Specifications

4-2
Cisco ONS 15530 Planning Guide
OL-7708-01
Chapter 4 Optical Loss Budgets
Overall Optical Loss Budget
Keep in mind that the decibel expresses a ratio of signal powers. This requires a reference point when
expressing loss or gain in dB. For example, the statement “there is a 5 dB drop in power over the
connection” is meaningful, but the statement “the signal is 5 dB at the connection” is not meaningful.
When you use dB you are not expressing a measure of signal strength, but a measure of signal power
loss or gain.
It is important not to confuse decibel and decibel milliwatt (dBm). The latter is a measure of signal power
in relation to 1 mW. Thus a signal power of 0 dBm is 1 mW, a signal power of 3 dBm is 2 mW, 6 dBm
is 4 mW, and so on. Conversely, –3 dBm is 0.5 mW, –6 dBm is 0.25 mW, and so on. Thus the more
negative the dBm value, the closer the power level approaches zero.
Overall Optical Loss Budget
An optical signal degrades as it propagates through a network. Components such as OADM modules,
fiber, fiber connectors, splitters, and switches introduce attenuation. Ultimately, the maximum allowable
distance between the transmitting laser and the receiver is based upon the optical loss budget that
remains after subtracting the power losses experienced by the channels as they traverse the components
at each node.
Table 4-1 lists the laser transmitter power and receiver sensitivity range for the transponder line cards,
ITU trunk card, the OSC (Optical Supervisory Channel) module, and the PSM (protection switch
module).
Note Add the proper system-level penalty to the receive power based on your actual network topology
characteristics, such as dispersion.
The goal in calculating optical loss is to ensure that the total loss does not exceed the overall optical (or
span) budget. The optical budget is determined by subtracting the minimum receiver sensitivity from the
minimum laser launch power on the cards. The OSC has an optical budget of 24 dB, which is equal to
the minimum OSC receiver sensitivity (–19 dBm) subtracted from the minimum OSC laser launch power
(5 dBm) on the OSC module.
Table 4-1 Trunk Side Transmitter Power and Receiver Ranges
Card or Module Type
Transmitter Power (dBm) Receiver Sensitivity (dBm)
Minimum Maximum Minimum Overload
Transponder line card 5 10 –28 –8
8-port multi-service muxponder 5 10 –28 –8
2.5-Gbps ITU trunk card 5 10 –28 –8
10-Gbps ITU trunk card 1 6 –22 –8
OSC module 5 10 –19 –1.5
PSM –31 17
1
1. The receiver detector only reports up to 0 dBm in the CLI (command-line interface). To measure the actual input
power to the receiver, use an optical power meter on the optical monitoring port.