Abstract:
In a method for transmitting data from a first node to a second node through an interlinking network including data transmission equipments: the data is transmitted from the first node to one of the data transmission equipments together with a first request for storage of the data in the one of the data transmission equipments; the data is stored in a storage unit provided in the one of the data transmission equipments in response to the first request; a second request for the data is transmitted from the second node to the one of the data transmission equipments; the data is read out from the storage unit in response to the second request; and the data is transmitted from the one of the data transmission equipments to the second node.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1) Field of the Invention  
           [0002]    The present invention relates to a method for transmitting data from a first network node (e.g., an information server) to a second network node (e.g., a client) through an interlinking network which includes a plurality of transmission equipments. The present invention also relates to a data transmission system including a plurality of transmission equipments and transmitting data from a first network node to a second network node. The present invention further relates to a transmission equipment used in a data transmission system interconnecting first and second network nodes.  
           [0003]    2) Description of the Related Art  
           [0004]    Recently, the bandwidths of local networks have been increased, and the bandwidths of the access networks have also been increased by the development of the technologies of XDSL (X Digital Subscriber Line), FTTH (Fiber To The Home), and the like. With the increase in the bandwidths of the local and access networks, business opportunities are increasing, and various services are becoming available. In such a situation, increase in the bandwidths of backbone or interlinking networks (e.g., use of broadband networks) is also required.  
           [0005]    In particular, attempts are currently made to transmit data requiring wide bandwidth (e.g., sound data or image data) by using the IP (Internet Protocol) platform. For example, in some applications such as delivery of radio programs or music, the attempted data transmission has become commercially practical. In addition, low-quality image delivery services have already become commercially practical.  
           [0006]    Currently, in order to realize the above data transmission using the IP platform, many distributed systems have been constructed by arranging a number of cache servers in the Internet. In such distributed systems, data are delivered by network elements using the IP multicast technique.  
           [0007]    In addition, in order to increase the overall throughput of the data transmission system and realize music or images delivery services to a wide area, attempts have been made to directly connect a LAN (Local Area Network) to a broadband, backbone or interlinking network such as a SONET (or SDH) network or a WDM (Wavelength Division Multiplex) network. It is considered that the direct connection of a LAN to a SONET (or SDH) network or a WDM network will be widely used in the future.  
           [0008]    Next, the data transmission using the IP platform and the transmission through the SONET (or SDH) network are explained below.  
           [0009]    (i) Data Transmission Using IP Platform  
           [0010]    [0010]FIG. 20 is a diagram illustrating an example of a conventional image delivery or cable television system which uses the Internet. The example of FIG. 20 has a typical LAN-WAN (Wide Area Network)-LAN configuration.  
           [0011]    In FIG. 20, reference numeral  10  denotes a camera,  11  denotes an image transfer equipment,  12  denotes an image server,  13  denotes an L 2  (Layer  2 ) switch,  14  denotes an IIP (Internet Imaging Protocol) router,  15  denotes a WAN,  16  denotes an IIP router,  17  denotes an image server,  18  denotes an L 2  switch, and  19 - 1  to  19 - 3  each denote a client.  
           [0012]    The camera  10  captures an image of an object and sound, converts the image and sound into digital data, and outputs the digital data. The image transfer equipment  11  generates an IP packet containing the digital data output from the camera  10 , and transmits the IP packet through the L 2  switch  13 . Each of the image servers  12  and  17  temporarily stores image data, and transmits the image data in response to a request from the clients  19 - 1  to  19 - 3 . Each of the L 2  switches  13  and  18  performs packet switching in the data link layer. Each of the IIP routers  14  and  16  transfers image data in accordance with a protocol for exchanging image data and related information.  
           [0013]    For example, the WAN  15  is realized by an ATM (Asynchronous Transfer Mode) network, a SONET (or SDH) network, an ISDN network, or the like, and uses the MPLS (Multiprotocol Label Switching) technique.  
           [0014]    Each of the clients  19 - 1  to  19 - 3  receives image data and displays an image.  
           [0015]    The image data and the sound data output from the camera  10  are delivered to the clients  19 - 1  to  19 - 3  in different manners according to the services requested by the clients  19 - 1  to  19 - 3 .  
           [0016]    In a first type of service, the image data and the sound data output from the camera  10  are temporarily stored in the image server  17 . Thereafter, when one of the clients  19 - 1  to  19 - 3  makes a request for the image data and the sound data, the image data and the sound data are transmitted to the one of the clients  19 - 1  to  19 - 3  through the L 2  switch  18 .  
           [0017]    In a second type of service (real-time delivery service), the image data and the sound data output from the camera  10  are first contained in an IP packet by the image transfer equipment  11 , and then multicast delivered to the clients  19 - 1  to  19 - 3  by using the IP multicast technique.  
           [0018]    However, the volume of image data transmitted in the conventional image delivery or cable television system is basically great. In particular, television broadcasting constantly requires for each channel a bandwidth of 3 to 10 Mbit/s in the case of standard definition television data and 20 Mbit/s in the case of high definition television data. In the case of sound data, a bandwidth of 80 to 130 Kbit/s is required.  
           [0019]    Therefore, the conventional system is sufficient to deliver the sound data. However, in order to deliver the image data, it is necessary to control the bandwidth and increase the transmission rate in the entire system including the LANs and WANs. For example, in the case where 10 channels of high definition television data are delivered, a bandwidth of 200 Mbps (=20 Mbps×10 ch) is constantly required.  
           [0020]    Nevertheless, the constant flow of the above traffic impedes transmission of other information, and is not economical.  
           [0021]    It is well known that bottlenecks in networks are typically caused by differences in the interface speed between LANs and WANs. For example, many WANs have ISDN (64/1,544 Kbps DS1), ATM (155 Mbps), or XDSL interfaces for connection to users. In addition, in an increasing number of cases of transmission, data transmitted from the users through the ISDN, ATM, or XDSL interfaces are multiplexed by using the SONET or SDH technology and transmitted through transmission lines of OC-3 (155 Mbps), OC-12 (622 Mbps), OC-48 (2.5 Gbps), OC-192 (10 Gbps), or the like. In order to realize the above operation, for example, a POS (Packet Over Sonet) unit is installed in IP routers. On the other hand, recently, many LANs uses the Ethernet having a transmission rate of 100 Mbps or 1 Gbps. That is, the bandwidths of the interfaces between the WANs and the LANs are much smaller than those of the LANs and WANs, and the transmission is delayed by the small bandwidths of the interfaces between the WANs and the LANs.  
           [0022]    Further, in transmission of a great amount of images and sound data, images and sound received by clients (users&#39; PCs) may be interrupted when packets are delayed, discarded, or disordered by the influence of the reduction of the bandwidth in the WAN and the best effort transmission of IP packets.  
           [0023]    In order to solve the above problem, conventionally, cache servers are distributed in the Internet so that information is stored by the cache servers, and the delay, discard, and disorder of the packets are absorbed.  
           [0024]    [0024]FIG. 21 is a diagram illustrating an example of a configuration of cache servers in the Internet. In FIG. 21, the other network elements such as IP routers and hubs are not shown.  
           [0025]    As illustrated in FIG. 21, the cache servers  30   a  to  30   d  are distributed in the WAN  30 , the clients  31  and  32  are connected to the cache server  30   d,  the clients  33  and  34  are connected to the cache server  30   c,  and the client  35  is connected to the cache server  30   b.    
           [0026]    Since the cache servers  30   a  to  30   d  are arranged near the corresponding clients  31  to  35 , it is possible to avoid inefficient access to original contents in response to each connection request, improve response, and reduce traffic.  
           [0027]    (ii) Conventional SONET or SDH System  
           [0028]    The SONET transmission equipments are high-speed transmission equipments mainly used in backbone networks of network providers, and various data such as data of telephones, leased lines, and IP packets, for which the network providers provide service, are transmitted through the SONET transmission equipments. The SONET transmission equipments multiplex such data into a high-speed transmission signal having a transmission rate of 2.5 Gbps, 10 Gbps, 40 Gbps, or the like, and transmit the high-speed transmission signal through a single optical fiber, by using the SONET or SDH technology. In SONET or SDH networks, each path has uniform traffic, and synchronism is maintained.  
           [0029]    [0029]FIG. 22 is a diagram for explaining the bandwidths of incoming lines and corresponding outgoing lines. In the path indicated by the double lines with arrows the example of FIG. 22, the transmission line incoming into the SONET transmission equipment  40  and the transmission line outgoing from the SONET transmission equipment  41  have an identical transmission rate 2.5 Gbps. On the other hand, in the path indicated by the single lines with arrows, the transmission line incoming into the SONET transmission equipment  43  has an transmission rate 2.5 Gbps, and the transmission line outgoing from the SONET transmission equipment  42  has an transmission rate 622 Mbps. This is impractical.  
           [0030]    In the packet transmission of data, basically, more than one network terminal shares a transmission line (i.e., a bandwidth of a transmission line), and the synchronism is not secured. On the other hand, in the SONET networks, bandwidth allocation to each transmission line is predetermined, and the synchronism is maintained.  
           [0031]    However, conventionally, in order to provide a broadband service in a wide area, cache servers and other network elements are required to be distributed over the wide area under various conditions. Therefore, sufficient bandwidths are not necessarily secured between a data source server and the cache servers. In this case, the amount of data output from the data source server is limited. Thus, even when the bandwidth is great in only a portion (e.g., in an interlinking network such as a SONET or SDH network) of a transmission path between the data source server and each cache server, the great bandwidth of the portion cannot be efficiently used.  
           [0032]    Further, in a broadband service such as an image or sound delivery service, data sent to the respective cache servers are often identical. However, conventionally, the data source server is required to send the identical data to each cache server separately. FIG. 23 is a diagram illustrating an example of a configuration for transmitting data from a data source server to a plurality of cache servers in a conventional data delivery service system. In the example of FIG. 23, the contents server (data source server)  51  is required to send identical data to each of the cache servers  52  to  54 , i.e., the contents server  51  is required to transmit the identical data three times. Therefore, utilization efficiency of the bandwidth of the WAN  50  is low.  
         SUMMARY OF THE INVENTION  
         [0033]    An object of the present invention is to provide a method for efficiently transmitting a great volume of data such as image data or sound data from a first network node to a second network node through an interlinking network.  
           [0034]    Another object of the present invention is to provide a data transmission system which can efficiently transmit a great volume of data such as image data or sound data from a first network node to a second network node through an interlinking network.  
           [0035]    A further object of the present invention is to provide a data transmission equipment which can be used in a data transmission system, and realize efficient transmission of a great volume of data such as image data or sound data in the data transmission system.  
           [0036]    (I) According to the first aspect of the present invention, there is provided a method for transmitting data from a first network node to a second network node through an interlinking network including a plurality of data transmission equipments. The method comprises the steps of: (a) transmitting the data together with a first request for storage of the data in one of the plurality of data transmission equipments, from the first network node to the one of the plurality of data transmission equipments; (b) receiving the data and the first request by the one of the plurality of data transmission equipments; (c) storing the data in a storage unit provided in the one of the plurality of data transmission equipments, based on the first request; (d) transmitting a second request for the data, from the second network node to the one of the plurality of data transmission equipments; (e) receiving the second request by the one of the plurality of data transmission equipments; (f) reading out the data from the storage unit based on the second request; and (g) transmitting the data from the one of the plurality of data transmission equipments to the second network node.  
           [0037]    According to the first aspect of the present invention, data transmitted from the first network node is temporarily stored in the storage unit in the one of the plurality of data transmission equipments in accordance with the first request, and is thereafter transmitted to the second network node in response to a second request from the second network node. Therefore, the transmission of data from the first network node to the one of the plurality of data transmission equipments can be performed independently of the transmission rate between the one of the plurality of data transmission equipments and the second network node, and can be therefore performed at high speed. In addition, the data can be transmitted from the one of the plurality of data transmission equipments to the second network node at a transmission rate which is optimum for the second network node.  
           [0038]    Further, the operations for readout and transmission of the data to the second network node do not affect traffic between the first network node and the one of the plurality of data transmission equipments.  
           [0039]    In the method according to the first aspect of the present invention, the step (d) may comprise the substeps of: (d1) transmitting the second request from the second network node to the first network node; and (d2) transferring the second request from the first network node to the one of the plurality of data transmission equipments.  
           [0040]    In addition, in the step (a), the data and the first request may be multiplexed.  
           [0041]    (II) According to the second aspect of the present invention, there is provided a data transmission system comprising: a plurality of network nodes; and an interlinking network which includes a plurality of data transmission equipments, interlinks the plurality of network nodes, and transmits data between the plurality of network nodes in a multiplexed form. In the data transmission system, one of the plurality of data transmission equipments includes: a reception unit which receives first data; a control information extraction unit which extracts control information from the first data received by the reception unit; a data acquisition unit which determines whether or not the control information includes a request for storage of the first data, and acquires the first data when the control information includes the request for storage of the first data; and a data storage unit which stores the first data acquired by the data acquisition unit.  
           [0042]    In the data transmission system according to the second aspect of the present invention, the one of the plurality of data transmission equipments may further include: a data readout unit which determines whether or not the control information includes a request for readout of second data stored in the data storage unit, and reads out the second data from the data storage unit when the control information includes the request for readout of the second data; and a transmission unit which transmits the second data to a destination.  
           [0043]    (III) According to the third aspect of the present invention, there is provided a data transmission equipment for use as a constituent of a multiplex transmission network. The data transmission equipment comprises: a reception unit which receives first data; a control information extraction unit which extracts control information from the first data received by the reception unit; a data acquisition unit which determines whether or not the control information includes a request for storage of the first data, and acquires the first data when the control information includes the request for storage of the first data; and a data storage unit which stores the first data acquired by the data acquisition unit.  
           [0044]    The data transmission equipment according to the third aspect of the present invention may also have one or any possible combination of the following additional features (i) to (x).  
           [0045]    (i) The data transmission equipment according to the third aspect of the present invention may further comprise: a data readout unit which determines whether or not the control information includes a request for readout of second data stored in the data storage unit, and reads out the second data from the data storage unit when the control information includes the request for readout of the second data; and a transmission unit which transmits the second data to a destination.  
           [0046]    (ii) The reception unit receives the first data at a first transmission rate, the transmission unit transmits the second data at a second transmission rate, and the first and second transmission rates can be determined independently of each other.  
           [0047]    (iii) The transmission unit transmits the second data to a plurality of destinations.  
           [0048]    (iv) The data transmission equipment according to the third aspect of the present invention may further comprise a control information insertion unit which inserts control information in the second data transmitted by the transmission unit.  
           [0049]    (v) The data transmission equipment according to the third aspect of the present invention may further comprise a data erasing unit which determines whether or not the control information includes a request for erasing of third data stored in the data storage unit, and erases the third data from the data storage unit when the control information includes the request for erasing of the third data  
           [0050]    (vi) The data storage unit is arranged in the reception unit.  
           [0051]    (vii) The data storage unit is arranged in the transmission unit.  
           [0052]    (viii) The data transmission equipment according to the third aspect of the present invention may further comprise a switch unit which cross-connects transmission lines, and the data storage unit is arranged in the switch unit.  
           [0053]    (ix) The reception unit receives the first data in accordance with one of SONET and SDH technologies.  
           [0054]    (x) The transmission unit transmits the second data in accordance with one of SONET and SDH technologies.  
           [0055]    (IV) The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiment of the present invention by way of example. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0056]    In the drawings:  
         [0057]    [0057]FIG. 1 is a diagram illustrating a basic construction of the data transmission system according to the present invention;  
         [0058]    [0058]FIG. 2 is a diagram illustrating an example of a data transmission system as a first embodiment of the present invention;  
         [0059]    [0059]FIG. 3 is a diagram illustrating an example of a construction of the transformation equipment  71 ;  
         [0060]    [0060]FIG. 4 is a diagram illustrating an example of a construction of the transformation equipment  76 ;  
         [0061]    [0061]FIG. 5 is a diagram illustrating an example of a construction of the SONET transmission equipment  73 ;  
         [0062]    [0062]FIG. 6 shows examples of information items which can be included in control information;  
         [0063]    [0063]FIG. 7 shows the fields (bytes) in a POH in a virtual tributary;  
         [0064]    [0064]FIG. 8 is a flow diagram indicating an example of a sequence of operations performed by the transformation equipment  71 ;  
         [0065]    [0065]FIG. 9 is a flow diagram indicating an example of a sequence of operations performed by the SONET transmission equipment  72 ;  
         [0066]    [0066]FIG. 10 is a flow diagram indicating an example of a sequence of operations performed by the SONET transmission equipment  73 ;  
         [0067]    [0067]FIG. 11 is a flow diagram indicating details of the data storage processing in step S 46  in FIG. 10;  
         [0068]    [0068]FIG. 12 is a flow diagram indicating details of the data readout processing in step S 48  in FIG. 10;  
         [0069]    [0069]FIG. 13 is a flow diagram indicating details of the data erasing processing in step S 50  in FIG. 10;  
         [0070]    [0070]FIG. 14 is a flow diagram indicating details of the data lock processing in step S 52  in FIG. 10;  
         [0071]    [0071]FIG. 15 is a flow diagram indicating details of the data unlock processing in step S 54  in FIG. 10;  
         [0072]    [0072]FIG. 16 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in a second embodiment of the present invention;  
         [0073]    [0073]FIG. 17 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in a third embodiment of the present invention;  
         [0074]    [0074]FIG. 18 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in a fourth embodiment of the present invention;  
         [0075]    [0075]FIG. 19 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in a fifth embodiment of the present invention;  
         [0076]    [0076]FIG. 20 is a diagram illustrating an example of a conventional image delivery or cable television system which uses the Internet;  
         [0077]    [0077]FIG. 21 is a diagram illustrating an example of a configuration of cache servers in the Internet;  
         [0078]    [0078]FIG. 22 is a diagram for explaining the bandwidths of incoming lines and corresponding outgoing lines; and  
         [0079]    [0079]FIG. 23 is a diagram illustrating an example of a configuration for transmitting data from a data source server to a plurality of cache servers in a conventional data delivery service system.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0080]    Embodiments of the present invention are explained in detail below with reference to drawings.  
         [0081]    (1) Basic Construction  
         [0082]    [0082]FIG. 1 is a diagram illustrating a basic construction of the data transmission system according to the present invention. In FIG. 1, reference numeral  61  denotes a contents server,  60  and  62  each denote a data transmission equipment, and  63  denotes a client. As illustrated in FIG. 1, the data transmission equipments  62  and  60  are connected between the contents server  61  and the client  63 , and constitute a WAN. The contents server  61  transmits data (e.g., image data) to be supplied to the client  63 . For example, the client  63  is a terminal of a user, and realized by a personal computer. The WAN may include other data transmission equipments, and the data transmission system of FIG. 1 may include other client.  
         [0083]    The data transmission equipment  60  comprises a reception unit  60   a,  a control information extraction unit  60   b,  a data acquisition unit  60   c,  a storage unit  60   d,  a readout unit  60   e,  and a transmission unit  60   f.    
         [0084]    The reception unit  60   a  in the data transmission equipment  60  receives data transmitted from the data transmission equipment  62 . The control information extraction unit  60   b  extracts control information from the data received by the reception unit  60   a.  When the control information extracted by the control information extraction unit  60   b  includes a request for storage of data, the data acquisition unit  60   c  acquires the data from the data received by the reception unit  60   a.  The storage unit  60   d  stores the data acquired by the data acquisition unit  60   c.  When the control information extracted by the control information extraction unit  60   b  includes a request for readout of the data stored in the storage unit  60   d,  the readout unit  60   e  reads out the data from the storage unit  60   d.  The transmission unit  60   f  transmits to the client  63  the data read out by the readout unit  60   e.    
         [0085]    (2) Operation of Basic Construction  
         [0086]    First, the contents server  61  supplies to the data transmission equipment  62  data (for example, image data) and a request for storage of the image data in the storage unit  60   d  in the data transmission equipment  60 . When the data transmission equipment  62  receives the image data and the request for storage, the data transmission equipment  62  generates first control information which indicates the requests for storage of the image data in the storage unit  60   d  in the data transmission equipment  60 , based on the image data, and transmits the image data and the first control information to the data transmission equipment  60 . The reception unit  60   a  in the data transmission equipment  60  receives the image data and the first control information transmitted from the data transmission equipment  62 . The first control information extraction unit  60   b  in the data transmission equipment  60  extracts the first control information, recognizes that the first control information includes a request for storage, and supplies the first control information to the data acquisition unit  60   c.  The data acquisition unit  60   c  acquires a portion or all of the image data associated with the first control information, and stores the portion or all of the image data in an area of the storage unit  60   d.    
         [0087]    In the above operations, the transmission of data from the contents server  61  to the data transmission equipment  60  can be performed independently of the transmission rate between the data transmission equipment  60  and the client  63 , and can be therefore performed at high speed.  
         [0088]    Next, when the client  63  sends to the contents server  61  a request for readout of the image data from the storage unit  60   d  in the data transmission equipment  60  in the situation in which the image data is stored in the storage unit  60   d  in the data transmission equipment  60 , the contents server  61  sends to the data transmission equipment  60  through the data transmission equipment  62  second control information which indicates the request for readout of the image data from the storage unit  60   d  in the data transmission equipment  60 . The control information extraction unit  60   b  in the data transmission equipment  60  extracts the second control information, recognizes that the second control information includes a request for readout, and supplies the second control information to the readout unit  60   e.  The readout unit  60   e  reads out from the storage unit  60   d  a portion or all of the image data stored in the storage unit  60   d  and designated by the second control information, and supplies to the transmission unit  60   f  the portion or all of the image data read out from the storage unit  60   d.  The transmission unit  60   f  transmits to the client  63  the portion or all of the image data read out from the storage unit  60   d.    
         [0089]    During the above operations for readout and transmission to the client  63 , only the second control information is transmitted between the contents server  61  and the data transmission equipment  60 . Therefore, the operations for readout and transmission to the client  63  do not affect data transmission for the other users.  
         [0090]    In addition, the transmission rate between the transmission unit  60   f  in the data transmission equipment  60  and the client  63  can be determined independently of the transmission rate between data transmission equipment  62  and the data transmission equipment  60 . Therefore, the data can be transmitted from the data transmission equipment  60  to the client  63  at an optimum transmission rate for the client  63 .  
         [0091]    When a plurality of clients are connected to the transmission unit  60   f,  the transmission unit  60   f  can concurrently transmit identical data to the plurality of clients.  
         [0092]    As explained above, in the data transmission system according to the present invention, the storage unit  60   d  is provided in the data transmission equipment  60  for storing data supplied by the contents server  61 , and thereafter, the data is read out and transmitted to the client  63  in response to a request from the client  63 . Therefore, the transmission of data from the contents server  61  to the data transmission equipment  60  is not affected by the transmission rate between the data transmission equipment  60  and the client  63  and can be performed at high speed. In addition, the transmission of data from the data transmission equipment  60  to the client  63  can be performed at an optimum transmission rate for the client  63 .  
         [0093]    (3) Construction of First Embodiment  
         [0094]    [0094]FIG. 2 is a diagram illustrating an example of a data transmission system as the first embodiment of the present invention. In FIG. 2, reference numeral  70  denotes a contents server,  71  and  76  each denote a transformation equipment,  72  to  75  each denote a SONET transmission equipment, and  77  to  79  each denote a client.  
         [0095]    The contents server  70  stores image data and sound data, and transmits a portion of the image data and sound data in response to a request from the clients  77  to  79  or an information provider, which provides information delivery services and is not shown in FIG. 2.  
         [0096]    Hereinafter, for simplicity of explanation, it is assumed that the data transmitted in the data transmission system according to the present invention are image data. However, any data which do not require real-time transmission can be transmitted in the data transmission system according to the present invention.  
         [0097]    The transformation equipment  71  receives IP packets being transmitted from the contents server  70  and containing image data and control information, transforms the IP packets into SONET frames, and transmits the SONET frames to the SONET transmission equipment  72 . In the SONET frames, the image data and the control information are associated with each other.  
         [0098]    [0098]FIG. 3 is a diagram illustrating an example of a construction of the transformation equipment  71 . The transformation equipment  71  of FIG. 3 comprises input ports  71   a  to  71   c,  a transformation circuit  71   d,  a control information extraction circuit  71   e,  and an output port  71   f.  The input ports  71   a  to  71   c  are connected to the contents server  70  and other nodes (not shown), and the transformation equipment  71  receives IP packets through the input ports  71   a  to  71   c.    
         [0099]    The control information extraction circuit  71   e  extracts control information from the IP packets received through the input ports  71   a  to  71   c,  and supplies the extracted control information to the transformation circuit  71   d.  The transformation circuit  71   d  transforms the IP packets into SONET frames, and supplies the SONET frames to the output port  71   f.  The control information extracted by the control information extraction circuit  71   e  is inserted in the SONET frames as a portion of overhead information. The output port  71   f  transmits the SONET frames to the SONET transmission equipment  72 .  
         [0100]    Referring back to FIG. 2, the transformation equipment  76  receives SONET frames from the SONET transmission equipment  73 , transforms the received SONET frames into IP packets, and transmits the IP packets to the clients  77  to  79  according to control information contained in the received SONET frames.  
         [0101]    [0101]FIG. 4 is a diagram illustrating an example of a construction of the transformation equipment  76 . The transformation equipment  76  of FIG. 4 comprises an input port  76   a,  a transformation circuit  76   b,  a control information extraction circuit  76   c,  and output ports  76   d  to  76   f.  The transformation equipment  76  receives SONET frames from the SONET transmission equipment  73  through the input port  76   a.  The control information extraction circuit  76   c  extracts control information from the SONET frames received through the input port  76   a,  and supplies the extracted control information to the transformation circuit  76   b.  The transformation circuit  76   b  transforms the received SONET frames into IP packets, and transmits the IP packets to the output ports  76   d  to  76   f.  The control information extracted by the control information extraction circuit  76   c  is inserted in the headers of the IP packets. The IP packets are transmitted to the clients  77  to  79  through the output ports  76   d  to  76   f.  Alternatively, the output ports  76   d  to  76   f  in the transformation equipment  76  may be replaced with a single output port. In this case, the IP packets output from the single output port can be transferred to the clients  77  to  79  by using a router or the like.  
         [0102]    Referring back to FIG. 2, the SONET transmission equipments  72  to  75  each have a function of a SONET ADM (add-drop multiplexer), and realize high-speed data transmission by multiplexing and demultiplexing received data. In the example of FIG. 2, the SONET transmission equipments  72  to  75  constitute a RING network which transmits, for example, OC-768 signals having a transmission rate of 40 Gbps.  
         [0103]    [0103]FIG. 5 is a diagram illustrating an example of a construction of the SONET transmission equipment  73 . The SONET transmission equipment  73  of FIG. 5 comprises input buffers  100  to  103 , POH termination units  104  to  107 , a control information extraction unit  108 , a path switch unit  109 , a hard disk unit  110 , a data storage control unit  111 , a controller  112 , a path selection unit  113 , POH insertion units  114  to  117 , a control information insertion unit  118 , framers  119  to  122 , and E/O conversion units  123  to  126 . Although not shown in FIG. 5, the SONET transmission equipment  73  further comprises a TDM (Time Division Multiplex) demultiplexing unit which receives SONET frames, and demultiplexes the SONET frames into virtual tributaries and other overhead information.  
         [0104]    The input buffers  100  to  103  temporarily store the virtual tributaries of the received SONET frames, and respectively supply the temporarily stored virtual tributaries to the POH termination units  104  to  107 . The POH termination units  104  to  107  extracts POHs (path overheads) each including control information from the virtual tributaries supplied from the input buffers  100  to  103 , under control of the control information extraction unit  108 , and supply the extracted POHs to the control information extraction unit  108  and data portions of the SONET frames to the path switch unit  109 . The control information extraction unit  108  receives the POHs extracted by the POH termination units  104  to  107 , extracts the control information from the POHs, and supplies the control information and the other portions of the POHs to the data storage control unit  111  and the controller  112 .  
         [0105]    The path switch unit  109  switches the data portions of the SONET frames under control of the data storage control unit  111  or the controller  112 . The hard disk unit  110  stores data supplied from the path switch unit  109 , and supplies data stored in the hard disk unit  110  to the path selection unit  113 , under control of the data storage control unit  111  or the controller  112 . The data storage control unit  111  controls the operations of storing data in the hard disk unit  110  and reading data from the hard disk unit  110 . The controller  112  monitors and controls the entire SONET transmission equipment and flows of data in the SONET transmission equipment. The path selection unit  113  selects paths of data under control of the data storage control unit  111  or the controller  112 .  
         [0106]    The POH insertion units  114  to  117  insert POHs in virtual tributaries containing the data portions output from the path selection unit  113 , respectively, under control of the control information insertion unit  118 . The control information insertion unit  118  controls the POH insertion units  114  to  117  so that control information is included in the POHs inserted in the virtual tributaries. The framers  119  to  122  produce SONET frames containing the virtual tributaries output from the POH insertion units  114  to  117 , respectively. The E/O conversion units  123  to  126  generate optical signals conveying the SONET frames produced by the framers  119  to  122 , by electrical-to-optical conversion.  
         [0107]    Referring back to FIG. 2, each of the clients  77  to  79  sends to the contents server  70  a request for transmission of image data (or sound data), receives the image data (or sound data), and displays an image represented by the received image data on a display device (not shown) or outputs sound data by a speaker (not shown).  
         [0108]    (4) Operations of First Embodiment  
         [0109]    The operations of the first embodiment are explained below.  
         [0110]    First, the operations of the data transmission system of FIG. 2 in response to a request for storage are explained below.  
         [0111]    When the aforementioned information provider sends to the contents server  70  a request for transmission of data (e.g., image data) to the SONET transmission equipment  73  and storage of the data in the SONET transmission equipment  73 , the contents server  70  supplies the data which is designated to be stored, to the transformation equipment  71  through an input port which is predetermined to be used when data designated to be stored is input into the transformation equipment  71 . In the example of FIG. 3, the data supplied from the contents server  70  is input into the transformation equipment  71  through the input port  71   b.  In addition, the contents server  70  supplies control information to the transformation equipment  71 . At this time, the control information includes information items Nos.  1 ,  6 ,  7 ,  8 ,  9 , and  10  illustrated in FIG. 6.  
         [0112]    [0112]FIG. 6 shows examples of information items which can be included in control information. The information item No.  1  is a data storage request, which is a request for storage of object data in a storage medium, the information item No.  6  is a data ID, which designates the object data, the information item No.  7  is data amount information, which indicates the size of the object data, the information item No.  8  is data position information, which indicates the position of the object data in a main signal, the information item No.  9  is destination information, which indicates a destination of the object data, and the information item No.  10  is source information, which indicates a source of the object data.  
         [0113]    In addition, the information item No.  2  is a data readout request, which is a request for readout of object data from a storage medium, the information item No.  3  is a data erasing request, which is a request for erasing of object data in a storage medium, the information item No.  4  is a data lock request, which is a request for locking (i.e., protection against erasing) of the object data stored in a storage medium, and the information item No.  5  is a data unlock request, which is a request for unlocking (removal of protection against erasing) of object data stored in a storage medium. Further, the information item No.  11  is reserved for future extension.  
         [0114]    When the transformation equipment  71  receives the control information as above, the control information extraction circuit  71   e  extracts the control information, and supplies the control information to the transformation circuit  71   d.  The transformation circuit  71   d  transforms the data (e.g., image data) into SONET frames. At this time, the transformation circuit  71   d  inserts the control information supplied from the control information extraction circuit  71   e,  in predetermined fields of POHs in virtual tributaries which contain the data.  
         [0115]    [0115]FIG. 7 shows the fields (bytes) in a POH in a virtual tributary. The control information extraction circuit  71   e  inserts the data ID (the information item No.  6 ) of 4 bits and the data amount information (the information item No.  7 ) of 4 bits in the J 1  byte  150  in the POH, the data storage request (the information item No.  1 ) of 2 bits in the F 2  byte  154  in the POH, the source information (the information item No.  10 ) of 1 byte in the Z 3  byte  156  in the POH, and the destination information (the information item No.  9 ) of 1 byte in the Z 4  byte  157  in the POH.  
         [0116]    The SONET frames generated as above are converted into optical signals in the output port  71   f,  and transmitted to the SONET transmission equipment  72 . The SONET transmission equipment  72  cross-connects the optical signals according to the control information, and transfers the optical signals to the SONET transmission equipment  73 , which is indicated in the control information as the destination. When the SONET transmission equipment  73  receives the optical signals, the optical signals are converted into electric signals, and are then supplied to an input buffer in the SONET transmission equipment  73  (e.g., input buffer  100  illustrated in FIG. 5). For example, the input buffer  100  temporarily stores virtual tributaries of the received SONET frames, and supplies the temporarily stored virtual tributaries to the POH termination unit  104 . The POH termination unit  104  extracts the POHs including the control information from the virtual tributaries supplied from the input buffer  100 , under control of the control information extraction unit  108 , and supplies the extracted POHs to the control information extraction unit  108 . The control information extraction unit  108  receives the POHs extracted by the POH termination unit  104 , extracts the control information from the POHs, and supplies the control information to the data storage control unit  111 .  
         [0117]    In this example, the data storage request is included in the control information. Therefore, the data storage control unit  111  controls the path switch unit  109  so that the data (e.g., image data) supplied from the POH termination unit  104  is supplied to the hard disk unit  110  and stored in an area of the hard disk. At this time, the data ID, the data amount information, and storage area information which indicates the area in which the data is stored and registered so that the data ID, the data amount information, and the storage area information are associated with each other.  
         [0118]    Thus, the data stored in the contents server  70  can be transferred to the SONET transmission equipment  73  and stored in the hard disk unit  110  in the SONET transmission equipment  73 .  
         [0119]    Next, the operations of the data transmission system of FIG. 2 in response to a request for download of data which is sent from a client are explained below.  
         [0120]    When the client  77  sends to the contents server  70  a request for download of data (e.g., image data), the request is transferred to the controller  112  in the SONET transmission equipment  72 . When the controller  112  in the SONET transmission equipment  72  receives the request, the controller  112  generates a virtual tributary addressed to the SONET transmission equipment  73 , and inserts a data ID of data to be read out and data amount information in the J 1  byte  150 , a data readout request in the F 2  byte  154 , source information in the Z 3  byte  156 , and destination information in the Z 4  byte  157 , in the POH of the virtual tributary. Then, the SONET transmission equipment  72  transmits a SONET frame containing the virtual tributary to the SONET transmission equipment  73 .  
         [0121]    When the SONET transmission equipment  73  receives the SONET frame, the virtual tributary is supplied to, for example, the POH termination unit  104  through the input buffer  100 . The POH termination unit  104  extracts the POH including the control information from the virtual tributary supplied from the input buffer  100 , under control of the control information extraction unit  108 , and supplies the extracted POH to the control information extraction unit  108 . The control information extraction unit  108  receives the POH extracted by the POH termination unit  104 , extracts the control information from the POH, and supplies the control information to the data storage control unit  111 .  
         [0122]    Then, the data storage control unit  111  detects the data readout request and the data ID included in the control information, and controls the hard disk unit  110  so that the data corresponding to the data ID is read out from the hard disk unit  110 , and also controls the path selection unit  113  so that the path selection unit  113  selects the data read out from the hard disk unit  110 . In addition, the path selection unit  113  supplies destination information (the information item No.  9 ) to the control information insertion unit  118 . Thus, the data read out from the hard disk unit  110  is selected by the path selection unit  113 , and supplied to, for example, the POH insertion unit  114 . On the other hand, the POH insertion unit  114  inserts POHs in virtual tributaries containing the data read out from the hard disk unit  110  and selected by the path selection unit  113 , under control of the control information insertion unit  118 , where the destination information is inserted in the Z 4  byte  157  in each POH. Then, the virtual tributaries are supplied to, for example, the framer  119 , which produces SONET frames containing the virtual tributaries output from the POH insertion unit  114 . The E/O conversion unit  123  generates an optical signal conveying the SONET frames produced by the framer  119 , by electrical-to-optical conversion.  
         [0123]    Thus, the optical signal generated as above is transmitted from the SONET transmission equipment  73  to the transformation equipment  76 , and received by the transformation equipment  76  through, for example, the input port  76   a.  The control information extraction circuit  76   c  in the transformation equipment  76  extracts the control information included in the POH in each virtual tributary, and supplies the POH to the transformation circuit  76   b.  The transformation circuit  76   b  transforms the received SONET frames into IP packets. At this time, the destination information included in the POHs in the received SONET frames is inserted in the headers of the IP packets. Then, the IP packets are supplied to the output port  76   d  which is connected to the client  77 , and transmitted to the client  77 . At this time, the transmission rate can be arbitrarily determined according to the transmission line between the transformation equipment  76  and the client  77 .  
         [0124]    Thus, the data stored in the hard disk unit  110  can be read out, and transmitted to the client  77 . When a request for readout of data is sent from the client  78  or  79  to the contents server  70 , the data can be read out and transmitted to the client  78  or  79  in a similar manner. Further, when the multicast technique is used, identical data can be concurrently transmitted to the clients  77  to  79 .  
         [0125]    As described above, data supplied from the contents server  70  is temporarily stored in the hard disk unit  110  in the SONET transmission equipment  73 , and is thereafter transmitted to a client in response to a request from the client. Therefore, the transmission lines from the contents server  70  to the SONET transmission equipment  73  are not used during the transmission of data from the SONET transmission equipment  73  to the client. Thus, the data can be transmitted from the SONET transmission equipment  73  to the client without affecting the traffic in the transmission lines from the contents server  70  to the SONET transmission equipment  73 .  
         [0126]    In addition, the data can be transmitted from the SONET transmission equipment  73  to the client at a transmission rate of the transmission line between the SONET transmission equipment  73  and the client. That is, the data can be transmitted to the client at a transmission rate which is optimum for the client.  
         [0127]    When data stored in the hard disk unit  110  becomes unnecessary, the information provider sends to the transformation equipment  71  a request for erasing of the data. When the transformation equipment  71  receives the request for erasing, the transformation equipment  71  inserts a data erasing request in the F 2  byte  154  and a data ID of the data and data amount information in the J 1  byte  150 , in a POH, and sends a SONET frame containing the POH to the SONET transmission equipment  73  through the SONET transmission equipment  72 . Then, the SONET transmission equipment  73  detects the data erasing request, the data ID, and the data amount information, and erases the data in the hard disk unit  110 .  
         [0128]    On the other hand, it is possible to lock data stored in the hard disk unit  110  so that the data stored in the hard disk unit  110  is protected against unintended erasing of the data. In order to realize the protected against erasing, the transformation equipment  71  inserts a data lock request in the F 2  byte  154  and a data ID of the data and data amount information in the J 1  byte  150 , in a POH, and sends a SONET frame containing the POH to the SONET transmission equipment  73  through the SONET transmission equipment  72 . Then, the SONET transmission equipment  73  detects the data lock request, the data ID, and the data amount information, and locks the data in the hard disk unit  110 .  
         [0129]    Further, it is possible to unlock (i.e., remove protection against erasing of) data stored in the hard disk unit  110 . In order to unlock data, the transformation equipment  71  inserts a data unlock request in the F 2  byte  154  and a data ID of the data and data amount information in the J 1  byte  150 , in a POH, and sends a SONET frame containing the POH to the SONET transmission equipment  73  through the SONET transmission equipment  72 . Then, the SONET transmission equipment  73  detects the data unlock request, the data ID, and the data amount information, and unlocks the data in the hard disk unit  110 .  
         [0130]    (5) Operation Flows  
         [0131]    The operations of the data transmission system of FIG. 2 can be realized in accordance with the following operation flows of the transformation equipment  71 , the SONET transmission equipment  72 , and the SONET transmission equipment  73 , which are explained below with reference to FIGS.  8  to  15 .  
         [0132]    [0132]FIG. 8 is a flow diagram indicating an example of a sequence of operations performed by the transformation equipment  71 .  
         [0133]    In step S 10 , the control information extraction circuit  71 e determines whether or not the SONET transmission equipment  72  receive a data storage request from the contents server  70 . When yes is determined in step S 10 , the operation goes to step S 11 . When no is determined in step S 10 , the sequence of FIG. 8 is completed.  
         [0134]    In step S 11 , the transformation circuit  71   d  acquires data which is requested to be stored in the hard disk unit  110 , from the input port  71   b.    
         [0135]    In step S 12 , the transformation circuit  71   d  inserts in a POH control information supplied from the control information extraction circuit  71   e.    
         [0136]    In step S 13 , the transformation circuit  71   d  transmits the data with the POH through the predetermined output port  71   f.    
         [0137]    [0137]FIG. 9 is a flow diagram indicating an example of a sequence of operations performed by the SONET transmission equipment  72 .  
         [0138]    In step S 20 , the control information extraction unit  108  extracts control information from a POH in a received SONET frame, refers to the control information, and determines whether or not the control information includes storage control information (i.e., one of a data storage request, a data readout request, a data erasing request, a data lock request, and a data unlock request). When yes is determined in step S 20 , the operation goes to step S 21 . When no is determined in step S 20 , the operation goes to step S 22 .  
         [0139]    In step S 21 , the controller  112  controls the control information insertion unit  118  so that the control information insertion unit  118  inserts the extracted control information in a POH, and a SONET frame including the POH is transmitted to a destination of the request included in the control information.  
         [0140]    In step S 22 , the controller  112  determines whether or not the SONET transmission equipment  72  has received the SONET frame through a predetermined input port. When yes is determined in step S 22 , the operation goes to step S 24 . When no is determined in step S 22 , the operation goes to step S 23 .  
         [0141]    In step S 23 , the controller  112  executes the normal SONET frame processing.  
         [0142]    In step S 24 , the control information extraction unit  108  extracts from the extracted control information necessary information including destination information.  
         [0143]    In step S 25 , the controller  112  controls the path switch unit  109  and the path selection unit  113  based on the destination information, and controls the corresponding input buffer so that data portions of SONET frame stored in the input buffer are supplied to the path switch unit  109  through the corresponding POH termination unit.  
         [0144]    In step S 26 , the controller  112  controls the control information insertion unit  118  so that control information is inserted in a POH.  
         [0145]    In step S 27 , the corresponding framer produce a SONET frame containing the data portions output from the corresponding POH insertion unit.  
         [0146]    In step S 28 , the corresponding E/O conversion unit generates an optical signal conveying the SONET frame produced by the framer, by electrical-to-optical conversion, and transmits the optical signal.  
         [0147]    [0147]FIG. 10 is a flow diagram indicating an example of a sequence of operations performed by the SONET transmission equipment  73 .  
         [0148]    In step S 40 , the SONET transmission equipment  73  receives a SONET frame, and virtual tributaries contained in the SONET frame are input into one of the input buffers.  
         [0149]    In step S 41 , the control information extraction unit  108  extracts control information from a POH in each of the virtual tributaries.  
         [0150]    In step S 42 , the control information extraction unit  108  determines whether or not the control information extracted in step S 41  includes storage control information (i.e., one of a data storage request, a data readout request, a data erasing request, a data lock request, and a data unlock request). When yes is determined in step S 42 , the operation goes to step S 45 . When no is determined in step S 42 , the operation goes to step S 43 .  
         [0151]    In step S 43 , the controller  112  executes the normal SONET frame processing.  
         [0152]    In step S 44 , the corresponding E/O conversion unit transmits a SONET frame produced by the normal SONET frame processing in step S 43 .  
         [0153]    In step S 45 , the data storage control unit  111  determines whether or not the storage control information extracted by the control information extraction unit  108  is a data storage request. When yes is determined in step S 45 , the operation goes to step S 46 . When no is determined in step S 45 , the operation goes to step S 47 .  
         [0154]    In step S 46 , the data storage control unit  111  executes data storage processing, which is explained later.  
         [0155]    In step S 47 , the data storage control unit  111  determines whether or not the storage control information extracted by the control information extraction unit  108  is a data readout request. When yes is determined in step S 47 , the operation goes to step S 48 . When no is determined in step S 47 , the operation goes to step S 49 .  
         [0156]    In step S 48 , the data storage control unit  111  executes data readout processing, which is explained later.  
         [0157]    In step S 49 , the data storage control unit  111  determines whether or not the storage control information extracted by the control information extraction unit  108  is a data erasing request. When yes is determined in step S 49 , the operation goes to step S 50 . When no is determined in step S 49 , the operation goes to step S 51 .  
         [0158]    In step S 50 , the data storage control unit  111  executes data erasing processing, which is explained later.  
         [0159]    In step S 51 , the data storage control unit  111  determines whether or not the storage control information extracted by the control information extraction unit  108  is a data lock request. When yes is determined in step S 51 , the operation goes to step S 52 . When no is determined in step S 51 , the operation goes to step S 53 .  
         [0160]    In step S 52 , the data storage control unit  111  executes data lock processing, which is explained later.  
         [0161]    In step S 53 , the data storage control unit  111  determines whether or not the storage control information extracted by the control information extraction unit  108  is a data unlock request. When yes is determined in step S 53 , the operation goes to step S 54 . When no is determined in step S 53 , the sequence of FIG. 10 is completed.  
         [0162]    In step S 54 , the data storage control unit  111  executes data unlock processing, which is explained later.  
         [0163]    [0163]FIG. 11 is a flow diagram indicating details of the data storage processing in step S 46  in FIG. 10.  
         [0164]    In step S 70 , the data storage control unit  111  controls the path switch unit  109  so that the corresponding path in the path switch unit  109  is switched and data contained in the virtual tributary is supplied to the hard disk unit  110 .  
         [0165]    In step S 71 , the hard disk unit  110  stores the data supplied through the path switch unit  109 .  
         [0166]    [0166]FIG. 12 is a flow diagram indicating details of the data readout processing in step S 48  in FIG. 10.  
         [0167]    In step S 90 , the data storage control unit  111  reads out from the hard disk unit  110  data which is designated by the control information.  
         [0168]    In step S 91 , the data storage control unit  111  controls the path selection unit  113  so as to supply the data read out from the hard disk unit  110 , to one of the POH insertion units corresponding to source information included in the control information. Then, the the POH insertion unit inserts control information in a POH in each virtual tributary.  
         [0169]    In step S 92 , the framer corresponding to the POH insertion unit executes processing for generating a SONET frame which contains the virtual tributary.  
         [0170]    In step S 93 , the E/O conversion unit corresponding to the above framer generates an optical signal conveying the SONET frame produced by the framer, by electrical-to-optical conversion, and transmits the optical signal.  
         [0171]    [0171]FIG. 13 is a flow diagram indicating details of the data erasing processing in step S 50  in FIG. 10.  
         [0172]    In step S 110 , the data storage control unit  111  searches the hard disk unit  110 , and determines whether or not data corresponding to a data ID included in the control information is locked. When yes is determined in step S 110 , the operation goes to step S 112 . When no is determined in step S 110 , the operation goes to step S 111 .  
         [0173]    In step S 111 , the data storage control unit  111  erases the data in the hard disk unit  110 .  
         [0174]    In step S 112 , the data storage control unit  111  executes exception processing since the SONET transmission equipment  73  is requested to erase the locked data.  
         [0175]    [0175]FIG. 14 is a flow diagram indicating details of the data lock processing in step S 52  in FIG. 10.  
         [0176]    In step S 130 , the data storage control unit  111  searches the hard disk unit  110 , and determines whether or not data corresponding to a data ID included in the control information is locked. When yes is determined in step S 130 , the operation goes to step S 132 . When no is determined in step S 130 , the operation goes to step S 131 .  
         [0177]    In step S 131 , the data storage control unit  111  locks the data in the hard disk unit  110  so that the data cannot be erased. For example, in a typical storage system, each data item is locked and unlocked by setting a predetermined bit in a data management table in a hard disk unit to ON or OFF, respectively. That is, the data can be locked by setting the predetermined bit to ON.  
         [0178]    In step S 132 , the data storage control unit  111  executes exception processing since the SONET transmission equipment  73  is requested to lock the already locked data.  
         [0179]    [0179]FIG. 15 is a flow diagram indicating details of the data unlock processing in step S 54  in FIG. 10.  
         [0180]    In step S 150 , the data storage control unit  111  searches the hard disk unit  110 , and determines whether or not data corresponding to a data ID included in the control information is locked. When yes is determined in step S 150 , the operation goes to step S 152 . When no is determined in step S 150 , the operation goes to step S 151 .  
         [0181]    In step S 151 , the data storage control unit  111  unlocks the data in the hard disk unit  110  so that the data can be erased.  
         [0182]    In step S 152 , the data storage control unit  111  executes exception processing since the SONET transmission equipment  73  is requested to unlock the already unlocked data.  
         [0183]    (6) Other Embodiments  
         [0184]    [0184]FIG. 16 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in the second embodiment of the present invention. In FIG. 16, reference numeral  170  denotes a first interface module,  171  denotes a switch module, and  172  denotes a second interface module.  
         [0185]    The first interface module  170  in FIG. 16 contains a TDM demultiplexing unit  170   a,  which receives SONET frames, and demultiplexes the SONET frames into virtual tributaries and other overhead information. In addition, although not shown in FIG. 16, the first interface module  170  contains the input buffers  100  to  103 , the POH termination units  104  to  107 , and the control information extraction unit  108  in FIG. 5. The switch module  171  in FIG. 16 contains the path switch unit  109 , the data storage control unit  111 , the controller  112 , and the path selection unit  113  in FIG. 5. The second interface module  172  in FIG. 16 contains the POH insertion units  114  to  117 , the control information insertion unit  118 , the framers  119  to  122 , and the E/O conversion units  123  to  126  in FIG. 5. In addition, the second interface module  172  includes a plurality of data storage units  172   a  to  172   d,  which are provided corresponding to the plurality of SONET transmission lines, respectively, instead of the common hard disk unit  110  in FIG. 5. The SONET transmission equipment having the construction of FIG. 16 can achieve the substantially the same functions as those of the SONET transmission equipment of FIG. 5.  
         [0186]    [0186]FIG. 17 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in the third embodiment of the present invention. In FIG. 17, reference numeral  180  denotes a first interface module,  181  denotes a switch module, and  182  denotes a second interface module.  
         [0187]    The first interface module  180  in FIG. 17 contains a TDM demultiplexing unit  180   e,  which receives SONET frames, and demultiplexes the SONET frames into virtual tributaries and other overhead information. In addition, although not shown in FIG. 17, the first interface module  180  in FIG. 17 contains the input buffers  100  to  103 , the POH termination units  104  to  107 , and the control information extraction unit  108  in FIG. 5. The switch module  181  in FIG. 17 contains the path switch unit  109 , the data storage control unit  111 , the controller  112 , and the path selection unit  113  in FIG. 5. The second interface module  182  in FIG. 17 contains the POH insertion units  114  to  117 , the control information insertion unit  118 , the framers  119  to  122 , and the E/O conversion units  123  to  126  in FIG. 5. In addition, the first interface module  180  includes a plurality of data storage units  180   a  to  180   d,  which are provided corresponding to the plurality of SONET transmission lines, respectively, instead of the common hard disk unit  110  in FIG. 5. The SONET transmission equipment having the construction of FIG. 17 can also achieve the substantially the same functions as those of the SONET transmission equipment of FIG. 5.  
         [0188]    [0188]FIG. 18 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in the fourth embodiment of the present invention. In FIG. 18, reference numeral  190  denotes a first interface module,  191  denotes a switch module, and  192  denotes a second interface module.  
         [0189]    The first interface module  190  in FIG. 18 contains the input buffers  100  to  103 , the POH termination units  104  to  107 , and the control information extraction unit  108  in FIG. 5. The switch module  191  in FIG. 18 contains the path switch unit  109 , the data storage control unit  111 , the controller  112 , and the path selection unit  113  in FIG. 5. The second interface module  192  in FIG. 18 contains the POH insertion units  114  to  117 , the control information insertion unit  118 , the framers  119  to  122 , and the E/O conversion units  123  to  126  in FIG. 5. In addition, the switch module  191  includes a data storage unit  191   a  and a switch unit  191   b.  The switch unit  191   b  corresponds to the path switch unit  109  and the path selection unit  113  in FIG. 5.  
         [0190]    Although the data storage unit  191   a  is provided corresponding to the hard disk unit  110  in FIG. 5, data are exchanged between the data storage unit  191   a  and the switch unit  191   b  as follows. When the SONET transmission equipment receives a data storage request, received data is supplied to the data storage unit  191   a  through the first interface module  190  and the switch module  191 , and stored in the data storage unit  191   a.  In addition, when the SONET transmission equipment receives a data readout request, data stored in the data storage unit  191   a  is read out from the data storage unit  191   a,  and supplied to the input side of the switch unit  191   b.  The SONET transmission equipment having the construction of FIG. 18 can also achieve the substantially the same functions as those of the SONET transmission equipment of FIG. 5.  
         [0191]    [0191]FIG. 19 is a diagram illustrating an outline of a construction of a SONET transmission equipment used in the fifth embodiment of the present invention. In FIG. 19, reference numeral  200  denotes a first interface module,  201  denotes a switch module,  202  denotes a data storage module, and  203  denotes a second interface module. The SONET transmission equipment of FIG. 19 is different from the SONET transmission equipment of FIG. 18 in that the data storage unit is provided in a separate module  202  from the switch module  201 . The SONET transmission equipment having the construction of FIG. 19 can also achieve the substantially the same functions as those of the SONET transmission equipment of FIG. 5.  
         [0192]    (7) Other Matters  
         [0193]    (i) Although the above embodiments are explained based on the SONET systems, substantially the same constructions can also be realized based on the SDH systems.  
         [0194]    (ii) The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.  
         [0195]    (iii) All of the contents of the Japanese patent application, No.2001-344277 are incorporated into this specification by reference.