Abstract:
Light carrying information for access terminals is carried via optical fibers. Information for terminals from groups is multiplexed over different time-slots and different communication wavelengths of the light in the fiber for a group. The information is passed between the fibers and the transport network via transceivers. The use of the transceivers is multiplexed between the optical fibers. Each transceiver passes information for selectable light guides at selectable communication wavelengths in different timeslots.

Description:
CROSS-RELATED TO RELATED APPLICATION  
         [0001]    This application claims the benefit of priority from corresponding European Application Serial No. 01305491.1, filed Jun. 25, 2001.  
         TECHNICAL FIELD  
         [0002]    The invention relates to a communications network and, in particular, to connections between a transport network and access terminals.  
         BACKGROUND OF THE INVENTION  
         [0003]    Communications networks are known that use a passive optical network to connect a transport network, such as the telephone transport network, to access terminals, such as end user terminals (e.g., see European Patent Application No. 1061764). As described in this reference, the passive optical network contains a bundle of N optical fibers and each of the fibers connects to a plurality of access terminals. Each single optical fiber is able to carry light of a number of M different communication wavelengths, so the network is capable of transmitting information at a number M of wavelengths in each fiber. Since a passive optical network is used, information may be transported to or from a terminal through the N optical fibers.  
           [0004]    This type of network may employ statistical multiplexing of the use of the optical fibers and wavelengths to provide a large information transport capacity at relatively low cost. Statistical multiplexing is based on the activity pattern that is characteristic of access terminals. These terminals are only active intermittently and, as a result, only need a low transport capacity on average. However, when active, an access terminal may need a maximum transport capacity that is much larger than the average capacity. This allows multiplexing of the use of fibers and wavelengths between terminals, fibers and wavelengths being allocated to specific access terminals only when needed. Thus, much less capacity is needed than the product of the maximum capacity per access terminal and the number of access terminals, although each individual terminal may use the maximum capacity from time to time.  
           [0005]    Although the assignment of terminals to times of transport and wavelengths is addressed in the aforementioned patent application, prior art arrangements do not address the way that light of different wavelengths is optically provided in the different fibers in such a network. One possible way to do this would be to provide a set of M light sources, light modulators and light demodulators for M different wavelengths for each of the N optical fibers. However, such a solution is expensive because it would require a large number (N×M) of light sources, light modulators and light demodulators.  
         SUMMARY OF THE INVENTION  
         [0006]    The number of components needed for multiplexing transport of optically modulated information to and from access terminals is substantially reduced according to the principles of the invention. More specifically, a method is provided for multiplexing information transport between a plurality of access terminals and a transport network. Light carrying information for (i.e. to or from) different access terminals is transported via N light guides (e.g. optical fibers) and passed to and/or from a transport network by L transceivers. Information for a group of different access terminals is multiplexed over the same fiber by time slot multiplexing and wavelength multiplexing. Each transceiver passes information from a selectable communications wavelength.  
           [0007]    According to the principles of the invention, the use of transceivers is multiplexed between the different light guides. Depending on the demand for information transport, different transceivers may at one time pass information with different communications wavelengths for the same fiber and, at another time, they may pass information with the same wavelength (or different wavelengths) for different fibers. At other times, mixtures of these extremes may occur. Thus, the number of transceivers that is needed to pass information from the access terminals to the transport network can be reduced. Preferably, the number of transceivers (L) is smaller than the number of light guides (N). In any case, the number L of transceivers can be considerably smaller than the number M of communications wavelengths used times the number of light guides N.  
           [0008]    In an illustrative embodiment, a connection between the light guides and the transceivers is realized by means of a wavelength independent cross-connect unit and filters which pass a selectable wavelength between the cross-connect unit and the transceivers. Thus, multiplexing can be realized with a relatively small number of components.  
           [0009]    In another embodiment, the connection between the light guides and the transceivers is realized by means of a set of cross-connect units, each performing the cross-connection for one of the communications wavelengths, and wavelength splitters and combiners to split the light from the light guides for use by the different cross-connect units and to merge the light from the different cross-connect units respectively.  
           [0010]    Preferably both the use of the receivers that pass information “upstream”, from the access terminals to the network, and of transmitters that pass information “downstream”, from the transport network to the access terminals, is multiplexed over the N different light guides and M different wavelengths. For this purpose, a further cross-connect unit may be provided, so that there are different cross-connects for upstream and downstream information transport.  
           [0011]    As an alternative, the cross-connect unit that is used for upstream information might also be used for “downstream” communication. However, this may reduce the transport capacity to half-duplex, compared to full-duplex with the use of two cross-connect units, one for upstream traffic and one for downstream traffic. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0012]    A more complete understanding of the invention may be obtained from consideration of the following detailed description of the invention in conjunction with the drawing, with like elements referenced with like reference numerals, in which:  
         [0013]    [0013]FIG. 1 is an exemplary communication network according to an illustrative embodiment of the invention;  
         [0014]    [0014]FIG. 2 shows an optical cross connect according to an illustrative embodiment of the invention;  
         [0015]    [0015]FIG. 2A shows a cross-connect unit according to an illustrative embodiment of the invention;  
         [0016]    [0016]FIG. 3 shows an optical cross connect according to another illustrative embodiment of the invention; and  
         [0017]    [0017]FIG. 4 shows a modulator filter according to an illustrative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    [0018]FIG. 1 shows an exemplary communication network. The network contains a transport network  10 , a local exchange  12 , a passive optical network  14  and a number of groups  16  of optical network units  18 . For sake of clarity, only one optical network unit  18  is numbered explicitly, while the other optical network units are shown below and above it. A similar convention, for sake of clarity and brevity, will also be used for numbering other types of similar elements. The local exchange  12  contains an optical line termination  120 , a plurality of transceivers (transmitters and/or receivers)  122 , an optical cross-connect  124  and a control unit  126 . The passive optical network  14  contains optical fibers  140   a - c,  wavelength multiplexer/demultiplexers  142   a - c  and splitters/combiners  144 . Although three transceivers  122  are shown by way of example, two transceivers or a larger number of transceivers may of course be used. The same holds, mutatis mutandis, for the number of fibers  140   a - c,  the number of multiplexers/demultiplexers  142   a - c,  the number of splitter/combiners  144 , the number of groups  16 , the number of optical network units  18 , and so on. The transceivers  122  may be transmitters for passing information “downstream” from the transport network  10  to the optical network units  18 , or receivers for passing information “upstream” from the optical network units  18  the transport network  10 .  
         [0019]    In operation in the “downstream” direction, information from transport network  10  is received by optical line termination  120  and passed to transmitters  122 . The transmitters  122  generate light onto which the information has been modulated. Optical cross-connect  124  passes this light to selected ones of the fibers  140   a - c.  Control unit  126  controls the optical cross-connect  124 , so as to determine from which transceiver  122  light is passed to which fiber  140   a - c.  Each fiber  140   a - c  corresponds to a multiplexer/demultiplexer  142   a - c  and a group of splitter/combiners  144 , through which it serves a group  16   a - c  of optical network units  18 . The respective fibers  140   a - c  pass light to the respective, corresponding multiplexer/demultiplexers  142   a - c.  The multiplexer/demultiplexers  142   a - c  split the light into different wavelength components, each in a different wavelength range. Although outputs for two wavelength components are shown by way of example, a larger or different number may be preferably used. Multiplexers/demultiplexers  142   a - c  pass each wavelength component to a respective one of the splitter/combiners  144 , which distributes the light of the component to the optical network units  18  of the relevant group  16   a - c.    
         [0020]    Conversely, in the “upstream” direction each splitter/combiners  144  combines light that carries information from different optical network units  18  and passes the combined light to a multiplexer/demultiplexer  142   a - c.  The multiplexer/demultiplexers  142   a - c  multiplex the light from a group of different splitter/combiners  144 , each splitter/combiner in the group contributing a different wavelength in the combined signal. The fibers  120   a - c  pass the multiplexed light to the optical cross-connect unit  124 . The optical cross connect units pass the light to the receivers  122 , which read the information and pass it to the transport network  10  via optical lint termination  120 . The control unit  126  controls the optical cross connect, so as to determine from which fiber  140   a - c  light is passed to which transceiver  122 .  
         [0021]    The optical cross-connect  124  serves to multiplex the use of the transceivers  122  for reception of light of different wavelengths from different fibers  140   a - c  and/or transmission of light of different wavelengths to fibers  140   a - c.  This is controlled by control unit  126 . The control unit bases the selection of the wavelength and fiber  140   a - c  for which the transceivers  122  are used on the need for communication of the optical network units  18 .  
         [0022]    If many optical network units  18  in a same group  16   a - c  need communication capacity at the same time, control unit  126  switches optical cross connection  124  so that transceivers  122  connect to the same fiber  140   a - c,  but at different wavelengths, so as to serve multiple optical network units  18  from the same group  16   a - c  simultaneously. When the communication capacity demanded for the optical network units  18  is distributed more evenly over the groups  16   a - c,  the transceivers are connected to different fibers  140   a - c,  operating for selected wavelengths, which may be the same or different, as required by the optical network units  18  that need service. Thus, communication to and/or from the optical network units  18  is multiplexed over time-slots and wavelengths. Any known allocation scheme for statistical multiplexing may be used to allocate the transceivers to fibers  140   a - c  and wavelengths. The multiplexing scheme can be applied either to downstream communication or to upstream communication or to a combination of both.  
         [0023]    [0023]FIG. 2 shows an optical cross-connect  124  for use in a network according to FIG. 1, in either the “upstream” or the “downstream” direction. Cross-connect  124  contains inputs/outputs  20   a,b  for transceivers (only two outputs are shown by way of example), first and second multiplexers/demultiplexers  22   a - b,    26   a - c  and sub cross-connect units  24   a - c.  The input/outputs  20   a,b  form the multiplexed input/output of respective ones of the first multiplexers/ demultiplexers  22   a - b.  Each first multiplexers/demultiplexers  22   a - b  has a number of demultiplexed input/outputs, each for a different range of wavelengths (this range corresponds to a communication wavelength). Similarly, the multiplexed input/outputs of each of the second multiplexers/demultiplexers  26   a - c  is coupled to a respective one of the fibers  140   a - c  and the second multiplexers/demultiplexers  26   a - c  have a number of demultiplexed input/outputs, each for a different range of wavelengths. Each sub cross-connect unit  24   a - c  cross connects demultiplexed input/outputs from the first and second multiplexers/demultiplexers  22   a - b,    26   a - c  for a respective one of the ranges of wavelengths. It will be understood that any number of outputs  20   a,b,  multiplexers/demultiplexers  22   a - b,    26   a - c,  cross-connect units  24   a - c  may used.  
         [0024]    [0024]FIG. 2A shows an embodiment of a sub cross-connect unit  24 . The sub cross-connect units  24  contains first and second splitter/combiners  240 ,  244  and optical switches  242  (only one numbered explicitly). The first splitter/combiner  240  has combination inputs/outputs to demultiplexed input/outputs from respective ones of the first multiplexer/demultiplexers  22   a - b.  The second splitter/combiner  240  has combination inputs/outputs to demultiplexed input/outputs from respective ones of the second multiplexer/demultiplexers  26   a - c.  The optical switches  242  each interconnect a split input/output of the first splitter/combiner  240  with a split input/output of the second splitter combiner. The optical switches  242  operate under control of control unit  126  (not shown). It will be clear that cross-connect units with different numbers of connections can be realized by using a different number of splitter/combiners, with different numbers of input outputs and more switches.  
         [0025]    In operation. each optical switch  242  receives liqht from one wavelength range from one transceiver  122  and one optical fiber  140   a - c.  The optical switch  242  selectively passes or blocks this light under control of control unit  126 .  
         [0026]    [0026]FIG. 3 shows a further optical cross-connect  124  for use in a network according to FIG. 1. Cross-connect  124  includes transceiver inputs/outputs  30   a - b  for different ones of the transceivers  122 , filters  32   a - b  and a sub-cross connect unit  34 . The sub cross-connect unit  34  is of a similar structure as the sub cross-connect units  24  of FIG. 2A. The sub cross-connect unit  34  has first input/outputs coupled to the fibers  140   a - c  and second input/outputs coupled to respective ones of the transceiver inputs/outputs  30   a - b  via respective ones of the filters  32   a - b.  Each filter contains a first and second multiplexer/demultiplexer  320 ,  324  and optical switches  322 . The input/outputs  30   a - b  for transceivers  122  form the multiplexed input/output of the first multiplexer/demultiplexer  320 . The first multiplexer/demultiplexer  320  has a number of demultiplexed input/outputs, each for a different range of wavelengths (this range corresponds to a communication wavelength). The second multiplexer/demultiplexer  320  has a number of demultiplexed input/outputs, each for a different range of wavelengths, coupled each coupled to a corresponding demultiplexed input/output of the first multiplexer/demultiplexer  320  via a respective one of the optical switches  322 . A multiplexed input/output of the second multiplexers/demultiplexers  324  is coupled to a respective one of the input/outputs of the sub cross-connect unit  34 . The optical switches  322  operate under control of the control unit  126  (not shown).  
         [0027]    In operation, sub-cross connect unit  34  passes light from a selected one of the fibers to a selected one of the transceivers  122  under selection by control unit  126 . The filters  32  ensure that only a selected one of the wavelengths is passed to or from the transceivers  122 . Compared with the optical cross connect of FIG. 2, considerably fewer optical switches are needed.  
         [0028]    In case statistical multiplexing is applied to upstream communication, the sub cross-connect unit  34  passes light of all wavelengths from a fiber  140   a - c  to selected ones of the filters  32 . The light of a single fiber  140   a - c  may contain multiple wavelengths carrying information from different optical network units  18  in the same group  16   a - c.  In this case, the control unit  126  will control the optical switches in the sub cross-connect unit  34  so that the light from this fiber  140   a - c  is passed to more than one of the filters  32 . The control unit  126  controls each filter so that light components with different wavelengths are passed to different receivers  122 . The receivers  122  are not wavelength-specific, i.e., they can decode information from light of any wavelength. Thus, control unit  126  controls which wavelength from which fiber  140   a - c  is decoded in each receiver  122 .  
         [0029]    Similarly, in case of downstream communication, if the cross connect of FIG. 2 is used, the control unit  126  controls optical switches in the sub-cross connect units  240   a - c  to determine which wavelength from transmitter  122  is passed to fiber  140   a - c  and, again, transmitters  122  may be wavelength unspecific.  
         [0030]    In case statistical multiplexing is applied to downstream communication, transmitters  122  may be used that modulate light of all available wavelengths. Only a single modulator is needed for all wavelengths in this case. Modulated light is transmitted to the sub cross-connect unit  34  via filters  32  which select one wavelength and pass light of this wavelength to a selected fiber  140   a - c  via sub-cross connect unit. A similar effect is achieved with the cross connect  124  of FIG. 2.  
         [0031]    [0031]FIG. 4 shows a combined modulator/filter  41  in which, as an alternative, the function of the filter  32  and the modulator of the transceiver  122  may be combined in the downstream case. The modulator/filter contains a plurality of single wavelength light sources  40   a - c  for different wavelengths, a plurality of optical modulators  42   a - c  and a multiplexer  44 . Each source  40   a - c  is connected to a respective demultiplexed input of the multiplexer  44  via a respective one of the optical modulators  42   a - c.  The multiplexed output of the multiplexer  44  may be connected to fibers  140   a - c  via a sub cross connect unit  46 , which is implemented, for example, as one of the sub cross connect units of FIG. 2A. In operation, a selected one of the modulators  42   a - c  is enabled under control of control unit  126  and information from the transport network  10  is used to control modulation by the enabled modulator  42   a - c.  Thus, the modulator/filter produces modulated light of a single wavelength. The sources  40   a - c  may be shared by different transceivers. Thus a minimum cost is required for the sources.  
         [0032]    In principle, transceivers  122  may be used that can handle both upstream and downstream traffic; simultaneously if necessary. In this case, the optical cross-connects of FIGS. 2 and 3 will provide selection for upstream and downstream traffic simultaneously. However, this may reduce the transmission capacity from full-duplex to half-duplex. In one embodiment, cross connect  146  contains two arrangements as shown in FIG. 2 or  3  in parallel, one for multiplexing of downstream communication and one for multiplexing upstream communication.