Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1994-05-24
1995-09-26
Boudreau, Leo H.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359140, 359152, 359177, 371 201, H04B 1008
Patent
active
054538653
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a system for monitoring the performance of a network. The system is suitable for use, for example, in a TPON (Telephony on a Passive Optical Network) system, but also may be used in other contexts.
BACKGROUND OF THE INVENTION
Conventionally, installed optical fibre networks have been tested to detect a fault such as a break in the network by connecting a dedicated optical time domain reflectometer (OTDR) to the network. The OTDR typically comprises a high power optical source which transmits a pulse onto the network. Light back-scattered from faults in the network is detected and correlated with the transmitted pulse to determine the location of the fault giving rise to the back-scattered signal.
As described in the present applicant's earlier international application WO 91/02959 the use of dedicated OTDR equipment has significant disadvantages. In particular it requires the interruption of the service normally carried on the network and so is normally used only after a failure occurs and is not suitable for continual monitoring of the network. The system described and claimed in the above cited application proposes the transmission of an appropriately coded OTDR signal as a part of each downstream data frame sent onto the network. In the preferred example the test pulse sequence is a Barker or Golay code transmitted in a header portion of a multiplexed data frame. Although this arrangement does make it possible to monitor the network continually there is then the disadvantage of a significant transmission overhead associated with the dedicated OTDR codes and a subsequent loss of bandwidth which might otherwise be used for data or for other system control functions.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a system for monitoring a telecommunications network, the system comprising a station connected to the network and including a transmitter for outputting a signal modulated with outgoing traffic onto the network and a receiver for receiving a signal from the network, the system further comprising a monitoring circuit including means for introducing a variable delay in said modulated signal from the transmitter, and means for combining the delayed signal with the received signal so as to produce an output dependent upon the correlation of said modulated signal from the transmitter and the received signal at the given delay.
The system of the present invention rather than using a dedicated OTDR probe signal as with the prior art systems, continually monitors the correlation between the output signal and the received signal as a function of a variable delay between those signals. In this manner, at a delay corresponding to the location of a fault in the network a peak in the correlation output is detected. This approach is able to provide improved sensitivity by comparison with the prior art systems since the entire signal power transmitted is monitored for reflections.
If the network is duplex then the receiver is arranged to receive a signal modulated with incoming traffic, and if the network is simplex then there will be a respective transmitter/receiver pair for both the downstream (transmit) and upstream (receive) paths of the network, each pair having an associated monitoring circuit. The transmitter associated with the receive path may also be arranged to transmit outgoing traffic.
Preferably the network is an optical network and the transmitter and receiver are arranged to transmit and receive optical signals. Preferably the system further comprises means for progressively varying the delay.
The system of the present invention brings particular advantages when used in the context of an optical network. It often happens in such a network that a local fault in one branch of the network leaves other parts of the network functioning normally. Accordingly using the system of the present invention it is possible for transmission and reception of data to continue as normal on the other pa
REFERENCES:
patent: 5022752 (1991-06-01), Bu-Abbud
patent: 5327277 (1994-07-01), Van Der Plas et al.
Patent Abstracts of Japan, vol. 4, No. 145 (E-29)(627) 14 Oct. 1980, JP A, 55 097753 (Matsushita Denki).
Patent Abstracts of Japan, vol. 7, No. 138 (P-204)(1283) 16 Jun. 1983, JP A,58053772 (Nippon Denshin).
Patent Abstracts of Japan, (E-77) (8340) 12 Dec. 1977, JP A 52098402 (Kumio Kasahara).
Okada et al., "Optical Cable Fault Location Using Correlation Technique", Electronics Letters, vol. 16, No. 16, Jul. 1980, Stevenage GB, pp. 629-630.
Ballance John W.
Clarke Donald E. A.
Faulkner David W.
Hale Michael A.
Boudreau Leo H.
British Telecommunications public limited company
Negash Kinfe-Michael
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