Wireless and Mobile networks Flashcards
List some of the problems with wireless
Attenuation
Noise
Reflection, scattering, and diffraction
Interference
What are some ways to combat problems with wireless (Attenuation, reflection, noise)
hint: MEEMo
Modulation
Error control coding
Equalisation
MIMO
What is AMC or Adaptive modulation coding
Dynamically adjusts modulation and coding to current channel condition
What is MIMO?
uses multiple antennas to point signals strongly in a direction
What is mobile IP? What are the types?
Enables connectivity while moving from one AP to another
Mobile - Dynamic changes and maintenance
Nomadic - Termination and new connection with new temp IP address
What is WiMax?
Worldwide Interoperability for Microwave Access Forum
Promotes 801.2 standards and develops interoperability specifications for WMAN and WAN
With Bluetooth and PAN, devices within ___m can share up to ___ mbps or ____ mbps of capacity
10m, 2.1, 24
What is an electromagentic signal?
a function of time that can also be a function of frequency
What is time-domain?
The effect time has on a signal
What is frequency domain?
The effect components and power have on a signal
What is an analog signal?
A signal that varies smoothly over time, continuous values
What is a digital signal?
A signal that maintains a constant level for a period of time and changes to another constant level, discrete values
What is a periodic signal? what is the period of the signal represented by?
Analog or digital signal pattern that repeats over time. The period of the signal is represented by T
What is peak amplitude?
The maximum value or strength of a signal, represented as A
What is frequency?
Rate of repetition of the cycle, represented by f
What is a phase?
Measure of the relative position in time within a signal period (t) represented by phi ϕ
What is wavelength?
Distance occupied by a signal cycle, represented by Lambda λ
Whatis the fundamental frequency?
The base frequency on which data is modulated, or when all the components of a signal are multiples of one frequency
What is a spectrum?
The range of frequencies contained in a signal
What is the absolute Bandwidth?
The width of the spectrum of a signal
What is effective bandwidth? (or bandwidth)
The narrow band of frequencies that most of a signal’s energy is contained in
What is the relationship between bandwidth (Hz) and the Bits per second (bps)?
The greater the bandwidth the higher the information-carrying capacity
Any electromagnetic signal can contain a collection of periodic analog signals (____ _____), at different __________, ___________, and ______
Sine waves at different amplitudes, frequencies and phases
What are the differences between analog and digital transmission
Analog
No regard for content
Can tolerate distortion
Amplifiers can boost the signal
Digital
Concerned with the content
Repeaters achieve distance
Attenuation endangers data integrity
What is channel capacity?
The maximum rate at which data can be transmitted over a given channel under given conditions
What is the data rate?
Rate at which data can be communicated (bps)
What is the number of discrete signal or voltage levels?
M
What is the Nyquist Bandwidth formula for two voltage levels/binary signals?
C = 2B
What is the Nyquist Bandwidth formula for multilevel signaling?
C = 2B log2 M
What is SNR?
The ratio of the power in a signal to the power of noise at a particular point in the transmission
What is the formula for SNR?
(SNR)db = 10log<10> signal power/noise power
What is the Shannon capacity formula?
C = B log<2> (1+SNR)
What is the problem with the Shannon Capacity formula?
Assumes white noise, impulse noise and attenuation is not accounted for
What is multiplexing?
Carrying multiple signals on a single medium
What are the advantages of using multiplexing?
Reduces cost and maximises bandwidth
What are the multiplexing techniques?
Frequency Division multiplexing
Time-division multiplexing
What is an antenna?
An electrical conductor or system of conductors
What is a radiation pattern?
A graphical representation of radiation properties of an antenna
What is beam width?
Measure of antenna directivity (where most of the power from the antenna goes)
What are omnidirectional antennas (with eg)
Antennas used to transmit to receivers whose location is not fixed, and transmit in all directions.
examples are isotropic and dipole antennas
What are directional antennas, with examples
Unidirectional antennas used to transmit to fixed receivers
Example: parabolic reflective antenna (or dish)
what is antenna gain?
Power in a particular direction, compared to that produced in any direction by an omnidirectional antenna
What are ISM bands?
Industrial, scientific and medical bands used without a license as long as regulations are followed, used for WLAN, IoT and WPAN
What is multiple access scheme? What are the types?
Dividing spectrums into time, frequency and signal encodings
Time division multiple access,
Frequency division multiple access
Code division multiple access
What are the propagation modes?
Ground wave propagation
Sky wave propagation,
Line of sight propagation
What is ground wave propagation?
Follows the earth’s contour to propagate frequencies up to 2 Mhz (AM radio)
What is sky wave propagation?
Signals propagated by reflecting off the ionosphere through a number of hops
What is LOS propagation?
Signals above 30MHz propagated by antennas within line of sight
What is diffraction?
Secondary waves behind objects with sharp edges
What is refraction?
Bending of signals as they change medium
What are the factors that affect LOS transmission?
Attenuation, noise, atmospheric absorption, multipath and refraction
What are the factors of attenuation for unguided media?
The signal must be be strong enough for the receiver to interpret
The signal must be stronger than the strength of the noise
Attenuation is greater at higher frequencies
What is free space loss
The attenuation of a transmitted signal over distance
What are the types of noise?
Thermal
Intermodulation
Crosstalk
Impulse noise
What is thermal noise?
Noise present in all electronic devices and media, and cannot be eliminated
What is intermodulation noise
Noise that occurs if signals with different frequencies share the same medium
What is Crosstalk?
Unwanted coupling between signal paths
What is impulse noise?
Irregular pulses or noise spikes of short duration and high amplitude
What is fading?
Variation of signal power caused by changes in the medium or path - can be in fixed environments or mobile environments
What are the types of fading?
Large scale and small scale
With small scale fading, what is the Doppler Spread?
Fluctuations caused by movement. If Coherence time, Tc is greater than Tb, it is slow fading
With small scale fading, what is multipath fading, and what are the slow and fast types called?
Multiple signals arriving at receiver - If coherence bandwidth Bc is greater than Bs or signal bandwidth, it is flat fading. If not, it is frequency selective fading
What are some ways to correct a channel?
Adaptive modulation and coding (AMC)
Bandwidth expansion,
diversity
Forward error correction
MIMO
What is forward error correction?
Transmitter adds error correcting code to data block, which is calculated and compared to the incoming code
How are diversity techniques used to channel correct?
Selection diversity - selecting the best signal
Combining diversity - Combining signals
What do MIMO antennas do?
Multiple input multiple output antenn arrays are used for:
Diversity, beamforming, multi user mimo
How is bandwidth expansion achieved?
Carrier aggregation
Frequency reuse based on coverage areas
mmWave for high frequencies
What are CMs and what do they do?
Control modules are WLAN interfaces that provide bridge or router functionality
What are WLAN motivations?
Cellular data offloading
Sync and file transfer
Multimedia streaming
What are the requirements for WLAN?
Throughput
Connection to backbone LAN
Service area
Transmission security
Dynamic configuration
What are the functions of the physical layer?
- Encoding and decoding signals
- Preamble generation and removal
- Bit transmission and reception
What are the sublayers of protocol architecture?
Physical medium dependent sublayer (PMD)
Physical layer convergence procedure
Medium access control (MAC)
LLC (logical link control)
What does the PMD (Physical medium dependent sublayer) do?
Physical medium dependent sublayer transmits and receives data through a wireless medium
What does the physical layer convergence procedure do?
Maps MPDU (MAC layer Protocol Data Units) into a framing format
Send and receive between stations using the same PMD sublayer
What does the medium access control do?
Assemble data into a frame with address and error detection fields
On reception, disassemble frame and perform address recognition and error detection
Govern access to the LAN transmission medium
What does the LLC (Logical Link Control) do?
Provide an interface to higher layers and perform flow and error control
What is the format of a MAC frame?
MAC Control - MAC protocol information
Destination MAC
Source MAC
Cyclic redundancy check
What does IEEE 802.11 MAC cover?
Reliable data delivery
Security
Access control
What does MAC do for reliable data delivery?
(4 frame exchange)
Source issues request to send (RTS)
Destination responds with clear to send (CTS)
Source retransmits data
Destination responds with ACK
How does IEEE 802.11 MAC handle access control?
Distributed coordination function and point coordination function
Distributed foundation wireless MAC
What is DCF? (Distributed coordination function)
Decentralised access control
CSMA with collision awareness
What are the factors that increase the risk for wireless networks
Channel: broadcasted messages are more susceptible to interception and jamming
Mobility: Mobility increases security risk
Resources: mobile devices have limited memory and processing power to counter threats
Accessibility: some wireless devices may operate unattended, increasing the risk of physical attacks
