Pulse or digital communications – Testing – Data rate
Reexamination Certificate
1999-05-06
2003-04-15
Chin, Stephen (Department: 2734)
Pulse or digital communications
Testing
Data rate
Reexamination Certificate
active
06549572
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to optical networks and, more particularly, to a method and apparatus for automated time domain monitoring in optical networks.
Generally, digital transmission systems deliver information, which is encoded as quantized signals, from a sender to a receiver. There are several parameters that characterize the quality of the transmission in these systems. One such parameter is the “bit error ratio” or BER. BER can be measured at any point in a transmission system, and may be used for fault detection and isolation.
There are many effects that contribute to degradation of a measured BER. For example, signal attenuation reduces signal amplitude, dispersion alters pulse shape, receiver and amplifier noise increase signal level ambiguity, and jitter creates uncertainty in the sampling point and affects other aspects of synchronization.
In an optical network, such as a Wavelength Division Multiplexing (WDM) network, data passes through many different types of network elements: wavelength converters and filters, wavelength add-drop multiplexer (ADM), cross-connects, and optical amplifiers. The network elements may perform multiple optical to electronic (O/E) and electronic to optical (E/O) conversions, or alternatively, may process the data in optical form. Although it is preferable to perform signal monitoring at network elements that include O/E and E/O converters, one can also tap and detect a signal at any point in a WDM network.
FIG. 1
illustrates a prior art optical signal monitoring system
100
, which monitors optical signals on a fiber link by performing indirect SNR or average power level measurement. As shown, optical monitoring system
100
includes an optical fiber
110
carrying an input optical signal, optical power splitter
120
, optical fiber
130
carrying output optical signal, wavelength selective filter
140
, photodetector
150
, electrical amplifier
160
, and a spectrum analyzer
170
. An incident optical signal present in optical fiber
110
is conveyed to output fiber
130
via coupler
120
.
To monitor the input optical signal, an optical tap in coupler
120
extracts a small amount of signal power from optical fiber
110
. Wavelength selective filter
140
selects a desired wavelength, and photodetector
150
converts the light associated with the selected wavelength into an electrical signal. The current from photodetector
150
is amplified by electrical amplifier
160
, and subsequently measured by spectrum analyzer
170
, or alternatively, an average value meter.
One of the signal characteristics measured by spectrum analyzer
170
is an estimated signal-to-noise ratio (SNR), which is commonly used to characterize a link performance in a WDM network. Optical domain spectral monitoring is typically used to estimate the SNR in optical networks. The signal is spectrally narrow, typically a few GHz. Assuming that the noise is slowly varying with wavelength, the optical noise level is measured at a wavelength slightly away from the channel's signal, where the ratio of this noise to the optical signal represents the optical SNR.
Presently, an optical spectrum analyzer or an equivalent (e.g., a Hewlett Packard wavemeter) is used to measure the characteristics of an input optical signal in a WDM network. Such instruments, however, have a number of disadvantages. First, these instruments are expensive and slow (i.e., require scanning across all wavelengths). Second, there are well-known inaccuracies that result from such optical measurements. Third, some sources of signal noise are not detectable with these instruments. For example, interferometric intensity noise is one such source of signal noise, which is not detectable in the optical domain. Fourth, any induced jitter or wander cannot be detected using these instruments. Finally, the required optical SNR for low BER depends not only on the signal rate but also on the details of the receiver design.
Optical networks, such as WDM networks, can provide flexible broadband connectivity. A unique feature of WDM network technology is rate and format transparency. For example, the bit rate ƒ
bit
of a signal may range from 25 Mb/s to 10 Gb/s. Furthermore, reconfigurable WDM networks perform highly variable route selection, a feature that is profoundly different from traditional point-to-point networks. At the periphery of such networks, any one set of users may employ at most a few line rates and formats. Within the network core, however, the full mix of rates and formats is encountered by most network elements. The route, line-rate, and format can be highly unpredictable and rapidly changing at any network element within a reconfigurable WDM network.
Like all other communication networks, it is desirable to monitor network transmission performance in WDM networks to anticipate problems before a user experiences poor service. Signal integrity and network link performance are closely monitored in traditional transmission systems, which operate at fixed line rates. These systems possess embedded signaling channels for diagnosis of transmission impairment and exchange of fault information.
For example, in Synchronous Optical Network (SONET) systems, the frame interval is continuously monitored along with verification of parity calculated on subsets of the bits within a frame. In these systems, loss of signal, loss of frame, and Bit Interleaved Parity 8 (BIP8) error rates are monitored and reported. Other transmission formats have their own embedded error detection.
Unlike traditional networks, however, WDM networks have less direct control over critical transmission parameters. Although many of the proposed WDM networks perform signal level management, they do not perform retiming, which is highly rate dependent and usually restrictive. When a WDM network performs little or no retiming, jitter management is delegated entirely to receivers at the user end. Accordingly, if the quality of the signal could be verified without full regeneration so that faults could be isolated to one subnetwork, it would facilitate implementation of multi-vendor interfaces for reconfigurable WDM networks.
Finally, interaction among protocol layers, such as locating faults when Internet Protocol (IP) routers or Asynchronous Transfer Mode (ATM) switches connect directly to optical networks, would be easier if a WDM network could dynamically determine the type of traffic it carries and test the quality of the traffic by checking for proper frame format and presence of errors. For example, when a network element in a WDM network detects an error in a signal, the network element could generate an alarm, facilitating the identification of the fault at the higher layer.
DESCRIPTION OF THE INVENTION
It is desirable to have a method and apparatus for performing automated time domain monitoring in optical networks that overcome the above and other disadvantages of the prior art. Methods and apparatuses consistent with the present invention determine characteristics of an input optical signal by estimating a minimum time interval between transitions in the input signal, determining a clock signal based on the estimated minimum time interval, and performing a time domain measurement on the input signal based on the determined clock signal.
In one embodiment, an optical signal monitoring apparatus comprises a forward rate detector, clock recovery circuit, and a time domain measurement circuit. The forward rate detector estimates the minimum time interval between transitions in an input optical signal. Based on the estimated minimum time interval, the clock recovery circuit extracts a clock signal from the input signal. Using the extracted clock signal, the time domain measurement circuit samples the input signal, and determines in time domain the characteristics of the input signal.
Methods and apparatuses consistent with the invention have several advantages over the prior art. In an optical network, many different time domain measurements may be performed on an optical signa
Anderson William T.
Banwell Thomas C.
Cheung Nim K.
Hodge James E.
Chin Stephen
Falk James W.
Giordano Joseph
Kim Kevin
Telcordia Technologies Inc.
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