Abstract:
A method of localizing failures occurring along a transmission path is provided. A data signal is transmitted along the transmission path, which comprises a path segment. A first network node performs a tandem connection monitoring source function of a tandem connection. A second network node monitors the transmission along the path segment, by performing a tandem connection monitoring sink function. When the first network node detects a failure of the data signal, the first network node enters information indicating a data signal failure into the data signal&#39;s overhead field. When the second network node detects a failure by the tandem connection monitory sink function, the second network node enters information into the overhead field. When a third network node detects a failure of the data signal, the third network node uses the information in the overhead field to determine whether the failure occurred within or outside of the path segment.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method of localizing a failure occurring along a transmission path. 
     BACKGROUND 
     When transmitting a data signal along a transmission path, a failure of a network resource may result in a failure of the transmission of the data signal. The transmission path may contain different segments. For different reasons, it is useful to know within which segment a failure occurred. 
     SUMMARY 
     A method of localizing a failure occurring along a transmission path is proposed. 
     A data signal, which contains an overhead, is transmitted along the transmission path. The transmission path comprises a path segment. 
     A first network node is located at the beginning of the path segment. The first network node performs a tandem connection monitoring source function of a tandem connection. 
     A second network node is located at the end of the path segment. The second network node monitors the transmission along the path segment, by performing a tandem connection monitoring sink function, which corresponds to the tandem connection monitoring source function of the first network node. 
     In the case, that the first network node detects that a failure of the data signal occurs before the path segment, the first network node enters information indicating a data signal failure into the overhead field of the overhead. 
     In the case, that the second network node detects a failure of the tandem connection, by performing the tandem connection monitory sink function, the second network node enters information indicating a tandem connection failure into the overhead field. 
     A third network node is located after the path segment at the transmission path. In the case, that the third network node detects that a transmission failure of the data signal occurs along the transmission path, the third network node uses the information that is contained in the overhead field for determining whether the detected failure occurred within the path segment or whether the detected failure occurred outside of the path segment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows network nodes located along a transmission path. 
         FIG. 2  shows a data field comprising an overhead payload. 
         FIG. 3  shows the network nodes located along the transmission path and data signals in the case of a failure occurring along a path segment. 
         FIG. 4  shows the network node located and data signals in the case of a failure occurring outside of the path segment of the transmission path. 
         FIG. 5  shows the network nodes as well as an assignment of the network nodes to different network domains. 
         FIG. 6  shows a data signal according to a known telecommunication standard. 
         FIG. 7  shows an allocation of overhead bytes. 
         FIG. 8  shows an allocation of overhead bytes to different path segments. 
         FIG. 9  shows hardware elements of a network node. 
         FIGS. 10 a , 10 b    shows elements of processing units which contained within network nodes. 
         FIG. 11  shows a network node. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows network nodes of a network NT that are located along a transmission path P. The transmission path P contains a path segment PS. As shown in  FIG. 1 , the transmission is performed from left to right. At the beginning BPS of the path segment PS, a network node N 1  is located. At the end EPS of the path segment PS another network node N 2  is located. A further network node N 3  is located along the transmission path P behind the path segment PS. 
     Further network nodes N are located along the transmission path P. These network nodes N monitor a transmitted data signal and generate an alarm signal in the case that a failure of the transmitted data signal occurs. Once a network node N generates an alarm signal, this alarm signal is then passed on until the end of the path P by the following network nodes. 
       FIG. 2  shows the general frame structure of the data signal DS which is transmitted along the transmission path. The data signal DS comprises a payload PL as well as an overhead OH. The overhead OH contains a tandem connection monitoring field TCMF for carrying parity information of a tandem connection monitoring layer. Furthermore, the overhead OH contains an overhead field OHF. The overhead field OHF contains a first overhead subfield OHF 1  and a second overhead subfield OHF 2 . 
     Coming back to  FIG. 1 , it is now explained in which way the data signal is transmitted along the transmission path P. 
     Starting on the left hand side, at the beginning BP of the path P, the data signal is transmitted towards the right hand side to the end EP of the transmission path P. The network nodes N, N 1 , N 2 , N 3  receive the data signal and pass it on to their next neighboring network node. 
     The network node N 1  performs a tandem connection monitoring source function of a tandem connection monitoring layer. The network node N 1  does so, by computing parity information over the payload of the data signal, and writing this parity information into the tandem connection monitoring field TCMF of the overhead OH as shown in  FIG. 2 . According to an alternative solution, the network node N 1  writes the computed parity information into a tandem connection field of a frame of the data signal, which is sent by the network node N 1  at a later point of time. 
     The network node N 2  receives the data signal DS. Furthermore, the network node N 2  monitors the transmission along the path segment PS, by performing a tandem connection monitoring sink function. The network node N 2  does so, by computing parity information over the payload PL of the received data signal DS, and comparing the computed parity information with parity information which is contained within the tandem connection monitoring field TCMF of the data signal DS. In the case, that a failure occurred along the path segment PS, the payload PL is modified such that a parity information computed at the network node N 2  would not be equal anymore to a party information contained within the tandem connection monitoring TCMF. Thus, the network node N 2  is able to detect a transmission failure occurring along the path segment PS. According to an alternative solution, the network node N 2  compares a computed parity information with parity information contained within a tandem connection monitoring field of a data signal which is received at a later point of time. 
     The tandem connection monitoring field TCMF of the data signal DS is a field that is assigned to the tandem connection monitoring layer which observes the transmission of the data signal along the path segment PS. According to an alternative solution, the data signal DS contains multiple fields for carrying parity information of multiple tandem connection monitoring layers, wherein each of the fields is assigned to an individual tandem connection monitoring layer used for monitoring transmission along an individual path segment. 
     In the case, that the network node N 1  detects that a failure of the data signal occurs before the path segment PS, the network node N 1  enters information indicating a data signal failure into the overhead subfield OHF 1 . The network node N 1  detects a failure of the data signal, either by receiving an alarm signal that is generated by a network node N located at the transmission path P before the path segment PS. Furthermore, the network node monitors a received data signal and derives from the received data signal itself, or from an absence of the data signal, that a failure of the date signal has occurred. 
     The network node N 2  terminates the tandem connection monitoring layer function that is performed along the path segment PS. In the case, that a parity information computed by the network node N 2  over the payload PL of the data signal DS is not equal to a parity information contained within the tandem connection monitoring field TCMF of the data signal, the network node N 2  derives that a failure of the tandem connection monitoring layer function is present. According to an alternative solution, the network node N 2  derives also from a certain bit pattern contained in the tandem connection monitoring field TCMF of the data signal DS, that an alarm signal has been generated by a network node N located before the network node N 2  along the path segment PS. Also in this case, due to the reception of the alarm signal, the network node N 2  concludes that a failure of the tandem connection monitoring layer function of the path segment PS is present. 
     In the case, that a failure of the tandem connection monitoring layer function performed along the path segment PS is detected by the network node N 2 , the network node N 2  enters information indicating a tandem connection failure into the overhead subfield OHF 2 . 
     The network node N 3  monitors the data signal, which is received by the network node N 3 , and carries out functions for detecting a failure of the data signal. In the case, that the network node N 3  detects a failure of the data signal, the network node N 3  uses the information contained within the overhead field OHF for determining whether the detected failure occurred within the path segment PS or outside of the path segment PS. By doing so, the network node N 3  is able to determine, whether a network resource has failed within the path segment PS or outside of the path segment PS. Thus, the network node N 3  is able to derive whether protection and/or restoration actions have to be initiated within the path segment PS or outside of the path segment PS. 
     In the case that the overhead subfield OHF 1  contains information that indicates no data signal failure, the network node N 3  derives from this that no data signal failure occurred before the path segment PS. Furthermore, in the case that the overhead subfield OHF 2  contains information indicating a tandem connection failure, the network node N 3  derives from this that a detected failure is caused by a failure that occurred along the path segment PS. 
       FIG. 3  shows the network NT together with the network nodes placed along the transmission path P and the path segment PS as already shown in  FIG. 1 . 
     Furthermore,  FIG. 3  shows a failure F 2  that occurs at the path segment PS at a location which is located between network node N 1  and a network node N placed within the path segment PS. 
     Furthermore,  FIG. 3  shows data signals DS 1 , DS 2  at different stages of the transmission path P. 
     At the beginning of the transmission of the data signal DS 1 , the information that is present within the overhead fields OHF 1 , OHF 2  is set to the value 0. A value of 0 indicates that no failure has occurred. 
     The network node N, which is located between the beginning BP of the path and the first network node N 1 , monitors the data signal DS 1 . Since no failure of the data signal DS 1  is detected by the network node N, the node N does not generate an alarm signal. 
     The network node N 1  monitors the data signal DS 1  and also checks whether an alarm signal is received. Since the data signal DS 1  has no failure at this stage, and since also no alarm signal has been received by network node N 1 , the network node N 1  does not modify the information contained in the overhead subfield OHF 1 . Furthermore, the network node N 1  generates parity information TCMN 1  that is inserted into the tandem connection monitoring field TCMF. 
     Within the path segment PS a failure F 2  occurs. At a network node N, which is located behind the location at which the failure F 2  occurs, the failure F 2  is detected. Therefore, the network node N generates an alarm signal called Alarm Indicating Signal (AIS), that is propagated along the transmission path P. The originally transmitted data signal DS 1  is replaced by a data signal DS 2  carrying the alarm signal. The payload PL of the data signal DS 2  contains only ones ALL 1  in its bit pattern. Furthermore, the overhead OH of the data signal DS contains in the tandem connection monitoring field TCMF only ones ALL 1  as its bit pattern. The overhead subfields OHF 1 , OHF 2  of the data signal D 2  contain zeros. 
     The network node N 2  receives the data signal DS 2  and derives from the bit pattern ALL 1  within the tandem connection monitoring field TCMF that the data signal DS 2  carries an alarm signal that was generated within the path segment PS. Thus, the network node N 2  concludes that a failure of the tandem connection monitoring layer function of the path segment PS has occurred. Therefore, the network node N 2  enters into the overhead subfield OHF 2  information that indicates a failure of a tandem connection. This information is provided in the form of a bit pattern “01”, representing a one, entered into the overhead subfield OHF 2 . The network node N 2  then transmits the data signal DS 2  further on. 
     The network node N 3  receives the data signal DS 2 . The network node N 3  derives from the fact that the payload PL of the received data signal DS contains only ones ALL 1 , that the received data signal is an alarm signal. Thus, the network node N 3  derives that a transmission failure has occurred along the transmission path P. 
     The network node N 3  considers the information provided by the overhead subfields OHF 1 , OHF 2 . Since this information indicates that no data signal failure has occurred before the path segment PS, but that a tandem connection failure has occurred within the path segment PS, the network node N 3  concludes that the detected failure has occurred within the path segment PS. 
       FIG. 4  shows the network NT together with the network nodes located along the transmission path P and the path segment PS as already shown in  FIG. 1 . 
     Furthermore,  FIG. 4  shows a failure F 1  occurring before the path segment PS and also before a network node N, which is located before the path segment PS. 
     Furthermore, the  FIG. 4  shows transmitted data signals DS 1 ′, DS 2 ′ at the different stages of the network. 
     At the beginning of the transmission path P, the overhead subfields OHF 1  and OHF 2  contain bit patterns consisting of only zeros. 
     A network node N is placed along the transmission path P before the path segment PS. Between this network node N and the beginning BP of the transmission path P, a failure F 1  occurs. This failure F 1  is detected by the network node N, which therefore replaces the original data signal DS 1 ′ by a data signal DS 2 ′ carrying an alarm signal. The data signal DS 2 ′ is of the same structure as the data signal DS 2  previously described. 
     At the beginning BPS of the path segment PS, the network node N 1  receives the data signal DS 2 ′. By analyzing the bit pattern contained in the payload PL of the data signal DS 2 ′, the network node N 1  derives that an alarm signal is received. Therefore, the network node N 1  concludes that a failure of the data signal DS 1 ′ is present. Thus, the network node N 1  enters into the overhead subfield OHF 1  information indicating a data signal failure. This information is provided in the form of a bit pattern “01”, representing a one, entered into the overhead subfield OHF 1 . Furthermore, the network node N 1  generates parity information TCMN 1 , that is computed over the payload PL of the data signal DS 2 ′, and enters this information into the tandem connection monitoring field TCMF of the data signal DS 2 ′. 
     At the end EPS of the path segment PS, the network node N 2  receives the data signal DS 2 ′. The network node N 2  calculates over the payload PL of the data signal DS 2 ′ parity information and compares this calculated parity information with the parity information TCMN 1  given in the tandem connection monitoring field TCMF of the data signal DS 2 ′. Since no failure occurred along the path segment PS, the calculated parity information matches the parity information TCMN 1  given in the tandem connection monitoring field TCMF. Therefore, the network node N 2  concludes that no failure of the tandem connection along the path segment PS is present. Therefore, the network node N 2  does not modify the information within the overhead subfield OHF 2  of the overhead OH. 
     The network node N 3  receives the data signal DS 2 ′ and derives from fact that the payload PL of the received signal DS 2 ′ contains only ones, that an alarm signal is received and that a failure along the transmission path P has occurred. 
     The network node N 3  considers the information given in the overhead subfields OHF 1 , OHF 2 . Since the overhead subfield OHF 1  indicates a data signal failure, and since the overhead subfield OHF 2  indicates that no tandem connection failure has occurred, the network node N 3  concludes that the detected failure has occurred outside of the path segment PS. 
       FIG. 5  shows network nodes of a network NT′ placed along a transmission path P. The network NT′ shown in  FIG. 5  is in general the same as the network NT shown in  FIG. 1 , but contains furthermore a network node N 4  that is located after the beginning BP of the transmission path P and before the beginning BPS of the path segment PS. The network node N 4  performs a tandem connection monitoring source function, while the network node N 3  performs a tandem connection monitoring sink function that corresponds to the source function of the network node N 4 . Thus, the network nodes N 4 , N 3  perform a tandem connection monitoring layer function TCM 2  between them for monitoring the transmission of the data signal between them. 
     The network nodes N 1 , N 2  perform a tandem connection monitoring layer function, as previously described above, which is indicated in  FIG. 5  as a tandem connection monitoring layer function TCM 1 . 
     For performing the different tandem connection monitoring functions TCM 1 , TCM 2 , the overhead of the data signal contains multiple tandem connection monitoring fields. Each tandem connection monitoring function TCM 1 , TCM 2  is assigned an individual tandem connection monitoring field within the overhead. 
     The network node N 3  is able to detect a failure occurring between the network node N 4  and itself by performing the tandem connection monitoring sink function of the tandem connection monitoring layer function TCM 2 . According to an alternative solution, the network node N 3  concludes that a failure occurred between the network node N 4  and itself, in the case that a received data signal contains within its payload only ones, indicating the presence of an alarm signal. 
     The network node N 3  analyzes information provided by the overhead subfields OHF 1 , OHF 2  as previously described above, for determining whether a detected failure occurred within the path segment PS or outside of the path segment PS. 
     The network nodes N 1 , N, N 2 , which are located within the path segment PS, belong to a network domain DOMAIN  1  as indicated in  FIG. 5 . Other network nodes N 3 , N 4 , N, which are located along the transmission path P between the network nodes N 4 , N 3 , but which do not belong to the network domain DOMAIN 1 , belong to another network domain DOMAIN  2 , as indicated in  FIG. 5 . 
     By being able to derive whether a detected failure occurred within the path segment PS or not, the network node N 3  is able to decide whether upon detection of a failure protection and/or restoration actions have to be carried out within the DOMAIN  1  or within the DOMAIN  2 . The network node N 3  initiates protection and/or restoration within the DOMAIN  2  only in the case that the failure was determined not to have occurred within the path segment PS. 
     According to a further embodiment, the transmitted data signal is an optical data unit (ODU) as proposed by the standard ITU-T G.709/Y.1331 (March 2003), briefly called G.709. An ODU contains an overhead called optical data unit overhead (ODU Overhead), as well an optical payload unit (OPU). An OPU contains payload data and an optical payload overhead (OPU Overhead). 
     The data signal in the form of an ODU is transported within a data signal called optical transport unit (OTU). An OTU contains furthermore additional overhead called OTU Overhead, as well as a Frame Alignment Overhead. The frame alignment overhead 
     The overall structure OTU-OH of overhead information contained in an OTU can be found in G.709 in section 15.8.1, and is shown in  FIG. 6 . The overhead OTU-OH consists of the Frame Alignment Overhead, the OTU Overhead, the OPU Overhead and further fields, which belong to the ODU Overhead ODU-OH. 
     The ODU Overhead ODU-OH contains six different fields for storing parity information of six different tandem connection monitoring layer functions. These fields are marked within the overhead ODU-OH as the fields TCM 1 , TCM 2 , TCM 3 , TCM 4 , TCM 5 , TCM 6 . Thus, the overhead ODU-OH supports up to six different tandem connection monitoring layers. Furthermore, the overhead ODU-OH contains fault type/fault localization overhead bytes which are stored within a field marked as FTFL. This field FTFL is described in the standard G.709 within the section 15.8.2.5.1. 
     The standard G.709 does not propose to use all of the possible bytes within the field FTFL, but leaves a number of bytes free to be used for other purposes. As described in the standard G.709, section 15.6.2.2, a data signal may span multiple frames of OTUs. Such signals require multiframe alignment processing to be performed, in addition to the usual OTUk/ODUk frame alignment. Within the OTUk Overhead OUT-OH, a byte within the Frame Alignment Overhead in row1, column7 is reserved for enabling multiframe alignment. The value of this byte is incremented each OTUk/ODUk frame and provides as such a 256-frame multi-frame. Since the field FTLF of one frame contains one byte, all FTFL fields of a 256-frame mulit-frame contain 256 bytes. A structure of the multi-frame structure of the field FTFL is shown in  FIG. 7  in detail. 
       FIG. 7  illustrates, that the multi-frame structure of the field FTFL contains 256 bytes. Out of these 255 bytes, the last four bytes are used for carrying information indicating a failure of a data signal or indicating a failure of a tandem connection. Four different bytes provide 32 bits which are indexed within  FIG. 7  as the bits  0  up to  31 . Out of these 32 bits, 24 bits are used for storing the appropriate information for six different path segments monitored by six different tandem connection monitoring functions. A first overhead field TC# 1  is made up of the bits  0  to  3  for storing the appropriate information with regard to a first path segment. Further overhead fields TC# 2  are used to TC# 6  store the appropriate information with regard to further path segments monitored by further tandem connection monitoring functions. The bits  24  to  31  are not used and kept as spare bits. 
       FIG. 8  illustrates the detailed structure of the overhead field TC# 3  as an example. The field TC# 3  contains a first overhead subfield INCOMING AIS made up of two bits 0, 1 and a further overhead subfield TC ALARM made up of two further bits  2 ,  3 . In the case, that a network node at a beginning of a path segment detects a failure of a data signal, the network node enters into the corresponding overhead subfield INCOMNIG AIS the bit pattern ‘01’, which represents a one. In the case, that a network node located at the end of a path segment detects a failure of a tandem connection, the network node enters into the corresponding overhead subfield TC ALARM the bit pattern ‘01’, which represents a one. 
     The network nodes N, N 1 , N 2 , N 3  detect a failure of a data signal, as described in the telecommunication standard ITU-T G.798 (December 2006), briefly called G.798, within the section 14.2 and 14.5, by carrying out monitoring of the received data signal and deriving from fault conditions that a failure of the data signal is present. 
     An alarm signal in the form of a data signal is generated as described in G.709 in section 16.5.1. Such an alarm signal is from its structure the same as the transmitted data signal, but contains within its payload only ones and within its overhead also only ones, except for the fields Frame Alignment Overhead, OTUk Overhead and the field FTFL. 
       FIG. 9  shows the general structure of a network node NN. The network node NN contains a physical interface PI 1  for receiving a data signal in the form of an optical transport unit (OTU), as described in the telecommunication standard G.709. 
     Furthermore, the network node NN contains an OTU framer OTUF for obtaining from an optical transport unit OTU an optical data unit ODU. Furthermore, the network node NN contains a processing unit PU, which processes the optical data unit ODU. Within the processing unit PU, an ODU framer ODUF and an ODU overhead processor OHP is contained. The ODU framer passes on an optical data unit to an optical switching matrix OXC. From the optical switching matrix OXC, a data signal path is established by switching an incoming optical data unit coming from a processing unit to a further processing unit PU. 
     The processing unit PU contains an ODU framer ODUF and an optical data unit overhead processor OHP. The ODU framer ODUF passes on an optical data unit ODU to an OTU framer OTUF, which then passes on an optical transport unit OTU to the physical interface PI 2  for sending the data signal. 
     The general structure of a processing unit PU 1  of a network node located at a beginning of a path segment is illustrated in detail in  FIG. 10 a   .  FIG. 10 a    shows, that an ODU framer ODUF 1  exchanges information with a tandem connection processor TCP 1 , which is contained within the overhead processor OHP shown in  FIG. 9 . The ODU framer ODUF 1  complies with the standard G.798. The ODU framer ODUF 1  monitors the received data signal ODU and is configured such, that in case that the payload of the data signal ODU contains only ones, indicating an alarm signal which is equal to a data signal failure, the ODU framer ODUF 1  indicates the reception of the alarm signal to the tandem connection processor TCP 1 , by providing appropriate alarm information ALARM. The ODU framer ODUF 1  furthermore monitors the received data signal, and derives from conditions, which are described in the telecommunication standard G.798 within the section 14.2 and 14.5. in detail, whether a data signal failure is present. If a data signal failure is present, the ODU framer ODUF 1  indicates this to the tandem connection processor TCP 1  via the alarm information ALARM. 
     In case of receiving alarm information ALARM, indicating a data signal failure, the tandem connection processor TCP 1  writes information INC AIS, indicating a data signal failure, into the overhead of the data signal ODU. 
     The tandem connection processor TCP 1  performs furthermore a tandem connection monitoring sink function, by computing parity information TCPI over the payload of the data signal ODU and writing this parity information TCPI into the overhead of the data signal ODU. 
     The general structure of a processing unit PU 2  of a network node located at an end of a path segment is illustrated in detail in  FIG. 10 b   .  FIG. 10 b    shows, that an ODU framer ODUF 2  exchanges information with a tandem connection processor TCP 2 , which is contained within the overhead processor OHP shown in  FIG. 9 . The ODU framer ODUF 2  complies with the standard G.798. The ODU framer ODUF 2  extracts tandem connection parity information TCI from the overhead of the data signal ODU and provides this information TCI to the tandem connection processor TCP 2 . 
     The tandem connection processor TCP 2  computes further parity information over the payload of the data signal ODU. In the case, that the computed parity information is not equal to the provided parity information TCI, or in the case that the provided parity information contains a bit pattern of only ones, the tandem connection processor TCP 2  concludes that a failure of the tandem connection is present. Thus, the tandem connection processor TCP 2  writes information TC ALARM indication a failure of a tandem connection into an overhead field of the data signal ODU. 
     A network node NN′ located after a path segment is shown in  FIG. 11 . The network node NN′ contains an interface PI for receiving an optical transport unit OTU. An OTU framer OTUF retrieves from the OTU an optical data unit ODU and passes the ODU on to a processing unit PU. The processing unit PU contains an ODU framer ODUF and an overhead processor OHP. The ODU framer ODUF complies with the standard G.798. The ODU framer ODUF extracts overhead information from the overhead of the ODU. 
     The ODU framer ODUF monitors the received data signal ODU and is configured such, that in case that the payload of the data signal ODU contains only ones, indicating an alarm signal which is equal to a data signal failure, the ODU framer ODUF concludes that a transmission failure is present. The ODU framer ODUF furthermore monitors the received data signal, and derives from conditions, which are described in the telecommunication standard G.798 within the section 14.2 and 14.5. in detail, whether a transmission failure is present. 
     The overhead processor determines from information contained in the overhead of the data signal ODU, whether a detected transmission failure occurred within a path segment or outside of a path segment.