Transcript ITS_7_Signal loss

Signal Loss on Copper
Cable
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.
A pulse
5
Volts
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
impedance
• Make 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
=
????????
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