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