Bits (0s and 1s) need to be transmitted from one host to another. Each bit is placed on the cable as an electrical signal or pulse. On copper cable the pulses are electrical signals of different voltage levels. The simplest signalling schemes have only two voltage levels, representing 1 and 0. Signal Loss on Copper Cable

A pulse Volts 5 0 Time

Signal reference ground Both hosts must measure volts from the same starting point so that 0 volts at one host is the same as 0 volts at the other host. They need a reference point, so this point is called the signal reference ground. It is created by grounding circuit boards to the computer case and then literally to the ground.

An ideal system We would like a 5 volt signal from one host to arrive as an unchanged, identical 5 volt signal at the other host. But it’s not like that in real life

Attenuation As a signal propagates, or travels, it becomes weaker. This is attenuation. If a signal becomes too weak then the receiving host cannot tell if it is truly a 0 or a 1.

Attenuation The maximum length allowed for UTP cable is 100 meters, as longer cables give too much attenuation causing signals to be too weak to be read. Energy leaks through the insulation, as well as bad connectors causes attenuation. Attenuation is measured in decibels.

Repeater Repeaters can be used to regenerate and re-time signals. While this allows signals to travel longer distances, a signal can only be regenerated a limited number of times.

Reflection If a voltage pulse hits an area where some of the energy can be reflected, we create confusing extra signals on the cable. This is why certain types of networks, such as bus topologies need terminators placed at the ends of the cable runs. These extra signals can also reflect and cause multiple echo effects called jitter.

Reflection  Impedance mismatches (differences in resistance) can cause reflection also. To avoid problems with reflection Choose cables with the correct impedanceChoose cables with the correct impedance Make sure that all connectors are properly installedMake sure that all connectors are properly installed  Cat 5 cable normally has 100 ohms impedance

Dispersion Dispersion is where a signal spreads out over time. It can be limited by good cable design, correct impedance and limiting cable lengths.

Propagation Propagation is the movement of a signal. As a signal takes a certain amount of time to travel, this time is called propagation time or propagation delay.

Noise Noise is unwanted additional electrical signals on a cable. The extra signals are added to the data signals and thus distort them. Signal + Noise = ???????? Signal + Noise = ????????

Noise It is not possible to avoid all noise, although it is important to keep the signal as high as possible, and to keep the noise as low as possible. This is called the signal to noise ratio. If there is too much noise then the receiving device will not be able to decode the signals correctly.

Noise from inside As voltage signals pass along a wire, they generate energy that radiates out, like a radio signal. Magnetic fields are induced. Other nearby wires can pick up the signal as crosstalk.

Noise from inside The purpose for the twisting of the pairs in a twisted pair data cable is to negate these fields and reduce the possibility of crosstalk.

Cancellation A signal needs two wires for transmission in order to make a complete electrical circuit. The pair of wires that are twisted are carrying current in opposite directions, the electromagnetic fields that they create tend to cancel each other out.

Cancellation The higher the category of cabling, the more twisting is needed. When putting on a connector or installing cable, you must insure that the wire pairs stay twisted as much as possible.

Good and bad connectors Good connector – wires untwisted just enough to put the connector on. Bad connector – wires untwisted too far, exposing them to crosstalk.

Noise from outside Noise comes from outside the system as well. One source of noise is (EMI) Electromagnetic interference. EMI is caused through electrical motors, Fluorescent lights, or Power cables. Another source of noise is Radio Frequency interference (RFI). RFI comes from Radio or TV signals.

Shielding UTP, or unshielded twisted pair, cables have no shielding against external noise or crosstalk. Shielded twisted pair (STP) cables have shielding but must be properly grounded. Shielding that is not grounded can act as an antenna to pick up signals and make the noise problem worse.

Shielding Since the twisting cancels out most crosstalk, and STP is more expensive to make and use, most networks use UTP. STP