IT 300 MIDTERM
Shannon Hartley Equation
C (bps) = B*log2(1+SNR)
Multiplexing: FDM, TDM
Analog baseband channels can only be multiplexed using Frequency Division Multiplexing (FDM) However, digital baseband channels can also be multiplexed by FDM Analog baseband channels can not be multiplexed using Time Division Multiplexing (TDM) TDM is a digital baseband multilexing technique based upon time slots
Multiple Access: DAMA, FDMA, CDMA, TDMA
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 frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA).
Bit Error Rate (BER)
is a measure of the probability of experiencing bit errors during digital transmission A bit error is an unintended change in logical state (ex., 0 to 1) Example: 1 bit error experienced for every 1000 bits sent is a BER=1/1000 or typically written: BER 10^-3
Differences : Acoustic, Electromagnetic and Electrical, and Optical signals
Acoustic: acoustic pressure wave is created that begins to vibrate air molecules along its path to the microphone. The microphone detects the pressure of the acoustic wave front, and converts this energy into changes in resistance within the microphone. The human ear typically between 20Hz and 20kHz (20,000 Hz) The human voice ranges from 100Hz to 8kHz voice grade ranges from 300Hz to 3400Hz Acoustic waves cannot travel in a vacuum An electromagnetic (E-M) signal carries information through unguided medium such as air or a vacuum. A radio frequency (RF) signal is a subset of the E-M spectrum. (RF) and optical signals are both considered EM waves. Weakens when traveling by air. Of course, if we want to send electrical signals that represent information, there must be changes in voltage and current that correspond to the information we wish to send. In this case, the changes in voltage and current are dependent upon the information, carries information using changes in current and voltage over conductive mediums. It goes without saying that optical signals are very different from RF signals. Major differences include: Optical communications are immune from RFI (radio frequency interference) since photons are neutral in charge.Light wave frequencies (THz) greater information carrying ability Communications systems typically use a combination of electrical, E-M and optical signals.
FSL attenuation
As the signal propagates from the transmit antenna to the receive antenna, the signal experiences attenuation due to signal spreading. This attenuation can be determined using the Friis free space loss equation (5.13), which is dependent upon transmission frequency, f = c/wavelength, and the distance between antennas, d, in meters. Finally, once the signal reaches the receive antenna, it is combined with the receive
Timing
Asynchronous transmission 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 Synchronous transmission, 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 Isochronous data are synchronous transmitted without a clock source Timing is recovered from the transitions in the received data stream (data to recover timing) Plesiochronous devices run at nominally the same rate with defined parameters of tolerance for variations
PCM, DPCM, ADPCM
By following Nyquist, PCM avoids signal aliasing, and although the use of 8 bits per sample (M 5 256) will cause some quantization error, it is not severe enough to prevent accurate voice grade digital trans- mission from occurring. Differential Pulse Code Modulation (DPCM), dramatically reduces the data rate capacity needed for transmission by decreasing the bit depth per sample from 8 to 4 bits. This is accomplished by only transmitting the differences detected from one sample to the next. This is useful for voice signals where changes between samples are not significant, thus reducing the required capacity from 64 kbps to 32 kbps. Adaptive Differential Pulse Code Modulation (ADPCM) can be applied where the bit depth per sample can dynamically change as the differences between samples are detected.
Types of circuit
Dedicated Circuit: Dedicated circuits are distinct, physical circuits dedicated to directly connecting devices across a network, 24/7 access to the physical path. Switched Circuits: Physical circuits that are allocated to users when needed. Circuit switching provides physical connectivity for the duration of a specific call. Virtual Circuits: Logical circuits that provide connectivity to users. Unlike switched circuits, many users can share the same physical circuit simultaneously. sharing is transparent to all users. Hint: This can only be used with digital circuits. Packet Switched: Logical connections in which information in the form of data packets, do not necessarily following the same physical paths through a network. Separated into smaller segments, and where each segment can take different network paths to its destination.
Full versus Partial Mesh Networks
Each node in the network is required to communicate with every other node in the network. As such, each node must have a dedicated link to every other node in the network. This is termed a full mesh network. (very expensive and complex network to implement!) equation: n(n-1)/2 Partial: Today, partial mesh networks are used extensively. The Internet is an example of a partial mesh network that is resilient to both nodal or link failures.
Error Control: Error Detection versus Error Correction
Error detection codes, Detects the presence of an error Error correction codes, or forward correction codes (FEC), Designed to detect and correct errors
RF Propagation: Multipath LOS, Refraction, etc.
Line-of-Sight (LOS) Susceptible to attenuation, weather, etc. - and must be amplified/repeated frequently Refraction: he fact or phenomenon of light, radio waves, etc., being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density. Multipath reflections - causes signal distortion
Link, circuit
Link: two or more nodes in order to exchange analog or digital information. Circuit represents- sent a complete communications path, end-to-end, between entities and are comprised of several links.
BPSK, QPSK, QAM, ASK, FSK, PSK
M'ary ASK (simplified equations for M=4, N=2 bits per symbol) Binary Phase Shift Keying (BPSK) - Two phase shifts used separated by 180 degrees (π radians) Quadrature Phase Shift Keying (QPSK) - four phase shifts of the waveform are used: φ = [0, 90, 180, 270 degrees] Quadrature Amplitude Modulation (QAM) is a combination of ASK and PSK. Two carriers at the same frequency, but 90 degrees (π/2 radians) apart Two amplitudes (one for each carrier) Four phase changes per carrier (0, 90 180, and 270 degrees) (8 different symbols)
Nyquist Sampling Theorem -Aliasing
Nyquist sampling theorem [eq. (3.11)]. The Nyquist sampling theorem tells us that if we have a sample rate (i.e., number of samples taken of the analog signal per second) that is greater than or equal to twice the frequency bandwidth of the analog signal, then we can faith- fully represent the signal in digital form. Equation: fs ≥ 2 (fHIGH - f LOW) ≈ 2*BW Nyquist tells us that we must sample the analog voice grade stream at approximately double the frequency bandwidth in order to avoid "aliasing" the signal which results in distortion at the other end.
OSI Reference Model (7 Layers)
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.) 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.) Network Layer: Provides an end-to-end transmission capability to transport data from source to destination over disparate Layer 2 networks. (ex., IP, etc.) 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.) Session Layer: Manages communication sessions between hosts (ex., NetMeeting, etc.) Presentation Layer: Provides information on how data should be presented (ex., .jpg, .tiff, .gif, ASCII, Unicode, .mpg, etc.) Application Layer: User applications (ex., .doc, .xls, .xml, telnet, ftp, .ppt, etc.)
Power and power density
Power (Watts) represents signal strength Power is equal to the amount of energy expended over time (E/t) Power (Volt-Amps) = V*I Power Density (Watts/meter2) is used to measure signal strength over distance (described as signal power density (i.e., watts per area).
Propagation Delay
Propagation delay also called latency, refers to the amount of time a signal travels from transmitter to receiver (medium + delays in processing)
Sinusoidal equations (sine and cosine)
Sinusoidal wave (frequency, amplitude and wavelength) The speed of sound is approximately 343 meters/second (767 mph) that are used to "carry" information from sender to receiver at a specified frequency. Modulation is the process of taking our information signal, and using this signal to modify certain aspects of our carrier wave such as the amplitude, frequency, or phase angle. cosine wave lags the sine wave
Spread spectrum vs OFDMA
Spread Spectrum With spread spectrum, the communications channel is spread over a very wide frequency bandwidth Spreading the signal has two major benefits: Signal survivability - Interference (ex., noise) or intentional signal jamming, both typically in a narrow band, only disrupts part of the signal vice the entire signal Use of available frequency bandwidth is more efficient - more than one user can occupy the same frequency bandwidth Use of individual user Pseudorandom codes (PN code) improves the security of the communications from eavesdropping 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
SNR
The Signal-to-Noise (SNR, S/N) is a measure of how much signal power is available to the receiver. "N" represents the noise-floor, which we will treat as thermal noise
Receive sensitivity
The ability of the receiver to detect a signal is termed receive sensitivity which is specific to the receiver model. The better the receiver, the better its sensitivity and ability to detect a weak signal. Example: A receiver requires a Receive Sensitivity in SNR of 10dB for signal detection. The SNR in dBs that reaches the receiver needs to be ≥10dB.
Quantization (bit depth), Quantization Noise (Error)
The process of mapping this bit sequence to the intensity or amplitude of the analog signal is called quantization and the number of bits we apply to each sample is termed the bit-depth. S This mismatch between analog and digital values is termed quantization error. The greater bit-depth we apply to each sample the smaller our quantization error, and the smaller the bit-depth the larger the quantization error. 8 bits give you 2^8=256 quantization levels, Each sample is quantized (i.e. rounded off to the nearest binary number) (assume if not give to you)
Signal Noise
Thermal Noise: - caused by the activity of electrons which increase with temperature Intermodulation Noise - caused by signals with different frequencies sharing a common medium where bandwidth overlap and unintended frequency harmonics may exist Crosstalk - occurs on guided medium where electrical coupling between electrical signals occur Impulse Noise - irregular noise spikes with high amplitudes (ex., lightening)
Simplex, Half Duplex (HDX), Full Duplex (FDX)
With one-way "simplex" communications, the source sends transmissions to one or more receivers over a single channel or circuit, with no expectation of a response back from the receiver. An example of simplex communications is broadcast radio or television. Half duplex: With "half- duplex" (HDX), communications occur between two or more entities over a single circuit or channel. Since the channel is shared between all users, only one user can send a transmission at a time. Push- to-talk (PTT) radios are an example of HDX communications where an RF channel is shared between communicating parties. Full Duplex: With "full-duplex" (FDX) communica- tions, communicating entities can send and receive simultaneously. FDX communications typically involve separate frequency channels or circuits that provide different send and receive paths. This enables users to transmit and receive at the same time.
dbw to dbm
add positive 30
Antenna and EM wave Polarization
antenna along the line of propaga- tion. Likewise, the rotation of left hand circular polarized (LHCP) waves can be determined using the "left hand rule" in which the EM wave appears to rotate in a counterclockwise direction when viewed from the antenna along the propagation path. EM polarization is defined by the direction of the electric field. In fig. 5.3, the electric field (E-field) is parallel to the y-axis and therefore the EM wave is considered vertically polarized. The magnetic field (H-field), which lies along the x-axis, mirrors the E-field and is always perpendicular to it. H
Baseband, passband, broadband
baseband2 signal, and raising its frequency to an even higher frequency by combining it with a carrier wave. A baseband signal, which can either be analog or digital, is combined with a carrier wave to create a passband signal. (true) The modulated signal is sometimes called the passband signal, which describes the modulated signal after it has passed through the system's frequency band-pass filter. multiplex these signal channels together using frequency division multiplexing. The multiplexed output, which is the aggregate of all combined channels, is termed the broadband.
Frequency and Time Domains
f (frequency of the wave in Hz) 5 1 (1.6) T A represents the peak amplitude of the sine wave, f is the frequency of the wave which equals 1/T in equation (1.6), t is the time variable, and is the phase angle of the sinusoid. Wavelength is As an example, frequency increase results in an increase of signal attenuation as well as an increase in the capacitive interaction between adjacent wires (i.e., causing interference or crosstalk between wires). Time domain gives you the profile of the signal over time, while viewing the signal in the frequency domain tells you the frequency bandwidth that the signal occupies. Viewing a signal in both domains is important when determining the overall quality of the signal. (true)
Baud verse data rate
hartley's law
Channel,
similar to links, typically support a single communications pair, or in the case of a broadcast, a single station from a radio broadcast tower.
Line Coding: NRZ, B8ZS, Manchester, AMI, 4b/5b
look at the charts Example 4B/5B (used in some Ethernet standards), maps (encodes) a four bit dataword into a five bit sequence. This always ensures that there will be a binary transition
Antenna Reciprocity Theorem
the transmit and receive patterns for an antenna are identical
UTP, Coax., Fiber Optic (TIR)
(UTP) copper wires, each pair is twisted in a helical pattern at different ratios in order to reduce the crosstalk between adjacent wire pairs. signals are opposite in polarity, reducing the effect of radiated noise energy and reducing overall attenuation Coaxial: Coaxial cable is an example of an unbalanced guided medium in which the center signal conductor is protected by an outer layer shield connected to ground. unbalanced conductor which guides the signal through the center conductor of a cable Fiber Optic (TIR):con- structed with an optical core made of either glass or plastic, and surrounded by a cladding made from similar optical material, but with a different refractive index from that of the core. Doing this enables TIR to occur between the core and the cladding boundary. can be bent without losing the signal that is propagating within the core.
