Coded data generation or conversion – Digital code to digital code converters – To or from nrz codes
Reexamination Certificate
2002-02-28
2003-09-30
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
To or from nrz codes
C341S050000
Reexamination Certificate
active
06628213
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital-signal coding and decoding, and more particularly, to coding and decoding of digital signals with the use of a Coded-Mark-Inversion (CMI) code.
2. Description of the Related Art
In the CMI code specified by Recommendation G.703-Annex A of the International Telecommunication Union (ITU-T) and others, block encoding is performed such that a “0 (L)” bit having a period of T in a binary digital signal (having “0” or “1”, or “L” or “H”) is converted to two bits “01 (LH)”, each bit having a period of T/2, and a “1 (H)” bit having a period of T is converted alternately to two bits “00 (LL)” and two bits “11 (HH)”, each bit having a period of T/2. In the present specification, a bit having a period of T or two bits each having a period of T/2 is called a time slot in some cases.
With the use of the CMI code, the transmission rate of a digital signal is substantially doubled. However, bit sequence independence (BSI) is obtained for consecutive 0's and consecutive 1's in an input signal, the size of hardware required for coding/decoding can be reduced, and transmission errors can be easily checked by code rule checking. The CMI code has been widely used in small- and medium-capacity fiberoptic transmission systems, intra-office transmission systems, fiberoptic access systems, optical data links, and others having transmission rates from several megabits per second to several tens of megabits per second.
The CMI code is a code with redundancy, in which the code is made to superpose a lower speed signal than the main, according to a rule called coding rule violation (CRV). More specifically, CRV is used for a system to transmit a supervisory signal, an 8-kHz frame signal, a voice signal, or a data signal by superimposing them on a main signal. CRV includes CRV0, in which “0 (L)” in the main signal is coded to “10 (HL)”, and CRV1, in which “1 (H)” in the main signal is coded to either “11 (HH)” or “00 (LL)”, whichever has the same polarity as a code to which the previous “1” was coded.
In the above description, signal states “0” and “1” are associated with “L” and “H”, respectively. Depending on a component (such as that formed by ECL) actually used for a CMI coding circuit/decoding circuit, signal states “0” and “1” may be associated with “H” and “L”, respectively. In this case, “L” and “H” in the above description should be read reversely. The above relationships also apply to the following description of the present specification.
As a system in which the CRV technique is applied to the CMI code, such as a fiberoptic transmission system, a synchronous CRV superposition (transmission) system is generally used, in which a signal to be superposed by the CRV technique is synchronized with the main signal, the transmission side forms CRV frames to superpose the signal, and the receiving side detects a CRV-frame synchronization pattern to extract the signal.
To superpose an asynchronous signal on the main signal, and to perform CRV superposition by a simpler configuration than the synchronous CRV superposition (transmission) system, an asynchronous CRV superposition (transmission) system has been examined, which does not require CRV-frame synchronization. As an example, a scheme is shown in
FIG. 4
of “Asynchronous Superposition Technique of Low Speed Signal Upon CMI-Coded Main Signal Using Coding Rule Violation”, Proceedings of 1982 General Convention of the Institute of Electronics and Communication Engineers, Kanagawa, Japan, p. 2186, in which, in order to superpose an asynchronous signal on the main signal, the asynchronous signal to be superposed is sampled each m (positive integer) bits of the main signal, CRV is applied when the sampled data is “1 (H)”, and CRV is not applied when the sampled data is “0 (L)”.
In the asynchronous CRV superposition (transmission) system, it is not easy for the receiving side to differentiate a CRV-indication time slot from a time slot having a transmission error. More specifically, in the asynchronous CRV superposition (transmission) system, when there is CRV1 preceded by n (positive integer) consecutive 0's further preceded by “1” (indicated by a solid line), as shown in
FIG. 11
, if one bit indicated by (a) in the consecutive 0's or the foregoing “1” indicated by (b) suffer transmission error (indicated by dotted lines), CRV1 is erroneously recognized as “1 (H)”. In the case of (b), the foregoing “1” is erroneously recognized as CRV0. In other words, a CRV discrimination error is propagated (spread). When bad conditions exist on a transmission line, the quality of a superposed signal regenerated at the receiving side may deteriorate.
The asynchronous CRV superposition (transmission) system superposes an asynchronous signal having a lower speed than the main signal. To improve the performance of the transmission system, it is preferred that communications be performed by superposing a signal having as high a speed as possible. In other words, there are demands for development of a signal coding/decoding system which does not adversely affect the main signal and which allows a high-speed asynchronous signal to be transmitted with high quality.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a CMI coding method, a CMI decoding method, a CMI coding circuit, and a CMI decoding circuit which do not adversely affect a main signal and which always allow an asynchronous signal having as high a speed as possible to be superposition-transmitted by CRV of the CMI code, with a simple procedure and a simple hardware structure.
In a CMI coding method according to the present invention, an asynchronous signal is superposed on a main CMI-coded signal with the use of only CRV0 as CRV. In addition, when the main CMI-coded signal is replaced with CRV0 for a signal to be superposed, since the main signal cannot be transmitted, the CMI-code coding method is configured such that two binary bits of the main signal, which should be originally CMI-coded and transmitted in a time slot in which CRV0 is to be disposed and in the next time slot thereof, are two-bit-coded according to a predetermined rule, and the two-bit code is transmitted by the use of the first half and the second half of the next time slot.
In the present specification, a time slot in which CRV is disposed is called a CRV-indication time slot. The next time slot of a CRV-indication time slot, in which a two-bit-coded code by a coding method according to the present invention is disposed, is called a crammed main-signal time slot. A CRV-indication time slot and the crammed main-signal time slot may be collectively called extra time slots.
More specifically, in the CMI coding method, whether CRV0 is substituted for the main CMI-coded signal is determined according to the state of a signal to be superposed. For example, when the signal polarity to be superposed is positive (it has a level of “1” or “H”), it is determined that CRV0 is disposed. Instead of the main CMI-coded signal, CRV0 (“HL”) which is a two-bit code, the two bits having opposite polarities to those of a two-bit code (“LH”) in which each bit has a period of T/2 and in which the two bits indicate a main signal state of “0” or “L” is disposed as required. In the next time slot of the time slot where CRV0 is disposed, a code is, disposed which is coded such that, when two binary bits of the main signal, which should be originally CMI-coded and transmitted in the time slot where CRV0 is to be disposed and in the next time slot thereof, have the same polarity, the two bits of the main signal are coded to a two-bit code, each bit having a period of T/2 and different polarity from each other.
A CMI decoding method according to the present invention performs processing in the order reverse to that of the CMI coding method described above. More specifically, when CRV0 is detected in a received CMI code, it is discriminated that a signal different from the main signal has been superposed, and a two-bit code dispos
Hitachi , Ltd.
Jean-Pierre Peguy
Lauture Joseph J
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