Abstract:
According to the invention, working signals are transmitted between network elements via a first communication path. The protection signals are transmitted continuously or in case of a fault at the same wavelength as the working signals via a second communication path of an optical network while the polarization of the protection signals is adjusted orthogonal to the polarization of additional working signals which are transmitted at the same wavelength via sections of the second communication path such that no wavelengths need to be kept clear for protection signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is the US National Stage of International Application No. PCT/EP2004/000573, filed Jan. 23, 2004 and claims the benefit thereof. The International Application claims the benefits of German application No. 10308306.5, filed Feb. 26, 2003, both applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION  
       [0002]     The invention relates to method and an arrangement for the transmission of working signals and protection signals via optical data networks.  
       SUMMARY OF THE INVENTION  
       [0003]     Today, because of their wide transmission bandwidth and their low attenuation, glass fibers are used for the transmission of digital signals with high data rates. To enable the transmission capacity of the glass fiber to be utilized, a number of signals featuring different wavelengths (WDM signals) are transmitted in parallel over a fiber. In this case a number of transmission channels are combined into bands. 2-fiber systems are mostly used in which the data signal is transmitted in one direction in each case. Single-fiber systems are however also known, with which different frequency bands or interleaved data signals with particular frequencies are transmitted in either direction. 4-fiber systems are also known for increasing the transmission capacity or for providing protection switching.  
         [0004]     A ring structure shown in  FIG. 1  is described in a contribution to the “22nd Conference on Optical Communication—ECOC 96, Oslo, WeB.2.3, 178 pages 3.51-3.54 “First results of an experimental Coloured Section Ring”, in which adjacent ADD/DROP multiplexers are connected to each other bidirectionally via two fibers in each case. For each transmission section between two adjacent DROP multiplexers only one wavelength is needed for both directions of transmission on each fiber. However different wavelengths are used on all transmission sections of the ring. The signals are added or dropped via optical ADD/DROP multiplexers which contain optical fibers. If for example a working connection is interrupted by a broken fiber, a protection connection is established via the (mostly) longer intact ring section using the same wavelength, i.e. the working signals previously sent over the interrupted section are “looped back” and transmitted over the intact sections. The advantage of this is that the wavelength does not need to be converted. Instead of for an individual wavelength this method can naturally also be employed for a number of wavelengths and transmission bands. Although the method offers the advantage that the wavelength for protection connections does not need to be converted, it does however sharply reduce the transmission capacity.  
         [0005]     The object of the invention is to provide protection connections which do not have an adverse effect on transmission capacity.  
         [0006]     This object is achieved by the claims, resulting in orthogonally polarized protection signals being transmitted. Furthermore, a suitable arrangement for this is also claimed.  
         [0007]     The method in accordance with the invention is particularly advantageous to implement in ring networks in which merely the transmitted signals of the two elements adjacent to an interruption point are looped back in the opposite direction using a polarization setter.  
         [0008]     The method in accordance with the invention can advantageously be used for 1:1 protection (the disturbed signal is diverted via an undisturbed connection path) and for 1+1 protection (a protection signal is always transmitted as well) for all network structures, especially for ring structures.  
         [0009]     The invention will now be explained in more detail below on the basis of an exemplary embodiment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows a 2-fiber ring network,  
         [0011]      FIG. 2  shows the 2-fiber ring network with protection switching,  
         [0012]      FIG. 3  shows a network element,  
         [0013]      FIG. 4  shows a receive section of the network element  
         [0014]      FIG. 5  shows a 2-fiber ring network with 1+1 protection,  
         [0015]      FIG. 6  shows a protection-switching device, and  
         [0016]      FIG. 7  shows a 4-fiber ring network with span protection. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0017]      FIG. 1  shows an extended 2-fiber ring network structure. A ring network formed with two fibers F 1  and F 2  features the network elements NE 1  to NE 6 . For a connection (channel) between the network element NE 1  and the network element NE 3  a wavelength λ 1  is used, with a working signal λ 1 E being transmitted in an easterly direction over the first fiber F 1  and a working signal λ 1 W with the same wavelength being transmitted in the opposite direction. The same wavelength can also be used for transmission for example between the network elements in NE 4  and NE 6 . The corresponding signals are labeled λ 1 S and λ 1 N. Naturally there will generally be more channels with other wavelengths present for connecting the other network elements, but these can be left out of our considerations for the purposes of explaining the invention.  
         [0018]      FIG. 2  shows a fiber break at an interruption point between the network elements. The connection path NE 1 -NE 2 -NE 3  is interrupted. In the known way the send signals must now be “Iooped back” by the network elements NE 2  and NE 3  adjacent to the interruption point through switchover devices U 1  and U 2  (possibly there is also already a loopback in the network elements NE 2  and NE 3 ) and is transmitted in the opposite direction via the undisturbed part of the ring network, the second connection path NE 1 -NE 6 -NE 5 -NE 4 -NE 3 . The signal λ 1 E is consequently transmitted over the other fiber F 2  as protection signal λ 1 EP and the signal λ 1 W is transmitted over fiber F 1  as protection signal λ 1 WP. So that a signal of this wavelength does not collide with other signals of the same wavelength, in a conventional system either this wavelength would have to be kept free on the remaining part of the ring, which results in the Coloured Section Ring described at the start, or the wavelength must be converted into another wavelength used for protection data connections only.  
         [0019]     In the case shown in  FIG. 2  signals with the same wavelength λ 1  are transmitted between the network elements NE 1  and NE 3  and also the network elements NE 4  and NE 6 . The working signals transmitted between the network elements NE 4  and NE 6  are labeled λ 1 S and λ 1 N in order to distinguish between them. Before the merging of the signals λ 1 S and λ 1 EP or λ 1 N and λ 1 WP the signals transmitted over a common fiber in each case must be (at least approximately) aligned orthogonally polarized to each other. This is undertaken for the signals λ 1 S and λ 1 EP expediently in the network element NE 6  by changing the polarization of the protection signal λ 1 EP.  
         [0020]     The main parts of the network element NE 6  are shown in  FIG. 3 . The demultiplexer DMUX splits a received WDM (Wavelength Division Multiplex) signal up into individual signals λ 1  to λn. The signal λn is (together with other signals) “looped through” and merged in the multiplexer MUX again with possibly newly added signals into a WDM signal.  
         [0021]     The protection signal λ 1 S fed via the series circuit of a polarization setter POLS 1 , a polarization divider POLD and a polarization multiplexer PMUX. The polarization divider POLD is not required here for the circuit to function but must be present in each network element in order to separate a working signal from the protection signal and enable one of the signals to be dropped. In this example the protection signal λ 1 EP is however looped through the network element. In the polarization multiplexer PMUX the protection signal λ 1 EP is merged with the working signal λ 1 S of the same wavelength. If the polarization of the signal λ 1 S is also not known, the two polarization setters POLS 1  and POLS 2  are required. The same applies to the protection signal λ 1 WP, for which the polarization is set in the network element NE 4  orthogonally to the polarization of the signal λ 1 N.  
         [0022]     In the network element NE 4  the signal λ 1 S is dropped and the protection signal λ 1 EP looped through. In  FIG. 4  only the parts of the network element NE 4  significant for the splitting of the working and protection signal are shown. These are the polarization setter POLS 4  and the polarization divider POLD 4 , which may have a polarization multiplexer PMUX 4  connected downstream if necessary.  
         [0023]     The working signal λ 1 S and the protection signal λ 1 EP are fed to the polarization setter POLS 4  which matches the polarizations of these signals to the orientation of the polarization divider POLD 4 . This splits the signal mixture into the working signal λ 1 S which is dropped here and the protection signal λ 1 EP which is forwarded to the network element NE 3 .  
         [0024]     The signal λ 1 N sent in the opposite direction is merged in accordance with  FIG. 3  with the protection signal λ 1 WP.  
         [0025]     The network element NE 3 , like all network elements, has the same circuit arrangement. The protection signal λ 1 EP is received after being fed back to the same port and is dropped there.  
         [0026]     The mutual influence of working signal and orthogonally polarized protection signal is slight in transmission links with Polarization Mode Dispersion (PMD) whenever the transmitted data signals exhibit the same data rates and (their bits or modulation section) have a specific phase angle to each other (with NRZ 0°). Therefore a synchronization of the protection signal can be worthwhile.  
         [0027]     Instead of the 1:1 protection described, a 1+1 protection can be used, in which the protection signal is transmitted continuously and therefore a faster switchover is made possible. A ring network with 1+1 protection is shown in  FIG. 5 . The signal λ 1 E—shown by a dashed line—is transmitted from the network element NE 1  via the network element NE 2  to the network element NE 3  in the opposite direction—shown by dashed and dotted line—the signal λ 1 W. Simultaneously the protection signal λ 1 EP, also shown by a dashed line, is transmitted via the network element NEG, NE 5  and NE 4 , and in the opposite direction the protection signal λ 1 WP, also shown as a dashed and dotted line is transmitted. In the event of a fault no loop is created through the network elements NE 2  and NE 3 , since the protection signals, also shown dashed or dashed and dotted, can already be sent and received via the intact loop section. In the network element there only needs to be a switchover between the working signal and the associated orthogonally polarized protection signal. This is shown simplified in  FIG. 6 . The working signal is fed from a first access port via a polarization setter POLS 3  with downstream polarization divider POLD 4  while the protection signal is fed via a second input port and a polarization setter POLS 4  with downstream polarization divider POLDS. In the protection case there only needs to be a switchover between these two receiver signals λ 1 E and λ 1 EP by a switchover device UE.  
         [0028]      FIG. 7  shows a 4-fiber ring network. Two fiber pairs F 1 , F 2  and F 3 , 4 are laid spatially separated. In the case of a fault or interruption of one of the fiber pairs F 1 , F 2  the signals λ 1 E and λ 1 W transmitted between the network elements NE 1  and NE 3  on the fibers F 1  and F 2  are diverted in the network elements NE 2  and NE 3  (NE 1  and NE 3  is also possible) via the fibers F 3  and F 4 , in which case they are polarized orthogonally to the further working signals λ 1 S and λ 1 N exhibiting the same working signals. Thus the disturbed fiber section (span) NE 2 -NE 3  is bridged without adversely affecting the further working signals.  
         [0029]     It should also be added that both with 2-fiber ring networks and also with 4-fiber ring networks all the wavelengths of the orthogonal protection “channels” can be used for low-priority traffic, which is then interrupted however in the event of a fault, in order to transmit protection signals with higher priority.