Optical network loss-of-signal detection

Details Optical network loss-of-signal detection Optical network loss-of-signal detection

1997-12-03

2001-01-23

Chan, Jason (Department: 2733)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200, C370S222000

Reexamination Certificate

active

06178025

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the detection of a loss of signal condition in an optical communications system and in particular, but not exclusively, to the detection of a loss of signal condition in an all optical synchronous communications network.
BACKGROUND TO THE INVENTION
Existing optical communications networks comprise a large number of system elements connected by optical fibres. A defect in such a network resulting in a loss of signal can be detected at a receiver by digital processing of the received signal bits after photoelectric conversion as described for example in U.S. Pat. No. 5,563,893 and U.S. Pat. No. 5,572,515. The majority of modern communications networks are synchronous communications systems conforming to frame format specifications such as those defined in accordance with the SONET standard specified by the American National Standards Institute (T1.105-199X, “American National Standard for Telecommunications—Digital Hierarchy—Optical Interface Rates and Formats Specification (SONET)”).
It is anticipated that the next generation of optical networks will rely increasingly on system elements such as cross connects (optical switches) which are all optical in that they function without conversion to the electrical domain. There is also a tendency towards propagation at higher bit rates of 10 Gbit/s or higher making it increasingly more attractive to rely upon all optical processing, particularly where an element is remotely located, in view of the complexity and cost of ultra high speed electronic circuits.
As noted by Mathias Bischoff et al in IEEE Communications Magazine, November 1996, “Operation and Maintenance For An All Optical Transport Network”, an important aspect of operation and maintenance of such all optical transport networks is likely to be the provision of optical failure detectors at various parts of the network, to enable defects in the network to be rapidly identified and remedial action taken appropriately. It is proposed for example that loss of signal may be detected by measurement of optical channel power. Channel power is however an unreliable indicator of signal presence because, in the absence of a data carrying signal, optical amplifiers and repeaters in a span of the network will tend to compensate by amplifying random noise, thereby fully or partially restoring the level of optical channel power in the absence of the data carrying signal. It is alternatively proposed that loss of signal may be detected as a result of the decoding process since the decoding apparatus will be unable to maintain synchronisation with a frame structure of received signals when a loss of signal condition exists. Other forms of signal degradation may also be detected at the decoding stage by measurement of signal to noise ratios or analysis of eye pattern statistics. Such decoding however requires conversion to the electrical domain and processing at the full bit rate of the data transmission.
Other known methods of monitoring the performance of optical communications systems include the modification of transmitted signals by the addition of a signature which can subsequently be traced through the system, as for example described in U.S. Pat. No. 5,513,029 which proposes the use of low frequency dither signals. The use of such dither or other forms of tracing signatures however may not be acceptable in a highly complex network accessed by many users and it would be preferable to avoid the need to modify the content of the optical signals carried by the system.
There remains a need to provide for loss of signal detection in such optical networks in a manner which is relatively simple to implement and cost-effective, thereby enabling loss of signal detection to be implemented at a large number of distributed monitoring locations of the network.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus for monitoring the transmission of an optical signal to determine the existence of a loss of signal condition in a communications system.
It is a further object of the present invention to provide loss of signal detection without reliance upon the processing of electronic signals at the bit rate of the communications carried by the optical signal.
It is a further object of the present invention to provide an optical communications system in which defects are identified and located using loss of signal detectors and in which the routing of data traffic is controlled to avoid such defects.
According to one aspect of the present invention there is disclosed a method of monitoring the transmission of an optical signal to determine the existence of a loss of signal condition in a communications system in which the optical signal is modulated at a bit rate with a digitally coded data stream; the method comprising the steps of:
(a) monitoring the optical signal at a monitoring location of the system to obtain a monitor signal representative of the optical signal;
(b) detecting the presence of features of the monitor signal occurring at a detection frequency lower than the bit rate and corresponding to a periodicity of recurrence of a feature of the digitally coded data stream; and
(c) determining whether a loss of signal condition exists in dependence upon the results of said detecting step.
Preferably the optical signal is monitored such that the monitor signal has a bandwidth lower than a bit rate frequency corresponding to the bit rate. In a synchronous communications system where a protocol defines a frame structure the data stream will typically contain features such as bit sequences defining framing bytes occurring at a predetermined frame rate, further features typically occurring at frequencies which are harmonics of the frame rate frequency. Since the frame rate is substantially lower than the bit rate, the bandwidth of the monitor signal need only be sufficient to allow adequate detection at the detection frequency. The electronics required to implement such detection and determining of loss of signal is thereby substantially simplified in complexity due to the relaxation in bandwidth requirement when compared with processing at the full bit rate.
Preferably the detecting step comprises an auto-correlation process using either a single delayed monitor signal or a series of delayed monitor signals which are delayed by respective integral multiples of a delay period. In the case of detection at the frame rate frequency, the delay period corresponds to the frame period.
Wavelength multiplexed optical signals may be demultiplexed prior to photoelectric conversion and may be subject to separate monitoring, detecting and determining steps in order to determine whether a loss of signal condition exists in respect of each of the separate components.
Detection of loss of signal may be utilised to operate an optical switch to effect re-routing of optical signals or to output a control signal to a network management system controlling the routing of traffic within the system to avoid a defect identified by the existence of the loss of signal condition.
Complex networks may be provided with a plurality of loss of signal detectors at monitoring locations distributed throughout the network.
The detection step may alternatively utilise a band pass filter passing the detection frequency, provided that the filter has a lower cut off frequency which is greater than zero frequency (DC).
The present invention also comprises apparatus and systems for use in the above method.
According to a second aspect of the present invention there is disclosed a method of monitoring the transmission of an optical signal to determine the existence of a loss of signal condition in a communications system in which the optical signal is modulated at a bit rate with a digitally coded data stream; the method comprising the steps of:
(a) monitoring the optical signal at a monitoring location of the system to obtain a monitor signal representative of the optical signal;
(b) measuring a characteristic of the monitor signal within a de

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