used and also allows a more flexible bus reconfiguration in the case when a larger part of the ordinary ring is erroneous. 
     In FIG. 3 a  if a first fault F 1  occurs for example between the first node N 1  and the second node N 2  the deactivated segment S 1 , wherever it is placed from the beginning, will move to the faulty part of the communication network, that is to the link between the first node N 1  and the second node N 2 , as shown in FIG. 3 b.    
     A flag will be set, indicating that the deactivated segment S 1  contains a fault and thus cannot be moved until the error is repaired. The bus will work, but if another fault should occur the bus will not work anymore, since the deactivated segment should not be moved if there is a fault in the link. Thus, a “non-working” and unmoveable deactivated segment S 1  containing a fault is obtained. 
     This problem is solved by the invention in that when a fault has occurred a connection is created over protecting links bypassing the non-working deactivated segment S 1 . 
     An extra “working” and movable deactivated segment S 2  is thus created between the first node N 1  and the second node N 2  using the links L 1  and L 2  via the first cross point C 1 . The non-working deactivated segment S 1  is deconnected from the bus and bypassed by the working deactivated segment S 2 , which forms part of the bus instead. 
     Of course it is not strictly necessary to place the new working deactivated segment S 2  exactly between the same two nodes N 1 , N 2  as the old non-working deactivated segment S 1  as shown in FIG. 3 b . It is possible to bypass the non-working deactivated segment S 1  with a usual active link and place the new working deactivated segment S 2  somewhere else on the bus, for example between the sixth N 6  and seventh N 7  node. 
     When the fault is repaired, the non-working deactivated segment S 1  is reset and once again working and a part of the bus. The protection paths and the second deactivated segment S 2  are no longer used, and the situation is once more that of FIG. 3 a , but with the deactivated segment S 1  between the first and the second nodes N 1 , N 2 . 
     Now, it could happen that a second fault F 2  occurs before the first fault F 1  is repaired, for example between the fourth node N 4  and the fifth node N 5 , see FIG. 3 c . In this case the working deactivated segment S 2  moves to the faulty part of the bus and becomes non-working and no longer a part of the bus. A new working deactivated segment S 3  is created via the fifth L 5  and sixth L 6  links and the second cross point C 2 , thus bypassing the now non-working deactivated segment S 2 . 
     It could instead happen that the second fault F 2  occurs in the link next to the link with the first fault F 1 , that is between the second node N 2  and the third node N 3 , as shown in FIG. 3 d . In this case an alternative can be to bypass both the first fault F 1  and the second fault F 2  at the same time if the node N 2  between the two faults can be dispensed with. This is done by rearranging the working deactivated segment S 2 , so as to instead encompassing the protecting links L 1  and L 3 . 
     The construction of a multi-direction cross point is simple, see FIG. 4 for an example with a possibility to connect four external bi-directional links. There are four optional preamplifiers P 1 , P 2 , P 3 , P 4 , four booster amplifiers B 1 , B 2 , B 3 , B 4 , four power splitters  21 ,  22 ,  23 ,  24  and four power combiners  25 ,  26 ,  27 ,  28 . On each incoming external link there is a preamplifier P 1 , P 2 , P 3 , P 4  connected to a power splitter  21 ,  22 ,  23 ,  24  connected with three of the power combiners  25 ,  26 ,  27 ,  28 . Each power combiner  25 ,  26 ,  27 ,  28  is in its turn connected to a booster amplifier B 1 , B 2 , B 3 , B 4 . 
     This arrangement permits connection with any two external bi-directional links, by simply activating the preamplifiers P 1 , P 2 , P 3 , P 4  and booster amplifiers B 1 , B 2 , B 3 , B 4  on the two external links that are to be connected and by deactivating the other amplifiers. 
     To determine which amplifiers are to be activated and not, for example a separate signalling channel may be used, which does not carry any traffic, but only information of the status of the network, instructions for flexing and connecting protection links and such. 
     If the net of protecting links is not too complicated the instructions for connecting the protecting links may also be simple. For example, if the fault is on the west side of the node, as for the first node N 1  in FIG. 3 b , the instructions may in this case be “turn left until the next node is reached”, which in this case would be the second node N 2 . The instructions from the second node N 2  to the first node N 1  would then at the same time be “turn right until the next node is reached”. The connections in the multi-direction cross points, in this case only one, will be set up accordingly. 
     A node in a network using the invention may look in many ways. Some of them will be described below, which all more or less use the WDM node in FIG. 2 as a basis. The same reference numbers will be used for corresponding features. It is possible to construct new nodes by combining features from different figures. Of course, also nodes in FDM and TDM networks may be modified in a corresponding way. 
     One embodiment of a node is shown in FIG. 5 a , which is rearranged in the equivalent FIG. 5 b  for sake of clearness only. In addition to the two usual optional preamplifiers Pe, Pw and the two booster amplifiers Be, Bw, there is a third optional preamplifier Pp and a third booster amplifier Bp for connection with a protection link. There are also transmitters  6  with multiplexers  15 , receivers  5  with demultiplexers  13 , power splitters  11   e ,  11   w ,  11   p ,  16 ,  18   p ,  18   e ,  18   w , power combiners  12 ,  14   p ,  14   e ,  14   w  and optionally three blocking filters  17   p ,  17   e ,  17   w.    
     Each of the preamplifiers Pp, Pw, Pe is connected with one of the three first power splitters  11   p ,  11   w   11   e . Each first power splitter  11   p ,  11   w ,  11   e  is in its turn connected with, on the one hand the demultiplexers  13  via a power combiner  12 , and on the other hand one of the three second power splitters  18   p ,  18   e ,  18   w , either directly or via one of the optional blocking filters  17   p ,  17   e ,  17   w.    
     Each of the second power splitters  18   p ,  18   e ,  18   w  is connected with two of the power combiners  14   p ,  14   e ,  14   w . Each of the power combiners  14   p ,  14   e ,  14   w  is thus connected with two of the second power splitters  18   p ,  18   e ,  18   w , but also with one of the booster amplifiers Bp, Be, Bw and with a power splitter  16  connected to the multiplexers  15 . 
     The connections are thus made so as to provide connection between east and west, between east and the protection link or between west and the protection link. 
     Of course, it would also be possible to have two protection links connected to the node; one for the east direction and one for the west direction, compare with the upper parts of FIGS. 10 and 11. 
     The invention may also be used, in more complicated nodes. In the following description FIGS. 6,  8  and  10  show nodes that are useful for all traffic cases; while in FIGS. 7,  9  and  11  nodes are shown for use in a system that reuse wavelengths in combination with a situation when all traffic channels are between adjacent nodes. They are all drawn in the rearranged way as in FIG. 5 b  for clearness only. The method of reusing wavelengths is beyond the scope of this application and will only be described briefly. Interested readers are instead referred to the Swedish applications SE9802070-4 and SE9802071-2. 
     A wavelength channel is said to be terminated in a node if it is received in the node, i.e. if there is a receiver  5  for this channel in the node. A wavelength channel is said to be reused if it is used for transmission from a node, i.e. if there is a transmitter  6  for this wavelength in said node, and at the same time the same wavelength is used for transmitting information in the same direction by another node without overlap of the transmission paths. Termination or reusing of wavelengths may be achieved by using inline blocking filters  17   e ,  17   w ,  17   pe ,  17   pw.    
     Each of the blocking filters  17   e ,  17   w ,  17   pe ,  17   pw  in the considered node blocks only those wavelengths respectively which are terminated in the node for the respective direction. All wavelengths not terminated in the node, are just passed through the node in the east or west direction in a basically unaffected way. 
     In FIG. 6 each receiver  5   e ,  5   w  is connected to a switch  37   e ,  37   w , which chooses if the wavelength of the receiver is to be received from the west preamplifier Pw, via west demultiplexers  13   w  or from the east preamplifier Pe, via east demultiplexers  13   e . In a corresponding way each transmitter  6   e ,  6   w  is connected to a switch  38   e ,  38   w , which chooses if the wavelength of the transmitter is to be transmitted to the west booster amplifier Bw, via west multiplexers  15   w  or the east booster amplifier Be, via east multiplexers  15   e.    
     In this case the optional protectional preamplifier Pp and the protectional booster amplifier Bp, each are connected to a switch  31 ,  32 , respectively. Those switches  31 ,  32  serve the purpose of choosing east or west going wavelengths. 
     In order to achieve this the two outputs of the switch  31  of the protectional preamplifier Pp are connected each to a power combiner  33   w ,  33   e  followed by a power splitter  34   w ,  34   e . Each power combiner  33   w ,  33   e  is also connected to the output of a preamplifier Pw, Pe, while each power splitter  34   w ,  34   e  is connected to a band-blocking filter  17   e ,  17   w  and to one of the demultiplexers  13   w ,  13   e.    
     Thus, if the link east of the node is non-working the switch  31  of the protectional preamplifier Pp is placed in its “east” position and the wavelengths coming from the east direction are received over the protecting link. 
     In a similar way the two inputs of the switch  32  of the protectional booster amplifier Bp are connected each to a power splitter  36   w ,  36   e  preceded by a power combiner  35   w ,  35   e . Each power splitter  36   w ,  36   e  is also connected to the input of a booster amplifier Bw, Be, while each power combiner  34   w ,  34   e  is connected to one of the band-blocking filters  17   e ,  17   w  and to one of the multiplexers  15   w ,  15   e.    
     Thus, if the link east of the node is non-working the switch  32  of the protectional booster amplifier Bp is placed in its “east” position and the east wavelengths are transmitted over the protecting link. 
     FIG. 7 is very much like FIG. 6. A difference is that the receivers  5   e ,  5   w  are connected directly to the demultiplexers  13   e ,  13   w  and that switches  41   e ,  41   w  instead are connected between the demultiplexers  13   e ,  13   w  and the power splitters  34   w ,  34   e . In a corresponding way the transmitters  6   e ,  6   w  are connected directly to the multiplexers  15   e ,  15   w  and switches  42   e ,  42   w  are instead connected between the multiplexers  15   e ,  15   e  and the power splitters  35   w ,  35   e.    
     FIG. 8 is also very much like FIG.  6 . In this case, however, there are two protection preamplifiers Ppw, Ppe and two booster amplifiers Bpw, Bpe, instead of one preamplifier/booster amplifier with a switch. 
     FIG. 9 is in its turn similar to FIG. 7, with the exception that there are two protection preamplifiers Ppw, Ppe and two booster amplifiers Bpw, Bpe, instead of one preamplifier/booster amplifier with a switch. 
     FIG. 10 is a combination of FIG.  8  and FIG. 5 b . Each preamplifier Ppw, Ppe, Pw, Pe is connected to demultiplexers  13   e ,  13   w , switches  37   e ,  37   w  and receivers  5   e ,  5   w  as in FIG. 8, via a first power splitter  51   pw ,  51   pe ,  51   w ,  51   e  each and one of two power combiners  54   e ,  54   w . The first power splitters  51   pw ,  51   pe ,  51   w ,  51   e  are also each connected to a second power splitter  52   pe ,  52   pw ,  52   e ,  52   w , via a band-blocking filter  17   pe ,  17   pw ,  17   e ,  17   w  each. The second power splitters  52   pe ,  52   pw ,  52   e ,  52   w  are also each connected to one protective and one “normal” booster amplifier Bpe, Bpw, Be, Bw, via power combiners  53   pe ,  53   pw ,  53   e ,  53   w . Finally the power combiners  53   pe ,  53   pw ,  53   e ,  53   w  are also connected to multiplexers  15   e ,  15   w , switches  38   e ,  38   w  and transmitters  6   e ,  6   w  as in FIG. 8, via one of two power splitters  55   e ,  55   w.    
     The same goes for FIG. 11, which is a combination of FIG.  9  and FIG. 5 b . Each preamplifier Ppw, Ppe, Pw, Pe is connected to switches  41   e ,  41   w , demultiplexers  13   e ,  13   w , and receivers  5   e ,  5   w  as in FIG. 9, via a first power splitter  51   pw ,  51   pe ,  51   w ,  51   e  each and two of four power combiners  61   e ,  61   w ,  62   e ,  62   w . The first power splitters  51   pw ,  51   pe ,  51   w ,  51   e are also each connected to a second power splitter  52   pe ,  52   pw ,  52   e ,  52   w , via a band-blocking filter  17   pe ,  17   pw ,  17   e ,  17   w  each. The second power splitters  52   pe ,  52   pw ,  52   e ,  52   w  are also each connected to one protective and one “normal” booster amplifier Bpe, Bpw, Be, Bw, via power combiners  53   pe ,  53   pw ,  53   e ,  53   w . Finally the power combiners  53   pe ,  53   pw ,  53   e ,  53   w  are also connected to switches  42   e ,  42   w , multiplexers  15   e ,  15   w , and transmitters  6   e ,  6   w  as in FIG. 8, via two of four power splitters  63   e ,  63   w ,  64   e ,  64   w.
A communications network comprising at least two nodes is disclosed. The network has two communication paths that carry traffic in opposite directions and work as a bi-directional bus. The network has a first inactive segment that carries no traffic and is arranged to allow the first inactive segment to be made active and another segment to be made inactive, especially in the case of fault in the other segment. The activation and inactivation logically "move" the first inactive segment. Additional protection paths are provided between the nodes.