Abstract:
A data connection switching unit is provided for use in an optical communication network, as well as an optical communication method and an optical communication network, in which, via a number of interconnected network node devices of an optical transport network, data is transferred to a second transceiver via a first data connection via optical signals, wherein a data connection switching unit is connected between the first transceiver and one of the network node devices, which independently instigates the setting up of a second data connection between first and second transceivers by virtue of the fact that it sends a data connection setup signaling signal to the corresponding network node device in which information referring to the desired course of the second data connection is included.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical communication network, a data connection switching unit for use in such a communication network, and an optical communication method.  
           [0002]    Optical communication networks generally have a first transceiver from which optical signals are transferred to a second transceiver via a data connection with the interposition of a number of interconnected network node devices. Network node devices can be interconnected, for example, in each case via one or more optical conductors.  
           [0003]    The data transmission within the communication network can be performed, for example, with the aid of optical WDM (wavelength division multiplex) binary signals. In this case, a number of wavelength-multiplexed, pulsed optical signals are transmitted via a single optical conductor.  
           [0004]    In the communication networks currently used, the data connections used within the network are set up not via signaling signals sent via separate signaling channels, but via a central control device or a central network management.  
           [0005]    For the purpose of connecting transceivers to such networks, it is possible to provide so-called protection switching units (PSUs) that, upon the occurrence of disturbances on a first data connection, cause the optical signals emitted by the first transceiver to be transmitted henceforth via a second data connection different from the first data connection.  
           [0006]    It is an object of the present invention to make available a novel optical communication network, a novel data connection switching unit for use in an optical communication network, and a novel optical communication method.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with a basic idea of the present invention, an optical communication network is provided in which, via a number of interconnected network node devices of an optical transport network, data is transferred to a second transceiver via a first data connection via optical signals, characterized in that a data connection switching unit is connected between the first transceiver and one of the network node devices, which independently instigates the setting up of a second data connection between first and second transceivers by virtue of the fact that it sends a data connection setup signaling signal to the corresponding network node device in which information referring to the desired course of the second data connection is included.  
           [0008]    The optical transport network is advantageously an ASON (automatically switched optical network)network. It is particularly preferred for the data connection switching unit to be connected to a network address storage device in which one or more optical network addresses assigned to the second transceiver in the address space of the optical transport network are stored.  
           [0009]    The data connection switching unit for the first transceiver preferably fulfils a proxy function; for example, with reference to network address interrogation and/or (connection switching) signaling. For example, upon the occurrence of disturbances or excessively strong disturbances on the first data connection, the data connection switching unit can switch over, without influencing the first transceiver, automatically from the first data connection to the second (undisturbed or strongly disturbed) data connection.  
           [0010]    Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]    [0011]FIG. 1 shows a schematic of an optical communication network with standby data connection switching units in accordance with the prior art.  
         [0012]    [0012]FIG. 2 shows a schematic of an automatically switched optical network (ASON).  
         [0013]    [0013]FIG. 3 shows a schematic of an optical communication network in accordance with a first exemplary embodiment of the present invention.  
         [0014]    [0014]FIG. 4 shows a schematic of the time sequence of signaling signals exchanged between the subscriber line unit shown in FIG. 3, the data connection switching unit, the network address storage device and two network node devices.  
         [0015]    [0015]FIG. 5 shows a schematic of an optical communication network in accordance with a second exemplary embodiment of the present invention.  
         [0016]    [0016]FIG. 6 shows a schematic of an optical communication network in accordance with a third exemplary embodiment of the present invention.  
         [0017]    [0017]FIG. 7 shows a schematic of the time sequence of signaling signals exchanged between the subscriber line unit shown in FIG. 6, the data connection switching unit, two network address storage devices and two network node devices. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    In accordance with FIG. 1, an optical communication network or optical transport network (OTN)  1  according to the prior art has a number of network node devices  8 ,  9 ,  10 ,  11 , and a number of data connection switching units or PSUs (Protection Switching Unit)  2 ,  3 , to which a subscriber line unit  6 ,  7  is connected in each case via corresponding optical conductors  4 ,  5 . The first data connection switching unit  2  is connected via an optical conductor  12  to a first network node device  8 , and via a further optical conductor  13  to a second network node device  10 . In a corresponding way, the second data connection switching unit  3  is respectively connected via optical conductors  14 ,  15  to a third and fourth network node device  9 ,  11 . The network node devices  8 ,  9 ,  10 ,  11  (and a multiplicity of further network node devices (not illustrated)) are interconnected within the optical transport network (OTN)  1  via a multiplicity of optical conductors.  
         [0019]    In the case of the transport network (OTN)  1  shown in FIG. 1, the data connections respectively used in the transmission of data between the individual network node devices are set up by a central network management system.  
         [0020]    Upon the occurrence of disturbances (or upon the occurrence of excessively strong disturbances) on a “working” data connection (illustrated in the representation in accordance with FIG. 1 by the arrows consisting of continuous lines) routed via the optical conductor  13  and the second (and fourth) network node device  10 ,  11 , the data connection switching units  2 ,  3  then, instead of this, relay the optical signals emitted by a corresponding subscriber line unit  6 ,  7  via a “standby” data connection routed via the optical conductor  12 , and the first (and third) network node device  8 ,  9  (illustrated in the representation in accordance with FIG. 1 by the arrows consisting of dashed lines).  
         [0021]    By contrast with FIG. 1, FIG. 2 shows an automatically switched optical network or ASON network  16 , which has a multiplicity of network node devices  17 ,  18 , interconnected via corresponding optical conductors, and a number of subscriber line units  19 ,  20 . The latter are connected via, in each case, one (or, for example, two) optical conductor  21   a,    21   b  and  22   a,    22   b,  respectively, to a corresponding final network node device  17 ,  18 , use being made in each case of a first optical waveguide channel for transmitting useful signals, and of a second optical waveguide channel for transmitting signaling signals used for switching useful signals, for example.  
         [0022]    [0022]FIG. 3 shows an optical communication network  30  (here: an automatically switched optical network or ASON network) in accordance with a first exemplary embodiment of the present invention. This has a multiplicity of network node devices  31 ,  32 ,  33 ,  34  interconnected via an optical conductor network  35  (illustrated in the representation in accordance with FIG. 3 by a dash-dotted ellipse), and a multiplicity of subscriber line units  36 ,  37 . The latter can be, for example, IP (Internet Protocol) routers with MPLS signaling.  
         [0023]    Within the optical conductor network  35 , each network node device  31 ,  32 ,  33 ,  34  is connected via, in each case, one or more optical conductor bundles to, in each case, a number of (for example, two, three or four) further network node devices  31 ,  32 ,  33 ,  34 . In each case, one or more optical conductor bundles (running in parallel) are arranged in a pipe (laid entirely or partially underground, for example). Each optical conductor bundle has one or more optical conductors.  
         [0024]    A WDM (wavelength division multiplex) data transmission method can be used for transmitting data between the first subscriber line unit  36  and the second subscriber line unit  37 .  
         [0025]    Because of the wavelength division multiplexing, it is possible to transmit a number of different, pulsed optical binary signals simultaneously in different wavelength regions over each optical conductor present in the network (the signals serving, for example, to transmit data between a number of further subscriber line units (not illustrated here) as well as to transmit data between the first and second subscriber line units  36 ,  37 ).  
         [0026]    As is further shown in FIG. 3, the first and second subscriber line units  36 ,  37  are connected to a data connection switching unit (SU)  40 ,  41  via one or more optical conductor  38 ,  39 , in each case. The first data connection switching unit  40  is connected to a corresponding (first) network node device  31  via one or more optical conductors  42 , and to a corresponding (second) network node device  32  via one or more further optical conductors  43 . In a corresponding way, the second data connection switching unit  41  is connected via one or more optical conductors  44  to a corresponding (third) network node device  33 , and via one or more further optical conductors  45  to a corresponding (fourth) network node device  34 .  
         [0027]    Used in each case between the subscriber line units  36 ,  37  and the data connection switching units  40 ,  41  respectively connected thereto, and between the data connection switching units  40 ,  41  and the network node devices  31 ,  32 ,  33 ,  34  respectively connected thereto are, in each case, a first optical waveguide channel  38   a,    39   a,    42   a,    43   a,    44   a,    45   a  for transmitting useful signals (illustrated in the representation in accordance with FIG. 3 by continuous lines), and a second optical waveguide channel  38   b,    39   b,    42   b,    43   b,    44   b,    45   b  for transmitting signaling signals, which are explained in further detail below (illustrated in the representation in accordance with FIG. 3 by dashed lines). In addition, a further optical waveguide channel  38   c,  which is used to transfer (address interrogation) signaling signals, is provided between the first subscriber line unit  36  and the first data connection switching units  40 .  
         [0028]    The different, parallel optical waveguide channels  38   a,    39   a,    42   a,    43   a,    44   a,    45   a  and  38   b,    39   b,    42   b,    43   b,    44   b,    45   b  can be switched respectively via a number of (for example, two or three) different optical conductors or, for example, via one and the same optical conductor (for example, via wavelength division or time division multiplexing).  
         [0029]    The optical communication network  30  also has a central network address storage device  46  of a network address directory services server computer (REG or Registry) that is connected in each case to the data connection switching units  40 ,  41  via an optical waveguide channel  47  routed via a corresponding further optical conductor.  
         [0030]    The optical network address of a specific subscriber line unit  37  in the address space of the optical communication network  30  is not known to the remaining subscriber line units  36 . Consequently, before the setting up of an appropriate data connection, the first step is, with the interposition of the data connection switching device  40 , to use the appropriate subscriber line unit  36  to interrogate the optical network address of that subscriber line unit  37  to which a connection is to be set up, doing so via appropriate signaling signals.  
         [0031]    In accordance with FIG. 4, the first step for this purpose is to send a first signaling signal S 1  (ALOOKUP(TB)) to the data connection switching unit  40  (SU 1 ) from the first subscriber line unit (TA)  36  via the optical waveguide channel  38   c  via appropriate optical binary pulses. Included in this address request signaling signal S 1  is an identifier TB that, in the address space of the subscriber line unit  36 , identifies the destination subscriber line unit  37 .  
         [0032]    The identifier TB is stored in a storage device (not illustrated) of the switching unit  40  under the control of a control device (likewise not illustrated) of the data connection switching device  40 . Thereupon, a second signaling signal (ALOOKUP(TB)) corresponding to the above-named first signaling signal S 1  is sent to the central network address storage device  46  (REG) from the data connection switching unit  40  via the optical waveguide channel  47 . The network address TB 1  corresponding to the identifier TB in the address space of the optical communication network  30  is read out of the network address storage device under the control of the associated network address directory service server computer.  
         [0033]    The network address directory service server computer then causes a third signaling signal S 3  (ARESULT (TB: TB 1 )) to be sent to the data connection switching unit  40  via the optical waveguide channel  47 . The network address TB 1  of the destination subscriber line unit  37  is, inter alia, included in this address check-back signaling signal S 3 .  
         [0034]    The network address TB 1  is stored in the storage device of the data connection switching unit  40 . The next step is to send the subscriber line unit  37  a fourth signaling signal S 4  (ARESULT (TB: TB 1 )) which contains, inter alia, the network address TB 1  of the destination subscriber line unit  37  and corresponds to the above-named address check-back signaling signal S 3 , from the data connection switching unit  40  via the optical waveguide channel  38   c.  The network address TB 1  is then stored, while being assigned to the subscriber line unit identifier TB, in a storage device of the subscriber line unit  36 .  
         [0035]    In the address interrogation operation described, the data connection switching unit  40  functions as a “proxy”, such that the subscriber line unit  36  cannot distinguish whether it is connected directly to a network node device of the optical conductor network  35 , or to the data connection switching unit  40 .  
         [0036]    In accordance with FIG. 4, the next step for setting up a (first, unassured “working”) data connection between first and second subscriber line units  36 ,  37  from the first subscriber line unit (TA)  36  is to use corresponding optical binary pulses transmitted via the optical waveguide channel  38   b  to send a fifth signaling signal S 5  (SETUP (dest=TB)) to the data connection switching unit (SU 1 )  40 . The above-named identifier TB identifying the destination subscriber line (TB) or the optical network address TB 1  thereof is included in this (connection setup request) signaling signal S 5 .  
         [0037]    Thereupon, a sixth (connection setup request) signaling signal S 6  (SETUP (dest=TB)) containing the destination subscriber line identifier TB (or the destination subscriber line network address TB 1 ) is sent to the network node device  31  from the data connection switching unit  40  via the optical waveguide channel  42   b.    
         [0038]    The next step is for a network node control device (not illustrated) to select a connection identifier (here: V 1 ) identifying the connection to be set up, and to store it in a network node storage device (likewise not illustrated).  
         [0039]    The network node device  31  (or the network node control device) then selects one of the further network node devices connected to it as that network node device via which the connection is to be extended. The next step is for the network node control device to cause a further signaling signal corresponding to the above-named signaling signal S 6  to be sent from the network node device  31  to the selected further network node device, etc.  
         [0040]    In this way, a “working” data connection, routed via the path TA-SU 1 -N 1 -N 2 -SU 2 -TB, is set up successively between the first subscriber line unit  36  and the second subscriber line unit  37 .  
         [0041]    If the connection has been set up successfully as far as the second subscriber line unit  37 , this is communicated to the second data connection switching unit  41  by the second subscriber line unit  37  via a signaling signal sent via the optical waveguide channel  39   b,  where this communication is relayed to the network node device  33  via a connection setup confirmation signaling signal sent via the optical waveguide channel  44   b,  and, from there, to the network node device  31  via further connection setup confirmation signaling signals. Via the optical waveguide channel  42   b,  the network node device  31  sends a connection setup confirmation signaling signal S 7  (PATH_OK (ref=V 1 )) to the data connection switching unit  40 , which signal includes, inter alia, the above-named connection identifier V 1 .  
         [0042]    Via the optical waveguide channel  38   b,  the data connection switching unit  40  then sends the connection setup confirmation signaling signal S 8  (PATH_OK (ref=V 1 )) shown in FIG. 5, and likewise included in the above-named connection identifier V 1 , to the first subscriber line unit  36 . The connection identifier is stored in the subscriber line storage device under the control of a control device (not illustrated) of the first subscriber line unit.  
         [0043]    As explained above, the “working” data connection switched via the “working” path TA-SU 1 -N 1 -N 2 -SU 2 -TB is set up with the interposition of the data connection switching unit  40 . Because of its “proxy” function, the subscriber line unit  36  cannot distinguish whether the “working” data connection is switched directly via a network node device of the optical conductor network  35 , or, indirectly, via the data connection switching unit  40 .  
         [0044]    The next step is for the data connection switching unit  40  to cause, in addition to the above-named “working” data connection routed via the path TA-SU 1 -N 1 -N 2 -SU 2 -TB, a further “standby” data connection, routed via a “standby” path, to be set up to the second subscriber line unit  37 .  
         [0045]    The “standby” path is to run disjointly within the optical conductor network  35  with reference to the above-named “working” path; that is to say, different paths between the individual network node devices  31 ,  32 ,  33 ,  34  are to be used in each case for the “standby” and the “working” paths. Alternatively, or in addition, the “standby” data connection is to be distinguished in another way from the “working” data connection: for example, in the case of both connections, the path can certainly run sequentially via two identical network node devices  31 ,  32 ,  33 ,  34 , but the aim thereby is to make use in each case of two different optical conductor bundles or optical conductors arranged in different pipes (duct diversity). Alternatively, or in addition, it is certainly possible in the case of both connections to make use between two network node devices  31 ,  32 ,  33 ,  34  of an identical pipe, but different optical conductor bundles arranged in the same pipes can be used, or, for example, an identical optical conductor bundle, but different optical conductors contained therein (fiber diversity). Alternatively, or in addition, the “working” and the “standby” data connections can also run through different buildings in each case (building diversity).  
         [0046]    Before the setting-up of the “standby” data connection, the data connection switching unit  40  firstly interrogates an alternative optical network address TB 2  for the second subscriber line unit  37  at the network address storage device (REG)  46 . In accordance with FIG. 4, for this purpose the data connection switching unit  40  sends a ninth signaling signal (ALOOKUP(TB,alt_addr)) to the central network address storage device (REG)  46  via the optical waveguide channel  47 . The signal includes, inter alia, the identifier TB identifying the destination subscriber line (TB). In reaction to the receipt of the signaling signal S 9 , the abovementioned optical destination subscriber line network address TB 1  corresponding to the above-named identifier TB is read out from the network address storage device (REG)  46  under the control of the associated network address directory service server computer, as are one or more further optical network addresses TB 2 , differing therefrom, of the destination subscriber line  37  in the address space of the optical communication network  30 .  
         [0047]    The network address directory service server computer then causes a tenth signaling signal S 10  (ARESULT_LIST (TB: TB 1 , TB 2 , . . . )) to be sent to the data connection switching unit  40  via the optical waveguide channel  47 . Included in this address list check-back signaling signal S 10  are the various optical network addresses TB 1 , TB 2 , . . . of the destination subscriber line unit  37 .  
         [0048]    Sending the above-named ninth and tenth signaling signals S 9 , S 10  is omitted in the case of an alternative exemplary embodiment. Instead of this, all the optical network addresses TB 1 , TB 2 , . . . of the second subscriber line unit  37  are directly interrogated as early as during the step of “looking up the address of TB” (compare FIG. 4) by sending signals corresponding to the ninth and tenth signaling signals S 9 , S 10  in place of the above-named second and third signaling signals S 2 , S 3  (ALOOKUP(TB)), ARESULT(TB: TB 1 )) from the data connection switching unit  40 . One of the optical network addresses (for example, the network address TB 1 ) is then used to set up the “working” data connection, and another optical network address (for example, the network address TB 2 ) is used to set up the “standby” data connection.  
         [0049]    The network addresses TB 1 , TB 2 , . . . received by the network address storage device (REG)  46  are stored in the storage device of the data connection switching unit  40 .  
         [0050]    In accordance with FIG. 4, the control device of the data connection switching unit  40  thereupon causes an eleventh signaling signal S 11  (GET_PATH (ref=V 1 )) to be sent from the latter to the network node device  31  via optical binary pulses transmitted via the optical waveguide channel  42   b.  This serves the purpose of interrogating information, stored in the storage device of the network node device  31  (or elsewhere in the optical conductor network  35 ), referring to the resources used by the “working” data connection (that is to say, information referring to the respectively used “working” path, or to the respectively used pipes, optical conductor bundles, optical conductors, etc.).  
         [0051]    Included inter alia in the (resource request) signaling signal S 11  is the identifier V 1  identifying the “working” data connection set up.  
         [0052]    If the network node device  31  receives the resource interrogation signaling signal S 13 , its control device reads out the above-named information stored in the network node storage device and referring to the resources used by the “working” data connection (for example, the identifiers of the network nodes via which the “working” path is routed, or their optical network addresses).  
         [0053]    In accordance with FIG. 4, a further signaling signal S 12  (PATH_LIST (ref=V 1 , nodeList=L 1 )) is sent to the data connection switching unit  40  from the network node device  31  via the optical waveguide channel  42   b.  As well as the identifier V 1  identifying the “working” data connection, this includes, inter alia, a list with the identifiers of the network nodes via which the “working” path is routed.  
         [0054]    After receipt of the resource communication signaling signal S 12 , the control device of the data connection switching unit  40  causes the “standby” data connection to be set up. In accordance with FIG. 4, for this purpose a signaling signal S 13  (SETUP (dest=TB 2 ; avoidList=Li)) is sent to the network node device (N 3 )  32  from the data connection switching unit  40  via optical binary pulses transmitted via the optical waveguide channel  43   b.  Included in this (standby connection setup request) signaling signal S 13  is one of the above-named (alternative) network addresses TB 2  of the destination subscriber line unit (TB)  37 , as well as the resources to be avoided in setting up the “standby” data connection (here: a list of the identifiers of the network nodes via which the “working” path is routed, and which are to be avoided by the “standby” path).  
         [0055]    After receipt of the standby connection setup request signaling signal S 13 , the network node control device of the network node device  32  selects a connection identifier (here: V 2 ) identifying the “standby” data connection to be set up, and stores it in the network node storage device. The network node device  32  (or the control device thereof) then selects one of the network node devices connected to the network node device  32  as that network node device via which the “standby” data connection is to be extended, specifically in such a way that the “standby” path thereby produced is disjoint relative to the above-named “working” path (that is to say here: that the next network node device used is not included in the above-named list of network node devices to be avoided).  
         [0056]    The next step is for the network node control device to cause a further standby connection setup request signaling signal, which corresponds to the above-named signaling signal S 13 , to be sent from the network node device  32  via a signaling optical waveguide channel to the selected further network node device, which signal includes, inter alia, the above-named network address TB 2  of the destination subscriber line unit, as well as the resources to be avoided in setting up the “standby” data connection.  
         [0057]    In this way, a “standby” data connection, routed via the path N 3 -N 4 -SU 2 -TB, is set up successively to the second subscriber line unit  37 .  
         [0058]    If the connection has been set up successfully as far as the second subscriber line unit  37 , this is communicated to the second data connection switching unit  41  by the second subscriber line unit  37  via a signaling signal sent via the optical waveguide channel  39   b,  from where this communication is relayed to the network node device  34  via a connection setup confirmation signaling signal sent via the optical waveguide channel  45   b,  and, from there, to the network node device  32  via further connection setup confirmation signaling signals. Via the optical waveguide channel  43   b,  the network node device  32  sends a connection setup confirmation signaling signal S 14  (PATH_OK (ref=V 2 )) to the data connection switching unit  40 , which signal includes, inter alia, the above-named connection identifier V 2  of the “standby” data connection.  
         [0059]    The connection identifier V 2  is stored, under the control of the control device of the data connection switching unit  40 , in the storage device thereof.  
         [0060]    The connection identifiers V 1 , V 2  are used when emitting the actual useful data via the optical waveguide channels  42   a,    43   a  to identify the connection respectively to be used.  
         [0061]    The above-named “standby” data connection can be used, for example, for data transmission only whenever disturbances on the “working” data connection occur (or the disturbances on the “working” data connection become excessively large). Consequently, it is possible for the data connection switching unit  40  to switch the data transmission over quickly to the “standby” data connection in the event of (strong) disturbances occurring on the “working” data connection.  
         [0062]    For this purpose, the quality of the data transmitted via the “working” data connection is measured under the control of the control device of the first and/or the second data connection switching unit  40 ,  41  (for example, by determining the bit error rate thereby occurring, such as by emitting a pseudo-random bit sequence via the first data connection switching unit  40 , and comparing the received bit sequence with an expected bit sequence stored in advance in the storage device of the second data connection switching unit). The quality of the data transmission via the “working” data connection (for example, in the case of SDH or Ethernet transmission) also can be established by evaluating the check sum data transmitted together with the (useful) data in the respective transmission frame.  
         [0063]    Instead of setting up the “standby” data connection (with the aid of the above-named signals S 9 , S 10 , S 11 , S 12 , S 13 , S 14 ) directly and automatically after setting up the “working” data connection, the “standby” data connection can, for example, not be set up until the quality of the data transmitted via the “working” data connection falls below a first, predetermined threshold value. If the data quality drops still further (for example, below a second, predetermined threshold value), the data transmission is switched over from the “working” data connection to the “standby” data connection as appropriate by the data connection switching unit  40 , as described above.  
         [0064]    Alternatively, the “standby” data connection can, for example, not be set up until after a collapse of the “working” data connection. The data connection switching unit  40  can thus emulate the quick new setup of a connection for a transport network that is actually not designed therefor. Sending the signals S 11  and S 12  can be omitted in the case of this alternative; the data connection switching unit  40  then switches the data transmission over to the “standby” data connection from the “working” one whenever it has received the signal S 14  (PATH_OK (ref=V 2 )).  
         [0065]    In the case of the above-named exemplary embodiment and of the above-named alternative exemplary embodiments, the connection setup attempt can be repeated several times if the setting up of the “standby” data connection was unsuccessful. The “working” data connection already can be used during this time, but is then not yet protected.  
         [0066]    Because of the “proxy” function of the data connection switching unit  40 , the subscriber line unit  36  cannot distinguish whether the data connection switching unit  40  is switching the data further via the “working” data connection, or via the “standby” one.  
         [0067]    [0067]FIG. 5 shows an optical communication network  130  in accordance with a second exemplary embodiment of the present invention. This is designed in a way corresponding to the communication network  30  shown in FIG. 3, except that, instead of being connected to a single optical conductor network of a single operating company, the first and second data connection switching units  140 ,  141  are connected to two optical transport networks  135   a,    135   b  that are separate organizationally (illustrated in the representation in accordance with FIG. 5 by two dash-dotted ellipses). In a way corresponding to the optical conductor network  35  shown in FIG. 3, transport networks  135   a,    135   b  are automatically switched optical networks or ASON networks.  
         [0068]    The first data connection switching unit  140  is connected to a corresponding network node device  131  of the first transport network  135   a  via one or more optical conductors  142 , and to a corresponding network node device  132  of the second transport network  135   b  via one or more further optical conductors  143 . In a corresponding way, the second data connection switching unit  141  is connected to a (second) final network node device  133  of the first transport network  135   a  via one or more optical conductors  144 , and to a corresponding (second) final network node device  134  of the second transport network  135   b  via one or more further optical conductors  145 .  
         [0069]    Used respectively between the subscriber line units  136 ,  137  and the data connection switching units  140 ,  141  respectively connected thereto, and between the data connection switching units  140 ,  141  and the network node devices  131 ,  132 ,  133 ,  134  respectively connected thereto, are a first optical waveguide channel  138   a,    139   a,    142   a,    143   a,    144   a,    145   a  for transmitting useful signals (illustrated in the representation in accordance with FIG. 5 by continuous lines), and a second optical waveguide channel  138   b,    139   b,    142   b,    143   b,    144   b,    145   b  for transmitting signaling signals (illustrated in the representation in accordance with FIG. 5 by dashed lines).  
         [0070]    Furthermore, a further optical waveguide channel  138   c  that is used to transfer (address interrogation) signaling signals is provided between the first subscriber line unit  136  and the first data connection switching unit  140 .  
         [0071]    The optical communication network  130  also has for both transport networks  135   a,    135   b  a central network address storage device  146  of a central network address directory service server computer (REG or Registry), which is connected, in each case, to the data connection switching units  140 ,  141  via an optical waveguide channel  147  routed via an appropriate further optical conductor.  
         [0072]    The optical network addresses of the subscriber line units  136 ,  137  are stored, jointly for both transport networks  135   a,    135   b,  in the network address space in the central network address storage device  146 . Therefore, they can be interrogated accordingly in a fashion corresponding identically to the first exemplary embodiment in accordance with FIG. 4 via signaling signals S 1 , S 2 , S 3 , S 4  from the respective subscriber line unit  136 . Apart from looking up the address of the destination subscriber line unit  37  (signals S 1 , S 2 , S 3 , S 4 ), it is also possible to set up the “working” and the “standby” paths in a correspondingly identical way as explained in conjunction with FIG. 4 for the first exemplary embodiment (that is to say, via appropriate signals S 5 , S 6 , S 7 , S 8  or S 9 , S 10 , S 11 , S 12 , S 13 , S 14 ).  
         [0073]    An optical communication network  230  in accordance with a third exemplary embodiment of the present invention is shown in FIG. 6. This is designed in a way corresponding to the communication network  130  shown in FIG. 5 (that is to say, the first and second data connection switching units  240 ,  241  are connected to two organizationally separate optical transport networks  235   a,    235   b ), except that two separate network address storage devices  246   a,    246   b  for the networks  235   a,    235   b  are provided instead of a single central network address storage device  246  for both transport networks  235   a,    235   b.  Different optical network addresses respectively stored in the first or second network address storage device  246   a,    246   b  are therefore respectively assigned in each network  235   a,    235   b  to each subscriber line unit  236 ,  237 .  
         [0074]    The first network address storage device  246   a  of a network address directory service server computer (REG 1 ) of the first transport network  235   a  is connected to the data connection switching unit  240  via an optical waveguide channel  247   a.  The switching unit  240  is connected in a corresponding way via a further optical waveguide channel  247   b  to the second network address storage device  246   a  of a network address directory service server computer (REG 2 ) of the second transport network  235   b.    
         [0075]    In the optical communication network  230  shown in FIG. 6, the optical network address of the destination subscriber line unit  237  is looked up in a fashion corresponding identically to the first and second exemplary embodiments, specifically in accordance with FIG. 7 via signaling signals S 101 , S 102 , S 103 , S 104  (that are identical to the signals S 1 , S 2 , S 3 , S 4  shown in FIG. 4) exchanged between the first subscriber line unit  236 , the first data connection switching unit  240 , and the first network address storage device  246 .  
         [0076]    Furthermore, the “working” path is designed in an identical way to the case of the optical communication networks  30 ,  130  shown in FIGS. 3 and 5 (and specifically, in accordance with FIG. 7, via signaling signals S 105 , S 106 , S 107 , S 108  exchanged between the first subscriber line unit  236 , the first data connection switching unit  240 , and the first network node device  231  of the first transport network  235   a  (which are identical to the signals S 5 , S 6 , S 7 , S 8  shown in FIG. 4)).  
         [0077]    A signaling signal S 109  (ALOOKUP(TB)) is sent to the second network address storage device  246   b  (REG 2 ) from the data connection switching unit  240  via the optical waveguide channel  247   b  in order to set up the “standby” data connection. This includes, inter alia, the identifier TB that identifies the destination subscriber line unit  237 , in the address space of the subscriber line unit  236 .  
         [0078]    Thereupon, the network address TB 2  corresponding to the identifier TB in the address space of the second transport network  235 b is read out of the second network address storage device  246   b  (REG 2 ) under the control of the associated network address directory service server computer.  
         [0079]    The network address directory service server computer then causes a further signaling signal S 110  (ARESULT (TB: TB 2 )) to be sent to the data connection switching unit  240  via the optical waveguide channel  247   b.  Included in this address check-back signaling signal S 110  is, inter alia, the above-named network address TB 2  of the destination subscriber line unit  237 .  
         [0080]    Thereupon, a (connection setup request) signaling signal S 111  (SETUP (dest=TB 2 )) including the network address TB 2  of the destination subscriber line unit  237  is sent to a network node device  232  of the second transport network  235   b  from the data connection switching unit  240  via the optical waveguide channel  243   b.  The next step is for a network node control device (not illustrated) to select the connection identifier (not yet allocated) (here: V 2 ) identifying the connection to be set up, and to store it in a network node storage device (likewise not illustrated).  
         [0081]    A “standby” data connection, routed via the path N 3 -N 4 -SU 2 -TB, is then set up successively to the second subscriber line unit  237  in accordance with the above representation by sending a further connection setup request signaling signal from the network node device  232 .  
         [0082]    If the connection has been successfully set up, this is firstly communicated to the second data connection switching unit  241  from the second subscriber line unit  237  via an appropriate signaling signal, then to the network node device  232  via a number of further signaling signals, for example of the network node device  234 , and finally to the first data connection switching unit  240 . In accordance with FIG. 7, for this purpose the network node device  232  sends a connection setup confirmation signaling signal S 112  (PATH_OK (ref=V 2 )) to the data connection switching unit  240  via the optical waveguide channel  243   b,  which signal includes, inter alia, the above-named connection identifier V 2 .  
         [0083]    In the case of further, alternative exemplary embodiments (not illustrated), the subscriber line unit  36 ,  136  shown in FIGS. 3 and 5, respectively, is connected directly to the network address storage device  46 ,  146  (for example, via the optical waveguide channels  38   c,    138   c ). In this case, the proxy function of the data connection switching unit  40 ,  140  is omitted when looking up the network address TB 1  of the destination subscriber line unit  37 ,  137 . In this case, during setting up of the “standby” data connection, the first data connection switching unit  40 ,  140  uses a signal corresponding to the signal S 9  to interrogate a network address TB 2  that is an alternative to the network address TB  1  (instead of the signal S 9  ALOOKUP(TB, alt_addr), use is then made of, for example, the signal S 9 ′ ALOOKUP(TB 1 , alt_addr)).  
         [0084]    Alternatively, or in addition, the data connection switching unit  40 ,  140  shown in FIGS. 3 and 5 can generate a dedicated connection identifier V 1 ′ different from the connection identifier V 1 , and transfer it to the subscriber line unit  36 ,  136  via a signal S 8 ′ (PATH_OK(ref=V 1 ′)) corresponding to the signal S 8  (PATH_OK(ref=V 1 )).  
         [0085]    Furthermore, in the case of the exemplary embodiment shown in FIG. 6, it is possible, in a fashion corresponding to the exemplary embodiments shown in FIGS. 3 and 5, to make use of signals corresponding to the signals S 11 , S 12  and S 13  (GET_PATH (ref=V 1 ), PATH_LIST (ref=V 1 , nodeList=L 1 ) and SETUP(dest=TB 2 , avoidList=L 1 )) in accordance with FIG. 4, in order to rule out that the “working” and the “standby” data connections are possibly no longer mutually disjoint in a third transport network (for example, run via two identical network node devices).  
         [0086]    In the case of further alternative exemplary embodiments, the working connection setup confirmation signaling signal S 8  or S 108  (PATH_OK(ref=V 1 )), for example, is not emitted until the “standby” connection has been successfully set up.  
         [0087]    Moreover, instead of the above-named data connection switching units  40 ,  41 ,  140 ,  141 , it is possible to make use of data connection switching units corresponding to these, in order to protect subregions of a transport network (and not, as in the case of the above-named exemplary embodiments, the entire transport network in each case). It is possible for this purpose to connect such data connection switching units, for example, upstream of further network node devices (not illustrated in FIGS. 3, 5,  6 ) within the respective transport network, for example, and thus to protect parts (that are particularly at risk of failure) of a path within a transport network.  
         [0088]    In the case of the exemplary embodiments described in conjunction with FIGS. 3, 5 and  6 , it has been assumed that the actual useful data transmitted via the “working” or the “standby” data connection, and the signaling information (for example, the signals S 1 , S 2 , S 3 , S 4 , etc.) are transmitted in each case via appropriate optical pulses and, in each case, via one and the same optical conductor. In the case of alternative exemplary embodiments, by contrast, the signaling information is transmitted via separate optical conductors and/or separate paths, by comparison with the useful information. It is likewise conceivable to transmit the signaling information via a separate network; for example, an electrical transmission network. Likewise, instead of as illustrated between the affected network nodes, the exchange of the signaling information also can be performed between the respectively affected network nodes and one or more central network nodes in which processing of the signaling information is carried out.  
         [0089]    Indeed, although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the present invention as set forth in the hereafter appended claims.