In a segmented HFC network, it is possible for each port of a node to carry unique information. Therefore, it is critical that information from one port not appear on the others. Digital information, in the form of QPSK or QAM modulated carriers is relatively immune to interference. As there are many sources of interference in a system, the interfering signal should be at least 45 dB below the digital information to ensure that the interference does not effect the system performance. If one port is operated at lower output, the isolation to that port must be greater.
Video is much more sensitive to interference.
Therefore, if all ports of a node are operated a the same output level, the required isolation is 70 dB. Again, we need to consider the effect of unequal output levels. Isolation A block diagram of a typical node is shown in Figure 1. If the diplex filter were perfect and there was no pick up between the forward and return paths, the forward information would not appear at all on the return. In real circuits, the diplex filter allows some leakage of the forward information into the return and the forward information can radiate through the air in the node as well as through the circuit board. This information can be easily picked up by the return circuitry which allows the forward information to appear on the return path.