AU688693B2 - Method for identification of a path trace and of a section trace in SDH signal sequences - Google Patents

Description

S F Ref: 353814

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIRCATION FOR A STANDARD PATENT

ORIGINAL

909 *9 90 9 0**9 909 9*~ 9 9 *99 Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Siemens Aktiengesellschaft Wittelsbacherplatz 2 80333 Muenchen

GERMANY

Wolfgang Drews Spruson Ferguson, Pater: Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Method for Identification of a Path Trace and of a Section Trace in SDH Signal Sequences The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845 I I 1 Description Method for identification of a path trace and of a section trace in SDH signal sequences A synchronous digital hierarchy SDH which is integrated in a communications network allows the transportation of various signal sequences, which are characterized by different bit rates and structures, without changes having to be made to the entire communications network in each case. Transmission of signal sequences at different bit rates or with different structures became possible by means of a subsignal form.

This sub-signal form is formed by so-called virtual containers The virtual containers are transported through the network nodes of the communications network 15 by leans of synchronous transport modules (STM). A virtual container comprises a path overhead POH and a container A virtual container is characterized by a path trace. The path trace is transmitted in the path overhead (J1 byte) and this is assigned to the container after receipt of the wanted signal, and remains with it until the wanted load is demultiplexed. In general, a N' path comprises more than one transmission section. In the synchronous digital hierarchy, these transmission sections are called multiplex or regenerator sections, 25 which are each characterized by a section trace, which is noted in a section overhead. The section overhead is part of an STM frame. Recommendations G.708, G.709 and G.783 Sdescribe the characterization of a virtual container and of a regenerator section. Operating, administration and maintenance functions are based, inter alia, on an evalfation of the path overhead or section overhead.

Furthermore, according to CCITT, one byte of a section overhead (JO byte) is provided, which defines the transmission of a section trace in the synchronous transport modules.

A transmission format which comprises a sequence of 16 bytes is desirable for both trace types I s 2 path/section. The transmission of the bytes takes place in 16 successive frames in the respective overhead byte.

The trace is monitored for correctness in the synchronous digital hierarchy network elements of the communication network at the termination points of the transmission section at the receiving end and at possible internal observation points in multiplex equipment and cross connectors. This monitoring is carried out by comparing the received trace with the trace to be expected in each case, which is predetermined by the equipment operator in the configuration of the individual network elements. In the event of mismatching, an alarm is produced, an Alarm Indication Signal AIS signal being transmitted instead of the received wanted signal in the transmission direction, and a Far End Receive Failure FERF indication being transmitted in the opposite direction to the transmission direction.

The object of the invention is to specify a further method for identification of a path trace and of a section trace in the case of SDH signal transmission.

The object is achieved according to the invention by the features stated in Patent Claim 1.

The measures according to the invention result in the advantage of reliable and robust identification of a correct trace.

The method furthermore provides the advantage that false alarms during or after immediate reproduction of a signal interruption if the trace is still unknown are avoided.

30 The method provides the further advantage that little computation complexity is required to support and to evaluate a trace.

Further advantageous developments of the invention are stated in the subclaims.

Further pecial features of the invention can be seen from the following relatively detailed explanatior.

of an exemplary embodiment, with reference to schematic drawings in which: Fig.' 1 shows a block diagram, 3 Fig. 2 shows a flow chart for calculation of the trail status, Fig. 3 shows a flow chart for deciding whether a trace can be defined, Fig. 4 shows a flow chart for deciding whether the previous trace should be confirmed or a new trace should be defined, Fig. 5 shows a flow chart for definition of a new trace, Fig. 6 shows a flow chart for confirmation of an existing trace, and Fig. 7 shows a flow chart for deciding whether the current trace should be compared with the expected trace.

In the case of the method according to the invention, path/section trace bytes which are selected by an application-specific integrated circuit ASIC from the *data flow are investigated under microprocessor control with respect to reliable and robust identification of a trace. If a trace mismatch is reliably present, an Alarm Indication Signal AIS is overlaid, under microprocessor control, in the receiving direction and a -signalling activity Far End Receive Failure FERF is initiated in ,the opposite direction to the receiving direction.

The method described in the following text is based on an algorithm which can be designed for implementation in hardware and software.

i. The function of the hardware in the ASIC module is to extract the trace byte (JI/JO) of the respective SDH signal in successive STM frames and to store it in a 30 byte-oriented data buffer RAM (see Fig. 1) A 3 x 60 byte RAM buffer may be used, for example, for this purpose in order to allow a persistence check, which will be described in the following text, in a simple manner.

Furthermore, the ASIC module has monitoring sensors for identification of signal interrtptions within a path or in a regenerator section. These monitoring sensors signal a Loss of Signal LOS, Loss of Frame LOF, Multiplex Section AIS MS-AIS, Loss of Pointer LOP and AU-AIS, and, in addition, a control signal for 4 overlaying the Alarm Indication AIS and a control signal for overlaying the Far End Receive Failure FERF signal (only in the case of a path) are produced in accordance with program procedures. The monitoring sensors and the contents of the trace buffer are interrogated via a processor.

Fig. 1 describes a higher level structure of the algorithm on the basis of function blocks and the data flow which takes place between the function blocks. The monitoring sensors TAD are used for identification of the signal interruptions mentioned above. These are interrogated cyclically by a processor, which is not illustrated explicitly here, using a first interrogation interval T1, and recorded signal interruptions are stored in a defect status register DSR. Furthermore, an alarm TRC-MISMATCH state TRC-MM which is still present is read from a state memory SM and is likewise stored in the defect status register DSR. The contents of the defect status register DSR are further processed in the function 20 block or Trail Status Evaluation TSE module. The alarm 'JRC-MISMATCH state TRC-MM is defined in this function block, and the overlaying of the AIS signal is also controlled by the state variable SET-AIS.

Furthermore, the trail status evaluation module TSE supplies the following state variable TRL-AV and TRC-RX- ST, which are used for controlling the sequences in the C. etrace evaluation TE function block: TRL-AV: State of the transmission route (path/section) under consideration, this route being S.i 30 designated as "available" or "unavailable".

TRC-RX-ST: Receiving state of the trace. This can assume one of the values "confirmed", "unconfirmed" or "unknown".

The trace evaluation TE function block represents the core of the evaluation algorithm for th trace (path/section). This module TE is likewise initiated cyclically by the processor, but asynchronously with respect to the first interrogation interval on the "Trail Status Evaluation" function block TSE, using a second 0 5 interrogation interval T2. As a result of the greater computation complexity, it is expedient for this module TE block to be initiated less frequently than the first interrogation interval (T2 Tl). The module TE likewise influences the state signals TRC-RX-ST and the alarm signal TRC-MISMATCH. The module TE furthermore supplies the trace (TRC-RX) which is identified in the data stream.

The "Trail Status Evaluation" TSE and "Trace Evaluation" TE modules are described in detail in the following sections.

The program sequence of the "Trail Status Evaluation" TSE module which is illustrated in Figure 2 initially checks the availability of the transmission 135 section using the signaled defects on the transmission route: LOS (Loss of Signal, for availability of the section and path) LOF (Loss of Frame, for availability of the section and 20 path) MS-AIS (Multiplex Section AIS, only for availability of the path) The availability of a regeneration section is also given on reception of MS-AIS; for the section trace, the situation is that, in accordance with ITU-T, the corresponding trace bytes are reset in each regenerator .'section.

LOP (Loss of Pointer, only for availability of the path) AU-AIS (only for availability of the path) If at least one of the signaled defects is present in a transmission section under consideration, the following actions are carried out: The state variable TRL-AV is selected to "false" (transmission section not available), in order in this way to inhibit the evaluation of the trace in the "Trace Evaluation" module or function block.

The state variable TRC-RX-ST is selected to "unknown" in order to initiate redefinition cf the trace after 6 the defect disappears.

The state variables TRC-MISMATCH and SET-AIS are set to "false" in order to avoid erroneous alarm production and AIS/FERF overlaying, particularly in the transitional time after disappearance of the abovementioned defects, for as long as it has not been possible to redefine the trace.

If none of these defects which relate to the transmission section under consideration is currently present, the following actions are carried out: The state variable TRL-AV is set to "true" (transmission section available) in order thus to enable the evaluation of the trace in the "Trace Evaluation" function block.

If the value of the state variable TRC-MISMATCH is "true" and overlaying of AIS/FERF has been enabled for this case by a configuration parameter set by the equipment operator, AIS/FERF is initiated by setting the state variable SET-AIS to "true".

In those program steps of the Trace Evaluation Smodule TE which are illustrated in Figure 3, the instantaneous availability of the transmission section is interrogated using the state signal TRL-AV, whose value is that which has been defined in the "Trail Status Evaluation" function block. In the event of nonavailability, no further actions are carried out and all the output parameters retain their previous values.

If the transmission section is available, the "Search for Trace" and "Trace Mismatch Detection" 30 subroutines are carried out successively. The program steps carried out there lead to the definition of the state variables TRC-MISMATCH, TRC-RX-ST and of the current trace TRC-RC.

Figure 4 illustrates the program module Search for Trace SFT. This program module controls the definition of the current trace, this being carried out in a different manner depending on the current value of the state variables TRC-RX-ST.

For TRC RX ST "unknown" (trace unknown): 7 In this state, the "Find new Trace" function block is called up for the initial or renewed definition of the trace. If definition is successful, the newly found value TRC-NEW is stored as the current trace in the variables TRC-RX, and the state variable TRC-RX-ST is set to "confirmed". If the definition is not successful, the previous state is retained, so that the definition is attempted once again when the algorithm is next called up.

For TRC RX ST "confirmed": In this state, it is assumed that the current trace TRC- RX was either found or confirmed when the algorithm was last called up. The "Confirm current Trace" function block is thus called up in the expectation of confirming the previous value once again. A less strict criterion than that for redefinition is use' for confirmation (see description Fig. If confirmation is successful, the 'previous state is retained.

If confirmation is unsuccessful, the state S variable TRC-RX-ST is set to "unconfirmed", but the previous trace TRC-RX is still regarded as valid and thus remains unchanged.

For TRC RX ST "unconfirmed": In this state, it is assumed that the current trace TRC- RX was not confirmed when the algorithm was last called up. The "Confirm current Trace" function block is thus called up again, in the expectation of confirming the previous value once again. If confirmation is successful, the state variable TRC-RX-ST is set to "confirmed", and 0 the previous trace continues to remain valid. If confirmation is once again unsuccessful, the state variable TRC-RX-ST is set to "unknown", and the previous trace is thus declared to be invalid, so that a redefinition is initiated when the algorithm is next called up.

This state-controlled monitoring of the trace results in the redefinition of the trace being initiated at the start and after identification of signal interruptions while, in contrast, in interruption-free "II- 8operation, a redefinition is not initiated until the current trace has not been confirmed twice. This results in a robust behaviour for a trace subject to interference in the case of short bit error bursts and in the case of statistical error rates up to 10E-3.

Figure 5 illustrates the program sequence of the Find new Trace program module FNT, in which the following steps are carried out to redefine the trace: 48 trace bytes from successive frames of the SDH signal are read into the trace buffer of the ASIC.

Subsequently, the trace buffer of the ASIC is read from by the microprocessor in the form of 3 x 16-byte data blocks, each of which normally contains the trace but in a phase which is still unknown.

A persistence check of the 3 data blocks is carried out with the condition that the mutually corresponding .,.bytes in each data block are exactly the same. If the persistence check fails, then the trace is regarded as not having been found.

If the persistence check is successful, then any one of the 3 data blocks is extracted for further processing.

An attempt is made in the extracted data block to find the byte whose MSB is 1 as the start byte of the trace (byte 1 in Table 1) If this is not successful, then the trace is regarded as not having been found, otherwise the 16-byte sequence with the identified start byte is regarded as the newly found trace.

The evaluation of CRC bits which are defined by the Standard and are contained in the start byte becomes 30 superfluous as a result of a persistence check of the 3 x 16-byte data blocks since a very robust behaviour is achieved just in this way for transmission bit errors having the following characteristics: In the event of a bit error rate of 10 3 in the transmission section unde 7 consideration, the identification of a false trace (apparently identified trace does not match the trace existing in the signal) occurs with a probability of 8.7 x 10 8 In the case of a bit error rate of 10- 3 in the 9transmission section under consideration, the trace which is present in 'the signal is identified with a probability of 0.68.

In the Confirm current Trace module CCT which is shown in Figure 6, the confirmation of an already known trace is illustrated using the following method: 48 trace bytes from successive frames of the SDH signal are read into the trace buffer of the ASCI. The trace buffer of the ASIC is then read from by the microprocessor in the form of 3 x 16-byte data blocks, each individual one of which normally contains the trace, but in a phase which is still unknown.

The i-th 16-byte data block is in each case extracted in a loop with the numerical index i=1, 3, and an attempt is made to find the byte where MSB is 1 as the start byte of the trace. If this is not successful, then the following (i+l)-th data block is extracted and o. a correspondingly investigated. If the start byte has been found, then the 16-byte sequence with the 20 identified start byte is regarded as a found trace and is compared with the previously known trace -TRC-RX. In the event of matching, the previous trace is regarded as being confirmed, and in the event of mismatching, t; the (i+1)-th data block is extracted and the procedure corresponds to that above.

The check of the CRC bit using a CRC-7 check need not be carried out since this is already covered in the confirmation of the current trace TRC-RX.

If no trace which matches the current value TRC-RC is found in any of the 3 x 16-byte data blocks, then the trace is regarded as being unconfirmed.

The following behaviour in the case of transmission bit errors is achieved in conjunction with the "Search for Trace" function block: In the case of a bit error rate of 10' in the transmission section under consideration, nonconfirmation of the current trace twice successively (this leads to the state of an unknown trace as a result of TRC-RX-ST being set to "unknown" and 10 initiates a redefinition) occurs with a probability of 3 x 10- 6 The value of the state variable TRC-MISMATCH is determined in the Trace Mismatch Detection module TMD illustrated in Figure 7, as follows: If the monitoring of the trace is enabled by a configuration parameter set by the equipment operator, the following interrogations are also carried out, otherwise the state variable TRC-MISMATCH is set to "false" (alarm off).

If the state variable TRC-RX-ST has the value "unknown" (current trace unknown), then TRC-MISMATCH is set to "true" (alarm on).

If the state variable TRC-RX-ST has the value "confirmed" or "unconfirmed" (current trace known), then the current trace is compared with the expected trace (configured by the equipment operator), and the state variable TRC-MISMATCH is set to "false" (alarm off) or "true" (alarm on) in the event of a match or 20 mismatch respectively. The comparison can be limited to bytes 2 to 16 of the trace, since the CRC bits are likewise the same in the event of matching.

The error probabilities quoted in the preceding descriptions of the figures are calculated using the following formulae: The calculation formulae for the error probabilities stated in the "Find new Trace" and "Confirmed Current Trace" function blc, are derived in the following text. N is assumed to be the number of bits 30 which make up the trace. In this exemplary embodiment, N is assumed to be 8 x 16 128, and p is assumed to be the probability of a transmission bit error, the limit for p being assumed to be 10" 3 in this case. The number of corrupted trace bits is given by a binomial distribution in the case of statistically independent bit errors.

A persistence check of three received trace blocks should be carried out for identification of the trace in accordance with the function block which is described under Figure 5. On the assumption that each I 11 block is interfered with by exactly the same bit error pattern, it is possible to identify a false trace. If, in this case, one considers all the n bit error patterns having only one bit subject to interference (this is the "most probable" case), then the probability of identification P, is given by:

P

1 N [p(l 3 8.7*10-8 (1) The correct trace is identified on reception of 3 error-free trace blocks. The probability of identification P 2 in this case is given by:

P

2 (1 p) 3 N 0.68 (2) Non-confirmation occurring twice in accordance with the function block which is illustrated under Figure 6 is carried out on reception of 6 trace blocks which are subject to interference from random bit error patterns. This is given by the probability P 3

P

3 [1 (1 6 3*10-6 (3) The method is designed in particular for a mixed implementation in hardware and software, as occurs 20 conventional SDH technology implementation forms, but a pure hardware implementation is also conceivable. In the case of the software-aided implementation, it is particularly advantageous that it is possible to dispense with the computation-intensive evaluation of CRC-7.

a *i 00

Claims (4)

1. A method for identification of a path trace or section trace which belongs to a data package and is transmitted within a predetermined frame structure in a synchronous digital hierarchy in a communications network, said method comprising the steps of: selecting trace and buffering data and fault messages from an overhead of the data package, checking the availability of a transmission section in a first program module; passing said selected trace data to a second program module; and 10 determining a current trace, as well as its state variable in accordance with subroutine procedures. I 2. The method according to claim 1, wherein: said selected fault messages are read out and evaluated in accordance with said first pg!'gram module, an alarm state is determined and the overlaying of an alarm 15 signal is initiated on the basis of an alarm state variable, and further state variables of S: the trace are buffered in a state memory unit in order to control the subroutine procedures which are assigned to the second program module.

3. The method according to claim 1, wherein the availability of said 'transmission section is checked, using a first state variable, in said second program module in a first program section, and if the first state variable is not available, no further program steps are carried out and all the output parameters of the second program module retain their previous values, and if the first state variable is available, a first and a second subroutine procedure are carried out, said further state variables of the trace being determined and being stored in s.'d state memory unit.

4. The method according to one of the preceding claims, wherein: in said first said subroutine procedure, a definition of said current trace is determined on the basis of a current value of said second state variable, wherein said e- value indicates the received state of a trace. N:\LIB)pOO0777:SEC I I

13- The method according to one of the preceding claims, wherein: an alarm is produced by a second subroutine procedure if a state variable which indicates a current trace is not found, and if said second state variable is known, this state variable is compared with an expected trace and said state variable which initiates said alarm is withdrawn in the event of matching, or an alarm is set in the event of a mismatch. 6. The method according to one of the preceding claims, wherein: a comparison is carried out with a respectively to be expected trace and said trace assigned to an equipment, an alarm signal being carried out in an onward transmission direction in the event of n.ismatching. 7. Method according to one of the preceding claims, wherein: trace bytes of the operating data of a respective SDH signal are extracted, and a memory unit is provided which buffers said extracted trace bytes, and a monitoring unit is provided for identification of signal interruptions or fault messages. 15 8. A method for identification of a path trace or section trace which belongs to a data package and is transmitted within a pre-determined frame structure in Sa synchronous digital hierarchy in a communications network substantially as herein described with reference to Figs. 1 to 7. C DATED this Thirtieth Day of December 1997 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON IN:AL.ppl00777SEC Abstract Method for identification of a path trace and of a section trace in SDH signal sequences Method for reliable and robust identification of a path trace and of a section trace, possible transition states before and aftor signal interruptions also being identified, so that the initiation of a false alarm and the fault signalling activities linked thereto are avoided. Fig. 1 *9*9 6eer 4 *e* «e* dr f« a.