Multiplex communications – Fault recovery – Bypass an inoperative channel
Reexamination Certificate
1998-04-06
2003-04-22
Kizou, Hassan (Department: 2662)
Multiplex communications
Fault recovery
Bypass an inoperative channel
C370S216000, C370S227000, C340S870030
Reexamination Certificate
active
06552998
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a two-way communication system that adjusts a transmission delay or transmission level when a subscriber station starts its operation over, for example, a two-way CATV network or passive optical star network, and more particularly to a delay or transmission level setting method suitable at a transmission link switching time in a two-way communication system. Moreover, the present invention relates to a delay setting method and a transmission level setting method suitble for two-way communication systems.
In conventional time division multiplex communication systems such as two-way CATV networks, passive optical star networks, or the like, since subscriber stations are respectively located at different distances from a center station, variations occur in the receive timings and the receive signal levels of up-stream signals received by the center station. In order to avoid such a problem, a respective transmission delay is set to each subscriber station by measuring the round-trip propagation delay time between a subscriber station and the center station when the subscriber station starts its operation.
By previously setting respective transmission delays to subscriber stations, the center station can receive up-stream signals transmitted from different subscriber stations at short intervals. In other words, the network transmission efficiency can be improved.
A typical prior-art time division multiplex communication system is disclosed, for example, in JP-A-299278/1992. According to the prior art, a subscriber station first receives a round-trip propagation delay measurement signal transmitted from a center station and then sends back an echo signal to the center station. The center station measures an interval between the time a measurement signal is transmitted and the time an echo signal is received, that is a round-trip propagation delay time.
Variations in the level of an up-stream signal received by a center station restrict the maximum laying distance and the configuration of relay equipment of the network because of the limited receivable signal level of the center station. For that reason, the requirement for receive signal levels is relieved by measuring the receive signal level in the center station when a subscriber station starts to operate and then adjusting it to a constant level.
JP-A-39787/1986, for example, discloses the prior-art level measuring method in which the center station measures the receive level of an echo signal.
In the above-mentioned prior art, where the distance between the center station and a subscriber station is long, a relay station is installed for signal transmission in the middle of the transmission line. In this case, the center station is connected to a relay station via a relay transmission line such as an optical fiber or a coaxial cable. In most cases, the relay transmission line is duplex systematized by a working system and a stand-by system in preparation for a failure of a network. If the relay transmission line is switched to the stand-by system because of a link failure in the relay transmission line in a working system, it is needed to reset the transmission delays or transmission level set amounts of all subscriber stations connected to the relay station.
The number of subscriber stations connected to one relay station depends on the network. However, in the case of the two-way CATV network, the maximum number of subscriber stations is estimated to be about 1000. In order to reset the delay or transmission level, the measurement signal and the measurement response signal must be transmitted and received to respective subscriber stations several times.
Therefore, when a link switching operation occurs in a relay transmission line, resetting respectively the delay or transmission level to each subscriber station very prolongs a failure recovery time of the system.
SUMMARY OF THE INVENTION
The present invention is made to solve the abovementioned problems in the prior art. The objective of the present invention is to provide a two-way communication system that can improve the shorten the system recovery time by adjusting the transmission delay or transmission level before an operation of a subscriber station and shortening the system recovery time when a link switching operation between a center station and a relay station occurs, over two-way CATV networks, passive optical star networks and the like.
Another objective of the present invention is to provide a method of setting a delay in a center station that can improve the shorten the system recovery time.
Further objective of the present invention is to provide a method of setting a transmisson level in a center station that can improve the shorten the system recovery time.
In order to achieve the above-mentioned problems, according to the present invention defined in claim
1
, a two-way communication system comprises a center station, subscriber stations each for mutually exchanging signals with the center station, a relay station for relaying signals between the center station and the subscriber stations, a relay transmission line connected between the center station and the relay station and duplex systematized by a working system and a stand-by system, and a subscriber transmission line connected between the relay station and the subscriber stations, wherein the center station measures a round-trip propagation delay when a subscriber station starts its operation and then sets a transmission delay of said subscriber station so as to equalize the round-trip propagation delay measured to said subscriber station to a fixed system delay, and wherein the center station measures a round-trip propagation delay of a subscriber station after a link switching operation when a link switching operation from a working system to a stand-by system occurs due to a failure in the relay transmission line, and resets transmission delays of all subscriber stations connected to the relay station at a time based on a difference between the measured delay and a round-trip propagation delay of a signal to the subscriber station before the link switching operation.
According to the two-way communication system of claim
2
, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line based on a time division multiplex system; and when a link switching operation from the working system to the stand-by system occurs due to a failure of one of the plural relay transmission lines or when a link switching operation from the stand-by system to the working station occurs due to a recovery of a failed one of said plural relay transmission lines, the center station measures a round-trip propagation delay of a signal to a subscriber station connected to the relay station involved in the link switching operation and resets round-trip propagation delays of all subscriber stations connected to the relay station involved in the link switching operation at a time, based on the difference between the measured delay and a round-trip propagation delay to the subscriber station before the link switching operation.
According to the present invention defined in claim
3
, a two-way communication system comprises a center station, subscriber stations each for mutually exchanging signals with the center station, a relay station for relaying signals between the center station and the subscriber stations, a relay transmission line connected between the center station and the relay station and duplex systematized by a working system and a stand-by system, and a subscriber transmission line connected between the relay station and the subscriber stations, wherein the center station measures a round-trip propagation delay when a subscriber station starts its operation and then sets a transmission delay of the subscriber station so as to equalize the round-trip propagation delay measured to the subscriber station to a fixed system delay; and wherein t
NEC Corporation
Odland David
Sughrue & Mion, PLLC
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