Patent Publication Number: US-2011051595-A1

Title: Transmission apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-195963, filed on Aug. 26, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a transmission apparatus and standby-system line switching. 
     BACKGROUND 
     Conventional transmission systems include a system that uses a transmission apparatus having a 1-to-n redundant configuration in which one station is connected to another station through normally used n working-system transmission paths and one standby-system transmission path. When a fault occurs on a working-system path (transmission path, unit, or line), the transmission apparatus switches from the working-system line (working line) to a standby-system line (protection line). An optical signal or electrical signal is transmitted through the transmission path. 
     1-to-1 or 1-to-n line switching carried out upon the occurrence of a fault is defined in the communication standard provided in GR-253 and ITU-T Standard G. 841. In line switching, a fault countermeasure command is transmitted and received between a monitoring device and the transmission apparatus, and 1-to-1 or 1-to-n line switching is carried out. Switching information is defined as such a command in K1 and K2 bytes included in the header of a transmission frame. When a fault occurs on the working system, the K1 and K2 bytes representing switching information are transmitted and received between one transmission apparatus and another transmission apparatus, using a frame transmitted through the standby-system line. 
       FIG. 29  is a diagram of a definition of K1 and K2 bytes in conventional line switching. In the K1 byte, four bits (bits  1 - 4 ) define a switching request, while the other four bits (bits  5 - 8 ) define a switch request channel (switch request channel number). In the K2 byte, four bits (bits  1 - 4 ) define information of a channel switched to a standby-system channel (bridging channel number), one bit (bit  5 ) defines the line switching method (architecture), and three bits (bits  6 - 8 ) define the mode. 
     In the diagram, switching requests defined by the bits  1 - 4  of the K1 byte are listed in descending order. Line switching is thus carried out according to the switching request highest in priority, i.e., highest on the list. For example, Forced Switch represented by the value of the bits  1 - 4  “1110” is higher in priority than Manual Switch represented by “1000”. 
     Among conventional techniques for carrying out line switching according to such a definition is a technique in which a standby-system line is allocated dynamically in a standby system in a 1-to-n redundant configuration to prevent instantaneous disconnection that occurs in a switchback process ensuing restoration from a fault to enable long hours of operation using a standby-system transmission path. The technique provides a configuration such that a standby-system line is not established statically upon the occurrence of a fault and such that a transmission path used as a standby-system path is determined dynamically based on the quality and priority of a transmission path at the time of the occurrence of a fault and restoration from the fault (see, e.g., Japanese Patent Application Laid-Open Publication No. 2001-339370). 
       FIG. 30  is a diagram of state transition that is made in response to a switching request based on a conventional definition.  FIG. 30  depicts transition between different states, with attention being focused on switching in slots (units or lines) of the station. For convenience, only some of defined states of  FIG. 29  are depicted in  FIG. 30 . The depicted states include No Request P 1 , Manual Switch P 2 , Signal Fail (SF) P 3 , and Forced Switch P 4 . Signal Degrade (SD), etc., is also included as a defined state. 
     In a state of Manual Switch P 2 , redundant path switching is executed automatically, irrespective of a line switching instruction by a maintenance person when a path error rate exceeds a threshold for the bit error rate specified for a state of SD or when the path enters a state of SF. In contrast, in a state of Forced Switch P 4 , even when the path enters the state of SD, where the path error rate is higher than the threshold for the bit error rate specified in the state of SD, or enters the state of SF, automatic redundant switching in response to such transition is not executed consequent to the Forced Switch having a higher priority than SD and SF (P 4 ), as depicted in  FIG. 29 . 
     A problem arises, however, when removal or a fault of a unit of the transmission apparatus, or a line fault, such as the disconnection of a cable from the unit, occurs after transition to the state of Manual Switch P 2  defined in  FIG. 30 . This is a situation, for example, where such a fault occurs when the maintenance person has set the unit to the state of Manual Switch for a given reason. This situation is equivalent to a condition for a state having a higher priority than the state of Manual Switch P 2  (SF (High) P 3  in  FIG. 30 ), resulting in transition p 2  (depicted in  FIG. 30 ) to automatically clear the state of Manual Switch P 2  and make transition to the state of SF (High) P 3  (depicted in  FIG. 30 ). This consequently means that the state of Manual Switch set by the maintenance person changes into another state without being noticed by the maintenance person. 
     After the occurrence of such a fault, when the maintenance person carries out restoration work, such as replacement of the unit, a Small Form Factor Pluggable (SFP), which is equivalent to a port and serves as an optical transmitting/receiving module mounted on the unit, an optical fiber, etc., and mounts or connects a new unit, SFP, cable, etc., the corresponding path abruptly switches back from a standby-system line to a working-system line. At this time, the maintenance person is not able to check the switchback or errors, leading to a problem in that if trouble occurs with a newly mounted unit or cable, signal disconnection occurs. 
     To prevent such a problem, the maintenance person may set the unit in the state of Forced Switch P 4  rather than the state of Manual Switch. Although the state of Forced Switch P 4  has a high priority, this state is incapable of relieving a different unit or line from a fault and may cause serious trouble if the maintenance person errantly forgets to clear the state of Forced Switch P 4 . It is desirable, therefore, for the state of Forced Switch P 4  to not be used if possible. In this manner, dealing with a specific event may be difficult if the conventional definition of states alone is applied. 
     SUMMARY 
     According to an aspect of an embodiment, a transmission apparatus switches to a line of one standby system upon occurrence of a fault on any one of n lines of a working system. The transmission apparatus includes a switching controller that when switching from a line of the working system to a line of the standby-system upon the occurrence of the fault and executing a given command to put the line causing the fault in a given state of line switching according to the given command, causes the working system to maintain the state of line switching according to the given command even after restoration from the fault. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of state transition of a switching request in a transmission apparatus according to an embodiment. 
         FIG. 2  is a diagram of an exemplary configuration of a transmission system incorporating the transmission apparatus. 
         FIG. 3  is a diagram of an exemplary configuration of an optical transmission system incorporating the transmission apparatus. 
         FIG. 4  is a diagram of a configuration of a unit mounted on the transmission apparatus. 
         FIG. 5  is a block diagram of an internal configuration of a control-system unit. 
         FIG. 6  is a diagram of the priority of each command set for the transmission apparatus. 
         FIG. 7  depicts a hardware configuration of the transmission apparatus in which the ports are disposed on a shelf. 
         FIG. 8  depicts a hardware configuration of the transmission apparatus in which the ports are disposed on the interface units. 
         FIG. 9  is a flowchart of a state switching process by each interface unit. 
         FIG. 10  is a flowchart of a switching process carried out by the entire the transmission apparatus. 
         FIG. 11  is a diagram of an example of an interface unit being removed from a slot executing Manual Switch. 
         FIG. 12  is a flowchart of a process of state switching depicted in  FIG. 11 . 
         FIG. 13  is a diagram of an example of removing an interface unit from a slot and then executing Manual Switch on the slot. 
         FIG. 14  is a flowchart of a process of the state switching depicted in  FIG. 13 . 
         FIG. 15  is a diagram of an example of removing an interface unit from a slot and then executing Advanced Manual Switch on the slot. 
         FIG. 16  is a diagram of an example of removal of an interface unit from a different slot after execution of Manual Switch. 
         FIG. 17  is a diagram of an example of cable detachment from a port executing Manual Switch. 
         FIG. 18  is a diagram of an example of executing Manual Switch on a port after cable detachment from the port. 
         FIG. 19  is a diagram of an example of executing Advanced Manual Switch on a port after cable detachment from the port. 
         FIG. 20  is a diagram of an example of cable detachment from a different port after execution of Manual Switch. 
         FIG. 21  is a diagram of an application example of switching information K1 and K2. 
         FIG. 22  is a diagram of another application example of switching information K1 and K2. 
         FIG. 23  is a diagram of the respective operations carried out between the transmission apparatus and an existing apparatus. 
         FIG. 24  is another diagram of the respective operations carried out between the transmission apparatus and the existing apparatus. 
         FIG. 25  is a diagram of an allocation example of the switching information K1 and K2. 
         FIG. 26  is another diagram of an allocation example of the switching information K1 and K2. 
         FIG. 27  is a diagram of an example of bit allocation for Advanced Manual Switch in the switching information K1 and K2. 
         FIG. 28  is a flowchart for explaining an example of a process of transmitting the switching information K1 and K2. 
         FIG. 29  is a diagram of a definition of K1 and K2 bytes in conventional line switching. 
         FIG. 30  is a diagram of state transition that is made in response to a switching request according to a conventional definition. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to the accompanying drawings. The transmission apparatus carries out 1-to-n redundant switching to execute line switching to one standby-system line when a fault occurs on any one of n working-system lines. For example, the transmission apparatus is applicable as a transmission apparatus that carries out switching operation conforming to GR-253, ITU-T Standard G. 841, etc. 
       FIG. 1  is a diagram of state transition of a switching request in the transmission apparatus according to an embodiment.  FIG. 1  depicts transition between different states, with attention being focused on switching in slots (equivalent to units) and lines of the station. For convenience, only some major states among multiple states are depicted in  FIG. 1 . 
     This transmission apparatus is characterized by a state of Advanced Manual Switch P 10  that is added to the various commands depicted in  FIG. 30  as a fault countermeasure command used in the transmission apparatus. Advanced Manual Switch P 10  is added for state transition inside the transmission apparatus, and is set to have a priority that is between the priority of Manual Switch P 2  and the priority of SF (High) P 3 . 
     This means that the transmission apparatus in the state of Advanced Manual Switch P 10  reports its state, to an external apparatus, as the state of Manual Switch P 2 , and the state information indicating Manual Switch P 2  is read by the external apparatus. Within the transmission apparatus, Advanced Manual Switch P 10  is defined as a state that has a higher priority than the state of Manual Switch P 2  (but is a state equivalent to neither Manual Switch P 2  nor Forced Switch P 4  inside the transmission apparatus). States and commands are set so that a state or command having a higher priority corresponds to a fault having a higher degree of seriousness. 
     As described, the state of Advanced Manual Switch P 10  is reported to external apparatuses, as state information indicative of Manual Switch P 2 . In state transition within the transmission apparatus, the state of Manual Switch P 2  transitions automatically to the state of Advanced Manual Switch P 10  when an SF (High) fault occurs. When switching requests are made to slots or lines, the switching requests are compared between all (n) slots or lines to carry out switching that satisfying the request of highest priority to achieve line relief. This operation is specified in the communication standard provided in GR-253, ITU-T G. 841. 
     Addition of the state of Advanced Manual Switch P 10  results in the addition of the following state transition inside the transmission apparatus. For example, if an SF (High) fault occurs on the same slot in the state of Manual Switch P 2 , the state of Manual Switch P 2  transitions to the state of Advanced Manual Switch P 10  (p 21 ). If an SF (High) fault of the same slot is cleared in the state of Advanced Manual Switch P 10 , the state of Advanced Manual Switch P 10  transitions to the state of Manual Switch P 2  (p 22 ). If a switching request in response to the occurrence of a fault having a higher priority, such as Forced Switch P 4 , is made to a different slot in the state of Advanced Manual Switch P 10 , the state of Advanced Manual Switch P 10  transitions to the state of SF (High) P 3  (p 101 ). If the maintenance person, etc., sets the state of Advanced Manual Switch P 10  in the state of SF (High) P 3 , the state of SF (High) P 3  transitions to the state of Advanced Manual Switch P 10  (p 102 ). If a switching request is made in response to the occurrence of a Forced Switch fault in the state of Advanced Manual Switch P 10 , the state of Advanced Manual Switch P 10  transitions to the state of Forced Switch P 4  (p 103 ). 
     The state of Advanced Manual Switch P 10  described with reference to  FIG. 1  functionally combines the states of Manual Switch P 2  and SF (High) P 3 . In this embodiment, the state of Advanced Manual Switch P 10  not only has the function of SF (High) P 3  of  FIG. 1  but further defines the state of SF (Low), of SD (Signal Degrade) (High), and of SD (Low) depicted in  FIG. 29  (which will be described in detail hereinafter). 
     Once transition is made from the state of Manual Switch P 2  to the state of Advanced Manual Switch P 10 , as depicted in  FIG. 1 , the state of Manual Switch P 2  is not cleared immediately even if a switching request is made. The state of Manual Switch P 2  is not cleared until the state of Advanced Manual Switch P 10  has been cleared and has transitioned back to the state of Manual Switch P 2 . 
     According to the configuration, in executing a procedure of replacing a unit or cable of the transmission apparatus in a fault-causing system, the maintenance person first carries out state switching to Manual Switch (or directly to Advanced Manual Switch, which will be described hereinafter) to switch from a working-system (work) component (unit, slot, or port) to a standby-system (protection) component. Subsequently, even if the working-system component to be replaced (SFP module on a unit or port, or connected cable) is removed, the state of Manual Switch (or Advanced Manual Switch) set in advance by the maintenance person is maintained from the perspective of external apparatuses. 
     As a result, when the working-system (unit or port) is restored, the transmission apparatus maintains the state of Manual Switch to prevent automatic switchback, thereby preventing unintended state switching. Even if the fault is resolved and a unit, port, cable, etc., is found to be normal, the maintenance person first confirms that the transmission apparatus has no problem and then clears the Manual Switch (Advanced Manual Switch) from outside the transmission apparatus. At this point, switchback to a working unit or port is allowed. Through this process, maintenance is carried out without causing instantaneous line disconnection or line fault. 
       FIG. 2  is a diagram of an exemplary configuration of a transmission system incorporating the transmission apparatus. The configuration example of  FIG. 2  depicts two transmission apparatuses  100 , each having a shelf  101  provided with ports  102 . The shelf  101  includes an interface unit  110  that is connected to a transmission path and that is the subject of 1-to-n switching, and a control-system unit  111 . The interface unit  110  includes n working-system units ( 4  units in  FIGS. 2 )  110   a  to  110   d  and one standby-system unit  110   e.    
     When a fault occurs on a working-system component (transmission path, unit, etc.), relief (protection) from the fault using a line of the standby-system unit  110   e  is carried out through line switching by the interface unit  110 . In this configuration, the working-system ports  102  are fixed to the shelf  101 . The working-system ports  102  are disposed on an upper aspect of the shelf  101 , and consist of 4 transmission ports (Tx) corresponding to 4 working-system units  110   a  to  110   d  and 4 reception ports (Rx) and thus, consist of 8 ports in total. Between two of the transmission apparatuses  100 , the transmission ports and the reception ports of the working-system ports  102  are connected to each other via optical fibers or electrical cables (not depicted). 
     The control-system unit  111  carries out cross connection control of switching a route for a signal traveling through a transmission path, 1-to-n unit switching control of switching to one standby-system transmission path upon the occurrence of a fault on any one of the n working-system transmission paths, and alarm monitoring on a transmission path and in the transmission apparatus. The control-system unit  111  includes multiple units, and is connected to a monitoring device  170 , such as a PC, via a communication line  160 , such as a LAN, craft cable, and USB. The monitoring device  170  monitors the state of the transmission apparatus  100 , and is caused to read out state information by an input command. Upon the occurrence of a fault, the maintenance person operates the monitoring device  170  to input a switching request command to the transmission apparatus  100 . 
       FIG. 3  is a diagram of an exemplary configuration of an optical transmission system incorporating the transmission apparatus. In the configuration example depicted in  FIG. 3 , the ports  102  are disposed directly on the interface units  110  mounted on the shelves  101  of two of the transmission apparatuses  100 . According to this configuration example, if a fault occurs on a working-system component (disconnection of a cable from the port  120 , etc.), the working-system component is relieved from the fault by line switching using the standby system, where the port  102   e  disposed on the standby-system unit  110   e  is used. 
       FIG. 4  is a diagram of a configuration of a unit mounted on the transmission apparatus. The interface unit  110  placed in a slot of the shelf  101  of the transmission apparatus  100  is connected to the control-system unit  111  inside the shelf  101 . The control-system unit  111  collects information concerning the state of the interface unit  110 , etc., through a built-in CPU and a state monitoring program running on the CPU, and is capable of setting operation, such as unit switching on the interface unit  110 , based on a command sent from the monitoring device  170 . The interface unit  110  has multiple removable transmitting/receiving modules  401  of Small Form Factor Pluggable (SFP) type that are equivalent to the ports. Multiple lines are arranged on the interface unit  110  to form rows from an upper part to a lower part thereof, and pairs including a transmitting SFP (Tx) and a receiving SFP (Rx) are arranged on the lines. 
       FIG. 5  is a block diagram of an internal configuration of the control-system unit. The control-system unit  111  carries out detection of a fault on a transmission path or unit and 1-to-n line switching control upon occurrence of a fault. The control-system unit  111  controls the interface unit  110  based on a command sent from the monitoring device  170 , and outputs to the monitoring device  170 , an alarm and report concerning the fault and operation state. The following constituent units each include a CPU, a memory, an I/O port, a hardware (HW) register, etc. The CPU executes a switching control program to carry out unit switching upon the occurrence of a fault. 
     Fault information from n working-system interface units  110   a  to  110   d  and one standby-system interface unit  110   e  is detected by fault detecting units  502 , respectively corresponding to the interface units  110  via a hardware register  501 . A transmitting/receiving unit  503  detects switching information K1 and K2 bytes. An input command given by an operator operating the monitoring device  170  is input via a command input/output unit  504 . Detected information and input information are output to a switching control unit (switching controller)  505 . 
     The switching control unit  505  determines the switching state of highest priority based on the input information, and carries out processes of setting switching control on the hardware register  501  via the I/O setting unit  506 , transmitting the switching information K1 and K2 bytes via the transmitting/receiving unit  503 , and outputting an alarm and report to the monitoring device  170  via the command input/output unit  504 . 
       FIG. 6  is a diagram of the priority of each command set for the transmission apparatus. For comparison, conventionally used commands are also included in the diagram. According to the priority setting depicted in  FIG. 6 , definitions of Advanced Manual Switch are added between Manual Switch set to have low priority order and Forced Switch set to have high priority order. 
     Advanced Manual Switch is set with respect to a definition of SF and to a definition of SD, thus is set to have priority that is one rank lower than that of SF and SD. For example, Advanced Manual Switch+SF (High) P 10   a  is set between SF (High) P 3   a  and SF (Low) P 3   b,  Advanced Manual Switch+SF (Low) P 10   b  is set between SF (Low) P 3   b  and SD (High) P 3   c,  and Advanced Manual Switch+SD (High) P 10   c  is set between SD (High) P 3   c  and SD (Low) P 3   d,  and Advanced Manual Switch+SD (Low) P 10   d  is set between SD (Low) P 3   d  and Manual Switch P 2 . 
     The overall priority order is thus set in this order of priority: Forced Switch P 4 &gt;SF (High) P 3   a &gt;Advanced Manual Switch+SF (High) P 10   a &gt;SF (Low) P 3   b &gt;Advanced Manual Switch+SF (Low) P 10   b &gt;SD (High) P 3   c &gt;Advanced Manual Switch+SD (High) P 10   c &gt;SD (Low) P 3   d &gt;Advanced Manual Switch+SD (Low) P 10   d &gt;Manual Switch P 2 . Newly set commands of Advanced Manual Switch  10 P (P 10   a  to P 10   d ) are given by newly carrying out bit setting # 1  to # 4  for the K1 and K2 bytes so that the commands do not overlap other commands (which will be described in detail hereinafter). 
       FIG. 7  depicts a hardware configuration of the transmission apparatus in which the ports are disposed on the shelf. In the example, the ports  102  are disposed on the shelf  101 , as depicted in  FIG. 2  and the working-system interface units  110  ( 110   a  to  100   d ) are execute line switching to the standby-system interface unit  110   e  by switching selectors  701  and  702 . In this configuration example, the working-system ports  102  fixed to the shelf  101  are used. Reference numeral  703  in  FIG. 7  denotes a matrix switch, which is disposed on the control-system unit  111  and switches a route for a signal from a transmission path to input/output the signal. 
       FIG. 8  depicts a hardware configuration of the transmission apparatus in which the ports are disposed on the interface units. The configuration is an example of the ports  102  being disposed on the interface units  110  ( 110   a  to  110   e ), as depicted in  FIG. 2 . In this case, the interface units  110  ( 110   a  to  100   d ) are able to execute line switching to the standby-system interface unit  110   e  by switching one selector  701 . 
       FIG. 9  is a flowchart of a state switching process by each interface unit. The flowchart depicts operations that are carried out until a state for each interface unit  110  becomes determinate, when a switching request command based on the manipulation of the monitoring device  170  by a maintenance person is input to the interface units  110 . If a fault or restoration from a fault occurs on a working-system interface unit  110  (step S 901 ), the following operations are carried out sequentially from commands higher in priority until the states become determinate. 
     For example, if a switching request input in response to the occurrence of a fault is Forced Switch having a high priority (step S 902 : YES), a Forced Switch request is settled (step S 903 ). If the switching request is not Forced Switch (step S 902 : NO), whether the switching request is SF (High) is determined (step S 904 ). If the switching request is SF (High) (step S 904 : YES), whether the current state is the state of Manual Switch is determined (step S 905 ). If the current state is not the state of Manual Switch (step S 905 : NO), the state of SF (High) is settled (step S 906 ). If the current state is the state of Manual Switch (step S 905 : YES), the state of Advanced Manual Switch+SF (High) is settled (step S 907 ). 
     If the switching request is not SF (High) at step S 904  (step S 904 : NO), whether the switching request is Manual Switch is determined (step S 908 ). If the switching request is Manual Switch (step S 908 : YES), the state of Manual Switch is settled according to the switching request (step S 909 ). If the switching request is not Manual Switch (step S 908 : NO), whether the switching request is No Request is determined (step S 910 ). If the switching request is No Request (step S 910 : YES), the state of No Request is settled according to the switching request (step S 911 ). This process includes additional steps related to newly set Advanced Manual Switch (steps S 904  to S 907 ). 
       FIG. 10  is a flowchart of a switching process carried out by the entire the transmission apparatus. The shelf  101  has n slots into which the interface units  110  are placed, and the switching control unit  505  of  FIG. 5  detects each switching request to each slot to process switching requests in descending order of priority. 
     For example, when detecting the occurrence of a switching factor or restoration corresponding to a fault or restoration occurring on the interface units  110  (step S 1001 ), the switching control unit  505  executes the following operations. First, a constant k representing the number of switching requests is set to 1 (step S 1002 ) to set a switching request to be executed by the entire apparatus (step S 1003 ). At this time, the slot number of the slot subject to the switching request and a state subject to the switching request (e.g., SF-High, etc.) are set. 
     The constant k is then incremented by 1 (step S 1004 ), and it is determined whether the switching request for a different slot (k slot) has a higher priority as a command (step S 1005 ). If the switching request for the different slot (k slot) has a higher priority (step S 1005 : YES), the switching request to the entire apparatus set at step S 1003  is updated (step S 1006 ). As a result of the updating, the slot number of the different slot having a higher priority and the state subject to the switching request are set. 
     If the switching request for the different slot (k slot) does not have a higher priority (step S 1005 : NO), the switching request setting is not updated, maintaining the setting made at step S 1003 . Subsequently, whether all n slots (k=n) have been processed through a series of the operations is determined (step S 1007 ). If all slots have not been processed (step S 1007 : NO), the process flow returns to step S 1004 , at which another slot is processed. If the processing of all the slots has been completed at step S 1007  (step S 1007 : YES), switching requests to the entire apparatus having n slots have been set. Hence, actual line switching (state transition) is carried out (step S 1008 ), after which the process is ended. 
     A first example describes carrying out line switching at the unit-level in the transmission apparatus of  FIG. 2  will be described.  FIG. 11  is a diagram of an example of an interface unit being removed from a slot executing Manual Switch. An example in which a fault occurs on the working-system interface unit  110   b  in the slot  3  of the shelf  101  is explained. 
     In this example, a switching request command “OPR-PROTNSW-EQPT (Manual)” for Manual Switch is input from the monitoring device  170  with the working-system interface unit  110   b  in a normal, mounted state in the slot  3 . Subsequently, even if the interface unit  110   b  is removed from the slot  3 , the state of Manual Switch of the slot  3  is not cleared but rather, continues. The monitoring device  170  external to the transmission apparatus is able to read out the state of Manual Switch using a command “RTRV-COND-ALL” to check an alarm and the state of the transmission apparatus. 
     In this situation, the slot  3  inside the transmission apparatus enters the state of Advanced Manual Switch+SF (High) or of SF (Low), but remains in the state of Manual Switch from the perspective of external apparatuses. The slot  3  thus maintains the state of Manual Switch with respect to external apparatuses even if the interface unit  110   b  is disconnected from the slot  3  or fails, or is mounted incorrectly. 
       FIG. 12  is a flowchart of a process of state switching depicted in  FIG. 11 . It is assumed that the slot  3  is initially in the state of Manual Switch (step S 1201 ). Following the initial state, although removal of the interface unit  110   b  from the slot  3  (step S 1202 ) leads to the state of SF (High) or of SF (Low), if the slot  3  is in the state of Manual Switch (step S 1203 : YES) at this point, the slot  3  requests for switching to the state of Advanced Manual Switch+SF (High or Low) inside the transmission apparatus (step S 1204 ). If the slot  3  is not in the state of Manual Switch (step S 1203 : NO), the slot  3  requests for switching to the state of SF (High or Low) (step S 1205 ). 
       FIG. 13  is a diagram of an example of removing an interface unit from a slot and then executing Manual Switch on the slot. Similar to  FIG. 11 , an example where a fault occurs on the working-system interface unit  110   b  placed in the slot  3  of the shelf  101  is explained. 
     It is assumed that a fault (EQPT fault on FLT, RMV, MEA, etc.) occurs on the interface unit  110   b  placed in the working-system slot  3 . The maintenance person removes the interface unit  110   b  to switch lines to the interface unit  110   e  of the standby-system slot  1 . Even after this, the command “OPR-PROTNSW-EQPT (Manual)” for Manual Switch is input from the monitoring device  170  to the slot  3  in the same manner as in the example of  FIG. 11  to be able to execute state switching. In this case, from the perspective of external apparatuses, the slot  3  remains in the state of Manual Switch, which allows the monitoring device  170  to read out the state of Manual Switch using the command “RTRV-COND-ALL”. Meanwhile, inside the transmission apparatus, the slot  3  enters the state of Advanced Manual Switch+SF (High or Low). 
     Subsequently, even if the interface unit  110   b  is removed from the slot  3 , the state of Manual Switch of the slot  3  is not cleared but rather, continues. Inside the transmission apparatus from which the unit has been removed, switching to the state of Advanced Manual Switch+SF (High or Low) is requested, so that switching occurs inside the transmission apparatus according to the switching request. After remounting of the interface unit  110   b,  state transition is made to the state of Manual Switch. According to a conventional technique, Manual Switch is cleared at the point that the interface unit  110   b  is removed upon occurrence of the fault. 
       FIG. 14  is a flowchart of a process of the state switching depicted in  FIG. 13 . It is assumed that initially, the interface unit  110   b  is removed from the slot  3  to bring about the state of SF (High or Low) (step S 1401 ) and then the command for causing the state of Manual Switch is input to the slot  3  (step S 1402 ). In this case, when the slot  3  is in the state of SF (High or Low) (step S 1403 : YES), the slot  3  requests switching to the state of Advanced Manual Switch+SF (High or Low) (step S 1404 ). When not in the state of SF (High or Low) (step S 1403 : NO), the slot  3  requests switching to the state of Manual Switch (step S 1405 ). 
       FIG. 15  is a diagram of an example of removing an interface unit from a slot and then executing Advanced Manual Switch on the slot. Different from the case of specifying switching to the state of Manual Switch using the command “OPR-PROTNSW-EQPT (Manual)”, switching to the state of Advanced Manual Switch may be input directly using a command “OPR-PROTNSW-EQPT (Advanced Manual)”, as depicted in  FIG. 15 . 
       FIG. 16  is a diagram of an example of removal of an interface unit from a different slot after execution of Manual Switch. If a fault occurs on the interface unit  110   b  placed in the working-system slot  3  and line switching has been made to the interface unit  110   e  of the standby-system slot  1 , the command “OPR-PROTNSW-EQPT (Manual)” for causing the state of Manual Switch is input to the working-system slot  3  to put the slot  3  in the state of Manual Switch. Subsequently, even if the interface unit  110  is removed from the slot  3 , the slot  3  maintains the state of Manual Switch in the same manner as described above. Following this, if a fault occurs on a working-system slot  4  different from the slot  3 , the state of Manual Switch is cleared and relief operation is carried out according to the priority set for the slots  3  and  4 . If the priority (PRI=High/Low) set for the slots  3  and  4  are the same (Low), as depicted in  FIG. 16 , the slot  3  having been switched first is given priority in relief operation to continue. 
     A second example describes executing line switching at the line-level in the transmission apparatus of  FIG. 3  will be described.  FIG. 17  is a diagram of an example of cable detachment from a port executing Manual Switch. The slot  3  of the shelf  101  is provided with the working-system interface unit  110   b.  An example of a fault resulting from cable detachment at the port  102  of the slot  3  is described. 
     In this example, it is assumed that a working-system port  3  is normal and in a state of No alarm. A switching request command “OPR-PROTNSW-OC3 (Manual)” for Manual Switch is input from the monitoring device  170 . After this, even if a cable connected to the port  3  is pulled out or a fault equivalent to SF (High), SF (Low), SD (High), SD (Low), etc. occurs, the state of Manual Switch is not cleared but rather, continues. The monitoring device  170  is able to read out the state of Manual Switch using a state checking command “RTRV-COND-ALL”. In this condition, inside the transmission apparatus, switching to Advanced Manual Switch+SF (High or Low) is requested, and state transition is made to the state of Advanced Manual Switch+SF (High or Low) according to the request. 
       FIG. 18  is a diagram of an example of executing Manual Switch on a port after cable detachment from the port. If a cable detaches from the working-system port  3 , the state of SF or SD (Signal Degrade) arises on the working-system port  3 , and line switching is made to the standby-system port  1 . In this situation, the command “OPR-PROTNSW-OC3 (Manual)” is input to the working-system port  3  to put the port  3  in the state of Manual Switch. The monitoring device  170  is able to read out the state of Manual Switch using the command “RTRV-COND-ALL”. Meanwhile, inside the transmission apparatus, a switching request for switching to Advanced Manual Switch+SF (High or Low) is made, and state transition is made to the state of Advanced Manual Switch+SF (High or Low) according to the request. 
     Subsequently, even if the cable connected to the port  3  is disconnected from the port  3  or a fault equivalent to SF or SD occurs, the state of Manual Switch is not cleared but rather, continues. The state inside the transmission apparatus from which the cable has been removed is Advanced Manual Switch+SF (High or Low), so that state switching occurs according to a switching request for switching to the state of Advanced Manual Switch+SF (High or Low). After the cable is reconnected, state transition is made to the state of Manual Switch. According to a conventional technique, Manual Switch is cleared at the point that the cable is detached. 
       FIG. 19  is a diagram of an example of executing Advanced Manual Switch on a port after cable detachment from the port. Different from the example of specifying Manual Switch using the command “OPR-PROTNSW-OC3 (Manual)”, Advanced Manual Switch may be input directly using the command “OPR-PROTNSW-OC3 (Advanced Manual)”, as depicted in  FIG. 19 . 
       FIG. 20  is a diagram of an example of cable detachment from a different port after execution of Manual Switch. If a cable becomes detached from the working-system port  3 , causing a fault equivalent to SF or SD and line switching is made to the standby-system port  1 , the command “OPR-PROTNSW-OC3 (Manual)” for switching to the state of Manual Switch is input to the working-system port  3  to put the port  3  in the state of Manual Switch. After this, even if the interface unit  110  of the port  3  is removed, the port  3  maintains its state of Manual Switch in the same manner as described above. Subsequently, if a fault equivalent to SF or SD occurs on a working-system port  4  different from the port  3  due to cable detachment, etc., the state of Manual Switch is cleared and relief operation is carried out according to the priority set for the ports  3  and  4 . If the priority set for the ports  3  and (PRI=High/Low) are the same (Low), as depicted in  FIG. 20 , the port  3  having been switched first is given priority in relief operation to continue. 
     A third example describes an application example of transmitting and receiving the switching information K1 and K2 bytes between transmission apparatuses will be described. As depicted in  FIGS. 2 and 3 , a transmission path can be switched freely from the working system to the standby system between a shelf  1  of the transmission apparatus  100  and a shelf  2  of the transmission apparatus  100 . 
       FIG. 21  is a diagram of an application example of switching information K1 and K2. It is assumed that initially, a working-system port W 4  of the shelf  2  is in the state of Advanced Manual Switch+SF (High), a working-system port W 2  of the shelf  2  is in the state of SD (High), and working-system ports W 1  and W 3  of the shelf  2  are in the state of No Request. MAN in  FIG. 21  is an abbreviation of Manual Switch. Here, the state of Advanced Manual Switch+SF (High) has a higher priority than the state of SD (High) and thus, switching to the state of Advanced Manual Switch+SF (High) is executed to switch the port W 4  to the working system. As a result, the switching information K1 and K2 bytes are transmitted and received between the shelves  1  and  2 , via the standby-system (protection) port  102   a.  The shelf  2  transmits switching information “MAN (Manual Switch)/W 4 /W 4 /1:n/Bi (Bi-directional mode, see FIG.  29 )” to the shelf  1 , using the switching information K1 and K2 bytes. 
     It is assumed that here that the working-system ports W 1  to W 4  of the shelf  1  are in the state of No Request, in which no specific fault factor requiring switching arises. In this situation, a line switching method of Bi-directional is set on the K2 byte, so that switching corresponding to the switching information “MAN/W 4 /W 4 /1:n/Bi” transmitted from the shelf  2  is executed with priority over the shelf  1 . 
     Although transmission of switching information of “SF (High)/W 4 /W 4 /1:n/Bi” from the shelf  2  is also conceivable, Manual Switch is transmitted as the switching information “MAN/W 4 /W 4 /1:n/Bi” to report the state of Manual Switch executed by a Far end request. 
       FIG. 22  is a diagram of another application example of switching information K1 and K2. It is assumed that following the situation depicted in  FIG. 21 , a fault of SF (Low) occurs on the working-system port W 2  of the shelf  1 . In this case, because of reception of the switching information “MAN/W 4 /W 4 /1:n/Bi” from the shelf  2 , the shelf  1  determines that the state of SF (Low) of the shelf  1  (the station) has a higher priority, and transmits switching information “SF-L/W 2 /W 4 /1:n/Bi” to the shelf  2 . As a result, switching to the state of SF (Low) according to the priority thereof is expected to occur on the shelf  2 . If no response is received from the shelf  2  within a given period, the shelf  1  detects the shelf  2  to be abnormal. The shelf  1 , however, does not output an alarm, such as Protection Switch Byte Failure, to an external apparatus, but determines that the shelf  2  is in the state of Advanced Manual Switch +SF (Low or High). The shelf  1  thus keeps transmitting the switching information “SF-L/W 2 /W 4 /1:n/Bi” to the shelf  2 . 
       FIG. 23  is a diagram of the respective operations carried out between the transmission apparatus and an existing apparatus to which the definition and setting of Advanced Manual Switch are have not been made. The shelf  1  ( 100   x ) is the existing apparatus without the definition of the state of Advanced Manual Switch, and the shelf  2  is the transmission apparatus having the definition and setting of the state of Advanced Manual Switch. The same circumstances as those of  FIG. 21  are assumed and the port W 4  is switched to the working system. In this example, the working-system port W 4  of the shelf  1  is in the state of Manual Switch, while the working-system port W 4  of the shelf  2  is in the state of Advanced Manual Switch+SF (High). Between the shelves  1  and  2 , the K1 and K2 bytes are transmitted and received via the standby-system (Protection) port  102   a.    
       FIG. 24  is another diagram of the respective operations carried out between the transmission apparatus and the existing apparatus. It is assumed that after the operation depicted in  FIG. 23 , a fault of SF (Low) occurs on the working-system port W 2  of the shelf  1 . As a result of the fault, the shelf  1  ( 100   x ) transmits the switching information “SF-L (Low)/W 2 /W 4 /1:n/Bi” to the shelf  2  for a given period, using the K1 and K2 bytes. If the shelf  2  does not carry out switching to a state of SF-H (High), the shelf  1  will detect the shelf  2  to be abnormal (Protection Switch Byte Failure APS). In this case, the shelf  1  continues to transmit the switching information “SF-L/W 2 /W 4 /1:n/Bi” to the shelf  2 , using the K1 and K2 bytes. In this manner, when the shelf  1 , as the existing apparatus, transmits the switching information “SF-L/W 2 /W 4 /1:n/Bi” to the shelf  2  but continues to receive a response from the shelf  2  indicative of the shelf  2  being in the state of Manual Switch even after an elapse of a given period, the shelf  1  may continue to transmit the same switching information. At this point in time, the shelf  2  is executing Advanced Manual Switch. Hence, even if the shelf  1  is an existing apparatus without an Advanced Manual Switch setting, the shelf  1  is able to continue operations with respect to the shelf  2 . 
     A fourth example relates to bit allocation for newly defined switching information K1 and K2 for Advanced Manual Switch. As described with reference to  FIG. 29 , “011” and “010” as the bits  6 - 8  of the K2 byte are not defined at present but are reserved for future use. These bits  6 - 8  are used in a new definition. 
       FIG. 25  is a diagram of an allocation example of the switching information K1 and K2. In the example depicted in  FIG. 25 , undefined “011” and “010” as the bits  6 - 8  of the K2 byte are used for Advanced Manual Switch in the following manner.
     “011”: Bi-directional Mode+Advanced Manual Switch   “010”: Uni-directional Mode+Advanced Manual Switch With such definitions, “011” and “010” as the bits  6 - 8  of the K2 byte are transmitted and received only when Advanced Manual Switch is executed.   

     If the bits  6 - 8  of the K2 byte are “011”, the reception-side determines that the switching information indicates a line switching method of Bi-directional Mode and the state of Advanced Manual Switch, and that the state of the counterpart node is any one of the following states.
     “1101”: Advanced Manual Switch+SF (High)   “1100”: Advanced Manual Switch+SF (Low)   “1011”: Advanced Manual Switch+SD (High)   “1010”: Advanced Manual Switch+SD (Low)   

     Similarly, if the bits  6 - 8  of the K2 byte are “010”, the reception-side determines that the switching information indicates a line switching method of Uni-directional Mode and the state of Advanced Manual Switch, and that the state of the counterpart node is any one of the following states.
     “1101”: Advanced Manual Switch+SF (High)   “1100”: Advanced Manual Switch+SF (Low)   “1011”: Advanced Manual Switch+SD (High)   “1010”: Advanced Manual Switch+SD (Low)   

     In the example depicted in  FIG. 25 , the working-system port W 2  of the shelf  2  is in the state of SD (High) and the port W 4  of the shelf  2  is in the state of Advanced Manual Switch+SF (High). In this case, the shelf  2  transmits “SF-H/W 4 /W 4 /1:n/011” as switching information using the K1 and K2 bytes. 
       FIG. 26  is another diagram of an allocation example of the switching information K1 and K2, a fifth example. The same circumstances as those depicted in  FIG. 21  are assumed for the shelf  2 , where the port W 4  is switched to the working system. In this example, the shelf  2  executes switching to the state of Advanced Manual Switch+SF (High) having the highest priority, based on switching requests of the ports W 1  to W 4  of the shelf  2 . The shelf  2  alternately transmits the following switching information to the shelf  1 , using the K1 and K2 bytes, in order to execute switching to the state of Advanced Manual Switch. 
     Switching information  1  “SF-H/W 4 /W 4 /1:n/Bi”
 
Switching information  2  “MAN/W 4 /W 4 /1:n/BI”
 
     As a result, the shelf  1  at the reception-side determines that the shelf  2  is in the state of Advanced Manual Switch+SF (High), and executes switching corresponding to the state of the shelf  2 . In the case of alternating transmission of the switching information  1  and  2 , the K1 and K2 bytes may seem to be unstable. To deal with this, an exceptional condition is set to the shelf  1 , a condition preventing the shelf  1  from detecting the switching information  1  and  2  to be Protection Switch Byte Failure, upon receiving the information. 
       FIG. 27  is a diagram of an example of bit allocation for Advanced Manual Switch in the switching information K1 and K2. For switching information for Advanced Manual Switch # 1  to # 4 , a code (bit information) allocated to a command different from Advanced Manual Switch is used in bit allocation for the K1 bytes of commands of  FIG. 6 . 
     In a first configuration example, the bits  1 - 4  of the K1 byte are allocated to represent the state of SF and of SD, and the bits  6 - 8  of the K2 byte are allocated to represent the command of Advanced Manual Switch by using undefined bits “010” and “011” as the bits  6 - 8 . In a second configuration example, only the bits  1 - 4  of the K1 byte are used to alternately transmit any one of bits “1101”, “1100”, “1011”, and “1010” representing setting for SD and SF and a bit “1000” representing setting for Manual Switch. 
       FIG. 28  is a flowchart for explaining an example of a process of transmitting the switching information K1 and K2.  FIG. 28  depicts an example of transmission of the command of Advanced Manual Switch+SF (High) among commands described in the second configuration example. When switching is made to Advanced Manual Switch+SF (High) (step S 2801 ), switching information K1 and k2 composed of the command of SF (High) “SF-H/W 4 /W 4 /1:n/Bi” and the command of Manual Switch “MAN/W 4 /W 4 /1:n/Bi” is made (step S 2802 ). 
     Subsequently, the transmission apparatus at the transmission-side sets the counter k to an initial value 1 (step S 2803 ), and determines whether the counter k is an odd number (step S 2804 ). If the counter k is an odd number (step S 2804 : YES), the transmission apparatus first transmits the command for SF (High) “SF-H/W 4 /W 4 /1:n/Bi” (step S 2805 ), and increases the counter k by 1 (step S 2806 ). Subsequently, steps following step S 2804  are repeated until the end of the transmission of the command of Advanced Manual Switch+SF (High), i.e., the occurrence of a request for the release of Advanced Manual Switch+SF (High) (step S 2807 ). The next transmission is carried out when the counter k is an even number (step S 2804 : NO), and the command of Manual Switch “MAN/W 4 /W 4 /1:n/Bi” is transmitted at the next transmission (step S 2808 ). Thereafter, the commands of SF (high) and Manual Switch are transmitted alternately. 
     According to the setting of Advanced Manual Switch described in the embodiments, when a fault-causing working-system unit or cable is removed for replacement, the state of Manual Switch executed before the removal can be maintained. This prevents the immediate occurrence of switchback after restoration from the fault of the working system and thus, no instantaneous line disconnection or line fault occurs. 
     The transmission apparatus disclosed herein offers an effect of preventing immediate line switchback that occurs at the time of restoration from a fault. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.