Error detection/correction and fault detection/recovery – Pulse or data error handling – Transmission facility testing
Reexamination Certificate
2000-05-23
2003-12-02
Ton, David (Department: 2784)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Transmission facility testing
C714S715000
Reexamination Certificate
active
06658607
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of detecting troubles or non stable transmission in transport telecommunication networks based on either SDH or SONET standards, and in particular to a method of determining a trace indication mismatch in a path.
BACKGROUND OF THE INVENTION
Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH) describe two families of closely related and compatible standards that govern interface parameters; rates, formats and multiplexing methods; operations, administration, maintenance and provisioning for high-speed signal transmission. SONET is primarily a set of North American standards with a fundamental transport rate beginning at approximately 52 Mb/s (i.e., 51.84 Mb/s), while SDH, principally used in Europe and Asia, defines a basic rate near 155 Mb/s (to be precise, 51.84×3=155.52 Mb/s). From a transmission perspective, together they provide an international basis for supporting both existing and new services in the developed and developing countries.
For transmitting data, SDH and SONET use frame formats transmitted every 125 &mgr;s (8000 frames/sec). Because of compatibility between SDH and SONET, their basic frames are similarly structured, but differ in dimension which fact reflects the basic transmission rates of 155.52 and 51.84 Mb/s, respectively. To be more specific, a basic frame format of SDH is 9 rows of 270 bytes, or 2430 bits/frame, corresponding to an aggregate frame rate of 155.52 Mb/s. For SDH systems, the mentioned basic frame transmitted at the rate 155.52 Mb/s forms the fundamental building block called Synchronous Transport Module Level-1 (STM-1). For SONET systems, the basic frame has dimensions of 9 rows by 90 byte columns and, being transmitted at the rate 51.84 Mb/s, forms the appropriate fundamental building block called Synchronous Transport Signal Level-1 (STS-1).
Lower rate payloads (data portions transmitted at rates smaller than the basic ones) are mapped into the fundamental building blocks, while higher rate signals are generated by synchronously multiplexing N fundamental building blocks to form STM-N signals (in SDH) or STS-N signals (in SONET).
Each basic frame in SONET or SDH comprises an information portion called Information Payload and a service portion called Overhead (OH), the latter being subdivided into a number of areas of overhead bytes (for example, Path Overhead—POH, Transport Overhead—TOH) predestined for various service and control functions. One of such areas is a column of Path Overhead (POH) usually residing within the Information Payload area. POH supports performance monitoring, status feedback, signal labeling, user channel and a tracing function in a path. This overhead is added and dismantled at or near the service origination/termination points defining the path, and is not processed at intermediary nodes. The SDH multiplexing structure, defined in ITU-T Recommendation G.707(03/96), comprises so-called virtual containers serving to combine lower rate payloads by mapping into these containers and adding POH. The combined payloads fitted with POH are further aligned and multiplexed in order to form an STM-N signal.
According to ITU-T Recommendation G.707(03/96), the POH of virtual containers consists of a plurality of bytes, most of which are used for end-to-end communication.
When an error is detected at a path terminating equipment (PTE), an Alarm Indication Signal (AIS) is to be transmitted to the downstream network elements, and a signal of Remote Defect Indication (RDI)—towards the upstream network elements. The AIS is stated to be in the form of all “1”-s sent in the payload associated with this path, and the RDI is a code sent in the path status byte G1 assigned in the POH for this purpose.
One of the important bytes of the POH is a Path Trace byte called J1. This byte is used to transmit repetitively a Path Access Point Identifier so that a path receiving terminal can verify its continued connection to the intended transmitter. In transport networks operating according to SONET, J1 is used to send a repetitive signal to form a 64 byte string (trace), while in networks utilizing SDH the repetitive signal produced by J1 byte is preferably in a format of 16 byte string (trace). However, in the SDH standard there is an option of a 64 byte free format string, and where the 16 byte format is transferred in the 64 byte field, it shall be repeated four times. The path terminating equipment (PTE), depending on the standard in use, must therefore be able to continuously compare either a 16 byte long string, or a 64 byte long string with an expected code of the J1 string (trace).
Traces having a standardized length of 16 bytes are created in all major layers of connection known for the SDH standard, for checking stability of transmission in a particular layer. For example, byte J0 of the SOH is used for producing such a standard trace at the Section layer for detecting troubles in transmission between a terminal element and the associated repeater; byte Z5 optionally produces a 16 byte long tandem trace at a so-called Tandem Connection layer and byte J2 is used for forming a 16 byte trace for checking transmission between SDH containers at a so-called Tributary layer.
A CRC
7
polynomial algorithm is presently utilized in SDH for detecting errors in the traces. It should be noted, that the CRC
7
procedure requires the beginning (and the end) of the trace under examination to be strictly defined, i.e., its result depends on which particular byte and bit is considered the beginning of the cyclically repeating trace.
In SONET, only the above-described J1 byte of POH is presently defined as a byte creating a standardized 64 byte trace (though not equipped with the CRC
7
procedure) for detecting troubles in the path transmission. However, the standardization process has a pronounced tendency to harmonization, and nowadays a number of developments to the SONET standard is being discussed by specialists. In all likelihood, the creating and checking of some additional traces will be proposed for SONET (and probably, for SDH) in the course of such developments.
U.S. Pat. No. 5,701,293 describes a trouble surveillance apparatus for a transmission system, comprising an input memory means for storing data representing lines and paths of the transmission system. Every time such data is input in the memory, a comparison element compares the previous state of the system stored in the memory with a state of the system currently input in the memory. If no change is found between them, the current state of the system is kept the same as the previous state thereof. It is therefore judged that no new trouble is caused. If any change is found, a trouble judging element seeks trouble corresponding to the change to store data as a link in a trouble memory. Unless any change is found, the previous input data is only compared with the currently input data. The algorithm is supposed to be effective also for high speed applications. On the one hand, it can appear to be too sensitive in cases where insignificant transition processes, temporarily affecting data, take place in the layer. On the other hand, the described method would require quite a large memory means at a terminating element, if long messages, such as 16 byte or 64 byte J1 strings, were to be compared directly. Moreover, a system designed to directly compare messages of any specific predetermined length is inapplicable “per se” for comparing messages having a different length.
Standards and Recommendations dealing with SDH and SONET transmission systems do not describe or suggest any universal method of checking data strings suitable both to SDH and SONET.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a universal process for detecting troubles in data transmission in SONET or SDH telecommunications systems by checking traces produced at various layers of the system and having standard leng.
One of the particular objects of the invention is to provide an
Dabby Amir
Michel David
Browdy and Neimark , P.L.L.C.
ECI Telecom Ltd.
Ton David
LandOfFree
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