Bits (0s and 1s) need to be transmitted Bits (0s and 1s) need to be transmitted from one host to another.from one host to another.

Each bit is placed on the cable as an Each bit is placed on the cable as an electrical signal or pulse.electrical signal or pulse.

On copper cable the pulses are electrical On copper cable the pulses are electrical signals of different voltage levels.signals of different voltage levels.

The simplest signalling schemes have The simplest signalling schemes have only two voltage levels, representing 1 only two voltage levels, representing 1 and 0.and 0.

Signal Loss on Copper Signal Loss on Copper CableCable

A pulseA pulse

Volts

5

0Time

Signal reference groundSignal reference ground

Both hosts must measure volts from the Both hosts must measure volts from the same starting point so that 0 volts at same starting point so that 0 volts at one host is the same as 0 volts at the one host is the same as 0 volts at the other host.other host.

They need a reference point, so this point They need a reference point, so this point is called the signal reference ground.is called the signal reference ground.

It is created by grounding circuit boards It is created by grounding circuit boards to the computer case and then literally to the computer case and then literally to the ground.to the ground.

An ideal systemAn ideal system

We would like a 5 volt signal from one host We would like a 5 volt signal from one host to arrive as an unchanged, identical 5 volt to arrive as an unchanged, identical 5 volt signal at the other host.signal at the other host.

But it’s not like that in real lifeBut it’s not like that in real life

AttenuationAttenuation

As a signal propagates, or travels, it As a signal propagates, or travels, it becomes weaker. This is attenuation.becomes weaker. This is attenuation.

If a signal becomes too weak then the If a signal becomes too weak then the receiving host cannot tell if it is truly a receiving host cannot tell if it is truly a 0 or a 1.0 or a 1.

AttenuationAttenuation

The maximum length allowed for UTP cable The maximum length allowed for UTP cable is 100 meters, as longer cables give too is 100 meters, as longer cables give too much attenuation causing signals to be much attenuation causing signals to be too weak to be read.too weak to be read.

Energy leaks through the insulation, as well Energy leaks through the insulation, as well as bad connectors causes attenuation.as bad connectors causes attenuation.

Attenuation is measured in decibels.Attenuation is measured in decibels.

RepeaterRepeater

Repeaters can be used to regenerate and re-Repeaters can be used to regenerate and re-time signals.time signals.

While this allows signals to travel longer While this allows signals to travel longer distances, a signal can only be regenerated distances, a signal can only be regenerated a limited number of times.a limited number of times.

ReflectionReflection

If a voltage pulse hits an area where some If a voltage pulse hits an area where some of the energy can be reflected, we create of the energy can be reflected, we create confusing extra signals on the cable. This confusing extra signals on the cable. This is why certain types of networks, such as is why certain types of networks, such as bus topologies need terminators placed bus topologies need terminators placed at the ends of the cable runs.at the ends of the cable runs.

These extra signals can also reflect and These extra signals can also reflect and cause multiple echo effects called jittercause multiple echo effects called jitter..

ReflectionReflection

Impedance mismatches (differences Impedance mismatches (differences in resistance) can cause reflection in resistance) can cause reflection also. To avoid problems with also. To avoid problems with reflectionreflection• Choose cables with the correct Choose cables with the correct

impedanceimpedance• Make sure that all connectors are Make sure that all connectors are

properly installedproperly installed Cat 5 cable normally has 100 ohms Cat 5 cable normally has 100 ohms

impedanceimpedance

DispersionDispersion

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

PropagationPropagation

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

NoiseNoise

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

Signal + Noise = ????????Signal + Noise = ????????

NoiseNoise

It is not possible to avoid all noise, It is not possible to avoid all noise, although it is important to keep the although it is important to keep the signal as high as possible, and to signal as high as possible, and to keep the noise as low as possible. keep the noise as low as possible. This is called the signal to noise ratio. This is called the signal to noise ratio.

If there is too much noise then the If there is too much noise then the receiving device will not be able to receiving device will not be able to decode the signals correctly.decode the signals correctly.

Noise from insideNoise from inside

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

Noise from insideNoise from inside

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

CancellationCancellationA signal needs two wires for transmission A signal needs two wires for transmission

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

CancellationCancellation

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

Good and badGood and bad connectorsconnectors

Good connector – wires untwisted just Good connector – wires untwisted just enough to put the connector on.enough to put the connector on.

Bad connector – wires untwisted too Bad connector – wires untwisted too far, exposing them to crosstalk.far, exposing them to crosstalk.

Noise from outsideNoise from outside

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

Another source of noise is Radio Another source of noise is Radio Frequency interference (RFI). RFI Frequency interference (RFI). RFI comes from Radio or TV signals.comes from Radio or TV signals.

ShieldingShielding

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

ShieldingShielding

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

STPSTP