IT 300 Midterm Exam

DSSS - PN Codes (ex., CDMA)

CDMA is a form of DSSS Communicating users are given a unique PN code that is shared between the source and destination The PN code is an "orthogonal" code that is used to expand the digital transmission (i.e., increase the number of bits transmitted). This results in a higher bit rate (called chip rate). The chip rate is larger than the information bit rate, which in turn increases the frequency bandwidth of the transmitted signal.

Periodic versus

Carrier waves are always periodic Represented by either a sine or cosine wave Periodic waves repeat some portion over and over again.

FM,

Frequency Modulation (FM) Voltage controlled oscillators (Kvco) converts message amplitude changes into frequency changes - Kvco unit isHz/volts

Aperiodic

Most baseband messages/information, are aperiodic (i.e., non-repeating pattern) For illustration, a simple periodic sinusoidal waveform is used Aperiodic waves are random

Plesichronous

Plesiochronous devices run at nominally the same rate with defined parameters of tolerance for variations

TIR, Refractive Index

(RI or n); n=c/Vs

MODEM (modulate/demodulate)

Analog baseband signal (message) Electrical or E-M signal Typically modulate this information onto a higher carrier frequency. MODEM (Modulator Demodulator)

Optical Signals at higher frequencies have ________.

At higher frequencies (THz range), greater capacity

Analog Information/Message to Analog Signal (modulated signal) eq

Information/message: m(t)=Amsin(2πfmt ±ɸ) Unmodulated Carrier: c(t)=Acsin(2πfct ±ɸ)

Intermodulation Noise

Intermodulation Noise - caused by signals with different frequencies sharing a common medium where bandwidth overlap and unintended frequency harmonics may exist

Layer 5 (Session Layer):

Manages communication sessions between hosts (ex., NetMeeting, etc.)

Optical Transmission Windows (wavelength)

Most commonly used wavelengths are 850, 1300, 1310 and 1550 nm High speed systems operate in various transmission windows specified by the ITU-T

Multiplexing

Multiplexing combines several baseband signals into an aggregate broadband signal

How we judge the quality of light source: Power,

Optical power. Needs to be as high as possible (higher power is better)

How we judge the quality of light source: Spectral Width,

Refers to range of wavelengths emitted by light source. LEDs have greatest spectral width (Narrower is better)

Foot Print,

The area of coverage on the earth's surface is called the "foot print"

optical signals are immune from _____.

Immune from RFI (Radio Frequency Interference) and EMI (Electromagnetic Interference) - photons

Resistance of the transmission line _______

attenuates the electrical signal

Most networks are comprised of a combination of

electrical, optical and RF Signals

Signal representation -

sinusoidal wave form

AM Index: eq

µAM=Am/Ac , 0≤µAM≤1

PM Index: eq

μp=Kp*Am, Kp (radians/volts)

c(t) = A sin(2πft ± ɸ), c(t) = A cos(2πft ± ɸ)

A sin(2πft + π/2) = A cos(2πft)

FDM

Analog baseband channels can only be multiplexed using Frequency Division Multiplexing (FDM)

Layer 1 (Physical Layer):

Physical interface/medium where information is exchanged in the form of electrical signals, optical and electromagnetic energy (hardware, transmission medium, interface specifications, etc.) - these are just signals, not at the data yet, no logical data

Aliasing

Sampling at lower than the Nyquist rate results in an effect called aliasing

Frequency Bandwidth

Signals that carry information will have a frequency bandwidth that can be plotted in the frequency domain Bandwidth = FrequencyHIGH - FrequencyLOW

Simplex, ,

Simplex Communications - communication flows in one direction only - from source to destination.

, ADPCM

Adaptive DPCM varies the size of the quantization step dynamically as needed based upon message amplitude and frequency in order to increase efficiency Used on fiber optics and CD-ROMs However, noise distortion may result when a signal change is significant from one sample to the next

Quantization Noise/Error

The difference between the actual analog level and the quantized binary value is called quantization error

Thermal Noise

Thermal Noise (aka white noise) - caused by the activity of electrons which increase with temperature

Noise

Transmitted signals, whether guided or un-guided, experience interference (noise) from many sources:

Chromatic Dispersion

(different colors of light arriving at the detector at slightly different times)

Data Security Firewalls

- application software that block access by unauthorized entities

Digital Information/Message to Analog Signal

ASK, FSK, PSK (BPSK, QPSK),QAM

TDM

Analog baseband channels can not be multiplexed using Time Division Multiplexing (TDM) TDM is a digital baseband multilexing technique based upon time slots

discrete frequency

Carrier wave that has not been modulated by a user's information/message is a

Pulse Code Modulation - sampling and quantization

Analog information/messages (ex., voice, music, images, videos) can be digitized into binary "1s" and "0s" Analog information must be sampled and quantized

Access Method - DAMA, TDMA, CDMA, OFDMA

Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA)

Decibel Measure

In communication systems, power in decibels is referenced to a standard unit of power such as 1 watt or 1 milliwatt (1E-3 watt).

Packet switched circuit

Logical connections in which information in the form of data packets, do not necessarily following the same physical paths through a network.

PM

Phase Modulation Similar to FM, we need to change message amplitudes to phase angle changes

Phase Angle

The phase angle of a carrier wave represents the shift of the wave in radians from a reference point zero.

Hartley's Law: eq

C(bps) = Baud * log2M

Shannon-Hartley: eq

C(bps)=BW * log2(1+SNR)

Data Security Port Security

denial of unauthorized remote access

Signals that carry information will have a frequency bandwidth that can be plotted ____

in the frequency domain

Frequency Units of Measure

1 kHz (kilo) = 1000 * 1 Hz = 1x103 = 1E3 Hz 1 MHz (mega) = 1,000,000 * 1 Hz = 1x106 = 1E6 Hz 1 GHz (giga) = 1,000,000,000 * 1Hz = 1E9 Hz 1 THz (tera) = 1E12 Hz 1 PHz (peta) = 1E15 Hz

Dedicated circuit

: Dedicated circuits are distinct, physical circuits dedicated to directly connecting devices across a network

Digital information and signals

A Digital signal changes according to discrete values Ex., Computer logic consist discrete binary steps of 0's and 1's

Baseband,

A baseband signal is the information or message from a single source (ex., computer, radio channel, etc.)

Broadband

A broadband signal is an aggregate of more than one baseband signal. multiplexing

Passband,

A passband signal is a baseband signal shifted to a higher frequency for transmission. modulation

Optical Detectors - APDs

APDs are more sensitive that PIN diodes Generate an avalanche of electrons hence have high gain More powerful and expensive Generally matched with LDs and SMF

ASCII,

ASCII (American Standard Code for Information Interchange), developed in 1963 by ANSI (American National Standards Institute) 7 bit coding scheme 2 7 = 128 characters

Digital Speech Interpolation (DSI)

Ability to compress data makes the assumption that there are predictable pauses in normal human speech During these pauses, additional voice signals are inserted (silence suppression)i t works effectively when there are a significant number of voice conversations supported More conversations supported, the more predictable the average pauses Performance degrades during heavy traffic

Analog

An Analog signal is continuously changing in value. Ex., hands on a clock, speedometer, etc. Any communications system intended for human users will typically have an analog interface (ex., microphones, monitors,speakers, etc.)

E-M polarization,

An antenna designed for a given polarization will receive no energy from a wave with an orthogonal polarization Note: orthogonal or opposite (ex. Right versus Left polarization)

Timing - Digital Synchronization

Asynchronous Synchronous Isochronous Plesichronous

Error Detection ARQ

Automatic repeat request (ARQ) protocols Block of data with error is discarded Transmitter retransmits that block of data

Bandwidth: eq

BW=2(μp+1)fm

Bandwidth: eq

BW=2fm

Bandwith: eq

BW=2fm(1+)

Error performance (BER)

Bit Error Rate (BER) is a measure of the probability of how many bit errors will be experienced during digital transmission

Shannon Hartley Equation:

C(bps)=BW*log2(1+SNR) The Shannon-Hartley equation gives us some insight regarding the theoretical maximum capacity in data rate that we can achieve under given noise and bandwidth constraints C (bps) = B*log2 (1+SNR), B is frequency bandwidth What does this equation tell you? Capacity is directly related to B. The greater the B, the greater the capacity Increasing SNR also increases your ability towards higher capacity

which wire has Unbalanced configuration

Coaxial has single unbalanced conductor

Link

Communications path between two nodes (a circuit will be comprised of one or more links).

E-M field

Current flowing through a conductive wire creates an E-M field that rotates in a circular fashion around the conductor

Error Detection CRC

Cyclic Redundancy Check (CRC) Transmitter For a k-bit block, transmitter generates an (n-k)-bit frame check sequence (FCS) Resulting frame of n bits is exactly divisible by predetermined number (polynomial) Receiver Divides incoming frame by predetermined polynomial number If no remainder, assumes no error

DAMA

Demand Assigned Multiple Access (DAMA) enables multiple devices to share access to the same network on a demand basis i.e. first come first served

Spread Spectrum - DSSS

Direct Sequence Spread Spectrum (DSSS) andEach requires unique Pseudorandom Numbers assigned to communicating pairs These methods also require the use of digital signal processors (DSP) which drive data through sophisticated algorithms CDMA is a form of DSSS Communicating users are given a unique PN code that is shared between the source and destination The PN code is an "orthogonal" code that is used to expand the digital transmission (i.e., increase the number of bits transmitted). This results in a higher bit rate (called chip rate). The chip rate is larger than the information bit rate, which in turn increases the frequency bandwidth of the transmitted signal.

Layer 4 (Transport Layer):

Enables the existence of multiple communication links between nodes and ensures that data is transmitted and received by the intended applications (ex., TCP, UDP, etc.) - once the packets have been delivered to the computer or server, the transport layer establishes the connection link between the source and destination - ensures the quality of the data - provides data delivered to the computer to the proper application

Digital Signal Encoding - symbols to binary data

Encoding (mapping) symbols to binary data Mapping combination of "1's" and "0's" to symbols such as alphanumeric, punctuation marks, special characters, etc. Popular Coding Schemes: Baudot, ASCII, EBCDIC, Unicode

Circuit

End-to-end communications path between two or more points. May consists of one or more links.

Error Control

Error Detection - parity, CRC ARQ Error Correction - Block, Convolutional

Error Detection -

Error detection codes Detects the presence of an error parity, CRC

Extended ASCII,

Extended ASCII 8 bit coding scheme 2 8 = 256 characters

EBCDIC Unicode

Extended Binary Coded Decimal Interchange Code (EBCDIC), developed by IBM in 1962 8 bit coding scheme 2 8 = 256 characters

Optical Fiber

Fiber Optic systems consist of light sources, fibers and light detectors Electrical Signals electrons; Optical Signals photons What are some of the differences between these two signals? Light sources convert electrical signals into optical ones Light detectors convert optical signals back to electrical ones Optical Fibers are made of either glass or plastic Construction: Core and Cladding RFI, EMI Immunity

FDD,

Frequency Division Duplex (FDD) provides full duplex communications - use of separate transmit and receive channels

Spread Spectrum - FHSS,

Frequency Hopping Spread Spectrum (FHSS) Each requires unique Pseudorandom Numbers assigned to communicating pairs These methods also require the use of digital signal processors (DSP) which drive data through sophisticated algorithms Invented Frequency Hopping Spread Spectrum (FHSS) in which the communications channel changes frequencies at a rapid rate across the frequency bandwidth.

Full Mesh Versus Partial Mesh networks

Full - Every node in the network is connected to every other node in the network. Expensive and complex! But extremely reliable because of much redundancy. Partial - Every node is NOT connected to every other node. Problem with first example is if central node goes down, whole network goes down. Second is more reliable.

Full Duplex (FDX)

Full-duplex communication - communications occurs in both directions simultaneously

Error Correction - Convolutional

Generates redundant bits continuously Error checking and correcting carried out continuously (n, k, K) code Input processes k bits at a time Output produces n bits for every k input bits K = constraint factor

Half Duplex (HDX)

Half-duplex communication - communications travel in both directions, but only in one direction at a time

Impulse Noise

Impulse Noise - irregular noise spikes with high amplitudes (ex., lightening)

AM,

In Amplitude Modulation (AM), the carrier's amplitude is modified by the message wave.

Optical Light Sources - LDs

LDs More expensive and complex to manufacture than LEDs Faster Coherent light - monochromatic Narrow spectral width They can couple efficiently onto single mode fibers (SMF) Typically used with SMF for long haul communications Ability to multiplex several wavelengths together (ex., WDM (Wavelength Division Multiplexing) s Commonly used with avalanche photodiodes (APD) detectors

Optical Light Sources - LEDs

LEDs Slower than LDs Large range of wavelengths (i.e. large spectral width) Produce incoherent light Relatively common in LANs (inexpensive and long lasting) Typically coupled to multimode fibers (MMF) & PIN detectors Not used with long haul systems

Satellite Communications LEO, MEO, GEO

LEO, MEO and GEO are orbits with progressively increasing altitudes with respect to the earth LEO and MEO systems are generally configured as constellations of multiple, small, low power satellites

Virtual circuit

Logical circuits that provide connectivity to users. Unlike switched circuits, many users can share the same physical circuit simultaneously

Transmission System Parameters

Modulation method Transmit power Attenuation and path impairments Resistance, Current, Voltage Wire diameter, Gauge SNR and receiver sensitivity Frequency bandwidth Capacity (bps)

Optical Detectors - PINs

Most common light detectors are PIN (Positive Intrinsic Negative diodes) and APD (Avalanche Photodiodes) PINs do not provide significant gain however they are inexpensive. Generally matched with LEDs and MMF

MMF

Multimode (MMF) Index of the MMF core may be step index or graded index core diameter of approximately 50-60 micro meters Used for relatively low speed, short haul applications Modal dispersion (Similar to Delay skew in twisted pairs) Pulse dispersion results from modal dispersion (i.e., some light not injected into the center)

Digital Signal Line Coding - binary data to electrical signal

NRZ, Bipolar AMI, Manchester B8ZS

Layer 2 (Data Link Layer):

Node-to-node communications. Information exists in logical digital format (1's and 0's) and is framed into symbols, and exchanged over a common network (ex., Ethernet, Token Ring, PPP, FR, ATM, etc.) here we can exchange data over a common network. you can communicate between computer to computer, etc.

Channel

One way communications path between transmitter and receiver (two channels are typically assigned for full-duplex communications).

SMF

Optical - Single Mode Fiber: core diameter of approximately 5-10 micrometers Used over longer distances and higher transmission rates - Modal dispersion is not an issue Suffers from chromatic dispersion (different colors of lightarriving at the detector at slightly different times)

EDFA

Optical amplifiers are purely optical devices used with SMF Erbium Doped Fiber Amplifier (EDFA) is a piece of fiber with Erbium material doped into it

Raman Amplification

Optical amplifiers are purely optical devices used with SMF Raman Amplification requires no fiber doping They perform better than EDFAs in terms of SNR High energy pump wavelength light beam transfers energy to signal beam More costly than EDFA amplifier

OEO Repeaters/Regenerators, Optical Switches & Amplifiers

Optical signals need to be repeated and/or amplified to overcome attenuation over long distances Optical repeaters/regenerators are optical-electrical-optical (OEO) devices Optical Amplifiers & Optical Switches

OEO Repeaters

Optical signals require boosting over long distances due to attenuation Optical repeaters/regenerators are optoelectronic devices Perform optical-electrical-optical (OEO) conversion

OFDM

Orthogonal Frequency Division Multiplexing (OFDM) - splits signal into many lower bit rate streams that are transmitted over carefully spaced frequencies Also called multicarrier modulation OFDM splits a user's signal into numerous parallel data streams called subcarriers Traditional FDM avoids frequency channel overlap OFDM allows overlap which greatly increases capacity

Error Detection parity,

Parity bit appended to a block of data Even parity - Added bit ensures an even number of 1s Odd parity - Added bit ensures an odd number of 1s Example, 7-bit character [1110001] Even parity [11100010] Odd parity [11100011]

Switched circuit

Physical circuits that are allocated to users when needed. Circuit switching provides physical connectivity for the duration of a specific call.

Unguided - ,E-M field Power Density

Power Density at distance D2: PD= P/(4πD22)

Parameters we use to judge the suitability when selecting guided medium for a communications systems:

Propagation delay Distance and Attenuation Bandwidth Error performance Security Cost and Overall Complexity

Propagation Delay

Propagation delay also called latency, refers to the time required for a signal to travel from transmitter to receiver (medium + delays in processing)

Layer 3 (Network Layer):

Provides an end-to-end transmission capability to transport data from source to destination over disparate Layer 2 networks. (ex., IP, etc.) facilitates exchanges between different source networks.

Layer 6 (Presentation Layer):

Provides information on how data should be presented (ex., .jpg, .tiff, .gif, ASCII, Unicode, .mpg, etc.)

Modal Dispersion

Pulse dispersion results from modal dispersion(i.e., some light not injected into the center

QAM

QuadratureAmplitude Modulation (QAM) is a combination of ASK and PSK. Two carriers at the same frequency, but 90 degrees (π radians) apart Two amplitudes (one for each carrier) Four phase changes per carrier (0, 90 180, and 270 degrees) Gives M=8 different levels per symbol

Bit Depth

Quantization with the proper assignment of bits per sample ("bit depth") avoids large quantization errors (quantizing noise)

quantization

Quantizing each sample to the closet binary number

E-M Propagation

Reflection, Refraction, Scattering/Dispersion, Diffraction, Absorption, Direct LOS

How we judge the quality of light source: Speed,

Related to rise and fall time of optical signal. LDs are faster than LEDs (faster is better)

Coaxial

Single unbalanced conductor which guides the signal through the center conductor of a cable Center conductor much thicker than twisted pair Heavier gauge increases available bandwidth, lowers resistance and increases distance of transmission A robust shielded copper cable - noise grounding Layer of dielectric material separates two conductors Entire cable protected by PVC or Teflon

SNR

The Signal-to-Noise (SNR, S/N) is a measure of how much signal power is available to the receiver. "N" represents thermal noise Signal-to-Noise Ratio (SNR) = Signal Power Received Noise Power present at the receiver In dBs: SNR(dBs) = Pr (dBs) - N(dBs), where Pr is the power received

Amplitude

The amplitude of an AC waveform is its height as depicted on a graph over time. An amplitude measurement can take the form of peak, peak-to-peak, average, or RMS quantity. Peak amplitude is the height of an AC waveform as measured from the zero mark to the highest positive or lowest negative point on a graph.

Antenna Polarization

The direction of the line traced out by the tip of the electric field vector determines the polarization of the wave

Unshielded Twisted Pair

The most common type of copper wire is the Unshielded Twisted Pair (UTP) UTP cables comprise two copper conductors separately insulated by a dielectric material Each conductor is used for signal transmission and reception as each conductor carries a similar electrical signal The signals are opposite in polarity, reducing the effect of radiated energy and therefore reducing attenuation (balanced) Crosstalk, delay skew, Balanced configuration

Frequency

The period of a wave is the amount of time it takes to complete one cycle. Frequency is the number of complete cycles that a wave completes in a given amount of time. Usually measured in Hertz (Hz), 1 Hz being equal to one complete wave cycle per second.

Period

The period of a wave is the time for a particle on a medium to make one complete vibrational cycle. Period, being a time, is measured in units of time such as seconds, hours, days or years.

Transponder,

The satellite's transceiver chain is called a transponder A transponder receives the weakened signals, amplifies and shifts the frequency of the signal for re-transmission

MIMO & DSP

These methods also require the use of digital signal processors (DSP) which drive data through sophisticated algorithms

Spread Spectrum - FHSS, DSSS

Three forms of spread spectrum techniques exist: Direct Sequence Spread Spectrum (DSSS) Frequency Hopping Spread Spectrum (FHSS) Time Hopping Spread Spectrum (THSS)

TDD

Time Division Duplex (TDD) provides two way communications over common frequency channel

Error Correction - Block

Transmitter Forward error correction (FEC) encoder maps each k-bit block into an n-bit block codeword (n,k) Receiver Block passed through an FEC decoder

PN Codes

Use of individual user Pseudorandom codes (PN code) improves the security of the communications from eavesdropping Communicating users are given a unique PN code that is shared between the source and destination All users occupy the same frequency bandwidth, but separated by unique PN codes

DPCM

With DPCM, only changes in signal level are encoded and transmitted The premise is that the signals levels within voice signals change at a relatively gradually Fewer bits are therefore used to represent each sample 4 bits are generally used in DPCM 8000 samples/sec x 4 bits/sample = 32,000 bps

E-M Power density (watts per m2) spreads and ____

and weakens as the E-M signal moves further away from the source

Electrical Signals emits

any electrical signal emits - an electromagnetic (EM signal) which goes through the air, allows for things like radio. information using changes in current and voltage over conductive mediums. Voltage and current carry the information

Acoustic signals

are pressure waves through a medium such as the atmosphere for water.

Isochronous

data are synchronous transmitted without a clock source Timing is recovered from the transitions in the received data stream

Synchronous

data is framed with one or more synchronization bits or bit patterns which identify the start and end of a logical block of data Master clock sources are used over the entire network

Antennas create

electromagnetic signals which carry information

Data Security Transmission Security

ensuring unauhorized entities from intercepting information

Wavelength

f a traveling wave has a fixed shape that repeats in space or in time, it is a periodic wave. Such waves are sometimes regarded as having a wavelength even though they are not sinusoidal. wavelength is measured between consecutive corresponding points on the waveform.

Nyquist Sampling Theorem:

fs≥ 2*BW > deals with aliasing

Data Security Authorization

identifying legitimate users

E-M

information through unguided medium such as air or a vacuum. any electrical signal emits - an electromagnetic (EM signal) which goes through the air, allows for things like radio

Signal wave

is a carrier that has been modulated by user's information/message, and therefore has a frequency bandwidth

Modulation

is a process in which a baseband information/message is increased in frequency through the modulation of the carrier waveform

OSI Reference Model (7 Layers)

model developed by the International Organization for Standardization (ISO) provides a conceptual, layered architecture for data communications. used to figure out the best protocols and interfaces to communicate.

Core and Cladding equation:

n(cladding) < n(core)

, Microfluidic

optical switches Microfluidic: Based on the movement of fluid in tiny channels etched on silicon chip

How we judge the quality of light source: Coherence,

refers to the extent by which the light signals are synchronized in phase. (want coherence but not too much in order to avoid speckling)

PM Signal: eq

s(t) = Ac cos (2πfct + μp cos (2πfmt))

FM Signal: eq

s(t)= Ac cos(2πfct +sin(2πfmt))

AM Signal: eq

s(t)=(Ac + Amsin(2πfmt ))sin(2πfct )

Current flow creates

the E-M wave (RH rule)

Electric field is perpendicular to

the magnetic field

What are the differences between acoustic and electrical?

the real difference is that acoustic waves depend on molecules and are slightly different in their propagation - longitudinal propagation - vs electrical traverse propagation method

Asynchronous

use start and stop frames, or adds start/stop bits to each byte of information This adds overhead to the transmission of each byte Parity bit is typically added for error detection

Data Security Authentication

validation of identity Consists of passwords, intelligent tokens, digital certificates

electrical signals

voltage (volts) and current (amperage) information using changes in current and voltage over conductive mediums.

..transducers?

yes, because if we have an electrical signal and you want to put it in an optical system, you need a way to transduce the electrical signal into an optical signal. an electrical is nothing but electrons, but optical is photons

FM Index: = eq

∆f/fm = (Kvco*Am)/fm, KVCO(Hz/volts)

Data Security Physical

access control (e.g., guards, locks, etc.)

MEMs

optical switches Mechanical: Based on arrays of tiny mirrors on a silicon chip

Examples of Unguided:

Microwave, WLL (BWA) - ex., WiMAX 802.16,

OFDMA

Orthogonal Frequency Division Multiple Access (OFDMA) uses OFDM to share the wireless channel Different users can have different slices of time and different groups of subcarriers

Layer 7 (Application Layer):

User applications (ex., .doc, .xls, .xml, telnet, ftp, .ppt, etc.)

Power "moves" the signal down the _______

transmission line