Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-10-07
2002-08-20
Marcelo, Melvin (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S507000
Reexamination Certificate
active
06438143
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a packet communications system, and more particularly to a communications system for sending and receiving image information and other information in real time over a high-speed packet network synchronizing between a transmitter side and a receiver side.
2. Description of the Prior Art
In recent years, development of communications systems using optical fibers and other high speed transmission paths has greatly increased communication capacity. This has spurred the development of digital communications systems, which are now used not only for digital computer data but also for transmitting digitized image signals, audio signals, and related additional information.
Asynchronous transfer mode (ATM) systems, for example, can operate at a speed in excess of 155 megabits/second, and practical ATM systems are now available for communicating digital image data in real time.
Formal standards for ATM systems have been adopted by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) and the ATM Forum, for example, and numerous related publications have been issued.
Such image communications systems employing high-speed packet networks are disclosed in, for example, Japanese Patent Laid-open Publication 5-14763, in gazette, as a first reference, Japanese Patent Laid-open Publication 5-14399, in gazette, as a second reference, and U.S. Pat. No. 5,386,436, as a third reference.
According to the first reference, clocks at transmitter and receiver terminals are synchronized in subordination to a network clock of a high-speed packet network. On the transmitter terminal, image data are transmitted in conjunction with transmission frame synchronizing signals that are generated based on the subordinately synchronized clock.
On the receiver terminal, meanwhile, reproduction-side synchronization is effected on the frames using both reception frame synchronizing signals generated on the basis of a subordinately synchronized clock and transmission-reproduction frame synchronizing signals generated on the basis of timing packets added to the transmission image packets.
In the disclosure of the second reference, synchronization packets added to transmitted image signals are detected on the receiver terminal, as in the first reference, and image frame synchronizing signals and image clocks are generated from moving averages of arrival times of the synchronizing information based on the synchronization packets, and the synchronization of image frames is thereby achieved.
The third reference provides a synchronization method using statistical characteristics of adaptive differential PCM (ADPCM) signals constituting transmission data.
The configurations described in the forgoing conventional systems, however, are subject to following problems.
As concerning the first reference:
(1) The transmission-reproduction frame synchronization signal (FST
1
) generated by a timing control means is generated based on a timing packet (PT), and therefore, when a PT cannot be detected because of an error such as a cell loss or packet destruction on the ATM network occurring on the packet communications network, the frame synchronization signal FST
1
cannot be generated normally. Thus, image data cannot be stored in a frame memory unit, which leads to the breakdown of the image.
(2) Also, intervals between timing packet receptions are not constant due to the influence of fluctuations on the ATM networks, etc., and therefore, there may be a problematic case where synchronization cannot be achieved because of the effects of network characteristics.
(3) Moreover, the system cited in the first reference will not function without the timing control means, resulting in very large circuit sizes.
As concerning the second reference:
(4) Image frame synchronizing signals are generated using an arrival time calculation means, moving average calculation means, observation window setting means, and frame synchronizing signal generation means, but this is all dependent on the synchronization packet Ps. Accordingly, although some degree of resilience to both packet destruction and fluctuations is effected by taking of moving averages, in the event that synchronization packets Ps are continually destroyed, for example, and if a limiting value thereof is exceeded, the images will break down.
(5) Moreover, the arrival time calculation means, moving average calculation means, observation window setting means, and frame synchronizing signal generation means are circuits involved in computations and thus result in a system having a very large circuit size.
(6) According to both the first and the second references, moreover, there is only one timing packet Pt or one synchronization packet Ps in one frame period, so that, when these are destroyed, it becomes impossible to receive any of the image data in the frame period, resulting in that the error is compounded.
The fundamental problem with the conventional systems disclosed in the first and the second references resides in the fact that the synchronization signals are generated on the receiver side based on unstable synchronization packets which have been detected on the receiver side.
In the conventional system disclosed in the third reference, unlike the art cited in the first and second references, there is no dependency on unstable synchronization packets, but there is a problem nevertheless in that this system only applies to a case where the data being communicated are adaptive differential PCM (ADPCM) signals.
SUMMARY OF THE INVENTION
The present invention has been developed to solve the problems inherent to the conventional systems, and an essential object of the present invention is to provide an improvement of an image packet communications system.
In the first aspect of the present invention, provided is an image packet communications system for communicating image data information in real time between a transmitter side and a receiver side through a network, wherein the transmitter comprises:
a transmission clock generator for generating a transmission clock subordinately synchronized to a given reference clock; and
a packet generator for generating image packets from input image signals based on the transmission clock, meanwhile, the receiver comprises:
a reception clock generator for generating a reception clock subordinately synchronized to the given reference clock, so that the reception clock is synchronized with the transmission clock;
a packet receiver receiving the image packets with a network delay to be stored in a buffer memory group; and
a buffer controller for generating a reception standard signal as a control signal for controlling the writing and reading of the image packets to and from the buffer memory group, based on the reception clock.
In this arrangement, the transmitter further comprises a transmission standard signal generator for generating a transmission standard signal based on the transmission clock and the packet generator generates the image packets with image packet headers attached thereto to produce transmission packets, with reference to the transmission standard signal, and wherein the packet receiver is comprised of a separator means for separating the image packets and the image packet headers from each other so that the buffer controller generates an address signal based on information of the image packet headers supplied from the packet receiver.
In the second aspect of the present invention, the buffer controller is supplied with a reference signal (REF) from the outside as a frame standard signal, and when detecting that a time differential between the reception standard signal generated based on the reception clock and the reference signal (REF) input from the outside exceeds a predetermined value, the writing and reading of the image packets to and from the buffer memory group is so controlled such that the image data output from the buffer memory group has a suitable volume for the reference signal.
In the third
Abelson Ron
Marcelo Melvin
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