Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
2000-05-23
2003-12-02
Ghayour, Mohammad H. (Department: 2634)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C375S363000, C370S505000
Reexamination Certificate
active
06658074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for reproducing a clock signal for a lower order group signal at a receiver side in a pulse stuffed synchronizing system.
2. Description of Related Art
In a conventional digital data transmission system, a transmitter converts digital signals into their high-speed digital signal form by time-division multiplexing and a receiver receives and demultiplexes the multiplexed signal to reproduce the original digital signals. The original digital signals to be multiplexed are however supplied from various devices and may not be matched in the clock rate without subjecting to a particular process. For matching or synchronizing the signals, a net synchronizing method or a stuffed synchronizing method may preferably be used.
The stuffed synchronizing method is not designed to directly synchronize the digital signals received from various devices. The stuffed synchronizing method stores the signals to be multiplexed in a memory and then reads out them by use of a common clock signal which is slightly faster than that of the digital signals to align the signals in the timing. A difference between the digital signal and the clock signal is compensated by inserting an extra number of pulses (referred to as stuffed pulses). It is thus needed at the receiver side to identify the position of stuffed pulses for removing the extra pulses.
More specifically, the pulse stuffed synchronizing system is depicted in “Simple Digital Data Transmission” by Makoto Yamashita et al, the Telecommunications Association in Japan, Ver. 4, Jun. 26, 1998. In this reference, there is a case where a clock signal for a lower order group signal and a clock signal for a higher order group signal are not synchronous with each other. At this time, extra pulses are inserted (stuffed) into the lower order group signal at the transmitter side to synchronize the clock signal for the lower order group signal and the clock signal for the higher order group signal. At the receiver side in the pulse stuffed synchronizing system, the extra or stuffed pulses are removed (de-stuffed) and then the clock signal for the lower order group signal is reproduced by a phase synchronization oscillating circuit.
Also, the pulse stuffed synchronizing system is described explicitly in “Waiting Time Jitter” by D. L. Duttweiler (The Bell System Technical Journal, Vol. 51, No. 1, January 1972). In this reference, it is described that the stuffed pulses can not fully be removed at the receiver side hence causing jitters.
Particularly, in a conventional pulse stuffed synchronizing method utilized with SONET (synchronous optical network) or SDH (synchronous digital hierarchy) in an advanced digital communication system, the extra pulses are inserted and removed on a byte-by-byte basis. As a result, a greater amplitude of jitter is generated.
A procedure of synchronously multiplexing existing DS3 signals under the SONET standard will now be described.
FIGS. 17
to
19
are diagrams showing a frame structure of an STS-1 signal, a frame structure of an STS-1 SPE signal and a byte structure of data signal, respectively.
The SONET specifications are defined in ANSI T1.105-1995 (Synchronous optical network-Basic description including multiplex structure, rates, and formats) and ANSI T1.105.02-1995 (Synchronous optical network-Payload mappings) of the American National Standards Institute. In the standards, the DS3 signal having a nominal bit repetitive frequency of 44.736 Mb/s is accommodated in the STS-1 (synchronous transport signal level 1) signal having a nominal bit repetitive frequency of 51.84 Mb/s. The STS-1 signal is the higher order group signal while the DS3 signal is the lower order group signal. As shown in
FIG. 17
, the STS-1 signal has a capacity of 810 bytes, 90 bytes in horizontal by 9 rows in vertical and accommodates a single STS-1 SPE (synchronous payload envelope) signal. In the STS-1 signal, the STS-1 SPE is accommodated in the region of a frame other than the overhead region without considering the stuffed pulses and its region is 783 bytes per frame.
Also, the STS-1 signal frame and the STS-1 SPE frame are not always matched relative to each other. As shown in
FIG. 17
, one frame of the STS-1 SPE signal may be accommodated in two frames of the STS-1 signal. In other words, the head location of the STS-1 SPE signal may be varied in the frame of the STS-1 signal. The head location of the STS-1 SPE signal in the STS-1 signal frame is indicated with pointers H
1
and H
2
which are accommodated in the overhead region of the STS-1 signal frame.
The STS-1 signal and the STS-1 SPE signal are not always synchronized with each other. For this reason, the STS-1 signal includes positive/zero
egative stuff data in units of bytes. The existence or non-existence of the stuffed pulses is also indicated with the pointers H
1
and H
2
. More specifically, the existence or non-existence of the stuffed pulses is indicated by inverting of specific bits of the pointers H
1
and H
2
.
Also, there is a pointer operation H
3
. When the positive stuffing is made, one byte of stuffed pulses is inserted after the pointer operation H
3
. In case of the negative stuffing, STS-1 SPE data is accommodated in one byte of the pointer operation H
3
. The zero stuffing means that neither the positive stuffing nor negative stuffing is made. In case of the zero stuffing, the stuffed pulses are inserted in one byte of the pointer operation H
3
represents and STS-1 SPE data is accommodated in one byte after the pointer operation H
3
.
As shown in
FIGS. 18 and 19
, the frame of the STS-1 SPE signal is composed of 783 bytes, 87 bytes in horizontal by 9 rows in vertical, and accommodates a DS3 signal. Since the STS-1 SPE signal and the DS3 signal are not synchronous with each other, the positive stuffing is defined in units of bits. The region of the STS-1 SPE signal where the DS3 signal is accommodated is represented by a combination of stuffed bits s and data bits i regardless of the positive stuffing, and 622 bits per row. The bit s indicates the positive stuffing location where the data of the DS3 signal is usually stored while the stuffed pulses are stored only in a positive stuffing mode. The existence or non-existence of the positive stuffing is indicated by converting all the stuff control bits c to 1. It should be noted that in
FIG. 19
, o indicates an overhead bit and r indicates a fixed stuff bit which is a type of overhead bit.
When the DS3 signal is accommodated in the STS-1 signal, there are carried out two stages of the stuffing processes, i.e., the positive/zero
egative stuffing process in units of bytes in the STS-1 signal and the positive stuffing process in units of bits in the STS-1 SPE signal. As described previously, the positive/zero
egative stuffing process in units of bytes in the STS-1 signal may generate a greater amplitude of jitter. It is hence crucial to remove such jitter.
For the purpose, an apparatus and a method for mapping and removing jitter are disclosed in Japanese Patent Laid Open Patent Application (JP-A-Heisei 9-505705) as shown by a circuit arrangement of FIG.
1
.
FIG. 1
is a block diagram showing the structure of a receiving unit (a de-synchronizer) for reproducing the inserted signals including asynchronous data from a high transmission rate synchronization signal in a predetermined clock rate.
Referring to
FIG. 1
, a first de-stuffing circuit
1
detects the positive/zero
egative stuffing in a received STS-1 signal and carries out the de-stuffing process to remove unnecessary bits such as of the overhead of the STS-1 signal. Thus, the first de-stuffing circuit
1
extract an STS-1 SPE signal
68
. Then, a second de-stuffing circuit
12
detects the positive stuffing in the STS-1 SPE signal and carries out the de-stuffing operation to remove unnecessary bits such as the overhead of the STS-1 SPE signal. Thus, the second de-stuffing circuit
12
extract a DS3 signal.
A stuff bit leak circuit
15
produces a data indica
Ghayour Mohammad H.
NEC Corporation
Sughrue & Mion, PLLC
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