Abstract:
Disclosed is a transmission apparatus for accommodating a plurality of asynchronous networks and a plurality of synchronous networks. The apparatus includes a plurality of asynchronous network interface units each having an asynchronous network termination unit for inputting and outputting a packet from and to one of the asynchronous networks and a mapping unit for carrying out a process to convert the packet into a first synchronous frame and vice versa, a plurality of synchronous network interface units each used for inputting and outputting a second synchronous frame from and to one of the synchronous networks, a buffer provided on at least a first asynchronous network interface unit selected among the asynchronous network interface units, and a control unit for storing a packet output by the asynchronous network termination unit for accommodating a specific one of the asynchronous networks and a packet output by the asynchronous network termination unit of the first asynchronous network interface unit into the buffer by adding identifiers for identifying the asynchronous network termination units in the event of a line failure occurring in one of the synchronous networks on a route related to the specific asynchronous network.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a transmission apparatus (or an NE, which is an abbreviation of Network Element). More particularly, the present invention relates to control to switch a line in the event of a line failure.  
           [0003]    2. Description of the Related Art  
           [0004]    A variety of switching techniques adopted at the present time to relieve a currently used line from a failure are defined in specifications of a SONET/SDH standard, which is referred to as the SONET standard instead of the SONET/SDH standard. Consider processing to transport a packet in accordance with the SONET standard. Originally, a SONET network is assumed to be a network operated for point-to-point communications of the connection type. For this reason, information flowing through a line in the network is treated equally without distinguishing the priority level of one information from that other information regardless of whether the information is audio information or packet information. Connection information operates in communication of a connectionless type till the information is accommodated in a SONET apparatus. Connection information operating in communication of a connectionless type can be moved in communication of a connection type, which permanently assigns channels for flowing information. If information is moved in communication of a connection type, the communication in which information does naturally flow all the time is converted into a communication for flowing information continuously in a forcible manner. If one considers the fact that the utilization rate of a communication line decreases due to a continuous flow of information, one will know that such a forced continuous flow of information is wasteful.  
           [0005]    As a spare line to be used as a relief line of a presently used line in accordance with the construction of the SONET standard, the same line as the presently used line needs to be prepared. If the act to relieve a presently used line by using a spare line is applied to a communication of the connectionless type, the waste of the presently used line used in a communication converted into the connection type is incurred by the spare line as it is. As a matter of fact, the spare line itself is, to the bitter end, a spare line, which is an inefficient line used only in the event of a line failure. During a communication of packet information, not only is the spare line itself inefficient, but the presently used line to be relieved by the spare line is also inefficient. Conventional techniques for relieving a presently used line include APS (Automatic Protection Switching) (1+1), APS (1:N), BLSR (Bidirectional Line Switched Ring), and so on.  
           [0006]    [0006]FIG. 25 is a diagram showing a change taking place in the event of a line failure with the conventional APS (1+1) technique adopted. As shown in FIG. 25, spare lines  4 P# 1  and  4 P# 2  having the same band as the presently used lines  2 W# 1  and  2 W# 2  between an NE 1 # 1  and an NE 1 # 2  are needed. As indicated by their names, the spare lines  4 P# 1  and  4 P# 2  are each a spare line. Thus, in a normal condition, the spare lines  4 P# 1  and  4 P# 2  are not used. It is not until a line failure that is detected on the presently used lines  2 W# 1  and  2 W# 2 , making it necessary to relieve the presently used lines  2 W# 1  and  2 W# 2  that the spare lines  4 P# 1  and  4 P# 2  are used.  
           [0007]    [0007]FIG. 26 is diagrams showing a change taking place in the event of a line failure with the conventional APS (1:N) technique adopted. In the (1:N) configuration, for a plurality of pairs of presently used lines  12 W# 1   j ,  12 W# 2   j  and  12 W# 3   j  between transmission apparatus NE 10 # and NE 10 # 2  where j=1 and 2, only a pair of spare lines  12 P#j where j=1 and 2 is provided. Thus, if failures occur on more than one pair of presently used lines, only one pair can be relieved. FIG. 26 shows typical failing line relieving control executed in the manner of granting priority to the earlier. Assume that a line failure is detected on the presently used lines  12 W# 11  and  12 W# 12 . In this case, the presently used lines  12 W# 11  and  12 W# 12  are replaced by the spare lines  12 P# 1  and  12 P# 2  as shown in the middle diagram of FIG. 26. Assume that a line failure thereafter is detected on the presently used lines  12 W# 21  and  12 W# 22 . In this case, the presently used lines  12 W# 21  and  12 W# 22  cannot be relieved as shown in the bottom diagram of FIG. 26.  
           [0008]    [0008]FIG. 27 is a block diagram showing a configuration adopting the APS (1:2) technique. As shown in FIG. 27, transmission apparatus NE 10 #i where i=1 and 2 each include ETHER interface units  13 #ij where j=1 and 2, a line exchange unit  14 #i, presently used OC 12  interface units  15 #ij where j=1 and 2, and spare OC 12  interface units  15 #iP. The ETHER interface units  13 #ij are typical interface units for interfacing with respect to data, LAN data, data frames and data packets. Spare OC 12  interface units  15 #iP where i=1 and 2 are in an available state of being not connected to the cards of the ETHER interface units for terminating signals of a terminal. The ETHER interface units  13 #ij inputs a packet from a data network such as an ETHER network, maps the packet onto a synchronous frame such as the STS 1 ×12 synchronous frame and passes on the frame to the line exchange unit  14 #i. The line exchange unit  14 #i carries out line exchange processing between the ETHER interface units  13 #ij where j=1 and 2 and the OC 12  interface units  15 #ij,  15 #iP where j=1 and 2. The presently used OC 12  interface units  15 #ij and the spare OC 12  interface units  15 #iP exchange an OC 12  packet with the SONET network.  
           [0009]    [0009]FIG. 28 is a block diagram showing a normal state of the configuration adopting the APS (1:2) technique. As shown in FIG. 28, ETHER termination units  16 #ij where j=1 and 2 receive a packet from an asynchronous network and outputs the packet to a SONET mapping unit  17 # 1   j . The SONET mapping unit  17 # 1   j  maps the packet onto an STS 12 ×12 synchronous frame. The STS 1 ×12 synchronous frame is output to the OC 12  interface units  15 # 1   j  via the line exchange unit  14 # 1 . Multiplexed in an OC 12  packet, the STS 12 ×12 synchronous frame is received by the OC 12  interface units  15 # 2   j . In this way, the communications between the ETHER interface units  13 # 1   j  where j=1 and 2 and the ETHER interface units  13 # 2   j  where j=1 and 2 are implemented through the presently used OC 12  interface units  15 #ij where i=1 and 2 whereas j=1 and 2. At that time, the spare OC 12  interface units  15 #iP where i=1 and 2 are not functioning.  
           [0010]    [0010]FIG. 29 is a diagram showing an internal state of the configuration adopting the APS (1:2) technique in the event of a line failure. Assume that a line failure is detected on the presently used line between the presently used OC 12  interface units  15 # 11  and  15 # 21 . In this case, the communication between the ETHER interface units  13 # 11  and  13 # 21  is implemented by the spare OC 12  interface units  15 P# 1  and  15 P# 2 .  
           [0011]    [0011]FIG. 30 is diagrams showing a change taking place in the event of a line failure with the conventional BLSR (NUT+PCA) technique adopted. The PCA is an abbreviation of Protection Channel Access, which is the name of a technology of using a spare side as an unredundant line in order to increase the efficiency of the line utilization. On the other hand, the NUT is an abbreviation of Non-preemptible Unprotected Traffic, which is a line operated as an unredundant line. The NUT is operated as a presently used line of the BLSR technique. By specifying the presently used side as a NUT line and specifying the spare side as a PCA line, it is possible to provide a configuration requiring no line switching even in the event of a line failure occurring on either side. In this case, since the spare side is operated as a PCA line while the presently used side is in a state of being guarded by a NUT line, impossibility of utilization as a relieved line on the presently used side is indicated.  
           [0012]    If a PCA line is specified as described above, an independent signal can be flown also to the spare line. By merely specifying a PCA line, in order to relieve a presently used line in the event of a line failure occurring on the presently used line, the independent signal flowing to the PCA line is cut off. If the presently used line is specified as a NUT line, on the other hand, a spare line cannot be switched in even if the presently used line fails. By specifying the presently used side as a NUT line and specifying the spare side as a PCA line, an independent signal can be flown to the spare line and, in addition, no such crushing will occur even in the event of a line failure. However, the presently used line cannot be relieved.  
           [0013]    The NEs  20 # 1 ,  20 # 2 ,  20 # 3  and  20 # 4  shown in FIG. 30 form a ring network. A signal from the NE  20 # 2  reaches the NE  20 # 4  through line ( 1 ) connecting the NE  20 # 2  to the NE  20 # 1  and line ( 2 ) connecting the NE  20 # 1  to the NE  20 # 4 . At that time, assume that line ( 2 ) is operated as a NUT line, a line failure is detected on line ( 2 ) and the BLSR function works. In this case, if the transmission lines are used normally, a signal from the NE  20 # 2  reaches the NE  20 # 4  through lines ( 1 ), ( 3 ), ( 4 ) and ( 5 ). Since line ( 3 ) is operated as a PCA line, however, line ( 2 ) operated as a NUT line cannot be relieved.  
           [0014]    [0014]FIG. 31 is a block diagram showing the conventional BLSR switching technique. As shown in FIG. 31, the NE  20 #i has ETHER interface units  30 #ij where j=1 and 2, a line exchange unit  32 #i, an OC 48  east  34 #iE and an OC 48  west  34 #iW. With respect to the NE  20  # 1 , route ( 1 ) includes a line between the ETHER interface unit  30 # 11  and the OC 48  west  34 # 1 W, a line between the OC 48  west  34 # 1 W and the OC 48  east  34 # 2 E and a line between the OC 48  east  34 # 2 E and the ETHER interface unit  30 # 21 . Route ( 2 ) includes a line between the ETHER interface unit  30 # 12  and the OC 48  east  34 # 1 E, a line between the OC 48  east  34 # 1 E and the OC 48  west  34 # 3 W and a line between the OC 48  west  34 # 3 W and the ETHER interface unit  30 # 31 . Route ( 3 ) includes a line between the ETHER interface unit  30 # 13  and the OC 48  west  34 # 1 W, a line between the OC 48  west  34 # 1 W and the OC 48  east  34 # 2 E and a line between the OC 48  east  34 # 2 E and the ETHER interface unit  30 # 22 .  
           [0015]    [0015]FIG. 32 is a block diagram showing operations in a normal condition of the NE  20 # 1  shown in FIG. 31. The ETHER interface units  30 #ij where j=1, 2 and 3 have ESTHER termination units  40 # 1   j  where j=1, 2 and 3 and SONET mapping units  42 # 1   j  where j=1, 2 and 3, respectively. The ETHER termination units  40 # 1   j  where j=1, 2 and 3 receive 1 Gbps packets from ETHER networks  50 # 1   j  respectively where j=1, 2 and 3, and output the packets to the SONET mapping units  42 # 1   j  respectively where j=1, 2 and 3. The SONET mapping units  42 # 1   j  where j=1, 2 and 3 map the 1 gbps packets onto STS 1 ×24 frames and output the frames to the line exchange unit  32 # 1 . The line exchange unit  32 # 1  outputs an STS 1  frame input from the SONET mapping units  42 # 1   j  where j=1, 2 and 3 to the OC 48  west  34 # 1 W, the OC 48  east  34 # 1 E and the OC 48  west  34 # 1 W respectively in accordance with the line setting. When the OC 48  west  34 # 1 W and the OC 48  east  34 # 1 E receive STS 1 ×48 frames, the OC 48  west  34 # 1 W and the OC 48  east  34 # 1 E map STS 1 ×48 frames onto an OC 48  packet and transmit the packet to a SONET network-BLSR right handed unit  60 #R and a SONET network-BLSR left handed unit  60 #L.  
           [0016]    [0016]FIG. 33 is a block diagram showing operations in a normal condition of the NE  20 # 2  shown in FIG. 31. The OC 48  east  34 # 2 E separates an STS 1 ×24 frame from an OC 48  packet received from the SONET network-BLSR left handed unit  60 #L, and the line exchange unit  32 # 2  supplies the STS 1 ×24 frame to the ETHER interface units  30 # 21  and  30 # 22 . The ETHER interface units  30 # 21  and  30 # 22  demap the STS 1 ×24 frame back onto a packet, which is transmitted to ETHER networks  50 # 21  and  50 # 23 .  
           [0017]    [0017]FIG. 34 is a block diagram showing operations in a normal condition of the NE  20 # 3  shown in FIG. 31. The OC 48  west  34 # 3 W separates a STS 1 ×24 frame from an OC 48  packet received from the SONET network-BLSR right handed unit  60 #R and the line exchange unit  32 # 3  supplies the STS 1 ×24 frame to the ETHER interface unit  30 # 31 . The ETHER interface unit  30 # 31  demaps the STS 1 ×24 frame back onto a packet, which is transmitted to an ETHER network  50 # 31 .  
           [0018]    [0018]FIG. 35 is a diagram showing routes established in the event of a line failure occurring in the BLSR network. The line failure is detected on a line connecting the NE  20 # 1  to the NE  20 # 3 . Since route  1  is not affected by the line failure, route  1  remains the same as the normal condition, including a line between the ETHER interface unit  30 # 11  and the OC 48  west  34 # 1 W, a line between the OC 48  west  34 # 1 W and the OC 48  east  34 # 2 E and a line between the OC 48  east  34 # 2 E and the ETHER interface unit  30 # 21 . On the other hand, a signal on a line between the OC 48  east  34 # 1 E and the OC 48  west  34 # 3 W is lost from route  2 . Thus, route  2  is newly established as follows to comprise a line between the ETHER interface unit  30 # 12  and the OC 48  west  34 # 1 W, a line between the OC 48  west  34 # 1 W and the OC 48  east  34 # 2 E and a line between the OC 48  east  34 # 2 E and the ETHER interface unit  30 # 31 . Since route  3  is specified as a PCA route and there is no other NUT specification, no spare line is available. Thus, the route is cut off. At that time, the NEs  20 #i where i=1, 2 and 3 enter the following states.  
           [0019]    [0019]FIG. 36 is an explanatory diagram showing operations carried out by the NE  20 # 1  shown in FIG. 31 in the event of a line failure. As shown in FIG. 36, the line exchange unit  32 # 1  carries out exchange processing on a line between the ETHER interface unit  30 # 12  and the OC 48  west  34 # 1 W. Since a connection destination is being used by the ETHER interface unit  30 # 12 , the ETHER interface unit  30 # 13  does not have a connection destination.  
           [0020]    [0020]FIG. 37 is an explanatory diagram showing operations carried out by the NE  20 # 2  shown in FIG. 31 in the event of a line failure. As shown in FIG. 37, the line exchange unit  32 # 2  carries out exchange processing on a line between the OC 48  east  34 # 2 E experiencing exchange processing with the ETHER interface unit  30 # 22  and the OC 48  west  34 # 2 W. Since a connection destination is being used by the ETHER interface unit  30 # 12 , the ETHER interface unit  30 # 22  does not have a connection destination.  
           [0021]    [0021]FIG. 38 is an explanatory diagram showing operations carried out by the NE  20 # 3  shown in FIG. 31 in the event of a line failure. As shown in FIG. 38, the line exchange unit  32 # 3  carries out exchange processing on a line between the ETHER interface unit  30 # 31  and the OC 48  east  34 # 3 E.  
           [0022]    As described above, in the conventional BLSR switching technique, the PCA technique treating a spare line like a presently used line is not capable of relieving a presently used line even if a line failure is detected. This is because, from the first, the PCA concept assumes that a spare line must be used for flowing a signal that must be utilized effectively and a relief operation is not taken into consideration. With this conventional technique, however, the following problem arises. In the case of the conventional technique, much like the packet over SONET technique, information propagating along a line is treated like information of a connection type in spite of the fact that the information is a connectionless type. Thus, the relief operation can be carried out only in line units as described above. As a result, the relief operation cannot be implemented in a way with a spare line used effectively.  
         SUMMARY OF THE INVENTION  
         [0023]    It is thus an object of the present invention to provide a transmission apparatus that has a configuration of implementation not using any original standard, and effectively uses a spare line of a SONET/SDH standard network for transmitting not only information of a connection type, but also information of a connectionless type.  
           [0024]    In accordance with an aspect of the present invention, there is provided a transmission apparatus for accommodating a plurality of asynchronous networks and a plurality of SONET/SDH networks, the transmission apparatus including a plurality of asynchronous network interface units each having an asynchronous network termination unit for inputting and outputting a packet from and to one of the asynchronous networks, and a mapping unit for carrying out a conversion process between the packet and a SONET/SDH frame, a plurality of SONET/SDH network interface units each used for interfacing with one of the SONET/SDH networks, a line exchange unit for carrying out line exchange processing between the mapping units and the SONET/SDH network interface units on the basis of line setting, a buffer provided on a first asynchronous network interface unit selected among the asynchronous network interface units to be used as a spare line and a control unit for storing a packet output by the asynchronous network termination unit for accommodating a specific one of the asynchronous networks and a packet output by the asynchronous network termination unit of the first asynchronous network interface unit into the buffer by adding identifiers for identifying the asynchronous network termination units, and reading out the packets from the buffer in the event of a line failure occurring in one of the SONET/SDH networks on a route related to the specific asynchronous network.  
           [0025]    Preferably, a packet may be output to one of the asynchronous network termination units that is identified by an identifier added to a packet output by the mapping unit of the first asynchronous network interface unit.  
           [0026]    More preferably, the control unit may allocat a first line capacity to the specific asynchronous network related to the route, a synchronous network on which has generated a line failure, and a second line capacity to one of the asynchronous networks that is accommodated by the asynchronous network termination unit of the first asynchronous network interface unit and may control a first packet output by the asynchronous network termination unit accommodating the specific asynchronous network related to the route, a synchronous network on which has generated a line failure, by discarding a second packet output by the asynchronous network termination unit of the first asynchronous network interface unit whose communication traffic exceeds the second line capacity and thus letting the first packet take precedence of the second packet output by the asynchronous network termination unit of the first asynchronous network interface unit.  
           [0027]    The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a block diagram showing the principle of the present invention;  
         [0029]    [0029]FIG. 2 is a block diagram showing the configuration of a network implemented by a first embodiment of the present invention;  
         [0030]    [0030]FIG. 3 is a block diagram showing the configuration of an NE used in the network shown in FIG. 2;  
         [0031]    [0031]FIG. 4 is a diagram showing control to receive a packet;  
         [0032]    [0032]FIG. 5 is a diagram showing the format of a packet;  
         [0033]    [0033]FIG. 6 shows a flowchart representing packet write control;  
         [0034]    [0034]FIG. 7 shows a flowchart representing packet read control;  
         [0035]    [0035]FIG. 8 shows a flowchart representing frame receive control;  
         [0036]    [0036]FIG. 9 is an explanatory diagram used for describing operations carried out by an NE  200 # 1  in a normal condition;  
         [0037]    [0037]FIG. 10 is an explanatory diagram used for describing operations carried out by an NE  200 # 2  in a normal condition;  
         [0038]    [0038]FIG. 11 is an explanatory diagram used for describing operations carried out by an NE  200 # 3  in a normal condition;  
         [0039]    [0039]FIG. 12 is a diagram showing a route established in the event of a line failure;  
         [0040]    [0040]FIG. 13 is an explanatory diagram used for describing operations carried out by the NE  200 # 1  in the event of a line failure;  
         [0041]    [0041]FIG. 14 is a diagram showing a technique to share a line;  
         [0042]    [0042]FIG. 15 is an explanatory diagram used for describing operations carried out by the NE  200 # 2  in the event of a line failure;  
         [0043]    [0043]FIG. 16 is a diagram showing a technique to share a line;  
         [0044]    [0044]FIG. 17 is an explanatory diagram used for describing operations carried out by the NE  200 # 3  in the event of a line failure;  
         [0045]    [0045]FIG. 18 is a block diagram showing the configuration of a network implemented by a second embodiment of the present invention;  
         [0046]    [0046]FIG. 19 is a block diagram showing the configuration of an NE used in the network shown in FIG. 18;  
         [0047]    [0047]FIG. 20 is an explanatory diagram used for describing operations carried out in the network shown in FIG. 18 a normal condition;  
         [0048]    [0048]FIG. 21 is an explanatory diagram used for describing operations carried out in the network shown in FIG. 18 in the event of a line failure;  
         [0049]    [0049]FIG. 22 is a diagram showing a technique to share a line;  
         [0050]    [0050]FIG. 23 is a block diagram showing the configuration of a network implemented by a third embodiment of the present invention;  
         [0051]    [0051]FIG. 24 is a block diagram showing the configuration of an NE used in the network shown in FIG. 23;  
         [0052]    [0052]FIG. 25 is a diagram showing the conventional APS (1+1) technique;  
         [0053]    [0053]FIG. 26 is a diagram showing the conventional APS (1:2) technique;  
         [0054]    [0054]FIG. 27 is a block diagram showing the configuration of a network adopting the conventional APS (1:2) technique;  
         [0055]    [0055]FIG. 28 is an explanatory diagram showing operations carried out in accordance with the conventional APS (1:2) technique in a normal condition;  
         [0056]    [0056]FIG. 29 is an explanatory diagram showing operations carried out in accordance with the conventional APS (1:2) technique in the event of a line failure;  
         [0057]    [0057]FIG. 30 is a diagram showing a (NUT+PCA) technique of the conventional BLSR method;  
         [0058]    [0058]FIG. 31 is a diagram showing a switching technique of the conventional BLSR method;  
         [0059]    [0059]FIG. 32 is an explanatory diagram used for describing operations carried out by an NE  20 # 1  shown in FIG. 31 in a normal condition;  
         [0060]    [0060]FIG. 33 is an explanatory diagram used for describing operations carried out by an NE  20 # 2  shown in FIG. 31 in a normal condition;  
         [0061]    [0061]FIG. 34 is an explanatory diagram used for describing operations carried out by an NE  20 # 3  shown in FIG. 31 in a normal condition;  
         [0062]    [0062]FIG. 35 is a diagram showing a route established in the event of a line failure occurring in a BLSR network;  
         [0063]    [0063]FIG. 36 is an explanatory diagram used for describing operations carried out by an NE  20 # 1  shown in FIG. 31 in the event of a line failure;  
         [0064]    [0064]FIG. 37 is an explanatory diagram used for describing operations carried out by an NE  20 # 2  shown in FIG. 31 in the event of a line failure; and  
         [0065]    [0065]FIG. 38 is an explanatory diagram used for describing operations carried out by an NE  20 # 3  shown in FIG. 31 in the event of a line failure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0066]    Before some preferred embodiments of the present invention are described, the principle of the present invention is explained. FIG. 1 is a block diagram showing the principle of the present invention. As shown in FIG. 1, a plurality of NEs  100 #i where i=1, 2 and 3 form a network. The NE  100 #i comprises a plurality of asynchronous interface units  102 #ij where j=1, 2 and so on, a first asynchronous network interface unit  104 #i, a line exchange unit  106 #i, a plurality of SONET/SDH network interface units  108 #ij where j=1, 2 and so on and a control unit  109 #i. The asynchronous network interface unit  102 #ij has an asynchronous network termination unit  110 #ij and a mapping unit  112 #ij. By the same token, the first asynchronous network interface unit  104 #i has an asynchronous network termination unit  120 #i and a mapping unit  122 #i. The first asynchronous network interface unit  104 #i also includes a buffer  124 #i. The NEs  100 #i where i=1, 2 and 3 form a ring network.  
         [0067]    As shown in FIG. 1, with respect to the NE  100 # 1 , route  1  comprises a line between the asynchronous network termination unit  110 # 11  and the SONET/SDH network interface unit  108 # 11 , a line between the SONET/SDH network interface unit  108 # 11  and the SONET/SDH network interface unit  108 # 21  and a line between the SONET/SDH network interface unit  108 # 21  and the asynchronous network termination unit  110 # 21 . Route  2  comprises a line between the asynchronous network termination unit  110 # 12  and the SONET/SDH network interface unit  108 # 12 , a line between the SONET/SDH network interface unit  108 # 12  and the asynchronous network termination unit  110 # 32  and a line between the SONET/SDH network interface unit  108 # 32  and the asynchronous network termination unit  110 # 31 . Route  3  comprises a line between the asynchronous network termination unit  120 # 1  and the SONET/SDH network interface unit  108 # 11 , a line between the SONET/SDH network interface unit  108 # 11  and the SONET/SDH network interface unit  108 # 21  and a line between the SONET/SDH network interface unit  108 # 21  and the asynchronous network termination unit  120 # 2 .  
         [0068]    Assume that a line failure is detected on the line between the SONET/SDH network interface unit  108 # 12  and the SONET/SDH network interface unit  108 # 32  on route  2 . In this case, the control unit  109 # 1  newly establishes route  2 ′ to comprise a line between an asynchronous network interface unit  110 # 12  and the SONET/SDH network interface unit  108 # 11 , a line between the SONET/SDH network interface unit  108 # 11  and the SONET/SDH network interface unit  108 # 21 , a line between the SONET/SDH network interface unit  108 # 21  and the buffer  124 # 2 , a line between the buffer  124 # 2  and the SONET/SDH network interface unit  108 # 22 , a line between the SONET/SDH network interface unit  108 # 22  and the SONET/SDH network interface unit  108 # 31  and a line between the SONET/SDH network interface unit  108 # 31  and the asynchronous network interface unit  102 # 31  as a substitute for route  2 .  
         [0069]    The control unit  109 # 1  writes a packet received by the asynchronous network termination unit  110 # 12  and  120 # 1  into the buffer  124 # 1  by adding an identifier to each of the packets, and then reads back the packets, supplying the packets to the mapping unit  122 # 1 . The mapping unit  122 # 1  maps each of the packets onto a SONET/SDH frame. The SONET/SDH network interface unit  108 # 11  multiplexes the SONET/SDH frame with a SONET/SDH frame on the SONET/SDH network side and outputs the multiplexed SONET/SDH frames to a synchronous network.  
         [0070]    The SONET/SDH network interface unit  108 # 2  receives a SONET/SDH frame and passes on the frame to the mapping unit  122 # 2  by way of the line exchange unit  106 # 2 . The mapping unit  122 # 2  demaps the SONET/SDH frame back onto a packet. The control unit  109 # 2  examines an identifier added to the packet in order to form a judgment as to whether or not the packet is to be supplied to the asynchronous network termination unit  120 # 2  or relay to another asynchronous network as part of control of processing to output a packet to the asynchronous network termination unit  120 # 2  or relay the packet to a SONET/SDH network. In this way, the buffer  124 # 1  is shared to raise the efficiency of the utilization of the lines in the event of a line failure.  
         [0071]    First Embodiment  
         [0072]    [0072]FIG. 2 is a block diagram showing the configuration of a network implemented by a first embodiment of the present invention. As shown in FIG. 2, a plurality of NEs  200 #i where i=1, 2 and 3 form a ring network. In this embodiment, in the event of a line failure, a line is switched in accordance with a BLSR recommendation. In a normal state with no line failure generated, routes  1 ,  2  and  3  with respect to the NE  200 # 1  are shown as a broken line, a chain line and a single-dotted chain line respectively like those shown in FIG. 31. An ETHER interface unit  214 #i uses a spare line in a PCA mode. An ETHER interface is an example of connectionless interface.  
         [0073]    [0073]FIG. 3 is a block diagram showing a typical configuration of the NE  200 #i used in the network shown in FIG. 2. As shown in FIG. 3, the NE  200 #i comprises a plurality of presently used line units  210 #ij and  212 #ij where j=1 and 2, the spare line unit  214 #i, a line exchange unit  32 #i and a control unit  218 #i. The presently used line unit (the ETHER interface unit)  210 #i is an ETHER interface board for accommodating an ETHER network. The ETHER interface board is inserted into a slot in the NE  200 #i. In the NE  200 #i, two presently used line units  210 #ij where j=1 and 2 are employed. The number of presently used line units  210 #ij can be increased or decreased in dependence on the operation. In addition, also in dependence on the operation, the NE  200 #i can also be designed to include no spare line units  214 #i and  216 #i as is the case with the conventional transmission apparatus. The presently used line unit  210 #ij comprises ETHER termination units  220 #ijk where k=1 and 2 and SONET mapping units  222 #ijk where k=1 and 2.  
         [0074]    The ETHER termination unit  220 #ijk has the following functions.  
         [0075]    (i): Receive a packet from an ETHER network.  
         [0076]    (ii): Output a packet received from a buffer  234 #i or the SONET mapping unit  222 #ijk to an ETHER network.  
         [0077]    The SONET mapping unit  222 #ijk has the following functions.  
         [0078]    (i): Map an input packet onto a synchronous frame such as STS 1 ×24 and output the frame to the line exchange unit  32 #i.  
         [0079]    (ii): Input a synchronous frame such as STS 1 ×24 from the line exchange unit  32 #i and demap the frame onto a packet.  
         [0080]    The output side of the ETHER termination unit  220 #ijk is connected to the buffer  234 #i by a signal line  224 #ijk. In the event of a line failure occurring on a route involving the ETHER termination unit  220 #ijk, the signal line  224 #ijk is used for allowing the buffer  234 #i to be shared by the alternative route involving the ETHER termination unit  220 #ijk and a route including the spare line  214 #i by writing a packet received by the ETHER termination unit  220 #ijk and a packet received by the ETHER termination unit  230 #i into the buffer  234 #i. Switching control using the signal line  224 #ijk can be executed as follows.  
         [0081]    (i): There is provided a method whereby, in a normal condition, a packet received by the ETHER termination unit  220 #ijk is output to the SONET mapping unit  222 #ijk by way of the buffer  234 #i but, in the event of a line failure occurring on a route involving the ETHER termination unit  220 #ijk, on the other hand, a packet received by the ETHER termination unit  220 #ijk is output to the SONET mapping unit  232 #i 1 . In order to implement this method, the control unit  218 #i controls the buffer  234 #i.  
         [0082]    (ii): There is provided another method whereby, in a normal condition, a switch provided between the signal line  224 #ijk and the ETHER termination unit  220 #ijk is turned off to output a packet directly to the SONET mapping unit  222 #ijk but, in the event of a line failure occurring on a route involving the ETHER termination unit  220 #ijk, on the other hand, the switch is turned on to output a packet to the SONET mapping unit  232 #i 1  by way of the buffer  234 #i. The control unit  218 #i executes control to turn the switch on and off.  
         [0083]    In accordance with control executed by the control unit  218 #i, the line exchange unit  32 #i outputs an input STS 1  frame to a port for the frame. The presently used line unit  212 #ij has SONET input/output units  240 #i 1   k  where k=1 and 2. The SONET input/output unit  240 #ijk has the following functions:  
         [0084]    (i): Multiplex an STS 1 ×24 frame received from the line exchange unit  32 #I into OC 48 , and output the multiplexed frame to a SONET network.  
         [0085]    (ii): Separate an STS 1  frame from an OC 48  packet received from a SONET network and output the frame to the line exchange unit  32 #i.  
         [0086]    The spare line unit  214 #i has an ETHER termination unit  230 #i, SONET mapping units  232 #ij where j=1 and 2 and a buffer  234 #i. The ETHER termination unit  230 #i is essentially identical with the ETHER termination unit  220 #ijk. The SONET mapping units  232 #ij where j=1 and 2 are virtually identical with the SONET mapping unit  222 #ijk. In the event of no line failure, the SONET mapping unit  232 #ij inputs and outputs a packet from and to an ETHER network accommodated by the ETHER termination unit  230 #i. In the event of a line failure, on the other hand, the SONET mapping unit  232 #ij inputs and outputs a packet from and to an asynchronous network accommodated by the ETHER termination unit  230 #i and an asynchronous network accommodated by the ETHER termination unit  220 #ijk involved in a line failure. Since STS 1 ×12 frames received by the SONET mapping unit  232 #i 1  are shared by a protection line and a switched route, on the other hand, the SONET mapping unit  232 #i 2  relays the frame to a SONET network on a route including the switched route.  
         [0087]    The buffer  234 #i is a buffer memory for holding the following packets. A packet received by the ETHER termination unit  220 #ijk involved in a line failure or a packet received by the ETHER termination unit  230 #i involved in a line failure in the event of the line failure.  
         [0088]    The buffer  234 #i may also be provided or each of the ETHER termination unit  230 #i, the ETHER termination unit  220 #ijk and the SONET mapping unit  232 #i 1  or shared by the ETHER termination unit  230 #i, the ETHER termination unit  220 #i and the SONET mapping unit  232 #i 1 .  
         [0089]    The control unit  218 #i has an APS switching control unit  250 #i, a line exchange control unit  252 #i and a buffer control unit  254 #i. The APS switching control unit  250 #i has the following functions:  
         [0090]    (i): Monitor a line failure that may occur on a route. In a BLSR configuration, the NEs  200 #i where i=1, 2 and so on share switching information. The NEs are managed as NEs in an idle state, a switched state and a pass-through state. In the event of no line failure, the NEs  200 #i where i=1, 2 and so on are regarded as NEs in an idle state. In the event of a line failure, on the other hand, the NE  200 # 1  typically functions as an add/drop switch station having a PCA spare line. The NE  200 # 2  typically functions as an add/drop pass-through station having a PCA spare line. The NE  200 # 3  typically functions as a switch station having an add/drop.  
         [0091]    (ii): Determine a switched route in accordance with a BLSR recommendation in the event of a line failure. At that time, if a switched route includes a line on a route operated as a PCA spare line, control is executed so as to allow communications through a failing line to be shared by a switched route and a PCA line.  
         [0092]    (iii): Give a command indicating a line setting change accompanying a route change to the line exchange control unit  252  #i.  
         [0093]    (iv): Give a control command accompanying a route change to the buffer  234 #i. The control command is explained as follows.  
         [0094]    The buffer control unit  254 #i has the following functions.  
         [0095]    (1): Control to Receive a Packet  
         [0096]    [0096]FIG. 4 is a diagram showing the control to receive a packet;  
         [0097]    (i): Packet Write Control  
         [0098]    [0098]FIG. 5 is a diagram showing the format of a packet and FIG. 6 shows a flowchart representing packet write control. As shown in the figure, the flowchart begins with a step S 2  to form a judgment as to whether or not a line failure has been detected. If a line failure has been detected, the flow of the control goes on to a step S 4 . If no failure has been detected, on the other hand, the flow of the control goes on to a step S 14 . At the step S 4 , the line involved in the line failure is examined to form a judgment as to whether or not the line should be relieved. If the line involved in the line failure is determined to be a line that should be relieved, the flow of the control goes on to a step S 6 . If the line involved in the line failure is determined to be not a line that should be relieved, on the other hand, the flow of the control goes on to the step S 14 . A line requiring a relief is a presently used line that is involved in a line failure and needs a relief, or a PCA line used for relieving such a presently used line. An instruction may explicitly indicate that a line does not have to be relieved. An example of such a line is a line with a line failure of a NUT specification. Such a presently used line does not have to be relieved. By providing such a line, the number of variations in operation can be increased.  
         [0099]    At the step S 6 , an identifier is added to the frame at the head of the frame whereas an identifier and an FCS (Frame Check Sequence) found from a transmitted packet are added to the frame at the tail of the frame as shown in FIGS. 4 and 5. These identifiers indicate the source of the packet and a destination to which the packet is to be output. The identifiers are required because, in the event of a line failure, the buffer  234 #i for storing frames is shared by a PCA line and a switched route. The identifiers are dependent on the number of ETHER termination units  220 #ijk including the ETHER termination unit  230 #i where j=1, 2 and so on and k=1, 2 and so on, which are accommodated by the NE  200 #i. If the NE  200 #i accommodates 8 lines, which are 7 presently used lines and 1 spare line, a 4-bit identifier is required. For example, an identifier having a value of 0 is defined for the spare line. It is to be noted that, in a normal condition, if a line is occupied by the PCA line, it is not necessary to assign an identifier to such a line.  
         [0100]    Then, at the next step S 8 , the packet is stored in the buffer  234 #i. Assume for example that the buffer  234 #i is provided for each of the ETHER termination units  220 #ijk and the ETHER termination unit  230 #i as shown in FIG. 4. In this case, the packet is stored in a buffer  234 #i in one of and the ETHER termination unit  230 #i and the ETHER termination units  220 #ijk, that is identified by the identifier. Then, at the next step S 10 , the packet is read out from the buffer  234 #i in accordance with buffer read control to be described later. The flow of the control then goes on to the step S 14 . At the step S 14 , the packet is passed on to the SONET mapping unit  232 #i 1 . It is to be noted that, if the packet has been stored into the buffer  234 #i, the packet is passed on to the SONET mapping unit  222 #ijk,  232 #i 1  corresponding respectively to the ETHER termination unit  220 #ijk,  230 #i each receiving the packet. If the outcome of the judgment formed at the step S 2  indicates that no line failure has been detected or if the outcome of the judgment formed at the step S 4  indicates that the line involved in the line failure is not a line that should be relieved, on the other hand, at the step S 14 , the packet is passed on to the SONET mapping unit  222 #ijk as is the case with a normal state of the line without carrying out a line switching operation. Then, at the next step S 16 , a frame output by the SONET mapping unit  222 #ijk is accommodated in a packet OC 48 , which is output from a SONET network.  
         [0101]    (ii): Packet Read Control  
         [0102]    Packet read control is control to discard a packet of either a PCA line or a switched route in case the capacity of the buffer is exceeded in the event of a line failure. This is because, in the event of a line failure, the line is shared by the PCA line and the switched route so that the capacity of the buffer may be exceeded. As shown in FIG. 4, priority levels assigned to packets are used in control to be executed when the capacity of the buffer is exceeded. Packets that can be held in the buffer within the capacity of the buffer are accepted without taking priority levels assigned to the packets into consideration. Packets that will be held in the buffer beyond the capacity of the buffer are accepted on a priority basis. Priority levels are assigned to packets for each identifier in accordance with destination addresses of the packets, source addresses of the packets and whether or not the packet is a network monitoring packet. As a result, when the capacity of the buffer is exceeded, packets are converted into a SONET frame starting with a packet having the highest priority level, and packets with low priority levels are discarded.  
         [0103]    [0103]FIG. 7 shows a flowchart representing the packet read control. As shown in the figure, the flowchart begins with a step S 20  at which the number of packets is counted for each passing identifier to measure the throughput for each identifier. Then, the flow of the control goes on to a step S 22  to form a judgment as to whether or not a throughput (the minimum capacity cited above) set for an identifier has been exceeded. If the throughput has been exceeded, the flow of the control goes on to a step S 24 . If the throughput has not been exceeded, on the other hand, the flow of the control goes on to a step S 26 . At the step S 24 , the packet is examined to form a judgment as to whether or not the packet can be discarded. If the packet cannot be discarded, the flow of the control goes on to the step S 26 . Examples of a packet that cannot be discarded are a packet having a specific transmission destination or a packet having a MAC address as its transmission destination, a packet generated by a specific transmission source or a packet having a MAC address as its transmission source and a packet having a high priority level such as a packet mapped onto a network monitoring frame. As described above, in the case of such an undiscardable packet, the flow of the control goes on to the step S 26 . If the packet can be discarded, on the other hand, the flow of the control goes on to a step S 30 . At the step S 26 , the packet is read out from the buffer  234 #i and passed on to the SONET mapping unit  222 #i 1 . Then, at the next step S 28 , the packet is accommodated in an OC 48  frame, which is then output from a SONET network. At the step S 30 , the packet is discarded.  
         [0104]    (2): Buffer  234 #i Read Control Relevant to Frame Reception  
         [0105]    [0105]FIG. 8 shows a flowchart representing frame receive control. As shown in the figure, the flowchart begins with a step S 30  to form a judgment as to whether or not a packet output from the SONET mapping unit  232 #i 1  is a packet including an attached identifier. If the packet output from the SONET mapping unit  232 #i 1  is a packet including an attached identifier, the flow of the control goes on to a step S 32 . If the packet output from the SONET mapping unit  232 #i 1  is not a packet including an attached identifier, on the other hand, the flow of the control goes on to a step S 40 . A packet including no attached identifier indicates a packet output in a normal state. At the step S 32 , the packet is examined to form a judgment as to whether or not the FCS including in the packet is correct. If the FCS including in the packet is correct, the flow of the control goes on to a step S 34 . If the FCS including in the packet is not correct, on the other hand, the flow of the control goes on to a step S 44 . At the step S 34 , the packet is stored in the buffer  234 #i.  
         [0106]    Then, the flow of the control goes on to a step S 36  to form a judgment as to whether or not the packet stored in the buffer  234 #i is a packet destined for this NE  200 #i on the basis of an identifier included in the packet. If the packet stored in the buffer  234 #i is not a packet destined for this NE  200 #i, the flow of the control goes on to the step S 40 . If the packet stored in the buffer  234 #i is a packet destined for this NE  200 #i, on the other hand, the flow of the control goes on to a step S 38 . At the step S 38 , the identifier at the head of the packet is fetched. At the step S 40 , the packet is passed on to an ETHER termination unit identified by the attached identifier if the packet stored in the buffer  234 #i is a packet destined for this NE  200 #i, or the packet is passed on to the SONET mapping unit  232 #i 2  if the packet stored in the buffer  234 #i is not a packet destined for this NE  200 #i. Then, at the next step S 42 , the received packet is output from a line. At the step S 44 , the packet is discarded.  
         [0107]    The line exchange control unit  252 #i has the following functions:  
         [0108]    (i): Control the line exchange unit  32 #i in accordance with line setting in the event of no line failure.  
         [0109]    (ii): Control the line exchange unit  32 #i in accordance with a switching command output by the APS switching control unit  250 #i in the event of a line failure.  
         [0110]    Next, operations of the network shown in FIG. 2 are explained.  
         [0111]    (1): Operations in a Normal State  
         [0112]    As shown in FIG. 2, with respect to the NE  200 # 1 , an ETHER signal flows through the following three routes, namely, routes  1 ,  2  and  3 . As shown by a broken line in FIG. 2, route  1  (which is presently used lines) comprises a line between the ETHER interface unit  210 # 11  and the OC 48  west  212 # 11 , a line between the OC 48  west  212 # 11  and the OC 48  east  212 # 22  and a line between the OC 48  east  212 # 22  and the ETHER interface unit  210 # 21 . As shown by a broken line in FIG. 2, route  2  (which is also presently used lines) comprises a line between the ETHER interface unit  210 # 12  and the OC 48  east  212 # 12 , a line between the OC 48  east  212 # 12  and the OC 48  west  212 # 31  and a line between the OC 48  west  212 # 31  and the ETHER termination unit  220 # 31 . As shown by a single-dotted chain line in FIG. 2, route  3  (which is a PCA route) comprises a line between the ETHER interface unit  214 # 1  and the OC 48  west  212 # 11 , a line between the OC 48  west  212 # 11  and the OC 48  east  212 # 22  and a line between the OC 48  east  212 # 22  and the ETHER interface unit  214 # 2 .  
         [0113]    [0113]FIG. 9 is an explanatory diagram used for describing operations carried out by the NE  200 # 1  in a normal condition. A packet received by the ETHER termination unit  220 # 11  from an ETHER network  300 # 11  is supplied to the SONET mapping unit  222 # 11  either by way of the buffer  234 # 1  or directly. The SONET mapping unit  222 # 11  maps the packet onto an STS 1 ×24 frame, and outputs the frame to the line exchange unit  32 # 1 . A packet received by the ETHER termination unit  220 # 12  from an ETHER network  300 # 12  is supplied to the SONET mapping unit  222 # 12  either by way of the buffer  234 # 1  or directly. The SONET mapping unit  222 # 12  maps the packet onto an STS 1 ×24 frame, and outputs the frame to the line exchange unit  32 # 1 . A packet received by the ETHER termination unit  230 # 1  from an ETHER network  300 # 13  is supplied to the SONET mapping unit  232 # 11  either by way of the buffer  234 # 1  or directly. The SONET mapping unit  232 # 11  maps the packet onto an STS 1 ×24 frame, and outputs the frame to the line exchange unit  32 # 1 . The line exchange unit  32 # 1  carries out line exchange processing as shown in FIG. 9 in accordance with line setting.  
         [0114]    [0114]FIG. 10 is an explanatory diagram used for describing operations carried out by the NE  200 # 2  in a normal condition. A packet received by the ETHER termination unit  220 # 21  from the ETHER network  300 # 21  is supplied to the SONET mapping unit  222 # 21  either by way of the buffer  23442  or directly. The SONET mapping unit  222 # 21  maps the packet onto an STS 1 ×24 frame, and outputs the frame to the line exchange unit  32 # 2 . A packet received by the ETHER termination unit  230 # 2  from the ETHER network 300422 is supplied to the SONET mapping unit 232421 either by way of the buffer  23442  or directly. The SONET mapping unit 232421 maps the packet onto an STS 1 ×24 frame, and outputs the frame to the line exchange unit  3242 . The line exchange unit  3242  carries out line exchange processing as shown in FIG. 10 in accordance with line setting.  
         [0115]    [0115]FIG. 11 is an explanatory diagram used for describing operations carried out by the NE  200 # 3  in a normal condition. A packet received by the ETHER termination unit  220 # 31  from the ETHER network  300 # 31  is supplied to the SONET mapping unit  222 # 31 . The SONET mapping unit  222 # 31  maps the packet onto an STS 1 ×24 frame, and outputs the frame to the line exchange unit  32 # 3 . The line exchange unit  32 # 3  carries out line exchange processing as shown in FIG. 11 in accordance with line setting.  
         [0116]    (2): Operations Carried Out in the Event of a Line Failure  
         [0117]    [0117]FIG. 12 is a diagram showing a route established in the network shown in FIG. 2 in the event of a line failure. Assume that a line failure is detected on a line connecting the OC 48  east  212 # 12  of the NE  200 # 1  to the OC 48  west  212 # 31  of the NE  200 # 3  as indicated by a cross mark X shown in FIG. 12. When the NEs  200 # 1 ,  200 # 2  and  200 # 3  detect the line failure, routes  1 ,  2  and  3  are switched as shown in FIG. 12 in accordance with a BLSR recommendation as follows. Since route  1  is not affected by the line failure, route  1  remains the same as that for the normal state. That is to say, as shown by a broken line in FIG. 12, route  1  comprises a line between the ETHER interface unit  210 # 11  and the OC 48  west  212 # 11 , a line between the OC 48  west  212 # 11  and the OC 48  east  212 # 22  and a line between the OC 48  east  212 # 22  and the ETHER interface unit  210 # 21 . As shown by a dotted line in FIG. 12, on route  2 , a signal is cut off between the OC 48  east  212 # 12  and the OC 48  west  212 # 31 . Thus, route  2  is newly established to comprise a line between the ETHER interface unit  210 # 12  and the OC 48  west  212 # 11 , a line between the OC 48  west  212 # 11  and the OC 48  east  212 # 22 , a line between the OC 48  east  212 # 22  and the OC 48  west  212 # 21 , a line between the OC 48  west  212 # 21  and the OC 48  east  212 # 32  and a line between the OC 48  east  212 # 32  and the ETHER interface unit  210 # 31 . As shown by a single-dotted chain line in FIG. 12, on route  3 , effects by the line failure are shared with the ETHER interface unit  210 # 12 . Thus, much like that of the normal state, route  3  comprises a line between the ETHER interface unit  214 # 1  and the OC 48  west  212 # 11 , a line between the OC 48  west  212 # 11  and the OC 48  east  212 # 22  and a line between the OC 48  east  212 # 22  and the ETHER interface unit  210 # 22 .  
         [0118]    [0118]FIG. 13 is an explanatory diagram used for describing operations carried out by the NE  200 # 1  in the event of a line failure. FIG. 14 is a diagram showing a technique to share a line. A packet received by the ETHER termination unit  220 # 12  from the ETHER network  300 # 12  includes an additional identifier for identifying the ETHER termination unit  220 # 12  as shown in FIG. 5 and is stored in the buffer  234 # 1  as shown in FIG. 14. On the other hand, a packet received by the ETHER termination unit  230 # 1  from the ETHER network  300 # 13  includes an additional identifier for identifying the ETHER termination unit  230 # 1  and is stored in the buffer  234 # 1 . For each identifier, the number of packets read out from the buffer  234 # 1  is counted to form a judgment as to whether or not the throughput has exceeded an upper limit for the identifier. If the throughput has exceeded an upper limit, the packet is output to the SONET mapping unit  232 # 11  in accordance with the control executed on a priority basis as described above. The SONET mapping unit  232 # 11  maps the packet onto an STS 1 ×24 frame, which is subjected to line exchange processing to the OC 48  west  212 # 11  in the line exchange unit  32 # 1 .  
         [0119]    [0119]FIG. 15 is an explanatory diagram used for describing operations carried out by the NE  200 # 2  in the event of a line failure. FIG. 16 is a diagram showing a technique to share a line. The OC 48  east  212 # 22  separates an STS 1 ×48 frame from an OC 48  packet received from the SONET network-BLSR left handed unit  310 #L, and the line exchange unit  32 # 2  supplies the STS 1 ×24 frame to the SONET mapping unit  232 # 21 . The SONET mapping unit  232 # 21  demaps the STS 1 ×48 frame back onto a packet, which is then stored in the buffer  234 # 2 . The additional identifier included in the packet stored in the buffer  234 # 2  is examined to form a judgment as to whether or not the flow of the packet is to be terminated at the ETHER termination unit  230 # 2 . If the flow of the packet is to be terminated at the ETHER termination unit  230 # 2 , the packet is output to the ETHER termination unit  230 # 2  with the identifier removed. If the flow of the packet is not to be terminated at the ETHER termination unit  230 # 2 , on the other hand, the packet is output to the SONET mapping unit  232 # 22  with the identifier kept in the packet. The packet is mapped by the SONET mapping unit  232 # 22  onto an STS 1 ×24 frame, which is then subjected to line exchange processing to the OC 48  west  212 # 21  in the line exchange unit  32 # 2 .  
         [0120]    [0120]FIG. 17 is an explanatory diagram used for describing operations carried out by the NE  200 # 3  in the event of a line failure. Since the NE  200 # 3  does not employ a spare line, the operations are the same as the conventional transmission apparatus. That is to say, the OC 48  east  221 # 32  separates an STS 1 ×24 frame from an OC 48  packet received from the SONET network-BLSR left handed unit  310 #L, which has received the packet from the ETHER network  300 # 31  through the ETHER termination unit  220 # 31 . The STS 1 ×24 frame is supplied to the line exchange unit  32 # 3 . The line exchange unit  32 # 3  then supplies the STS 1 ×24 frame received from the OC 48  east  221 # 32  to the ETHER interface unit  210 # 31  in accordance with line switching.  
         [0121]    It is to be noted that, even when a plurality of line failures occur, lines are shared by one of a plurality of presently used lines and a spare line. In the embodiment described above, in the case of the BLSR switching technique, a line is shared by a PCA line and a switched route. Thus, the efficiency of the line utilization can be improved. In this embodiment, the BLSR switching technique is adopted. However, the present invention can also be applied to an UPSR switching technique. In addition, the present invention can also be applied to transmissions of data information with the connectionless type through SONET such as Packet over SONET, ATM over SONET, ETHER over SONET or IP over SONET. In addition, in the transmission apparatus, by merely connecting an audio interface unit, which is used for terminating an audio network and accommodating audio information received from the audio network in a SONET/SDH frame, to a line exchange unit, it is possible to construct a SONET/SDH network in which audio signals and data signals coexist without modifying other configurations at all.  
         [0122]    Second Embodiment  
         [0123]    [0123]FIG. 18 is a block diagram showing the configuration of a network adopting an 1:N (APS) technique in accordance with a second embodiment of the present invention. A typical case for which N=2 is explained. It is to be noted that, by setting N at 1, the configuration shown in FIG. 18 can be applied to an APS (1+1) configuration. As shown in FIG. 18, ETHER interface units  360 # 1   j  where j=1 and 2 are connected to ETHER interface units  360 # 2   j  where j=1 and 2 by OC 12  interface units  362 # 1   j  where j=1 and 2 and 362# 2   j  where j=1 and 2. OC 12  interface units  370 # 1  and  370 # 2  are each a spare line. In the case of this embodiment, however, the OC 12  interface units  370 # 1  and  370 # 2  are used for communications in a normal state in order to increase the efficiency of the line utilization. In the event of a line failure occurring on any one of lines between the OC 12  interface units  362 # 1   j  where j=1 and 2 and the OC 12  interface units  362 # 2   j  where j=1 and 2, a line is shared by the ETHER interface units  370 # 1  and  370 # 2  and ETHER interface units  360 # 1   j  and  360 # 2   j.    
         [0124]    [0124]FIG. 19 is a block diagram showing the configuration of the NE  350 #i used in the network shown in FIG. 18. ETHER termination units  380 #ij where j=1 and 2 and 390#i are each essentially identical with the ETHER termination unit  220 #ijk employed in the NE shown in FIG. 3. SONET mapping units  382 #ij where j=1 and 2 and  392 #i are each essentially identical with the SONET mapping unit  222 #ijk employed in the NE shown in FIG. 3 except that the SONET mapping units  382 #ij where j=1 and 2 and  392 #i each map a packet onto an STS 1 ×12 frame instead of an STS 1 ×24 frame. A buffer  394 #i is essentially identical with the buffer  234 #i employed in the NE shown in FIG. 3. A control unit  400 #i has an APS control unit  410 #i, a line control unit  412 #i and a buffer control unit  414 #i. Basic operations of the control unit  400 #i are essentially identical with those of the control unit  218 #i employed in the NE shown in FIG. 3 except that the adopted switching technique is an APS technique instead of the BLSR technique.  
         [0125]    The operations of the network shown in FIG. 18 are explained as follows.  
         [0126]    (1): Normal Operations  
         [0127]    [0127]FIG. 20 is an explanatory diagram used for describing operations carried out in the network shown in FIG. 18 in a normal condition. As shown in FIG. 20, there are established routes comprising lines between the ETHER interface units  360  # 1   j  where j  1  and  2  and the OC 12  interface units  362 # 1   j  where j  1  and  2 , lines between the OC 12  interface units  362 # 1   j  where j=1 and 2 and the OC 12  interface units  362 # 2   j  where j=1 and 2 and lines between the OC 12  interface units  362 # 2   j  where j=1 and 2 and the ETHER interface units  360 # 2   j  where j=1 and 2.  
         [0128]    (2): Operations Carried Out in the Event of a Line Failure  
         [0129]    [0129]FIG. 21 is an explanatory diagram used for describing operations carried out in the network shown in FIG. 18 in the event of a line failure. As indicated by a cross mark X in FIG. 21, a line failure is detected on a line between the OC 12  interface unit  362 # 11  and the OC 12  interface unit  362 # 21 . FIG. 22 is a diagram showing a technique to share a line. The control unit  400 # 1  adds an identifier to each of packets output by the ETHER termination units  380 # 11  and  390 # 1  before storing the packets into the buffer  394 # 1 . Subsequently, a packet is read out from the buffer  394 # 1  and output to the SONET mapping unit  392 # 1 , which then maps the packet onto an STS 1 ×12 frame. The STS 1 ×12 frame is subjected to line exchange processing in the line exchange unit  32 # 1  and multiplexed in an OC 12  packet, which is then supplied to the OC 12  interface unit  372 # 2 . The OC 12  interface unit  372 # 2  separates the STS 1 ×12 frame from the OC 12  packet. The STS 1 ×12 frame is subjected to line exchange processing in the line exchange unit  32 # 2  and supplied to the SONET mapping unit  392 # 2 , which then demaps the frame onto the original packet. The packet is stored in the buffer  394 # 2  to be output later to the ETHER termination units  380 # 1   j  where j=1 and 2 with the identifiers removed.  
         [0130]    Furthermore, if a line failure is also detected on a line between the OC 12  interface unit  362 # 12  and the OC 12  interface unit  362 # 22 , communications through a spare line between OC 12  interface unit  372 # 1  and  37242  are carried out, sharing the spare line by the line between the ETHER interface units  360 # 1   j  and  360 # 2   j  where j=1 and 2, and the line between the ETHER interface units  37041  and  37042  in order to improve the efficiency of the line utilization.  
         [0131]    Third Embodiment  
         [0132]    [0132]FIG. 23 is a block diagram showing the configuration of a network adopting a 1:N (APS) technique in accordance with a third embodiment of the present invention. A typical case for which N=2 is explained. In the case of this embodiment, lines of the SONET network have different bands, namely, OC 12 , OC 3  and OC 48  bands. In addition, the OC 48  band of a spare line is broader than the other OC 12  and OC 3  bands of presently used lines. In the event of a line failure, a spare line between an OC 48  interface unit  514 # 1  and an OC 48  interface unit  514 # 2  is shared by a route between ETHER interface units  360 # 1   j  and  360 # 2   j , which is involved in the line failure, and a line between ETHER interface units  370 # 1  and  370 # 2 . For this reason, a spare line with a broad bandwidth is used in order to prevent a packet from being discarded.  
         [0133]    [0133]FIG. 24 is a block diagram showing the configuration of an NE  500 #i used in the network shown in FIG. 23. Configuration elements virtually identical with their counterparts employed in the NE shown in FIG. 19 are denoted by the same reference numerals as the counterparts. A SONET mapping unit  600 #i 2  of an ETHER interface unit  510 #i 2  is substantially identical with the SONET mapping unit  380 #i 1  except that the SONET mapping unit  600 #i 2  maps a packet onto an STS 1 ×3 frame instead of an STS 1 ×12. By the same token, a SONET mapping unit  602 #i of an ETHER interface unit  514 #i is substantially identical with the SONET mapping unit  392 #i except that the SONET mapping unit  602 #i maps a packet onto an STS 1 ×24 frame instead of an STS 1 ×12. An OC 3  interface unit  510 #i 2  is substantially identical with the OC 12  interface unit  362 #i 2  except that the OC 3  interface unit  510 # 12  multiplexes a frame in an OC 3  packet instead of an OC 12  packet.  
         [0134]    If a line failure is detected on a line between an OC 12  interface unit  362 # 11  and an OC 3  interface unit  362 # 21  and/or a line between an OC 3  interface unit  512 # 12  and an OC 3  interface unit  512 # 22 , the spare line between the OC 48  interface unit  516 # 1  and the OC 48  interface unit  516 # 2  is shared by a line between an ETHER interface unit  360 # 11  and an ETHER interface unit  360 # 21  and/or a line between an ETHER interface unit  510 # 12  and an ETHER interface unit  510 # 22  and the line between the ETHER interface unit  514 # 1  and the ETHER interface unit  514 # 2 . Since the line capacity of the spare line is big, communications can be carried out without discarding packets.  
         [0135]    In the case of this embodiment, a presently used line, on which a line failure has been detected, is replaced by a spare line. However, a presently used line, on which a line failure has been detected, can also be replaced by another presently used line. In addition, a spare line, on which a line failure has been detected, can also be replaced by a presently used line. In this case, the presently used lines and the spare line are each provided with a buffer and a spare SONET mapping unit, which are controlled by the control unit to carry out a switching operation. In accordance with the present invention, in a transmission of connectionless type information called a packet by mapping the packet onto information with a connection type, in the event of a line failure, communications through the failing line are shared by a spare line and a presently used line.  
         [0136]    The present invention is not limited to the details of the above described preferred embodiments. The scope of the present invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.