This first post is two classes combined.
Chapter 2- Wireless Data Transmission
Notes and stuff after the break
-Data is represented by the binary digits 1 and 0.
-
-Be able to convert binary to decimal and
back 128 64 32 16 8 4 2 1
-The original ASCII was 128 characters- values from
0-127. It takes 7 bits to represent -128
items/characters. There is now an
extended ASCII table that deals with 128-255.
Wireless
signals
-Wireless data signals travel on electromagnetic
waves at the speed of light- 300,000kmps/186,000mps.
-There are two types of waves for wireless use-
infrared light and radio waves.
-The longer the wavelength, the lower the frequency
is. The shorter waves don’t even have a
name for the frequencies.
Infrared
Light
-It’s easy to transmit information with light, as
computers and data communication equipment use binary. A 1 in binary code could result in a light
quickly flashing on or off. This method
is much like Morse Code, just with on/off instead of dots and dashes.
-Infrared light is adjacent to visible light, but
invisible. It is, in general, a better
medium for data transmission because it is less susceptible to
interference.
-These systems require an emitter that transmits an LED signal and a detector that receives the signal.
-Infrared wireless systems send data by the intensity
of the light wave. The detector senses
the higher intensity pulse of light and produces a proportional electrical
current.
-Infrared wireless transmission types include directed transmission (line of sight,
or LOS) and diffused transmissions,
which rely on reflected light.
-Advantages:
It does not interfere with other types of communication signals, it does
not penetrate walls
-Limitations:
lack of mobility, short range of coverage- only 50ft/15m, it can only be
used indoors. Speed of transmission is
limited as well- diffused transmissions can only send data up to 4mbps. Due to the wide beam, the signal becomes weak
quickly, resulting in loss of data.
Radio
Waves
-Radio waves are the most common and effective means
of wireless communications today. Energy
travels through space or air in electromagnetic waves.
-Radio (radiotelephony) waves are created when am
electric current passes through a wire.
It creates a magnetic field around the wire. As this magnetic field radiates out, it
creates the waves. The wires could be
made of copper, aluminum, silver, gold, ect.
-Advantages of radio waves are: travel great distances, penetrate nonmetallic
objects and they are invisible. They can
be used to transmit data over long distances and without the need for
wires. The types of data are analog and
digital.
Analog
and Digital
-Analog signal intensity (voltage or amplitude)
varies and is broadcast (occurs) continuously.
Examples are audio, video, voice and light.
-You turn it on, ramps up like a lightbulb, when the power is cut, it powers down and the signal ramps down.
-You turn it on, ramps up like a lightbulb, when the power is cut, it powers down and the signal ramps down.
-How do you get a negative voltage with a
battery? Turn it around ;)
-LED cannot use a backward battery. The way they are built, it only conducts in
one direction, so if the flow is reversed, it is blocked, sometimes catastrophically.
-The flow of energy in a battery is out the negative
side (gtfo!) and in the positive.
-DC (direct current) Resistance causes electrons to
lose their energy over time.
-AC The highest points are called peaks, the lowest
is also called a peak. P-P (peak to
peak) voltage is calculated by the distance between the two.
-Digital signal consists of discrete or separate
pulses. It has numerous starts and stops
throughout the signal stream, for example; Morse code. Computers operate using digital signals. The analog signal must be converted into a
digital format before it can be stored and processed or interpreted by a
computer.
-A Modem (modulator/DEModulator) is a piece of
equipment that converts the distinct digital signals from a computer and
encodes them into a continuous analog signal for transmission over analog phone
lines. Modulation is the process of
encoding the digital signals (bits) onto an analog wave.
Frequency
-Frequency is calculated by how long it takes for a
cycle to complete. If a cycle takes 10
seconds, the frequency is .1Hz. If it takes 1/10th of a second, its
10Hz. And so on and forth
-A Cycle is composed of one top and one bottom
peak.
-A Carrier signal is sent by radio transmitters in a
continuous wave (CW) of constant amplitude and frequency. The signal is an up and down wave called an
oscillating signal or a sine wave.
-A change in voltage does not create a change in
frequency. The term Hertz (Hz) is used
to measure frequency. The wave measured
as 710,000 Hz is referred to as 710 KHz.
-Kilohertz
(KHz) is 1,000 Hz
-Megahertz
(MHz) is 1,000,000 Hz
-Gigahertz
(GHz) is 1,000,000,000 Hz
This wave is 3Hz.
-Antennas are a length of copper wire or similar
material with one end free and the other connected to a receiver or
transmitter. Electrical current moves
the antenna at the same frequency as the radio waves. The electric and magnetic fields are always
at a 90 degree angle of each other.
Transmission
Speed
-The speed of transmission is usually shown in bits
per second (bps). The baud rate is
another term used in measuring the speed of radio transmission. It is the number of signal units per second
that are required to represent the bits transmitted. The Baud is a change in the carrier
signal. It is possible to have a change
in signal (a baud) represent more than 1bit.
-Analog modems transmit at a rate of 4800 baud, the maximum
number of signal changes per second that a phone line can support. Current modems can transmit up to 33,600 bps
using more complex modulation techniques along with data compression.
-Bandwidth is the range of frequencies that can be
transmitted by a particular system or medium.
It also refers to the maximum data transmission capacity and is only
accurate when referring to purely digital systems.
-IEEE 802.3 is what covers the transmission of data
over cables.
-Baseband is dc on a wire. Un-modulated pulses on a wire. Broadband is modulated
pulses. Broadband is the range of frequencies that must be carried on a
line. The bandwidth is the space between
the top and bottom frequencies.
Analog
Modulation
-Analog Modulation is a representation of analog
information by an analog signal.
-Types of Analog Modulation:
Amplitude
Frequency
Phase
-Amplitude modulation
(AM); the height of a carrier wave is known as the amplitude and can be
measured in volts. The height of the
carrier wave is changed in accordance with the height of the modulating
signal. This is used by broadcast radio
stations and are very susceptible to interference from outside sources.
-Frequency modulation (FM) is the number of waves
that occur in one second change and is based on the amplitude of the modulating
signal. It is often used by broadcast
radio stations and is not as susceptible to interference from outside
sources. FM carrier has a wider
bandwidth that allows it to carry Hi-Fi as well as stereophonic signals.
-Phase modulation (PM) changes the starting point of
the cycle. It is not generally used to
represent analog signals. A signal
composed of sine waves has a phase associated with it. Phase is measured in degrees and one complete
wave cycle covers 360 degrees. A phase
change is always measured with reference to some other signal. PM systems almost always use the previous
wave cycle as the reference signal.
Digital
Modulation
-A method of encoding a digital signal onto an
analog wave for transmission over a medium that does not support digital
signals. In a digital system, the
changes are distinct using binary signals which exist in one of two states, a 1
or 0. For a computer to be able to
understand these signals each bit must have a fixed duration to represent 1 or
0.
-Advantages are:
Makes
better use of the bandwidth available
Requires
less power to transmit
Performs
better when the signal experiences interference from other signals
Its error-correcting
techniques are more compatible with other digital systems
-There are three basic types of digital modulations;
amplitude, frequency and phase. As users
demand more transmission speed; today there are dozens of different types of
modulation.
Next time: finishing
up digital modulation and the rest of chapter 2, start on chapter 3, review
chapter 2 on Friday. Eval for ch2 2 Mondays
from now. Review questions going
out. Go over pictures for review.
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