Activity Interactivity: Noise - White and
Narrowband This activity allows the user to enter a value for
amplitude and a value for frequency and then generate the white
noise and narrowband noise.
Content 4.1
Background for Studying Frequency-Based Cable Testing
4.1.8 Bandwidth Bandwidth is an extremely important
concept in communications systems. Two ways of considering
bandwidth that are important for the study of LANs are analog
bandwidth and digital bandwidth. Analog bandwidth typically
refers to the frequency range of an analog electronic system.
Analog bandwidth could be used to describe the range of
frequencies transmitted by a radio station or an electronic
amplifier. The units of measurement for analog bandwidth is
Hertz, the same as the unit of frequency. Examples of analog
bandwidth values are 3 kHz for telephony, 20 kHz for audible
signals, 5 kHz for AM radio stations, and 200 MHz for FM radio
stations. Digital bandwidth measures how much information can
flow from one place to another in a given amount of time. The
fundamental unit of measurement for digital bandwidth is bits
per second (bps). Since LANs are capable of speeds of millions
of bits per second, measurement is expressed in kilobits per
second (Kbps) or megabits per second (Mbps). Physical media,
current technologies, and the laws of physics limit bandwidth.
During cable testing, analog bandwidth is used to determine the
digital bandwidth of a copper cable. Analog frequencies are
transmitted from one end and received on the opposite end. The
two signals are then compared, and the amount of attenuation of
the signal is calculated. In general, media that will support
higher analog bandwidths without high degrees of attenuation
will also support higher digital bandwidths. Web Links
Bandwidth Basics – A Comparison of Connection Speeds
http://www.wesonline.com/techsystems/ bandwidth.htm
Content 4.2 Signals and Noise 4.2.1
Signaling over copper and fiber optic cabling On copper
cable, data signals are represented by voltage levels that
represent binary ones and zeros. The voltage levels are
measured with respect to a reference level of zero volts at
both the transmitter and the receiver. This reference level is
called the signal ground. It is important that both
transmitting and receiving devices refer to the same zero volt
reference point. When they do, they are said to be properly
grounded. In order for the LAN to operate properly, the
receiving device must be able to accurately interpret the
binary ones and zeros transmitted as voltage levels. Since
current Ethernet technology supports data rates of billions of
bits per second, each bit must be recognized, even though
duration of the bit is very small. The voltage level cannot be
amplified at the receiver, nor can the bit duration be extended
in order to recognize the data. This means that as much of the
original signal strength must be retained, as the signal moves
through the cable and passes through the connectors. In
anticipation of ever-faster Ethernet protocols, new cable
installations should be made with the best available cable,
connectors, and interconnect devices such as punch-down blocks
and patch panels. There are two basic types of copper cable:
shielded and unshielded. In shielded cable, shielding material
protects the data signal from external sources of noise and
from noise generated by electrical signals within the cable.
Coaxial cable is a type of shielded cable. It consists of a
solid copper conductor surrounded by insulating material, and
then braided conductive shielding. In LAN applications, the
braided shielding is electrically grounded to protect the inner
conductor from external electrical noise. The shielding also
helps eliminate signal loss by keeping the transmitted signal
confined to the cable. This helps make coaxial cable less noisy
than other types of copper cabling, but also makes it more
expensive. The need to ground the shielding and the bulky size
of coaxial cable make it more difficult to install than other
copper cabling. There are two types of twisted-pair cable:
shielded twisted-pair (STP) and unshielded twisted pair (UTP).
STP cable contains an outer conductive shield that is
electrically grounded to insulate the signals from external
electrical noise. STP also uses inner foil shields to protect
each wire pair from noise generated by the other pairs. STP
cable is sometimes called screened twisted pair (ScTP). STP
cable is more expensive, more difficult to install, and less
frequently used than UTP. UTP contains no shielding and is more
susceptible to external noise but is the most frequently used
because it is inexpensive and easier to install. Fiber optic
cable is used to transmit data signals by increasing and
decreasing the intensity of light to represent binary ones and
zeros. The strength of a light signal does not diminish like
the strength of an electrical signal does over an identical run
length. Optical signals are not affected by electrical noise,
and optical fiber does not need to be grounded. Therefore,
optical fiber is often used between buildings and between
floors within the building. As costs decrease and demand for
speed increases, optical fiber may become a more commonly used
LAN media. Web Links Fiber Optic Page
http://www.eco.utexas.edu/faculty/Norman/
long.extra/Student.F98/ modem/FourthRight.html
Content
4.2 Signals and Noise 4.2.2
Attenuation and insertion loss on copper media Attenuation
is the decrease in signal amplitude over the length of a link.
Long cable lengths and high signal frequencies contribute to
greater signal attenuation. For this reason, attenuation on a
cable is measured by a cable tester using the highest
frequencies that the cable is rated to support. Attenuation is
expressed in decibels (dB) using negative numbers. Smaller
negative dB values are an indication of better link
performance. There are several factors that contribute to
attenuation. The resistance of the copper cable converts some
of the electrical energy of the signal to heat. Signal energy
is also lost when it leaks through the insulation of the cable
and by impedance caused by defective connectors. Impedance is a
measurement of the resistance of the cable to alternating
current (AC) and is measured in ohms. The normal, or
characteristic, impedance of a Cat5 cable is 100 ohms. If a
connector is improperly installed on Cat5, it will have a
different impedance value than the cable. This is called an
impedance discontinuity or an impedance mismatch. Impedance
discontinuities cause attenuation because a portion of a
transmitted signal will be reflected back to the transmitting
device rather than continuing to the receiver, much like an
echo. This effect is compounded if there are multiple
discontinuities causing additional portions of the remaining
signal to be reflected back to the transmitter. When this
returning reflection strikes the first discontinuity, some of
the signal rebounds in the direction of the original signal,
creating multiple echo effects. The echoes strike the receiver
at different intervals making it difficult for the receiver to
accurately detect data values on the signal. This is called
jitter and results in data errors. The combination of the
effects of signal attenuation and impedance discontinuities on
a communications link is called insertion loss. Proper network
operation depends on constant characteristic impedance in all
cables and connectors, with no impedance discontinuities in the
entire cable system. Web Links What is Signal
Attenuation? http://www.vislab.usyd.edu.au/photonics/
fibres/fibre/ attenuation0.html
Content 4.2
Signals and Noise 4.2.3 Sources of noise on
copper media Noise is any electrical energy on the
transmission cable that makes it difficult for a receiver to
interpret the data sent from the transmitter. TIA/EIA-568-B
certification of a cable now requires testing for a variety of
types of noise. Crosstalk involves the transmission of signals