Chapter 12. Flashcards
What is often a good tool that can be used to diagnose layer 1 issues.
Spectrum Analyser.
What is the mortal enemy of WLAN performance?
Retransmissions.
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.
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.
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.
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 5%.
How are layer 2 retransmissions recorded or detected?
Via using a protocol analyzer.
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.
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.
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.
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.
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.
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.
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.
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
What is delay spread?
Difference in time between the primary signal and the reflected signals
What kind of interference does multipath cause?
Multipath can cause intersymbol interference (ISI), which causes data corruption.
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.
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.
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).
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.
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.
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
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.
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.
How many channels are correctly available in 5 GHz?
25 channels in the U-NII bands.
What is considered non overlapping in 5 GHz for channel size?
20 MHz of separation between the center frequencies.
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.
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.
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.
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.
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.
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.
What did the 802.11k amendment do to relieve 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.
