The interactive service shown is a cable modem system. Four nodes of service are combined at the headend. Assuming an input level of 18 dBmV at the input to the return path amplifier in the node, there is a 35 dBmV RF level out of the optical receiver at the headend. For proper operation, the input level for the cable modem equipment controller must be 0 dBmV. Figure 5. This can be accomplished using integrated or modular RF signal management products.
Thermal noise is caused by active components like amplifiers.
The noise is caused by thermal fluctuations in the device and is characterized by the noise figure of the device. Fiber optic link noise comes from a number of sources, including the fiber transmitter, the receiver and the fiber itself. The most common source is usually ingress. Unlike the other two sources, ingress can be difficult to control because it is most often introduced within the subscriber's home. There have been documented cases of optical nodes and amplifiers being completely overwhelmed by nearby arc welding machines.