Abstract:
In a protection switching method for a passive optical network system, a communication abnormality is detected in at least one active-system virtual path established between an optical line terminal and a subscriber terminal through a transmission path and a network unit. When a communication abnormality is detected in an active-system virtual path, a switch is controlled to switch the transmission paths to establish a standby-system virtual path between the optical line terminal and the subscriber terminal as a communication partner. A protection switching apparatus for a passive optical network system is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a protection switching method and apparatus for a PON (Passive Optical Network) system in which a plurality of ONUs (Optical Network Units) are star-connected to an OLT (Optical Line Terminal) through a photocoupler. 
     FIG. 24  shows the basic arrangement of a PON system. 
   As shown in  FIG. 24 , in an OLT  106 , a transmission/reception section  101  is connected to a port of a switch (SW)  104 , and the transmission/reception section  101  is connected to transmission/reception sections  103 - 1  to  103 - n  of a plurality of ONUs  107 - 1  to  107 - n  through a photocoupler  102  and optical fibers  112 - 1  to  112 - n . The ONUs  107 - 1  to  107 - n  are star-connected to the single OLT  106 . 
   The transmission/reception sections  103 - 1  to  103 - n  of the ONUs  107 - 1  to  107 - n  are respectively connected to subscriber terminals  109 - 1  to  109 - n . When the transmission/reception section  101  of the OLT  106  is to communicate with one of the subscriber terminals  109 - 1  to  109 - n , a control section  110  of the OLT  106  controls switching operation of the switch  104 . With this operation, identical signals are distributed from the photocoupler  102  to the ONUs  107 - 1  to  107 - n  through the optical fibers  112 - 1  to  112 - n , and one of the ONUs  107 - 1  to  107 - n  extracts the signal upon determining that the signal is self-addressed. 
   Assume that the ONU  107 - 1  determines that the signal is self-addressed. In this case, when a virtual path is established between the subscriber terminal  109 - 1  and the transmission/reception section  101  of the OLT  106 , the transmission/reception section  101  of the OLT  106  can communicate with the subscriber terminal  109 - 1 . That is, the transmission/reception section  101  of the OLT  106  can communicate with the subscriber terminal  109 - 1  through the photocoupler  102 , optical fibers  112 - 1  to  112 - n , and the transmission/reception section  103 - 1  of the ONU  107 - 1 . 
     FIG. 25  shows a dual arrangement designed to ensure the reliability of a PON system having such an arrangement. In the dual arrangement shown in  FIG. 25 , an OLT  106  has sections of two systems, i.e., a 0-system transmission/reception section  101   a  and 1-system transmission/reception section  101   b . Likewise, ONUs  107 - 1  to  107 - n  respectively have sections of two systems, i.e., 0-system transmission/reception sections  103 - 1   a  to  103 - na  and 1-system transmission/reception sections  103 - 1   b  to  103 - nb . In order to implement two systems in this manner, the OLT  106  has a selector (SEL)  105  in addition to a switch  104  and control section  110 . 
   The SEL  105  selectively switches between the 0-system transmission/reception section  101   a  and 1-system transmission/reception section  101   b  to connect the 0-system transmission/reception section  101   a  to the 0-system transmission/reception sections  103 - 1   a  to  103 - na  of the ONUs  107 - 1  to  107 - n  through a photocoupler  102   a  and optical fibers  112 - 1   a  to  112 - na . In addition, the 1-system transmission/reception section  101   b  of the OLT  106  is connected to the 1-system transmission/reception sections  103 - 1   b  to  103 - nb  of the ONUs  107 - 1  to  107 - n  through optical fibers  112 - 1   b  to  112 - nb.    
   The 0-system transmission/reception sections  103 - 1   a  to  103 - na  and 1-system transmission/reception sections  103 - 1   b  to  103 - nb  of the ONUs  107 - 1  to  107 - n  are respectively selected by SELs  108 - 1  to  108 - n  of the ONUs  107 - 1  to  107 - n  in accordance with selection of 0-system or 1-system by the SEL  105  of the OLT  106 . The selected 0-system transmission/reception sections  103 - 1   a  to  103 - na  or 1-system transmission/reception sections  103 - 1   b  to  103 - nb  are respectively connected to subscriber terminals  109 - 1  to  109 - n.    
   Assume that the PON system is operating with the 0-system transmission/reception section  101   a  and 0-system transmission/reception sections  103 - 1   a  to  103 - na  belonging to an active system, and the 1-system transmission/reception section  101   b  and 1-system transmission/reception sections  103 - 1   b  to  103 - nb  belonging to a standby system. 
   Note that the terms “0-system” and “1-system” are added to physically identify the respective sections. However, the 0-system transmission/reception section  101   a  and 0-system transmission/reception sections  103 - 1   a  to  103 - na  do not always belong to the active system, and the 1-system transmission/reception section  101   b  and 1-system transmission/reception sections  103 - 1   b  to  103 - nb  do not always belong to the standby system. That is, the 0-system and 1-system are irrelevant to the active and standby systems. The active system is a currently used system, and the standby system is a system that is used upon switching from the active system. 
   Assume that the 0-system is an active system in the following description. Referring to  FIG. 25 , the 0-system transmission/reception section  101   a  of the OLT  106  is now capable of communicating with the subscriber terminals  109 - 1  to  109 - n  through the photocoupler  102   a , the 0-system transmission/reception sections  103 - 1   a  to  103 - na  and SELs  108 - 1  to  108 - n  of the ONUs  107 - 1  to  107 - n . Assume that a virtual path is established between the subscriber terminal  109 - 1  and the 0-system transmission/reception section  101   a  of the OLT  106  through the photocoupler  102   a  and transmission/reception section  103 - 1   a  and SEL  108 - 1  of the ONU  107 - 1 , and the 0-system transmission/reception section  101   a  is now communicating with the subscriber terminal  109 - 1 . 
   When an abnormality occurs in the virtual path between the subscriber terminal  109 - 1  and the 0-system transmission/reception section  101   a  of the OLT  106  during this communication owing to some cause, no data is transmitted from the subscriber terminal  109 - 1  to the 0-system transmission/reception section  101   a  of the OLT  106 . As a result, the 0-system transmission/reception section  101   a  detects the occurrence of the abnormality in the virtual path, and sends a warning signal to the control section  110 . 
   Upon reception of the warning signal, the control section  110  outputs a switching instruction to the SEL  105  to switch from the 0-system transmission/reception section  101   a  to the 1-system transmission/reception section  101   b . With this operation, all the virtual paths between the OLT  106  and subscriber terminals  109 - 1  to  109 - n  are switched to the 1-system at once. That is, virtual paths are established between the 1-system transmission/reception section  101   b  of the OLT  106  and the subscriber terminals  109 - 1  to  109 - n  through the photocoupler  102   b , optical fibers  112 - 1   b  to  112 - nb , and the 1-system transmission/reception sections  103 - 1   b  to  103 - nb  and SELs  108 - 1  to  108 - n  of the ONUs  107 - 1  to  107 - n.    
   With this operation, the communication between the OLT  106  and the subscriber terminal  109 - 1 , which has been interrupted due to the occurrence of the abnormality, is resumed upon switching to the virtual path constituted by the 1-system transmission/reception section  101   b , photocoupler  102   b , optical fiber  112 - 1   b , and 1-system transmission/reception section  103 - 1   b  and SEL  108 - 1  of the ONU  107 - 1 . 
     FIG. 26  shows another example of the dual arrangement of a conventional OPN system. 
   In the arrangement shown in  FIG. 26 , control sections  111 - 1  to  111 - n  are added to the ONUs  107 - 1  to  107 - n  in  FIG. 25 . These control sections  111 - 1  to  111 - n  control SELs  108 - 1  to  108 - n  to switch (select) between 0-system transmission/reception sections  103 - 1   a  to  103 - na  and 1-system transmission/reception sections  103 - 1   b  to  103 - nb . Since the arrangement of the remaining portion is the same as that in  FIG. 25 , the same reference numerals as in  FIG. 25  denote the same parts in  FIG. 26 , and a description thereof will be omitted. 
   Assume that as in the case shown in  FIG. 25 , a fault has occurred in one of the following components of the 0-system: a transmission/reception section  101   a , photocoupler  102   a , optical fibers  112 - 1   a  to  112 - na , and the transmission/reception sections  103 - 1   a  to  103 - na  of ONUs  107 - 1  to  107 - n , while the PON system is operating with the 0-system serving as an active system, and the 1-system serving as a standby system. 
   The 0-system transmission/reception section  101   a  always monitors signals between the OLT  106  and ONUs  107 - 1  to  107 - n , and notifies a control section  110  of the OLT  106  of an abnormality upon detecting a signal abnormality. Upon reception of the abnormality notification, the control section  110  outputs a switching command to an SEL  105  of the OLT  106  to switch the transmission path from the 0-system to the 1-system. As a consequence, connection between the OLT  106  and the ONUs  107 - 1  to  107 - n  is restored by using the 1-system optical transmission path. 
   Upon outputting the switching command to the SEL  105 , the control section  110  outputs switching commands to the ONUs  107 - 1  to  107 - n  through the 1-system connection, i.e., the SEL  105 - 1 , 1-system transmission/reception section  101   b , photocoupler  102   b , and the 1-system transmission/reception sections  103 - 1   b  to  103 - nb  of the ONUs  107 - 1  to  107 - n . With this operation, the control sections  111 - 1  to  111 - n  of the ONUs  107 - 1  to  107 - n  control switching operation of the SELs  108 - 1  to  108 - n  to restore the transmission paths to subscriber terminals  109 - 1  to  109 - n.    
   In each of the dual arrangements of the conventional PON systems shown in  FIGS. 25 and 26 , however, even if a fault occurs in only the transmission/reception  103 - 1   a  of the 0-system ONU  107 - 1 , which is part of the PON system, the overall PON system must be simultaneously switched from the 0-system to the 1-system in order to restore a path under communication. That is, switching is performed even for the ONUs  107 - 2  to  107 - n  that are operating normally. As a result, the communication quality deteriorates due to short breaks and the like caused in this operation. 
   In each of the arrangements of the conventional PON systems shown in  FIGS. 25 and 26 , the active and standby systems are physically discriminated from each other, and the standby system cannot be used until it is selected by the SEL  105 . In addition, the active system is switched to the standby system by only interchanging the physical transmission paths, and only the same connection as the preceding connection is restored. 
   A star type light subscriber transmission device using a star coupler is disclosed in Japanese Patent Laid-Open No. 05-153053 (reference 1). According to reference 1, a fault detection circuit and fault detection signal generation circuit are connected to the terminal on an N branching side of the star coupler having a branching of 2: N via an optical directional coupler. A first-station-side light subscriber transmission device is connected to one terminal of the branching side of the star coupler, and a fault detection signal extraction circuit and a second-station-side light subscriber transmission device are connected to the other terminal via an optical branching device. The first- and second-station-side light subscriber transmission devices are switched and controlled by a selection circuit which received the output signal from the fault detection signal extraction circuit. 
   A dual changeover system using a star coupler is disclosed in Japanese Patent Laid-Open No. 10-294753 (reference 2). According to reference 2, a phase difference calculation means calculates a reception phase difference between an active-system transmission/reception section and a standby-system transmission/reception section of a subscriber-side device while the reception states of the active-system transmission/reception section and standby-system transmission/reception section are normal. A pointer control means then calculates a standby-system transmission phase by using the calculated reception phase difference and the active-system transmission phase. 
   In references 1 and 2, there is no description about switching of only a virtual path having undergone a fault to the standby system, and there is provided no solution to the above problem associated with switching of normal virtual paths. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a protection switching method and apparatus for a PON system, which can easily switch only a path to be restored to a standby-system path without affecting communication through normal virtual paths in the PON system. 
   In order to achieve the above object, according to the present invention, there is provided a protection switching method for a passive optical network system including an optical line terminal for switching between a first active-system transmission/reception section and a first standby-system transmission/reception section by using a switch, a plurality of network units for selectively connecting second active-system transmission/reception sections and second standby-system transmission/reception sections to subscriber terminals upon switching the sections through selectors in the event of a communication abnormality, and transmission paths for star-connecting the second active-system transmission/reception sections to the first active-system transmission/reception section, and also star-connecting the second standby-system transmission/reception sections to the first standby-system transmission/reception section, comprising the steps of detecting a communication abnormality in at least one active-system virtual path established between the optical line terminal and the subscriber terminal through the transmission path and the network unit, and upon detection of a communication abnormality in the active-system virtual path, causing the switch to switch the transmission paths to establish a standby-system virtual path between the optical line terminal and the subscriber terminal serving as a communication partner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is block diagram showing a protection switching apparatus for a PON system according to the first embodiment of the present invention; 
       FIG. 2  is a block diagram showing virtual paths established between an OLT and subscriber terminals in the protection switching apparatus in  FIG. 1 ; 
       FIG. 3  is a block diagram showing a normal established state of virtual paths between the OLT and a plurality of subscriber terminals in the protection switching apparatus in  FIG. 1 ; 
       FIG. 4  is a block diagram for explaining the flows of ATM signals in the normal established state in  FIG. 3 ; 
       FIG. 5  is a block diagram showing a state wherein an abnormality has occurred in a virtual path (VP 1 ) in the normal established state in  FIG. 3 ; 
       FIG. 6  is a block diagram showing a state wherein a new virtual path (VP 2   a ) is established from the abnormal state in  FIG. 5 ; 
       FIG. 7  is a block diagram showing the flows of ATM signals in the established state of the new virtual path (VP 2   a ) in  FIG. 6 ; 
       FIG. 8  is a block diagram showing the operation of a selector of an ONU at the time of switching to the new virtual path (VP 2   a ) in  FIG. 6 ; 
       FIG. 9  is a block diagram showing a protection switching apparatus for a PON system according to the second embodiment of the present invention; 
       FIG. 10  is a block diagram showing virtual paths established between an OLT and subscriber terminals in the protection switching apparatus in  FIG. 9 ; 
       FIG. 11  is a block diagram showing the first virtual path setting example in the protection switching apparatus in  FIG. 10 ; 
       FIG. 12  is a block diagram showing the second virtual path setting example in the protection switching apparatus in  FIG. 10 ; 
       FIG. 13  is a block diagram showing the third virtual path setting example in the protection switching apparatus in  FIG. 10 ; 
       FIG. 14  is a block diagram showing the fourth virtual path setting example in the protection switching apparatus in  FIG. 10 ; 
       FIG. 15  is a block diagram showing a protection switching apparatus for a PON system according to the third embodiment of the present invention; 
       FIG. 16  is a block diagram showing a protection switching apparatus for a PON system according to the fourth embodiment of the present invention; 
       FIG. 17  is a block diagram showing virtual paths established between an OLT and subscriber terminals in the protection switching apparatus in  FIG. 16 ; 
       FIG. 18  is a block diagram showing a normal established state of virtual paths between the OLT and a plurality of subscriber terminals in the protection switching apparatus in  FIG. 16 ; 
       FIG. 19  is a block diagram showing the flows of ATM signals in the normal established state in  FIG. 16 ; 
       FIG. 20  is a block diagram showing a state wherein an abnormality has occurred in a virtual path (VP 1 ) in the normal established state in  FIG. 16 ; 
       FIG. 21  is a block diagram showing a state wherein a new virtual path (VP 2   a ) is established from the abnormal state in  FIG. 20 ; 
       FIG. 22  is a block diagram showing the flows of ATM signals in the state wherein the new virtual path (VP 2   a ) is established in  FIG. 21 ; 
       FIG. 23  is a block diagram showing the operation of a selector of each ONU at the time of switching to the new virtual path (VP 2   a ) in  FIG. 21 ; 
       FIG. 24  is a block diagram showing the basic arrangement of a PON system; 
       FIG. 25  is a block diagram showing the first example of a conventional PON system having a dual arrangement; and 
       FIG. 26  is a block diagram showing the second example of a conventional PON system having a dual arrangement. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described in detail below with reference to the accompanying drawings. 
     FIG. 1  shows a protection switching apparatus for a PON system according to the first embodiment of the present invention. Referring to  FIG. 1 , an OLT  6  incorporates a switch  4 , 0-system transmission/reception section  1   a , and 1-system transmission/reception section  1   b.    
   The 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  detect the communication states of virtual paths established between the OLT  6  and subscriber terminals  91  to  9   n  connected to ONUs (Optical Network Units)  71  to  7   n , and send warning signals to a control section  10 . The 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  are connected to different ports of the switch  4 . The switch  4  switches virtual paths to be established under the control of the control section  10 . The switch  4  and control section  10  constitute a virtual path establishment switching means. 
   Assume that virtual paths constitute an ATM (Asynchronous Transfer Mode) PON in this embodiment. Note that a virtual path is a bundle of virtual channels, and switching of connections using virtual channels can be executed in the same manner as described above. 
   The 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  are respectively connected to photocouplers  2   a  and  2   b . The photocoupler  2   a  is connected to 0-system transmission/receptions  31   a  to  3   na  of the ONUs  71  to  7   n  through optical fibers  11   a  to  1   na . That is, the ONUs  71  to  7   n  are star-connected to the photocoupler  2   a  (the 0-system transmission/reception section  1   a  of the OLT  6 ). Likewise, the photocoupler  2   b  is connected to 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n  through optical fibers  21   b  to  2   nb . That is, the ONUs  71  to  7   n  are star-connected to the photocoupler  2   b  (1-system transmission/reception section  1   b  of the OLT  6 ). 
   The ONUs  71  to  7   n  respectively include SELs  81  to  8   n  for making connection to the subscriber terminals  91  to  9   n  by switching from the 0-system transmission/receptions  31   a  to  3   na  to the 1-system transmission/receptions  31   b  to  3   nb  (or switching in the reverse direction) in accordance with switching request signals transmitted from the OLT  6 . Switching request signals from the OLT  6  are sent from the control section  10  to the ONUs  71  to  7   n  through the optical fibers  11   a  to  1   na  or optical fibers  21   b  to  2   nb  at the time of virtual path switching control by the switch  4 . 
   The operation of the protection switching apparatus having the above arrangement will be described next, assuming that the 0-system and 1-system respectively serve as an active system and standby system.  FIG. 2  shows a case wherein a virtual path is established to allow communication between the subscriber terminal  91  and the OLT  6 . 
   Referring to  FIG. 2 , when the 0-system transmission/reception section  1   a  of the OLT  6  is connected to the 0-system transmission/reception section  31   a  of the ONU  71  through the photocoupler  2   a  and optical fiber  11   a , a virtual path VP 1   a  is established between the subscriber terminal  91  and the 0-system transmission/reception section  1   a  of the OLT  6  in accordance with the setting of the switch  4 . When the 1-system transmission/reception section  1   b  of the OLT  6  is connected to the 1-system transmission/reception section  1   b  of the ONU  71  through the photocoupler  2   b  and optical fiber  21   b , a virtual path VP 1   b  is established between the subscriber terminal  91  and the 1-system transmission/reception section  1   b  of the OLT  6  in accordance with the setting of the switch  4 . 
   In the case shown in  FIG. 2 , the subscriber terminal  91  can communicate with either the 0-system transmission/reception section  1   a  or the 1-system transmission/reception section  1   b  of the OLT  6 . 
   Switching operation from an active-system virtual path under normal communication operation to a standby-system virtual path will be described next with reference to  FIG. 3 . 
   Referring to  FIG. 3 , the bold lines indicate established virtual paths VP 1  and VP 2 . The virtual path VP 1  is established by the following route: switch  4 —0-system transmission/reception section  1   a  of the OLT  6 —photocoupler  2   a —optical fiber  11   a —0-system transmission/reception section  31   a  of the ONU  71 —SEL  81 —subscriber terminal  91 . Communication is performed between the subscriber terminal  91  and the 0-system transmission/reception section  1   a  of the OLT  6  through the virtual path VP 1 . 
   The virtual path VP 2  is established by the following route: switch  4 —0-system transmission/reception section  1   a  of the OLT  6 —photocoupler  2   a —optical fiber  12   a —0-system transmission/reception section  32   a  of the ONU  72 —SEL  82 —subscriber terminal  92 . Communication is performed between the 0-system transmission/reception section  1   a  and the subscriber terminal  92  through the virtual path VP 2 . 
   Referring to  FIG. 3 , therefore, the virtual path VP 1  takes charge of communication between the OLT  6  and the subscriber terminal  91 , and the virtual path VP 2  takes charge of communication between the OLT  6  and the subscriber terminal  92 . 
   In such a normal communication state, the switch  4  operates in the manner shown in  FIG. 4 . Referring to  FIG. 4 , ATM cell #1 to be transmitted to the ONU  71  is transmitted to the 0-system transmission/reception section  31   a  of the ONU  71  through the switch  4 , 0-system transmission/reception section  1   a , photocoupler  2   a , and virtual path VP 1 . ATM cell #2 to be transmitted to the ONU  72  is transmitted to the 0-system transmission/reception section  32   a  of the ONU  72  through the switch  4 , 0-system transmission/reception section  1   a , photocoupler  2   a , and virtual path VP 2 . 
   In this case, both ATM cells #1 and #2 pass through the 0-system transmission/reception section  1   a  of the OLT  6 . In accordance with the values of headers H 1  and H 2  of ATM cells #1 and #2, the switch  4  outputs ATM cells #1 and #2 to the corresponding ports. 
   When an abnormality (e.g., disconnection) occurs in the virtual path VP 1  between the 0-system transmission/reception section  31   a  of the ONU  71  and the photocoupler  2   a  as indicated by an “X” in  FIG. 5  in the above normal communication state, only a signal from the ONU  71  does not arrive at the 0-system transmission/reception section  1   a  of the OLT  6 . The 0-system transmission/reception section  1   a  of the OLT  6  therefore detects an abnormality in the communication state of the virtual path VP 1 , and sends a warning signal to the control section  10 . 
   Upon reception of the warning signal, the control section  10  controls the switch  4  to change the setting of the virtual path. With this operation, the switch  4  switches the virtual path VP 1  to the virtual path VP 1   a , as shown in  FIG. 6 . That is, the virtual path VP 1   a  is established by the following route: 1-system transmission/reception section  1   b  of the OLT  6 —photocoupler  2   b —optical fiber  21   b— 1-system transmission/reception section  31   b  of the ONU  71 . As a consequence, the communication between the OLT  6  and the ONU  71  is resumed. 
   At this time, the virtual path VP 2  is kept used for the communication between the OLT  6  and the ONU  72  without being affected. 
     FIG. 7  shows the flows of signals through the switch  4  at the time of switching from the virtual path VP 1  to the virtual path VP 1   a  in  FIG. 6 . 
   Referring to  FIG. 7 , in accordance with the setting of a virtual path with respect to the switch  4 , ATM cell #1 to be transmitted to the ONU  71  is transmitted to the virtual path VP 1   a  formed by the following route: switch  4 —1-system transmission/reception section  1   b  of the OLT  6 —photocoupler  2   b —optical fiber  21   b . ATM cell #2 to be transmitted to the ONU  72  is transmitted to the virtual path VP 2  formed by the following route: switch  4 —0-system transmission/reception section  1   a  of the OLT  6 —photocoupler  2   a —optical fiber  12   a– 1-system transmission/reception section  32   a  of the ONU  72 . 
   In this case, in accordance with the values of the headers H 1  and H 2  added to ATM cells #1 and #2, ATM cells #1 and #2 are distributed to the corresponding ports of the switch  4 . 
   The operation of the SEL  81  of the ONU  71  at the time of the above switching from the virtual path VP 1  to the virtual path VP 1   a  will be described next with reference to  FIG. 8 . 
   Referring to  FIG. 8 , the control section  10  transmits a switching request signal for the SEL  81  to the ONU  71  through the virtual path VP 1   a  established by the control operation of the switch  4 . Upon reception of the switching request signal from the OLT  6 , the ONU  71  selects and connects the SEL  81  through the 1-system transmission/reception section  31   b , thus connecting the subscriber terminal  91  to the 1-system transmission/reception section  31   b  through the SEL  81 . 
   With this operation, a signal from the 1-system transmission/reception section  31   b  of the ONU  71  is transmitted to the subscriber terminal  91  through the SEL  81 . As a result, the communication between the OLT  6  and the subscriber terminal  91  is resumed through the virtual path VP 1   a.    
     FIG. 9  shows a protection switching apparatus for a PON system according to the second embodiment of the present invention. The same reference numerals as in  FIG. 1  denote the same parts in  FIG. 9 . 
   The arrangement of an OLT  6  in  FIG. 9  is the same as that in  FIG. 1 . In the OLT  6 , a 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  are connected to different ports of a switch  4 . The 0-system transmission/reception section  1   a  of the OLT  6  is connected to a photocoupler  2   a  and connected to 0-system transmission/receptions  31   a  to  3   na  of ONUs  71  to  7   n  through the photocoupler  2   a  and optical fibers  11   a  to  11   na . The 1-system transmission/reception section  1   b  of the OLT  6  is connected to a photocoupler  2   b  and connected to 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n  through the photocoupler  2   b  and optical fibers  21   b  to  2   nb.    
   In this connection arrangement, the 0-system transmission/reception section  1   a  of the OLT  6  exchanges signals with the 0-system transmission/receptions  31   a  to  3   na  of the ONUs  71  to  7   n . The photocoupler  2   a  branches an optical signal from the 0-system transmission/reception section  1   a  into signals to the optical fibers  11   a  to  11   na  so as to transmit them to the 0-system transmission/receptions  31   a  to  3   na  in the ONUs  71  to  7   n . Likewise, the 1-system transmission/reception section  1   b  of the OLT  6  exchange signals with the 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n . The photocoupler  2   b  branches an optical signal from the 1-system transmission/reception section  1   b  into signals to the optical fibers  21   b  to  2   nb  so as to transmit them to the 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n.    
   The ONUs  71  to  7   n  respectively incorporate SELs  81  to  8   n  as in the arrangement shown in  FIG. 1 . The SELs  81  to  8   n  select signals from the 0-system transmission/receptions  31   a  to  3   na  or 1-system transmission/receptions  31   b  to  3   nb  and output them to subscriber terminals  91  to  9   n . The 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  of the OLT  6  have the function of always monitoring signals. With this function, each of the 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  detects an abnormality in a transmission path and notifies the control section  10  of the abnormality. 
   Upon reception of the abnormality from the 0-system transmission/reception section  1   a  or 1-system transmission/reception section  1   b , the control section  10  outputs a switching control signal to the switch  4 . Upon reception of the switching control signal from the control section  10 , the switch  4  discriminates header information, and outputs an input ATM cell to a corresponding port. 
   The ONUs  71  to  7   n  respectively incorporate controls sections  111  to  11   n . The 0-system transmission/receptions  31   a  to  3   na  and 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n  notify the controls sections  111  to  11   n  of switching request signals transmitted from the OLT  6  through transmission paths. Upon notification of the switching request signals, the controls sections  111  to  11   n  perform switching control on the SELs  81  to  8   n.    
   As shown in  FIG. 10 , therefore, when a virtual path is to be established to exchange signals with the subscriber terminal  91  through the ONU  71 , a route running through the 0-system transmission/reception section  1   a  of the OLT  6 , i.e., a virtual path VP 1   a , and a route running through the 1-system transmission/reception section  1   b  of the OLT  6 , i.e., virtual path VP 1   b , can be selectively used. 
   With this arrangement, therefore, when any one of the transmission elements constituting the active-system virtual path fails, the standby-system virtual path is selected by switching the virtual paths at the switch  4 , thereby resuming the communication with the target subscriber through a transmission path that is physically different from the faulty path. 
   Since the 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  are connected to different ports of the switch  4  in the OLT  6 , the two virtual paths VP 1   a  and VP 1   b  can be set to have different bands. In addition, since the resources of the two systems, i.e., the 0-system and 1-system, can be selectively used, one of the systems through which an active virtual path is to be established can be arbitrarily determined. By using these features, flexible network design like that described below can be realized. 
   In the first example of this case, a standby-system virtual path having a band narrower than that of an active-system virtual path is prepared through one route, and the remaining band is used for services. By setting a limitation that minimum services are ensured in the event of a fault in this manner, the total band allocated to the physical resources of the 0-system and 1-system is effectively distributed. This makes it possible to use a broader band for services than in a case wherein the resources of only one system are fully used. 
   In the second example, since faults do not always occur in all virtual paths in operation at once, there is no need to always ensure a band for setting standby-system virtual paths for all the active-system virtual paths. That is, a given band is ensured as a common standby band for all virtual paths in operation, and is used for only a virtual path having undergone a fault to set a standby-system virtual path. 
   This makes it possible to broaden the band that can be used for services as compared with a case wherein a band is ensured for standby-system virtual paths in a one-to-one correspondence with all active-system virtual paths. If a band broader than the common standby-system band is required in case faults have occurred in a plurality of virtual paths at once, the band for virtual paths in normal operation is limited to ensure a necessary band for virtual paths. 
   In the third example, no band for setting standby-system virtual paths is ensured during normal operation, and a maximum band allowed for physical resources is used for services. A band for other normal virtual paths is limited in the even of a fault to ensure a band necessary for standby-system virtual paths, thus establishing standby-system virtual paths. 
   In the fourth example, a single system accommodates subscribers who require no dual protection and subscribers who requires dual protection. 
   The operation of the second embodiment having this arrangement will be described next. For the sake of simplicity, assume that the number of ONUs is four, i.e., n=4. 
     FIG. 11  shows an example of how a first virtual path is set.  FIG. 11  shows a state wherein the virtual path VP 1   a  is established for the ONU  71  by using the 0-system, and a virtual path VP 2   a  is established for the ONU  72 . In this state, the OLT  6  exchanges signals with the subscriber terminals  91  and  92  through the virtual paths VP 1   a  and VP 2   a . At this time, the SELs  81  and  82  of the ONUs  71  and  72  respectively select the 0-system transmission/reception sections  31   a  and  32   a.    
   In addition, virtual paths VP 3   a  and VP 4   a  are respectively established for the ONUs  73  and  74  by using the 1-system. In this state, the OLT  6  exchanges signals with the subscriber terminals  93  and  94  through the virtual paths VP 3   a  and VP 4   a . At this time, the SELs  83  and  84  of the ONUs  73  and  74  respectively select 1-system transmission/receptions  33   b  and  34   b.    
   On the other hand, standby-system virtual paths VP 3   b  and VP 4   b  corresponding to the virtual paths VP 3   a  and VP 4   a  are set by using the 0-system. In addition, standby-system virtual paths VP 1   b  and VP 2   b  corresponding to the virtual paths VP 1   a  and VP 2   a  are set by using 1-system. 
   In a normal case, communication is performed by using the virtual paths VP 1   a , VP 2   a , VP 3   a , and VP 4   a . If faults occur in the respective virtual paths VP 1   a , VP 2   a , VP 3   a , and VP 4   a , the control section  10  of the OLT  6  instructs the switch  4  to switch to the standby-system virtual paths VP 1   b , VP 2   b , VP 3   b , and VP 4   b  and reconnect them to the corresponding ONUs. 
   The control section  10  sends switching request signals for the SELs  81  and  82  to the 1-system transmission/reception sections  31   b  and  32   b  of the ONUs  71  and  72  by using the standby-system virtual paths VP 1   b  and VP 2   b , respectively. At the same time, the control section  10  sends switching request signals for the SELs  83  and  84  to the 0-system transmission/reception sections  33   a  and  34   a  of the ONUs  73  and  74  by using the standby-system virtual paths VP 3   b  and VP 4   b , respectively. 
   The transmission/reception sections  31   b ,  32   b ,  33   a , and  34   a  of the ONUs  71  to  74  receive the switching request signals from the OLT  6 , and notify the control sections  111  to  114  of the signals. Upon reception of these notifications, the respective control sections  111  to  114  switch the SELs  81  to  84  to resume the communication with target subscriber terminals. 
   Assume that each of the 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  of the OLT  6  has a band of “100” (a band of “100” per system), and the bands are assigned as follows:
         virtual path VP 1   a =40   virtual path VP 2   a =40   virtual path VP 3   a =40   virtual path VP 4   a =40   virtual path VP 1   b =10   virtual path VP 2   b =10   virtual path VP 3   b =10   virtual path VP 4   b =10       

   In this case, each of the subscriber terminals  91  to  94  can use only a band of “10” in the event of a fault, but can use a band of “40” in normal operation. That is, a band of “160 (=40×4)” can be used for services as a whole. Therefore, services can be provided beyond a band of “100” that is obtained by fully using one system in the conventional scheme, and protection against faults can be guaranteed. 
   The second virtual path setting example will be described next with reference to  FIG. 12 . As in the first virtual path setting example, the virtual paths VP 1   a  to VP 4   a  are respectively set for the subscriber terminals  91  to  94 . In this state, a virtual path VP 6  is set to switch the virtual path VP 1   a  or VP 2   a  that has become abnormal. In addition, a virtual path VP 5  is set to switch the virtual path VP 3   a  or VP 4   a  that has become abnormal. 
   Assume that each of the 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  of the OLT  6  has a band of “100”, and the respective bands are assigned as follows:
         virtual path VP 1   a =33   virtual path VP 2   a =33   virtual path VP 3   a =33   virtual path VP 4   a =33   virtual path VP 5 =33   virtual path VP 6 =33       

   In this case, in normal operation, the total band that can be used for services is “132 (=33×4)”. That is, services can be provided beyond a band of “100” that is obtained by fully using one system in the conventional scheme. 
   In this setting example, if one of the virtual paths VP 1   a  and VP 2   a  become abnormal, the virtual path can be restored by directly switching to the virtual path VP 6 . In addition, if both the virtual paths VP 1   a  and VP 2   a  become abnormal at once, the respective virtual paths are set again such that the subscriber terminals  91  to  94  share a band of “100” held by the 1-system transmission/reception section  1   b  of the OLT  6 . With this operation, protection against faults can be guaranteed. The same applies to switching operation for the virtual paths VP 3   a  and VP 4   a  in the event of faults. 
   The third virtual path setting example will be described next with reference to  FIG. 13 . As in the first virtual path setting example, the virtual paths VP 1   a  to VP 4   a  are set for the subscriber terminals  91  to  94 . 
   Assume that each of the 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  of the OLT  6  has a band of “100”, and the respective bands are assigned as follows:
         virtual path VP 1   a =50   virtual path VP 2   a =50   virtual path VP 3   a =50   virtual path VP 4   a =50       

   In this case, in normal operation, the total band that can be used for services is “200 (=50×4)”. That is, services can be provided beyond a band of “100” that is obtained by fully using one system in the conventional scheme. 
   In this setting example, no band is set in advance to reset a virtual path when it becomes abnormal. Every time an abnormality occurs in a virtual path, the band that has been used by normal virtual paths is limited, and the surplus band is used to restore the abnormal virtual path. This makes it possible to guarantee protection against faults. 
   The fourth virtual path setting example will be described next with reference to  FIG. 14 . As in the first virtual path setting example, the virtual paths VP 1   a  to VP 4   a  are set for the subscriber terminals  91  to  94 . Assume that there is no need to provide protection for only services given to the subscriber terminal  92 . Therefore, the ONU  72  requires only one transmission/reception section (1-system transmission/reception section  32   a  is used in  FIG. 14 ). 
   In this setting example, communication is performed by using the virtual paths VP 1   a , VP 2   a , VP 3   a , and VP 4   a  in normal operation. When faults occur in the virtual paths VP 1   a , VP 3   a , and VP 4   a , except for the virtual path VP 2   a , the control section  10  of the OLT  6  instructs the switch  4  to switch to the standby-system virtual paths VP 1   b , VP 3   b , and VP 4   b  so as to resume the communication with the target subscriber terminals. 
   In this case as well, active- and standby-system virtual paths can be set in arbitrary bands within the range of the maximum band held by the physical resources. 
   As described above, in the second embodiment, virtual paths are switched by the switch  4  without discriminating physically different systems between an active system and a standby system. Since this makes it possible to establish a standby-system path to a target ONU through physically different transmission paths, network design can be performed with a high degree of freedom. As a consequence, active-system virtual paths can be provided by efficiently using limited physical resources, and standby-system virtual paths can be flexibly prepared in accordance with services. 
     FIG. 15  shows a protection switching apparatus for a PON system according to the third embodiment of the present invention. 
   Referring to  FIG. 15 , in an OLT  6 , transmission/reception sections  11  to  13  are connected to different ports of a switch  4 . The transmission/reception section  11  of the OLT  6  is connected to transmission/reception sections  311   a  to  31   na  of ONUs  711  to  71   n  through a photocoupler  21 . The transmission/reception section  12  of the OLT  6  is connected to transmission/reception sections  321   a  to  32   na  of ONUs  721  to  72   n  through a photocoupler  22 . The transmission/reception section  13  is connected to transmission/receptions  311   b  to  31   nb  of ONUs  711  to  71   n  and transmission/reception sections  321   b  to  32   nb  of the ONUs  721  to  72   n  through a photocoupler  23 . 
   In normal operation, virtual paths are set for subscriber terminals  911  to  91   n  through the transmission/reception section  11  of the OLT  6 . In addition, virtual paths are set for subscriber terminals  921  to  92   n  through the transmission/reception section  12  of the OLT  6 . It is taken for granted that these virtual paths rarely become abnormal at once. For this reason, every time an abnormality occurs in an active-system virtual path, a standby-system virtual path is set again by using the band held by the transmission/reception section  13 . That is, the transmission/reception sections  11  and  12  serve as 0-system transmission/reception sections, whereas the transmission/reception section  13  serves as a 1-system transmission/reception section. 
   With this arrangement, the single transmission/reception section  13  can provide protection for the ONUs  711  to  771   n  and  721  to  72   n  connected to the transmission/reception sections  11  and  12  in the OLT  6 . Note that the number of transmission/reception sections in the OLT  6  may be four or more, and the number of ONU groups may be three or more. 
     FIG. 16  shows a protection switching apparatus for a PON system according to the fourth embodiment of the present invention. 
   The fourth embodiment is a modification to the first embodiment. More specifically, control sections  111  to  11   n  are additionally arranged in the ONUs  71  to  7   n  in  FIG. 1 , and the control sections  111  to  11   n  perform selective control on SELs  81  to  8   n  to switch 0-system transmission/receptions  31   a  to  3   na  and 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n . The arrangement of the remaining portion is the same as that in  FIG. 1 . The same reference numerals as in  FIG. 1  denote the same parts in  FIG. 16 , and a description thereof will be omitted. 
   The operation of the protection switching apparatus having the above arrangement will be described below. 
   Referring to  FIG. 17 , when a 0-system transmission/reception section  1   a  of a OLT  6  is connected to the 0-system transmission/reception section  31   a  of the ONU  71  through a photocoupler  2   a  and optical fiber  11   a , a virtual path VP 1   a  is established between a subscriber terminal  91  and the 0-system transmission/reception section  1   a  of the OLT  6  in accordance with the setting of a switch  4 . When a 1-system transmission/reception section  1   b  of the OLT  6  is connected to the 1-system transmission/reception section  31   b  of the ONU  71  through a photocoupler  2   b  and optical fiber  21   b , a virtual path VP 1   b  is established between the subscriber terminal  91  and the 1-system transmission/reception section  1   b  of the OLT  6  in accordance with the setting of the switch  4 . 
   In the case shown in  FIG. 17 , the subscriber terminal  91  can communicate with both the 0-system transmission/reception section  1   a  and 1-system transmission/reception section  1   b  of the OLT  6 . 
   Switching operation from an active-system virtual path under normal communication operation to a standby-system virtual path will be described next with reference to  FIG. 18 . 
   Referring to  FIG. 18 , the bold lines indicate established virtual paths VP 1  and VP 2 . The virtual path VP 1  is established through the following route: switch  4 —0-system transmission/reception section  1   a  of the OLT  6 —photocoupler  2   a —optical fiber  11   a —0-system transmission/reception section  31   a  of the ONU  71 —SEL  81 —subscriber terminal  91 . Communication is performed between the 0-system transmission/reception section  1   a  and the subscriber terminal  91  through the virtual path VP 1 . The virtual path VP 2  is established through the following route: switch  4 —0-system transmission/reception section  1   a  of the OLT  6 —photocoupler  2   a —optical fiber  12   a —0-system transmission/reception section  32   a  in an ONU  72 —SEL  82 —subscriber terminal  92 . Communication is performed between the 0-system transmission/reception section  1   a  and the subscriber terminal  92  through the virtual path VP 2 . 
   In the case shown in  FIG. 18 , therefore, the virtual path VP 1  takes charge of communication between the OLT  6  and the subscriber terminal  91 , and the virtual path VP 2  takes charge of communication between the OLT  6  and subscriber terminal  92 . 
   In a normal communication state shown in  FIG. 18 , the switch  4  operates in the manner shown in  FIG. 19 . Like  FIG. 4 ,  FIG. 19  shows the flows of signals. More specifically, ATM cell #1 to be transmitted to the ONU  71  is transmitted to the 0-system transmission/reception section  31   a  of the ONU  71  through the switch  4 , 0-system transmission/reception section  1   a , photocoupler  2   a , and virtual path VP 1 . ATM cell #2 to be transmitted to the ONU  72  is transmitted to the 0-system transmission/reception section  32   a  of the ONU  72  through the switch  4 , 0-system transmission/reception section  1   a , photocoupler  2   a , and virtual path VP 2 . 
   In this case, both the ATM cells #1 and #2 pass through the 0-system transmission/reception section  1   a . In addition, the switch  4  has the function of outputting ATM cells #1 and #2 to corresponding ports of the switch  4  in accordance with the values of headers H 1  and H 2  of ATM cells #1 and #2. 
   Assume that in the normal communication state shown in  FIG. 18 , an abnormality (e.g., disconnection) occurs in the virtual path VP 1  between the photocoupler  2   a  and the 0-system transmission/reception section  31   a  of the ONU  71 , as indicated by “X” in  FIG. 20 . In this case, only a signal from the ONU  71  does not arrive at the 0-system transmission/reception section  1   a  in the OLT  6 . The 0-system transmission/reception section  1   a  of the OLT  6  detects the abnormality in the communication state of the virtual path VP 1 , and sends a warning signal to the control section  10  of the OLT  6 . 
   Upon reception of the warning signal from the 0-system transmission/reception section  1   a , the control section  10  controls the switch  4  to change the setting of virtual paths. As a consequence, the switch  4  switches the virtual path VP 1  to the virtual path VP 1   a , as shown in  FIG. 21 . That is, the virtual path VP 1   a  is established through the following route: 1-system transmission/reception section  1   b  of the OLT  6  photocoupler  2   b —optical fiber  21   b– 1-system transmission/reception section  31   b  of the ONU  71 . As a consequence, the communication between the OLT  6  and the ONU  71  can be resumed. 
   At this time, the virtual path VP 2  used for communication between the OLT  6  and the ONU  72  is kept used for the communication without being affected. 
     FIG. 22  shows the flows of signals through the switch  4  at the time of switching from the virtual path VP 1  to the virtual path VP 1   a.    
   Referring to  FIG. 22 , in accordance with the setting of a virtual path for the switch  4 , ATM cell #1 to be transmitted to the ONU  71  is transmitted to the virtual path VP 1   a  formed by the following route: switch  4 —1-system transmission/reception section  1   b  in the OLT  6 —photocoupler  2   b —optical fiber  21   b . ATM cell #2 to be transmitted to the ONU  72  is transmitted to the virtual path VP 2  formed by the following route: switch  4 —0-system transmission/reception section  1   a  of OLT  6 —photocoupler  2   a —optical fiber  12   a —0-system transmission/reception section  32   a  of the OLT  6 . 
   In accordance with the values of the headers H 1  and H 2  added to the cells, these ATM cells #1 and #2 are distributed to the corresponding ports of the switch  4 . 
   The operation of the SEL  81  of the ONU  71  upon the above switching from the virtual path VP  1  to the virtual path VP 1   a  will be described next with reference to  FIG. 23 . 
   Referring to  FIG. 23 , the control section  10  of the OLT  6  controls the switch  4 , and transmits a switching request signal to the ONU  71  through the virtual path VP 1   a  established in the above manner. Upon reception of the switching request signal from the OLT  6 , the 1-system transmission/reception section  31   b  of the ONU  71  notifies the control section  111  of the SEL switching signal. 
   Upon reception of the SEL switching signal, the control section  111  outputs a switching control signal to the SEL  81  of the ONU  71 . In accordance with the switching control signal from the ONU  71 , the SEL  81  switches the 0-system transmission/reception section  31   a  of the ONU  71  to the 1-system transmission/reception section  31   b , and connects the subscriber terminal  91  to the 1-system transmission/reception section  31   b  through the SEL  81 . With this operation, a signal from the 1-system transmission/reception section  31   b  of the ONU  71  is transmitted to the subscriber terminal  91  through the SEL  81 . As a consequence, the communication between the OLT  6  and the subscriber terminal  91  can be resumed through the virtual path VP 1   a.    
   According to the fourth embodiment, since the control sections  111  to  11   n  of the ONUs  71  to  7   n  perform switching control on the 0-system transmission/reception section and 1-system transmission/reception section more reliably, communication between each subscriber terminal and the OLT can be reliably established, and a network can be flexibly designed. 
   In each embodiment described above, optical transmission paths constituted by the photocouplers  2   a  and  2   b  and optical fibers  11   a  to  1   na  and  21   b  to  2   nb  are used as transmission paths. However, the present invention is not limited to this, and may be applied to a case wherein a coaxial cable or another kind of metal line is used as a transmission path. 
   In each embodiment described above, an abnormality due to disconnection of the virtual path VP 1  is assumed to be an example of an abnormal communication state. However, in addition to this abnormal state, the present invention can be applied to abnormal states wherein, for example, one or a plurality of the 0-system transmission/receptions  31   a  to  3   na  and 1-system transmission/receptions  31   b  to  3   nb  of the ONUs  71  to  7   n  has failed, the photocouplers  2   a  and  2   b  have failed, and SELs  81  to  8   n  have failed. 
   In each embodiment described above, when the virtual path VP 1  is to be switched to the virtual path VP 1   a  by using the ATM system, this switching operation is performed by distributing ATM cells #1 and #2 to ports of the switch  4  in accordance with the values of the headers H 1  and H 2  of the ATM cells #1 and #2. However, the present invention is not limited to this and can be applied to an STM (Synchronous Transfer Mode) PON system. In this case, a virtual path may be established by determining specific ports of the switch  4  for which data are to be destined in accordance with the time slots of frames output from the control section  10  in a predetermined cycle. 
   Each embodiment described above has been described in association with the occurrence of an abnormality in the virtual path VP 1 . Even if, however, an abnormality occurs in another portion, e.g., in the optical fiber  22   b  before the establishment of a virtual path, a virtual path can be established by switching the switch  4  to allow communication between the OLT  6  and the subscriber terminal  92 . 
   In this case, when the 1-system transmission/reception section  1   b  in the OLT  6  detects the abnormality in the optical fiber  22   b  while the virtual path VP 2  shown in  FIG. 21  is not established, and outputs a warning signal, the control section  10  controls the switch  4  to switch ports. With this operation, the virtual path VP 2  is established by the following route: switch  4 —0-system transmission/reception section  1   a  of the OLT  6 —photocoupler  2   a —optical fiber  12   a —0-system transmission/reception section  32   a  of the ONU  72 . At the same time, the 0-system transmission/reception section  32   a  of the ONU  72  is selected by the SEL  82  and connected to the subscriber terminal  92 . This allows communication between the subscriber terminal  92  and the OLT  6 . 
   In each embodiment described above, virtual paths are switched in the event of an abnormality in one optical fiber, i.e., a transmission path of one system. However, the virtual paths between the OLT  6  and the ONUs  71  to  7   n  can be simultaneously switched by the switch  4 . 
   In this case, when the 0-system transmission/receptions  31   a  to  3   na  of the ONUs  71  to  7   n  simultaneously transmit warning signals indicating communication abnormalities, the control section  10  of the OLT  6 , which has received these warning signals, simultaneously switches the ports of the switch  4  to simultaneously switch the virtual paths from the active system between the OLT  6  and the ONUs  71  to  7   n  to the standby system. In addition, when the ONUs  71  to  7   n , which have accessed signals distributed from the OLT  6 , simultaneously output warning signals, the control section  10  may simultaneously switch predetermined ports of the switch  4  upon reception of the signals. 
   As has been described above, in the protection switching method and apparatus for the PON system according to the present invention, only a path to be restored can be easily switched to a standby-system path without affecting communication through a normal virtual path in the PON system, and the communication can be continued. This makes it possible to easily switch virtual paths without causing any short break and the like in normal apparatuses.