Patent Publication Number: US-2022217572-A1

Title: Signal transfer device, signal transfer method, signal transfer control device, signal transfer control method and signal transfer program

Description:
TECHNICAL FIELD 
     The present invention relates to a signal transfer technology using Time Aware Shaper (TAS). 
     BACKGROUND ART 
     In recent years, in usual cellular systems, a radio control device and a radio device are separately provided as a base station configuration. In this configuration, the radio control device and the radio device are connected by an optical path involving an optical device and an optical fiber; the optical path involving an optical device and an optical fiber is referred to as mobile fronthaul (MFH). 
     For MFH, point-to-point connections have been used, but networking is being studied for the purpose of reducing costs for MFH. As examples of networking, wavelength division multiplexed (WDM) networking, time division multiplexing passive optical network (TDM-PON), a configuration in which layer 2 switches (L2SWs) are coupled in multiple stages have been developed (refer to, for example, Non-Patent Literature 1). In particular, with the network system in which layer 2 switches serving as signal transfer devices are connected in multiple stages (hereinafter referred to as “L2NW”), a plurality of paths can be constructed between a radio device and a radio control device in a pair, and thus, it is considered that the L2NW can achieve higher redundancy as compared to other kinds of network systems. 
     Incidentally, low delays are required for MFH, and thus, to handle traffic with strict delay requirements, the standardization of time sensitive network (TSN) has proceeded and the application of TSN is being studied (refer to, for example, Patent Literature 1 and 2). TAS studied for TSN is effective especially when high priority traffic has the periodicity. With the TAS technology, traffic streams are scheduled in accordance with priority so as to enable or disable communication. Specifically, while high priority traffic is arriving at SW, only high priority traffic is forwarded but low priority traffic is not forwarded; while high priority traffic is not arriving, low priority traffic is forwarded; these operations are periodically repeated. As such, high priority traffics can be forwarded without waiting for traffics of other priorities, and as a result, the TAS technique is suitable to achieve low delays. 
       FIG. 7  illustrates an example of a configuration of a usual signal transfer device using TAS. In  FIG. 7 , the signal transfer device includes a frame differentiation unit  901 , a high priority buffer  902 , a low priority buffer  903 , an output unit  904 , and a scheduler unit  905 . The frame differentiation unit  901  distinguishes input traffic between low priority frames and high priority frames. The high priority buffer  902  stores a distinguished high priority frame. The low priority buffer  903  stores a distinguished low priority frame. The output unit  904  outputs an output frame from the high priority buffer  902  or the low priority buffer  903  to a forwarding destination. The scheduler unit  905  provides, in accordance with present time information, an output stop instruction for the high priority buffer  902  and an output instruction for the low priority buffer  903  during a low priority signal transmission period; in contrast, the scheduler unit  905  provides, in accordance with present time information, an output instruction for the high priority buffer  902  and an output stop instruction for the low priority buffer  903  during a high priority signal transmission period. 
       FIG. 8  illustrates an example in which high priority radio devices (A 1 , A 2 ) and high priority radio control devices (S 1 , S 2 ) are accommodated by using an L2NW. In the example in  FIG. 8 , an upstream signal from the high priority radio device A 1  to the high priority radio control device S 1  is forwarded along a path from an L2SW(1) to an L2SW(2) to an L2SW(3) to an L2SW(4); an upstream signal from the high priority radio device A 2  to the high priority radio control device S 2  is forwarded along a path from the L2SW(2) to the L2SW(3). 
       FIG. 9  illustrates an example of traffic streams in the L2NW.  FIG. 9  corresponds to the L2NW employing the TAS technique illustrated in  FIG. 8 . Every L2SW repeats a period that allows high priority traffic to be transmitted (high priority signal transmission period) and a period that allows low priority traffic can be transmitted (low priority signal transmission period). For example, two traffic streams of frames  1  from the radio device A 1  and frames  2  from the radio device A 2  both flow into the L2SW(2) and the both kinds of frames are alternately output. Here, when only either frames from the radio device A 1  or the radio device A 2  are brought into focus with respect to the output of the L2SW(2), the interval of the frame  1  (or the interval of the frame  2 ) is extended as compared to the interval of the output of the radio device A 1  (or the output of the radio device A 2 ); in other words, the frame interval is extended. 
     As described in Patent Literature 2, it is assumed that burst traffic transmission occur at MFH. In addition, the amount of traffic at MFH keeps changing in accordance with communication environments surrounding terminals. Thus, as indicated as the output of the L2SW(1), the output of the L2SW(2), and the output of the L2SW(4) illustrated in  FIG. 9 , when traffic is relatively less, no traffic flows in the latter period of the high priority signal transmission period (for example, periods TS 1 , TS 2 , and TS 3  in  FIG. 9 ). With this regard, to effectively use the high priority signal transmission period when traffic is less, technologies have been developed in which, when the end of high priority traffic is detected, the high priority signal transmission period is released to transmit low priority traffic (refer to, for example, Patent Literature 2). 
       FIG. 10  illustrates an example of traffic streams when the high priority signal transmission period is released. Similarly to  FIG. 9 ,  FIG. 10  corresponds to the L2NW illustrated in  FIG. 8 . It can be seen that, in  FIG. 10 , after the signal transfer devices L2SW(1), L2SW(2) and L2SW(4) each detect the end of traffic in the high priority signal transmission period (for example, TD 1 , TD 2  and TD 3  in  FIG. 10 ), the signal transfer devices L2SW(1), L2SW(2) and L2SW(4) release the high priority signal transmission period and extend the low priority signal transmission period to allocate the period to transmit low priority signals. 
     To detect the end of high priority traffic as described above, for example, the two following methods can be considered. The first method is that the high priority radio device or the high priority radio control device outputs a detection signal indicating the end of traffic. However, to implement this method, the high priority radio device or the high priority radio control device needs to have a function of outputting a detection signal. Furthermore, the signal transfer device also needs to have a function of correctly recognizing the detection signal output by the radio device or the radio control device, and thus, laborious work is necessary to match specifications among various types of signal transfer devices and implement the signal transfer devices in accordance with the specifications. The second method is that the signal transfer device autonomously detects the end of high priority traffic. On the assumption that high priority traffic flows in a burst manner, it can be considered that, when no frame is received for a predetermined time or longer after the final frame was received, high priority traffic has finished. Here, the time used to assume that high priority traffic has finished after the final frame was received is referred to as a frame interval threshold. 
       FIG. 11  illustrates an example of a configuration of a known signal transfer device for autonomously detecting the end of high priority traffic. In  FIG. 11 , the blocks assigned the same reference numerals as those of  FIG. 7  operate in the manner similar to the blocks in  FIG. 7 . In  FIG. 11 , the configuration additionally includes a frame arrival time information acquisition unit  906  for obtaining the arrival time information of a frame that have most recently arrived at the high priority buffer  902 . In  FIG. 11 , when the time of the frame interval threshold or longer elapses after the arrival time of a frame that have most recently arrived in the high priority signal transmission period, information of the arrival time being obtained by the frame arrival time information acquisition unit  906 , the scheduler unit  905  provides an output stop instruction for the high priority buffer  902  and an output instruction for the low priority buffer  903 . This reduces the idle time in the high priority signal transmission period so as to effectively use the communication band. 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: “Time-Sensitive Networking for Fronthaul”, IEEE Std 802.1CM-2018. 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2018-129661 
     Patent Literature 2: Japanese Patent Laid-Open No. 2019-004329 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     To reduce the idle time in the high priority signal transmission period and effectively use the communication band, it is necessary to make the frame interval threshold as small as possible. However, for example, in  FIG. 9 , the output of the L2SW(1) is only the frames  1  from the radio device A 1 ; in contrast, the output of the L2SW(4) is both the frames  1  from the radio device A 1  and the frames  2  from the radio device A 2 , and thus, the intervals between the frames are extended; consequently, the frame interval may vary among the signal transfer devices. In this case, if the frame interval thresholds of all the signal transfer devices are set to the frame interval of the L2SW(1), since the frame interval of the L2SW(4) is relatively long, the high priority signal transmission period in the L2SW(4) may be released although the high priority traffic has not ended. Hence, it is necessary to set an optimum frame interval threshold for each signal transfer device. 
     Moreover, when the radio device A 2  does not exist and different frame streams do not flow into the signal transfer device, the frame interval of traffic output from a particular radio device is determined in accordance with installation conditions of a corresponding radio device, and thus, it is difficult to preset the frame interval threshold by using the signal transfer device. 
     Further, after the frame interval threshold is set, if flows increase in number because radio devices are added or if the number of frame streams flowing into the signal transfer devices change because the L2SW connection configuration changes, the frame interval threshold needs to be set again, which makes the user operation complicated. 
     An object of the present invention is to provide a signal transfer device, a signal transfer method, a signal transfer control device, a signal transfer control method, and a signal transfer program for determining optimum frame interval thresholds for individual signal transfer devices autonomously without any user operation by setting optimum frame interval thresholds in accordance with frame arrival intervals calculated for corresponding signal transfer devices, when flows change due to addition of radio devices or communication conditions. 
     Means for Solving the Problem 
     A signal transfer device of a first invention for forwarding high priority traffic frames and low priority traffic frames includes a control unit configured to periodically switch between a period that allows high priority traffic frames to be transmitted and a period that allows low priority traffic frames to be transmitted and, when the control unit detects that no high priority traffic frame arrives for a preset frame interval threshold or longer in the period that allows high priority traffic frames to be transmitted, release the period that allows high priority traffic frames to be transmitted and allocate the released period to the period that allows low priority traffic frames to be transmitted, a frame arrival time information acquisition unit configured to obtain information of frame arrival times of the high priority traffic frames, a frame interval calculation unit configured to calculate, in accordance with the information of frame arrival times obtained by the frame arrival time information acquisition unit, frame intervals between frames of the high priority traffic frames input in chronological order, a frame interval threshold calculation unit configured to calculate a new frame interval threshold in accordance with the frame intervals, and a frame interval threshold configuration unit configured to change the frame interval threshold to the new frame interval threshold. 
     As for a second invention according to the signal transfer device of the first invention, the frame interval threshold calculation unit may be configured to calculate the new frame interval threshold in accordance with the frame intervals excluding a frame interval including the period that allows low priority traffic to be transmitted. 
     A signal transfer method of a third invention for forwarding high priority traffic frames and low priority traffic frames includes control processing of periodically switching between a period that allows high priority traffic frames to be transmitted and a period that allows low priority traffic frames to be transmitted and, when the control unit detects that no high priority traffic frame arrives for a preset frame interval threshold or longer in the period that allows high priority traffic frames to be transmitted, releasing the period that allows high priority traffic frames to be transmitted and allocating the released period to the period that allows low priority traffic frames to be transmitted, frame arrival time information acquisition processing of obtaining information of frame arrival times of the high priority traffic frames, frame interval calculation processing of calculating, in accordance with the information of frame arrival times obtained by the frame arrival time information acquisition processing, frame intervals between frames of the high priority traffic frames input in chronological order, frame interval threshold calculation processing of calculating a new frame interval threshold in accordance with the frame intervals, and frame interval threshold configuration processing of changing the frame interval threshold to the new frame interval threshold. 
     As for a fourth invention according to the signal transfer method of the third invention, the frame interval threshold calculation processing may include calculating the new frame interval threshold in accordance with the frame intervals excluding a frame interval including the period that allows low priority traffic to be transmitted. 
     A signal transfer control device of a fifth invention controls at least one signal transfer device configured to, when the signal transfer device periodically switches between a period that allows high priority traffic frames to be transmitted and a period that allows low priority traffic frames to be transmitted so as to forward high priority traffic frames and low priority traffic frames, in a case in which the signal transfer device detects that no high priority traffic frame arrives for a preset frame interval threshold or longer in the period that allows high priority traffic frames to be transmitted, release the period that allows high priority traffic frames to be transmitted and allocate the released period to the period that allows low priority traffic frames to be transmitted. The signal transfer control device includes a frame interval calculation unit configured to, when the at least one signal transfer device includes a plurality of signal transfer devices, obtain from the plurality of signal transfer devices information of arrival times of high priority traffic frames and calculate, for the respective signal transfer devices, frame intervals between frames of the high priority traffic frames input in chronological order, a frame interval threshold calculation unit configured to calculate, for the respective signal transfer devices, new frame interval thresholds in accordance with the frame intervals, and a frame interval threshold configuration unit configured to send notifications to the plurality of signal transfer devices to change the frame interval threshold to a corresponding one of the new frame interval thresholds. 
     A signal transfer control method of a sixth invention controls at least one signal transfer device configured to, when the signal transfer device periodically switches between a period that allows high priority traffic frames to be transmitted and a period that allows low priority traffic frames to be transmitted so as to forward high priority traffic frames and low priority traffic frames, in a case in which the signal transfer device detects that no high priority traffic frame arrives for a preset frame interval threshold or longer in the period that allows high priority traffic frames to be transmitted, release the period that allows high priority traffic frames to be transmitted and allocate the released period to the period that allows low priority traffic frames to be transmitted. The signal transfer control method includes frame interval calculation processing of, when the at least one signal transfer device includes a plurality of signal transfer devices, obtaining from the plurality of signal transfer devices information of arrival times of high priority traffic frames and calculating, for the respective signal transfer devices, frame intervals between frames of the high priority traffic frames input in chronological order, frame interval threshold calculation processing of calculating, for the respective signal transfer devices, new frame interval thresholds in accordance with the frame intervals, and frame interval threshold configuration processing of sending notifications to the plurality of signal transfer devices to change the frame interval threshold to a corresponding one of the new frame interval thresholds. 
     A seventh invention is a signal transfer program for causing a computer to execute a process of the signal transfer method according to the third or fourth invention or the signal transfer control method according to the sixth invention. 
     Effects of the Invention 
     The signal transfer device, the signal transfer method, the signal transfer control device, the signal transfer control method, and the signal transfer program according to the present invention can determine optimum frame interval thresholds for individual signal transfer devices autonomously without any user operation when flows change due to addition of radio devices or communication conditions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example in which high priority radio devices and high priority radio control devices are accommodated by using an L2NW. 
         FIG. 2  illustrates an example of a configuration of a signal transfer device according to a first embodiment. 
         FIG. 3  illustrates an example in which the L2NW additionally includes a new high priority radio device. 
         FIG. 4  illustrates an example of a process of the signal transfer device according to the first embodiment. 
         FIG. 5  illustrates an example of an operation of the signal transfer device according to the first embodiment. 
         FIG. 6  illustrates an example of a configuration of a remote control device according to a second embodiment. 
         FIG. 7  illustrates an example of a configuration of a usual signal transfer device using TAS. 
         FIG. 8  illustrates an example in which high priority radio devices and high priority radio control devices are accommodated by using an L2NW. 
         FIG. 9  illustrates an example of traffic streams in the L2NW. 
         FIG. 10  illustrates an example of traffic streams when a high priority signal transmission period is released. 
         FIG. 11  illustrates an example of a configuration of a known signal transfer device for autonomously detecting the end of high priority traffic. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of a signal transfer device, a signal transfer method, a signal transfer control device, a signal transfer control method, and a signal transfer program according to the present invention will be described with reference to the drawings. In the following description about the embodiments, the signal transfer device corresponds to a device such as a layer 2 switch (L2SW), and the signal transfer control device corresponds to a device for controlling operation of the L2SW or the like. 
     The signal transfer device described in the embodiments is suitable for the case in which, for example, a network (L2NW) is used as mobile fronthaul (MFH) for forwarding frames among devices that are radio control devices and radio devices separately provided as base stations in a cellular system. Here, since low delays are required to transmit frames among radio control devices and radio devices, the signal transfer device according to the embodiments is based on Time Aware Shaper (TAS). By using TAS, a period that allows high priority traffic frames to be transmitted (high priority signal transmission period) and a period that allows low priority traffic frames to be transmitted (low priority signal transmission period) are periodically repeated; in this configuration, frames to be transmitted and received between a radio control device and a radio device are scheduled to be forwarded in the high priority signal transmission period. 
     The signal transfer device described in the embodiments has a function of, when no high priority frame arrives for a predetermined time (frame interval threshold), releasing the remainder of the high priority signal transmission period and allocate the remainder to the low priority signal transmission period for the purpose of avoiding the existence of useless remaining time in which no frame is forwarded within the high priority signal transmission period in the case in which, for example, relatively less frames are forwarded. In particular, the signal transfer device described in the embodiments can autonomously set optimum frame interval thresholds for individual signal transfer devices by appropriately changing frame interval thresholds, which are used to determine that no high priority frame arrives, in accordance with frame arrival intervals calculated for the corresponding signal transfer devices constituting the L2NW, when flows change due to addition of radio devices or communication conditions. As a result, the signal transfer device can precisely determine the end of frame of high priority traffic when the frame interval of high priority traffic is relatively long, which prevents erroneous determination that the frame of high priority traffic has ended before a subsequent frame arrives. 
       FIG. 1  illustrates an example in which high priority radio devices (A 1 , A 2 ) and high priority radio control devices (S 1 , S 2 ) are accommodated by using an L2NW. While  FIG. 1  illustrates a network (L2NW) having a configuration similar to the configuration in  FIG. 8  described as a known technology, L2SW(1) to L2SW(7) are not a signal transfer device  800  or a signal transfer device  800 A illustrated in  FIG. 7  or  FIG. 11  as known technologies but a signal transfer device  101  described in the embodiments. In  FIG. 1 , all the L2SWs are the signal transfer device  101 . The respective L2SWs are identified by adding a numeral in parentheses at the end of reference characters. For example, the signal transfer device  101  as the L2SW(1) is indicated as the signal transfer device  101 ( 1 ). 
     In  FIG. 1 , the user can determine which path a traffic stream between a radio device and a radio control device flows along. Here, it is assumed that a path with a minimum number of hops is selected for every traffic. In the example in  FIG. 1 , an upstream signal from the high priority radio device A 1  to the high priority radio control device S 1  is forwarded along, for example, a path from the L2SW(1) to the L2SW(2) to the L2SW(3) to the L2SW(4); an upstream signal from the high priority radio device A 2  to the high priority radio control device S 2  is forwarded along, for example, a path from the L2SW(2) to the L2SW(3). 
     First Embodiment 
       FIG. 2  illustrates an example of a configuration of the signal transfer device  101  according to the first embodiment. In  FIG. 2 , the signal transfer device  101  includes a frame differentiation unit  201 , a high priority buffer  202 , a low priority buffer  203 , an output unit  204 , a scheduler unit  205 , a frame arrival time information acquisition unit  206 , a frame interval calculation unit  207 , a frame interval threshold calculation unit  208 , and a frame interval threshold configuration unit  209 . 
     The frame differentiation unit  201  distinguishes frames (input traffic) received from other signal transfer devices, radio devices, radio control devices, and the like in accordance with priority stored in the frame header to output high priority frames to the high priority buffer  202  and low priority frames to the low priority buffer  203 . For example, in the case in which the signal transfer device  101  is the L2SW(2) in  FIG. 1 , the frame differentiation unit  201  distinguishes frames as input traffic received from the L2SW(1) and frames as input traffic received from the high priority radio device A 2  to route the frames to the high priority buffer  202  or the low priority buffer  203 . The frame differentiation unit  201  may use, for example, a VLAN Identifier (VID), a media access control (MAC) address, or an internet protocol (IP) address to distinguish frames. 
     The high priority buffer  202  is a buffer memory for storing high priority frames distinguished by the frame differentiation unit  201 . In accordance with instructions provided by the scheduler unit  205 , the high priority buffer  202  receives and temporarily retain high priority frames output by the frame differentiation unit  201 . 
     The low priority buffer  203  is a buffer memory for storing low priority frames distinguished by the frame differentiation unit  201 . In accordance with instructions provided by the scheduler unit  205 , the low priority buffer  203  receives and temporarily retain low priority frames output by the frame differentiation unit  201 . 
     The output unit  204  outputs high or low priority frames read by the scheduler unit  205  from the high priority buffer  202  or the low priority buffer  203  to a forwarding destination. For example, in the case in which the signal transfer device  101  is the L2SW(3) illustrated in  FIG. 1 , the output unit  204  outputs the frames from the high priority radio device A 1  to the L2SW(4) and the frames from the high priority radio device A 2  to the high priority radio control device S 2 . 
     The scheduler unit  205  provides, in accordance with present time information, an output stop instruction for the high priority buffer  202  and an output instruction for the low priority buffer  203  during a low priority signal transmission period; in contrast, the scheduler unit  205  provides, in accordance with present time information, an output instruction for the high priority buffer  202  and an output stop instruction for the low priority buffer  203  during a high priority signal transmission period. The scheduler unit  205  operates as a control unit for control processing the signal transfer device  101 . Details of operation of the scheduler unit  205  will be described later. 
     The frame arrival time information acquisition unit  206  obtains the arrival time information of a particular frame most recently arrived at the high priority buffer  202  with reference to present time information obtained by a clock outside or inside the signal transfer device  101  (frame arrival time information acquisition processing). 
     The frame interval calculation unit  207  calculates intervals between frames input to the high priority buffer  202  in chronological order with respect to each frame stream from a particular source in accordance with corresponding frame arrival time information obtained by the frame arrival time information acquisition unit  206  (frame interval calculation processing). For example, in the case in which the signal transfer device  101  is the L2SW(3) in  FIG. 1 , the frame interval calculation unit  207  calculates frame intervals between frames from the high priority radio device A 1  and frame intervals between frames from the high priority radio device A 2 . 
     The frame interval threshold calculation unit  208  calculates a frame interval threshold in accordance with the frame intervals calculated by the frame interval calculation unit  207  (frame interval threshold calculation processing). The scheduler unit  205  uses the frame interval threshold to detect when a series of frames have been completely transmitted from a particular source in a burst manner. Details of operation of the frame interval threshold calculation unit  208  will be described later. 
     The frame interval threshold configuration unit  209  provides a frame interval threshold configuration instruction for the scheduler unit  205  in accordance with a frame interval threshold calculated by the frame interval threshold calculation unit  208  (frame interval threshold configuration processing). 
     Next, details of operation of the scheduler unit  205  will be described. When no frame arrives when the time of a preset frame interval threshold or longer elapses after a frame arrival time indicated by information obtained by the frame arrival time information acquisition unit  206 , the scheduler unit  205  provides an output stop instruction for the high priority buffer  202  and an output instruction for the low priority buffer  203  and changes a frame interval threshold in accordance with an instruction provided by the frame interval threshold configuration unit  209 . Here, it is assumed that an initial value of the frame interval threshold is preset because the frame interval threshold configuration unit  209  has not configured the frame interval threshold at the start of operation of the signal transfer device  101  and before the start of reception of frames. It is also assumed that the initial value of the frame interval threshold is set to a value sufficiently larger than a realistic frame interval such as a maximum value that can be achieved on a memory (or infinity). 
     As the information of the frame arrival time obtained by the frame arrival time information acquisition unit  206 , a frame leading end arrival time and a frame trailing end arrival time can be considered. In the case in which it is possible to obtain information of both the frame leading end arrival time and the frame trailing end arrival time, the frame interval calculation unit  207  can calculate the frame interval as a difference between the frame trailing end arrival time of a particular frame and the frame leading end arrival time of a subsequent frame. This difference equals an inter frame gap (IFG). By contrast, in the case in which the frame arrival time information acquisition unit  206  can obtain information of only either the frame leading end arrival time or the frame trailing end arrival time, the frame interval calculation unit  207  can calculate the frame interval as a difference between the frame leading end arrival time of a particular frame and the frame leading end arrival time of a subsequent frame or a difference between the frame trailing end arrival time of a particular frame and the frame trailing end arrival time of a subsequent frame. These differences equal the total value of a frame length and an IFG. 
     Next, details of operation of the frame interval threshold calculation unit  208  will be described. The frame interval threshold calculation unit  208  can set the frame interval threshold to a longest interval or an average interval of the frame intervals calculated by the frame interval calculation unit  207 . Alternatively, the frame interval threshold calculation unit  208  can set the frame interval threshold to the total value of a longest interval and a margin or the total value of an average interval and a margin, where the longest interval and the average interval are obtained from the frame intervals calculated by the frame interval calculation unit  207 . In the case described above, the calculation target may be a frame interval determined across a low priority signal transmission period between a particular high priority signal transmission period and a subsequent high priority signal transmission period. For example, the frame interval may be calculated as an interval between the final frame of a particular high priority signal transmission period and the first frame of a subsequent high priority signal transmission period. Hence, it is desirable that, by setting a threshold determining the calculated frame interval as an out-of-target interval (referred to as an observation threshold), when a calculated frame interval exceeds the observation threshold, the frame interval threshold calculation unit  208  excludes the calculated frame interval and determines a largest or average value of frame interval values less than the observation threshold. 
     The observation threshold can be identical in length to the low priority signal transmission period. In the case in which the length of the low priority signal transmission period varies as in the present embodiment, for example, a minimum or average value of the length of the low priority signal transmission period can be used as a standard length of the low priority signal transmission period. Alternatively, the calculation of frame interval may be suspended when the high priority signal transmission period is changed to the low priority signal transmission period; and the calculation of frame interval may be restarted from a first frame after the low priority signal transmission period is changed to the high priority signal transmission period. Furthermore, a largest or average value may be calculated in accordance with frame intervals in one high priority signal transmission period. The frame interval threshold calculation unit  208  can set any cycle to calculate a frame interval threshold. 
     The frame interval threshold configuration unit  209  can provide a configuration instruction whenever information of a frame interval threshold calculated by the frame interval threshold calculation unit  208  is obtained or after a plurality of pieces of information have been obtained. For example, when the frame interval calculation unit  207  or the frame interval threshold calculation unit  208  causes a temporary malfunction and a particular value significantly different from other frame interval thresholds is thus temporarily notified, it is possible to hinder malfunctions by obtaining a plurality of pieces of information of frame interval thresholds and excluding the particular value significantly different from the other frame interval thresholds. 
     (Case of Adding Radio Device) 
       FIG. 3  illustrates an example in which the L2NW additionally includes a high priority radio device A 3 . In  FIG. 3 , the blocks with reference numerals identical to those in  FIG. 1  are identical to or similar to the blocks in  FIG. 1 . Since in  FIG. 3  the high priority radio device A 3  is newly added and a flow is thus added, the frame interval may be extended at an L2SW on the path along which the additional flow flows. With this respect, the signal transfer device  101  according to the present embodiment can calculate a frame interval after change autonomously without any user operation so as to set again an appropriate frame interval threshold. For example, in the L2NW in  FIG. 3 , the high priority radio device A 3  belonging to the high priority radio control device S 2  is newly added under the L2SW(2) under which the high priority radio device A 2  is also provided. In this case, not only frames from the high priority radio device A 1  and the high priority radio device A 2  but also frames from the high priority radio device A 3  are alternately output in the direction from the L2SW(2) to the L2SW(3), and as a result, when frames from the high priority radio device A 1  are brought into focus, the frame interval is extended. Thus, it is necessary to increase the frame interval threshold for the L2SW(4). With this respect, the signal transfer device  101  according to the present embodiment can autonomously set again the frame interval threshold by regularly calculating the frame interval, and consequently, no malfunction occurs when the frame interval is extended. It should be noted that, for the period from when a radio device is added to when the frame interval threshold is reconfigured, the signal transfer device  101  according to the present embodiment still operates with an original frame interval threshold configured before the radio device is added, and thus, the high priority signal transmission period may be released while high priority traffic still flows; however, it is possible to reduce the effect by setting the cycle for reconfiguring the frame interval threshold to a relatively short cycle. Alternatively, by adding, when a new flow is detected, a process of setting the frame interval threshold again to, for example, a maximum value that can be achieved on a memory (or infinity), it is possible to prevent the high priority signal transmission period from being released when high priority traffic still flows, although the releasable length of the high priority signal transmission period is temporarily shortened. When a new flow is detected denotes when, for example, a frame having a MAC address from which no traffic has been forwarded is detected. 
     (Case of Removing Radio Device) 
     When a radio device is removed so that a corresponding flow is eliminated, the frame interval at an L2SW on the path along which the corresponding flow flows may be shortened. With this respect, the signal transfer device  101  according to the present embodiment can calculate a frame interval after change autonomously without any user operation so as to set again an appropriate frame interval threshold. For example, it is assumed that the high priority radio device A 2  belonging to the high priority radio control device S 2  is removed from the L2NW in  FIG. 1 . In this case, since only frames from the high priority radio device A 1  are output in the direction from the L2SW(2) to the L2SW(3), when the frames from the high priority radio device A 1  are brought into focus, the frame interval is shortened. Thus, it is necessary to decrease the frame interval threshold for the L2SW(4). With this respect, the signal transfer device  101  according to the present embodiment can autonomously set again the frame interval threshold by regularly calculating the frame interval, and consequently, processing is properly carried out when the frame interval is shortened. 
       FIG. 4  illustrates an example of a process of the signal transfer device  101  according to the first embodiment. The process illustrated in  FIG. 4  is performed by the units of the signal transfer device  101  illustrated in  FIG. 2 . 
       FIG. 5  illustrates an example of an operation of the signal transfer device  101  according to the first embodiment, in which  FIG. 5( a )  illustrates an example of frame intervals and  FIG. 5( b )  illustrates an example of an operation in which the high priority signal transmission period is released. 
     Hereinafter, the process of the signal transfer device  101  according to the present embodiment illustrated in  FIG. 4  will be described in detail with reference to  FIG. 5 . 
     In step S 101 , the signal transfer device  101  starts transfer. 
     In step S 102 , the scheduler unit  205  sets the frame interval threshold to a sufficiently large value such as a maximum value that can be achieved on a memory (or infinity). 
     In step S 103 , the scheduler unit  205  starts processing for the high priority signal transmission period. It is assumed that the length of the high priority signal transmission period and the length of the low priority signal transmission period are preset in the scheduler unit  205  and the preset length of the high priority signal transmission period and the preset length of the low priority signal transmission period are alternately allocated in a repeating manner. 
     In step S 104 , the frame arrival time information acquisition unit  206  obtains information of the arrival time of a particular frame arrived at the high priority buffer  202 , and the frame interval calculation unit  207  calculates a frame interval from a preceding frame immediately before the particular frame. 
     In step S 105 , after calculating a frame interval, the frame interval calculation unit  207  determines at least whether the number of calculated frame intervals exceeds a predetermined value or whether the calculated time exceeds a predetermined value; in the case in which the predetermined value is not exceeded, the processing in step S 104  is repeated; in the case in which the predetermined value is exceeded, the process proceeds to step S 106 . 
     In step S 106 , the frame interval threshold calculation unit  208  determines whether the frame interval calculated in step S 104  exceeds an observation threshold to extract the frame interval when the frame interval is equal to or shorter than the observation threshold or exclude the frame interval when the frame interval exceeds the observation threshold. For example, in  FIG. 5( a ) , when it is assumed that the length of the low priority signal transmission period is set as the observation threshold, only a frame interval Δt 3  out of frame intervals Δt 1 , Δt 2 , Δt 3 , and Δt 4  exceeds the observation threshold, and thus, the frame interval threshold calculation unit  208  excludes the frame interval Δt 3 . 
     In step S 107 , the frame interval threshold calculation unit  208  extracts a longest frame interval from frame intervals equal to or shorter than the observation threshold. For example, in  FIG. 5( a ) , a longest frame interval out of the frame intervals Δt 1 , Δt 2 , and Δt 4  is extracted. Here, when these frame intervals are in the following relationship: Δt 1 &gt;Δt 2 &gt;Δt 4 , the frame interval Δt 1  is extracted as the longest frame interval. 
     In step S 108 , the frame interval threshold configuration unit  209  changes the frame interval threshold and sets the frame interval threshold for the scheduler unit  205 . For example, in  FIG. 5( b ) , the frame interval threshold is changed to Δt 1 . In accordance with the frame interval threshold Δt 1 , the scheduler unit  205  performs processing of releasing the high priority signal transmission period. It should be noted that the processing operations in steps S 104  to S 108  are performed in parallel with frame transfer processing, and the frame interval threshold changed in the processing operation in step S 108  is referred to in the processing operation in step S 109 . 
     In step S 109 , the scheduler unit  205  determines whether, after the time of the frame interval threshold or longer elapses after a latest frame has arrived, no subsequent frame arrives. In the case in which no frame arrives for the frame interval threshold or longer, the scheduler unit  205  proceeds to step S 110 ; in the case in which any frame arrives, the scheduler unit  205  proceeds to step S 111 . For example, in  FIG. 5( b ) , the scheduler unit  205  determines whether a subsequent frame arrives after Δt 1  or longer elapses after the third frame  1  has finally arrived; since no frame subsequently arrives, the scheduler unit  205  proceeds to step S 110 . 
     In step S 110 , the scheduler unit  205  releases the high priority signal transmission period. For example, in  FIG. 5( b ) , the scheduler unit  205  releases a period D (period from a time T 1  to a time T 2 ) that is the remainder of the high priority signal transmission period. 
     In step S 111 , the scheduler unit  205  determines whether the high priority signal transmission period has ended. In the case in which the high priority signal transmission period has ended, the scheduler unit  205  proceeds to step S 112 ; in the case in which the high priority signal transmission period has not ended, the scheduler unit  205  returns to step S 109  and repeats the same processing. 
     In step S 112 , the scheduler unit  205  starts the low priority signal transmission period. In the case in which the high priority signal transmission period is released in step S 109  and the case in which the predetermined high priority signal transmission period ends in step S 111 , the low priority signal transmission period starts. For example, in  FIG. 5( b ) , in the case in which the high priority signal transmission period is released in step S 110 , the low priority signal transmission period starts from the time T 1 ; in the case in which the predetermined high priority signal transmission period ends in step S 111 , the low priority signal transmission period starts from the time T 2 . 
     In step S 113 , the scheduler unit  205  determines whether the low priority signal transmission period has ended. In the case in which the low priority signal transmission period has not ended, step S 113  is repeated. In the case in which the low priority signal transmission period has ended, the process of the signal transfer device  101  returns to step S 103  and the same processing is repeated. 
     As described above, after the frame interval threshold elapses after the arrival time of the final frame of high priority traffic, the signal transfer device  101  according to the present embodiment releases the high priority signal transmission period, such that the low priority signal transmission period can be extended. Particularly, since the signal transfer device  101  according to the present embodiment calculates the frame interval and changes the frame interval threshold, when the frame interval increases, the frame interval threshold also increases; similarly, when the frame interval decreases, the frame interval threshold also decreases. As such, it is possible to use an optimum frame interval threshold in response to changes in condition such as addition or removal of radio device. 
     Incidentally, the method of presetting the frame interval threshold as the method described as a known technology can be applied to only the case in which traffic streams from the L2SW and the high priority radio device flow in a burst manner, the frame intervals are almost the same, and the value is specified in advance. This means that, in the case in which frame intervals in traffic streams output from the L2SW and the high priority radio device are not specified in advance and the frame intervals differ from each other, the frame interval threshold cannot be determined. By contrast, the signal transfer device  101  according to the present embodiment can autonomously change the frame interval threshold in accordance with frame intervals in traffic streams output from the L2SW and the high priority radio device, in the case in which the frame intervals in traffic streams output from the L2SW and the high priority radio device are not specified in advance and the frame intervals differ from each other. 
     With this configuration, the signal transfer device  101  according to the present embodiment can prevent the high priority signal transmission period from being released although high priority traffic frames have not ended and from being uselessly maintained although high priority traffic frames have ended. 
     While the signal transfer device  101  according to the present embodiment includes the blocks illustrated in the  FIG. 2 , it is possible to perform the processing operations of the blocks by running a program of the signal transfer method corresponding to the processing operations of the blocks with the use of a computer. The program may be provided by being stored in a storage medium or through a network. 
     Second Embodiment 
     The signal transfer device  101  according to the first embodiment has been described as an example in which the signal transfer device per se calculates the frame interval threshold and determines whether to release the high priority signal transmission period. In a second embodiment, not the signal transfer device per se but the signal transfer control device remotely connected to the signal transfer device calculates the frame interval threshold and determines whether to release the high priority signal transmission period so as to control the signal transfer devices. 
       FIG. 6  illustrates an example of a configuration of a remote control device  300  according to the second embodiment. The remote control device  300  corresponds to a signal transfer control device for controlling a signal transfer device using TAS. 
     In  FIG. 6 , the remote control device  300  includes a frame arrival time information acquisition unit  401 , a frame interval calculation unit  402 , a frame interval threshold calculation unit  403 , and a frame interval threshold configuration unit  404 . The remote control device  300  corresponds to a signal transfer control device that controls a plurality of signal transfer devices  101  from a remote area away from installation locations of the plurality of signal transfer devices  101  in the L2NW. 
     The frame arrival time information acquisition unit  401  obtains information of frame arrival times from the signal transfer devices. 
     The frame interval calculation unit  402  calculates frame intervals with respect to each frame stream from a particular source in accordance with the information of frame arrival times obtained by the frame arrival time information acquisition unit  401 . For example, in the case in which the remote control device  300  controls the L2SW(3) in  FIG. 1 , the frame interval calculation unit  402  calculates frame intervals between frames from the high priority radio device A 1  and frame intervals between frames from the high priority radio device A 2 . 
     The frame interval threshold calculation unit  403  calculates a frame interval threshold in accordance with the frame intervals calculated by the frame interval calculation unit  402 . The frame interval threshold calculation unit  403  performs almost the same operation as the operation of the frame interval threshold calculation unit  208  illustrated in  FIG. 2 . 
     The frame interval threshold configuration unit  404  provides frame interval threshold configuration instructions for the signal transfer devices in accordance with frame interval thresholds calculated by the frame interval threshold calculation unit  403 . 
     The signal transfer device used in the second embodiment has a function of notifying the remote control device  300  of information of frame arrival time. For example, three blocks of the frame interval calculation unit  207 , the frame interval threshold calculation unit  208 , and the frame interval threshold configuration unit  209  are removed from the configuration of the signal transfer device  101  in  FIG. 2  while only the frame arrival time information acquisition unit  206  is included in the configuration of the signal transfer device  101 ; the scheduler unit  205  notifies the remote control device  300  of information of frame arrival time obtained by the frame arrival time information acquisition unit  206 . The remote control device  300  can be connected to the individual signal transfer devices through dedicated lines or a dedicated network, or, for example, the L2NW illustrated in  FIG. 1 . 
     It is assumed that the scheduler unit of the signal transfer device used in the second embodiment has a function of changing the frame interval threshold in accordance with an instruction provided by the frame interval threshold configuration unit  404  of the remote control device  300 , similarly to the scheduler unit  205  of the signal transfer device  101  according to the first embodiment. 
     While the remote control device  300  according to the present embodiment includes the blocks illustrated in the  FIG. 6 , it is possible to perform the processing operations of the blocks by running a program of the signal transfer control method corresponding to the processing operations of the blocks with the use of a computer. The program may be provided by being stored in a storage medium or through a network. 
     While the first and second embodiments are described by using a ring network configuration as an example, the present invention is not limited to this configuration but may be applied to other network configurations, such as a honeycomb network and a mesh network. 
     As described above as the embodiments, the signal transfer device, the signal transfer method, the signal transfer control device, the signal transfer control method, and the signal transfer program according to the present invention can determine optimum frame interval thresholds for individual signal transfer devices autonomously without any user operation by setting optimum frame interval thresholds in accordance with frame arrival intervals calculated for corresponding signal transfer devices, when flows change due to addition of radio devices or communication conditions. 
     REFERENCE SIGNS LIST 
       101 ,  800 ,  800 A Signal transfer device 
       201 ,  901  Frame differentiation unit 
       202 ,  902  High priority buffer 
       203 ,  903  Low priority buffer 
       204 ,  904  Output unit 
       205 ,  905  Scheduler unit 
       206 ,  401 ,  906  Frame arrival time information acquisition unit 
       207 ,  402  Frame interval calculation unit 
       208 ,  403  Frame interval threshold calculation unit 
       209 ,  404  Frame interval threshold configuration unit 
       300  Remote control device 
     A 1 , A 2 , A 3  High priority radio device 
     S 1 , S 2  High priority radio control device