Abstract:
A packet switching apparatus is disclosed which is adapted to determine and control an allowed transmission rate of a transmitting terminal in a packet switching network. The packet switching apparatus comprises a rate discriminator to monitor an interval of packet reception for each of connections, judge, based on the interval of packet reception, whether the transmitting terminal having sent the packet is in pause of transmission and determine a first allowed transmission rate for the transmitting terminal correspondingly to a result of the judgment, an ER calculator to calculate, based on a predetermined algorithm, a second allowed transmission rate for the transmitting terminal, and an ER setter to select one of the first allowed transmission rate determined by the rate discriminator and second allowed transmission rate calculated by the ER calculator, whichever is smaller, and write it into a packet returned to the transmitting terminal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a packet switching apparatus adapted to determine and control an allowed transmission rate to be informed to a transmitting terminal in a packet switching network in which a band management is done by feedback control, and a method of controlling the allowed transmission rate. 
     2. Description of the Related Art 
     As one of the communication systems in which feedback control is done, an available bit rate (ABR) service in an asynchronous transfer mode (ATM) network has been proposed. The ABR service is disclosed in the document “ATM forum Traffic Management Specification Version 4.0 R11” (Shirish S. Sathaye, March 1996). In the ABR service, an allowed cell rate (ACR) being a transmission cell rate at a terminal is controlled by a feedback control in which a resource management (RM) cell is used. 
     FIG. 4 is a schematic block diagram showing the configuration of an ATM network for an ABR service. The operation of the ABR service as in the document “ATM forum Traffic Management Specification Version 4.0 R11” will be described herebelow with reference to FIG.  4 . 
     In FIG. 4, an ATM network is generally indicated with a reference  401 . The ATM network  401  incorporates an ATM switch  402  and has connected thereto a transmitting terminal  403  and receiving terminal  404 . For sending data to the receiving terminal  404 , the transmitting terminal  403  mixes a forward resource management (FRM) cell in a data cell. Upon reception of the data cell through the ATM switch  404 , the receiving terminal  404  will change the FRM cell to a backward resource management (BRM) cell and send it back to the transmitting terminal  403 . 
     When the FRM cell sent from the transmitting terminal  403  and the BRM cell sent back from the receiving terminal  404  are passed through the ATM switch  402 , the ATM cell  402  will write into either the FRM or BRM cell or into both an explicit rate (ER) indicative of how much congested the connections from the transmitting terminal  403  to the receiving terminal  404  are currently. The ER is a maximum transmission rate at which such connections routed through the ATM switch  402  are allowable without any congestion and which is currently allowed for the transmitting terminal  403 . When the connections through the ATM switch  402  are totally a low load, a higher ACR is allowable for the transmitting terminal  403 . Therefore, the ER value calculated by the ATM switch  402  will be a large one. On the contrary, when such connections are a larger load, the ATM switch  402  will calculate a smaller ER value. The transmitting terminal  403  will increase or decrease the ACR based on an ER written in the BRM cell sent from the ATM switch  402  to effect a data communications at a rate within the range of increased or decreased ACR. 
     As mentioned above, the transmitting terminal  403  decreases the ACR because of a received ER value which is lower. In addition, the ACR is decreased due to an initialization thereof. More particularly, if a length of time for which the transmitting terminal  403  sends no FRM cell exceeds an ACR decrease time factor (ADTF), the ACR will be decreased to an initial cell rate (ICR). As described in the aforementioned document “ATM forum Traffic Management Specification Version 4.0 R11”, this initialization is an operation effected by the transmitting terminal  403  controlling itself. One of the FRM cell sending conditions is that one FRM cell is sent for a predetermined number of data cells. Thus, since an extremely small number of FRM cells is sent when the transmitting terminal  403  has no further data to send, the ACR initialization will be effected. The purpose of this ACR initialization is for the transmitting terminal  403  to resume with an ICR, not any ACR, a data transmission of which it has once been in pause, in a predetermined time from the pause, thereby preventing the network from being abruptly applied with a large load. 
     FIG. 5 is a schematic block diagram of an ATM switch in the ATM network, showing the role of the ATM switch. In the network illustrated in FIG. 5, transmitting terminals  502  equal in peak cell rate (PCR) to each other are connected to an ATM switch  504  to links  503 , respectively, each having a linkage capacity which is same as the PCR of the transmitting terminals  501  and  502 , they are multiplexed by an ATM switch  504 , and connected to an ATM network  505  via another link  503 . Of the transmitting terminals, one  501  will always send data at a full ACR as the actual transmission rate while the other  502  will send data initially at the full ACR, then at a rate extremely low (LCR) but at which the ACR is not initialized by the transmitting terminal itself, and at the full ACR again in a predetermined time after that. 
     FIG. 6 is a time chart showing the operations of the transmitting terminal  502  shown in FIG.  5 . FIG. 6 shows values of an ER received by the transmitting terminal  502 , and an ACR and an actual transmission rate, respectively, of the terminal  502 . First, when the transmitting terminals  501  and  502  are continuously sending data, the ER for no congestion of the ATM switch  504  will take a half of a PCR value (will be referred to simply as “PCR” hereinafter), that is, “PCR/2”. Accordingly, the ACR of the transmitting terminals  501  and  502  having received that ER will also be “PCR/2”. The transmitting terminals  501  and  502  will send data at an actual transmission rate “PCR/2”. 
     In this condition, the actual transmission rate of the transmitting terminal  502  will be reduced to “LCR” at a time t0. The “LCR” is a very small value at which however the ACR will not be initialized. As the actual transmission rate is thus decreased, the ATM switch  504  will be applied with little load for the data transmission by the transmitting terminal  502 . Therefore, the ATM  504  will calculate a new ER. Conventionally, the ER will be equal to “PCR” when calculated by the commonly used ER calculation method stated in “Explicit Rate Indication for Congestion Avoidance; ERICA)” in the aforementioned document “ATM forum Traffic Management Specification Version 4.0 R11”. The ATM switch  504  will inform the new ER thus calculated thereby to the transmitting terminals  501  and  502 . 
     It should be noted for FIG. 6 that the ER value of “PCR” is informed to the transmitting terminal  502  at a time t2 a predetermined time after the time t0 at which the actual transmission rate of the transmitting terminal  502  has decreased to “LCR”. This is because for a time period from the reduction of the actual transmission rate of the transmitting terminal  502  until the terminal  502  is informed of an ER reflecting the reduced actual transmission rate, a propagation delay is caused by a distance between the transmitting terminal  502  and ATM switch  504 . 
     The transmitting terminal  501  has an actual transmission rate equal to “PCR” since the received new ER will lead to an ACR which takes a value “PCR”. On the other hand, the actual transmission rate of the transmitting terminal  502  is very low being “LCR”, but the ACR will be maintained at “PCR” because the received ER has a value of “PCR”. 
     Next, assume that a factor having reduced to “LCR” the actual transmission rate of the transmitting terminal  502  has been eliminated at a time t5 as shown in FIG.  6 . Thus, the actual transmission rate of the transmitting terminal  502  will be raised up to the full ACR. The ACR is not be initialized but maintained at “PCR”, so that the actual transmission rate will abruptly jump up to an ACR value, that is, “PCR”. Therefore, both the transmitting terminals  501  and  502  will send data at the actual transmission rate “PCR” after the time t5 and so the amount of data arriving at the ATM switch  504  and ATM network  505  will exceed the capacity (equal to “PCR”) of the link  503  connecting the ATM switch  504  and ATM network  505  to each other. Thus, a serious congestion will take place at the ATM switch  504 . Then, the ATM switch  504  will reduce the ER value to “PCR/2” and inform it to the transmitting terminals  501  and  502 . 
     Because of the delay of a propagation between the transmitting terminals  501  and  502  and the ATM switch  504  as mentioned above, however, the transmitting terminals  501  and  502  will receive the ER of “PCR/2” and starts decreasing the ACR at a time t7 which is a predetermined time from the time t5 at which the congestion has first taken place. For a time period from the time t5 until t7, an extremely high possibility of cell loss will exist in the ATM switch  504 . 
     As having been described in the foregoing, the conventional ATM network is disadvantageous in that if the actual transmission rate of a certain one of the transmitting terminals connected to the ATM network falls, the ACR has an higher ACR reflecting the reduced actual transmission rate, and if the transmitting terminal in question abruptly raises the actual transmission rate up to the full ACR, the ATM network will suddenly be applied with a large load so that a cell loss will take place with a result that the ATM network will incur a lower efficiency of data transmission. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a packet switching apparatus destined for use in a packet transmission network and adapted to prevent the network from being suddenly applied with a high load without any rapid change of the actual transmission rate to a high value and avoid any cell loss even if the actual transmission rate of a certain one of transmitting terminals involved in an ABR service becomes lower and that transmitting terminal has a suddenly increased amount of data to send after the ACR of the transmitting terminal is initialized, and a method of controlling the transmission rate. 
     The present invention has another object to provide a packet switching apparatus adapted to improve the safety of a network in which it is incorporated, by initializing the ACR of a transmitting terminal under control of the network, and a method of controlling the transmission rate. 
     The present invention has a still another object to provide a packet switching apparatus adapted to judge the pause of transmission for each virtual path or virtual channel with no influence on the ACR of other virtual paths or virtual channels, and a method of controlling the transmission rate. 
     The present invention have a yet another object to provide a packet switching apparatus in which a time of protection is set to judge whether a transmitting terminal is in pause of transmission to prevent the throughput from being reduced due to a frequent initialization of ACR, and a method of controlling the transmission rate. 
     According to the first aspect of the invention, a packet switching apparatus adapted to determine and control an allowed transmission rate for a transmitting terminal connected in a packet switching network in which a band is controlled by a feedback control, comprises 
     means for monitoring the interval of packet reception for each connection, and judging, based on the interval of packet reception, whether the transmitting terminal having sent the packet is in pause of transmission and determining a first allowed transmission rate for the transmitting terminal correspondingly to a result of the judgment, 
     means for calculated a second allowed transmission rate based on a predetermined algorithm, and 
     means for selecting a smaller one among the first allowed transmission rate determined by the rate discriminating means and the second allowed transmission rate calculated by the rate calculating means, and writing it into a packet returned to the transmitting terminal. 
     In the preferred construction, the apparatus further comprises means for storing a threshold arbitrarily set for the interval of packet reception, 
     the rate discriminating means judging, when the interval of packet reception is smaller than the threshold, the transmitting terminal having sent the packet to be in pause of transmission. 
     In the preferred construction, the apparatus further comprises means for storing the arbitrarily set threshold for the interval of packet reception and an arbitrarily set protection time, 
     the rate discriminating means judging, when the status in which the interval of packet reception is smaller than the threshold lasts for a time equivalent to the protection time, the transmitting terminal having sent the packet to be in pause of transmission. 
     In another preferred construction, when the rate discriminating means judges the transmitting terminal to be in pause of transmission, a preset value is taken for the first allowed transmission rate, 
     when the transmitting terminal is not in pause of transmission, a maximum value allowed for a transmission in the network is taken for the first allowed transmission rate. 
     In another preferred construction, when the rate discriminating means judges the transmitting terminal to be in pause of transmission, a preset value which is approximately an initial transmission rate of the transmitting terminal is taken for the first allowed transmission rate, 
     when the transmitting terminal is not in pause of transmission, a maximum value allowed for a transmission in the network is taken for the first allowed transmission rate. 
     According to the second aspect of the invention, a packet switching apparatus adapted to determine and control an allowed transmission rate for a transmitting terminal connected in an ATM network being a packet switching network in which a band is controlled by a feedback control, and in which an ABR service is done in which a band control by a resource management cell is effected, comprises 
     means for monitoring the interval of cell reception for each virtual path or channel of a reception cell, and judging, based on the interval of cell reception, whether the transmitting terminal having sent the cell is in pause of transmission and determining a first ER value for the transmitting terminal correspondingly to a result of the judgment, 
     means for calculated a second ER value based on a predetermined algorithm, and 
     means for selecting a smaller one among the first ER value determined by the rate discriminating means and the second ER value calculated by the rate calculating means, and writing it into a backward resource management cell returned to the transmitting terminal. 
     In the preferred construction, the apparatus further comprises means for storing a threshold arbitrarily set for the interval of cell reception, 
     the rate discriminating means judging, when the interval of cell reception is smaller than the threshold, the transmitting terminal having sent the cell to be in pause of transmission. 
     In the preferred construction, the apparatus further comprises means for storing the arbitrarily set threshold for the interval of cell reception and an arbitrarily set protection time, 
     the rate discriminating means judging, when the status in which the interval of cell reception is smaller than the threshold lasts for a time equivalent to the protection time, the transmitting terminal having sent the cell to be in pause of transmission. 
     In another preferred construction, when the rate discriminating means judges the transmitting terminal to be in pause of transmission, a preset value is taken for the first ER value, 
     when the transmitting terminal is not in pause of transmission, a peak cell rate allowed for a transmission in the network is taken for the first ER value. 
     In another preferred construction, when the rate discriminating means judges the transmitting terminal to be in pause of transmission, a preset value which is approximately an initial cell rate of the transmitting terminal is taken for the first ER value, 
     when the transmitting terminal is not in pause of transmission, a peak cell rate allowed for a transmission in the network is taken for the first ER value. 
     According to another aspect of the invention, a method of determining and controlling an allowed transmission rate for a transmitting terminal connected in a packet switching network in which a band is controlled by a feedback control, comprising the steps of 
     monitoring the interval of packet reception for each connection, 
     judging, based on the interval of packet reception, whether the transmitting terminal having sent the packet is in pause of transmission and determining a first allowed transmission rate for the transmitting terminal correspondingly to a result of the judgment, 
     calculating a second allowed transmission rate based on a predetermined algorithm, and 
     selecting, a smaller one among the first allowed transmission rate and the second allowed transmission rate, and writing it into a packet returned to the transmitting terminal. 
     In the preferred construction, at the step of determining the first allowed transmission rate, when the interval of packet reception is smaller than the threshold, the transmitting terminal having send the packet is judged to be in pause of transmission. 
     In the preferred construction, at the step of determining the first allowed transmission rate, when the status in which the interval of packet reception is smaller than the threshold lasts for a time equivalent to the protection time, the transmitting terminal having sent the packet is judged to be in pause of transmission. 
     In another preferred construction, the step of determining the first allowed transmission rate further comprises the steps of 
     taking a preset value for the first allowed transmission rate when the transmitting terminal is judged to be in pause of transmission, and 
     taking, otherwise, for the first allowed transmission rate a maximum value allowed for a transmission in the network. 
     In another preferred construction, the step of determining the first allowed transmission rate further comprises the steps of 
     taking for the first allowed transmission rate a preset value which is approximately an initial transmission rate of the transmitting terminal when the transmitting terminal is judged to be in pause of transmission, and 
     taking, otherwise, for the first allowed transmission rate a maximum value allowed for a transmission in the network. 
     Other objects, features and advantages of the present invention will become clear from the detailed description given herebelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only. 
     In the drawings: 
     FIG. 1 is a schematic block diagram of an embodiment of the packet switching apparatus according to the present invention; 
     FIG. 2 is a flow chart of operations of a rate discriminator in the embodiment in FIG. 1; and 
     FIG. 3 is a time chart of rate control operations in the embodiment of the present invention in FIG. 1; 
     FIG. 4 is a schematic block diagram of an ATM network for an ABR service, showing the configuration of the network; 
     FIG. 5 is a schematic block diagram of an ATM switch in the ATM network, showing the role of the ATM switch; 
     FIG. 6 is a time chart of the conventional rate control operations. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention will be discussed hereinafter in detail with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to unnecessary obscure the present invention. 
     FIG. 1 is a schematic block diagram showing the configuration of an embodiment of the packet switching apparatus according to the present invention. As shown, the packet switching apparatus according to this embodiment comprises a cell detector  101 , a rate discriminator  102 , an operational memory controller  103  and operational memory  104 , a parameter memory controller  105  and parameter memory  106 , an ABR service class output buffer  107 , an address manager  108 , an ER calculator  109 , a timer keeper  110 , and an ER setter  111 . It should be noted that only the components characteristic of the present invention are illustrated with other common components omitted. An ATM switch works in units of VP (virtual path) or in units of VC (virtual channel) as the case may be. The following description will refer to an ATM switch in which the packet switching apparatus according to the present invention is used works in units of VC (virtual channel). Of course, the embodiment of the present invention is applicable for an ATM switch which works in units of VP. 
     In this embodiment, the cell detector  101  receives a cell from a switch (not shown), sends a cell reception flag and VC value to the rate discriminator  102 , and the cell reception flag, VC value and a cell class information to the ER calculator  109 . The cell class information is an information indicating which the cell received from the switch is, a data cell or an RM cell. 
     Based on a current time received from the time keeper  110  and the VC value and cell reception flag received from the cell detector  101 , the rate discriminator  102  controls an arrival time of a cell for each VC. The operational memory controller  103  and operational memory  104  are used to calculate, based on a time at which a cell has been received, and keep in a same VC a time at which a next cell is to be received. Further, based on a time of cell interval thus calculated, a threshold rate (TCR) previously stored in the parameter memory  106  and a time of protection T TCR , an ER is determined and sent to the ER setter  111 . The values of TCR and protection time  TTCR  can arbitrarily be set. The function of the rate discriminator  102  will be apparent from the detail description given later. 
     Using a predetermined algorithm of ER calculation, the ER calculator  109  calculates an ER from the cell reception flag, VC value and cell class information received from the cell detector  101 , and a queue length information received from the ABR service class output buffer  107  via the address manager  108 . The ER calculation algorithm may be a conventional one. 
     The time keeper  110  always counts a time and sends a current time to the rate discriminator  102  and ER calculator  109 . 
     Upon reception of a BRM cell from an input circuit interface (not shown), the ER setter  111  determines an ER value which is to be informed to a transmitting terminal on a VC to which the BRM belongs, writes it into the ER area of the BRM cell, and sends it to a switch (not shown). The ER value of the BRM cell is determined by selecting either of the ER values received from the rate discriminator  102  and ER calculator  109 , whichever is smaller. 
     The packet switching apparatus according to this embodiment functions as will be discussed in detail with reference to FIG.  1 : 
     Upon arrival, through the switch, of a cell of a predetermined VC delivered from a predetermined transmitting terminal connected to the network, the cell detector  101  will send a flag indicating the reception of the cell and the value of the VC to the rate discriminator  102 . Then the rate discriminator  102  will read a TCR and protection time T TCR  from the parameter memory  106  by means of the parameter memory controller  105  to judge whether the transmitting terminal having sent the VC is substantially in pause of transmission. If the actual transmission rate of the received cell is lower than the TCR and the time for which the actual transmission rate lower than the TCR lasts reaches the protection time T TCR  the transmitting terminal is judged to be in pause of transmission. 
     The actual transmission rate of the transmitting terminal is kept low for a certain length of time in some cases (the transmitting terminal is substantially in pause of transmission), while it is caused by a bursting traffic to temporarily be lower than the TCR in other cases. If it is assumed that the ACR is also initialized in case the rate is momentarily low, the initialization will be effected very frequently and the throughput become extremely low. To avoid this, a protection time T TCR  is set for no ACR initialization to be done when a time period for which the actual transmission rate is lower than the TCR is shorter than the protection time T TCR . Since a TCR is set for initializing the ACR (a set value of TCR will be referred to as “TCR” herebelow), the TCR should desirably be same as the ICR of the transmitting terminal. 
     When having judged the transmitting terminal not to be in pause of transmission, the rate discriminator  102  will set the ER value to be informed to the ER setter  111  to “PCR” of the transmitting terminal in consideration. This is intended for selecting, in selecting either an ER value determined by the rate discriminator  102  or a one calculated by the ER calculator  109 , whichever is smaller, as an ER value of a BRM to be sent to the transmitting terminal in consideration, the ER value calculated by the ER calculator  109 . On the other hand, when the transmitting terminal is judged to be in pause of transmission, the ER value to be sent to the ER setter  111  is set to “TCR”. Upon request from the ER setter  111 , the ER calculator  109  will inform the ER setter  111  of a currently latest ER value. 
     Using the conventional algorithm of ER calculation, the ER calculator  109  calculates an ER from a VC value and cell class information received from the cell detector  101 , a queue length information received from the address manager  108 , and a time information received from the time keeper  110 . Upon request from the ER setter  111 , the ER calculator  109  will inform the ER setter  111  of a currently latest ER value. 
     The ER setter  111  detects a BRM cell from among the cells received from an input circuit interface (not shown) and informs the rate discriminator  102  and ER calculator  109  of a VC of the detected BRM cell to request them for ER values for the VC. Upon reception of the ER values sent from the rate discriminator  102  and ER calculator  109 , the ER setter  111  will compare the ER values with each other and selects a smaller one of them for writing into the BRM cell which will be sent to the switch (not shown). 
     With the above operations, the ER value for sending to the transmitting terminal is normally a same one as in the prior art that indicates an ATM switch-allowable rate. However, when the transmitting terminal is in pause of transmission and the actual transmission rate is decreased, the ER value will be changed over to a one intended for initialization of the ACR. If a congestion has caused the actual transmission rate to temporarily be lower, an ER value indicative of an ATM switch-allowable rate and smaller than the ER value intended for the ACR initialization, will be chosen. 
     The rate discriminator  102  in this embodiment functions as will be discussed in detail with reference to FIG. 2 being a flow chart of its operations. 
     As in the flow chart of FIG. 2, the rate discriminator  102  first resets a flag indicating that the rate of reception from the transmitting terminal is lower than TCR (the flag will be referred to as “status flag” hereinafter), and sets to PCR of the transmitting terminal an ER of which the ER setter  111  is to be informed (at step  201 ). Then, based on the cell reception flag and VC value received from the cell detector  101 , the rate discriminator  102  will check whether after a cell of a predetermined VC value is received, a next cell of the same VC value is received within a time of “1/TCR” (at step  202 ). If such a cell is received within the time “1/TCR”, it is meant that the actual transmission rate of the transmitting terminal on the VC is not less than the TCR. Therefore, the transmitting terminal is judged not to be in pause of transmission. If the status flag is off, that is, if the transmission rate has already been higher than TCR before reception of the next cell, the operation returns to step  202  with no further operation (at step  207 ). On the other hand, if the status flag is on, it is turned off, the counting of the protection time T TCR  is interrupted (at steps  207  and  208 ) and an ER value to be sent to the ER setter  111  is set to the PCR of the transmitting terminal on the VC in consideration (at step  209 ). 
     If no next cell is received within a time “1/TCR” at step  202 , the rate of reception from the transmitting terminal on the VC can be judged to be lower than “TCR”. In this case, if the status flag is off and the protection time T TCR  is not yet counted, the status flag is turned on, the counter is reset, then the counting of the protection time T TCR  is started (at steps  203  and  204 ), and it is judged again at step  202  whether the rate of cell reception is still lower than “TCR”. On the other hand, if the rate of cell reception has already been less than “TCR” before the cell reception, namely, if the status flag is on (at step  203 ), it is judged whether the counting of the protection time T TCR  is complete (at step  205 ). If the counting of the protection time T TCR  is not yet complete, it is judged again at step  202  whether the rate of cell reception is lower than “TCR”. If the counting of the protection time T TCR  is complete, the rate of reception from the transmitting terminal on the VC becomes lower than “TCR” and the status has already lasted until the protection time T TCR , SO that an ER value for sending to the ER setter  111  is set to “TCR” (at steps  205  and  206 ). 
     The ACR control in this embodiment will further be described with reference to FIG. 3 being a time chart of the operations of the transmitting terminal  502  in the network in FIG. 5 using the ATM switch in which the packet switching apparatus according to the present invention is incorporated. FIG. 3 shows values of ER, ACR and actual transmission rate received by the transmitting terminal  502 . In the network illustrated in FIG. 5, the transmitting terminals  502  equal in PCR to each other are connected to the ATM switch  504  to the links  503 , respectively, each having a linkage capacity which is same as the PCR of the transmitting terminals  501  and  502 , they are multiplexed by the ATM switch  504 , and connected to the ATM network  505  by means of the single link  503 . Of the transmitting terminals, one  501  will always send data at a full ACR as the actual transmission rate while the other  502  will send data initially at the full ACR, then at a rate extremely low (LCR) but at which the ACR is not initialized by the transmitting terminal itself, and at the full ACR again in a predetermined time after that. 
     As seen from FIG. 3, when the transmitting terminals  501  and  502  are continuously transmitting data, the ER value is “PCR/2” for no congestion of the ATM switch  504 . Therefore, the ACR of the transmitting terminals  501  and  502  is also “PCR/2”. Namely, the transmitting terminals  501  and  502  transmit data at the actual rate of “PCR/2”. 
     In this condition, the actual transmission rate of the transmitting terminal  502  is reduced to “LCR” at a time t0. Since the distance between the transmitting terminal  502  and ATM switch  504  causes a propagation delay, the ATM switch  504  will receive the actual transmission rate “LCR” of the transmitting terminal  502  at the time t1 at which the ATM switch  504  will detect that the actual transmission rate is less than “TCR”, and start counting the protection time T TCR . 
     The ER value of which the transmitting terminal  502  is informed is a one calculated by the conventional ER calculator. Therefore, the load to the ATM switch  504  is halved because of the reduced actual transmission rate of the transmission terminal  502 , so that the ER value increases up to “PCR”. It is at a time t2 delayed a predetermined time caused by the propagation delay that the ER value arrives at the transmitting terminals  501  and  502 . 
     At a time t3 delayed by a protection time T TCR  from the detection that the actual transmission rate has fallen to “LCR”, the ATM switch  504  will judge that the transmitting terminal  502  is substantially in pause of transmission and send an ER set to the value “TCR” to the transmitting terminal  502 . The ER value sent to the transmitting terminal  501  remains as “PCR” with no change. When an ER of the value “TCR” is received at a time t4 delayed a predetermined time from the time t3 because of the propagation delay, the value of ACR having increased toward “PCR” has to be decreased to “TCR” correspondingly to the received ER value. The ER value sent from the ATM switch  504  is not changed from “PCR” so long as the transmitting terminal  502  keeps transmitting data at a rate lower than “TCR”. 
     Next, if the factor having caused the actual transmission rate of the transmitting terminal  502  to LCR is removed at a time t5, the actual transmission rate of the transmitting terminal  502  will be increased up to “TCR”. At a time t6 delayed a predetermined time from the time t5, the ATM switch  504  will detect the increased actual transmission rate, change the ER value from a one set to “TCR” to a one calculated by the conventional method of ER calculation, and send the ER value to the transmitting terminal  502 . At a time t7 delayed a predetermined time from the time t6, the transmitting terminal  502  will receive the new ER value, and increase the actual transmission rate little by little from “TCR” as the ACR value is increased. With the above operations, the actual transmission rate received by the ATM switch  504  from the transmitting terminal  502  can be prevented from abruptly increasing, so that the ATM switch  504  will not easily be congested. 
     As having been described in the foregoing, even if lasting, for a predetermined length of time, of a low actual transmission rate of a predetermined one of the transmitting terminals connected in a network has led to ACR initialization and thereafter the amount of transmitted data from the transmitting terminal suddenly increases, the present invention permits to prevent the actual transmission rate of the transmitting terminal from abruptly changing to a larger value. Thus, the present invention permits to effectively prevent a cell from being discarded when the network is suddenly applied with a large load. 
     Also, since the ACR of the transmitting terminal is initialized under control of the network, the present invention can assure a higher network safety than the prior art in which the increase or decrease of the actual data transmission rate is affected only by the operation of the transmitting terminal. 
     Since it is judged whether the transmitting terminal is in pause of transmission for each virtual path or virtual channel, ACR of other virtual paths or channels is not affected as by the conventional method of ER calculation. 
     Further, since a time of protection is set as a factor to judge whether a transmitting terminal is in pause of transmission, a temporary reduction of the actual transmission rate due to a bursting traffic cannot be judged to be in pause of transmission. Therefore, even frequent ACR initialization will not lead to any deduction of the throughput. 
     Furthermore, since ACR initialization is done by sending an ER value set nearly to ICR to a transmitting terminal judged to be in pause of transmission, any high ACR of the transmitting terminal will not be maintained. Therefore, even if the transmitting terminal has a suddenly increased amount of data to send, the actual transmission rate may be raised little by little. 
     Although the invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodies within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims.