Wireless Fundamentals

Here you will find answers to Wireless Fundamentals Questions – Part 1

If you are not sure about Wireless, please read my Wireless tutorial and Basic Wireless Terminologies

Question 1

What is fading?

A. Another signal source is producing energy on the channel in which you are trying to operate.
B. The desired signal reaches the receiving antenna via multiple paths, each of which has a different propagation delay and path loss.
C. A time-varying change in the path loss of a link with the time variance governed by the movement of objects in the environment, including the transmitter and receiver themselves.
D. A function of the frequency and should be provided in the cable specification by the vendor.
E. The minimum signal level for the receiver to be able to acceptably decode the information.
F. The time delay from the reception of the first instance of the signal until the last instance.

 

Answer: C

Explanation

Fading is a time-varying change in the path loss of a link with the time variance governed by the movement of objects in the environment, including the transmitter and receiver themselves. For example, you might be sitting in a conference room with a wireless laptop and be connected to an AP in the hallway. If someone closes the door to the conference room, the path loss drops, resulting in a lower received signal level because the signal has to go through different paths to reach the destination. This scenario is a fade.

The different signal paths between a transmitter and a receiver correspond to different transmission times. For an identical signal pulse from the transmitter, multiple copies of signals are received at the receiver at different moments. The signals on shorter paths reach the receiver earlier than those on longer paths.

In wireless communications, signal fading is caused by multi-path effect. Multi-path effect means that a signal transmitted from a transmitter may have multiple copies traversing different paths to reach a receiver. Thus, at the receiver, the received signal should be the sum of all these multi-path signals.

(Notice that the resulting received signal has similar form of direct signal)

Question 2

In what frequency band does 802.11n operate in?

A. 5.0Ghz
B. 2.4Ghz
C. 3.7Ghz
D. 2.4Ghz and 5.0Ghz

 

Answer: D

Explanation

802.11n operates in the 2.4-GHz and 5-GHz bandwidths and is backward-compatible with 802.11a and 802.11b/g.

Question 3

Which three elements define the 802.11n implementation of MIMO? (Choose three)

A. Channel Bonding
B. Dynamic Frequency Selection
C. Maximal Ratio Combining
D. Packet Aggregation
E. Spatial Multiplexing
F. Transmit Beam Forming

 

Answer: C E F

Explanation

Multiple-Input Multiple-Output (MIMO) is a technology that is used in the new 802.11n specification. A device that uses MIMO technology uses multiple antennas to receive signals (usually two or three) as well as multiple antennas to send signals. Three elements that are implemented in MIMO are:

+ Maximal Ratio Combining (MRC): used by the receiver with multiple antennas to optimally combine energies from multiple receive chains. An algorithm eliminates out-of-phase signal degradation.

In this example, the radio receiver uses 3 antennae to take advantage of the multiple signals that each carry an identical copy of the data, by combining the received signals and performing algorithms that increase the sensitivity to the received signal.

+ Spatial Multiplexing: Multiple antennas are used on the transmitting or receiving end. Since the same channel is used, each antenna receives the direct component intended for it as well as the indirect components for the other antennas. However it requires the client to have multiple antennae and be 802.11n compliant.

We can also use Spatial Multiplexing to transmit multiple data streams at the same time. They are transmitted on the same channel, but by different antenna. This technique can double, triple, or quadruple the data rate depending on the number of transmit antennas.

+ Transmit Beam Forming: The transmitter on a MIMO AP is able to adjust the transmitted signal by modifying the transmitted beam from each of its antenna according to the reflective environment in order to improve the signal to noise ratio (SNR) at the receiver, thereby improving network performance and predictability.

Question 4

Effective Isotropic Radiated Power is calculated by using which three values? (Choose three)

A. antenna bandwidth
B. antenna gain
C. cable loss
D. receiver sensitivity
E. SSID
F. transmission power

 

Answer: B C F

Explanation

Effective Isotropic Radiated Power (EIRP) is a way to measure the amount of energy radiated from an antenna. EIRP is calculated using the following formula:

EIRP = transmitter output power – cable loss + antenna gain

in which:

+ Transmitter output power: the total power radiated by the antenna.
+ Cable loss: when an access point sends energy to an antenna to be radiated, a cable might exist between the two. A certain degree of loss in energy is expected to occur in the cable.
+ Antenna gain: To counteract cable loss, an antenna adds gain, thus increasing the energy level.

Example 1:

Transmitter power = 35 dBm
Antenna gain = 10 dBi
Line loss = 8 dB

EIRP (dBm or dBW) = P (dBm or dBW) + G (dBi) – L (dB)

EIRP = 35 + 10 – 8
EIRP = 37 dB

Example 2:

Transmitter with 100 mW output power (+20 dBm)
Yagi antenna with a 13.5 dBi gain rating
50 foot of cable with a loss of 2.2 dB

EIRP = 20 + 13.5  – 2.2  = 31.3 dBm

Question 5

What increases bandwidth and resists multipath problems by carrying data in subcarriers?

A. Direct Sequence Spread Spectrum
B. Frequency Hopping Spread Spectrum
C. Narrow Band Frequency
D. Orthogonal Frequency Division Multiplexing

 

Answer: D

Explanation

Orthogonal Frequency Division Multiplexing (OFDM) defines a number of channels in a frequency range. It transmits several high speed communication channels through a single communication channel using separate sub-carriers ( frequencies) for each radio channel.

(“Orthogonal” here means the peak of each signal coincides with the trough of other signals)

With OFDM (Orthogonal Frequency Division Multiplexing), sub-carriers in the signal wave can be overlapping without causing any interference. This is accomplished by making the sub-carriers in an OFDM exactly orthogonal to each other, meaning that while one is at its peak the others are all zero. As a result, OFDM systems are able to maximize spectral efficiency without causing adjacent channel interference. The frequency domain of an OFDM system is represented in the diagram below.

Comparing to traditional FDM, each channel is spaced by about 25% of the channel width. This is done to ensure that adjacent channels do not interfere.

Because of the overlapping ability without interfering, OFDM systems are able to maximize spectral efficiency and this allows for a greater data throughput for a given bandwidth.

OFDM helps resisting multipath

The great advantage of OFDM is that if there are 48 channels carrying data, each of them can transmit slower than a CCK channel and the group of 48 will still achieve a higher throughput. For example, if each subcarrier sends at 1 Mb/s, the total speed achieved will be 48 Mb/s. The result is that not only is OFDM faster but, as each channel transmits slower, it becomes more resistant to multipath. Because there are many channels, some of them can be affected by interferences but the others can still offer normal communications. The control channels allow the receiver to detect which channels are unusable and provide feedback to the sender.

Question 6

What is derived from measuring the RF duty cycle?

A. dynamic channel selection
B. LWAPP header length
C. RF utilization
D. transmit power control

 

Answer: C

Explanation

Duty cycle is a percentage, and is directly related to RF utilization. If something other than a Wi-Fi radio is transmitting on a channel, then a Wi-Fi radio can’t use the channel until it’s free. So duty cycle tells us how often (what percentage of time) the channel is in use by something that is seen as noise by the Wi-Fi network.

Question 7

In 802.1X, which is the supplicant?

A. the point of access
B. the machine in the network that keeps a list of conditions
C. the machine that attempts to access the network
D. the device that performs the authentication

 

Answer: C

Explanation

The IEEE 802.1X protocol defines port-based access control. It defines three roles:
+ Supplicant: The machine that wishes to access the network.
+ Authenticator: The point of access, typically a switch. It is the point of entrance to the network. The supplicant, typically a PC, would connect to the authenticator.
+ Authentication server: A machine somewhere in the network that keeps a list of conditions by which access should be granted or refused.

Question 9

What three roles are defined by 802.1X? (Choose three)

A. AAA Server
B. Authenticatee
C. Authenticator
D. Authentication Server
E. Supplicant

 

Answer: C D E

Question 10

Which modulation technique allows you to achieve a data rate of 54Mb/s in the 2.4GHz-band?

A. Complimentary Code Keying
B. Differential Binary Phase Shift Keying
C. Differential Quadrature Phase Shift Keying
D. Quadrature Amplitude Modulation

 

Answer: D

Explanation

Quadrature Amplitude Modulation (QAM) is a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. In a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (so they are called quadrature carriers). Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave. The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information.

With QAM method, 802.11g standard delivers the same 54 Mb/s maximum data rate as 802.11a, but operates in the
same 2.4-GHz band as 802.11b.