Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
2001-10-23
2004-12-07
Tu, Christine T. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S701000
Reexamination Certificate
active
06829741
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to communication systems, and more particularly, to techniques for applying forward error correction to frames of a synchronous optical communication system.
BACKGROUND OF THE INVENTION
In general, the noise associated with a communication channel can cause transmission errors. The error probability of a digital communication system is the number of errors per total number of bits received. For instance, if one error bit per 100 bits occurs, the error probability is {fraction (1/100)}, or 10
−2
. Forward error correction (FEC) techniques, such as Reed-Solomon or BCH encoding, employ the concept of redundancy in order to maintain an acceptable error rate. In this context, redundancy is the fraction of total message content that can be eliminated without loss of essential information. A FEC code is an encoding algorithm that uses more signal elements (e.g., redundant bits) than necessary to represent the essential information to be communicated. The intended receiver can decode the transmitted information (including both essential information and redundant elements) using a complementary FEC decoding algorithm to recover the intrinsic information. By using redundancy, the probability of error is spread out over a greater number of signal elements, and a more robust communication is generally achieved.
FEC and Optical Transmission
Historically, the use of forward error correction (FEC) in the context of transmitting optical signals has been limited. FEC has been used, for instance, in submarine systems to decrease the number of necessary 3R (re-time, re-shape, re-transmit) regenerators for transoceanic transmission of optical signals. Recommendation G.975 of the Telecommunications Standards Section of the International Telecommunication Union (ITU-T) addresses this particular application. FEC is also being used in the context of dense wavelength division multiplexing (DWDM) optical systems. By using FEC in such systems, it is possible to increase the number of the transmitted colors significantly thereby resulting in an increase in the overall transmission rate of the optical system.
Recommendation G.975 Coding
Recommendation G.975 defines the architecture of an encoder. The encoder produces FEC frames. A digital de-multiplexer and a multiplexer are used before and after the encoding stage, respectively. The multiplexer effects the interleaving operation. The de-multiplexer, on the other hand, provides a complementary de-interleaving operation needed for data integrity. The corresponding decoder architecture uses the same digital multiplexer and digital de-multiplexer. The input of the decoder is passed through a de-multiplexer to implement de-interleaving stage resulting in N parallel signals. After passing these signals through a bank of N RS decoders, they are multiplexed back to the original signal. A typical value for N, for example, is 16 corresponding to an interleaving of depth sixteen.
In applying Recommendation G.975, a coding gain of about 6 dB can be achieved. This coding gain translates into transforming a signal with a bit error ratio (BER) of about 10
−4
to a signal with a BER of about 10
−15
. The input signal to the encoder is a multiple of STM-
16
(synchronous transport module level
16
) where each STM-
16
is processed in parallel. A SMT-
16
frame has a bit rate of 2.48832 gigabits-per-second (Gbps). The input signal (referred to as payload or STM-
16
data) is first segmented into blocks of 238 bytes each. After adding an overhead byte, the resultant 239 bytes are passed through a Reed-Solomon (
255
,
239
) encoder thereby generating FEC frames of 255 bytes each. Based on the properties of Reed-Solomon (RS) redundancy codes, the added 16 bytes of parity can correct up to eight erroneously received byte-symbols within that FEC frame.
If the number of byte-symbols in error in a received FEC frame is greater than eight, then the decoder is not able to correct the errors and decoding failure is declared. To reduce such “burst errors,” it is therefore necessary to use interleaving where bytes from each FEC frame are spread during transmission so that the probability of receiving more than eight bytes in error for each FEC frame is minimized. Recommendation G.975 proposes blocks of N consecutive FEC frames where the first column of the frames is transmitted first followed by the second column, and so on. Using an interleaving of depth N, it is possible to correct burst error lengths of up to 8N bytes. The penalty paid for this error correction capability is the extra 255*N bytes of delay introduced by the interleaver.
Optical Transport Network (OTN)
In general, optical networks operate to provide transport, multiplexing, routing, supervision, and survivability of client signals that are processed in the physical domain. The optical transport network (OTN) initiative of the ITU-T (as well as the T1X1.5 technical subcommittee) uses the extra overhead byte added to each FEC frame to propose a new digital hierarchy (e.g., ITU-T G.709) and the corresponding network architecture (e.g., ITU-T G.872). To form an OTN frame, the interleaving depth is set to 16 to arrive at a FEC super-frame. This super-frame has 16 bytes of overhead followed by 3808 bytes of payload. The tail end of this super-frame includes 256 parity bytes. Four super-frames are used to form an OTN frame. The OTN frame is defined for three different line rates k (where k=1, 2 and 3, corresponding to line rates of 2.5, 10 and 40 Gbps, respectively). Note that the format of an OTN frame is the same for all the line rates, but the frame rate depends on the line rate. For purposes of clarity, this line rate k is distinct from and not to be confused with the pre-encoding block length k, which is used in reference to the selected coding algorithm. Synchronous Optical Network (SONET)
One type of client signal in an OTN is generally referred to as synchronous optical network (SONET). SONET is defined by a set of American National Standards Institute (ANSI) standards defining the rates and formats for synchronous optical networks. Such standards include ANSI T1.105, ANSI T1.106 and ANSI T1.117. In addition, Telecordia Technologies' GR-253-CORE standard provides generic requirements on SONET transport systems. Similar standards, referred to as Synchronous Digital Hierarchy (SDH), have been established by the ITU-T (e.g., G.707). Note that SONET and SDH are technically compatible standards having differences that are readily understood by one skilled in the art. The SONET frame rate is always the same (e.g., 8000 frames/sec) regardless of the line rate.
SONET's synchronous, flexible, optical hierarchy allows many signals of different capacities to be carried by a transmission system. This is accomplished by a byte-interleaved multiplexing scheme. Generally, byte-interleaving simplifies multiplexing, and facilitates end-to-end network management. The first step in the SONET multiplexing process involves the generation of the lowest level or “base” signal. In SONET, this base signal is referred to as synchronous transport signal level-
1
(STS-
1
).
The STS-
1
signal operates at a bit rate of 51.84 Megabits per second (Mbps). The bit rate of each of the higher level signals is the signal integer level number multiplied by 51.84 Mbps. Thus, a hierarchal family of STS-N signals provided as illustrated by Table 1. An STS-N signal is formed from N byte-interleaved STS-
1
signals. The base signal for a SDH system is referred to as synchronous transport module level-
1
(STM-
1
), which has a bit rate of 155.52 Mbps. The hierarchal family of STM-M signals is also illustrated by Table 1. Note that N typically ranges from 1 to 192, where only particular values are recognized (e.g., 1, 3, 12, 48, and 192). However, higher values of N, as well as other recognized values are possible. Note the relationship of M to N (M equals N divided by 3).
TABLE 1
SONET STS Hierarchy
Signal
Bit Rate
Capacity
STS-1, OC-1
51.840 Mbps
28 DS1s or 1 DS3
STS-3, OC-
Abaye Alireza
Khansari Masoud
Centillium Communications Inc.
Fenwick & West LLP
Tu Christine T.
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