Abstract:
A media converter allowing the response test to be activated from cable side and easy detection of occurrence of a failure is disclosed. A pair of physical-layer interfaces to different types of transmission media are connected through a memory for temporarily storing data to be transferred between the physical-layer interfaces. The media converter determines whether a received block of data stored in the memory includes trigger data uniquely assigned to the media converter in a source address field of the received block of data. Only when the received block of data stored in the memory includes the trigger data, a response block of data corresponding to the received block of data is sent back to a source that has transmitted the received block of data.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a media converter for converting from one type of media to another, for example, from an electrical conductor cable such as unshielded twisted pair (hereafter, abbreviated as UTP) to optical fiber. The present invention further relates to a link test method using the media converter and a control method of the media converter.  
           [0003]    2. Description of the Related Art  
           [0004]    There has been a lot of talk recently about FTTH (Fiber To The Home) allowing high-speed transmission of multimedia data such as music, moving picture, and medical data by the installation of optical fiber directly to the home or office. In an era of FTTH, a media converter is an indispensable communication device to connect a fiber-optic line to a computer in the home or office.  
           [0005]    In general, a media converter has a pair of ports that are to be connected to a fiber-optic cable and a UTP cable, respectively. For each of the ports, a physical-layer device is provided, which supports MII (Media Independent Interface) conforming to IEEE802.3 standards.  
           [0006]    In addition, since a media converter converts from one type of media to another, it usually has a missing-link function such that, in case of disconnection in one link, the other link is automatically disconnected. For example, in the event that the link on the fiber-optic side has been disconnected due to some failure on the fiber-optic cable, the media converter automatically disconnects the other link on the UTP cable side.  
           [0007]    In the case where such a media converter is used to connect the UTP cable to the fiber-optic cable, it is necessary to perform a link test to check whether each cable is properly connected to an opposite device on a corresponding link.  
           [0008]    A conventional media converter is provided with a link-test switch by which a link test function is activated to determine whether each link is properly established. The link status for each port is indicated by a light-emitting diode (LED) provided for each port.  
           [0009]    There have been proposed various link test techniques of networks. For example, Japanese Patent Application Unexamined Publication No. 8-331126 discloses a link test method using a special control code to be transmitted between two adjacent switches connected by a link. More specifically, one switch transmits a control code to the other switch, which in turn transmits a response message back to the source switch. The source switch detects the presence or absence of a response to the control code and, when receiving the response message, analyzes it, to determine whether the link normally functions.  
           [0010]    However, the above link test technique is used on network switches and is fundamentally different in structure and function from a media converter designed to convert from one type of media to another with a missing-link function.  
           [0011]    As described above, since a conventional media converter is set to the test mode by operating the link-test switch, it is impossible to activate the link test from cable side (UTP cable or fiber-optic cable) and therefore difficult to perform the link test rapidly and easily. In other words, the conventional media converter is not designed to be controlled from network side.  
           [0012]    When the missing-link function of media converter is activated, the host computer cannot monitor any status of the media converter, even if the media converter operates normally.  
           [0013]    Accordingly, when some error occurs in the link through the media converter, the host side cannot specify the location of a failure: in the link from the host to the media converter; in the media converter itself; or in the link distant from the media converter.  
         SUMMARY OF THE INVENTION  
         [0014]    An object of the present invention is to provide a media converter and its control method allowing the response test to be activated from cable side.  
           [0015]    Another object of the present invention is to provide a failure detecting system and method allowing easy detection of occurrence of a failure and the location thereof in the link including the media converter.  
           [0016]    Still another object of the present invention is to provide a media converter suitable for a failure detecting system allowing easy detection of occurrence of a failure and the location thereof in the link including the media converter.  
           [0017]    According to the present invention, a media converter for converting from one type of media to another, includes: a first physical-layer interface to a first transmission medium; a second physical-layer interface to a second transmission medium; a memory connected between the first and second physical-layer interfaces, for temporarily storing data to be transferred between the first and second physical-layer interfaces; a determiner for determining whether a received block of data stored in the memory includes predetermined data at a predetermined position of the received block of data; and a controller controlling such that, when it is determined that the received block of data stored in the memory includes the predetermined data, a response block of data corresponding to the received block of data is sent from a corresponding one of the first and second physical-layer interfaces back to a source that has transmitted the received block of data.  
           [0018]    According to another aspect of the present invention, a control method for controlling a media converter including: a first physical-layer interface to a first transmission medium; a second physical-layer interface to a second transmission medium; and a memory connected between the first and second physical-layer interfaces, for temporarily storing data to be transferred between the first and second physical-layer interfaces, wherein the control method includes the steps of: a) determining whether a received block of data stored in the memory includes predetermined data at a predetermined position of the received block of data; and b) when it is determined that the received block of data stored in the memory includes the predetermined data, generating a response block of data corresponding to the received block of data; and c) transmitting the response block of data from a corresponding one of the first and second physical-layer interfaces back to a source that has transmitted the received block of data.  
           [0019]    According to still anther aspect of the present invention, a control method for controlling a media converter including: a first physical-layer interface to a first transmission medium; a second physical-layer interface to a second transmission medium; and a memory connected between the first and second physical-layer interfaces, for temporarily storing data to be transferred between the first and second physical-layer interfaces, wherein each of the first and second physical-layer interfaces supports MII (Media Independent Interface) conforming to IEEE802.3 standards, wherein the control method includes the steps of: a) determining whether a received block of data stored in the memory includes predetermined data at a predetermined position of the received block of data; b) when it is determined that the received block of data stored in the memory includes the predetermined data, generating a response block of data corresponding to the received block of data; c) determining whether the media converter is in a missing link state such that, when one of the first and second physical-layer interfaces comes into link disconnection, the other one of the first and second physical-layer interfaces also comes into link disconnection; and d) when it is determined that the received block of data stored in the memory includes the predetermined data under missing link state, disabling the missing link state to transmit the response block of data from a corresponding one of the first and second physical-layer interfaces back to a source that has transmitted the received block of data.  
           [0020]    According to the present invention, a method for detecting a failure on a link including a plurality of media converters, each of which converts from one type of media to another, includes the steps of: a) transmitting a block of data to each of the media converters, the block of data having identification data of the media converter written in a predetermined position of the block of data; b) determining whether a response block of data is received from a corresponding media converter within a predetermined time period; and c) determining a location of a failure based on a determination result of the step (b).  
           [0021]    In the step (c), when a response block of data is not received from a corresponding media converter within a predetermined time period, it may be determined that a failure occurs at a location beyond the corresponding media converter.  
           [0022]    According to the present invention, the system for detecting a failure on a link including a plurality of media converters, each of which converts from one type of media to another, includes: a test manager connected to one of the media converters, wherein each of the media converters includes: a first physical-layer interface to a first transmission medium; a second physical-layer interface to a second transmission medium; a memory connected between the first and second physical-layer interfaces, for temporarily storing data to be transferred between the first and second physical-layer interfaces; and a media converter controller determining whether a received block of data stored in the memory includes the identification data of its own at a predetermined position of the received block of data; when it is determined that the received block of data stored in the memory includes the identification data, generating a response block of data corresponding to the received block of data; and transmitting the response block of data from a corresponding one of the first and second physical-layer interfaces back to a source that has transmitted the received block of data. The test manager includes: an interface to a network manager; and a test manager controller transmitting a block of data to each of the media converters, the block of data having identification data of the media converter written in a predetermined position of the block of data; determining whether a response block of data is received from a corresponding media converter within a predetermined time period; and determining a location of a failure based on a determination result.  
           [0023]    A media converter with a test manager for use in a failure detection of a link composed of a plurality of types of transmission media, includes: a first physical-layer interface to a first transmission medium; a second physical-layer interface to a second transmission medium; a memory connected between the first and second physical-layer interfaces, for temporarily storing data to be transferred between the first and second physical-layer interfaces; a media converter controller determining whether a received block of data stored in the memory includes the identification data of its own at a predetermined position of the received block of data; when it is determined that the received block of data stored in the memory includes the identification data, generating a response block of data corresponding to the received block of data; and transmitting the response block of data from a corresponding one of the first and second physical-layer interfaces back to a source that has transmitted the received block of data; an interface to a network manager; and a test manager controller transmitting a block of data to each of the media converters, the block of data having identification data of the media converter written in a predetermined position of the block of data; determining whether a response block of data is received from a corresponding media converter within a predetermined time period; and determining a location of a failure based on a determination result. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a block diagram showing the internal circuit of a media converter according to a first embodiment of the present invention;  
         [0025]    [0025]FIG. 2 is a diagram showing a format of a trigger packet used in the first embodiment;  
         [0026]    [0026]FIG. 3 is a diagram showing a sequence of response test operation in the first embodiment;  
         [0027]    [0027]FIG. 4 is a flowchart showing a test control operation in the media converter;  
         [0028]    [0028]FIG. 5 is a flowchart showing a test control operation in a management switch;  
         [0029]    [0029]FIG. 6 is a block diagram showing the internal circuit of a media converter with test manager according to a second embodiment of the present invention;  
         [0030]    [0030]FIG. 7 is a schematic diagram showing a network system for explanation of a failure detection method according to a third embodiment of the present invention;  
         [0031]    [0031]FIG. 8 is a diagram showing a sequence of response test operation in the third embodiment;  
         [0032]    [0032]FIG. 9 is a flowchart showing a test control operation in the media converter with test manager;  
         [0033]    [0033]FIG. 10 is a flowchart showing a test control operation in the test manager; and  
         [0034]    [0034]FIG. 11 is a block diagram showing the internal circuit of a media converter with test manager according to a fourth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0035]    Referring to FIG. 1, a media converter (MC)  10  according to a first embodiment of the present invention is connected to a host computer or management switch  20  through 100BASE-TX:UTP cable and to the other host computer or management switch  30  through 100BASE-FX:optical cable. Needless to say, this system is shown just as an example for simplicity.  
       Media Converter  
       [0036]    The media converter  10  has a pair of ports, which are provided with physical-layer devices (PHYs)  101  and  102 , each connected to UTP cable and optical cable, respectively. As described before, the physical-layer devices  101  and  102  support MII (Media Independent Interface) conforming to IEEE802.3 standards.  
         [0037]    The media converter  10  is further provided with a FIFO (First-in-first-out) memory  103  that is connected between the physical-layer devices  101  and  102  to absorb frequency deviations between transmission and reception. Data received at one physical-layer device are sequentially written into the FIFO memory  103  and then read out from the FIFO memory  103  in the same sequence to be output to the other physical-layer device.  
         [0038]    In addition, the FIFO memory  103  is connected to a PLD (programmable logic device)  104  that has been programmed to provide a predetermined logical function. The PLD  104 , as described later, is designed to check data stored in the FIFO memory  103  at a predetermined timing after a packet has been received and, only if the data of the FIFO memory  103  matches predetermined data, to output an enable signal ELB to a microprocessor  105 .  
         [0039]    The microprocessor  105 , when receiving the enable signal E LB  from the PLD  104 , generates a predetermined response packet and controls a corresponding physical-layer device to send it as a reply to the received packet back to the source.  
         [0040]    Further, the microprocessor  105  can access internal registers including farEF (far End Fault) register and Force Link register incorporated in the physical-layer devices  101  and  102  according to IEEE802.3 -standard MII. Accordingly, link information indicating link establishment status and/or half/full duplex can be acquired from each physical-layer device. Furthermore, it is possible to force the physical-layer device being in link disconnection status into transmittable state by accessing the Force Link register thereof.  
         [0041]    The management switch  20  is provided with a physical-layer device  201  that supports MII (Media Independent Interface) conforming to IEEE802.3 standards, MAC (Media Access Control) layer device  202 , and a microprocessor (CPU)  203 . The physical-layer device  201  is connected to the physical-layer device  101  of the media converter  10  through the UTP cable. The microprocessor  203  can access internal registers incorporated in the physical-layer device  201  according to IEEE802.3 -standard MII. Accordingly, link information indicating link establishment status can be acquired from the physical-layer device  201 . Furthermore, it is possible to force the physical-layer device  201  being in link disconnection status into transmittable state by accessing the Force Link register thereof.  
         [0042]    The management switch  30  has a circuit structure similar to the management switch  20 . Its physical-layer device is connected to the physical-layer device  102  of the media converter  10  through the optical cable. The physical-layer device also supports MII (Media Independent Interface) conforming to IEEE802.3 standards.  
         [0043]    In the case where a normal Ethernet packet is transferred, the media converter  10  performs only normal media conversion. More specifically, a normal Ethernet packet received from the management switch  20  is converted into optical data by the media converter  10  and the optical data is transmitted to the destination host computer or management switch  30  through the optical cable. Contrarily, normal optical data received from the management switch  30  is converted into normal Ethernet packet by the media converter  10  and the normal Ethernet packet is sent to the management switch  20  through UTP cable.  
         [0044]    In the case where a response test is activated, the management switch  20  generates a special Ethernet packet including predetermined trigger data (hereafter, called a trigger packet P TRG ) and transmits it to the media converter  10  through the UTP cable.  
       Trigger Packet  
       [0045]    Referring to FIG. 2, similar to a normal Ethernet packet, a trigger packet is composed of 8-byte preamble, 6-byte destination address field, 6-byte source address field, data field of 48-1502 bytes, and 4-byte FCS field. In the case of the trigger packet, predetermined trigger data is written in the source address field.  
         [0046]    The trigger data is preferably unique data such as identification number. Here, the ID number assigned to a circuit board incorporated in the media converter  10  is used as the trigger data because the circuit board number is a unique number assigned to the circuit board by the vendor. Such a circuit board number is used as the trigger data to generate a trigger packet having the circuit board number stored in the source address field to transmit it to the media converter  10 .  
         [0047]    When receiving a packet having its own circuit board number stored in the source address field thereof, the media converter  10  switches into response test mode and generates a response packet to send it back to the management switch  20 . When receiving a packet having data other than its own circuit board number stored in the source address field thereof, the media converter  10  passes the packet as a normal packet through.  
         [0048]    The response test operation of the media converter will be described hereafter.  
       Response Test Sequence  
       [0049]    Referring to FIG. 3, when link disconnection is detected in a management switch  20  (step S 301 ), a microprocessor  203  starts a test program (step S 302 ). In the test mode, the microprocessor  203  generates a trigger packet P TRG  having the circuit board number of the media converter  10  written in the source address field thereof, and transmits it to the UTP cable through the physical layer device  201 .  
         [0050]    When the physical layer device  101  receives the trigger packet P TRG , the media converter  10  is switched to the test mode (step S 303 ), and a missing link function is released (Disable) (step S 304 ) and a response packet P RPL  is returned through the same physical-layer device  101  to the management switch  20 . When a predetermined time has elapsed after the test mode, the media converter  10  returns to the normal mode (step S 305 )  
         [0051]    When the management switch  20  receives the specified response packet P RPL , it is determined that the link is normally established. When the management switch  20  does not receive the response packet P RPL  within the predetermined time or the response packet P RPL  is not the specified one, it is determined that a failure has occurred (step S 306 ).  
         [0052]    In FIG. 3, the response test in the normal mode is shown. However, in the case where the link is disconnected due to the missing link function, the similar response test can be also made. The test operations in the media converter  10  and management switch  20  in the missing-link mode will be described hereafter.  
       Test Control of Media Converter  
       [0053]    Referring to FIG. 4, when the media converter  10  is in missing link mode, the physical-layer devices  101  and  102  are set to be incapable of transmitting but receiving.  
         [0054]    When the physical layer device  101  or  102  has received data, the PLD  104  checks data stored in the FIFO memory  103  at a predetermined timing after the data has been received to determine whether the data stored in the FIFO memory  103  is the specified trigger data or not (step S 401 .) Here, the trigger data check is performed at the timing of the source address field as shown in FIG. 2.  
         [0055]    When the specified trigger data is found in the source address field of the received packet (YES at step S 401 ), the PLD  104  outputs the enable signal E LB  to the microprocessor  105 , and the microprocessor  105  switches its operation mode to the test mode and disables the missing link function (step S 402 .)  
         [0056]    Then, the microprocessor  105  accesses internal resistors of each physical-layer device to get link information (step S 403 .) Here, since the physical-layer device  101  has received the trigger packet P TRG , the microprocessor  105  accesses the internal resistors of the corresponding physical-layer device  102  to acquire link information of the optic cable side.  
         [0057]    Subsequently, the microprocessor  105  generates a response packet P RPL  having the acquired link information written in the source address field thereof, and transmits it from the physical-layer device  101  which has received the trigger packet P TRG , to the source (step S 404 .) After that, the microprocessor  105  returns to the normal mode (step S 405 .) When the received packet is not a packet having its own trigger data included therein (No at step S 401 ), the packet is transferred in the normal mode (step S 405 ).  
       Test Control of Management Switch  
       [0058]    It is assumed that the link of UTP cable side is disconnected due to some cause. In this case, the management switch  20  cannot know where a failure occurs. For example, the management switch  20  detects only the link disconnection of UTP cable side when the UTP cable is cut or when the media converter  10  enables the missing link function caused by disconnection of the optic cable side. In other words, the management switch  20  cannot discriminate between disconnection caused by the UTP cable cut and disconnection caused by the missing link function.  
         [0059]    Referring to FIG. 5, when the microprocessor  203  detects the link disconnection of UTP cable side (YES at step S 501 ), the microprocessor  203  accesses the force link register of the physical-layer device  201  to force the physical-layer device  201  into ForceLink Enable status (step S 502 .)  
         [0060]    When the physical-layer device  201  is switched to the ForceLink Enable status, the microprocessor  203  generates a trigger packet PTRG having the specified number (here, circuit board number) of the media converter  10  written in the source address field, and transmits it through the UTP cable. After the microprocessor  203  releases the ForceLink Disable status (step S 504 ), it returns to the normal mode (step S 505 ) and waits for a corresponding response packet to the trigger packet P TRG  to be received for the predetermined time (steps S 506 , S 507  and S 508 .)  
         [0061]    When the response packet R PRL  is received within in the predetermined time (YES at step S 506 ), the link information of the media converter  10  included in the response packet P PRL  is read to analyze (step S 507 ). After the predetermined time has passed (YES at step S 508 ), the judgment based on the received link information is made (step S 509 .)  
         [0062]    For example, in the case where the response packet PPRL was received within the predetermined time, it can be determined that at least the UTP cable and the media converter  10  operates normally and a failure occurs in more distant cable or devices. In this case, it can be determined from the link information included in the response packet PPRL that the failure may occur in the optic cable side.  
         [0063]    In the case the response packet P PRL  is not received within the predetermined time (NO at step S 506  and YES at step S 508 ), it can be determined that the UTP cable and/or the media converter  10  have some failure.  
         [0064]    As described above, the media converter according to the first embodiment is automatically switched to the test mode when receiving a predeteremined trigger packet, and a response packet to the trigger packet is sent back to the transmitting side. In this way, the response test can be activated from the cable side.  
         [0065]    Then, when the link disconnection is detected, the host side forces the physical-layer device into ForceLink enable status to transmit the trigger packet. When the media converter under the missing link status receives the trigger packet, the media converter disables the missing link function to send the response packet back to the transmitting side. Therefore, when receiving the response packet, the host side can determine that at least the media converter and the link to the media converter operate normally, while when the response packet is not returned, it can be determined that at least one of the media converter and the link has some failure.  
       Second Embodiment  
       [0066]    A media converter according to a second embodiment of the present invention will be described in detail hereafter.  
         [0067]    Referring to FIG. 6, a media converter  600  is composed of a media converter  601  and a test manager  602  that is connected to the microprocessor  105  of the media converter  601  through a dedicated bus  603 . Since the media converter  601  has the substantially same structure as the media converter  10  as shown in FIG. 1 (FIFO memory  103  and PLD  104  are not shown in FIG. 3), circuit blocks similar to those previously described with reference to FIG. 1 are denoted by the same reference numerals and the descriptions will be omitted.  
         [0068]    The test manager  602  includes a microprocessor  604  and a network interface card (NIC)  605 . The microprocessor  604  is connected to the microprocessor  105  of media converter  301  through the bus  603 . The network interface card  605  is connected with a network management tool (not shown) through a  10  M UTP cable.  
         [0069]    As will be described later, the test manager  602  monitors the link status, instructs test startup, and analyze and judge the collected information. Though the above-described media converter  601  works in a basically same manner as the converter  10 , the difference exists in exchanging control signals and data between the microprocessor  105  and the microprocessor  604  of the test manager  602  through the dedicated bus  603 .  
         [0070]    An operation of the test system using the media converter  600  will be described below in detail.  
       Test Sequence  
       [0071]    As shown in FIG. 7, it is assumed for simplicity that the media converter  600  is connected with the management switch  20  through UTP cable UTP 1  and with the other media converter  40  through the optic fiber cable FO and the media converter  40  is further connected with UTP cable UTP 2 .  
         [0072]    It is further assumed that a failure has occurred in UTP cable UTP 2 . In this case, since the missing link function is active at both media converters  601  and  40 , the entire link is in a disconnection status.  
         [0073]    Referring to FIG. 8, the test manager  602  monitors the link status through the media converter  601 , and when it detects the link disconnection (step S 801 ), the test manager  602  notifies the network management tool of the link disconnection status (step S 802 ). When the network management tool receives the notification of link disconnection, it instructs the test manager  602  to start the test program (step S 803 ).  
         [0074]    When instructed by the network management tool, the test manager  302  starts the test mode (step S 804 ) and causes the media converter  601  to switch to the test mode (Step S 805 ). Then, the media converter  601  disables the missing link function (step S 806 ), and switches the physical-layer device  102  to ForceLink Enable status (step S 807 ) to transmit the trigger packet having the unique number of the media converter  40  written in the source address field thereof to the media converter  40 . After transmitting the trigger packet, the media convert  601  disables the ForceLink (step S 808 ) and notifies the test manager  602  of the link information. Thereafter, the media converter  601  returns to the normal mode (step S 809 ) and then waits for the corresponding response packet to the trigger packet to be received.  
         [0075]    On the other hand, when having received the trigger packet from the media converter  601 , the media converter  40  is switched to the test mode (step S 810 ) and disables the missing link function to transmit a response packet back to the media converter  601  (step S 811 ). Thereafter, the media converter  40  returns to the normal mode (step S 812 ). When the media converter  601  receives the response packet from the media converter  40 , the media converter  601  reads the link information from the response packet and notifies the test manager  602 .  
         [0076]    The test manager  602  analyzes the link information received from the media converters  601  and  40  to judge the location of a failure (step S 513 ), and notifies the network management tool of the test result.  
       Test Control of Media Converter  
       [0077]    Referring to FIG. 9, when the test manager  602  starts the test mode (YES at step S 901 ), the microprocessor  105  of media converter  601  disables the missing link function (step S 902 ), and switches the physical layer device  102  to ForceLink Enable status to transmit a trigger packet to the next media converter  40  (step S 903 ). After transmitting the trigger packet addressed to the next media converter  40  (step S 904 ), the microprocessor  105  disables ForceLink status (step S 905 ) and then acquires the link information from each physical layer device (step S 906 ). The microprocessor  105  notifies the test manager  602  of the acquired link information through the dedicated bus  603  (step S 907 ).  
         [0078]    Then, the microprocessor  105  returns to the normal mode (step S 908 ) and waits for the response packet to the transmitted trigger packet to be received (step S 909 ). When the microprocessor  105  receives the response packet (YES at step S 909 ), it reads the link information stored in the response packet to notify the test manager  602  (step S 910 ). When the microprocessor  105  receives any packet other than the response packet (NO at step S 909 ), it just transfers it (step S 911 ).  
         [0079]    When the test mode is not started (NO at step S 901 ), the microprocessor  105  determines whether the trigger packet has been received or not (step S 912 ). When the trigger packet has not been received (NO at step S 912 ), the control goes to the step S 608 . Specifically, when the physical-layer device  101  or  102  receives a packet, the PLD  104  determines whether the data stored in the FIFO memory  103  is the predefined trigger data (the self-identified number) at the predetermined timing. In this case, the content of FIFO memory  103  is checked at the timing of the source address field thereof as shown in FIG. 2. When the trigger data addressed to itself is found in the source address field, it is determined that the trigger packet has been received.  
         [0080]    When the trigger packet is received (YES at step S 912 ), the microprocessor  105  of media converter  601  disables the missing link function (step S 913 ) and acquires the link information from each physical layer device (step S 914 ). As described above, the microprocessor  105  generates the response packet having the acquired link information written in the predetermined location, and transmits it to the source side of trigger packet (step S 915 ). After transmitting the response packet, the control goes to the step S 908 .  
       Test Control of Test Manager  
       [0081]    Referring to FIG. 10, the microprocessor  604  of the test manager  602  monitors the link status through the microprocessor  105  of media converter  601  (step S 1002 ) and determines whether the link is disconnected or not (step S 1003 ), as long as it is not instructed to start the test from the network management tool (No at step S 1001 ). When the link status is normal (NO at step S 1003 ), steps S 1001  and S 1002  are repeated until the test startup instruction is received.  
         [0082]    When the microprocessor  604  detects the link disconnection (YES at step S 1003 ), the microprocessor  604  notifies the network management tool of the link disconnection (step S 1004 ) and waits for the test startup instruction to be received from the network management tool.  
         [0083]    When the microprocessor  604  is instructed to start the test by the network management tool (YES at step S 1001 ), it starts the test mode of media converter  601  (step S 1005 ) and waits for link information to be received from media converters (here, MC 601  and MC 40 ) within a predetermined time-out period (steps S 1006 -S 1008 ).  
         [0084]    Upon receipt of link information from a media converter (YES at step S 1006 ), the microprocessor  604  arranges the link information (step S 1007 ) and, after an elapse of the predetermined time, determines a test result based on the acquired link information (step S 1009 ). For example, if the microprocessor  604  does not acquire link information from the media converter  40  within the predetermined time but from the media converter  601 , it can be determined that a failure occurs in the media converter  40  or the optic fiber cable connected between the media converters  601  and  40 . In the case where the microprocessor  604  receives the notification within the predetermined time from both media converters  601  and  40 , it can be determined from the link information received from the media converter  40  whether a failure occurs on the UTP cable UTP 2  (see FIG. 7). The test manager  602  notifies the network management tool of the test result (step S 1010 ).  
         [0085]    As described above, in the network composed of plural media converters, when trigger packets are sequentially transmitted to plural media converters, it is possible to detect a failure based on their response packets. Since the above-described failure detection can be made by using only media converter  600  without any additional functions to management switch  20 , the system structure is simplified.  
         [0086]    The test control of the media converter  40  is similar to that of the media converter  10  as shown in FIG. 4. Therefore, the details will be omitted.  
         [0087]    Another embodiment of the media converter with test manager is shown in FIG. 11. This media converter has a port P 0  connected to the network management tool and N pairs of port P 11  and P   (i−1, 2, 3, . . . , N) corresponding to N media converters MC 1  to MC N , respectively. The test manager  302  manages each of the media converters MC 1  to MC N  as described above.  
         [0088]    As described above, according to the present invention, by sequentially transmitting trigger packets to plural media converters, failure detection can be made based on their response packets. Particularly, when the response packet has not been received from a certain media converter within the predetermined time, it can be determined that a failure has occurred in more distant from this media converter. Therefore, the occurrence of a failure in a link including media converters can be easily detected and the location of the failure can be specified to some extent.  
         [0089]    Each media converter can discriminate a trigger packet addressed thereto from other trigger packets and normal packets. Accordingly, the test mode can be activated by receiving the trigger packet and the response packet in response to the trigger packet can be sent back to the source. Therefore the link test can be started from cable side and it can be verified by receiving the response packet that not only the link reach this media converter but this media converter itself are normally operating.  
         [0090]    Further, when each media converter receives any trigger packet other than the trigger packet addressed thereto in the status of missing link, the media converter is switched to the normal mode to pass this received packet through. Therefore, since a packet which does not target this media converter for the response test is passed through, the link test can be performed more distant from there.  
         [0091]    According to the media converter with test manager, when a link disconnection is detected, the test mode is started to transmit the trigger packet and can perform the failure detection test by receiving the response packet. Since no additional function to a management switch is required, the system structure becomes simplified.