4557 comm networks

cryptography

transforming data using complex algorithm called a cipher secret data used called keys allows us to control access to data

narrowband

transmission channel with a single voice channel

wideband

transmission channel with wider bandwidth than one voice channel

low earth orbit

travels at a faster speed than earth often used for mobile phones and GPS

middle earth orbit

travels at a lower speed than the earth often used for mobile phones and GPS

unguided media

travels from antennas through air, water and space wireless media everywhere must be shared

sound wave frequency

1 hertz=1 vibration per second number of pulses of a sound wave that reach your ear in one second applications like sound where we can hear the change

decimal notation

10 possible values in each place use place values each place to the left increases by power of 10 for example 506 - 6 ones, no tens and 5 100s 6*1+0+10+5*100

FM radio spectrum

200 khz wide baseband signal gets 15 kHz the fm dial ranges from 87.5 to 109 mhz

ASCII

American Standard Code for Information Interchange a way of representing characters, letters numbers or symbols with numbers or bits

Must hopping

MEO and LEO ground receivers must hop from satellite to satellite to get constant signal network of satellites in elliptical orbit requires inter satellite links and handoffs between satellites

symmetric lock and key

Same key for locking and unlocking message is inside the box and you use the key to lock the box allows for a truly secret convo between those who have identical keys

frequency modulation (FM)

The frequency of the carrier is made to vary in proportion to the changing amplitude of the modulating signal the amplitude of modulated carrier remains constant

signal

a time varying event (wave to convey info over a distance) traffic signals use light waves phone calls use waves that are transmitted over wire, optical fiber, air and through space

wave forms

a wave form shows the shape of a signal as it varies over time can be periodic or aperiodic

spectrum

a continuous range of frequencies that can propagate over a medium often subdivided into narrower bands

aliasing

a false pattern appears in the message signal when it is reproduced avoid by using higher resolution or sampling frequency when digitizing original

channel

a frequency band reserved for transmitting and receiving electromagnetic signals allows us to send multiple independent streams of data on a single transmission medium

asymmetric key rules

a key pair consists of a public and secret key belonging to a secret individual keys in a key pair are complements if public locks, corresponding secret unlocks if secret locks, corresponding public locks

waves

a mode of energy transfer think ripples in a pond

Local Area Network (LAN)

a network available over a limited area such as an office, building or campus

phase

a shift of a sine wave expressed in terms of degrees

sound

a signal of information transmitted as a wave interpretation of very rapid vibrations the ear drum transmitted as waves of pressure

fourier theorem

all waveforms can be broken down into a combo of sine waves

electromagnetism

alternating current electrical current repeatedly and regularly alternates alternating current creates radiation more alterations means more radiation

three parameters of sine waves

amplitude frequency phase

noise

any signal that does not convey useful signal may interfere with hat you are using to transmit information signal to noise ratio: how much the original signal is affected by noise

transceivers

are two way devices, acting as transmitters and receivers

electromagnetic wave propagation

as with magnets, the + and - aspects of the wave act like the poles on a magnet as each radio wave is emitted it peoples the preceding wave

guard bands

band of frequency left vacant between two channels to protect against interference this allows for a clear sound with no interferences am has practically no guard

significance of bandwidth

bandwidth is proportional to the rate of reliable communication higher bandwidth means more info can be transmitted FM gets more bandwidth than AM which is why it has better sound more bandwidth means better signal

broadband

can usually provide multiple channels of data over a single communications medium

AM Carrier

carriers waves frequency is always much higher than modulating signals frequency between those frequencies you have the frequencies of different AM stations

periodic waveforms

caused by oscillating option called sine waves

connecting endpoints

circuit (comm path over an established medium) line or link (one segment of a circuit) switch (establishes and maintains connections over physical circuits)

switched circuit

circuit temporarily established at request of one or more connected stations

telecommunication

communication at a distance can be accomplished with smoke or flags, light or electrons refers to sending and receiving electromagnetic waves by electronic devices

antennas

conduct electromagnetic energy factors influencing distance signal will travel - strength of originating broadcast - possible interference - terrain - vicinity of nearest repeaters

connections

connectionless - each packet processed individually - no guarantees packets will arrive at destination in sequence connection oriented - specific paths established prior to sending first packet - once a path is set, all packets flow in sequence down the same physical path

virtual circuit

data from source is passed to destination over various real circuit configurations

types of circuits

dedicated, switched, virtual

multiplexing

deriving two or more channels from a single transmission medium/circuit combing several signals into one allows us to send more info over medium

receiver

detects and converts the signal back into usable information

dedicated circuit line

directly connects devices across a network without assessing a switch

wavelength

distance wave travels in one period is a wavelength electromagnetic signals all travel at the same speed

vector representation of images

don't specify each pixel value specify instructions for determining pixel values such as line segment from pixel triangle with verticies

representing a picture

draw a grid of rectangles over the image represent each cell with a bit string that represents the cell color

types of signals

electrical, acoustical and light

dictionary techniques

encode sequence of symbols with location of sequence in a dictionary (8,8) could replace the word sequence

analog multiplexing

frequency division multiplexing each signal modulated onto its own frequency modulated signals are combined into composite signal

satellite types

geostationary earth orbit middle earth orbit low earth orbit

networking components

host: a computer on a network hub: a device for connecting multiple hosts to a common network router- a device for interconnecting multiple networks

frequency

how often a sine wave repeats itself measured in cycles/second

character strings

how to represent character strings a collection of adjacent bit strings can store a sequence of letters

limits of light and sound

humans hear acoustic waves between 20Hz and 20kHz humans can see electromagnetic waves between 450-750 thz

digital

info converted into 1s and 0s sent as impulse signals uses repeaters to reproduce signal for transmission over some distance internet, dvd, hdtv

analog

info represented as continuous physical quantities sent as wave signals uses amplifiers to boost signal for transmission over some distance

transmitter subcomponents

input converter (combines message with carrier) amplifier

modulation process

input: message bearing signal modulation: message bearing signal combined with carrier output: transports signal from one place to another as a modulated carrier

undersampling

insufficient samples collected to accurately reconstruct original message

frequency & wavelength

inversely proportional wavelengths vary greatly over spectrum radio waves long as a football field visible light waves smaller than a cell

digital transmission advantages

less chance of garbling because there are fewer signal variations to distinguish among

cryptography goals

message confidentiality message integrity remove authentication message non repudiation

AM demodulation

modulated signals positive peaks contain same info as negative peaks

information and communication technologies

more common term now for tools for transmitting and manipulating information

network

multiple computers communicating over shared transmission channel can be wired wireless or both

bitrate

number of bits per second used to represent info digitally

bandwidth

numerical difference between upper and lower frequencies used by a modulated signal on a transmission medium

binary notation

only 2 possible values in each place 0 or 1 use place value - right most place represents how many 1s - next place to the left for 2s - next place to the left for 4s each place increases by the power of 2

digital pros and cons

pro: no loss of quality during reproduction enhance security takes less bandwidth con: overhead expense

modulation

process done with electromagnetic signals alter characteristics of a periodic carrier signal to enable it to carry info allows us to move signals to other frequencies

Asymmetric lock and key

public keys can be widely distributed makes digital signatures possible have to have public key in order for someone to be able to verify a signature can only open with the public key that is given to people

antenna design

radio antennas are most efficient when antenna height equals signal wavelength

demodulation

rectify or eliminate the negative part (one way circuit allows one way flow of electrons) low pass filter eliminates the higher carrier frequency

remedying delay problems

reduce delay by moving closer or reducing network interconnections reduce number of data transfers by caching or local processing

geostationary earth orbit

remans stationary in one spot above the earth good for 24 hour non stop communication orbits farthest from earth

compression

removes redundancy in signals same info but in fewer bits saves bandwidth

bit string exchange technique

represent bits as pulses each group of pulses is a bit string receiver uses threshold pulse detection the receiver needs only detect the presence of a pulse or not

Nyquist Sampling Theorem

sample at least twice max frequency to reconstruct waveform without distortion

fm stereo spectrum

since fm is 20x wider than Am there is a lot of space left over multiplexing

transmission time factors

size of message bitrate

propagation delay factors

speed of propagation distance message latency

transmitter

takes info and coverts it to a signal for transmission

amplitude modulation

the amplitude of the carrier is made to vary in proportion to the changing amplitude of the modulating signal

audible frequencies

the audible range of hearing for humans is 20 Hz to 20,000 Hz any sound below this range is classified as infrasound

amplitude

the energy conveyed by the wave for sound, amplitude corresponds to the strength of the vibration or intensity of the sound

propagation delay

the time it takes for a signal to travel through the transmission system from sender to receiver things that influence the delay: - nature of the medium - length of the circuit - number of network elements on path - amount of processing done on signal

period

time it takes sine waves to make one full cycle

Receiver subcomponents

tuner detector output

lossy compression

use one code to represent more than one thing common for pictures group of four pixels can represent each by code for the average color losing info used by jpeg

pros and cons of satellite communication

used for voice, data, private network, GPS, Satellite radio and TV cheaper than cable global coverage con: prone to environmental interference significant propagation delay

variable length coding

uses shorter codes for common bit strings only useful if we are certain what the signal will convey

typical video bitrates

using MPEG 2 a common standard

some frequencies are better

wavelength influences a signals ability to get past obstacles signals with short wavelengths are blocked by obstacles signals with long wavelengths diffract, allowing them to travel around obstacles

filtering frequencies

waves can be filtered based on their frequency filters are used to eliminate noise to create clear, undisturbed signals/sounds or to amplify certain ranges of audio frequencies

useful qualities of waves

waves travel, the medium doesn't we can transmit signals without sending physical objects waves travel through a variety of media we can choose the medium that works best for our communication needs

waves in the wild

we use waves/signals to transmit a variety of messages face to face convo, fm radio, broadcast tv

oversampling

when additional samples yield no perceivable difference in reconstructed message results in unnecessarily large files

guided media

wire and cable requires a physical tether less interference more secure than wireless

analog pros and cons

world is analog con: have noise hard to secure