Panko 6 – Flashcards
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At what layers do wireless networks operate? [224]
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Physical Link and Data Link Layers.
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Are wireless network standards OSI standards or TCP/IP standards? Explain. [224]
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Wireless network are also single network, which require physical and data link layer standards so they are also OSI standards.
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What 802 working group creates 802.11 standards? [225]
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802.11 Working Group
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Why do wireless clients need access to the wired Ethernet LAN? [225]
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Wireless clients to access network resources it needs to be connect to wireless access point, which is usually connected to Ethernet LAN where resources such as Internet access router are usually located. 802.11 WLANs today are also primarily supplement wired LANs.
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How can firms provide WLAN coverage throughout a large building? [225]
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A firm can strategically place wireless access points judiciously throughout a building. In essence, the firm is creating a large 802.11 WLAN "cloud" .
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Is wireless radio transmission usually expressed in terms of wavelength or frequency? [226]
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Frequency
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What is a hertz? [226]
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One cycle per second
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Convert 3.4 MHz to a number without a metric prefix. [226]
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3400000 Hz
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Understand the inverse square law for attenuation. Be able to solve a problem such as (228): If the signal strength at 10 meters is 40 milliwatts (mW), what will the strength be at 20 meters? (answer: 10 mW)
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(1/r^2) -- for simple problems S2 = S1(r1/r2)^2 -- for problem given Note: r is the meters for the first and second signal
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What type of antenna normally is used in WLANs? Why? [227]
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Omnidirectional antennas are normally used in WLANs, because transmission of signal is equally strong in all direction, transmission signal spread in all three dimensions and receives incoming signal well from all direction. They are best for short distance.
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At what range of frequencies do most wireless systems operate? [226]
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500 megahertz 10 gigahertz
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Distinguish between omnidirectional and dish antennas in terms of operation. [227]
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Omnidirectional can transmit three-dimensional signal transmission but short distance while dish antennas can point transmission of signals in one direction. Signals can also travel longer distances because of focusability.
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Under what circumstances would you use an omnidirectional antenna? [227]
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Short distances, three dimensional signal transmissions.
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Which offers more reliable transmission characteristics—UTP or radio transmission? [228]
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Which offers more reliable transmission characteristics—UTP or radio transmission? [228]
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Which attenuates more rapidly with distance—signals sent through wired media or radio signals?
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Attenuation with radio signal rapidly increases as distance increases
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If the signal strength from an omnidirectional radio source is 8 mW at 30 meters, how strong will it be at 120 meters, ignoring absorptive attenuation? Show your work. [228]
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if S2 = S1 * (r1/ r2) 2 Then S2 = 8 mW* (30 meters /120 meters) 2 S2 = 8 mW* (1/4) 2 S2 = 8 mW* (1/16) S2 = 1/2 mw
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How are shadow zones (dead spots) created? [229]
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if there is a large or dense object (e.g., a brick wall), blocking the direct path between the sender and the receiver, the receiver may be in a ___________ (d___ spot), where the receiver cannot get the signal.
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Why is multipath interference very sensitive to location? [229]
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Radio waves tend to bounce off walls, floors, and other object therefore direct signal and out of bounced signal may arrive out of phase when they arrive to the receiver because they travel different distances. The signals then cancels out.
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What is the most serious propagation problem in WLANs? [229]
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Multipath interference may cause the signal to range from strong to nonexistent within a few centimeters
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List some sources of EMI[229]
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Cordless telephones, microwaves and devices near the wireless network
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What propagation problems become worse as frequency increases? [230]
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Shadow zone propagation problems tend to grow worse with frequency
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Distinguish among the frequency spectrum, service bands, and channels. [230]
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Frequency spectrum all possible frequencies from zero hertz to infinity. Service bands contiguous spectrum ranges within a frequency spectrum. Channels are subdivided service brands with smaller frequency ranges
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In radio, how can you send multiple signals without the signals interfering with one another? [230]
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Service bands are subdivided into smaller frequency ranges called channels. A different signal can be sent in each channel because signals in different channels do not interfere with one another. This is why you can receive different television channels successfully.
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Does a signal usually travel at a single frequency, or does it spread over a range of frequencies? [231]
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Spread over a range of frequencies called bandwidth.
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What is channel bandwidth? [231]
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A channel also has a bandwidth. For instance, if the lowest frequency of an FM channel is 89.0 MHz and the highest frequency is 89.2 MHz, then the channel bandwidth is 0.2 MHz (200 kHz).
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If the lowest frequency in a channel is 1.22 MHz and the highest frequency is 1.25 MHz, what is the channel bandwidth? (Use proper metric notation.) [231]
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1.25 - 1.22 = 0.03 MHz or 30 kHz
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Why is large channel bandwidth desirable? [231]
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The maximum possible speed is directly proportional to bandwidth, so if you double the bandwidth, you can potentially transmit up to twice as fast.
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What do we call a system whose channels have large bandwidth? [233]
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broadband channels (fast)
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What other types of system do we call broadband? [233]
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Transmission systems: They are very fast are usually called broadband systems even when they do not use channels.
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channels with small bandwidths
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narrowbands (slow)
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What other types of system do we call broadband? [233]
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Transmission systems that are very fast are usually called broadband systems even when they do not use channels.
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Write the Shannon Equation. List what each letter is in the equation. [231]
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C = B [Log^2 (1+S/N)] C = Maximum possible transmission speed in the channel (bps) B = Bandwidth (Hz) S/N = Signal-to-noise ratio measured as the power ratio, not as decibels Note that doubling the bandwidth doubles the maximum possible transmission speed Multiplying the bandwidth by X multiplies the maximum possible speed by X Wide bandwidth is the key to fast transmission Increasing S/N helps slightly, but usually cannot be done to any significant extent
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What information does C give you? [231]
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Maximum possible transmission speed in the channel (bps)
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What happens to the maximum possible propagation speed in a channel if the bandwidth is tripled while the signal-to-noise ratio remains the same? [231]
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Maximum possible transmission is trippled
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Given their relative bandwidths, about how many times as much data is sent per second in television than in AM radio? (The information to answer this question is in the text.) [231].
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Approximately 600 times more data are sent per second in television as opposed to AM radio.
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What is the golden zone in commercial mobile radio transmission? [234]
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The golden zone for commercial mobile services is 500 MHz to 10 GHz.
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Why is the golden zone important? [234]
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Golden zone frequencies are high enough for there to be large total bandwidth At higher frequencies, there is more available bandwidth Golden zone frequencies are low enough to allow fairly good propagation characteristics At lower frequencies, signals propagate better
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What is a clear line-of-sight limitation? [234]
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Absorption and shadow zone propagation problems are large. In addition, demand for channels and service bands in the golden zone is increasing rapidly.
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Do WLANs today use licensed or unlicensed bands? [235]
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WLANs use unlicensed bands
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What is the advantage of using unlicensed bands? [235]
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Any wireless host can be turned on or moved around without the need for any government approval.
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What is the disadvantage? [235]
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Users must be able to tolerate interference from others
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In what two unlicensed bands does 802.11 operate? [235-237]
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2.4 and 5 GHz
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Does the 2.4 GHz band or the 5 GHz band allow longer propagation distances for a given level of power? Justify your answer. [235-237]
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The power required for 2.4 Ghz band is lower allowing it to transmit longer distances however, 5 Ghz bands use higher frequencies therefore higher absorptive attenuation.
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How many non-overlapping 20 MHz channels does the 2.4 GHz band support? [235-237]
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2.4 GHz, three possible non-overlapping 20 MHz 802.11 channels, which are centered at Channels 1, 6, and 11.
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Why is the number of non-overlapping channels that can be used important? [235-237]
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If nearby access points operate in the same channel, their signals will interfere with each other unless the access points are far apart. This interference is called co-channel interference.
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How many non-overlapping 20 MHz channels does the 5 GHz band support? [235-237]
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the 5 GHz band provides between 11 and 24 non-overlapping 20 MHz channels, depending on the frequencies allocated to unlicensed operation in a country.
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In unlicensed bands, what type of transmission method is required by regulators [238]
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You are required by law to use spread spectrum transmission in unlicensed bands Spread spectrum transmission reduces propagation problems Especially multipath interference Spread spectrum transmission is NOT used for security in WLANs Normal Transmission versus Spread Spectrum Transmission
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What is the benefit of spread spectrum transmission for business communication? [238]
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Spread spectrum transmission is transmission that uses far wider channels than transmission speed requires therefore reducing propagation problems and channel interference.
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Is spread spectrum transmission done for security reasons in commercial WLANs? [239]
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Spread spectrum transmission is not done for security reasons in commercial WLANs.
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In normal radio operation, how does channel bandwidth usually relate to the bandwidth required to transmit a data stream of a given speed? [239]
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Normal Transmission versus Spread Spectrum Transmission (See Figure 6-16) Normal transmission uses only the channel bandwidth required by your signaling speed Spread spectrum transmission uses channels much wider than signaling speed requires
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How does this change in spread spectrum transmission? [239]
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Spread spectrum transmission is transmission that uses far wider channels than transmission speed requires.
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What spread spectrum transmission method is used for the most recent 802.11 standards? [240]
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Most recent 802.11 standards use Orthogonal Frequency Division Multiplexing (OFDM)
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OFDM Describe it. [240]
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Broadband channels are dived into smaller sub channels called carriers. Parts of each frame are transmitted in each subcarrier. OFDM sends data redundantly across the subcarriers, so if there is impairment in one or even a few subcarriers, all of the data usually will get through.
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List the elements in a typical 802.11 LAN today. [241]
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an 802.11 wireless LAN typically is used to connect a small number of mobile devices to a large wired LAN— typically, an Ethernet LAN—because the servers and Internet access routers that mobile hosts need to use usually are on the wired LAN.
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Why is a wired LAN usually still needed if you have a wireless LAN? [241]
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Wireless LANs is typically used to connect a small number of mobile devices to a large wired LAN typically Ethernet LAN because the servers and Internet access routers that mobile hosts need to use usually are on the wired LAN
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In the figure, what is the distribution system? [241]
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The network to which access point connect (Wired Ethernet LAN).
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Besides moving packets between wireless clients and the Ethernet network, what other control functions do access points have? [242]
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Access points also control hosts as wells as assigning transmission power levels to hosts within its range and other supervisory chores.
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What is a BSS? (Do not just spell out the acronym.) [242]
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Basic service set (BSS) A basic service set (BSS) consists of an access point and the set of hosts it serves
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What is an SSID? (Do not just spell out the acronym.) [243]
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service set identifier (SSID). (Note that the term basic is not in the name.) Wireless hosts must know the SSID to associate with the access point.
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Does the access point have an SSID? [243]
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The access point in a BSS has an identifier called the service set identifier (SSID). Wireless hosts must know the SSID to associate with the access point.
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Why must wireless devices know the access point's SSID? [243]
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So wireless devices knows how to associate the access point.
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What is a handoff in 802.11? [243]
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A handoff is when a user moves to a closer access point.
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What is the relationship between handoffs and roaming in WLANs? [243]
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The ability to use handoff ins is called roaming in WLANs
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What is an ESS? (Do not just spell out the abbreviations.) [243]
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An extended service set (ESS) is a group of BSSs that are connected to the same distribution system (network) and have the same SSID.
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What feature do all access points in an ESS share? [243]
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Connected to the same distribution system (network) and have the same SSID.
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How can access points communicate with each other? [243]
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Distribution system.
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What is the purpose of the 802.11r standard? [243]
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Access points to contact one another when it deals with roaming.
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What is a collision? [244] Why is it bad? [244]
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Channel Sharing The access point and all the hosts it serves transmit in a single channel. If two devices transmit at the same time, their signals will collide, becoming unreadable. (If two devices transmit in the same channel at the same time, their signals will interfere with each other.)
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What is the purpose of media access control? [244]
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Media access control (MAC) methods govern when hosts and access points may transmit so that collisions can be avoided. The access point and all of the wireless hosts it serves transmit and receive in a single channel. When a host or the access point transmits, all other devices must wait.
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Does media access control limit the actions of wireless hosts, the access point, or both? [244]
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MAC limits both the actions of wireless hosts and the access point.
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Of the four 802.11 transmission standards summarized in this section, which are full standards, and which are only draft standards?
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802.11g and 802.11n are full standards 802.11ac and 802.11ad are still draft standards.
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What is the maximum rated speed for each standard? [247-249]
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802.11g = 54 Mbps 802.11n Single Band = 100 to 600 Mbps 802.11n Dual Band = 100 to 600 Mbps 802.11ac = 433 Mbps to 6.9 Gbps 802.11ad = 7 Gbps
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Compare maximum speeds for 802.11g and 802.11n and the maximum distances at which each standard can provide these speeds. [247-249]
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802.11g, the maximum distance and speed are 30 m and 54 Mbps. 802.11n, distance and speed are much longer and faster at 70 m and 100-600 Mbps respectively.
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Which can bring gigabit speeds to clients? [247-249]
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802.11ac & 802.11ad
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What business application will gigabit transmission speed make feasible to do wirelessly?
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Data backup, sharing data, and syncing data
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What spread spectrum method do all four standards use as their main method?
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OFDM
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Why is wider channel bandwidth good? [250]
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doubling channel bandwidth doubles transmission speed. However, service bands have limited total bandwidth, so wider channels means fewer channels. ]
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What is the downside of wider channel bandwidth? [250]
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The wider channel bandwidth the fewer channels.
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What frequency band or bands do 802.11g, 802.11n single band, 802.11n dual band, 802.11ac, and 802.11 ad use? [250]
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2.4, 5, 60 GHz band
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For each, compare channel bandwidth and the number of possible channels. [250]
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2.4 802.11g use 20 MHz and 3 Channels. 802.11n Single Band has 20 MHz and 3 Channels 802.11n Dual Band uses 40 MHz has 3 in 2.4 GHz , 12 (US) in the 5 GHz 802.11ac has 80 MHz and 160 MHz uses 6 in 80 MHz and 3 Channels in 160 MHz 802.11ad uses 60 MHz and 3 Channels in the US & 4 in Europe. , 5, and 60 GHz band
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What are the benefits and problems of transmission in the 60 GHz band? [250]
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Winder channels but less channels available to use
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How does MIMO work? [251]
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By having the host or access point send two or more spatial streams (radio signals) in the same channel between tow or more different antennas on access point and wireless hosts.
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What is the main benefit of MIMO? What is its other benefit? [251]
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Greater transmission speed & greater transmission distance.
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Compare the range of rated speeds possible 802.11g with 40 MHz channels and 802.11ac. [251]
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Speeds for 802.11 with 40 MHz channels is 300 Mbps. 802.11ac doubles the possible spatial streams to eight. This creates a large number of possible rated speeds of 433 Mbps, 867 Mbps and 1.3 Gbps.
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What is beamforming? [252]
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having multiple antennas and changing the phase of waves coming from different antennas, an access point can focus signals toward individual hosts instead of broadcasting them,]
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Beamforming What two benefits can it bring? [252]
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Gives more effective power, stronger signal to each wireless host and greater propagation distance.
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Distinguish between MIMO and multiuser MIMO. [252]
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Multiuser MIMO is the possibility of simultaneously transmission in a single channel by multiple devices that are using a single access point as oppose to just one on MIMO. Beamforming can also bring multiuser MIMO, which is the possibility of simultaneous transmission in a single channel by multiple devices that are using a single access point. The 802.11n standard specified multiuser MIMO, but it did not specify a single multiuser MIMO technology. This led to a great deal of market confusion, and multiuser MIMO did not become popular with 802.11n. With 802.11ac, the Technical Group 802.11ac avoided market confusion by specifying a single multiuser MIMO technique. Beamforming is likely to be common in 802.11ac products.
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Distinguish between rated speed, aggregate throughput, and individual throughput. [253]
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RATED SPEED VERSUS THROUGHPUT Rated speed is the number of bits that the host or access point will transmit per second. access point throughput is aggregate throughput, which is shared by all users of an access point. Suppose that the aggregate throughput is 100 Mbps per second and there are 10 users of an access point. then individual throughput would be about 10 Mbps (actually somewhat less because of time lost in turn-taking). Of course, it would be rare for all stations to transmit simultaneously. However, even if three are sending and receiving simultaneously, the individual throughput they experience would be about 30 Mbps. Access point throughput is aggregate throughput This must be shared by all stations currently sending or receiving Individual throughput can be much lower than aggregate throughput
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What factors influence individual throughput, given a certain level of aggregate throughput? [253]
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Another consideration is that speed is highest when a user is very near an access point. As the user moves away, speed falls. The problem is that at maximum transmission speed, there must be almost perfect propagation characteristics. As a user moves away from an access point, signal strength falls, errors There are many other factors that will reduce individual throughput. For example, there are still some wireless devices that use the old 802.11b standard, which only has a rated speed of 11 Mbps.
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Why does transmission speed drop as a computer moves farther from an access point? [253]
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As distance increase, attenuation increases therefore the wireless access point have to switch to a more reliable frequency which results in slower speeds.
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How does the presence of a distant station harm all users of an access point? [253]
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Hosts connected with an access point are some that are farther away, throughput will fall because signal have to travel longer distance and will take longer to send and receive packets, and this time will be take away from other users.
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What is backward compatibility? [254]
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802.11g access point. In the same way, an 802.11n access point must be able to work with an older 802.11g access point. Of course, when an 802.11n device works with an 802.11g device, the transmission can take place only at 802.11g speeds. The 802.11n device must drop back to 802.11g operation. This Panko, Raymond; Panko, Julia (2012-10-09). Business Data Networks and Security (9th Edition) (Page 254). Prentice Hall. Kindle Edition.
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When a device designed to use a newer standard must work with a device that only uses an older standard, what standard do they use to communicate? [254]
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Older standard
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What is white space, and why is using it attractive? [255]
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The FCC is now considering how equipment can use empty channels (known as white space) on an opportunistic basis. This can provide more spectrum capacity for WLAN operation, cellular and similar service, or both. Regulatory agencies around the world are now considering how to allocate white space. Panko, Raymond; Panko, Julia (2012-10-09). Business Data Networks and Security (9th Edition) (Page 255). Prentice Hall. Kindle Edition.
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Why may spectrum scarcity be a problem?
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Spectrum scarcity may be a problem because unless more efficient wireless transmission methods emerge very rapidly, spectrum capacity will place limits on demand growth.
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) If spectrum scarcity becomes a problem, how will that affect users? [255]
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Slower growth and cost for transmission will rise
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How does Wi-Fi Direct differ from the traditional way in which two wireless hosts communicate?[255]
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Adhoc connection, no access point need and its simpler.
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802.11s standard Panko, Raymond; Panko, Julia (2012-10-09). Business Data Networks and Security (9th Edition) (Page 256). Prentice Hall. Kindle Edition.
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How does Wi-Fi Direct differ from the traditional way in which two wireless hosts communicate?[255] Adhoc connection, no access point need and its simpler. b) Can a large Wi-Fi network operate without an Ethernet switched backbone? [256] yes c) What technology allows this? [256] adhoc d) What is the current 802.11 standard for this technology? [256]
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What devices forward frames in a mesh network? [256]
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Access points or wireless hosts
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What three issues must be overcome to make mesh networking acceptable to corporations? [256]
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Self-organizing, overloading near the geographical center of the mesh, and securities
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Why can two products that comply with the same standard have different performance? [257]
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The number of hardware varies between manufacturer such as number of antenna and processor used.
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What is the advantage of having three antennas rather than two on an access point? [257]
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Transmit three spatial streams allowing for a faster data transfer and better reliability.
