|  | 4.3 | Basics of Signals and Noise in Communications Systems
| 4.3.4 | Network signal propagation

Propagation means travel. When a NIC puts out a voltage or light pulse onto a physical medium, that square pulse made up of waves travels along the medium (propagates). Propagation means that a lump of energy, representing 1 bit, travels from one place to another. The speed at which it propagates depends on the actual material used in the medium, the geometry (structure) of the medium, and the frequency of the pulses. The time it takes the bit to travel from one end of the medium and back again is referred to as the round trip time, (RTT). Assuming no other delays, the time it takes the bit to travel down the medium to the far end is RTT/2.

The fact that the bit takes a small amount of time to travel along the medium does not normally cause network problems. However, with the ever-increasing data transmission rates of today's networks sometimes you must account for the amount of time it takes the signal to travel. There are two extreme situations to consider. Either the bit takes "0" time to travel, meaning it travels instantaneously, or it takes "forever" to travel. The first case is wrong according to Albert Einstein, whose "Theory of Relativity" says no information can travel faster than the speed of light in a vacuum. This means that the bit takes at least a small amount of time to travel. The second case is also wrong, because with the right equipment, you can actually time the pulse. Lack of knowledge of propagation time is a problem, because you might assume the bit arrives at some destination either too soon, or too late. If the propagation time is too long, you should re-evaluate how the rest of the network will deal with this delay. If the propagation delay is too short, you may have to slow down the bits, or save them temporarily (known as buffering), so that the rest of the networking equipment can catch up with the bit.