BUS 499: Data Comms/Networking Final Ch. 6
Are the set of frequencies used for police communication in a city channels or a service band? Explain.
A service band, service bands are dedicated to specific services
Over what transmission system do access points communicate with each other to accomplish roaming?
A switched ethernet network also known as the distribution system
Does spread spectrum transmission increase transmission speed thanks to its wider channels?
Also no this method is used to control the multi interference problem
What is beamforming?
Antennas in advanced MIMO systems focus radio power instead of broadcasting it in all directions equally when the access point transmits to a wireless device, the signal will be stronger
What other types of systems do we call broadband?
Any fast system, even if it doesn't use radio channels Ex: Internet
Does media access control apply to wireless hosts, access points, or both?
Applies to both
What will happen to your speed as you move away from the access point?
As you travel away, the access point will transmit more slowly to be more easily understood
Distinguish between a BSS, and ESS, and an SSID.
BSS - basic service set; an access point and all the wireless hosts that this access point serves ESS - extended service set; group of basic service sets with the same SSID that connect via a distributed system SSID - service set ID; name of the access point. Is used to identify all access points that belong to the same ESS
Why must the access point remove an arriving packet from the frame and place the packet in a different frame when it sends the packet back out?
Because of the different protocols, a frame from 802.3 LAN cannot travel through the 802.11 LAN.
Why is this a problem?
Creates mutual channel interference (a lot of overlapping channels)
What is the main benefit of MIMO?
Faster signals, can send more info in the same channel It increases throughput, i.e., higher transmission speed.
Distinguish among the frequency spectrum, service bands, and channels.
Frequency spectrum - consists of all possible frequencies; can be divided into frequency ranges called service bands Service bands - specified for a set of services; can be divided into channels. Contiguous spectrum ranges created by regulators Channels - Channels are simply the result of the service bands being subdivided further into smaller frequency ranges.
What is its other benefit?
Greater transmission range. Therefore, fewer access points are needed, which results in a lower equipment and installation cost.
When does it make sense to use RTS/CTS?
If the clients cannot hear each other
Does a frame's receiver transmit an ACK immediately or after a random delay?
Immediately
What is the most serious propagation problem in WLANs? How is it controlled?
Multipath interference (direct and reflected signals interfere) controlled by using spread spectrum transmission, which spreads the signal over a wide range of frequencies so the effects average out to a 0
Describe RTS/CTS.
RTS/CTS = request to send / clear to send a turn taking protocol; others have to wait after a clear-to-send is sent to another host
Is wireless radio transmission usually described in terms of wavelength or frequency?
Radio transmission waves are represented by frequency
What is a transceiver?
Radios used for data transmission (they both transmit and receive)
Does a signal travel at a single frequency, or does it spread over a range of frequencies?
Range of frequencies called bandwidth
Distinguish between omnidirectional and dish antennas in terms of operation
Omnidirectional - antenna that transmits and receives signals from all directions Dish - transmits and receives signals in a specific and narrow direction Can receive and send stronger signals
What type of antenna normally is used in WLANs? Why?
Omnidirectional, it would be nearly impossible to always locate the signal if using dish
Is CSMA/CA+ACK reliable or unreliable?
Reliable protocol, thanks to the acknowledgment CSMA/CA+ACK = carrier sense multiple access / collision avoidance + acknowledgement
What spread spectrum transmission method dominates today?
Orthogonal frequency division multiplex (OFDM)
If the signal strength from an omnidirectional radio source is 20 mW at 10 meters, how strong will it be at 70 meters, ignoring absorptive attenuation? Show your work
P2 = 20 mW r2 = 10 meters r1 = 70 meters DR = r2/r1 = 10/70 = 0.1429 DR^2 = 0.0204 Signal strength at 200 m = 0.0204 * 20 = 0.4084
What characteristics do all access points in a corporate network share?
Same SSID, indicating they are of the same ESS
What problems does it pose for Wi-Fi?
Serious propagation problems, such as much higher attenuation and shadow zones.
Describe the steps that occur when the server transmits a packet back to the wireless client.
Server sends frame containing the packet, the access point removes the packet and places it in a new frame for respective protocol, and sends it through the host network
Wi-Fi operates in the 2.4 GHz ____ and the 5 GHz ____.
Service band, Service Band
If you increase propagation distance by a factor of 100, how much will signal intensity change at the receiver?
Signal will be (1/100)^2 or 1/10,000 of what it was
In the 2.4 GHz and 5 GHz service bands, what type of transmission method is required by regulators?
Spread spectrum transmission method is required
If you want to transmit seven times as fast, how much wider must the channel be?
The channel must be 7 times as wide
If you triple channel bandwidth, what happens to the number of channels in the service band?
The numbers will decrease proportionally (i.e., 20MHz no becomes 60MHz and you would only be able to have one (1) channel instead of three (3))
If you quadruple propagation distance, how much will signal intensity change at the receiver?
The signal will be only (1/4)^2 or 1/16 of what it was
Why are companies moving rapidly into the 5 GHz band?
There is more total bandwidth and more channels but they have to deal with more attenuation More non overlapping channels so you won't get as much co channel interference
Do WLANs today use licensed or unlicensed service bands
Unlicensed
Under what circumstances would you use an omnidirectional antenna?
Used for short distances such as LAN and cellphones.
What is roaming in 802.11?
handing off from one access point in a building to another changing access points in same service set Roaming is moving from one access point to another access point without losing wifi connection
Compare the market status of 802.11n and 802.11ac.
n still dominates the installed based today but ac dominates sales and will soon supplant n as the dominant Wi-Fi technology
Is CSMA/CA+ACK efficient?
no, waiting for transmission wastes time
Is RTS/TS usually required or optional?
optional
If the lowest frequency in a channel is 1.22 MHz and the highest frequency is 1.25 MHz, what is the channel bandwidth? Give your answer in kHz
1.25-1.22 = 0.03 mHz or 0.03*1000 = 30 kHZ
How many channels are there in the FM band? (You can compute this from information in the text.)
100
In what two unlicensed service bands does 802.11 usually operate?
2.4 GHz and 5 GHz
What is the actual difference between 802.11 and wifi?
802.11 create the standards with many options that govern wireless local area networks Wifi Alliance creates subsets of 802.11 standards with selected options Needs to pass a test (wifi logo)
Compare the rated speeds of 802.11n and 802.11ac.
802.11n products deliver speeds of 100 to 600 Mbps 802.11ac standards deliver speeds of 433Mbps to 6.9 Gbps
Distinguish between 802.3 standards and 802.11 standards
802.3 - standards created for ethernet networks 802.11 - standards created for wireless networks
What two propagation problems become worse as frequency increases?
Absorptive attenuation and dead zones
You are using an access point with a rated speed of 4 Gbps. Why will you experience much less speed?
Actual speed will always be lower than rated speed and the number of users will also decrease speed
Why is CA desirable?
CA = collision avoidance Desirable because it avoids or prevents the problem of collision which can cause errors
Which is more efficient, RTS/CTS or CSMA/CA+ACK?
CSMA/CA+ACK is more efficient
In radio, how can you send multiple signals without the signals interfering with one another?
Can send multiple signals in different channels, since they are in different channels there is no interference between channels If talking about sending different signals in one channel → use MIMO (multiple input, multiple output) technology to avoid interferences
What does CS mean?
Carrier sensing - must listen to the signal before attempting to transmit signal Carrier = signal Sensing = listening to traffic
An FM radio station is called moldy oldies 101.1 is this a channel or a service band
Channel
If you need an access point providing 3 Gbps service, what choice do you have?
Choose 802.11ac
If you triple channel bandwidth in a service band, what happens to the number of channels in a service band?
Decreases proportionally (in this case it will decrease to 1/3)
What causes dead zones?
Dense objects in which the signal cannot pass through
All wireless hosts and the access point that serves them transmit on the same channel. What problem does this cause?
Each device should transmit one at a time or else can cause collision unless using MIMO method
If the signal strength from an omnidirectional radio source is 8 mW at 30 meters, how strong will it be at 150 meters, ignoring absorptive attenuation? Show your work
Equation: P2 = P1/(r2/r1)^2 8 mW = strength P2 30 m = distance r2 150 m = r1 Distance ratio (DR) = r2/r1 = 30/150 = 0.2 DR^2 = 0.04 Signal strength at 150 m = 0.04 * 8 = 0.32
What will it be at 200 meters?
Equation: P2 = P1/(r2/r1)^2 P2 = 8 mW r2 = 30 m r1 = 200 m DR = r2/r1 = 30/200 = 0.15 DR^2 = 0.0225 Signal strength at 200 m = 0.0225 * 8 = 0.18
Why is the 60 GHz unlicensed band attractive?
In corporations, the 5 GHz band is close to being as saturated like the 2.4 GHz band in corporations. The new 60 GHz unlicensed band is attractive for high-end residential use because it can support wireless communication between a laptop and a television for streaming 4K video and make wireless connections to replace USB cords and do so at ultrahigh speeds. In residences, it is a high-speed cable replacement technology.
Contrast inverse square law attenuation and absorptive attenuation
In inverse square law attenuation, the signal spreads out as the surface of a sphere, thus decreasing signal strength. Absorptive attenuation simply means that the signal is being absorbed by air and water.
What is the main promise of 802.11ax over 802.11ac?
It addresses the problem of increasing density (the number of hosts per access point). It promises to serve four times as many hosts per access point as 802.11ac.
What is the benefit of spread spectrum transmission for business communication?
It improves transmission reliability
Why does it divide the channel into subcarriers?
It is easier to control signals in smaller channels than in one big channel Spread spectrum transmission divides channels into subcarriers and redundantly sends information within subcarriers If you have a problem in one subcarrier, the problem won't affect the signal transmission
How is 802.11ay likely to be better than 802.11ad?
It should bump basic speed to 20 to 30 Gbps over substantially longer distances than 802.11ad, and by bonding several channels together, it will be able to provide much higher speeds. Using MU-MIMO, it also can direct energy with beamforming to give much better range. This and other improvements should even allow it to penetrate walls and other obstacles, at least to some extent.
At what layers do wireless LANs operate?
Layer 1 and 2 (physical and data link)
Under what circumstances would you use a dish antenna?
Longer distances such as a stationary radio
How does media access control (MAC) address this problem?
MAC governs when you can transmit in order to avoid collisions MAC address implements rule that to ensure that only one device transmits at a time
How does MIMO use spatial streams to increase transmission speed?
MIMO increases transmission speed but does not increase bandwidth MIMO can send 2 signals in one channel called spatial streams If a device has two antennas then each antenna can send two signals in a single channel They will not interfere because they are sent from different antennas The receiving host will separate these signals because of different transmission times
Distinguish between MIMO and multiuser MIMO (MU-MIMO)
Multiple input/multiple output (MIMO)—radio transmission method that sends several signals simultaneously in a single radio channel. In MU-MIMO, device power and sensitivity is divided between 2 or more wireless devices and allows communications with 2 or more wireless devices simultaneously
What is the downside?
Must tolerate interference from others
How is carrier sensing used in multiple access?
Needs to first listen to traffic If there is traffic it has to wait a certain amount of time
In 802.11 wifi networks, can simple installation rules usually reduce propagation effects to non-issues?
No Wireless networks are unpredictable, making it hard to manage No standards or guidelines on how to set up wireless networks
Can a wireless access point and one of the wireless clients in its BSS transmit simultaneously?
No - they would collide, MAC prevents this
In a wifi LAN, do two wireless hosts usually send frames directly to one another? Explain
No, in wireless LAN, the host needs to send a frame to the access point and the access point retransmits to the destination host
Is spread spectrum transmission done for security reasons in commercial WLANs?
No-only in the military spread spectrum
What is a hertz?
Number or frequency of cycles per second
Are wireless LAN standards governed by OSI or TCP/IP standards?
OSI because it is a single network
What benefits can it bring?
Stronger signal so that the radio can operate at a lower power or send the signal farther. It also allows multi-user MIMO (MU-MIMO).
How many 20 MHz non overlapping channels does the 2.4 GHz band support?
Supports only 3 non overlapping channels (1, 6, 11)
Why is large channel bandwidth desirable?
Wide bandwidth is the key to fast transmission
Why was 802.11 made reliable?
Wireless networks have a lot of propagation problems and errors
Do we use the two terms interchangeably?
Yes
In 802.3 ethernet networks, can simple installation rules usually reduce propagation effects to non-issues?
Yes Ethernet propagation issues are predictable and we can also manage them With simple installation rules, we can reduce the propagation problem
What is the advantage of using unlicensed service bands?
You can add or drop access points anytime you choose and have as many wireless hosts as you wish. You don't need to get a license for every wireless device and you don't need to revise your license when the device moves
What do we call a system whose channels are wide?
broadband
Is CSMA/CA+ACK required or optional?
required
Why does the access point connect to the corporate ethernet LAN?
to give clients access to servers and internet access routers on the Wired LAN
