Abstract:
A network device of is a network device that transfers frames by repeating, in a constant cycle, a reserved transfer interval and a free transfer interval. The network device includes a transmission port, a cycle timer, a mode switching control unit that monitors a transfer state of the transmission port and selects a store-and-forward system when the transmission port is in the transfer process and selects a cut-through system when the transmission port is not in the transfer process, and a transfer prohibition control unit that selects the cut-through system as a transfer system when a non-reserved frame is transmitted and switches a transfer method of the non-reserved frame to the store-and-forward system when a reserved transfer interval is established, with reference to the cycle timer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a network device of a transfer switching type and a frame transfer method. 
     2. Description of the Related Art 
     Real-time communication technology such as Institute of Electrical and Electronic Engineers (IEEE) 1394 employs a transfer system (referred to hereinbelow as “cyclic transfer”) using a cycle including real time data and best effort data.  FIG. 5  shows a standard cycle pattern. As shown in  FIG. 5 , cycles are repeated by taking predetermined 125 μsec as one cycle. Packet data, that is, a frame, occupying a predetermined time band within this one cycle is transferred between network devices. Here, the first half of one cycle is taken as a reserved transfer interval and the second half is taken as a free transfer interval. 
     The reserved transfer interval is used for real time data communication. In this interval, for example, as shown in  FIG. 5 , a predetermined time, that is, bands  1  to  5  are reserved for frame transmission. The reserved bands  1  to  5  are used only between the respective set devices. Where frames A 1  to A 5  of real time data are arranged in the reserved bands  1  to  5 , a constant amount of data communication is possible within a constant time. By contrast, the free transfer interval is used for best effort data communication that has no real time property. In this interval, no band is reserved. For example, as shown in  FIG. 5 , where a band  6  of this interval is vacant during data transfer, a frame B 1  is arranged therein and data communication between the devices is performed. Frames B 2  to B 5  are similarly arranged. 
     For example, a daisy-chain connection composed of network devices  11  to  14  shown in  FIG. 6  and a star connection composed of network devices  11 ,  12 ,  13 , and  15  can be considered as a network configuration that realizes a cyclic transfer. Each network device has a bridge function, and network devices  12 ,  13 , and  15  can transfer a frame transmitted from a network device on one side of the device to a network device on the other side. As a result, communication can be performed by using a bridge function even between the network devices that are not directly connected to each other. 
     There is a trend to applying the above-described cyclic transfer to Ethernet (registered trademark), which is a Local Area Network (LAN) standard, and high speed and high reliability of data communication with the cyclic transfer are sought for a LAN using the Ethernet (registered trademark). 
     A cut-through system and a store-and-forward system are available as frame transfer systems. In the cut-through system, the transmitted data are immediately transferred without error checking. As a result, the delay time is small. However, data having errors can fill the network and transmission efficiency can drop. In the store-and-forward system, the transmitted data are transferred after accumulation. Therefore, errors can be checked. However, the delay time increases over that in the cut-through system by the time required for data accumulation. 
     Thus, the cut-through system and store-and-forward system have exactly opposite features. Accordingly, the cut-through system and store-and-forward system are sometimes used together so that switching therebetween is possible. For example, Japanese Patent Application Publication No. 9-18507 (JP-A-9-18507) and Japanese Patent Application Publication No. 9-149068 (JP-A-9-149068) disclose a method for switching from the cut-through system to the store-and-forward system when the number of errors or error frequency in data rises. 
     However, when the method disclosed in JP-A-9-18507 or JP-A-9-149068 is applied to the cyclic transfer, a delay can occur and there is a risk of data being not present in the reserved band. More specifically, where a network device that performs transferring is switched to the store-and-forward system due to error occurrence, transfer delay is caused by data accumulation. As a result, data are not present in the band reserved by the network device that is a transmission source, and a shift can occur in a band reserved by another network device. In this case, one of competing data has to wait, thereby further increasing the delay. 
     SUMMARY OF THE INVENTION 
     The first aspect of the invention relates to a network device that transfers frames by repeating, in a constant cycle, a reserved transfer interval that is a time band, in which a frame is transferred with a reservation, and a free transfer interval that is a time band, in which a frame is freely transferred. The network device includes a transmission port, a cycle timer, a mode switching control unit that monitors a transfer state of the transmission port and selects a store-and-forward system as a transfer method when the transmission port is in the transfer process and selects a cut-through system as a transfer method when the transmission port is not in the transfer process, and a transfer prohibition control unit that selects the cut-through system as a transfer system when a non-reserved frame is transmitted and switches a transfer method of the non-reserved frame to the store-and-forward system when a reserved transfer interval is established, with reference to the cycle timer. 
     The second aspect of the invention relates to a frame transfer method by which a frame is transferred between network devices by repeating, in a constant cycle, a reserved transfer interval that is a time band, in which a frame is transferred with a reservation, and a free transfer interval that is a time band, in which a frame is freely transferred. The frame transfer method includes monitoring a transfer state of a transmission port of the network device; selecting a store-and-forward system as a transfer method when the port is in a transfer process; selecting a cut-through system as a transfer method when the port is not in a transfer process; and selecting the cut-through system as a transfer system when a non-reserved frame is transmitted, and switching a transfer method of the non-reserved frame to the store-and-forward system when the reserved transfer interval is established. 
     The network device in accordance with the invention can reduce a delay of real-time data transfer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein: 
         FIG. 1  is a schematic diagram of a network of the embodiment; 
         FIG. 2  is a block diagram of a network device of the embodiment; 
         FIG. 3  is a schematic diagram of reservation priority control performed by the network device of the embodiment; 
         FIG. 4  is a schematic diagram of a frame transferred by the network device of the embodiment; 
         FIG. 5  illustrates an example of a cycle pattern; and 
         FIG. 6  is a schematic diagram of a network. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A specific embodiment employing the invention will be described below in greater detail with reference to the appended drawings. However, the invention is not limited to the below-described embodiment. Furthermore, the description and drawings below are appropriately simplified to clarify the explanation. 
       FIG. 1  shows a general network configuration and a network device of the embodiment. As shown in  FIG. 1 , a network  100  has network devices  101  to  106 . The network devices  101  to  106  perform transmission and reception of frames by cyclic transfer. Because the network devices  101  to  106  have identical configuration, the network device  101  will be explained herein by way of example. The network device  101  has an application  121 , a communication logic  122 , and ports  123  to  125 . 
     The application  121  generates data to be used in another network device in the network or uses data generated in another network device. Examples of the application include generation of video data by using a peripheral device such as a camera and transmission of the video data to another network device and display of video data transmitted by another network device on a display. 
     The communication logic  122  is configured by a Medium Access Control (MAC) bridge (including a switch, a rooting table, etc. for realizing bridge communication between a plurality of ports in the device itself) specified by IEEE 802.1 or a circuit performing operation and control specified by a protocol such as Spanning Tree Protocol (STP). In the embodiment, control information for determining where the frame is a reserved frame is further added to the frame. 
     The ports  123  to  125  perform transmission and reception of frames between network devices. For example, a connector or a cable specified by IEEE 802.3 and hardware conforming to a transmission-reception protocol such as MAC can be used as the ports  123  to  125 . 
     The communication logic  122  and application  121  connected to adjacent network devices via the ports  123  to  125 , thereby configuring the network  100 . The connection between the network devices may be a daisy-chain connection composed of network devices  101  to  104  or a star connection composed of network devices  101 ,  102 ,  103 , and  105 . 
     In each network device, a rooting table (not shown in the figure) located inside the device saves information indicating which port of the device is connected to which port of another network device. As a result, even when a plurality of ports are used, as in the network device  102  or  103 , each network device performs communication between the ports of the adequate network device on the basis of this information. 
       FIG. 2  shows in greater detail a configuration block diagram of the network devices  101  to  106  shown in  FIG. 1 . Because the network devices  101  to  106  having identical configuration, the network device  101  will be explained hereinbelow by way of example. In  FIG. 2  components denoted by the same reference numerals as in  FIG. 1  have similar configuration and explanation thereof is herein omitted. 
     Each port from among the ports  123  to  125  has a respective reception port  131  and a transmission port  132 . The reception port  131  sends a frame received from another network device to a below-described switch  140 . The transmission port  132  transmits a frame sent from the switch  140  to another network device. 
     The communication logic  122  has the switch  140 , a reservation table  141 , a cycle timer  142 , a transfer management unit  143 , a transmission unit  147 , and a reception unit  148 . Here, the transfer management unit  143  has a mode switching control unit  144 , a transfer prohibition control unit  145 , and a reservation priority control unit  146 . 
     The switch  140  performs bridge communication between a plurality of ports in the device itself, for example, between the reception port  131  of the port  123  and the transmission port  132  of the port  125 . Furthermore, the switch sends a frame received by the device to the reception unit  148  and sends a frame sent from the transmission unit  147  to the transmission port  132  of the designated port. Here, the reception unit  148  sends the received data to the adequate application  121  even when the data received from the switch  140  are the address of the device itself. The transmission unit  147  sends data received from the application  121  to the switch  140 . Furthermore, the switch  140  sends to the transfer management unit  143  information indicating whether the transmission port  132  connected to the frame transmission destination is in the process of frame transfer. 
     The reservation table  141  has information indicating which time band has been reserved. A reserved transfer interval is determined by the time band in which a reserved frame is reserved, as shown in  FIG. 5 . As shown in  FIG. 5 , the reserved frame is not necessarily reserved in all the time bands of the reserved transfer interval. For example, no reservation is made between A 3  and A 4 . The cycle timer  142  measures time information of the device and sends this time information to the transfer management unit  143 . 
     The mode switching control unit  144  of the transfer management unit  143  switches between the cut-through system and the store-and-forward system. More specifically, when the transmission port  132  connected to the frame transmission destination is in the frame transfer process, the store-and-forward system is selected, and when the transmission port is not in the frame transfer process, the cut-through system is selected. Thus, if the transmission port  132  is vacant, the cut-through system is, in principle, selected. Therefore, a delay time can be reduced. 
     The transfer prohibition control unit  145  of the transfer management unit  143  operates based on the transfer system selected by the mode switching control unit  144  and information from the cycle timer  142  and prohibits the transfer of a non-reserved frame when the cut-through system is selected as the transfer system (that is, the transmission port is not in a transfer process) and when the time band is a reserved time band. 
     More specifically, when a port is not in a frame transfer process, the cut-through system is selected by the mode switching control unit  144 . When the received frame is not a frame for which the band has been reserved (referred to hereinbelow as “band-reserved frame”) and when the time is a reserved band time, the transfer prohibition control unit  145  prohibits the cut-through system and switches to the store-and-forward system. Within the reserved band time, a store state with frame accumulation is assumed. When the time is not a reserved band time, the transfer of accumulated frames is started and a forward state is assumed. With such transfer prohibition control, a reserved band can be ensured for the band-reserved frame and the transfer delay thereof can be prevented. 
     Table 1 is an operation table of the mode switching and transfer prohibition control explained hereinabove. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Port is not in transfer process 
               
             
          
           
               
                   
                 Port is in transfer 
                   
                 Non-reserved 
               
               
                   
                 process 
                 Reserved band time 
                 band time 
               
               
                   
                   
               
             
          
           
               
                 Reserved frame 
                 Store-and-forward 
                 Cut-through system 
                 Cut-through 
               
               
                   
                 system 
                   
                 system 
               
               
                 Non-reserved 
                   
                 Transfer prohibition, 
                 Cut-through 
               
               
                 frame 
                   
                 store-and-forward 
                 system 
               
               
                   
                   
                 system 
               
               
                   
               
             
          
         
       
     
     The reservation priority control unit  146  of the transfer management unit  143  preferentially transmits a band-reserved frame at a point in time a transition to a forward state is made when the band-reserved frames have been accumulated in the store state. As shown in Table 1, even with the band-reserved frame, the store-and-forward system is selected if the transmission port  132  for transfer is in the transfer process. 
       FIG. 3  is a schematic diagram of reservation priority control. In  FIG. 3 , four frames are accumulated as accumulated data by the store-and-forward system. The second accumulated frame is a band-reserved frame. Even when a frame that has not been band reserved (non-reserved frame in  FIG. 3 ) was earlier accumulated, the reservation priority control unit  146  preferentially transfers a band-reserved frame (reserved frame in  FIG. 3 ). As a result, the delay of band-reserved frame can be minimized. 
       FIG. 4  shows an example of a transfer frame for performing the reservation priority control. In the MAC frame of IEEE 802.3, a 7-byte Preamble, a 1-byte Start of Frame Delimiter (SFD), a 6-byte Destination Address, a 6-byte Transmission Source Address, and a 2-byte Type are arranged in the header of each cycle. These are followed by DATA, and finally an Frame Check Sequence (FCS) is arranged. In the embodiment, 4-byte control data are arranged at the very end of the header, that is, at the leading end of DATA. Control information is information indicating whether the frame is a reserved frame. The reservation priority control unit  146  determines whether the frame is a reserved frame on the basis of this control information and preferentially extracts the reserved frame from the accumulated data. 
     As described hereinabove, in accordance with the invention, in a network device in which the cut-through system and the store-and-forward system can be switched, the cut-through system is, in principle, selected if the transmission port  132  connected to the frame transmission destination is vacant. As a result, a delay time can be reduced. Furthermore, the transfer prohibition control can ensure a reserved band for a band-reserved frame and prevent a transfer delay thereof. In addition, the delay of the brand-reserved frame can be minimized by a reservation priority control by which the accumulated band-reserved frames are transferred with the highest priority.