A controller or controllers don't communicate reliably on an I/NET LAN, often somewhere in the middle of the network.
As the signal travels down the cable it is affected the "characteristic impedance" at a relatively constant amount. As the signal approaches the end of the network, that impedance increases to completely "open" at the end. According to Ohms law, the higher the resistance/impedance, the higher the voltage will be as the current is applied. Therefore the signal will gain enough energy to be reflected back up the cable as noise. The controller(s) that is (are) most affected are located at the point where the reflection from the end of the network arrive during the original signal.
Placing a 120 - 180 OHM 1/2 Watt Resistor at both the physical ends of the Controller LAN will clamp those reflections since the resistor is fairly close to the characteristic impedance of most cables.
The Sub-LAN works differently than the Controller LAN, so DO NOT use a 120 - 180 OHM resistor on the Sub-LAN. Instead, use a 620 OHM resistor placed at both physical ends of the Sub-LAN.
The reason for this is that if you use a 120 - 180 OHM resistor at each end of the Sub-LAN wiring, you remove the bias needed by the "Mark-Space" signal on the Sub-LAN. The 620 OHM resistor is large enough to absorb some of the noise without messing with the Comm. However, you might notice a slight decrease in signal strength.
Also, refer to the article I/NET Controller LAN (C-LAN) Requirements for additional details.