|  | 4.3 | Basics of Signals and Noise in Communications Systems
| 4.3.8 | Dispersion, jitter, and latency
While dispersion, jitter, and latency are actually three different things that can happen to a bit, they are grouped together because they each affect the same thing - the timing of a bit. Since you are trying to understand what problems might occur as millions and billions of bits travel on a medium in one second, timing matters a lot. Dispersion is when the signal broadens in time. It caused by is the type of media involved. If serious enough, 1 bit can start to interfere with the next bit and confuse it with the bits before and after it. Since you want to send billions of bits per second, you must be careful not to allow the signals to spread out . Dispersion can be fixed by proper cable design, limiting cable lengths, and finding the proper impedance. In optical fibers, dispersion can be controlled by using laser light of a very specific wavelength. For wireless communications, dispersion can be minimized by the frequencies used to transmit. All digital systems are clocked, meaning clock pulses cause everything to happen. Clock pulses cause a CPU to calculate, data to store in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, you will get timing jitter. This means that bits will arrive a little earlier and later than expected. Jitter can be fixed by a series of complicated clock synchronizations, including hardware and software, or protocol synchronizations. Latency, also known as delay, has two main causes. First, Einstein’s theory of relativity states, "nothing can travel faster than the speed of light in a vacuum (3.0 x 108 meters/second)." Wireless networking signals travel at slightly less than the speed of light in vacuum. Networking signals on copper media travel in the range of 1.9x10^8 m/s to 2.4x10^8 m/s. Networking signals on optical fiber they travel at approximately 2.0x10^8 m/s. So to travel a distance, a bit takes at least a small amount of time to get to where it’s going. Second, if the bit goes through any devices, the transistors and electronics introduce more latency. The solution to the problem of latency is the careful use of internetworking devices, different encoding strategies, and various layer protocols. Modern networks typically work at speeds from 1 Mbps-155 Mbps and greater. Soon they will work at 1 Gbps or 1 billion bits in one second. If bits are broadened by dispersion, then 1s can be mistaken for 0s and 0s for 1s. If groups of bits get routed differently and there is no attention paid to timing, the jitter can cause errors as the receiving computer tries to reassemble packets into a message. If groups of bits are late, the networking devices and other destination computers might get hopelessly lost and overwhelmed by a billion bits per second.