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Ch. 5 Review Questions
1. Each of the following protocols is supported on an IEEE 802.11 network except
a. System Network Architecture (SNA)
b. TCP/IP
c. Appletalk
d. IPX/SPX
2. _____ is a group of wireless devices that is served by a single access point (AP).
a. Basic Service Set
b. ad hoc mode
c. Extended Service Set
d. Independent Service Set
3. Each WLAN must be assigned a unique identifier known as the _____.
a. Service Set Identifier (SSID)
b. Access Category Channel
c. Traffic Indication Set (TIS)
d. Beacon identifier
4. With dynamic rate shifting implemented in a WLAN, what happens when a user
moves farther away from the access point?
a. The connection is automatically dropped at 25 meters.
b. The connection speed decreases.
c. The connection speed increases.
d. The connection frequency changes from 2.4 GHz to 5 GHz.
5. In order to accommodate additional wireless users over a wider area, which
configuration should be used?
a. ad hoc
b. Basic Service Set
c. Extended Service Set
d. Channel Access Set
6. When a packet reaches the MAC layer (or the Data Link layer in the OSI model)
these segments are formally known as frames. True or False?
7. The IEEE 802.11 standard specifies six categories of MAC frame formats. True
or False?
8. At regular intervals the AP in an infrastructure network or wireless device in an
ad hoc network sends a data frame to both announce its presence and to provide
the necessary information for other devices to join the network. True or False?
9. In passive scanning, the wireless device must first send out a management probe
request frame on each available channel and then wait for an answer from all
available APs. True or False?
10. Because wireless LANs cannot limit access to the RF signal by walls or doors,
wireless authentication requires the wireless device to be authenticated prior to
being connected to the network. True or False?
11. _____ authentication requires the wireless device to correctly encrypt the
challenge text. shared key
12. Once a wireless device is authenticated the final step is to be accepted into the
wireless network is a process known as _____. association
13. The IEEE 802.11 standard specifies two procedures for transmitting on the
WLAN, distributed coordination function (DCF) and an optional _____. point
coordination function (PCF)
14. Instead of using Ethernet CSMA/CD, IEEE WLANs use _____. CSMA/CA
15. Because IEEE 802.11 authentication is weak, another option is to use _____.
digital certificates
16. Explain why Carrier Sense Multiple Access/Collision Detection cannot be used
on a WLAN.
CSMA/CD cannot be used for wireless networks for two reasons. First, collision
detection is very difficult with wireless transmissions. With CSMA/CD, the stations
must be able to transmit and listen at the same time. However, in radio systems the
signal from a transmitting station is so strong that it will overpower that same
station’s ability to simultaneously receive a transmission. In short, while it is
transmitting, a station “drowns out” its own ability to detect a collision. A second
factor that makes collision detection so difficult with wireless transmission is that all
stations would have to be able to detect transmissions from all other stations at all
times. In a wireless environment that covers a large area, a station may not be in
range of all other stations. Laptops may all be within range of the access point but
not within range of each other.
17. Explain how virtual carrier sensing works.
A Request to Send (RTS) frame is transmitted by a wireless device to the access
point. This frame contains a duration field that indicates the length of time needed
for both the transmission and the returning ACK frame. The access point as well as
all stations that can receive the RTS frame are alerted that the wireless device needs
to reserve the medium for a specific period of time. Each receiving station stores
that information in its net allocation vector (NAV). No station can transmit if the
NAV contains a value other than zero. The access point then responds back to the
wireless device with a Clear to Send (CTS) frame that alerts all devices that medium
is now being reserved and they should suspend any transmissions. Once the wireless
device receives the CTS frame it can then proceed with transmitting its frame
18. How does fragmentation reduce collisions?
Fragmentation involves dividing the data to be transmitted from one large frame
into several smaller ones. Sending many smaller frames instead of one large frame
reduces the amount of time that the wireless medium is being used and likewise
reduces the probability of collisions.
19. Why is Quality of Service (QoS) important to WLANs?
The DCF contention method was intended to be a “fair” approach to wireless
transmissions. Although DCF works well for it data transmissions, the same is not
true for real-time traffic that is time dependent. These types of transmissions, like
voice or video, depend heavily upon each frame arriving very quickly one after the
other, whereas data transmissions are not as sensitive to time. Delays in
transmission can result in a video that “freezes” on the screen or a conversation that
has gaps of dead space. DCF cannot distinguish between voice, video, and data
frames to assure that time-sensitive frames have a priority. This capability is known
as Quality of Service (QoS). QoS on WLANs has become an increasing area of
interest with the widespread adoption of Voice over IP (VoIP) telephony. VoIP uses
IP-based data packet switching networks to transmit voice communications. VoIP
over WLANs holds tremendous potential by providing workers who are not
regularly at their desks more flexibility. It can also reduce operating costs associated
with use of cell phones and private radio walkie-talkie systems
20. Explain how power management functions on an infrastructure network.
In an infrastructure WLAN when a mobile 802.11 devices is running on its battery
and goes into sleep mode, the access point is informed. The access point keeps a
record of which stations are awake and which are sleeping. As the access point
receives transmissions it first checks its records to determine if that station is in
sleep mode. If it is sleeping, the access point temporarily stores those frames. At set
times the access point will send out a beacon frame to all stations. This frame
contains a list, known as the traffic indication map (TIM), of the stations that have
buffered frames waiting at the access point. At that same set time all stations that
have been sleeping will switch from sleep mode into an active listening mode. This is
possible because a station’s local timer does not sleep. If a station learns that it has
buffered frames waiting for it, that station can send a request to the access point to
have those frames forwarded to it. If it has no buffered frames, it can return to sleep
mode.