in each of a plurality of adm nodes for sts path add / drop ( insertion / extraction ) in sonet , a spare route is set in correspondence to a working route , to which a path terminating point of each adm node belongs , and data of a switching destination path terminating point corresponding to the spare route is previously stored as path terminating point control data . in the event of a path trouble , a switching destination terminating point corresponding to the path terminating point control data is found , and the path terminating point corresponding to the trouble path is switched form the prevailing path terminating point over to the switching destination path terminating point . in response to restoration of the trouble path , control of switching - back to the original path terminating point is made . in this way , the transmission line in trouble can be automatically restored . fig1 is a block diagram showing an embodiment of the present invention . in the figure , parts like those in fig3 - 5 , 13 and 14 are designated by like reference numerals and symbols . the illustrated communication network comprises a plurality of dcss 2 - 1 to 2 - 6 , adms 3 - 1 to 3 - 10 each inserted between adjacent ones of the dcss , and a network controller 1 for system controlling the entire sonet network comprising the dcss and adms . fig2 is a block diagram showing a specific example of each of the adms 3 - 1 to 3 - 10 as nodes in fig1 . in the figure , parts like those in fig1 are designated by like reference numerals and symbols . line check frame inserting / extracting parts 40a and 40b periodically detect and extract only f2 bytes in the path over - heads extracted in sts path terminal / monitor parts 36a and 36b , and deliver three bytes among the extracted f2 bytes as a line check frame to respective line checking parts 41a and 41b , while periodically inserting predetermined three bytes of line check frame in f2 bytes of path over - head in each byte unit . line check parts 41a and 41b check whether the received line check frame is identical with a normal frame pattern . if the line check frame is not identical , this state is informed inclusive of its path terminating point id ( identification data ) to a path terminating point controller 38 . the path terminating point controller 38 is provided with a path terminating point control data memory 381 and a path switching / switching - back controller 382 . the path switching / switching - back controller 382 , in response to receipt of a notice of line check frame abnormality from the line checking parts 41a and 41b , retrieves and determines the switching destination path terminating point id of the terminating point ids of the abnormal path from the path terminating point control data memory 381 , and provides a control command for switching the path in trouble by reporting this switching destination path terminating point id together with the pertinent path terminating point id to a path setting part 372 in an sts path controller 37 . also , is response to receipt of a notice of line check frame normality from the line checking parts 41a and 41b , the path switching / switching - back controller 38 retrieves the switching designation path terminating point id set as normal path terminating point control data from the path terminating point control data memory 382 , and provides a path switching - back control command together with the pertinent path terminating point id to the path setting part 372 . the path setting controller 372 , in response to receipt of this path switching control command , executes switching control of the trouble path terminating point from the prevailing path terminating point over to the switching destination path terminating point or switching - back control from the switching destination path terminating point over to the prevailing path terminating point according to the command data , i . e ., prevailing path terminating point id and switching destination path terminating point id , and also switching destination path terminating point ( i . e ., prevailing opposite side path terminating point ) id previously stored in the sts path setting data memory 371 . the remainder of the structure is the same as in the example shown in fig1 . assuming now a working route of a path set between adjacent adms is as shown at 101 in fig3 when a trouble occurs in a spot a of the transmission line of the route 101 , the path of this route 101 has to be relieved . for the purpose of the relief , a spare route 102 as shown in fig4 is preliminarily set in as a bypassing route . when the trouble a occurs in the path 101 set between adms as shown in fig3 the path 101 is automatically switched over to the spare route 102 , thus forming a bypassing route 103 as shown in fig5 . to this end , automatic path terminating point switching control is necessary in adms 3 - 1 to 3 - 3 as nodes on the opposite sides of the spot of trouble a . a process of preparation such as the setting of the spare route 102 for the prevailing the working route 101 and the setting of various data for the switching of route over to the spare route 102 , will now be described with reference to the flow chart of fig6 . first , the controlling personnel controlling the communication network shown in fig1 predicts the number of paths , route , etc . predicted to be set between adms and designs a communication network which can ensure the band ( i . e ., number ) of spare routes capable of bypassing paths ( steps 61 to 64 ). then , the user executes path setting between adms according to the necessary traffic . the path setting is made by path setting control from the network control personnel 1 with respect to all the adms present on the route . the path setting ( i . e ., switch setting in the sts path switch 42 ) is executed with notification of a combination of path terminating point ids of the input and output of the sts path switch 42 to the sts path controller 37 . the combination data is stored in the sts path setting data memory 371 . assuming that , for instance , the working route 101 shown in fig3 is set through connection of the path terminating points ido and idi in the sts path switch 42 in the adm 3 - 1 as shown in fig9 a path terminating point connection relation as shown in fig1 is stored in the sts path setting data memory 371 using instance identifiers , i . e ., ids . then , the spare route 102 is set with respect to the working route 101 ( step 66 ). assuming that the spare route 102 is set through the switching of the path terminating point idi from the working path terminating point ido over to idj , as shown in fig9 in the path switch 42 of the adm 3 - 1 , data as shown in fig1 are stored in the path terminating point control memory 381 of the path terminating point controller 38 . referring to the table shown in fig1 , for the path terminating point with instance identifier ido , it is previously set that the switching destination path terminating point identifier is idj . in the case of path terminating point which require no connection change for the spare route switching , &# 34 ; 0 &# 34 ; is set as the switching path terminating point identifier . after the above process , operation for actual communication is made . in the operation , it becomes necessary for each adm to detect the transmission line trouble occurrence . in this embodiment , each adm transmits the line check frames for confirming the normality of inter - adm transmission lines ( step 67 ). the sending and extracting detection of the line check frames are executed by line check frame inserting / extracting parts 40a and 40b . fig1 ( a ) shows an example of line check frame . this example uses f2 byte in payload envelope over - head . three successive bytes as the f2 bytes , as shown in fig1 ( b ), are used as line check frame . the first and second bytes constitute a predetermined check code , and the third byte constitutes a crc code of the two bytes . in fig1 ( a ), reference numeral 110 designates a section over - head ( soh ), 111 a line over - head ( loh ), 113 a payload , and 114 a payload envelope . under the above assumption , the operation of the embodiment of the present invention will now be described with reference to the flow chart of fig7 . upon occurrence of transmission line trouble a as shown in fig3 influencing a path constituted by sole adms ( step 71 ), a line check frame to be received vanishes at the transmission line trouble a , so that a line check frame which is not expected is detected by the line checking parts 41a and 41b in the opposite side adms 3 - 1 and 3 - 2 . this line check frame is transmitted up to the adm , in which the path set between adms is detected . thus , all the adms on the path that is set detect the line check frame abnormality . each adm reports the abnormality to the path terminating point controller 38 via its line checking part . the path terminating point controller 38 checks whether the path terminating point id corresponding to the received line check abnormality identifies a path terminating point having to do with the path setting . this check is executed according to the data shown in fig1 stored in the path terminating point control data memory 381 . in other words , the controller 38 checks whether the switching destination path terminating point identifier with respect to the path terminating point id ( i . e ., instance identifier and ido in this embodiment ) corresponding to the path abnormality is &# 34 ; 0 &# 34 ; ( steps 73 and 74 ). if the identifier is &# 34 ; 0 &# 34 ;, no path switching is necessary , and nothing is done ( step 75 ). in this embodiment , the switching destination path terminating point identifier is &# 34 ; idj &# 34 ;, and subsequently the state of path switching is checked ( step 76 ). the prevailing state is &# 34 ; non - switched state &# 34 ; or &# 34 ; 0 &# 34 ;. thus , the path switching / switching - back controller 382 determines that the path switching is necessary , and sends out a path switching control command to the sts path controller 37 . at this time , the switching destination path terminating point id ( idj ) indicative of the switching destination is reported together with the path terminating point id ( ido ) corresponding to the path abnormality to the sts path controller 37 ( step 77 ). the sts path controller 37 retrieves the data ( fig1 ) in the sts path setting data memory 371 with reference to the reported path terminating point id ( ido ) corresponding to the path abnormality ( step 78 ), and determines that the prevailing opposite side instance path terminating point id corresponding to ido is idj . thus , the path switching part 372 switches the path terminating point idi in the sts path switch 42 from ido over to idj ( step 79 ). at this time , the data stored in the sts setting data memory 371 is re - written or switched from one representing the connection state ( ido - idi ) shown in fig9 over to one representing a new connection state ( idi - idj ) ( step 80 ). now , the switching - back process will be described with reference to the flow chart of fig8 . when the transmission line trouble a influencing the path constituted by the sole adms as shown in fig3 is removed ( step 81 ), an expected line check frame is detected by the transmission line checking parts 41a and 41b of the opposite side adms 3 - 1 and 3 - 2 . this line check frame is transmitted up to the adm , which the path set between adms is terminated in , so that all the adms on the path that is set detect the line check frame abnormality restoration . each adm reports the abnormality restoration to the path terminating point controller 38 while holding the normal state in its line checking parts 41a and 41b . the path terminating point controller 38 checks whether the path terminating point id pertaining to the received line check frame abnormality restoration identifies a terminating point having to do with the path setting , i . e ., a path setting subject path terminating point . this check is done with reference to the data shown in fig1 , stored in the path terminating point data memory 381 . that is , the controller 38 checks whether the path terminating point id ( i . e ., instance identifier and ido in this embodiment ) pertaining to the path abnormality restoration is &# 34 ; 0 &# 34 ; ( step 84 ). changes in construction will occur to those skilled in the art and various apparently different modifications and embodiments may be made without departing from the scope of the present invention . the matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only . it is therefore intended that the foregoing description be regarded as illustrative rather than limiting .