Wireless Network Interfaces Lecture 2
Signals
All signals have three components: Amplitude, Frequency and Phase
digital signal
Digital signal Consists of discrete or separate pulses Has numerous starts and stops throughout the signal stream
analog modulation
In order to transmit analog data, you can modulate the data onto a set of analog signals Analog modulation types Amplitude modulation Frequency modulation Phase modulation
emitter -> intense (1), weak pulse (0) -> detector
Intense pulse, weak pulse
infrared
Some specialized wireless local area networks are based on the infrared method Used in situations where radio signals would interfere with other equipment
Signals -> electromagnetic waves
Wireless data signals travel on electromagnetic waves.
frequency
A change in voltage does not create a change in frequency
Infrared light
Adjacent to visible light (although invisible). A much better medium for data transmission. Less susceptible to interference. On (1), off (0)
Direct Sequence Spread Spectrum (DSSS)
Advantages DSSS signal appears to an unintended narrow-band receiver to be low-powered noise Noise can cause some of the chips to change value Receiver can recover the original data bit Using statistical techniques and mathematical algorithms Thus avoiding the need for retransmission DSSS devices are typically higher-end products Because they are more expensive to manufacture than FHSS systems
infrared light
Advantages It does not interfere with other types of communication signals. It is not affected by other signals (except light). Infrared light does not penetrate walls Signals are kept inside a room
digital modulation
Advantages Makes better use of the bandwidth available Requires less power to transmit Performs better when the signal experiences interference from other signals Error-correcting techniques are more compatible with other digital systems
radio waves
Advantages of radio waves Can travel great distances Can penetrate most solid objects with the exception of metallic ones Invisible
amplitude
Amplitude The height of the wave above or below a given reference point
Transmitting Analog Data with Analog Signals
Amplitude modulation (AM) Height of the carrier wave is changed in accordance with the height of the modulating signal Used by broadcast radio stations Very susceptible to interference from outside sources
analog signal
Analog signal The intensity of the waves (voltage or amplitude) varies It is broadcast continuously Examples: Audio, video, voice, and light
antenna
Antenna Length of copper wire, or similar material with one end free and the other end connected to a receiver or transmitter
Barker code
Barker code (or chipping code) A particular sequence of 1s and 0s Ideal for modulating radio waves As well as for being detected correctly by the receiver It is also called a pseudo-random code
binary signals
Binary signals Digital data transmissions are typically sent in bursts of bits Three types of binary signaling techniques can be used: Return-to-zero (RZ) Non-return-to-zero (NRZ) Polar non-return-to-zero (polar NRZ) Also known as non-return-to-zero-level (NRZ-L) A variation of NRZ-L is non-return-to-zero, invert-on-ones (NRZ-I)
carrier signal
Carrier signal Sent by radio transmitters Continuous wave (CW) of constant amplitude (also called voltage) and frequency An up-and-down wave called an oscillating signal or a sine wave
Signals
Computer networks and data/voice communication systems transmit signals
digital signal
Computers operate using digital signals Analog signal must be converted into a digital format before it can be stored and processed or interpreted by a computer
Direct Sequence Spread Spectrum (DSSS)
DSSS system transmits combinations of multiple chips 11 chips are transmitted at a rate 11 times faster than the data rate Characteristics Frequency of the digital component of the signal is much higher than that of the original data (chip rate) A plot of the frequency spectrum of this signal would look similar to random noise All of the information contained in the original signal (a 0 or a 1 bit) is still there!
Data
Data are entities that convey meaning (computer files, music on CD, results from a blood gas analysis machine)
EM Wave -> analog
Electrical current moves through the antenna at the same frequency as the radio waves Electromagnetic wave (EM wave) - continuous (analog) combination of magnetism and electrical pressure moving away from an antenna
frequency
Frequency - Rate at which a radio circuit creates the waves The number of times a cycle occurs within one second
FHSS (Frequency Hopping Spread Spectrum)
Frequency hopping can reduce the impact of interference from other radio signals An interfering signal will affect the FHSS signal only when both are transmitting at the same frequency and at the same time Because FHSS transmits short bursts over a wide range of frequencies, the extent of any interference will be small The error can be detected through error checking Message can be easily retransmitted
Frequency Modulation (FM)
Frequency modulation (FM) Number of waves that occur in one second undergoes change based on the amplitude of the modulating signal Often used by broadcast radio stations Not as susceptible to interference from outside sources FM carrier has a wider bandwidth Allows it to carry Hi-Fi as well as stereophonic signals
infrared light
Infrared wireless systems send data by the intensity of the light wave. Detector senses the higher intensity pulse of light. And produces a proportional electrical current.
infrared light
Infrared wireless transmission types. Directed transmission (called line-of-sight or LOS). Diffused transmission - relies on reflected light
Infrared light
It is easy to transmit information with light. Computers and data communication equipment use binary code Light has two properties: off and on. A 1 in binary code could result in a light quickly flashing on; a 0 could result in the absence of light.
Infrared light
Light spectrum Types of light that travel from the Sun to the Earth
infrared light
Light waves cannot penetrate through materials like wood or concrete Heat rays are absorbed by most objects Solid objects, dust and humidity can limit the distance that light and infrared waves can travel
infrared light
Limitations Lack of mobility Range of coverage Can cover a range of only 50 feet (15 meters) Diffused infrared can only be used indoors Speed of transmission
infrared light
Limitations Lack of mobility Range of coverage. Can cover a range of only 50 feet (15 meters). Diffused infrared can only be used indoors. Speed of transmission.
digital modulation
Method of encoding a digital data onto an analog wave for transmission over a medium that does not support digital signals or digital data onto a digital signal In a digital system, the changes are distinct using binary signals which exist in one of two states, a 1 or a 0 For a computer to be able to understand these signals each bit must have a fixed duration to represent 1 or 0
modem
Modem (Modulator/Demodulator) Converts the distinct digital signals from a computer Encodes them into a continuous analog signal for transmission over analog phone lines
modulation
Modulation Process of encoding the digital signals (bits) onto an analog wave
narrow-band transmission
Narrow-band transmissions Each signal transmits on one radio frequency or a very narrow range of frequencies Vulnerable to outside interference from another signal Radio signal transmissions are narrow-band
amplitude shift key
One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)
frequency shift key
One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)
phase shift key
One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)
radio waves
Radio (radiotelephony) waves When an electric current passes through a wire, it creates a magnetic field in the space around the wire As this magnetic field radiates or moves out, it creates radio waves
how radio data is transmitted
Radio waves can be used to transmit data over long distances without the need for wires Types of data Analog data Digital data
radio waves
Radio waves provide the most common and effective means of wireless communications today Energy travels through space or air in electromagnetic waves
infrared light
Require: Emitter that transmits a signal (LED). Detector that receives the signal.
signals
Signals are the electric or electromagnetic encoding of data (telephone conversation, web page download)
sine wave
Sine wave (analog wave) and frequency
spread spectrum transmission
Spread spectrum transmission Takes a narrow band signal and spreads it over a broader portion of the radio frequency band Results in less interference and fewer errors Two common methods: Frequency hopping and direct sequence
diffused infrared transmission
getting refraction spot, wide angle, detector, emitter
Directed infrared transmission
line of sight - emitter, detector
hertz
The term Hertz (Hz) is used to measure frequency A Kilohertz (KHz) is 1,000 Hertz A Megahertz (MHz) is 1,000,000 Hertz A Gigahertz (GHz) is 1,000,000,000 Hertz. Ex: The wave measured as 710,000 Hz is referred to as 710 KHz
Transmitting Digital Data with Digital Signals: Digital Encoding Schemes
There are numerous techniques available to convert digital data into digital signals. NRZ-L, NRZI, Manchester. Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltages and 0s as positive voltages Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0 Fundamental difference exists between NRZ-L and NRZI With NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1, With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1
There are three basic types of digital modulations:
There are three basic types of digital modulations: Amplitude, frequency, and phase. Users demand more transmission speed Today there are dozens of different types of modulation
Speed of light -> electromagnetic waves -> signals
Through space at the speed of light 186,000 miles per second (300,000 kilometers per second)
Transmit digital signal over analog medium
To transmit a digital signal over an analog medium, a modem is used
Frequency Hopping Spread Spectrum (FHSS)
Uses a range of frequencies Changes frequencies several times during transmission Hopping code Need to know the sequence of changing frequencies The receiving station must also know the hopping code Multiple radios can each use a different sequence of frequencies within the same area and never interfere with each other If interference is encountered on a frequency, only a small part of the message is lost
Direct Sequence Spread Spectrum (DSSS)
Uses an expanded redundant code to transmit each data bit And then a modulation technique such as QPSK A DSSS signal is effectively modulated twice. Before transmission, add the original data bit to the chipping code
wavelength
Wavelength - The distance between any point in one wave cycle and the same point in the next wave cycle