Error detection/correction and fault detection/recovery – Pulse or data error handling – Error count or rate
Reexamination Certificate
2000-09-29
2004-05-11
Decady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Error count or rate
C714S752000
Reexamination Certificate
active
06735725
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for performance management in a multiplexed transmission system, particularly but not exclusively an optical transmission system.
BACKGROUND OF THE INVENTION
The demand for high speed and high capacity data transmissions has been rising. In long haul transport, as well as in metro ring applications, the use of dense wavelength-division-multiplexing (DWDM) or wavelength-division-multiplexing (WDM)allows for increases in the transmission bandwidth by two, three or more times.
DWDM, or equivalently, for the purposes of this specification, WDM, systems permit a number of signals to be carried along a single optical fiber by modulating each signal about a separate optical carrier wavelength. Typically, optical routing and signal regeneration are performed by passive and active optical elements.
DWDM optical fiber telecommunication systems can have extremely high overall data capacity since each channel is capable of carrying a high data rate signal. These high capacity signals can be carried cost-effectively over many hundreds of kilometers if Erbium Doped Fiber Amplifiers (EDFA) are used to boost the power of the optical signal periodically and overcome the loss incurred in the optical fiber and the passive optical elements. There is a growing requirement to increase the capacity of the existing communication systems.
While optical amplifiers are designed to produce a linear gain profile, as a practical matter, the wavelength dependent profile of EDFAs and other optical elements in the network is non-uniform, so that this objective cannot always be reached across the entire wavelength range over which signals will be transmitted. A significant challenge in carrying such multi-channel signals over many spans of fiber separated by boosting EDFAS has to do with the fact that the wavelength spectrum of the gain of the EDFAs is not flat. In fact, because of the physical properties of the Erbium ions that provide the gain, the shape of the gain spectrum changes from strong gain (about 23.5 dB at 1530 nm) to weak gain (about 21.5 dB at 1560 nm). The fiber span also shows non-uniform loss across the wavelength spectrum. Generally, the higher the wavelength, the higher the loss.
Moreover, even with a gain flattening filter, the gain profile of an optical amplifier across the wavelength range still shows ripple and gain tilt. The ripple (the slight variation in the gain) and tilt (the slope of the gain profile) are functions of input power and are intrinsic properties of the amplifier material.
Further, due to aging amplifiers and environmental factors, optical signal quality could degrade, resulting in a degradation of the system performance over time.
In a long multi-span cascade of fiber spans and EDFA line-amplifiers, the nominal gain of the EDFA is set equal to the span loss, so that a nominal channel does not rise or fall in power as it propagates downstream. This non-ideal gain (due to EDFAs) and loss (due to fiber and component lose) spectrum means that in a long multi-span cascade of fiber spans and EDFA line-amplifiers, some channels will have more gain or higher loss than the average and will grow in relative power as the multi-channel signal propagates down the link. However, some channels have less gain or lower loss than the average, and so the power of that channel will decrease as the multi-channel signal propagates down the link.
The non-linear nature of the overall gain or loss profile has a profound impact on the bit error rate (BER) of the optical link.
The amount of gain provided by an EDFA is controlled by the amount of pump laser power that is applied to the Erbium doped fiber, and typically covers a range of 15 dB to 35 dB. The amount of output power capability of the EDFA is also influenced by the amount of pump laser power. For any given amount of pump power, there is a certain limit to the total power over all of the channels, with 15 dBm as an example of a typical value. This is a natural physical limit at which the pump photon flux is just sufficient to replenish the depletion of the Erbium population inversion by the high signal output power. As well as this natural physical limit on the total power capability, there can also be an additional lower limit applied by design. For a given number of channels, it might be useful to limit the total power out of the EDFA and launched into the optical fiber in order to avoid certain nonlinearities in the fiber. This total power control (TPC) mode typically is implemented by tapping off a very small but controlled fraction of the light at the output of the EDFA and monitoring that with a photodetector.
Since all of the wavelength channels can carry revenue generating traffic, it is of interest to ensure that all of the channels meet a certain standard of performance. In a digital system, BER is typically used as a figure of merit, and 10
−12
is a common objective for BER. One of the main influences which will degrade the BER of multi-span EDFA links is the noise known as the Amplified Spontaneous Emission (ASE) which is generated inside the EDFAS. The amount of total noise (ASE, signal-to-spontaneous beat noise, spontaneous-to-spontaneous noise, etc.) relative to the signal power is typically quantified by the Optical Signal to Noise Ratio (OSNR), defined as:
OSNR
=Signal Power/(noise density*
BW
OSNR
) (1)
where BW
OSNR
is the spectral band over which the OSNR is defined (for example 0.1 nanometers).
To optimize the OSNR of any given channel in a multi-span link, the input powers to each EDFA should be kept as high as possible at all of the amplifiers. This influences the design of multi-channel links where some channels will be increasing in power going down span, and some channels will be decreasing in power. The simplest case to consider is one in which all of the channels are initially launched at the same power. In the case of a channel which has more than average EDFA gain, it increases in power after that initial launch point, up until the receiver. With such high powers going into the EDFAs, that channel will have a good OSNR and will then have a good BER, provided that fiber nonlinearities are not provoked. However, a channel which has less than average gain will drop in power at every span as it propagates down-link. This channel will have a poor OSNR and thereby will have a high BER, which may not meet an objective like 10
−12
.
At first, it might be thought that the simplest way to ensure that the weak channels do not severely hamper the system would simply be to turn all transmitters up to their highest launched power achievable. However, constraints (either natural or by design) on the total power available from the EDFA rule out this simple approach. Given that the total EDFA power is limited, the solution in the past has traditionally been to turn up all transmitters only by the appropriate amount such that the end performance (either OSNR or BER) is balanced between all channels. If any transmitters were launching more than the power necessary to achieve this balanced performance condition, then they would necessarily be taking more power than they need from at least one of the EDFAs. Because of the constraint on total EDFA power, this removal of power would then reduce the power available to the weaker channels. This means that the performance of the weaker channels would suffer if the strong channels were allowed to get better end performance than the average. In conclusion, when operating under total power constraints, adjusting the channel launched power of the transmitters to achieve equalization of the end performance of all of the channels is the optimum solution.
Therefore, it is important to have a method to adjust the launching power of the channels in order to equalize the BER performance of all the channels. Since aging and optical degradation happens over time, it is important to develop the equalization method so it can be used during the operation of the network and not simply during s
Wan Ping W.
Wu Chung Y.
De'cady Albert
Nortel Networks Limited
Torres Joseph D
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