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Flashcards in Chapter 12. Deck (85):
1

What is often a good tool that can be used to diagnose layer 1 issues.

Spectrum Analyser.

2

What is the mortal enemy of WLAN performance?

Retransmissions.

3

What are block ACK's? What happens when one frame is corrupted?

When aggregate frames are acknowledged by Block ACK.

If a frame is corrupted then the entire block will need to be sent again.

4

What is latency?

The time it takes to deliver a packet from the source destination to the destination device. A delay in the delivery (increased latency) of a VoIP packet due to layer 2 retransmissions can result in echo problems.

5

What is jitter? What could be the cause of jitter?

Jitter is the variation of latency. Measurement of latency that is not average. If all packets travel at exactly the same speed through the network, jitter will be zero. A high variance in the latency (jitter) is a common result of 802.11 layer 2 retransmissions.

6

What is a tolerable # for layer 2 retransmission rates for data? What about voice?

A layer 2 retransmission rate of up to 10 percent without any noticeable degradation in performance.

However, time-sensitive applications such as VoIP require that higher-layer IP packet loss be no
greater than 2 to percent 5%.

7

How are layer 2 retransmissions recorded or detected?

Via using a protocol analyzer.

8

What are some possible causes for layer 2 retransmissions?

Multipath, RF interference,, low signal to noise ratio, hidden node problems, near/far problems, and mismatched power issues, and adjacent channel interference.

9

What can happen if an AP is configured to transmit at high amplitude, and other clients are trying to transmit?

Severe degradation of service. During clear channel assessment if a station of detecting RF energy, it will defer transmission until medium is clear.

10

What is narrowband interference?
What kind of interference can it cause?
What kind of damage can it cause?
What can be used to detect it?
What can you do to work around it?

An RF signal occupies a smaller and finite frequency space and will not cause a denial of service (DoS) for an entire band, such as the 2.4 GHz ISM band.

A narrowband signal is usually very high amplitude and will absolutely disrupt communications in the
frequency space in which it is being transmitted. Narrowband signals can disrupt one or several 802.11 channels.

Damage : Narrowband RF interference can also result in corrupted frames and layer 2 retransmissions.

Spectrum Analyser.

To work around you, adjustments to the channel reuse plan will be necessary.

11

What is wideband interference?
How do you get rid of it?

A source of interference is typically considered wideband if the transmitting signal has the
capability to disrupt the communications of an entire frequency band.

The only way to eliminate wideband interference is to locate the source of the interfering device with a spectrum analyzer and remove the interfering device.

12

What is all-band interference?

Typically associated with frequency hopping spread
spectrum (FHSS) communications that usually disrupt the 802.11 communications at 2.4 GHz.

13

What is bluetooth?

A short-distance RF technology used in WPANs. Bluetooth uses FHSS and hops across the 2.4 GHz ISM band at 1,600 hops per second.

14

What do new bluetooth devices use to prevent band wide interference?
Why does this only work in a single channel single AP environment?

Newer Bluetooth devices utilize adaptive mechanisms to avoid interfering with 802.11 WLANs. Bluetooth adaptive frequency hopping is most effective at avoiding interference with a single AP transmitting on one 2.4 GHz channel.

Issue : If multiple 2.4 GHz APs are transmitting on channels 1, 6 and 11 in the same physical area, it is impossible for the Bluetooth transmitters to avoid interfering with the WLAN.

15

How large was the channel size in MHz that B radios used? What about g/n radios?

22 MHz. for B
20 MHz. for G\N

16

What is delay spread?

Difference in time between the primary signal and the reflected signals

17

What kind of interference does multipath cause?

Multipath can cause intersymbol interference (ISI), which causes data corruption.

18

Why does multipath have a constructive effect for MIMO systems?

Multipath has a constructive effect with 802.11n/ac transmissions that utilize multiple-input, multiple-output
(MIMO) antennas and maximum ratio combining (MRC) signal processing techniques.

19

What is the main purpose of active surveys?
What happens when no interference is detected?

The main purpose of the active site survey is to look at the percentage of layer 2 retries.
If it is determined during the spectrum analysis portion of the site survey that no RF interference occurred, the most likely cause of the layer 2 retransmissions will be multipath.

20

What are some locations for high multipath indoors?

High-multipath environments exist indoors in areas such as long corridors and anywhere metal is located (for example, warehouses with metal shelving or metal racks).

21

What is the purpose of indoor patch antennas?

The most common use case for using a MIMO patch antenna indoors is a high-density environment. MIMO patch and panel antennas are often mounted from the ceiling downward to provide tight “sectors” of coverage.

22

What is a high density environment?
What are some examples?

A high-density environment can be described as an area where numerous Wi-Fi client devices exist in a very small area. An example might be a gymnasium at a school or a meeting hall packed with people
using multiple Wi-Fi radios.

23

What is adjacent channel interference?

Degradation of performance resulting from overlapping frequency space that occurs due to an improper
channel reuse design. In the WLAN industry, an adjacent channel is considered to be the next or previous numbered channel. For example, channel 3 is adjacent to channel 2

24

How far apart do channel needs to be apart on 2.4 GHz to be considered non overlapping. What other channels could be considered non overlapping?

5 channels
Channels 2 and 7 are nonoverlapping, as well as 3 and 8, 4 and 9, and 5 and 10. The important thing to remember is that
there must be fve channels of separation in adjacent coverage cells.

25

What happens if you have overlapping coverage + frequency overlap?

If overlapping coverage cells also have frequency overlap from adjacent channels, the transmitted frames
will become corrupted, the receivers will not send ACKs, and layer 2 retransmissions will
significantly increase.

26

How many channels are correctly available in 5 GHz?

25 channels in the U-NII bands.

27

What is considered non overlapping in 5 GHz for channel size?

20 MHz of separation between the center frequencies.

28

What is SNR?
Give an example.

The SNR is not actually a ratio. It is simply the difference in decibels between the received signal and the background noise (noise floor)

If an 802.11 radio receives a signal of –70 dBm and the noise floor is measured at –95 dBm, the difference between the received signal and the background noise
is 25 dB. The SNR is therefore 25 dB.

29

What happens if the SNR is to low?

Data transmissions can become corrupted with a very low SNR. If the amplitude of the noise floor is too close to the amplitude of the received signal, data corruption will occur and result in layer 2 retransmissions.

30

What is considered a good SNR ratio? Bad? What do the vendors recommend?

An SNR of 25 dB or greater is considered good signal
quality.

SNR of 10 dB or lower is considered poor signal quality.

To ensure that frames are not corrupted, many vendors recommend a minimum SNR of 20 dB for data WLANs and a minimum SNR of 25 dB for voice WLANs.

31

Designing for coverage during a site survey, the normal recommended best practice is to provide for a ..... signal? What about voice?

-70 dBm for data
-67 for voice.

32

What is Mismatched Power Settings?
How does this happen?
What will happen if this occurs?
How do you prevent this?
Why is it a bad idea to configure an AP to transmit at full power?

Communications can break down if a client station’s transmit power level is less than the transmit power level of the access point. As a client moves to the outer edges of the coverage cell, the client can “hear” the AP; however, the AP cannot “hear” the client.

if an access point has a transmit power of 100 mW
and a client has a transmit power of 20 mW, the client will hear a unicast frame from the AP because the received signal is within the client station’s receive sensitivity capabilities. However, when the client sends an ACK frame back to the AP, the amplitude of the client’s transmitted signal has dropped well below the receive sensitivity threshold of the AP’s radio. The ACK frame is not “heard” by the AP, which then must retransmit the unicast frame. All of the client’s transmissions are effectively seen as noise by the AP, and layer 2 retransmissions are the result.

To prevent this make sure that the transmit power is not to far off from the capabilities of your stations radios.

With that in mind, configuring an access point to
transmit at full power is not a good idea and may cause this problem. Although an AP can usually be configured to transmit to as much as 100 mW, many clients such as smartphones and tablets may only have transmitters capable of 25 mW.

33

Why does dynamic RF capabilities used by many WLAN vendors create mismatches?

An access point might dynamically increase its transmit power to a level above the client’s transmit
power.

34

What did the 802.11k amendment do to aleve issues with power mismatches?

The ratified 802.11k amendment does make it possible for an AP to inform clients to use transmit power control (TPC) capabilities to change their transmit amplitude dynamically to match the AP’s power.

35

What is the near/far issue?
How does this occur?
Prevention?

Disproportionate transmit power settings between multiple clients may also cause communication problems within a basic service set. A low-powered client station that is at a great distance from the access point could become an unheard client if other high-powered stations are very close to that access point.

Occurs : The transmissions of the high-powered stations
could raise the noise floor near the AP to a higher level. The higher noise floor would corrupt the far station’s incoming frame transmissions and prevent this lower-powered station from being heard

Prevention : The half-duplex nature of the medium usually prevents most near/far occurrences. Please understand that the medium access methods employed by Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) usually averts the near/far problem. A well planned WLAN that provides for –70 dBm or stronger coverage should also negate any
worries about near/far issues.

36

What is Clear Channel Assessment?

CCA involves listening for 802.11 RF transmissions at the Physical layer; the medium must be clear before a station can transmit.

37

What is a problem with physical carrier sense on some networks? (Hidden Node)

The problem with physical carrier sense is that all stations may not be able to hear each other. Remember that the medium is half-duplex and, at any given time, only one radio can be transmitting. What would happen, however, if one client station that was about to transmit performed a CCA but did not hear another station that was already transmitting? If the station that was about
to transmit did not detect any RF energy during its CCA, it would transmit. The problem is that you then have two stations transmitting at the same time. The end result is a collision, and the frames will become corrupted. The frames will have to be retransmitted.

38

What is hidden node?

Occurs when one client station’s transmissions are heard by the access point but are not heard by any or all of the other client stations in the basic service set (BSS). The clients would not hear each other and therefore could transmit at the same time. Although the access point would hear both transmissions, because two client radios are transmitting at the same time on the same frequency, the incoming client transmissions would be corrupted.

39

What could be the loss rate and retransmission

A hidden node can drive retransmission rates above 15 to 20 percent or even higher.

40

How can a use create a hidden node problem?
Can this be caused by access points transmitting at too high of an amplitude?

A user moving behind some sort of obstruction can cause a hidden node problem. VoWiFi phones and other mobile Wi-Fi devices often become hidden nodes because users take the mobile device into quiet corners or areas where the RF signal of the phone cannot be heard by other client stations.

The hidden node problem can also occur when two client stations are at opposite ends of an RF coverage cell and they cannot hear each other, as shown in Figure 12.13. This often happens when coverage cells are too large as a result of the access point’s radio transmitting at too much power.

41

Why should you disable the lower data rates (Hidden Node).

Data rates is that a 1 and 2 Mbps coverage cell at 2.4 GHz can be quite large and often results in hidden nodes. If hidden node problems occur in a network planned for coverage, then RTS/CTS may be needed

42

What is RTS\CTS? How can this be effected by hidden node?

When clients request access to the medium.
The stations on the other side of the obstacle may not hear the RTS frame from the hidden node, but they will hear the CTS frame sent by the access point. The stations that hear the CTS frame will reset their NAV for the period of time necessary for the hidden node to
transmit the data frame and receive its ACK frame.

43

What is dynamic rate switching?
Why does this happen?

As client station radios move away from an access point. As mobile client stations move further away from an access point, both the AP and the client will shift down to lower rates that require a less complex modulation coding scheme (MCS).

44

What are the thresholds for dynamic rate switching?

proprietary and are defned by 802.11 radio manufacturers. Most vendors base DRS on receive signal strength indicator (RSSI) thresholds, packet error rates, and retransmissions. RSSI metrics are usually based on signal strength and signal quality.

45

Why is it recommended best practice to turn off the lowest data rates on 2.4 GHz?

Explain the 3.

1. Sticky client roaming problems.
2. Medium contention
3. Hidden Node problem.

When 802.11 radios transmit at very low data rates such as 1Mbps and 2Mbps, effectively they cause medium-contention overhead for higher data rate transmitters due to the long wait time. The time it takes them to transmit data is very long, making the clients close to the AP wait a long time.

46

How does a client make the decision to roam?
What is the most important criteria?

As the client station communicates on the network,
it continues to look for other access points via probing and listening on other channels and will hear received signals from other APs. (SNR, error rates, and retransmissions) The variable of most importance will always be received signal strength:

47

What is the thresholds required for higher data rate in dBm?

-70 dBm for high speed data.

48

How do you insure that the client roams and still is able to keep max transmission rates?

Typically, most vendor RSSI thresholds require a received signal of –70 dBm for the higher data rate communications. Each client needs to be able to hear the other AP nearby at -70 dBm as well.

49

What are some issues that can be caused by AP's being to close to one another?

On the flip side, too much duplicate coverage will also cause roaming problems. For example, a client station may stay associated with its original AP and not connect to a second access point even though the station is directly underneath the second access point. This can also create a situation in which the client device is constantly switching back and forth between the two or more APs on different channels. If a client station can also hear dozens of APs on the same channel with very strong signals, a degradation in
performance will occur due to medium contention overhead.

50

What is the important of latency in an 802.1X/EAP security solution?

Another design issue of great importance is latency. The 802.11-2012 standard suggests the use of an 802.1X/EAP security solution in an enterprise. The average time involved during the authentication process can be 700 milliseconds or longer. However, VoIP can only handle 120 millisecond delays.

51

What is a Mobile IP standard?

How does it work?

Is an Internet Engineering Task Force (IETF) standard protocol that allows mobile device users to move from one layer 3 network to another while maintaining their original IP address.

Mobile IP is defned in IETF request for comment (RFC) 3344. Layer 3 roaming solutions based on Mobile IP use some type of tunneling method and IP header encapsulation to allow packets to traverse between separate layer 3 domains with the goal of maintaining upper-layer communications.

52

What is a home address in layer 3 roaming?

The original IP address that it was given on the home network.

53

What is co-channel interference? Why is it bad?

If all of the APs are on the same channel, unnecessary medium contention overhead occurs. As you have learned, CSMA/CA dictates half-duplex communications, and only one radio can transmit on the same channel at any given time.if an AP on channel 1 is transmitting, all nearby access points and clients on the same channel will defer transmissions. The result is that throughput is adversely affected: Nearby APs and clients have to wait much longer to transmit because
they have to take their turn. The unnecessary medium contention overhead that occurs because all the APs are on the same channel.

54

Which is more disruptive? co-channel or adjacent channel?

Adjacent channel interference is a much more serious problem than co-channel interference because of the corrupted data and layer 2 retries.

55

Can client stations also partake in cochannel interference?

Please understand that client station transmissions also result in the medium contention performance downgrade known as co-channel interference. As shown in Figure 12.27, if a client is at the outer edges of a coverage cell, the client’s transmissions may propagate into another cell using the same channel. All of the radios in the other cell will defer if they hear
the original client’s transmissions.

56

How many available channels are there in the 5 GHz band? How many are available in the US?

25 channels are available but only 23 can be used in the USA.

57

What is recommended best practice for channel reuse in 5 GHz, and avoiding frequency side band overlap interference from adjacent channels?

It is a recommended practice that any adjacent coverage cells use a frequency that is at least two channels apart and not use an adjacent frequency.

58

What is the second recommended best practice for 5 GHz channel reuse design? Why is this recommended?

The second recommended practice for 5 GHz channel reuse design is that there should be at least two cells of coverage space distance between any two
access points transmitting on the same channel. Following this rule will prevent co-channel interference from APs and most likely also from clients. The client’s signal will have to propagate a greater distance and should attenuate to an amplitude level below the noise floor before the signal reaches another coverage cell using the same channel

59

What are the 2 things that you need to take into consideration when creating a channel plan?

1. The first factor is what channels are available legally in your country or region.
2. The second factor to consider is what channels the client population supports. Can your clients support DFS?

60

What are the capabilities of channel bonding two 20MHz channels together?

Introduced the capability of bonding two 20MHz channels to create a larger 40 MHz channel. Channel bonding effectively doubles the frequency bandwidth,
meaning double the data rates that can be available to 802.11n radios.

61

How many 40 MHz channels are available in the 5 GHz band?

10 since 2 of them can not be used due to TDWR band.

62

Why should you NOT enable channel bonding?

1. If four or fewer 40 MHz channels are available, you might not want to turn on channel bonding, especially if the 5 GHz radios are transmitting at a higher power level. There are not enough channels between each other to avoid adjacent channel overlap interference.

2. Also, many clients may not support channel bonding. The key is that the client radios must support DFS and should support channel bonding.

63

What is single channel architecture?

The client stations see transmissions on only a single channel with one SSID (logical WLAN identifier) and one BSSID (layer 2 identifier). From the perspective of the client station, only one access point exists. Uplink and downlink transmissions are coordinated by a WLAN controller on a single 802.11 channel in such a manner that the effects of co-channel interference are minimized.

64

What is a virtual BSSID?

Used in single channel architecture systems to make the WLAN appear as one giant AP. Because the multiple access points advertise only one single virtual MAC address (BSSID), client stations believe they are connected to only a single access point, although
they may be roaming across multiple physical APs. Roaming is handled by the controller.

65

What are the benefits of SCA?

The ability to have seamless handoffs and roaming is instant.

66

Why should you NOT use SCA?

1. Because opportunistic key caching (OKC) and other fast secure roaming mechanisms have become more commonplace and VoWiFi is deployed extensively within the more traditional MCA architecture. Eliminating the benefits of the past.

2. Also capacity. One single channel is used.

67

What is channel stacking on a SCA?

When each floor is a different channel. Creating layers of channels.

68

What is cell sizing?

stations, the cell sizes have been decreased while the number of cells has been increased. Adjusting the transmit power to limit the coverage area.

69

Why will clients, if given a choice, will connect to 2.4 GHz radios first before 5 GHz?

Because 5 GHz attenuates faster then 2.4 GHz; and by the time that a client sees the 5 GHz its not as strong as 2.4 and clients connect to the strongest signal.

70

How does band steering work?

When a dual-frequency client first starts up, it will transmit probe requests on both the 2.4 and 5 GHz bands looking for an AP. When a dual-frequency AP
hears probe requests on both bands originating from the same client radio, the AP knows that the client is capable of operating in the 5 GHz band. the AP will then try to steer the client to the 5 GHz band by responding to the client using only 5 GHz transmissions. Although the client is steered to the 5 GHz AP, there may be reasons for the client to connect to the AP using the 2.4 GHz radio. If the client radio continues to try to connect to the AP using the 2.4 GHz radio, the AP will ultimately allow the connection.

71

What kind of frame is sent to the AP when a client wants to connect?

An association frame.

72

What is load balancing on a wireless network? How does it work?

Why is this a TERRIBLE idea when roaming is needed?

Load balancing clients between access points ensures that a single AP is not overloaded with too many clients and that the total client population can be served by numerous APs with the final result being better
performance.

Works : If an AP is already overloaded with too many clients, the AP will defer the association response to the client. The hope is that the client will then send another
association request to another nearby AP with a lesser amount of client load.

Bad : Load balancing between access points is typically implemented in areas where there is a high density of clients and roaming is not necessarily the priority. In areas where roaming is needed, load balancing is usually not a good idea because the mechanisms may cause clients to become sticky and stay associated to the AP too long.

73

How many clients can an AP support on N with average usage?

If the access point is using 802.11n radios with
20 MHz channels, a good rule of thumb is that each radio could support 35 to 50 active devices for average use, such as web browsing and checking email.

74

In a VHD why should you cut out b data rates?

Disable the 802.11b data rates to cut down on medium contention overhead and protection mechanism overhead. Keep in mind that 802.11b clients will not be allowed to associate

75

In a VHD why select a high min data rate such as 18 Mbps?

Because they client can not roam far and stay connected.

76

In a VHD why lower the AP transmit power for cell sizing?

To limit co-channel interference

77

In a VHD why use load balancing and band steering?

To prevent roaming and to balance the AP load between frequencies.

78

In a VHD why use sector antennas?

To provide sectorized coverage and prevent the signal from traveling too far.

79

In a VHD why turn OFF some 2.4 GHz radios.

To cut down on 2.4 GHz co-channel interference.

80

Why are data rates always aggregate?

The medium access method that uses interframe spacing, physical carrier sense, virtual carrier sense, and the random back-off timer creates overhead and consumes bandwidth.

81

Why is lightning a problem to WLAN? What are solutions to prevent this?

Direct and indirect lightning strikes can damage WLAN equipment. Lightning arrestors should be used for protection against transient currents. Solutions such as lightning rods or copper/fiber transceivers may offer protection against lightning strikes

82

Why is wind an issue in focused beam RF? What are solutions?

Even slight movement of a highly directional antenna can cause the RF beam to be aimed away from the receiving antenna, interrupting the communications. In high-wind environments, a grid antenna will typically
remain more stable than a parabolic dish.

83

Why is water a problem to WLAN RF? Issues ? Solutions?

Water damage is often a serious problem with cabling and connectors. Connectors should be protected with drip loops and coax seals to prevent water damage. Cables and connectors should be checked on a regular basis for damage. A radome (weatherproof protective
cover) should be used to protect antennas from water damage.

Rain can also attenuate a signal. 0.08 dB per mile.

System Operating Margin of 20 dB is recommended.

84

Why is air stratification important to WLAM design?

A change in air temperature at high altitudes is known as air stratification (layering). Changes in air temperature can cause refraction. Bending of RF signals over long-distance point-to-point links can cause misalignment and performance issues. K-factor
calculations may be necessary to compensate for refraction over long-distance links.

85

Why is UV/SUN an issue to WLAN?

UV rays and ambient heat from rooftops can damage cables over time if proper cable types are not used