QUESTION: 249

Which of the following are likely causes of propagated RF signal degradation? Select three.

An amplifier in the RF signal path
Trees in the Line-of-Sight
Weak receiver sensitivity
Co-channel interference
Adjacent channel interference
Flocks of birds in the signal path

Answer(s): B,D,E

Explanation:

While it is true that absorption of RF by various materials (buildings, trees, water vapor, etc.) tends to increase with frequency, remember we are talking about "free space" here. The frequency dependence in this case is solely due to the decreasing effective aperture of the receiving antenna as the frequency increases. This is intuitively reasonable, since the physical size of a given antenna type is inversely proportional to frequency. If we double the frequency, the linear dimensions of the antenna decrease by a factor of one-half, and the capture area by a factor of one-quarter. The antenna therefore captures only one-quarter of the power flux density at the higher frequency versus the lower one, and delivers 6 dB less signal to the receiver. However, in most cases we can easily get this 6 dB back by increasing the effective aperture, and hence the gain, of the receiving antenna. For example, suppose we are using a parabolic dish antenna at the lower frequency A consideration when planning or troubleshooting an RF link is the Fresnel Zone. The Fresnel Zone occupies a series of concentric ellipsoid-shaped areas around the LOS path, as can be seen in Figure 2.10. The Fresnel Zone is important to the integrity of the RF link because it defines an area around the LOS that can introduce RF signal interference if blocked. Objects in the Fresnel Zone such as trees, hilltops, and buildings can diffract or reflect the main signal away from the receiver, changing the RF LOS. These same objects can absorb or scatter the main RF signal, causing degradation or complete signal loss.
///
A consideration when planning or troubleshooting an RF link is the Fresnel Zone. The
Fresnel Zone occupies a series of concentric ellipsoid-shaped areas around the LOS path, as can be seen in Figure 2.10. The Fresnel Zone is important to the integrity of the RF link because it defines an area around the LOS that can introduce RF signal interference if blocked. Objects in the Fresnel Zone such as trees, hilltops, and buildings can diffract or reflect the main signal away from the receiver, changing the RF LOS. These same objects can absorb or scatter the main RF signal, causing degradation or complete signal loss.
// To illustrate co-channel interference, assume a 3-story building, with a wireless LAN on each floor, with the wireless LANs each using channel 1. The access points' signal ranges, or cells, would likely overlap in this situation. Because each access point is on the same channel, they will interfere with one another. This type of interference is known as co-channel interference.
In order to troubleshoot co-channel interference, a wireless network sniffer will be needed. The sniffer will be able to show packets coming from each of the wireless LANs using any particular channel. Additionally, it will show the signal strength of each wireless LAN's packets, giving you an idea of just how much one wireless LAN is interfering with the others.
The two solutions for co-channel interference are, first, the use of a different, nonoverlapping channel for each of the wireless LANs, and second, moving the wireless LANs far enough apart that the access points' cells do not overlap. These solutions are the same remedy as for adjacent channel interference.
D, E:
Co-channel Interference
Co-channel interference can have the same effects as adjacent channel interference, but is an altogether different set of circumstances. Co-channel interference as seen by a spectrum analyzer is illustrated in Figure 9.17 while how a network configuration would produce this problem is shown in Figure

To illustrate co-channel interference, assume a 3-story building, with a wireless LAN on each floor, with the wireless LANs each using channel 1. The access points' signal ranges, or cells, would likely overlap in this situation. Because each access point is on the same channel, they will interfere with one another. This type of interference is known as co-channel interference.
In order to troubleshoot co-channel interference, a wireless network sniffer will be needed. The sniffer will be able to show packets coming from each of the wireless LANs using any particular channel. Additionally, it will show the signal strength of each wireless LAN's packets, giving you an idea of just how much one wireless LAN is interfering with the others.
The two solutions for co-channel interference are, first, the use of a different, nonoverlapping channel for each of the wireless LANs, and second, moving the wireless LANs far enough apart that the access points' cells do not overlap. These solutions are the same remedy as for adjacent channel interference

wireless clients are polled for data transmission
WHEN AN ACCESS POINT is operating in distributed coordination function mode,

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QUESTION: 250

Which types of spread spectrum technologies are allowed by the FCC for use in the ISM bands? Select three.

FHSS
DSSS
Infrared
Narrowband
OFDM

Answer(s): A,B,E

Explanation:

The license-free ISM bands have been set aside internationally for use in Industrial, Scientific and Medical applications.In each of these bands, the radio devices are required to use spread spectrum modulation techniques. The two types of allowed spread spectrum modulation are direct sequence (DSSS) and frequency hopping (FHSS). In the US, Canada, South and Central America, much of Asia, and most of Europe, the ISMbands fall in the900 MHz (L-band), 2.4 GHz (S-band) and 5.8 GHz (C-band) frequency spectrum.The largest ISM band, the 5.8 GHz band (C-band)is the least-used worldwide and offers a contiguous 125 MHz block of spectrum for high-speed network applications.

Reference:

http://www.airlinx.com/products.cfm/product/1-0-0.htm

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QUESTION: 251

The Wi-Fi Alliance added interoperability testing to the Wi-Fi certification process for 802.11a devices to ensure that they are interoperable with which device?

802.11 devices
802.11b devices
802.11a devices
802.11g devices
HiperLAN/2 devices

Answer(s): C

Explanation:

The lower 5 GHz UNII band and the 2.4 GHz ISM band are the same width - 100 MHz. 802.11a equipmentis new and significantly more expensive than 802.11b equipmentand is not compatible with 802.11b or 802.11g equipment in any capacity. The UNII bands (all three of them) allow for a larger useable portion than does the 2.4 GHz ISM band, yielding a maximum of 4 non-overlapping DSSS channels.
Wi-Fi is the hardware compatibility standard created and maintained by WECA for 802.11b devices. IEEE 802.11g devices use the 2.4 GHz ISM band are backwards compatible with 802.11b. 802.11a devices use a different set of frequencies and a different modulation type from 802.11b, and are thus incompatible.
A recent addition to the list of devices using direct sequence technology is the IEEE 802.11a standard, which specifies units that can operate at up to 54 Mbps. Unfortunately for 802.11 and 802.11b device users, 802.11a is wholly incompatible with 802.11b because it does not use the 2.4 GHz band, but instead uses the 5 GHz UNII bands. The 5 GHz UNII bands are made up of three separate 100 MHz-wide bands, which are used by 802.11a-compliant devices. The three bands are known as the lower, middle, and //
The standards ETSI has established, HiPerLAN/2 for example, directly compete against standards created by the IEEE such as 802.11A. There has been much discussion about IEEE and ETSI unifying on certain wireless technologies, but nothing has materialized as of this writing. This effort is referred to as the "5UP" initiative for "5 GHz Unified Protocol." The IEEE's attempt at interoperability with ETSI's HiperLAN/2 standard is the new forthcoming 802.11h standard.
ETSI's original HiPerLAN standard for wireless, dubbed HiperLAN/1, supported rates of up to 24 Mbps using DSSS technology with a range of approximately 150 feet. HiperLAN/1 used the lower and middle UNII bands, as do HiperLAN/2, 802.11a, and the new 802.11h standard. The new HiperLAN/2 standard supports rates of up to 54 Mbps and uses all three of the UNII bands.
ETSI's HiperLAN/2 standard has interchangeable convergence layers, support for QoS, and supports DES and 3DES encryption. The supported convergence layers are ATM, Ethernet, PPP, FireWire, and 3G.
HiperLAN/2 - An extension to the HiperLAN protocol developed by ETSI (European Telecommunications Standards Institute) that provides a 54 Mbps data rate in the 5GHz band.

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QUESTION: 252

Before a client can participate in a Wireless LAN, which of the following must occur? Select two.

The client must be associated.
The client must be issued an IP address by a DHCP server.
The client must negotiate an encryption algorithm with the Access Point.
The client must be authenticated.

Answer(s): A,D

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QUESTION: 249

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QUESTION: 250

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QUESTION: 251

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QUESTION: 252