PATENT ABSTRACT
Disclosed herewith is a PON system and a bandwidth controlling method capable of controlling congestion with use of an upstream bandwidth in a PON section efficiently when congestion occurs in a gateway (GW) connected to an OLT. An OLT connected to a plurality of ONUs through a passive optical network (PON) and to a gateway (GW) through a communication line, when receiving a congestion occurrence notice indicating a congestion occurred output number from a GW, identifies the identifier of the ONU that is using a GW output line having the congestion output port number and shifts the bandwidth controlling of the PON section in a normal mode for allocating a bandwidth to each ONU normally to that in a bandwidth suppression mode for allocating a congestion time allowable bandwidth that is less than the current bandwidth to the ONU having the identified ONU identifier and a bandwidth to each of other ONUs according to its transmission queue length.

PATENT DESCRIPTION
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application JP 2006-107790 filed on Apr. 10, 2006, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The present invention relates to a passive optical network (PON) system comprising an optical line terminal (OLT) and plural optical network units (ONUs) connected to each other through a passive optical network (PON), more particularly to a method for controlling the bandwidth of a PON section when line congestion occurs in any of the output ports of a frame transfer device connected to the OLT. 
     BACKGROUND OF THE INVENTION 
     A carrier network for providing services of connection to the Internet consists of an access network for holding a plurality of subscribers directly and a relay network (hereunder, to be referred to as a metro network) for bundling a plurality of access networks. And a passive optical network (PON) is one of such access network types. 
     A PON is a network system for connecting an OLT and a plurality of ONUs to each other through an optical fiber network provided with a function for branching an optical signal and enabling each ONU to transmit data to the OLT in a time division manner according to its access right (data transmission time band) information notified from the OLT. The PON branches an optical fiber connected to an OLT into a plurality of branch line optical fibers with use of an optical splitter (optical coupler) to enable an optical fiber section between an optical splitter and the OLT to be shared by a plurality of ONUs when an ONU disposed in the user&#39;s house is connected to a branch line optical fiber. Thus the PON can reduce the expenses for laying the optical fiber and hold many user terminals through the ONU. 
     The OLT of the PON system is connected to a metro network through, for example, a gateway (GW). The GW is a frame transfer device for holding a plurality of OLTs and transferring each variable length frame between each OLT and the metro network according to its header. In addition to the GW for housing the OLT, the metro network also includes another GW for housing, for example, an ISP network. Consequently, each user terminal connected to an ONU can access the Internet through the OLT, GW, and ISP network. 
     There are some types of PONs such as B-PON (Broadband PON) for transmitting information with use of fixed length ATM cells in an optical fiber section (PON section), G-PON (Gigabit PON) capable of realizing gigabit class high speed data transfer, and GE-PON (Giga-Ethernet PON) preferred to Ethernet (trade mark) services. 
     In the case of the G-PON capable of transferring variable length frames, an OLT generates GTC (G-PON Transmission Convergence) downstream frames specific to a PON section and maps GEM (G-PON Encapsulation Mode) frames including packet data addressed to each ONU in the payload of each GTC downstream frame. The GTC downstream frames are broadcast to a plurality of branch line optical fibers through an optical splitter (optical coupler). Each ONU checks the ONU identification information (ONU port ID) of the destination indicated by the header part of the GEM frame extracted from the GTC payload to receive the packet data addressed to itself selectively, then transfers the data to the subject target user terminal. 
     The PON OLT has a DBA (Dynamic Bandwidth Allocation) function for allocating a data transmission time band (time slot) for each ONU according to an amount of accumulated transmission data (transmission queue length) in each ONU. In the DBA, a time slot is allocated to one or a plurality of ONUs in a predetermined cycle ΔT. Consequently, the maximum number of time slots to be allocated to one ONU becomes ΔT. If B is assumed as a bandwidth of an optical fiber line, therefore, the maximum bandwidth B (limit) to be allocated to one ONU becomes ΔT×B. The OLT DBA function distributes the maximum bandwidth B (limit) to be allocated in a cycle ΔT to a plurality of ONUs according to the amount of the data in the transmission queue in each ONU. 
     In the case of the G-PON, each time slot is specified with a transmission starting time and a transmission ending time. The OLT sets the bandwidth control information of each ONU indicating both ONU identification information and a time slot in the header part of a GTC downstream frame, then notifies the information items to each ONU. Because the length varies among the branch line optical fiber sections in the PON, the transmission delay time of frames streaming from ONU to OLT also varies among ONUs. Consequently, the OLT measures the transmission delay time of each ONU and notifies an equivalent delay time to each ONU beforehand. Each ONU has a function for correcting the specified transmission starting time with the equivalent delay time upon receiving an allocated time slot from the OLT to start data transmission at a proper timing. 
     The OLT transfers upstream frames received from the ONU to the GW. In this case, if the OLT transmission buffer is insufficient in capacity, the frames might be discarded. To avoid such frame discarding, for example, JP-A No. 159203/2004 discloses a packet transfer device for limiting the transmission data amount from every ONU housed in the subject OLT by adjusting the access rights in a PON section in case where the accumulated data amount in the OLT built-in buffer exceeds a predetermined threshold value. In the description below, congestion is defined as a state in which frames may be discarded if an amount of accumulated data that exceeds a predetermined threshold value is left as is. 
     In the field of communication networks, a back pressure (BP) technique is known well as a technique for preventing such discarding of communication frames. According to the technique, congestion occurrence is notified from a congestion-occurred node device to a data transmission source device so that the data transmission source device stops data transmission or adjusts the transmission data amount. For example, JP-A No. 153505/2004 discloses a data frame transmission system that controls congestion with use of a pose frame conforming to the IEEE (Institute of Electrical and Electronic Engineers) 802.3. In JP-A No. 153505/2004, if congestion occurs, the subject data frame transmission device transmits a pose frame to the data transmission source device. Receiving the pose frame, the source device stops the data frame transmission or limits the bandwidth during a period specified with the payload part of the pose frame. 
     SUMMARY OF THE INVENTION 
     In the case of a carrier network, a logic path set with such a communication protocol as virtual local area network (VLAN) and multi-protocol label switching (MPLS) is used for Internet connection services. Using such a logic path enables user frames received from a specific subscriber terminal to be transferred along a predetermined route. If such a logic path is set, however, congestion might occur in a gateway (GW) in which logic paths handling much data respectively are concentrated as shown in  FIG. 19  and some frames to be transferred might be discarded in the worst case. 
     In a gateway (GW) that houses plural PONs, even if a specific output port goes into congestion, such frame discarding in the congestion port can be avoided by applying a back pressure to each PON OLT. In other words, when such congestion occurs, a back pressure is applied to the OLT provided with a function for adjusting access rights in a PON section as disclosed in JP-A No. 159203/2004, thereby the amount of data flowing from OLT to GW is reduced. Furthermore, according to the technique disclosed in JP-A No. 153505/2004, the OLT can adjust access rights in the PON section during a period specified in a pose frame. 
     However, because the GW that houses the OLT is provided with plural input/output ports (line interfaces), even when transmission data is concentrated at a specific output port, the buffer of each of other output ports is usually still sufficient in capacity. In the description below, a “congestion port” is defined as a port in which the output buffer is insufficient in capacity due to transfer data concentration as shown in  FIG. 19  and a “normal port” is defined as a port in which the output buffer is still sufficient in capacity even in such a congestion case. And a logic path that passes such a congestion port is referred to as a “congestion port path” and a logic path that passes a normal port is referred to as a “normal port path”. 
     If a congestion control method disclosed in JP-A No. 159203/2004 is applied in a case where a congestion port and a normal port coexist such way, the amount of data transmitted from every ONU to which one of the OLTs connected to a GW is connected is suppressed. Consequently, even when the congestion in the GW is eliminated due to the suppression of the data transmission from each OLT such way, even the bandwidth of the subscriber (ONU) who uses a normal port path comes to be limited while the congestion is limited, thereby the throughput of the whole access network is lowered. That has been a problem. 
     Furthermore, according to the congestion controlling method disclosed in JP-A No. 159203/2004, each ONU allocated bandwidth is reduced while the congestion is controlled without discriminating between the congestion port path user and the normal port user. Thus if an ONU transmission buffer overflows due to the limitation of the output bandwidth, the normal port path user also comes to suffer from the frame discarding problem. 
     Under such circumstances, it is an object of the present invention to provide a PON system and a bandwidth controlling method capable of controlling congestion by making good use of the upstream bandwidths of a PON section when congestion occurs at a gateway (GW) connected to an OLT. 
     It is another object of the present invention to provide a PON system and a bandwidth controlling method capable of controlling congestion without degrading the bandwidth allocated to each ONU that uses a GW normal port when congestion occurs at a gateway (GW) connected to an OLT. 
     In order to achieve the above objects, a PON system of the present invention is connected to plural optical network units (ONU) through a passive optical network (PON) and an optical line terminal (OLT) connected to a gateway (GW) through a communication line, when receiving a congestion occurrence notice indicating a number of the output port in which congestion occurred (congestion occurred output port number) from the GW, identifies the ID of the ONU that uses a GW output line having the congestion output port number and shifts the PON section bandwidth controlling in a normal mode for allocating a bandwidth to each ONU to that in a bandwidth suppressing mode for allocating a congestion time allowable bandwidth that is less than the current bandwidth to an ONU having the identified ONU identifier and allocating a bandwidth to each of other ONUs according to its transmission queue length. 
     More concretely, in the PON system of the present invention, the OLT includes a congestion control table that records a relationship between a GW output port number and the identifier of each ONU that uses a GW output line identified with the output port number; an OLT controlling part that records a transmission queue length notified in an upstream PON frame from each ONU beforehand and allocates a bandwidth distributed to each of the ONUs dynamically as the next upstream frame transmission bandwidth in a PON section according to its transmission queue length; and a downstream frame generation part that generates a downstream PON frame including bandwidth control information for indicating an upstream frame transmission bandwidth of each ONU according to the frame transmission bandwidth allocated by the OLT controlling part. When receiving a congestion occurrence notice that indicates a congestion occurred output port number from the GW, the OLT control part identifies the ONU ID corresponding to the congestion occurred output port number in the bandwidth control table and shifts the PON section bandwidth control in a normal mode for allocating a bandwidth to that in a bandwidth suppressing mode for allocating a congestion time allowable bandwidth that is less than the current bandwidth to an ONU having the identified ONU ID and allocating a bandwidth to each of other ONUs according to its transmission queue length. 
     In the first embodiment of the present invention, the OLT control part includes a bandwidth control table for recording a transmission queue length notified from an ONU, an allocated bandwidth calculated in a predetermined cycle according to the transmission queue length, and a congestion flag corresponding to the ONU ID respectively. When receiving a congestion occurrence notice from the GW, the OLT control part changes the state of the congestion flag corresponding to the ONU ID identified in the bandwidth control table to a congestion display state, then allocates a bandwidth to each ONU of which congestion flag indicates the normal state according to its transmission queue length and allocates a congestion time allowable bandwidth to each ONU of which congestion flag indicates the congestion display state during the bandwidth controlling in the bandwidth suppression mode. 
     Furthermore, in the first embodiment of the present invention, the downstream frame generation part generates bandwidth control information of each ONU according to its congestion flag state set in the bandwidth control table and, in the downstream PON frame, notifies each ONU of which congestion flag indicates the normal state in the downstream PON frame of a bandwidth according to its transmission queue length and notifies each ONU of which congestion flag indicates the congestion display state of a congestion time allowable bandwidth. 
     The OLT control part, when receiving a congestion reset notice indicating a number of the output port in which congestion is reset (congestion reset output port number) from the GW, identifies the ONU ID corresponding to the congestion reset output port number with reference to the bandwidth control table beforehand and changes the state of the congestion flag corresponding to the ONU ID identified in the bandwidth control table to the normal state at a predetermined timing. 
     According to the present invention, when congestion occurs at a specific output port of a GW to which an OLT is connected as shown in  FIG. 20 , the OLT can delete the bandwidth of the specific ONU that is transmitting communication frames to the congestion occurred output port selectively, thereby the OLT can distribute a surplus bandwidth in the PON section generated due to the deletion to other ONUs. Thus the GW congestion state can be eliminated without lowering the whole system throughput. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for showing a configuration of a network to which the present invention applies; 
         FIG. 2  is an example of a downstream frame format in a PON section; 
         FIG. 3  is an example of a format of a congestion occurrence/reset notification frame to be transmitted from a GW; 
         FIG. 4  is a block diagram of a configuration of an OLT; 
         FIG. 5  is an example of a bandwidth control table provided for an OLT; 
         FIG. 6  is an example of a congestion control table provided for an OLT; 
         FIG. 7  is a block diagram of a configuration of an ONU; 
         FIG. 8  is a block diagram of a configuration of a GW; 
         FIG. 9  is a flowchart of an example of bandwidth control processings executed by an OLT; 
         FIG. 10  is an explanatory diagram for describing bandwidth allocation by an OLT in a normal mode and in a bandwidth suppression mode; 
         FIG. 11  is a flowchart of downstream PON frame generation processings executed by an OLT; 
         FIG. 12  is a detailed flowchart of an example of processings for generating a bandwidth information field shown in  FIG. 11 ; 
         FIG. 13  is an example of a congestion control sequence executed among GW, OLT, and ONU; 
         FIG. 14  is a diagram for describing a relationship between a change of a GW output frame buffer queue length and bandwidth controlling by an OLT; 
         FIG. 15  is an explanatory graph for describing temporal changes of a transmission queue length and an allocated bandwidth in an ONU that is transmitting a frame that passes a GW congestion port; 
         FIG. 16  is an explanatory graph for describing temporal changes of a transmission queue length and an allocated bandwidth in an ONU that is transmitting a frame that passes a normal port of a GW; 
         FIG. 17  is a configuration of a communication network to which the present invention applies in another embodiment; 
         FIG. 18  is a block diagram of a configuration of a GW  500 - 1  shown in  FIG. 17 ; 
         FIG. 19  is an explanatory diagram for describing a place where a frame is to be discarded in a GW and how a bandwidth is used in each place according to an employed conventional bandwidth controlling method; and 
         FIG. 20  is an explanatory diagram for describing a place where a frame is to be discarded and how a bandwidth is used in each place in a GW to which the present invention applies. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereunder, the preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram for showing an example of a communication network to which the present invention applies. 
     A carrier network consists of an access network and a metro network. The access network includes optical line terminals (OLT)  10  ( 10 - 1 - 1  to  10 - 2 -n) and plural optical network units (ONU)  30  ( 30 - 1  to  30 -n) connected to each other through a passive optical network (PON) respectively. The metro network includes plural gateways (GW)  50  ( 50 - 1  to  50 - 4 ) connected to each other through lines  101  ( 101 - 1  to  101 - 4 ). 
     Each of subscriber terminals TE (TE- 1 - 1  to TE-n-m) is connected to the ONU  30  through one of the lines  109  ( 109 - 1  to  109 -n-m) and connected to the metro network through a PON and an OLT  10 . The gateways GW  50  of the metro network are divided into two types; GWs like  50 - 1  and  50 - 2  connected to an OLT  10  through the lines  105  ( 105 - 1 - 1  to  105 - 2 -n) and GWs like  50 - 3  and  50 - 4  connected to ISP networks  103  ( 103 - 1  and  103 - 2 ) through the lines  102  ( 102 - 1  and  102 - 2 ). Each IPS network  103  is provided with ISP servers  90  ( 90 - 1  and  90 - 2 ) and each subscriber terminal TE accesses the Internet  114  through an ISP server  90 . On each of connection lines  101 ,  102 ,  105 , and  109  are transferred communication frames according to the Ethernet (trade mark) protocol. 
     The passive optical network PON consists of optical fibers  106  ( 106 - 1  to  106 -n) housed in an OLT  10 , branch line optical fibers  108  ( 108 - 1 - 1  to  108 -n-m) connected to each ONU  30 , and optical splitters  107  ( 107 - 1  to  107 -n). The PON is structured, for example, as a G-PON conforming to the ITU-T Recommendation G.984.1. 
     A downstream optical signal transmitted from an OLT  10  through an optical fiber  106  is branched by an optical splitter  107  into plural branch line optical fibers  108  and broadcast to plural ONUs  30  connected to those branch optical fibers  108 . On the contrary, upstream optical signals transmitted from each ONU  30  through branch line optical fibers  108  are multiplexed by an optical splitter  107  and transferred to an OLT  10  through an optical fiber  106 . In this embodiment, it is assumed that both upstream and downstream optical signals are subjected to a frequency multiplexing process and transmitted through the same optical fiber. However, it is also possible to divide the optical fibers  106  and  108  in a PON section into an upstream one and a downstream one, thereby the same wavelength optical signal is applied to the upstream and downstream optical signals. Although the metro network connects plural gateways GW  50  to each other in a ring pattern with the line  101  in FIG.  1 , the metro network may also connects those gateways GW  50  in another pattern, for example, in a mesh pattern. 
     The present invention premises that a fixed data path is provided for each ONU  30  in the metro network. For example, assume now that a user connected to the ONU  30 - 1  has made a contract with a provider of the ISP network  103 - 1  for a connection service to the Internet. In this case, the GW  50 - 1  always transfers communication frames received from the ONU  30 - 1  through the OLT  10 - 1 - 1  to the connection line  101 - 2  of the ISP network  103 - 1 . At first, the ISP server  90 - 1  of the ISP network  103 - 1  executes predetermined communication procedures such as user authentication, IP address allocation, etc. with respect to each subscriber terminal TE, then enables the communication between the subscriber terminal TE and the Internet  104 . 
     In the same way, if the user connected to the ONU  30 -n has made a contract with a provider of the ISP network  103 - 1  for a connection service to the Internet, the GW  50 - 1  always comes to transfer frames received from the ONU  30 -n through the OLT  10 - 1 -n to the connection line  101 - 2 . 
     According to the present invention, each OLT  10  already knows a GW output port through which frames from each ONU  30  connected through the PON are transmitted. If congestion occurs in a GW transmission queue, therefore, the OLT  10  controls the bandwidth in the PON section by making good use of the relationship between the ONU  30  and the GW. 
       FIG. 2  is an example of a format of downstream frames in a PON section, transmitted from the OLT  10  to the ONU  30 . Here, a GTC downstream frame applied to the G-PON is shown. 
     A GTC downstream frame  60  consists of a header part (PCBd)  61  and a GTC payload  62 . The GTC payload  62  includes plural unicasting frames addressed to a specific ONU and broadcasting frames or multicasting frames to be received by plural ONUs in the GEM frame format specific to a PON section. Each ONU checks the ONU ID set in the header of the GEM frame extracted from the GTC payload  62  to select each frame it receives. 
     The OLT  10  transmits each GTC downstream frame  60  in the basic frame cycle ΔTF, for example, in cycles of 125 microseconds. The header part  61  of the GTC downstream frame consists of a Psync field  611  that includes a specific signal pattern to enable the receiving side (ONU  30 ) to synchronize with frames, a bandwidth information field  613  for notifying each ONU  103  of an upstream frame transmission time band, an ONU control information field  614  for notifying each ONU of such control information as starting, stopping, etc., and a field of other field information  612 . The frame information field  612  includes, for example, such information as whether to make an FEC processing in the subject frame, a frame counter, a BIP, a header length, etc. The contents in the frame information field  612  and the ONU control information field  614  are deleted in some cases according to the circumstances. 
     The bandwidth information field  613  consists of plural bandwidth control information fields  613 - 1 ,  613 - 2  . . . provided for each ONU. Each bandwidth control information field  613 -i indicates a bandwidth control ID (Alloc-ID)  621  for identifying an ONU, a transmission starting time  622  and transmission ending time  623  for indicating an upstream frame transmission time band (allocated bandwidth), and other control information  624 . The other control information  624  includes, for example, information about whether to notify the subject transmission queue state from ONU to OLT and control information for specifying whether to require an FEC in the subject upstream frame. 
     The destination ONU (Alloc-ID)  621  of the bandwidth information notified in the bandwidth information field  613  has no relation to the destination ONU in the GEM frame included in the payload  62 . The OLT  10  calculates a bandwidth to be allocated to each ONU  30  under its control in cycles of 125 microseconds×N (e.g., N=4 to 6). The OLT  10  divides the upstream transmission time band of 125 microseconds×N into plural time bands so as to allocate a longer transmission time band to each ONU having a lot of data in the transmission queue according to the state of the upstream transmission queue in each ONU (e.g., a queue length indicating an amount of data to be transmitted) to determine a bandwidth to be allocated to each ONU. 
     For example, if m units of ONUs  30  are connected to an OLT  10 , each GTC downstream frame includes bandwidth control information related to some of the m units of ONUs. The OLT  10  completes notice of an allocated bandwidth to each ONU by transmitting GTC downstream frames by N times consecutively. Each ONU finds bandwidth control information having the Alloc-ID of itself in a received GTC downstream frame to transmit upstream data according to the transmission starting time  622  and the transmission ending time  623  indicated by the bandwidth control information. 
     According to the present invention, the GW  50  monitors the transmission queue length QL at each output port and when the transmission queue length QL exceeds a predetermined first threshold HT 1 , the GW  50  transmits a congestion occurrence notification frame that specifies an output port number to the OLT  10  to prevent communication frame discarding to be caused by insufficient capacity of the buffer. Each OLT  10  allocates a bandwidth to each OLT in the bandwidth suppression mode for suppressing only the amount of transmission data from a specific ONU to be transmitted to the output port specified in the above congestion occurrence notification frame (hereunder, to be referred to as a congestion port). 
     When the congestion port transmission queue length falls below the second threshold TH 2  that is lower than the first threshold TH 1  due to a back pressure to the OLT, the GW  50  transmits a congestion reset notification frame that specifies the subject output port number to each OLT  10 . Each OLT  10  then resets the suppression of the amount of transmission data to be transmitted to the output port specified in the congestion reset notification frame and returns to the bandwidth allocation control in the normal mode according to the state of the upstream transmission queue in each ONU. 
     In the case of the network configuration shown in  FIG. 1 , for example, if the length of the transmission queue of an output port connected to the line  101 - 2  exceeds the first threshold value TH 1 , the GW  50 - 1  transmits a congestion occurrence notification frame to each of the OLTs  10 - 1 - 1  to  10 - 1 -n, thereby each of the OLTs  10 - 1 - 1  to  10 - 1 -n reduces the bandwidth allocated to the ONU that is communicating with the Internet  104  through an ISP network  103 . 
     As to be described later, in the first embodiment of the present invention, each OLT is provided with a congestion control table for recording a relationship between each output port number and each ONU ID of the connected GW  50 . Thus the OLT, when receiving a congestion occurrence notification frame from the GW  50 , identifies the ONU ID corresponding to the congestion port number with reference to the congestion control table and allocates a bandwidth to each OLT in the bandwidth suppression mode so as to reduce the allocated bandwidth to the ONU having this ONU ID. 
     Also in the first embodiment of the present invention, each OLT is provided with a bandwidth control table for indicating the relationship among a transmission queue length, the current allocated bandwidth, and a predetermined congestion time allowable bandwidth with respect to an ONU ID respectively. When an ONU ID corresponding to a congestion port number is identified, therefore, each OLT reduces the bandwidth allocated to the ONU having this ONU ID up to a congestion time allowable bandwidth specified in the bandwidth control table. In the bandwidth suppression mode, because the bandwidth in the PON section comes to have a surplus due to the reduction of the amount of data transmitted to the congestion port from the ONU, the OLT  10  uses the PON section bandwidth effectively by increasing the bandwidth allocated to another ONU that is transmitting data to a normal port. 
       FIG. 3  is an example of a format of the congestion occurrence/reset notification frame (back pressure control frame) transmitted from GW  50  to OLT  10 . 
     The back pressure control frame uses, for example, a frame format provided with an Ethernet VLAN (Virtual Local Area Network) tag. The back pressure control frame includes a destination MAC (Media Access Control) address  601 , a source MAC address  602 , a VLAN tag  603 , a congestion bit  604 , and a congestion port number  605 . 
     The destination MAC address  601  indicates the MAC address of each OLT  10  housed in a GW  50  and the source MAC address  602  indicates the MAC address of a network interface that houses a connection line between the GW  50  and OLT  10 . The VLAN tag  603  has a specific value set to indicate that the subject frame is a back pressure control frame. The congestion bit  604  has the value “1” for a congestion occurrence notification frame and “0” for a congestion reset notification frame. The congestion port number  605  indicates a congestion detected GW output port number. 
     Receiving a back pressure control frame, each OLT  10  can recognize the received frame as a back pressure control frame according to the value set in the VLAN tag  603  and determine the received frame as a congestion occurrence notification or congestion reset notice according to the value set in the congestion bit  604 . Each OLT  10  can also identify an ONU of which allocated bandwidth is to be reduced with reference to the congestion port number  605 , the congestion control table, and the bandwidth control table. 
       FIG. 4  is a block diagram of a configuration of an OLT  10 . 
     The OLT  10  consists of an OLT control part  11 , a parameter memory  12 , an optical transmission/receiving part  13  connected to a PON optical fiber  106 , an Ethernet interface (IF)  14  connected to an Ethernet line  105  for connecting a GW  50 , as well as an upstream signal processing circuit and a downstream signal processing circuit connected respectively between the optical transmission/receiving part  13  and the Ethernet interface IF  14 . The parameter memory  12  includes a bandwidth control table  120  to be described in  FIGS. 5 and 6  and a congestion control table  130  formed respectively in itself. 
     The upstream signal processing circuit consists of an optical/electrical conversion part  15  for converting each optical signal received by the optical transmission/receiving part  13  to an electrical signal, an upstream frame terminal part  16  connected to the optical/electrical conversion part  15 , an upstream data buffer  17  for temporarily storing upstream frames output from the upstream frame terminal part  15 , and an upstream frame transmission part  18  for reading frames from the upstream data buffer  17  and transmitting those frames to the Ethernet IF  14 . The upstream frame terminal part  16  regenerates an upstream frame from each output signal of the optical/electrical conversion part  15  and outputs such control information as a source queue length extracted from the frame header to the OLT control part  11 , as well as converts each received frame to a data frame in the Ethernet format and outputs the data frame to the upstream data buffer  304 . 
     The downstream signal processing circuit consists of a frame analysis part  19  connected to the Ethernet IF  14 , a downstream data buffer  20  for temporarily storing user frames output from the data analysis part  19 , a frame generation part  21  for generating a downstream PON frame (GTC downstream frame) described in  FIG. 2  in a predetermined cycle ΔTF and mapping each user frame read from the downstream data buffer  20  or control frame output from the OLT control part  11  in the GEM frame format, a downstream frame transmission part  22  for transmitting each downstream frame output from the downstream frame generation part  21  as an electrical signal, and an electrical/optical conversion part  23  for converting each electrical signal output from the downstream frame transmission part  22  to an optical signal. The frame analysis part  19  analyzes frames received from the Ethernet IF  14  and outputs user frames to the downstream data buffer  20  and back pressure control frames to the OLT control part  11  respectively. 
     The OLT control part  11  calculates a bandwidth to be allocated to each ONU  30  in a predetermined cycle according to the source queue length indicated by the bandwidth control table  120  formed in the parameter memory  12  and updates the bandwidth control table  120  with the result. How the OLT control part  11  calculates a bandwidth to be allocated such way will be described more in detail later with reference to  FIG. 9 . 
     As shown in  FIG. 5 , the bandwidth control table  120  consists of plural table entries, each corresponding to a bandwidth control ID (Alloc-ID)  121  that is an ONU ID. Each table entry indicates a congestion time allowable bandwidth (Bcon)  122  predetermined for each ONU, a maximum control bandwidth (Bmax)  123 , an allocation bandwidth (BW)  124  calculated by the OLT control part  11 , an ONU source queue length (SQ)  125 , a congestion flag  126 , and a carry-over bandwidth  127 . The source queue length (SQ)  125  indicates the latest value of the transmission queue of the ONU  30  notified from the upstream frame terminal part  303 . 
     As shown in  FIG. 6 , the congestion control table  130  consists of a congestion flag  132  corresponding to a GW output port number  131  and plural table entries, each indicating a relationship with an ONU ID (bandwidth control ID)  133  that uses a GW output line identified with the GW output port number  131 . 
     The OLT control part  11 , when receiving a back pressure control frame as shown in  FIG. 3  from the frame analysis part  19 , refers to the congestion control table  130  formed in the parameter memory  12  to update the congestion flag  132  of a table entry corresponding to the congestion port number  605  indicated in the back pressure control frame according to the congestion bit  605  in the received frame, then identifies the ID  133  of the ONU in which the congestion is to be controlled. After that, the OLT control part  11  searches for a table entry corresponding to the bandwidth control ID  133  in the bandwidth control table  120  and updates the value of the congestion flag  126 . If the back pressure control frame is a congestion occurrence notification frame (congestion flag=“1”), the value of the congestion flag  126  is changed to “1” immediately. If the back pressure control frame is a congestion reset notification frame (congestion flag=“0”), the value of the congestion flag  126  is changed to “0” at the next bandwidth calculation time. 
       FIG. 7  is a block diagram of a configuration of an ONU  30 . 
     The ONU  30  consists of an ONU control part  31 , an Ethernet interface (IF)  32  connected to an Ethernet line  109  for connecting a subscriber terminal TE, an optical transmission/receiving part  33  connected to a PON branch line optical fiber  108 , as well as an upstream signal processing circuit and a downstream signal processing circuit connected respectively between the Ethernet interface IF  32  and the optical transmission/receiving part  33 . 
     The ONU control part  31  consists of an upstream data buffer management part  311 , a control frame generation part  312 , and an upstream transmission timing generation part  313 . 
     The upstream signal processing circuit consists of an upstream data buffer  34  for temporarily storing upstream user frames output through the Ethernet IF, an upstream frame generation part  35  for generating upstream PON frames, an upstream frame transmission part  36  for transmitting upstream PON frames generated by the upstream frame generation part  35  in a time band indicated by the upstream transmission timing generation part  313 , and an electrical/optical conversion part  37  for converting each electrical signal output from the upstream frame transmission part  36  to an optical signal. 
     The downstream signal processing circuit consists of an optical/electrical conversion part  38  for converting each optical signal received by the optical transmission/receiving part  33  through a branch line optical fiber  108  to an electrical signal, a downstream frame terminal part  39  connected to the optical/electrical conversion part  38 , a downstream data buffer  40  for temporarily storing user frames addressed to the ONU of itself from the downstream frame terminal part  39 , and a downstream frame transmission part  41  for transmitting user frames read from the downstream data buffer  40  and control frames supplied from the control frame generation part  312  to the Ethernet IF  32  respectively. 
     The downstream frame terminal part  39  converts each signal output from the optical/electrical conversion part  38  to a downstream PON frame (GTC downstream frame) and supplies information of an allocated bandwidth to the self-ONU, extracted from the frame header (PCBd), to the upstream signal timing generation part  313 . The downstream frame terminal part  39  also extracts each user frame addressed to the self ONU from the GTC payload and converts the user frame to a data frame in the Ethernet format and outputs the frame to the downstream data buffer  40 . 
     The upstream transmission timing generation part  313  determines an upstream frame transmission time band according to the transmission starting time and the transmission ending time indicated by the allocated bandwidth information, as well as according to an equivalent delay time notified beforehand from the OLT  10 , then controls the upstream frame generation part  35 , the upstream frame transmission part  36 , and the upstream buffer management part  311  respectively. 
     The upstream data buffer management part  311  monitors the amount of data (transmission queue length) stored in the upstream data buffer  34 . When the transmission queue length exceeds a predetermined threshold value, the management part  311  issues a control signal to the control frame generation part  312 , which then generates a pose frame and notifies the upstream frame generation part  35  of the current transmission queue length at a timing specified by the upstream transmission timing generation part  313 . The upstream frame generation part  35  includes information of a transmission queue length notified from the upstream buffer management part  311  in the header according to a command from the upstream transmission timing generation part  313  and generates an upstream PON frame including a user frame read from the upstream data buffer  34  in the payload, then outputs the PON frame to the upstream frame transmission part  36 . 
     The control frame generation part  312  generates a control frame (pose frame) used to instruct an object subscriber terminal TE to suppress data transmission according to a control signal received from the upstream data buffer management part  311 , then outputs the control frame to the downstream frame transmission part  41 . The downstream frame transmission part  41  transmits each user frame read from the downstream data buffer  40  to the Ethernet IF  32  if there is no pose frame generated from the control frame generation part  312 . If a pose frame is generated, the control frame generation part  312  transmits the pose frame to the Ethernet IF  32 . 
     After suppressing data transmission in response to the pose frame, if the subscriber terminal TE has a queue length of the upstream data buffer  34 , which falls below the predetermined congestion reset threshold value, the upstream data buffer management part  311  issues a control signal for resetting the congestion to the control frame generation part  312 . In response to the control signal, the control frame generation part  312  generates a control frame to reset the suppression of the data transmission and outputs the control frame to the downstream transmission part  41 . 
       FIG. 8  is a block diagram of a configuration of a GW  50 - 1 . 
     The GW  50 - 1  consists of plural network interface (NIF) parts  51  ( 51 - 1  to  51 -q) connected to an Ethernet line ( 105  or  101 ) respectively, a frame relay part  52  for connecting those NIF parts  51  to each other, a GW management part  53 , and a management information table  54  for holding GW management information to be referred to from the GW management part  53 . Each NIF part  51  consists of an input line interface part  51 A connected to the Ethernet IF  511  for housing Ethernet input/output lines and an output line interface part  51 B. 
     The input line interface part  51 A consists of a parameter table  513  for holding parameter information used for header processing, an input header analysis part  512  for analyzing the header of each frame received from the Ethernet IF  511  and adding an internal header including internal routine information to the received frame according to the parameter table  513 , an input frame buffer  514  for temporarily storing frames output from the input header analysis part  512 , and an input frame reading part  515  for reading frames from the input frame buffer  514  and outputting the read frames to the frame relay part  52 . 
     The frame relay part  52  relays each frame received from each input line interface part  51 A to the output line interface part  51 B of a specific output port determined by the internal routine information. As described above, according to the present invention, because a fixed path is set for each ONU in a carrier network, the frame relay part  52  comes to transfer frames through a fixed output port in each ONU  103 . 
     The output line interface part  51 B consists of a parameter table  517 , an output header analysis part  516  for analyzing each frame received from the frame relay part  52  to remove the internal header and make a header processing according to the data set in the parameter table  517 , an output frame buffer  518  for temporarily storing frames output from the output header analysis part  516 , a queue length monitor  519  for monitoring the amount of data (queue length) stored in the output frame buffer  519  and outputting queue length information to the GW management part  53 , an output frame reading part  520  for reading frames output from the output frame buffer  518  and outputting the frames to the Ethernet IF  511 , and a back pressure control frame generation part  521 . 
     The parameter table  517  records parameter information required for a header processing, a destination MAC address (OLT MAC address) required to generate a back pressure control frame, and a source MAC address (Ethernet IF MAC address). However, the output line interface part  51 B to which no OLT is connected is not required to record those MAC addresses. The management information table  54  records the first threshold value TH 1  for detecting congestion occurrence, the second threshold value TH 2  for detecting congestion reset, and a congestion flag indicating a congestion state in each output port (output line interface part  51 B). The first and second threshold values are in a relationship of TH 1 &gt;TH 2 . 
     The GW management part  53 , when receiving a queue length QL from the queue length monitor  519  of each output line interface part  51 B, compares the queue length QL with the threshold values TH 1  and TH 2 . If the queue length QL of the output frame buffer  518  of an output port Pi exceeds the first threshold value TH 1 , the GW management part  53  sets “1” in the congestion flag corresponding to the output port Pi in the management information table  54  and notifies the congestion occurrence to the back pressure control frame generation part  521  belonging to each of other output ports. 
     As for the output port Pi of which congestion flag is set at “1”, the GW management part  53  compares the queue length QL of the output frame buffer  518  to be received later from the queue length monitor  519  with the second threshold value TH 2 . When the queue length QL is under the second threshold value TH 2 , the GW management part  53  changes the value of the congestion flag to “0” and notifies the back pressure control frame generation part  521  belonging to each of other output ports, of congestion reset. The congestion occurrence notice and the congestion reset notice respectively include a VLAN tag value, a congestion bit indicating occurrence/reset of congestion, an output port number indicating a congestion occurred (reset) output line interface part  51 B. 
     The back pressure control frame generation part  521  of each output line interface part  51 B, when receiving a congestion occurrence or reset notice from the GW management part  53 , reads the object MAC address information from the parameter table  517  to generate a back pressure control frame shown in  FIG. 3  and output the frame to the output frame reading part  520 . The output frame reading part  520 , when receiving the back pressure control frame and completing the transmission of one of the frames being read from the output frame buffer  518 , transmits the control frame to the back pressure control frame Ethernet IF  511 , then restarts reading/transmission of frames from the output frame buffer  518  later. 
       FIG. 9  is a flowchart of bandwidth control processings executed by the OLT control part  11  of an OLT  10 . The OLT control part  11  executes a bandwidth control processing  110  in the basic frame cycle ΔTF (in cycles of 125 microseconds) shown in  FIG. 2  to calculate a bandwidth to be allocated to each ONU in the ΔTF×N cycle. In the description below, the N value is represented as MAX and the number of execution times of the bandwidth control processing  110  is represented by a parameter i. 
     In the bandwidth control processing  110 , the OLT control part  11  increases the value of the parameter i (i=i+1) (step  111 ) and checks whether or not a new back pressure control frame is received from the GW (step  112 ). If no back pressure control frame is received, the OLT control part  11  determines whether or not the parameter i value has reached MAX (step  117 ). If the determination result is NO (&gt;MAX), the OLT control part  11  exits the processing. If the determination result is YES (=MAX), the OLT control part  11  calculates a bandwidth to be allocated (to be described later) and updates the bandwidth control table  120 . 
     If a new back pressure control frame is received from the GW, the OLT control part  11  checks the congestion bit  604  included in the received frame (step  113 ). If “1” is set in the congestion bit  604 , that is, if the received frame is a congestion occurrence notification frame, the OLT control part  11  sets “1” in the congestion flags  132  and  126  corresponding to the congestion port number  605  indicated in the received frame in the congestion control table  130  and bandwidth control table  120  (step  114 ), then determines whether or not the value of the parameter i has reached the MAX (step  117 ). 
     Concretely, in step  114 , the OLT control part  11  searches the subject table entry corresponding to the congestion port number  605  in the congestion control table  130  and sets “1” in the congestion flag  132 , then refers to the bandwidth control table  120  according to the bandwidth control ID  133  indicated by the table entry to set “1” in the congestion flag  126  of each table entry corresponding to the bandwidth control ID  133 . 
     In case where “0” is set in the congestion bit included in the received frame, that is, if the received frame is a congestion reset notification frame, the OLT control part  11  records the value of the congestion port number  605  in the work area as a congestion reset port number (step  116 ), then determines whether or not the parameter i value has reached the MAX (step  117 ). If the determination result is NO (&lt;MAX), the OLT control part  11  exits the processing. 
     If the parameter i value has reached the MAX, the OLT control part  11  checks the congestion reset port number in the work area (step  118 ). If no congestion reset port number is recorded in the area, the OLT control part  11  calculates a new bandwidth to be allocated to each ONU with reference to the bandwidth control table  120  and records the new allocation bandwidth  124  in the bandwidth control table  120  (step  121 ). After that, the OLT control part  11  sets the initial value “1” in the parameter i and exits the processing. 
     If a congestion reset port number is recorded in the work area, the OLT control part  11  sets “0” in the congestion flags  132  and  126  corresponding to the congestion reset port number in the congestion control table  130  and in the bandwidth control table  120  respectively (step  119 ), then clears the congestion reset port number in the work area (step  120 ). After that, the OLT control part  11  calculates a new bandwidth to be allocated to each ONU (step  121 ). 
     According to the present invention, the bandwidth allocation calculation mode executed by the OLT control part  11  includes a normal mode and a bandwidth suppression mode. In the normal mode, for example, a bandwidth is allocated to each ONU according to a source queue length  125  indicated in the bandwidth control table  120  with use of a dynamic bandwidth allocation (DBA) function ruled by the ITU-T Recommendations G983.4 and G984.3. Concretely, for example, the OLT control part  11  finds a total value of the source queue lengths  125  in the bandwidth control table  120  and calculates a weight of each ONU according to the ratio between the total queue length value and each ONU source queue length  125 , then distributes an upstream bandwidth to each ONU in the ΔTF×N period of the PON section. 
     In the bandwidth suppression mode, a bandwidth is allocated only when congestion occurs in any of the output ports of a GW. In the bandwidth suppression mode, the OLT control part  11  allocates a bandwidth indicated by the congestion time allowable bandwidth  122  to an ONU for which “1” is set in the congestion flag  126  in the bandwidth control table  120 . Because the congestion time allowable bandwidth  122  is smaller than the bandwidth  124  allocated in the normal mode, a surplus is generated in the upstream bandwidth of the PON section if a logical path communication bandwidth going to a congestion port of a GW is reduced to a congestion time allowable bandwidth  122  from an allocated bandwidth  124 . In the bandwidth suppression mode of the present invention, therefore, the OLT control part  11  reduces the total value of the congestion time allowable bandwidths  122  indicated by each table entry of which congestion flag  126  is set at “1” from the upstream bandwidth in the ΔTF×N period of the PON section and distributes the remaining bandwidth to each of other ONUs just like in the normal mode. 
       FIG. 10  is an explanatory diagram for describing how an OLT  10  allocates a bandwidth in the normal mode and in the bandwidth suppression mode respectively. 
     To simplify the description, it is assumed here that 8 units of ONUs  30  are connected to an OLT  10  and the OLT control part  11  calculates a bandwidth to be allocated to each ONU in cycles of ΔTD, which is three times the basic frame cycle ΔTF (N=3). And also to make it easier to understand the difference between the bandwidth allocation in the normal mode and that in the bandwidth suppression mode, it is assumed here that each ONU transmission queue length is fixed in each period shown in  FIG. 10 . Each of slash-marked frames F 2  (F 12 , F 22 ), F 3  (F 13 , F 23 ), and F 6  (F 16 , F 26 ) represents a bandwidth allocated to the ONU  2 , ONU  3  and an ONU  6  that are forwarding to a GW congestion port. 
     In this embodiment, the OLT control part  11  updates the value of the allocated bandwidth  124  in the bandwidth control table  120  in cycles of ΔTD and the downstream frame generation part  21  notifies each ONU of the allocated bandwidth  124  indicated in the bandwidth control table  120 . The OLT control part  11 , when receiving a congestion notice from the GW  50 , sets “1” in the congestion flag  126  corresponding to the congestion port in the bandwidth control table  120 . In this embodiment, the downstream frame generation part  21  checks the congestion flag  126  in the bandwidth control table  120  and notifies each ONU for which “0” is set in the congestion flag  126  of the allocated bandwidth  124  and notifies each ONU for which “1” is set in the congestion flag  126  of the congestion time allowable bandwidth  122 . 
     The frame cycles ΔTF( 1 ) to ΔTF( 3 ) shown in (A) of  FIG. 10  indicate the upstream frame transmission bandwidths of the ONU  1  to an ONU  8  allocated in the normal mode. (B) indicates an upstream frame transmission bandwidth in frame cycles ΔTF( 4 ) to ΔTF( 6 ) when the OLT control part  11  receives a congestion notice from the GW  50  while the ONU  2  is transmitting a frame F 12 . 
     As described above, because the OLT control part  11  updates the value of the allocated bandwidth  124  in the bandwidth control table  120  in cycles of ΔTD, the value of the allocated bandwidth  124  of each ONU is not changed in frame cycles of ΔTF( 4 ) to ΔTF( 6 ). In this embodiment, however, the downstream frame generation part  21  notifies each ONU for which “1” is set in the congestion flag  126  of the congestion time allowable bandwidth  122 . Consequently, the bandwidth of the upstream frames streaming from the ONU  6  to a GW congestion port is reduced, thereby an empty bandwidth BW (V) is generated at the end of the frame cycle TF( 6 ). 
     If the OLT control part  11  updates the value of the allocated bandwidth  124  in the bandwidth control table  120  between frame cycles ΔTF( 6 ) and ΔTF( 7 ), the bandwidths to be notified to ONU  1  to ONU  8  are changed in frame cycles ΔTF( 6 ) TF( 7 ) to ΔTF( 6 ) TF( 9 ) as shown in (C) of  FIG. 10 . Because of the bandwidth allocation in the bandwidth suppression mode, the congestion time allowable bandwidth  122  is assumed as the bandwidth of each of ONU  2 , ONU  3 , and ONU  6  that are transmitting upstream frames to the congestion port while the bandwidths of other ONUs that are transmitting upstream frames to normal ports increase more than in the normal mode. In  FIG. 10 , it is assumed that the OLT control part  11  receives a congestion reset notice from the GW  50  while the ONU  7  is transmitting an upstream frame F 27 . In this embodiment, transient periods T 1  and T 2  are generated before and after an optimal bandwidth control period T 2  in the bandwidth suppression mode. 
       FIG. 11  is a flowchart of downstream PON frame generation processings  210  executed by the downstream frame generation part  21  in basic frame cycles of ΔTF (in cycles of 125 microseconds). 
     The downstream frame generation part  21  generates a Psync field  611 , a frame information field  612  (step  211 ), a bandwidth information field  613  (step  212 ), and an ONU control information field  614  (step  213 ) of the GTC frame header (PCBd) shown in  FIG. 2 , then generates a GTC payload  62  in which a user frame read from the downstream data buffer  20  and a control frame generated by the OLT control part  11  are mapped in the GEM frame format (step  214 ). 
       FIG. 12  is an example of a detailed flowchart of the bandwidth information field generation processings (step  212 ). The allocated bandwidth  124  set by the OLT control part  11  in the bandwidth control table  120  indicates an upstream bandwidth of each ONU in the ΔTF×N period. The downstream frame generation part  21  divides an allocated bandwidth  124  indicated in the bandwidth control table  120  into N×PON frames to be notified to every ONU  30  indicated with the bandwidth control ID  121  in the bandwidth control table  120 . 
     In  FIG. 12 , the parameter i indicates the basic frame cycle ΔTF in a period ΔTD shown in  FIG. 10  and the parameter j indicates a position of a table entry in the bandwidth control table  120 . MAX indicates the number of basic frame cycles N included in a period ΔTD. 
     The downstream frame generation part  21  compares the value of the parameter i with “MAX+1” (step  220 ). If i=MAX+1 is satisfied, that is, if a new period ΔTD is set, the downstream frame generation part  21  sets “1” in the values of the parameters i and j (step  221 ) and initializes the total value of the bandwidths to be allocated to ONUs (usable BW) in the downstream PON frame (the first basic frame cycle) generated this time (step  222 ). 
     The downstream frame generation part  21  then compares the parameter j with the number of table entries in the bandwidth control table  120  (step  223 ). If the parameter j is over the number of table entries, the downstream frame generation part  21  exits the processing. If the parameter j is not over the number of table entries, the downstream frame generation part  21  reads the j-th table entry from the bandwidth control table  120  to check the congestion flag  126  (step  225 ). If “1” is set in the congestion flag  126 , the downstream frame generation part  21  sets the value of the congestion time allowable bandwidth  122  indicated by the j-th table entry in the variable BW(j) (step  226 ). If “0” is set in the congestion flag  126 , the downstream frame generation part  21  sets the value of the allocated bandwidth  124  indicated by the j-th table entry in the variable BW(j) (step  227 ). 
     After that, the downstream frame generation part  21  checks the carry-over bandwidth  127  of the j-th table entry (step  228 ). If “0” is set in the carry-over bandwidth  127 , the downstream frame generation part  21  compares the usable BW with BW(j) (step  231 ). If “0” is not set in the carry-over bandwidth  127 , the downstream frame generation part  21  sets the value of the carry-over bandwidth  127  in BW(j) (step  229 ) and clears the carry-over bandwidth  127  of the j-th table entry (step  230 ), then compares the usable BW with BW(j) (step  231 ). 
     Here, the carry-over bandwidth means a bandwidth notified to each ONU in the next basic frame period when a bandwidth (congestion time allowable bandwidth or allocated bandwidth) specified by the j-th table entry is over two basic frame cycles as shown in  FIG. 10 . In an actual case, however, it is possible to notify a bandwidth specified by the j-th table entry in one basic frame cycle first, then subtract the carry-over bandwidth from the usable BW in the next basic frame cycle. 
     If the usable BW is over BW(j), the downstream frame generation part  21  calculates both transmission starting time and transmission ending time on the basis of the BW(j) (step  234 ). If the usable BW is under BW(j), the downstream frame generation part  21  sets the usable BW in BW(j) (step  232 ) and records the insufficient bandwidth in the j-th table entry as a carry-over bandwidth (step  233 ), then calculates both transmission starting time and transmission ending time on the basis of the BW(j) (step  234 ). 
     After that, the downstream frame generation part  21  sets bandwidth control information that includes both transmission starting time and transmission ending time together with a bandwidth control ID indicated by the j-th table entry in a downstream PON frame (step  235 ), then subtracts the value of BW(j) from the usable BW (step  236 ) and compares the usable BW value with the predetermined bandwidth minimum value BWmin (step  237 ). If the usable BW value is over BWmin, the downstream frame generation part  21  increases the parameter j value (step  238 ) and returns the control sequence to step  223 . If the usable BW value is under BWmin, the downstream frame generation part  21  increases the parameter i value (step  239 ) and exits the processing. 
       FIG. 13  is a diagram for describing a congestion control sequence according to the present invention, to be executed among the GW  50 , OLT  10 , and ONU  30 . 
     The GW  50  monitors the queue length QL of each output port (output line interface). If the queue length QL exceeds the threshold TH 1 , the GW  50  issues a back pressure control frame (congestion occurrence notice) (congestion occurrence detection  401 ). If the queue length QL is under the threshold value TH 2 , the GW  50  issues a back pressure control frame (congestion reset notice) that specifies a congestion port (congestion reset detection  404 ). The back pressure control frame (congestion occurrence/reset notice) is transmitted to every OLT connected to the GW  50 . 
     The OLT  10  transmits downstream PON frames DF 1 , DF 2 , DF 3 , DF 4 , . . . in the basic frame cycles ΔTF (SQ 01 , SQ 02 , SQ 04 , and SQ 05 ). While the output port of the GW  50  is normal in state, the OLT  10  notifies each ONU of a bandwidth allocated in the normal mode. The ONU  30 -k connected to the OLT  10 , when receiving a downstream PON frame DF 2  including bandwidth control information addressed to itself, starts transmission of an upstream frame UFi at a proper timing (ΔTS) obtained by adding an equivalent delay to the transmission starting time indicated by the bandwidth control information (SQ 03 ). 
     The OLT  10 , when receiving a congestion occurrence notice from the GW  50 , sets “1” in the congestion flag corresponding to the notified GW output port number in the congestion control table  130  and in the bandwidth control table  120  respectively (step  402 ). After “1” is set in the congestion flag, a period until the OLT  10  allocates a bandwidth in the band allocation cycle ΔTD (step  403 ) and the bandwidth control table  120  is updated becomes a transient period T 1  shown in  FIG. 10 . 
     If the bandwidth control table  120  is updated while the GW  50  is in a congestion state, the bandwidth control by the OLT  10  becomes the optimal control period T 2 . The downstream PON frames (SQ 11 , SQ 12 , and SQ 14 ) transmitted in the optimal control period T 2  notify each ONU that uses a GW normal port of an extended allocation bandwidth (E-BWs) and notify each ONU that uses a congestion port of a congestion time allowable bandwidth (C-BWs). Consequently, if the ONU  30 -k uses a congestion port, the ONU  30 -k comes to transmit upstream frames UFi+1 (SQ 13 ) in the congestion time allowable bandwidth. 
     The OLT  10 , when receiving a congestion reset notice from the GW  50 , records a congestion reset port number (step  405 ), then sets “0” in the congestion flag in the next allocation processing in the bandwidth allocation cycle ΔTD (step  406 ). Consequently, a period between when the GW  50  issues a congestion reset notice and when the OLT  10  allocates a bandwidth (step  406 ) becomes a transient period T 3  in which a downstream PON frame (SQ 15 ) notifies each ONU of an extended allocation bandwidth or congestion time allowable bandwidth. 
     If the OLT  10  that has received a congestion reset notice updates the bandwidth control table  120  once, the OLT  10  enters the normal mode in which the LOT  10  notifies each ONU with a downstream PON frame (SQ 21  and SQ 22 ) of a bandwidth (U-BWs) to be allocated normally. Consequently, the ONU  30 -k for which the transmission bandwidth has been suppressed comes to be able to transmit upstream frames UFi+2 in the ordinary allocated bandwidth (SQ 23 ). 
       FIG. 14  is a graph for describing a relationship between bandwidth controlling by the OLT  10  and a temporal change of the amount of the accumulated data (transmission queue length) in an output frame buffer  518  observed by an output frame buffer  519  of the GW  50 . TH 1  indicates the first threshold value for detecting congestion occurrence and TH 2  indicates the second threshold value for detecting congestion reset. ΔTD indicates a period of bandwidth calculation by the OLT  10 . 
     The GW management part  53  reads a transmission queue length QL from the output frame buffer  519  of the each output port (output line interface part)  51 B of the GW  50  in a predetermined cycle, for example, in the basic frame cycle ΔTF and instructs the back pressure control frame generation part  521  to issue a congestion occurrence notification frame when the queue length QL exceeds the first threshold value TH 1  and instructs the back pressure control frame generation part  521  to issue a congestion reset notification frame when the congestion occurred output port queue length QL falls below the second threshold value TH 2 . Consequently, the GW  50  assumes a period between congestion occurrence and congestion reset as a congestion period. 
     The first threshold value TH 1  is set at a value smaller than the capacity of the output frame buffer  518 . Thus the congestion occurrence notification frame is issued before the output frame buffer becomes full. And the capacity of the output frame buffer  518  remains still enough to absorb an increase of the queue length until each OLT  10  suppresses the bandwidth in response to a congestion occurrence notice. Receiving a congestion occurrence notice from the GW  50 , the OLT control part  11  controls the bandwidth allocated to each ONU in the bandwidth suppression mode consisting of the above described transient periods T 1  and T 3 , as well as the optimal control period T 3 . 
       FIG. 15  is an explanatory diagram for describing the bandwidth suppression mode periods  151  and  152  in an OLT  10  and the temporal changes of both transmission queue length  153  and allocated bandwidth  154  in an ONU that is transmitting frames that have passed a congestion occurred GW port (hereunder, to be referred to as a congestion path ONU). 
       FIG. 16  is an explanatory graph for describing the bandwidth suppression mode periods  151  and  152  in an OLT  10  and the temporal changes of both transmission queue length  153  and allocated bandwidth  155  in an ONU that is transmitting frames that have passed a normal GW port (hereunder, to be referred to as a normal path ONU). 
     In order to make it easier to understand the effect of the bandwidth controlling according to the present invention, assume here that the transmission queue length is changed in the same way in both normal path ONU and congestion path ONU and only the change of an allocated bandwidth is checked between those ONUs. The bandwidth controlling in the OLT control part  11  is affected on each ONU just with a delay of the basic frame cycle ΔTF. In any period other than the periods  151  and  152  in the bandwidth suppression mode, an allocated bandwidth BW(j) is notified to each ONU according to the transmission queue length  153  calculated as a bandwidth in the normal mode. 
     A period between when the OLT control part  11  enters a bandwidth suppression mode period  151  between times t 1  and t 2  and when the next bandwidth is allocated becomes a transient period T 1 . If the next allocated bandwidth is notified to a congestion path ONU in the transient period T 1 , the congestion path ONU bandwidth is suppressed to the congestion time allowable bandwidth Bcon within the transient period T 1  as shown in  FIG. 15 . If the OLT control part  11  allocates a bandwidth just before the time t 2 , the bandwidth suppression mode enters the optimal control period T 2 . In this embodiment, the bandwidth allocated to a congestion path ONU is kept at the congestion time allowable bandwidth Bcon and this state is continued even after the bandwidth suppression mode goes into the transient period T 3  after the GW output port goes out of the congestion state. The congestion path ONU bandwidth returns to a bandwidth allocated according to the transmission queue length SQ when the bandwidth controlling of the OLT  10  goes into the normal mode. 
     According to the present invention, because only the bandwidth allocated to a congestion path ONU is deleted when the OLT  10  enters the bandwidth suppression mode, each normal path ONU is allocated a bandwidth as usually in a transient period T 1  of the bandwidth suppression mode as shown in  FIG. 16 . When the bandwidth suppression mode of the OLT control part  11  enters the optimal control period T 2 , a surplus bandwidth generated in the PON section due to the deletion of the congestion path ONU is distributed to each of plural normal path ONUs according to the transmission queue length. Consequently, as shown in  FIG. 16 , in the optimal control period T 2 , the bandwidth to be allocated to each normal path ONU increases. However, because a bandwidth for each normal path ONU is allocated just like in the normal mode according to the transmission queue length, the allocated bandwidth is reduced in proportion to the reduction of the transmission queue length SQ as shown at the time t 8 . 
     In the embodiment described above, the OLT control part  11  keeps the congestion path ONU bandwidth at the congestion time allowable bandwidth Bcon in the bandwidth suppression mode period. However, it is also possible to allocate a bandwidth to the congestion path ONU according to the source queue length within the congestion time allowable bandwidth. In the same way, it is also possible to allocate a bandwidth to each normal path ONU according to the source queue length within the maximum control bandwidth  123  indicated in the bandwidth control table  120 . 
     In this embodiment, the congestion time allowable bandwidth of each ONU is given as a fixed value. However, the allowable bandwidth may be a dynamic value obtained by multiplying the value of a bandwidth allocated to each ONU before the congestion occurs by a predetermined reduction rate. 
     Although a back pressure control frame generation part  521  is provided for each output line interface part  51 B of the GW  50  and the back pressure control frame generation part  521  generates both congestion occurrence notification frame and congestion reset notification frame according to the commands from the GW management part  53  in the above embodiment, the GW management part  53  may generate the back pressure control frames (congestion occurrence notification frame and congestion reset notification frame). 
     In such a case, a back pressure control frame buffer is provided for each output line interface part  51 B instead of the back pressure control frame generation part  521  and for example, an OLT connection port number, a connection port MAC address, an OLT MAC address corresponding to a GW output port number respectively are recorded in the management information table  54  beforehand. The GW management part  53  thus refers to the management information table  54  when detecting congestion occurrence in an output port Pi to generate a back pressure control frame and output the control frame to the back pressure control frame buffer of the output line interface part  51 B corresponding to the OLT connection port number. The output frame reading part  520  is allowed to read and transmit frames according to the priority given to the back pressure control frame buffer over the output frame buffer  518 . 
     Second Embodiment 
       FIG. 17  is another embodiment of the network to which the present invention applies. 
     In this embodiment, GWs  500 - 1  and  500 - 2  of a carrier network are provided with a line interface (PON interface) having OLT functions respectively and houses PON optical fibers  106  ( 106 - 1 - 1  to  106 - 2 -n) directly. 
       FIG. 18  is a configuration of the GW  500 - 1 . 
     The GW  500 - 1  consists of plural network interface parts  510  ( 510 - 1  to  510 -q), a frame relay part  52  for connecting those interface parts  510  to each other, a GW management part  53 , and a management information table  54 . The network interface part  51 - 1  for housing the PON optical fiber  106  is provided with a PON interface (IF)  5110  instead of the Ethernet IF  511  shown in  FIG. 8 . The network interface part for housing the carrier network lines  101  ( 101 - 1  and  101 - 2 ) is provided with an Ethernet IF  511  shown in  FIG. 8 . 
     The PON interface (IF)  5110  is the same as that of the OLT  10  shown in  FIG. 4  except for that the Ethernet IF  14 , the upstream frame transmission part  18 , and the frame analysis part  19  are excluded from the configuration. And the OLT control part  11  is connected to the GW management part  53  in this embodiment. 
     According to this embodiment, because the OLT control part  11  can receive both congestion occurrence notice and congestion reset notice from the GW management part  53  directly, it is possible to omit the management of the destination MAC address required for generating a back pressure control frame shown in  FIG. 3 . Furthermore, according to this embodiment, because the OLT control part  11  can quickly reduce a bandwidth allocated to a congestion path ONU when congestion occurs in an output port in a GW, it is possible to quicken avoiding of congestion with a back pressure for the congestion path ONU, thereby the GW output frame buffer memory can be reduced in capacity. 
     Although an example of application to the G-PON has been described in the first and second embodiments, the bandwidth controlling of the present invention may also apply to other types of PON, for example, to the GE-PON.