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