Term
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Definition
| all possible frequencis from zero to infinity |
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Term
|
Definition
| the frequency spectrum is divided into service bands that are dedicated for specific services |
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Term
|
Definition
| Service bands are divided into smaller frequency ranges called Channels |
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Term
| In radio, how can you send multiple signals without the signals interfering with one another? |
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Definition
| A different signal can be sent in each channel because signals in defferent channels do not interfere with each other |
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Term
| Does a signal usually travel at a single frequency, or does it spread over a range of frequencies? |
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Definition
| Singlas spread over a range of frequencies, this range is known as the Bandwidth |
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Term
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Definition
| the highest frequency in a channel minus the lowest frequency |
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Term
| If the lowest frequency in a channel is 1.22 MHz and the highest frequency is 1.25 MHz, what is the channel bandwidth |
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Definition
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Term
| Why is large channel bandwidth desirable? |
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Definition
| Because they are much faster than narrowband |
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Term
| What do we call a systemwhose channels have large bandwidth? |
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Definition
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Term
Write the Shannon Equation;
what does each letter represent |
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Definition
C=B[Log2(1+S/N)]
C = Maximum possible speed for a given bandwidth and signal to noise ratio
B = Bandwidth
S/N = Singal to noise ratio
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Term
| what happens to the maximum propagation speed in a channel if the bandwidth is tripled while the signal-to-noise ratio reamins the same? |
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Definition
| It could potentially transmit up to three times as fast |
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Term
| How many times as much data is sent pre second in television than in AM radio? |
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Definition
C=B[Log2(1+S/N)]
6,000,000[Log2 (1+ 3/1)] / 10,000 [Log2 (1+ 3/1)] = Approx. 600
Approx. 600 times more powerful
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Term
| In the Shannon Equation, should S/N be entered as a ratio or in decibels? |
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Definition
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Term
| What is the dividing line between narrowband and broadband speeds? |
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Definition
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Term
| What is the golden zone in commercial mobile radio transmission? |
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Definition
In the high megahertz to low gigahertz range
(approx. 800MHz - 6GHz) |
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Term
| What is a clear line of sight limitation |
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Definition
| Clear line of sight is an unobstructed direct path. This is needed because radio waves attenuate more rapidly with distance and cannot flow through or around objects as they do at lower frequencies. |
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Term
| Do WLANs today use licensed or unlicensed bands? |
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Definition
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Term
| What is the advantage of using unlicensed bands? |
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Definition
| The advantage of using unlicensed bands is there is no need for government approval, thus it is much less expensive. |
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Term
| What is the disadvatage of unlicensed bands? |
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Definition
| There is no way to stop intereference other than negotiating with others who are using the band and there is a limit to transmission power. |
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Term
| In what two unlicensed bands does 802.11 operate? |
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Definition
| 802.11 operates in the 5-gigahertz and the 2.4 gigahertz bands |
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Term
| how wide are 802.11 channels usually? |
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Definition
| each channel in both of the 802.11 bands is usually 20 megahertz wide |
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Term
| which licensed band is defined the same way in most countries around the world? |
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Definition
| the 2.4 gigahertz band is the one standardized throughout most of the world. Diffrerent countries use different parts of the 5 gigahertz band. |
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Term
| does the 2.4GHz band or the 5GHz band allow longer propagation distances for a given level of power? |
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Definition
2.4GHz band
(lower frequency) |
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Term
| How many overlapping channels does the 2.4GHz band support? |
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Definition
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Term
| Why is the number of non overlapping channels that can be used important? |
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Definition
| it eliminates the mutual channel interference problem |
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Term
| How many non overlapping channels does 5GHz channel support? |
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Definition
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Term
| In unlicensed bands, what type of transmission method is required by regulators? |
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Definition
| Spread Spectrum Transmission |
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Term
| What is the benefit of spread spectrum transmission for business communication |
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Definition
| uses far wider channels than required |
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Term
| Is spread spectrum transmission done for security reasons in commercial WLANs |
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Definition
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Term
| In normal radio operation, how does channel bandwidth usually relate to the bandidth required to transmit a data stream of a given speed? |
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Definition
| bandwidth is no wider than required for the signals speed |
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Term
| How does this change in spread spectrum transmission as apposed to normal radio operation |
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Definition
| Spread Spectrum transmission: Channel bandwidth is much wider than required for the signal's speed |
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Term
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Definition
| FHSS (Frequency Hopping Spread Spectrum) is the simplest form of spread spectrum transmission; FHSS uses only the bandwidth required by the signal, but hops frequently within the spread spectrum channel |
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Term
| what is the limitation of FHSS |
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Definition
| FHSS is only useful for relative low speed |
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Term
|
Definition
Direct Sequence Spread Spectrum, it spreads its signal over the entire bandwidth of a channel. Interference and mulipath interference will affect only small parts of the signal, allowing most of the signal to get through for correct reception.
An analogy used in the text: If an ocean wave hits an obstacle such as a pier, it will still hit the shore at almost full strength. |
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Term
| For what WLAN standard is DSSS used |
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Definition
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Term
| What spread spectrum transmission method is used for 54 Mbps 802.11g WLANs? |
|
Definition
Orthogonal Frequency Division Multiplexing
(OFDM) |
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Term
Describe Orthogonal Frequency Division Multiplexing
(OFDM) |
|
Definition
| each broadband channel is divided into many smaller sub channels called subcarriers. Parts of each frame are tranmitted in each subcarrier. OFDM send data redundantly across the subcarriers, so if there is impairment in one or even a few subcarriers, all of the data usually will still get through. |
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Term
| List the elements in a typical 802.11 LAN today. |
|
Definition
Wired LANs
Wireless access points
hosts with a wireless NIC |
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Term
| Why is a wired LAN usually still needed if you have a wireless LAN |
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Definition
| Because the serves and internet accass routers that mobile hosts need to use usually are on the wired LAN |
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Term
| Is a wireless access point a bridge or a router |
|
Definition
a bridge
bridges connect two different types of LANs |
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Term
| Why must the access point remove an arriving packet from the frame in which the packet arrives and place the packet in a different frame when it sends the packet back out? |
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Definition
| Different networks require different frames. |
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Term
| What is a handoff in 802.11? |
|
Definition
| A handoff is a transfer of a mobile stantion's service from one access point to another closer access point |
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Term
| What is the relationship between handoffs and roaming in WLANs |
|
Definition
| in WLANs the ability to use handoffs is called roaming |
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Term
| when there is an access point and serveral wireless hosts, why may only one device transmit at a time? |
|
Definition
| The access point and all the stations it serves transmit in a single channel. If two devices transmit at the same time, their signals will collide, becoming unreadable |
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Term
| What is the purpose of Media Access Control? |
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Definition
| MAC methods govern when devices transmit so that only one device transmits at a time. |
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Term
| Does media access control limit the actions of wireless hosts, the access point, or both? |
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Definition
| Both hosts and access points |
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Term
| Describe CSMA/CA+ACK transmission reliable or unreliable? Explain. |
|
Definition
In CSMA/CA+ACK, if a wireless NIC hears a transmission, it must not transmit. If a station does not hear traffic, it considers the last time it heard traffic. If the time is less than some critical value, the station sets a random timer and waits. If there still is no traffic after the random wait, the station may send. However, if the time since the last transmission exceeds the critical value, the station may transmit immediately.
ACK means that when an access point receives a frame from a station, or when a station receives a frame from an access point, the receiver immediately sends an acknowledgement frame (ACK). A frame that is not acknowledged is retransmitted.
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Term
| Is CSMA/CA+ACK tranmission reliable or unreliable |
|
Definition
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Term
| Why is CSMA/CA+ACK innefficient |
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Definition
| waiting for transmission wastes valuable time. |
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Term
|
Definition
| in Request to Send/Clear to Send (RTS/CTS), when a station wished to send and is able to send because of CSMA/CA, the station may send a RTS message to the access point. If the access point broadcasts a CTS message, then other stations must wait, and the station sending the RTS can then transmit, ignoring CSMA/CA |
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Term
| Is CSMA/CA+ACK required or optional |
|
Definition
|
|
Term
| is RTC/CTS required or optional |
|
Definition
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|
Term
| Which is more efficient, RTS/CTS or CSMA/CA+ACK? |
|
Definition
|
|
Term
| Distinguish between rated speed, aggregate throughput, and individual throughput in 802.11 WLANs |
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Definition
Rated Speed lies between 11Mbps-54Mbps.
Aggregate throughput can reduce the actual throughput significantly.
Individual throughput is substantially lower than actual |
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Term
| Why does transmission speed drop as a computer moves farther from an access point? |
|
Definition
| The computer and access point will switch to a lower speed to reduce errors |
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|
Term
What are the rated speed of:
802.11
802.11a
802.11b |
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Definition
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|
Term
| In what band do 802.11, 802.11a, and 802.11b operate? |
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Definition
|
|
Term
| Why was 802.11a not popular when it first appeared? |
|
Definition
| equipment to run 802.11a was significantly higher priced at the time of conception |
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Term
| what 802.11 standard has the largest market share today? |
|
Definition
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|
Term
| How is 802.11g better than 802.11b? |
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Definition
| 802.11g is backwards compatible |
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Term
| Can you use 802.11b hosts with an 802.11g access point? |
|
Definition
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|
Term
| Is there a speed penalty for using 802.11b hosts with an 802.11g access point? |
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Definition
| Yes, throughput plummets for every device associated with the access point. |
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Term
| What disadvantages does 802.11a have compared with 802.11g? |
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Definition
| 802.11a operates on the 5GHz band while 802.11g operates on the 2.4GHz band. This allows for less expensive equipment and proves better propagation characteristics than 5GHz while offering the same rated speed. 802.11g also provides for faster throughput at all distances. |
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Term
| Is 802.11a backward compatible with 802.11b or 802.11g? |
|
Definition
802.11a is not backward compatible with either 802.11b or 802.11g
however 802.11g is backward compatible with 802.11b |
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|
Term
what are the two benefits of MIMO
(Multiple Input Multiple Output) |
|
Definition
| By using more antennas and more signals it can substantially increase throughput. It also increases propagation distance. This provides better coverage and allows access points to be placed further apart. |
|
|
Term
| what will be 802.11n's rated speed |
|
Definition
|
|
Term
| In what two ways does 802.11n increase throughput? |
|
Definition
| It is able to use either a 20MHz or 40MHz channel. Using a 40MHz channel alone can double the max speed |
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Term
|
Definition
| MIMO sends two or more radio signals in the same channel between two or more different antennas on access points and wireless NICs. The two signals in the same channel will interfere with each other. However, the two signals sent by two different antennas will arrive at the two receiving antennas at slightly different times. Using special detection and separation methods the receiver can separate the signals on the same channel and read them individually |
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Term
| What is the risk of buying draft 802.11n equipment? |
|
Definition
| Financial risk and risk of being locked into a specific vendor. |
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Term
| For what unlicensed bands will the 802.11n standard work? |
|
Definition
| 2.4GHz now because its cheaper but its crowded, soon it will be on 5GHz because it will allow for 40MHz channels and simultaneous channels |
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|
Term
| What are two attractions for 5GHz 802.11n operation? |
|
Definition
| there should be no problems finding two channels to bond into a 40MHz channel and access points, many operate on several channels simultaneously. |
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Term
| What is the advantage of dedicating channels to VoIP service? |
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Definition
| Keep the channel relatively unpopulated so the VoIP users get good service |
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Term
| What is the goal of the 802.11e standard? |
|
Definition
| Allow managers to specify very high level of quality. |
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|
Term
| Describe how mesh networking would work in 802.11 WLANs. |
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Definition
| They work by organizing themselves into a mesh and routing packets from one to another. |
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Term
| What two problems mention in the text would 802.11 mesh networking designers have to overcome? |
|
Definition
| The mesh would have to be self organizing making it very difficult and expensive to maintain if hosts are frequently entering and leaving the network. Second, routers near the geographical center would easily become overloaded with traffic. |
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|
Term
| What benefit will smart antennas bring? |
|
Definition
| Smart antennas allow for directing of a signal so that each wireless host will receive a stronger signal |
|
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Term
| Distinguish between war drivers and drive by hackers? |
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Definition
War driver just discover unprotected access points.
Drive by hackers discover them and also hack into the network |
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Term
| What is an evil twin access point? |
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Definition
| When a drive by hacker mimics an access point so that internal hosts associate with it, and send it their credentials. The evil twin then uses those to hack into the network. |
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Term
| What is a rogue access point? |
|
Definition
| Unauthorized access points that are set up by a department or and individual. Often they have very poor security, making drive by hacking easier or operate at high power attracting too many hosts |
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|
Term
| When 802.11 was created, what security protocol did it offer? |
|
Definition
Wired Equivalent Privacy (WEP)
a very rudimentary security protocol |
|
|
Term
| How long does it take to crack WEP today? |
|
Definition
|
|
Term
Who created WPA?
(Wi-Fi Protected Access) |
|
Definition
The Wi-Fi Alliance
and industry trade group that certifies 802.11 products of interoperability |
|
|
Term
| What is WPA's attraction compared with 802.11i |
|
Definition
| WPA provides far stronger security than WEP |
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|
Term
| What is WPA's disadvantage compared with 802.11i |
|
Definition
| WPA requires more memory and processing power in wireless NICs and access points. |
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|
Term
| What should companies do if they have access points or NICs that cannot be upgraded to WPA? |
|
Definition
| Firms should discard legacy wireless NICs and access points that cannot be upgraded to WPA |
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|
Term
| What is the strongest security protocol for 802.11 today |
|
Definition
|
|
Term
| What does the Wi-Fi Alliance call 802.11i |
|
Definition
|
|
Term
| What encryption method does 802.11i use |
|
Definition
|
|
Term
| What is deterring companies from converting from WPA to 802.11i |
|
Definition
| It is expensive and there is no cracks with the WPA |
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|
Term
| In what mode of 802.11i WPA operation is a central authentication server used? |
|
Definition
|
|
Term
| Why does 802.1x not need security between the NAS and the computer in Ethernet? |
|
Definition
| Because the actual authentication is done by a central authentication server. |
|
|
Term
| What does the Wi-Fi Alliance call 802.1X mode? |
|
Definition
|
|
Term
| Why does 802.1X need security between the NAS and the computer in 802.11 wireless access? |
|
Definition
| because transmission between a wireless host and the access point are easy to intercept and mimic. |
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|
Term
| What standard governs authentication exchanges in 802.1X? |
|
Definition
| Extensible Authentication Protocol (EAP) |
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