Chapter 6: Wireless LANs I (Test Your Knowledge)

If the lowest frequency in a channel is 1.22 MHz and the highest frequency is 1.25 MHz, what is the channel bandwidth? (Write it properly in metric notation.)

0.03 MHz - 30 kHz

What three issues must be overcome to make mesh networking acceptable to corporations?

1.) Meshes must be self-organized; hosts and access points enter and leave the mesh frequently and the network must respond immediately to changes. 2.) It will be difficult to avoid overloading access points near the geographical center of the mesh 3.) Security. With no central control, there will have to be security existing on every device.

Convert 3.4 MHz to a number without a metric prefix. (The use of metric prefixes was discussed in a box in Chapter 1.)

3,400,000 Hz

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.

80 mW (30m/120m)^2 = 0.5 mW

Distinguish between 802.11 and Wi-Fi.

802.11 and Wi-Fi are the same exact thing; Wireless LAN standards were created by the IEEE 802.11 Working Group.

In what two service bands does 802.11 operate?

802.11 operates in the 2.4 GHz and 5 GHz unlicensed bands.

What is the market position of 802.11ac?

802.11ac is growing rapidly.

Among the four standards listed, which are obsolete?

802.11g and 802.11a are both obsolete now but are still used.

What frequency band or bands do 802.11g, 802.11a, 802.11n, and 802.11ac use?

802.11g uses 2.4 GHz, 802.11a uses 5 GHz, 802.11n uses both, and 802.11ac uses 5 GHz.

Which dominates the installed base today?

802.11n dominates the installed base today.

Compare the range of rated speeds possible with 802.11n and 802.11ac.

802.11n has a rated speed of 100 Mbps to 600 Mbps. 802.11ac has a rated speed of 433 Mbps to 6.93 Gbps. 802.11ac has a much greater range.

Distinguish between 802.3 standards and 802.11 standards.

802.3 are Ethernet LAN network standards. 802.11 are Wireless LAN standards.

What is an SSID? (Do not just spell out the acronym.)

A Service Set ID identifies an access point—actually a network.

What is a BSS? (Do not just spell out the acronym.)

A basic service set is an access point and its wireless hosts.

What is a handoff in 802.11?

A handoff in 802.11 is when a host travels too far from a wireless access point, and is handed off to a closer access point in the same extended service set.

What is a hertz?

A hertz is a measure of frequency - one cycle per second.

Does a signal usually travel at a single frequency, or does it spread over a range of frequencies?

A signal does not usually travel at a single frequency, it spreads over a range of frequencies; this range is also known as a signal's bandwidth.

What is a transceiver?

A transceiver is a radio for data transmission. Transceivers transmit and receive.

What characteristics do all access points in an ESS share?

All access points are connected to the same distribution system they all have the same SSID.

What devices forward frames in a mesh network?

All wireless devices forward frames in a mesh network.

What is an ESS? (Do not just spell out the abbreviation.)

An Extended Service Set is a group of Basic Service Sets that are connected to the same distribution system and in which all access points have the same SSID.

Why must an 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?

An access point must 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 because 802.11 frames cannot travel over 802.3 LANs and vice versa. 802.11 frames can only travel between wireless access points and the wireless host. 802.3 frames can only travel between the access point and the server.

What other types of system do we call broadband?

Any transmission system that is very fast is called broadband today even if it does not use channels.

What two propagation problems become worse as frequency increases?

As frequency increases, waves will suffer more rapidly from absorptive attenuation, and dead zone problems will increase. Higher frequencies are less able to travel through and around objects.

What is backward compatibility?

Backward compatibility is newer devices with newer standards that still implement older standards.

What is beamforming?

Beamforming is directing signal energy toward individual devices.

Why was 802.11 made reliable?

Because wireless transmission has so many errors that it makes sense to get rid of errors at the data link layer.

Does a frame's receiver transmit an ACK immediately or after a random delay?

Before the random delay, so that it will get through.

What do we call a system whose channels are wide?

Broadband.

Which is more efficient, RTS/CTS or CSMA/CA+ACK?

CSMA/CA+ACK

What does CS mean? (Do not just spell out the abbreviation.)

Carrier sense—listening to traffic.

Why is CA desirable?

Collision avoidance avoids collisions by only allowing devices to transmit at relatively safe times.

Why are companies moving rapidly into the 5 GHz service band?

Companies are moving rapidly into the 5 GHz band because it has more bandwidth.

List some sources of EMI.

Cordless telephones, microwaves, and other nearby access points are all examples of EMI sources.

How are dead zones created?

Dead zones are created by dense objects that block the direct signal path between the sender and the receiver.

Why is it important that governments add more bandwidth to the 5 GHz band?

Demand is increasing so fast that more channels are needed.

What type of antenna normally is used in WLANs? Why?

Distances are short, and you do not necessarily know where the access point is.

How can firms provide WLAN coverage throughout a large building?

Firms can provide WLAN coverage throughout a large building by placing multiple access points throughout that building.

Is wireless radio transmission usually expressed in terms of wavelength or frequency?

Frequency

Under what circumstances would you use a dish antenna?

I would use a dish antenna if I needed to send a signal very far away in a specific direction and new where the other radio was.

Under what circumstances would you use an omnidirectional antenna?

I would use an omnidirectional antenna if I am working with short distances such as a WLAN and if I did not know where the other radio was located.

How is carrier sensing used in multiple access?

If a device hears traffic, it does not transmit.

If you triple channel bandwidth, what happens to the number of channels in the service band?

If you triple channel bandwidth, you can only have one third as many channels in a service band.

In 802.11 Wi-Fi networks, can simple installation rules usually reduce propagation effects to non-issues?

In 802.11 Wi-Fi networks, simple installation rules usually do not reduce propagation effects to non-issues. This is the case because radio propagation is unreliable. The signal can be weakened in several different ways.

In 802.3 Ethernet networks, can simple installation rules usually reduce propagation effects to non-issues?

In 802.3 Ethernet networks, simple installation rules, such as respecting cord distance limits, usually reduce propagation effects to non-issues.

In normal radio operation, how does channel bandwidth relate to the bandwidth required to transmit a data stream of a given speed?

In normal radio operation channel bandwidth is only big enough to handle the desired speed. Any extra channel bandwidth would be a waste since it won't increase speed.

Why must wireless devices know the access point's SSID?

In order to connect to it, the devices needs to know the access point's SSID.

In radio, how can you send multiple signals without the signals interfering with one another?

In radio you can send multiple signals without the signals interfering with one another by sending each signal on a different channel. <Later, we will see that something called MIMO allows you to send two signals in the same channel without (too much) interference.

How does this change in spread spectrum transmission?

In spread spectrum transmission, the original signal, called a baseband signal, is spread over a much broader channel than is required by the transmission speed

What factors influence individual throughput, given a certain level of aggregate throughput?

Individual throughput is determined by the number of users using a specific access point. The more users on the access point, the less individual throughput each user receives.

Contrast inverse square law attenuation and absorptive attenuation.

Inverse square law attenuation occurs as the signal spreads out in a sphere and becomes weaker. Absorptive attenuation is when the signal is lost through energy absorption; for example air, plants, etc.

How many non-overlapping 20 MHz channels does the 5 GHz service band support?

It can provide between 11 and 24 non-overlapping 20 MHz channels.

What is the benefit of spread spectrum transmission for business communication?

It improves transmission reliability, especially with regard to multipath interferences.

Why does it use subcarriers instead of simply spreading the data over the entire channel?

It is easier to produce the necessary signal uniformity in smaller subcarriers than across a wide channel.

Why is it important?

It is important because without it if a new device with new standards came out, everyone would be forced to throw away their old device and buy the new device. This would be very expensive and inefficient.

All wireless hosts and the access point that serves them transmit on the same channel. How does media access control address this problem?

MAC methods govern when hosts and access points are allowed to transmit to avoid collisions.

Distinguish between MIMO and multiuser MIMO.

MIMO only allows transmission to one wireless device. MU-MIMO can focus on two wireless devices at the same time.

All wireless hosts and the access point that serves them transmit on the same channel. Does media access control apply to wireless hosts, access points, or both?

Media access control applies to both wireless hosts and access points.

At what range of frequencies do most wireless systems operate?

Most wireless systems operate between 500 MHz and 10 GHz.

What is the most serious propagation problem in WLANs?

Multipath Interference is the most serious propagation problem in WLANs because multiple signals can cancel each other out.

Is spread spectrum transmission done for security reasons in commercial WLANs?

No. Implementations in WLANs provide no security benefits.

Is CSMA/CA+ACK efficient?

No. There are many delays.

Distinguish between omnidirectional and dish antennas in terms of operation.

Omnidirectional antennas send a weaker signal that spreads in every direction, and can also receive a signal from a receiver from any direction. Dish antennas send a strong signal in a specific direction.

Describe RTS/CTS.

One wireless client sends a request-to-send message to the access point. The access point broadcasts a clear-to-send frame. The requesting device may now send. Other wireless devices, hearing the CTS signal, must wait.

Is RTS/CTS required or optional?

Optional.

Distinguish between rated speed, aggregate throughput, and individual throughput.

Rated speed is what an access point should get according to the 802.11 standards it uses. Aggregate throughput is what an access point actually gets and is usually 25%-50% lower than the Rated Speed. An access point's aggregate throughput is also shared with everyone that uses that access point. Individual throughput is the throughput each individual host gets while using an access point. If there is 1 host then it gets 100% of the aggregate throughput, if there are 2 they each get 50%.

Is CSMA/CA+ACK reliable or unreliable?

Reliable, thanks to acknowledgements.

Is CSMA/CA+ACK required or optional?

Required.

If you want to transmit seven times as fast, how much wider must the channel be?

Seven times as wide.

In unlicensed bands, what type of transmission method is required by regulators?

Spread spectrum transmission.

Which band dominated use initially?

The 2.4 GHz band dominated use initially.

How many 20 MHz non-overlapping channels does the 2.4 GHz band support?

The 2.4 GHz band supports 3 non-overlapping 20 MHz channels.

What is the relationship between handoffs and roaming in Wi-Fi?

The ability to use handoffs is called roaming. They are the same thing.

What benefits can it bring?

The benefits of beamforming is that stronger signals can be sent to individual wireless hosts.

For each, compare channel bandwidth and the number of possible channels.

The channel bandwidth of 802.11g and 802.11a is 20 MHz. 802.11g has 3 possible channels, and 802.11a has 20-25 channels. The channel bandwidth of 802.11n is 40 MHz but it will drop to 20 MHz if there is interference on the two selected channels. At 40 MHz 802.11n has 8-12 channels, and at 20 MHz it has 20-25. The channel bandwidth of 802.11ac is 80 MHz or 160 MHz. It has 4 -6 channels at 80 MHz and 1-2 at 160 MHz.

What is the current 802.11 standard for mesh networking?

The current 802.11 standard for mesh networking is 802.11s.

In Figure 6-16, what is the distribution system?

The distribution in Figure 6-16 is a Wired Ethernet LAN; the wired LAN to which access points connect is a distribution system.

What is the downside?

The downside is that you have no control over interference from others around you.

Distinguish among the frequency spectrum, service bands, and channels.

The frequency spectrum is the range of all possible frequencies from 0 Hz to infinity. A service band is a range of frequencies allocated to a certain purpose; GPS, AM Radio, etc. Channels are smaller frequency ranges in a specific Service Band.

What is the main benefit of MIMO?

The main benefit of MIMO is that it can greatly increase throughput.

What is its other benefit?

The other benefit of MIMO is that it has a greater transmission range. This allows for greater propagation distances which in turn, allows for fewer access points to be installed.

What is the purpose of the 802.11r standard?

The purpose of the 802.11r standard is that it enables access points in the same distribution system to communicate with each other in roaming.

How does MIMO use spatial streams to increase transmission speed?

The spatial streams sent from different antennas will arrive at the two receiving antennas at slightly different times. MIMO uses detection and separation methods based on differences in arrival times for the two spatial streams; the receiver can separate the two spatial streams in the same channel and so can read them individually.

What spread spectrum transmission method dominates today?

The spread spectrum transmission method that dominates today is called Orthogonal Frequency Division Multiplexing (OFDM), which divides each broadband channel into many smaller sub channels called subcarriers.

All wireless hosts and the access point that serves them transmit on the same channel. What problem does this cause?

This causes individual throughput to fall because when a wireless device tries to transmit a message all other devices on the same channel must wait so that collisions are avoided.

Why does transmission speed drop as a computer moves farther from an access point?

Transmission speed drops as a computer moves farther from an access point because as you get further away from an access point the signal weakens and your transceiver must switch to a less aggressive modulation method that is less sensitive to errors. This method transmits more slowly.

(How many 20 MHz non-overlapping channels does the 2.4 GHz band support? The 2.4 GHz band supports 3 non-overlapping 20 MHz channels.) Why is this a problem?

Two nearby access points may have to be set to the same channel. This will cause interference.

Why can two products that comply with the same standard perform differently?

Two products that comply with the same standard perform differently because there are extra options in each standard that could increase performance if implemented in a product.

How can access points communicate with each other?

Via the distribution system.

Do WLANs today use licensed or unlicensed bands?

WLANs today use unlicensed bands.

Why is multipath interference very sensitive to location?

Wavelengths are very short, so two waves may add or cancel over slightly different distances.

When a device that implements 802.11ac attaches to an 802.11n access point, what standard do they use to communicate?

When a device that implements 802.11n attaches to an 802.11a access point they will use the 802.11n standard to communicate.

(Why can two products that comply with the same standard perform differently? Two products that comply with the same standard perform differently because there are extra options in each standard that could increase performance if implemented in a product.) What implications does this have for making purchases? (The answer is not in the text.)

When making a purchase this implies that you should know the optional standards and which product has the standards that fit your needs.

What is the advantage of using unlicensed bands?

When using unlicensed bands, companies can add or drop access points whenever they want. They can also have as many wireless hosts as they want.

Why is wider channel bandwidth good?

Wider channel bandwidth is good because it increases the possible transmission speed.

What is the downside of wider channel bandwidth?

Wider channel bandwidth means fewer channels.

Do wireless LAN standards come from OSI or TCP/IP? Explain.

Wireless LAN standards come from OSI because that is the dominant architecture for layers 1 and 2.

At what layers do wireless LANs operate?

Wireless LANs operate at layers 1 and 2.

When does RTS/CTS make sense to use?

Wireless clients may not be able to hear one another; this will make CSMA/CA+ACK ineffective. All devices can hear the access point (or they could not communicate through it); this means the CTS/RTS will always work.

Why do wireless clients need access to the firm's main wired switched Ethernet network?

Wireless clients need access to the firm's main wired switched Ethernet network because the resources they need are located there. They also need to be able to connect to the Internet via the Internet access router, which is on the main wired LAN.

What is wireless mesh networking?

Wireless mesh networking is an all-wireless network in which there is no wired distribution system.

Does the access point have an SSID?

Yes.

Why is large channel bandwidth desirable?

You can transmit signals faster.