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Cisco ONS 15501 User Guide
78-14134-02, Release 2.0
Chapter 1 Product Overview
Key Features
.
Variable Gain
When the gain of an EDFA is fixed, the assumption is that all networks can be laid out with equally
spaced EDFAs. In reality, this is rarely the case. For designs in which the spacing must be flexible,
variable gain allows the network designer to tailor network requirements much more accurately. For
instance, when an Cisco ONS 15501 is used as a preamplifier for receivers having an overload point of
-8 dBm per wavelength, the output VOA (variable optical attenuator) can prevent overload by reducing
the signal going to the receiver. Alternately, when EDFA spacing is only 10 dB, the output VOA can be
enabled to avoid saturation of the next stage EDFA, ensuring that the entire network has good gain
flatness and virtually consistent OSNR across all wavelengths.
The variable gain capabilities of the Cisco ONS 15501 greatly enhance the flexibility of an optical
network. System operators can add or drop optical elements, such as OADM (optical add/drop
multiplexer), without drastic network redesigns or costly equipment changes. When a change occurs in
span loss, the adjustable gain can be used to reset the network to a better operating point.
Transient Suppression
Transients in the performance of EDFAs are inevitable whenever the number of signals or the relative
power of signals change. For example, when channel rerouting or system failure (caused by a fiber cut
or equipment malfunction) transfers all incoming power to a single “surviving channel,” that channel
will momentarily experience a higher gain, which can cause BER (bit error rate) problems due to
eye-pattern closure. The amount of time required by an amplifier to recover from such a change indicates
its suitability for add/drop applications.
The most important parameters in transient suppression are the recovery time and the overshoot and
undershoot amplitude. The recovery time for the signal amplitude to get within 10% of the “steady state”
amplitude after the switching event is referred to as the transient suppression time. Smaller values are
desirable. From a 10 dB change in power (simulating the adding or dropping of 29 out of 32 channels
present), the Cisco ONS 15501 never exceeds 100 microseconds and is typically below 50
microseconds. The Cisco ONS 15501 can respond to the most drastic power changes with overshoots or
undershoots of less than 1 dB.
Table 1-3 Relative OSNR in Cascading EDFAs
Number of
Cascaded
EDFAs
Number of
Wavelengths
Worst Case OSNR at 17 dB Gain
1
at
Gain Minimum
1. 0 dBm total input power.
OSNR, 23 dB Gain
2
2. -6 dBm total; -21 dBm per channel.
1 32 37.00 dB 31.00 dB
2 32 33.25 dB 27.25 dB
3 32 30.70 dB 24.70 dB
4 32 28.75 dB 22.70 dB
5 32 27.00 dB 21.00 dB
6 32 25.50 dB 19.50 dB