bus terminator (BT)
Transmission media must be terminated with their impedance. With regard to the classic Ethernet in bus topology, it must be noted that the bus must be terminated at both ends with a bus termination that has the impedance of the transmission medium.
For example, if the transmission medium has an impedance of 50 ohms, then both bus ends must be terminated with a resistor of 50 ohms. The termination with the correct characteristic impedance is necessary because reflections occur at the bus end if there are no or different impedances, and these reflections are present as standing waves on the transmission medium.
If there is no terminating resistor, the transported signals are reflected at the ends of the cable and interfere on the transmission medium with the outgoing signals. This usually leads to mutilations of the data stream up to complete extinction.