Abstract:
The present invention provides a data transmission method in which a SONET is applied to Ethernet LANs. This method realizes a redundant structure of the Ethernet while utilizing the bandwidth of the SONET. On a server side, a working path and a back-up path of the Ethernet are multiplexed onto the same path of the SONET, and data transmission is conducted through the multiplexed path. On a client side, a data packet transmitted through the multiplexed path is divided by a filtering operation through detection of a port ID added on the transmitting side.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention generally relates to data transmission methods and data transmission devices, and, more particularly, a SDH (Synchronous Digital Hierarchy)/SONET (Synchronous Optical Network) transmission system that complies with the Ethernet (a registered trade name) protection system, and increases a transmission efficiency by applying the Ethernet protection system to the SDH/SONET transmission system.  
           [0002]    In recent years, as Ethernet LANs have been widely used, there is an increasing demand for suitable protection provided to prevent packet transmission data from being damaged even when a fault occurs in a transmission path. In response to such a demand, there is a method called “dual-homing” method.  
           [0003]    [0003]FIGS. 1A and 1B illustrate a protection system in accordance with a dual-homing method. As shown in FIG. 1A, this system includes a master LAN switch LS-M and a slave LAN switch LS-S, and a server  10  is connected to clients  21  and  22  via working paths P-W 1  and P-W 2  and back-up paths P-P 1  and P-P 2 . In this system, actual data packets are normally transmitted through the working paths P-W 1  and P-W 2 , while control packets are transmitted through the back-up paths P-P 1  and P-P 2 .  
           [0004]    It is assumed here that a fault has occurred in the master LAN switch LS-M, to which the working paths P-W 1  and P-W 2  are connected, and, as a result, it has become impossible to transmit actual data packets through the working paths P-W 1  and P-W 2  in the above structure. In such a case, the fault is detected by the provided protection system, and switching control is then performed on the master LAN switch LS-M and the slave LAN switch LS-S, so that the actual data packets can be transmitted through the back-up paths P-P 1  and P-P 2 , through which the control packets have been transmitted until then, instead of the working paths P-W 1  and PW 2 . The switching control is performed at a high speed so as to switch the paths without causing a time lag. By doing so, an accident such as inadvertent discard of data packets can be prevented.  
           [0005]    In the above dual-homing system, the backup paths P-P 1  and P-P 2  need to take all data traffic, instead of the working paths P-W 1  and P-W 2 , but, in a normal operation, the back-up paths P-P 1  and P-P 2  transmit only the control packets. This implies that only a half of the capacity of the four paths P-W 1 , P-W 2 , P-P 1 , and P-P 2 , is utilized, which presents a problem of poorer path usability.  
           [0006]    To use the LANs of the above Ethernet type over a long distance, a communication system using a multiplexing communication network SONET has been developed.  
           [0007]    [0007]FIGS. 2A and 2B illustrate a structure in which LANs of the Ethernet type are applied to (or mapped onto) a SONET so as to realize LANs over a long distance.  
           [0008]    In this case, the above four paths P-W 1 , P-W 2 , P-P 1 , and P-P 2  need to be realized in the SONET, and, to do so, the corresponding transmission bandwidth needs to be ensured on the optical cable transmission paths that constitute the SONET. Furthermore, it is a known fact that a SONET has its own protection system (or redundant structure), and only a half of the transmission bandwidth of the SONET is utilized accordingly. When LANs of the dual-homing Ethernet type provided with the above protection are applied to a SONET, the resultant usability of the transmission bandwidth will be only a fourth of the usability of a structure that is not provided with a protection (or a redundant structure).  
         SUMMARY OF THE INVENTION  
         [0009]    A general object of the present invention is to provide data transmission methods and data transmission devices in which the above disadvantages are eliminated.  
           [0010]    A more specific object of the present invention is to provide a data transmission method in which LANs of the Ethernet type provided with a protection are applied to a SONET (or mapped onto the SONET) so as to realize a long-distance LAN and increase the usage efficiency of the transmission bandwidth of a SONET.  
           [0011]    The above objects are achieved by a structure in which a working path and a back-up path for realizing a redundant structure in a small-scale communication network are multiplexed onto the same path in a long-distance communication network, where small-scale communication networks, such as LANs, are connected with the long-distance communication network, such as a SONET, so as to obtain a system that can take advantage of the functions of the small-scale communication networks over a long distance.  
           [0012]    Among the working paths and the back-up paths that constitute the redundant structure in the small-scale communication network, each of the back-up paths normally has a very small amount of data transmission corresponding to a control packet, and, therefore, one path can substantially accommodate the amount of data transmission corresponding to a pair of a working path and a back-up path. In view of this, each corresponding pair of the working paths and back-up paths are multiplexed onto the same path in the long-distance communication network. Thus, the communication resources can be efficiently utilized.  
           [0013]    With this structure, the reliability of the system can be increased by virtue of the redundant structure of the small-scale communication networks, while the transmission bandwidth of the long-distance communication network can be efficiently utilized.  
           [0014]    The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIGS. 1A and 1B illustrate a structure in which a dial-homing method is applied to Ethernet LANs;  
         [0016]    [0016]FIGS. 2A and 2B illustrate an example of a data transmission system for connecting the Ethernet LANS shown in FIGS. 1A and 1B to each other with a SONET;  
         [0017]    [0017]FIG. 3 is a schematic view of a principal structure in accordance with the present invention;  
         [0018]    [0018]FIG. 4 is a schematic view of the SONET and the surroundings shown in FIG. 3;  
         [0019]    [0019]FIG. 5 is a schematic view of a structure in which the working paths and the back-up paths of FIG. 3 are multiplexed or separated;  
         [0020]    [0020]FIG. 6 is another schematic view of a structure in which the working paths and the back-up paths of FIG. 3 are multiplexed or separated;  
         [0021]    [0021]FIG. 7 is a block diagram showing the structure of the SONET-ADM device of FIG. 3;  
         [0022]    [0022]FIG. 8 is a block diagram showing the structure of the Ethernet unit of FIG. 3;  
         [0023]    [0023]FIG. 9 is a block diagram illustrating a situation where the ports of the Ethernet are in one-to-one correspondence with the STS paths of the SONET, with the redundant structure not recognized in the structure of FIG. 8;  
         [0024]    [0024]FIGS. 10A and 10B illustrate a process for mapping an Ethernet frame onto a SONET frame;  
         [0025]    [0025]FIGS. 11A and 11B illustrate the process for mapping the Ethernet frame onto the SONET frame. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    The following is a description of embodiments of the present invention, with reference to the accompanying drawings.  
         [0027]    Referring to FIG. 3 through  6 , a hardware structure in accordance with the present invention will first be described.  
         [0028]    [0028]FIG. 3 is a schematic view of the principal structure of this embodiment. In this structure, a SONET transmission system is employed so that dual-homing LANs can be used over a long distance, as in the Ethernet described with reference to FIGS. 1A and 1B. For ease of explanation, FIG. 3 shows the structure corresponding only to the client  21  shown in FIGS. 1A and 1B. However, it should be understood that the client  22  shown in FIGS. 1A and 1B can also be achieved by the same structure.  
         [0029]    As shown in FIG. 3 through FIG. 6, this system includes a server SONET-ADM(Add/Drop Multiplexer)  100  (hereinafter referred to as the server ADM  100 ), and a client ADM  200 , and optical cable transmission paths  300  that constitute a SONET.  
         [0030]    Referring to FIG. 7, the server ADM  100  connects line interface cards  160  and tributary interface cards (or an Ethernet unit)  180  with a pair of switch fabrics  130  and  140  each having two STSs(Synchronous Transport Signals) as one unit (hereinafter referred to as STS-SFs  130  and  140 ). The interface cards  160  and  180  are connected to the switch fabrics  130  and  140  by transmitting STS- 1  frame signals, the number of which depends on each corresponding bandwidth.  
         [0031]    The client ADM  200  also has the same structure. As indicated by the brackets in FIG. 7, the client ADM  200  connects line interface cards  260  and tributary interface cards (or an Ethernet unit)  280  with switch fabrics  230  and  240 . The structure of each switch fabric having a TSI (Time Slot Interchange) can be achieved by a known art relating to ADMs, and, therefore, explanation for the structure of each switch fabric is omitted from this description.  
         [0032]    Referring back to FIG. 3, the server ADM  100  includes: the Ethernet unit  180  that has a multiplexing unit  110  for multiplexing a signal sent from a master LSN switch LS-M and a signal sent from a slave LSN switch LS-S, and a separating unit  120  for separating a signal sent from the SONET into two signals and supplying the separated signals to the master LAN switch LS-M and the slave LAN switch LS-S, respectively; the switch fabric  130  that maps the multiplexed signal in an Ethernet frame onto a SONET frame and then transmits the mapped signal to a SONET transmission path  300 ; the switch fabric  140  that extracts the Ethernet frame from the SONET frame and supplies the extracted frame to the separating unit  120 ; and the line interface  160  that acts as an interface between the SONET transmission path and the switch fabrics  130  and  140 .  
         [0033]    Likewise, the client ADM  200  includes: the Ethernet unit  280  that has a multiplexing unit  210  for multiplexing a working path signal P-W 1  and a back-up path signal P-P 1  supplied from a client LAN switch LS-C 1 , and a separating unit  220  for separating a signal sent from the SONET into two signals and supplying the separated signals to a working signal port and a back-up signal port of the client LAN switch LS-C 1 ; the switch fabric  230  that maps the multiplexed in an Ethernet frame onto a SONET frame and then transmits the mapped signal to the SONET transmission path  300 ; the switch fabric  240  that extracts the Ethernet frame from the SONET frame and then supplies the extracted frame to the separating unit  220 ; and the line interface that serves as an interface between the SONET transmission path  300  and the switch fabrics  230  and  240 .  
         [0034]    Referring now to FIGS. 5 and 6, the multiplexing units  110  and  210 , and the separating units  120  and  220  will be described. Where a signal is transmitted from the server side to the client side in FIG. 5, the multiplexing unit  110  maps each Ethernet frame on the master side and the slave side onto a SONET frame to be transmitted through the same STS path on the SONET. More specifically, each ID inserting unit ID-INS allocates a port ID to a MAC (Media Access Control) frame of the Ethernet. In the example shown in the figure, a port ID of  5  is allocated to the working packet, and a port ID of  6  is allocated to the back-up packet. Further, each flag inserting unit FLAG-INS allocates a predetermined flag that will be described later.  
         [0035]    The separating unit  220  on the receiving side then separates the signal formed by multiplexing the working packet and the back-up packet onto the same STS path, so as to recover the original signals. There are two specific methods of doing this:  
         [0036]    1) The port ID allocated on the transmitting side is detected so as to separate the packets.  
         [0037]    2) The port ID allocated on the transmitting side is detected, and, if the detected port ID is identical to the receiving side port ID, the packet is passed on. On the other hand, if the detected port ID is not identical to the receiving side port ID, the packet is discarded.  
         [0038]    Either of the above two methods is conducted at each ID processing unit ID-MSK. More specifically, if the working side port ID is 5, the packet is passed on, but, if the working side port ID is not 5, the packet is discarded. Likewise, the back-up side port ID is 6, the packet is passed on, but if the back-up side port ID is not 6, the packet is discarded. As a result of this control operation, packets can be correctly separated and supplied to the working port (W) and the back-up port (P) of the LAN switch LS-C 1 .  
         [0039]    Meanwhile, each flag detecting unit FLAG-DET detects a flag. If a plurality of packets having a flag are detected in a row, it is determined that a fault has occurred in a corresponding signal transmission system, and the optical output toward the LAN side is shut down so as to automatically stop the packet transmission. As an example of the flag, a blank frame can be used. If a MAC frame is blank, the flag is considered to be set. For instance, if a fault such as a lost signal due to insufficient optical output from the LAN side is detected at each fault detecting unit LOS DET shown in FIG. 8, the corresponding flag inserting unit FLAG-INS inserts a blank in the MAC frame.  
         [0040]    In a case where transmission is conducted from the client side to the server side, as shown in FIG. 6, basically the same processes as in the case of FIG. 5 are performed, and, therefore, explanation for those processes is omitted from this description.  
         [0041]    Referring now to FIG. 8, the Ethernet units  100  and  200  will be described in greater detail.  
         [0042]    First, each of the multiplexing units  110 W of the working side and  110 -P of the back-up side includes a physical terminal unit Phy-R, a frame terminal unit MAC-R, an encapsulating unit ENCAP, one of the above described ID inserting units ID-INS, one of the above described flag inserting units FLAG-INS, and one of the above described fault detecting units LOS DET. Except for the ID inserting unit ID-INS, the flag inserting unit FLAG-INS, and the fault detecting unit LOS DET, the above components have the same structures as those in a conventional Ethernet unit, and, therefore, explanation for those components is omitted from this description.  
         [0043]    The encapsulating unit ENCAP extracts actual data from a standard Ethernet frame (or MAC frame) shown in FIG. 10A, and then allocates a flag part, an address part and a control part and a port ID, to the extracted actual data, as shown in FIGS. 10B and 11A. The extracted data is then mapped onto the payload of a standard SONET frame, as shown in FIG. 11B.  
         [0044]    Each of the separating units  120 -W of the working side and  120 -P of the back-up side includes one of the above described flag detecting units FLAG-DET, one of the ID processing units ID-MSK, a decapsulating unit DECAP, a frame terminal unit MAC-T, a physical terminal unit Phy-T, and a flag monitoring unit FLAG MON. Except for the flag detecting unit FLAG-DET, the ID processing unit ID-MSK, and the flag monitoring unit FLAG MON, the above components have the same structures as those in a conventional Ethernet unit, and, therefore explanation for those components is omitted from this description.  
         [0045]    When a flag is detected by the flag detecting unit FLAG-DET, the flag monitoring unit FLAG MON automatically stops the output of the packet at an electric/optical conversion module. The decapsulating unit performs an operation reverse to the operation performed by the encapsulating unit on the transmitting side, so that the Ethernet frame is extracted from the SONET frame.  
         [0046]    In FIG. 8, the signals outputted from the multiplexing units  110 -W and  110 -P are inputted into both the switch fabric  130  and the switch fabric  140  via an interface unit STS INF-R, but the input is normally effective only for the switch fabric  130  (as indicated by a circle on the path in FIG. 8) by virtue of the switching function of the switch fabrics. Likewise, although the signals outputted from the switch fabrics  130  and  140  are inputted into the separating units  120 -W and  120 -P via an interface unit STS INF-T in FIG. 8, only the signal outputted from the switch fabric  140  is supplied to the separating units  120 -W and  120 -P (as indicated by a circle on the path in ,FIG. 8) by virtue of the selecting function of a selector SELX.  
         [0047]    The client Ethernet unit  280  has the same structure as the above described server Ethernet unit  180 , and, therefore, explanation for the structure of the client Ethernet unit  280  is omitted from this description.  
         [0048]    Referring now to FIG. 9, an operation mode in which the Ethernet units  180  and  280  are used but the above protection is not provided will be described.  
         [0049]    In this non-protection mode, packets to be processed by the working multiplexing units  110 -W and  220 -W, the back-up multiplexing units  110 -P and  220 -P, the working separating units  120 -W and  220 -W, and the back-up separating units  120 -P and  220 -P, are not multiplexed or separated in the above described manner, but each of the packets is transmitted through an individual STS path of the SONET. Here, the paths of the Ethernets and the paths of the SONETS are in one-to-one correspondence. As shown in FIG. 9, the signals outputted from the multiplexing units  110 -W and  110 -P are separately supplied to the switch fabric  130  by virtue of the function of the selector SELX, and are then separately transmitted on individual paths P-D and P-Dx. Likewise, packets separately transmitted through individual paths are received by the switch fabric  140 , and are then separately supplied to the separating units  120 -W and  120 -P.  
         [0050]    With the selector SELX, it is possible to switch operation modes between a mode in which the protection is provided in a redundant structure, and a mode in which the protection is not provided where the ports of the Ethernets and the STS paths are in one-to-one correspondence. By such a switching function, it is possible to react flexibly to the external situation such as data traffic. As described above, only the paths provided circles in the drawings normally remain effective by virtue of the switch fabrics and the selector.  
         [0051]    The same effects as the effects of the present invention can be obtained by applying a conventional VLAN (Virtual LAN) system. However, in a case where the above described functions are obtained by the VLAN system, there is a problem that users cannot use the VLAN function at will. In this embodiment, on the contrary, a working packet and a back-up packet can be multiplexed onto the same STS path, and the multiplexed packets can be separated to the original individual packets, so that users can use the VLAN functions at will.  
         [0052]    As described so far, in accordance with the present invention, measures against faults (i.e. a redundant structure) on Ethernets are taken by providing dual-homing protection in a structure in which a SONET is applied to LANs of the Ethernet type. In this structure, a working (master) path and a back-up (slave) path are multiplexed onto the same STS path on the SONET. In this manner, the redundant structure inherent in the SONET can be utilized to the maximum, so that information packets can be certainly transmitted to the recipient. Also, since the transmission bandwidth of the SONET can be utilized, communication resources can be effectively used.  
         [0053]    Furthermore, by adding a flag, the recipient can be notified of a fault in the Ethernets, and the optical output to the receiving Ethernet LAN can be automatically shut down. Accordingly, Ethernet users do not necessarily notice the longer transmission path via the SONET.  
         [0054]    It should be noted that the present invention is not limited to the embodiments specifically disclosed above, but other variations and modifications may be made without departing from the scope of the present invention.