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Wireless Midterm Review Notes
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- 1Describe characteristics of ADhoc mode"No central control device
Distinguish is to be a triangle icon
BSS- name of the network when connection is p2p
IBSS- no central control device" - 2What is Independent Basic Service Set(IBSS)?Adhoc = IBSS
- 3BSASingle Access Point = Basic Service Area
- 4DSCable connection between AP and switch
- 5ESSMore than 1 Access point = Extended Service Set (ESS)
- 6Whats the purpose of Service set Identifier?" Logical seperation of WLAN
Must meet between AP:Client
AP broadcasting SSID = Beacon Fram
Case-sesnsitive" - 7What's Basic Service Set Identifier (BSSID) and MBSSID?"1 SSID radio signal will have a mac that single mac will the be the BSSID
Multiple SSIDs with single AP will be MBSSID" - 8Describe characteristics of Repeaters"Is used to extend wireless signal to complete coverage
Must overlap minimum of 50%
Can repeat signal only 3 times(3 hops)" - 9Describe characteristics of Workgroup bridgesAPs used to connect one or several non-wireless devices to the wireless network
- 10Describe characteristics of Bridges"Point to point bridge
Point to multi point
Only head office is root
clinet is to be Non-root" - 11Describe characteristics of Mesh networksMultiple AP for redundancy
- 12What's Radio Frequency (Radio wave)?"*Electromagnatic waves - Defined by frequency and wavelength
*Always sent at the speed of light
*A wave that travels A short/long amount of time is called one cycle
*A band is A small section of the spectrum of radio frequencies which channels are used or set asside for the same purpose" - 13Define RF principles - frequency"*measured in hertz
*lower frequency will travel further in the air
*high frequency will travel less distance but travels at greater speed
*Think Fans and speed settings
" - 14Define RF principles - Wavelength"Physical distance from one point of cyle to the same next point of cycle (top to top/bottom to bottom)
" - 15Define RF principles - Amplitude"*Amplitude is power
*Active amplification - injecting power to increase power (gain power)
*Passive amplification - focusing the energy in one direction, by using an antenna
*Attenuation - Decrease of amplitude
" - 16Absorbtion"Signal is transmitted but some enviromental factors such as walls or people can affect the legnth and strength of signal because it can be absorbed by those objects
" - 17Reflection"When signal is transmitted objects can reflect and bound around
can increase sength of signal" - 18Multipath"*Phenomenon which is a major concerin indoor enviroments
*When a single signal is sent but reflects off objects, though main signal wil reach destination on time sometimes the relections on the way will be recieved by destination out of phase.
*When signal is recieved twice at the exact same time the primary wave's power is increased by the secondary wave thus getting twice the positive and negatve energy.
*The result is that both waves add up to twice the amplitude of a single wave and both signals are in phase. This is called ""Upfade"".
" - 19Scattering"*Reflection off humidity in the air
*Scattered signal is weaker because its been reflected in a different direction along the path and diffused because of the micro reflections might hit the reciever
*scattering also depends on radio frequency
*affects shorter wavelengths more than longer ones
*can cause 2 effects
degradation of wave strength and quality of signal to reciever
unpredictable reflection
Outdoor enviroment usually" - 20Refraction"*the transition from one enviroment to another (air to water)
*when wave changes direction
*has a minor effect on indoor networks" - 21Free path loss"Loss of a signal when it further away from the source
Same ammount of energy but covering a large area
RF line of sight
-
Line of Sight
-straight line path to all objects but can be affected by mountains, treets, curve of earth
Fresnel Zone
-Needs 60% minimum to be clear of abstractions for signal to be recieved properly" - 22Define Recieved Signal Strength Indication (RSSI)"The power level of an RF signal required to be recieved successfully by the recieving radio
-Value that determines how much sinal is recieved by the endpoint" - 23Define Signal to Noise Ration (SNR)"Not useful signal recieved at the same frequency is called noise, devices such as Microwave ovens, Bluetooth devices, portable telephones
negative value measured in decibels (dB)
lower noie value indicates a quieter/better enviroment
Signal to noice ratio = The difference in strength between main signal and background noise" - 24Compare absolute and relative measurements"Absolute measurement is a value is a measurement taken a point in time (watts)
relative over a section of time which is compared to the absolute measurement" - 25Define relative Measurements:"dBm- The measurement of the intensity of the signal
is a reference against milliwatt
dBi-the effective gain of an atenna compared to an isotropic antenna
actual vs theoretical
Isotropic antenna is theoretical where coverage is equally
greater dBi = higher gain
higher gain = smaller angle of coverage
dBd- the difference between actual antenna transmission " - 26What's the purpose of the antenna in Wlan?Antennas convert electrical energy into radio frequency (RF) waves when it transmits, or RF waves into electrical energy when it receives
- 27Define polarization."The wave and how it can move in different directions
-Vertical Horizontal or circular directions?
When choosing an antenna is to pick the antenna radiation shape called radiation pattern
Most wireless networks use vertcal polarization
H-plane - Width of coverage
E-plane - Depth of coverage" - 28How does it influence on a WLAN?Is that the antenna is designed to give signals in a certain direction
- 29Define Omnidirectional antenna and list the types of antennas"Radiates on the H-plane and not so much on the E-plane
suited for small area ie. single floor
Can increase power of omnidirectional antenna to cover more than 1 floor" - 30Define directional Antenna and list the types of antennas"Directional antenna is usually mounded on walls
You can pick direction of the covereage but is aimed to where you want
Semi-directional -
Highly-directional - Used for point to point. More precise" - 31When to use interfrated antennas vs. external antennas?"Intergrated
Used for ceiling mounting where looks are a primary concern
When no requirement for directional antennas
When the temp range wont exceed 0 - +40c
External
Omni-directional coverage is not needed and greater range is needed
Temp rage -20 - +55c
Placed in an enclosure and the antennas need to be extended" - 32What considerations would you apply when you are choosing cables and connectors?"*Correct cable
-impedance of cable to match empedance of antenna/reciever
-Cable introduce signal loss into communication link
-Connectors also support specific ranges of frequencies and must mach impedance of RF equipment
-Connecors add loss to RF link
-Connectors on average cause about 1/2dB of insertion loss" - 33What's the spectrum"Spectrum is a range of frequencies
Each frequency has specific applications" - 34List characteristics of 2.4 GHz-ISM band?"Freemount overlap on channels
non-licensed
Used by 802.11,802.11b,802.11g, and 802.11n
Each channel is 22 Mhz wide
3 non-overlapping channels 1, 6, 11
Using any other channels will cause interferance
Europe uses channels 1,5,9 and 13 but accept the slight overlap for 1 more channel
" - 35"List characteristics of 5GHz-UNII band? ""4 UNII bands
UNII-1,UNII-2,UNII-3,UNII-2Extended
23 non-overlapping channels
" - 36Whats the role of the FFC, IEEE and Wi-fi alliance?To regulate the radio frencuency standars for wifi frencuency (not sure)
- 37What is the differences between Narrowband and Spread Spectrum transmissions methods?"Narrowband
Single frequency
High powerlevel to combat inteference issues
examples: Radio Station
Spread Spectrum Transmission
Spread accross a whole band of frequencies instead of of a single frequency
2.4 band uses 22MHz-wide signals
Doesnt have as much power to overcome narrowband interference
Signal can be degraded" - 38Define the FHSS modulation"*To avoid inteferece is to change frequency
-Interference is often caused by narrowband signals
-FHSS - Frequency Hopping Spread Spectrum
-The band divided into many small channels
-emitter chooses and tells the reciever about the Channel Hop Sequence
-Happens every 300-400ms
-If signal is destructed the only 300-400ms of info will be lost during the tranmission
-Doesnt scale well and the quantity of info that can be sent on each channel is limited
-Limitation is caused bt the fact that each channel is 1 MHz wide." - 39Define the DSSS"*DSSS
Info sent on a wider channel instead of hopping
sent in parallel
" - 40DSSS - BPSK"states the rule: When the next value to send is a zero, do nothing special but continue sending the wave without alteration. When the next value to send is a 1, change direction (if the wave was going up, suddenly go down, and vice versa
allows 1MB/S" - 41DSSS - QPSK"DQPSK groups bits by pairs (00, 01, 10, 11) and states the rule that, if the group to transmit is 00, do nothing (carry on sending the wave as it is). If the group to transmit is 01, turn 90 degrees
Allows 2Mbps" - 42Define the DSSS - CCK Modulation"*Complementart Code Keying(CCK)
*Each group of 4 buts is encoded into a 6-bit symbol
-64(2^6) differnt symbols
-Bits are grouped by 4 or by 8 and prepresented as 6 bits unique code sequence
-2 more complementatry bits are used
-Coding 4 bits per symbol allow 5.5 Mpbs
-coding 8 bits per symbol allow 11 Mps" - 43Define the OFDM"*Orthognal Frequency Division Multiplexing
-used in 802.11a/n/ac
-can achieve highest data rates with the max resistance to corruption of data caused by interference
-Subchannels
52 subcarriers
48 sub carriers are used to transmit data and 4 subcarriers used to pilot carriers for monitoring path shits and interference
20MHz wide ( each subcarrier)
each of the subcarriers has a lower data rate but the data is sent at the same time over the subcarriers in parallel that achieves higher data rate
-16-QAM: 24 & 36 Mbps
-64-QAM: 48 & 56 Mbps
-256-QAM: 802.11ac
-BPSK=9Mbps when using carriers
-QPSK=18Mbps when using carriers" - 44What's the "Dynamic Data Rate Shifting" in WLAN?"The farther away you get from the access point, the lower the data rates are that you can achieve.
It even happens without dropping your connection. Also, it is done on a transmission-by-transmission basis.
" - 45Describe medium acccess method in WLAN with DSMA/CA method."Not condiered available neoth the physical and virtual carrier report that is clear
ACK message for each unicast
If no ACK then retransmission is made
Request to send and Clear to send packets
RTS is used by sending device and CTS is used from recieving device
Used to figure out when to send and when not to send
CTS has a duration field which says how long the communication should take
Wireless is a half-duplex (HDX)
Collisions are avoided by using a contention mechanism
Non centralized way of deciding when a frame is transmitted, instead each device fends for them selves and decide them selves - method is called Distrbuted Coordination Function (DCF)" - 46Explain why there are different types of interframe space (IFS) and how do they work in WLAN?"IFS is a period that a station has to wait before it can send
Station always listens to medium as to not send if another station signal is detected
After a frame is completely sent there is always silence
This allows multipath issues to clear
Short interframe Space (SIFS): for high priority the silence between unicast frames and Acknowledge
Distributed Interframe Space (DIFS): Standard priority it is the silence between one transmission adn the next
Reduced Interframe Space (RIFS): used by 802.11n station between each segment of a burts
" - 47List Magement Frames and explain its functions"Beacons
Probe requests/response
Authentication
Association/Reassociation/Deassociation
Deauthentication" - 48Describe a communication process between a Wi-fi host and AP?"1.Recieve beacon frame
2.Probe request
3.probe response
4.Authentication request
5.Authentication response
6.Association request
7.Association response
8.Deauthentication request
9.Deauthentication repsonse
" - 49List control frames with its functions and explaining two cases that a control frame is used to protection against collisions."Control frames are special messages used in the cell to improve the efficiency of the connection.
The ACK frame is used to acknowledge unicast frames.
Request to Send/Clear to send (RTS/CTS) is used for protection against collisions in two cases
When 802.11b clients are present in a 802.11g cell:
To solve an issue called hidden node" - 50Whats the purpose of the mandatory and supported data rate settings on AP?"mandatory - that client MUST be able to preform that modulation to be allowed in the cell
Supported - The client CAN join the cell even if the NIC cant achieve that modulation rate" - 51How does Power save mode work between wireless clients and AP?"*to save battery power
-will snooze and when it has nothing to send
-will wake up periodically when reciving an unexpected message from AP
-starts by sending null/empty data frame with a header Power Management bit sent to 1
-this informs the AP that the station goes to dozing mode
-AP will then buffer incoming traffice for that station under a field called ""Traffic Indication Map (TIM)""
-A fieled called Delivery traffic indication map (DTIM) says the AP has broadcast or multicast traffice buffered
-for unicast traffice, stations hearing thier AID (associatied ID) in the TIM sends a PS (power save) pollcontrol frame to ask for the buffered frames" - 52Descrive a communication process between a host and AP?"Beacons
Probe Requests and Responses
Authentication
Association, Reassociation and Deassociation
Deauthentication" - 53802.11b StandardOperates in the 2.4-GHz band
Specifies four data rates up to 11 Mbps:
1, 2, 5.5, 11 Mbps.
Provides specifications for vendor interoperability.
Defines basic security, encryption, and authentication for the wireless link. - 54802.11g StandardOperates in the 2.4-GHz ISM band.
DSSS and OFDM transmission.
12 data rates of up to 54 Mbps.
1, 2, 5.5, 11 Mbps (DSSS / 802.11b)
6, 9, 12, 18, 24, 36, 48, 54 Mbps (OFDM)
Full backward compatiblity to 802.11b standard - 55802.11a StandardOperates in only the 5-GHz spectrum
Uses Orthogonal Frequency-Division Multiplexing (OFDM).
ach channel is 20 MHz wide.
52 subcarriers
48 subcarriers are used to transmit data, with 4 subcarriers used a pilot carriers for monitoring path shifts and interference
Uses eight data rates of up to 54 Mbps
6, 9, 12, 18, 24, 36, 48, 54 Mbps - 56802.11n ProtocolData rate up to 600Mbs.
Operates at both 2.4 and 5 GHz.
MIMO (Multiple-Input Multiple-Output ):
Spatial Multiplexing (SM)
Maximal Ratio Combining (MRC)
Transmit Beamforming (TxBF)
High Throughput (HT) Channels - Channel bonding.
HT Mac - Block Acknowledgement.
HT Operation - Backward compatible with 802.11g and 802.11a. - 57Multiple-Input Multiple-Output (MIMO)Uses multiple radios and antennas.
Three technologies are supporting MIMO:
Spatial Multiplexing (SM)
Multi Ratio Combining (MRC)
Transmit Beamforming (TxBF). - 58Spatial Multiplexing (SM)The emitter sends different simultaneous signals from different radios. The 802.11n receiver will receive these signals on all its radios. Each of the receive radios independently decode the arriving signals. Then, each receive signal is combined with the signals from the other radios. This results in additional throughput. This process is called spatial multiplexing .
- 59Multi Ratio Combining (MRC),Because of multipath, a signal travels along different paths before reaching the receiver. With a technique called Multi Ratio Combining (MRC), the receiver can combine the signals received on each antenna and radio chain, resulting in a stronger received signal, again increasing range or speed
- 60Transmit Beamforming (TxBF).The emitter can also send the same signal from several antennas. By carefully coordinating these signals based on the feedback transmitted by the 802.11n receiving station, the emitter aims at making these signals be received in phase, thus increasing the signal power level at the receiving station, allowing for longer range or higher throughput. This process is called Transmit Beamforming (TxBF).
- 61802.11ac*Channel Bonding 80 MHz (mandatory)
*Faster modulation 256-QAM (Optional)
*802.11ac is a 5 GHz-only technology, so dual-band APs and clients will continue to use 802.11n at 2.4 GHz.
*Multi-User MIMO (MU-MIMO)
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- Fichas sobre Wireless Midterm Review Notes, creado por itechnical2 el 29/02/2016.
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- Creado por Justo Juan Black · 29 feb 2016
- Copiado por itechnical2 · 29 feb 2016
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