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 capabilites 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.
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.
Low Noise Figure
The low noise characteristics of the Cisco ONS 15501 allow over six amplifiers to be cascaded and still
achieve an excellent OSNR at input powers as low as –21 dBm per channel. This enables seamless
migration to higher speeds beyond OC-48 and to a larger number of channels.
High Maximum Output Power
The high maximum optical power of the Cisco ONS 15501 increases the number of wavelengths that can
potentially be routed to it. The higher input power range available can be used to increase the number of
wavelengths to 128 from 32, without having any spectral gain tilt effects.
The Cisco ONS 15501 supports SNMP, and it has a console port to facilitate setup and monitoring. With
a customer-supplied network monitor and the provided MIB file, all monitorable and settable parameters
are available remotely.
Cisco ONS 15501 User Guide
78-14134-01, Release 1.0