CWNA Guide to Wireless LANs, Second Edition
5-1
Chapter 5
IEEE 802.11 Media Access Control and Network Layer
Standards
At a Glance
Instructor’s Manual Table of Contents

Overview

Objectives

Teaching Tips

Quick Quizzes

Class Discussion Topics

Additional Projects

Additional Resources

Key Terms
CWNA Guide to Wireless LANs, Second Edition
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Lecture Notes
Overview
The IEEE 802.11 standard specifies that all WLAN features are implemented in the
PHY and MAC layers. Because no modifications are needed at any of the other layers,
existing software designed to meet other 802.x standards will correctly operate on the
802.11 standard. Any network operating system or LAN application that functions on a
standard LAN will also run on a WLAN without modification. In fact, WLANs support
almost all Layer 3-7 protocols, such as TCP/IP, Novell IPX/SPX, Microsoft NetBEUI,
and AppleTalk.
Isolating all of the WLAN functions in the bottom two layers of the IEEE 802.11
standard does not mean that new features cannot be added to the existing upper layers.
A new feature can be added as long as it does not modify the two lower layers. In this
chapter students will learn about the three types of WLAN configurations, and look in
detail at the IEEE 802.11 MAC layer standard that implements WLAN features. They
will also learn about enhanced features in the Network layer that can provide benefits
for mobile wireless computing.
Chapter Objectives




List and define the three types of WLAN configurations
Tell the function of the MAC frame formats
Explain the MAC procedures for joining, transmitting, and remaining connected to a
WLAN
Describe the functions of Mobile IP
Teaching Tips
IEEE Wireless LAN Configurations
1. Provide an overview of the three IEEE WLAN configurations.
Basic Service Set
1. Explain that a Basic Service Set (BSS) is defined as a group of wireless devices that is
served by a single access point (AP). Mention that the BSS must be assigned a Service
Set Identifier (SSID), which serves as the “network name” for the BSS. Illustrate a BSS
with Figure 5-1.
Teaching
Tip
The SSID can be any alphanumeric string from 2 up to a maximum of 32
characters.
CWNA Guide to Wireless LANs, Second Edition
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2. Define the term basic service area (BSA). Explain that the BSA for a wireless LAN
depends on many factors.
Teaching
Tip
The diameter of the BSA for a BSS with two stations will be somewhere in the
range of twice the coverage-distance between two wireless stations.
3. Discuss the concept of dynamic rate shifting.
4. Explain that 802.11g networks have a similar BSA to 802.11b networks, while 802.11a
networks have a smaller BSA.
Teaching
Tip
Although it is possible for a single 802.11b BSS to support up to 100 devices,
having this many users in a single BSS will probably result in unacceptable
throughput rates because the devices all share the same medium (similar to an
Ethernet network using a hub instead of a switch). Some industry experts
recommend that a single BSS should have no more than 50 devices if the users
are performing basic network functions, or 20 devices if users often transfer large
files across the network.
Teaching
Tip
A portal is a device that interconnects between an 802.11 and another 802 LAN.
Extended Service Set
1. Explain that an Extended Service Set is comprised of two or more BSS networks that
are connected through a common distribution system. Illustrate with Figure 5-2.
Mention that ESS can accommodate additional users over a wider area.
2. Explain that in ESS, APs can be positioned so that the cells overlap to facilitate
roaming. Define the term handoff. Explain that mobile wireless devices generally
choose an AP to which to connect based on signal strength.
Teaching
Tip
One of the weaknesses of the IEEE 802.11 standard is that it does not specify
how a handoff should take place. Because roaming between APs of different
vendors can sometimes be a problem, some industry experts recommend that all
APs in an ESS be from the same vendor.
Teaching
Tip
In an ESS, the entire network looks like an independent basic service set to the
Logical Link Control layer (LLC). This means that stations within the ESS can
communicate or even move between BSS's transparently to the LLC.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
5-4
In an ESS, each access point is assigned a different channel wherever possible to
minimize interference. If a channel must be reused, it is best to assign the reused
channel to the access points that are the least likely to interfere with one another.
Independent Basic Service Set
1. Explain that an Independent Basic Service Set is a wireless network that does not use an
access point. Mention that IBSS is also referred to as peer-to-peer or ad hoc mode.
Illustrate with Figure 5-3.
2. Explain that BSS has more flexibility than IBSS in being able to connect to other wired
or wireless networks. Mention that IBSS is useful for quickly and easily setting up a
wireless network anywhere that users need to share data between themselves but do not
need a connection to the Internet or an external network.
IEEE 802.11 Media Access Control (MAC) Layer Standards
1. Provide an overview of the functions of the MAC layer in a WLAN. Mention that the
mechanics of how these functions are performed center around the frames sent and
received on a WLAN.
MAC Frame Formats
1. Define the term packet. Explain that, strictly speaking, other terms are actually used to
describe the smaller segments of data transmitted on a network.
2. Discuss the concept of frames as they relate to the MAC Layer. Stress that IEEE MAC
frames are not the same as 802.3 Ethernet frames.
3. Briefly discuss the purpose and contents of the control field of a frame. Mention that all
frames contain the MAC address of the source and destination device, a frame sequence
number, and a frame check sequence used for error detection.
4. Describe the purpose of management frames. Illustrate the format of a management
frame with Figure 5-4. Describe the purpose of each field illustrated in this Figure.
5. Using the list on page 158 of the text as a guide, discuss some of the types of
management frames and their functions.
6. Explain the purpose and role of control frames. Illustrate an example of the format of a
control frame with Figure 5-5.
7. Discuss the purpose and role of data frames. Illustrate a data frame with Figure 5-6, and
discuss the purpose of the fields in this type of frame. Mention that the exact contents of
a data frame will vary depending on the mode of transmission.
Discovering the WLAN
1. Provide an overview of the process of discovering a WLAN.
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Beaconing
1. Describe the process of beaconing. Explain that a beacon frame is a type of
management frame and its format follows the standard management frame format.
Illustrate the process of beaconing with Figure 5-7.
2. Using the list on pages 160 and 161 of the text as a guide, discuss the fields contained in
a beacon frame.
Teaching
Tip
To increase security many access point vendors have included an option to
disable the SSID from being broadcast in beacon frames. This will be covered in
detail in Chapter 7.
3. Explain that in an ad hoc network each wireless device assumes the responsibility for
sending the beacon. Discuss the process of beaconing on this type of network.
4. Describe the standard beacon interval on an infrastructure WLAN, and discuss how
adjusting this interval can affect the network.
Teaching
Tip
If a beacon frame is delayed because of other network traffic, the actual time
between beacons may be longer than the beacon interval. However, wireless
devices can compensate by resynchronizing with the timestamp in the next
beacon.
Scanning
1. Describe the purpose of scanning. Briefly introduce the two types of scanning (passive
and active).
2. Discuss passive scanning. Explain that wireless devices simple listen for a beacon
frame, and typically scan all available channels for a set time period. Discuss the
differences between scanning on a BSS versus an ESS.
Teaching
Tip
Even after a wireless device is connected to a wireless network it continues to do
passive scanning (remember that beacon frames are broadcast to all network
devices). In the event that the device is disconnected from the wireless network,
it can use that information to reconnect more quickly.
3. Discuss the process of active scanning. Illustrate with Figure 5-8.
4. Explain that in an ESS configuration with multiple access points, the device wanting to
join a network might have many different access points from which to choose. Discuss
some of the criteria that may be used in making this choice.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
5-6
If a device has not been configured to connect with a specific access point, it will
generally connect with the access point from which it has received the strongest
radio signal. However, on some occasions a device may try to connect to an AP
based on the first signal that it encounters.
5. Stress that the difference between passive scanning and active scanning is which device
initiates the discovery.
Joining the WLAN
1. Provide a brief overview of the two-step process of joining a WLAN.
Authentication
1. Mention that in a wired LAN, the user is authenticated after the network device is
already connected to the network. Explain that, in contrast, a wireless device (not
individual user) must be authenticated prior to being connected to the network. Define
the term authentication, as it relates to IEEE 802.11 standards.
2. Describe the process of open system authentication, mentioning that it is the most basic
(and the default) method of authentication. Explain how open system authentication is
performed. Illustrate with Figure 5-9.
3. Describe the process of shared key authentication. Define the term challenge text.
Illustrate with Figure 5-10. Mention that this type of authentication is based upon the
fact that only pre-approved wireless devices are given the shared key.
Teaching
Tip
The secret key is sometimes called the “shared secret.”
Teaching
Tip
Shared key authentication uses a WEP privacy algorithm.
4. Stress that both of the previously discussed authentication methods are weak, and
explain why.
5. Discuss the concept of a digital certificate, and explain how it might be used on a
wireless network. Stress that it is not possible to change any part of a digital certificate
without being detected.
Association
1. Define the term association. Explain that it is the final step in joining a wireless
network.
2. Explain that after the authentication process, the AP sends an association response
frame, which contains either an acceptance or rejection notice. Mention the information
contained in the association response frame.
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Quick Quiz 1
1. A(n) ____________________ is defined as a group of wireless devices that is served by
a single access point (AP).
Answer: Basic Service Set (BSS)
2. A(n) ____________________ is comprised of two or more BSS networks that are
connected through a common distribution system.
Answer: Extended Service Set (ESS)
3. True or False: IEEE MAC frames are the same as 802.3 Ethernet frames.
Answer: False
4. ____________________ frames are used to set up the initial communications between a
device and the access point (for infrastructure mode) or between devices (ad hoc mode),
and then maintain the connection.
Answer: Management
5. The ____________________ field of a beacon frame identifies a specific wireless
LAN.
Answer: Service Set Identifier (SSID)
6. With ____________________ scanning a wireless device simply listens for a beacon
frame.
Answer: passive
7. ____________________ authentication is the most basic and the default authentication
method supported by the 802.11 standard.
Answer: Open system
8. ____________________ are digital documents that associate an individual with a key
value.
Answer: Digital certificates
Transmitting on the WLAN
1. Explain that the MAC layer has the primary responsibility of controlling access to the
wireless medium. Briefly introduce the two procedures for transmitting on a WLAN
specified by the IEEE 802.11 standard.
Distributed Coordination Function (DCF)
1. Define the term channel access methods.
2. Describe the concept of contention, explaining that the philosophy of contention is to
have computers compete for access to the network medium. Explain that any computer
can send a message at any time, but collisions may results. Discuss the methods that can
be employed to avoid collisions.
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3. Explain that CSMA/CD specifies that before a networked device starts to send a frame
it should first listen on the wire to see if any other device is currently transmitting.
Describe what happens if traffic is detected, if it is not detected, and if two devices
listen at exactly the same time. Mention how collisions are handled when they do occur.
Illustrate with Figure 5-11.
4. Explain the reasons why CSMA/CD cannot be used on wireless networks. Illustrate the
hidden node problem with Figure 5-12.
5. Discuss the Distributed Coordination Function in detail. Explain that it is a modified
version of CSMA/CD that attempts to avoid collisions altogether rather than simply
detecting them.
6. Explain why most collisions occur immediately after a station completes a transmission.
Explain that with CSMA/CA, all stations are forced to wait a random amount of time
after the transmission medium is clear, mentioning how this significantly reduced the
number of collisions.
7. Discuss how CSMA/CA uses explicit acknowledgement frames to further reduce
collisions. Illustrate with Figure 5-13.
8. Explain that CSMA/CA does not eliminate the potential for collisions, mentioning that
the hidden node problem is not solved. Explain that the Request to Send/Clear to Send
(RTS/CTS) protocol can be used to solve the hidden node problem and provides
additional protection against collisions. Discuss this protocol with the aid of Figure 514.
Teaching
Tip
Request to Send (RTS) and Clear to Send (CTS) frames are control frames, not
management frames.
9. Stress that the RTS/CTS protocol imposes significant overhead upon the WLAN with
the transmission of its RTS and CTS frames, especially with short packets. Explain that,
when using RTS/CTS, the 802.11 standard allows the transmission of small data
packets without RTS/CTS. Discuss the concept of the RTS threshold.
Teaching
Tip
Because the RTS/CTS protocol imposes additional overhead, it is not used unless
there is no other way to improve network performance. Before using RTS/CTS,
the first approach might be to move the devices or access point if possible so that
the devices can sense each other’s transmissions.
Interframe Spacing
1. Introduce the concept of interframe spacing. Discuss the three types of interframe
spaces.
2. Using Figure 5-15 to illustrate, discuss the sequence of event involved in a transmission
by a wireless device using direct sequence spread spectrum (DSSS) that has been
assigned three slot times.
CWNA Guide to Wireless LANs, Second Edition
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3. Using Figure 5-16 to illustrate, discuss how the situation is complicated when two
stations need to transmit.
Fragmentation
1. Explain that Fragmentation involves dividing the data to be transmitted from one large
frame into several smaller ones, mentioning that this technique can also reduce
collisions.
There are several reasons why it is preferable to use smaller packets in a
Wireless LAN environment. Some reasons include:
- Due to the higher Bit Error Rate of a radio link, the probability of a packet
getting corrupted increases with the packet size.
Teaching
Tip
- In case of packet corruption (either due to collision or noise), the smaller the
packet, the less overhead it causes to retransmit it.
- On a Frequency Hopping system, the medium is interrupted periodically for
hopping, so, the smaller the packet, the smaller the chance that the transmission
will be postponed after dwell time.
2. Explain that if the length of a data frame to be transmitted exceeds a specific value, the
MAC layer will divide or fragment that frame into several smaller frames. Discuss how
these fragments are labeled, and mention that the receiving station uses the labels to
reassemble the fragments.
Teaching
Tip
The fragmentation frame size can be adjusted and is typically between 256 and
2,346 bytes.
3. Explain that fragmentation can be considered as an alternative to RTS/CTS, but that it
also has high overhead. Mention that, because of the overhead requirements of both
RTS/CTS and fragmentation, these techniques should only be used if absolutely
necessary
Teaching
Tip
Fragmentation does not always have to be used separately from RTS/CTS. The
802.11 standard permits them to be used simultaneously.
Teaching
Tip
To minimize overhead on the network, 802.11 does not fragment broadcast (e.g.,
beacons) and multicast frames.
Point Coordination Function (PCF)
1. Describe the concept of polling as a channel access method. Stress that it effectively
prevents collisions.
CWNA Guide to Wireless LANs, Second Edition
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2. Explain that the IEEE 802.11 standard specifies the Point Coordination Function as the
mechanism for implementing polling. Mention that, with PCF, the access point serves
as the polling device or “point coordinator.”
3. Explain that the point coordinator only has to wait through the point coordination
function IFS (PIFS) time gap, which is shorter than the DFIS time gap. Illustrate with
Figure 5-17.
4. Explain that if the point coordinator hears no traffic at the end of the PIFS time gap, it
sends out a frame, known as a beacon frame, to all stations. Mention that one field of
this frame contains a value that indicates the length of time that PCF (polling) will be
used instead of DCF (contention), and that after the stations receive this beacon frame
they must stop any transmission for that length of time.
Teaching
Tip
Stations that receive the beacon frame store the length of time field for PCF in
their own NAV field.
5. Explain that because each station can be told the length of time that PCF will be used
instead of DCF, the 802.11 standard allows a WLAN to alternate between PCF (polling)
and DCF (contention). Illustrate with Figure 5-18.
Teaching
Tip
PCF always has a higher priority than DCF.
Quality of Service (QoS) and 802.11e
1. Explain that DCF works well for data transmissions, but not for real-time traffic that is
time dependent. Explain how QoS proposes to solve this problem. Mention the role that
VoIP has played in the interest in QoS.
Teaching
Tip
According to Infonetics Research, the number of large enterprises that use VoIP
will increase an additional 33 percent by 2006.
2. Discuss the difficulties that have been encountered while trying to implement QoS on
wireless networks. Using Table 5-1 as a guide, discuss the WMM specification
proposed by the Wi-Fi Alliance.
3. Discuss the 802.11e draft, explaining that it intends to provide QoS over WLANs.
Describe EDCA, mentioning that it proposes to use four access categories to
differentiate between types of traffic. Explain that HCCA is a new form of PCF,
intended to serve as a centralized scheduling mechanism.
Remaining Connected to the WLAN
1. Provide a brief overview of the role that the MAC Layer plays in keeping stations
connected to the WLAN.
CWNA Guide to Wireless LANs, Second Edition
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Reassociation
1. Explain that reassociation occurs when a device drops the connection with one access
point and establish a connection with another.
2. Discuss the various reasons why reassociation may be necessary.
3. Describe how a device initiates the reassociation process. Mention that it can use
information from previous scans. Discuss how, once a new AP is located, reassociation
proceeds.
Teaching
Tip
Reassociation is always initiated by the station and never by the access point.
Power Management
1. Explain that when that laptop is part of a WLAN it must continue to remain “awake” in
order to receive network transmissions.
2. Discuss power management, as it relates to the 802.11 standards. Explain that power
management is transparent to all protocols and applications so that it will not interfere
with normal network functions.
3. Explain that the process of power management differs based on the WLAN
configuration. Discuss the concepts of buffering and the traffic indication map.
Illustrate power management in infrastructure mode using Figure 5-19.
Teaching
Tip
The amount of sleep time for a mobile station is generally 100 milliseconds,
although this parameter can be changed.
4. Discuss how power management operates in an Ad Hoc configuration. Explain the
events that occur at the ad hoc traffic indication message window.
5. Explain that laptops that are plugged in are in continuous aware mode, and discuss the
consequences of this fact.
6. Mention that power management can slow network response time.
WLAN Network Layer Standards
1. Provide an overview of WLAN Network Layer standards. Explain that the IEEE 802.11
standard specifies that the features of a WLAN be confined to the PHY and MAC
layers. Mention that a Network Layer enhancement is emerging that will extend
mobility while not altering the existing standard.
WLAN IP Addressing
1. Provide a review of the purpose of IP addresses and the role that they play in standard
networking. Stress that the traditional TCP/IP scheme prohibits a mobile user from
switching to another network and using the same IP number.
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Mobile IP
1. Explain that mobile IP provides a mechanism within the TCP/IP protocol to support
mobile computing. Stress that it does not alter the TCP/IP protocol suite.
2. Explain that in mobile IP, computers are given a home address, and that the computer
also has a home agent. Mention that the home agent is a forwarding mechanism that
keeps track of where the mobile computer is located.
Teaching
Tip
A home agent can be a router, firewall, or gateway.
3. Discuss the concept of a foreign network, and explain the role of a foreign agent.
Illustrate with Figure 5-20. Mention that the foreign agent assigns the mobile computer
a care-of address, and that this address is then registered with the home agent. Illustrate
with Figure 5-21.
4. Explain how the home agent tunnels frames to a foreign agent, which then transmits the
frame to the computer on the foreign network. Illustrate with Figure 5-22.
Quick Quiz 2
1. True or False: CSMA/CD cannot be used for wireless networks.
Answer: True
2. With wireless CSMA/CA the amount of time that a station must wait after the medium
is clear is called the ____________________.
Answer: slot time
3. The ____________________ is the time used by a device to access the medium after it
has been asked and then given approval to transmit.
Answer: Point Coordination Function IFS (PIFS)
4. ____________________ involves dividing the data to be transmitted from one large
frame into several smaller ones.
Answer: Fragmentation
5. The capability to prioritize
____________________.
Answer: Quality of Service (QoS)
different
types
of
frames
is
known
as
6. When a device drops a connection with one access point and establishes a connection
with another, it is referred to as ____________________.
Answer: reassociation
CWNA Guide to Wireless LANs, Second Edition
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Class Discussion Topics
1. Have the students read the paper entitled Techniques to Reduce IEEE 802.11b MAC
Layer Handover Time (April 2003) authored by Héctor Velayos and Gunnar Karlsson at
the Royal Institute of Technology in Stockholm, Sweden. This paper can be found
online at:
www.it.kth.se/~hvelayos/papers/TRITA-IMIT-LCN%20R%2003
02%20Handover%20in%20IEEE%20802.pdf
Once they have read the paper, lead a discussion of the techniques presented in this
paper for improving the handoff time at the MAC layer. Are the students able to
understand the concepts presented in this paper? Do they think that these techniques are
feasible in a non-laboratory environment? Do they think that they would be able to
implement these techniques given what they have learned so far in this course?
2. Have the students read the paper entitled Quality of Service Schemes for IEEE 802.11
Wireless LANs – An Evaluation (2003) authored by Anders Lindgren, Andraes
Almquist and Olev Schelen at the Luleå University of Technolog in Luleå, Sweden.
This paper can be found online at:
http://www.cs.ucsb.edu/~ebelding/courses/595/s05/papers/qos_schemes
Once they have read the paper, lead a discussion of the evaluation of QoS techniques
presented in this paper. Are the students able to understand the concepts presented in
this paper? Do they agree with the metrics used, and if not, what other types of metrics
do they feel would be useful and enlightening? According to the paper, what are the
advantages and disadvantages of various QoS schemes?
Additional Projects
1. The implementation of the distribution system connecting two BSSs is not specified by
802.11. Therefore, a distribution system may be created from existing or new
technologies. However, the 802.11 standard does specify the services that the
distribution system must support. Have the students research online to find out what
services must be supported. Have them write a short report detailing these services
(some of these services were detailed in this chapter).
2. A good method to find out if you should activate fragmentation is to monitor the
wireless LAN for collisions. Have the students first determine the number of collisions
that occur over a 30 minute time frame (can be achieved using Cisco Aironet client
adapter tools). Next, have them adjust the fragmentation threshold to various levels and
recheck the number of collisions that occur over a 30 minute time frame. The students
should chart the number of collisions as a function of the fragmentation threshold, and
estimate what the optimum collision threshold is for the WLAN. It would be best if this
network was performed during periods of high network traffic.
CWNA Guide to Wireless LANs, Second Edition
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Additional Resources
1. IEEE 802.11 architecture:
http://www.wirelessnetworkstutorial.info/ieee802.11/wifi_architecture.php
2. Wireless LAN setup in Extended Service Set (ESS) mode (PDF):
http://www.comp.nus.edu.sg/~cs3103/labs/lab_802_11.pdf
3. IEEE 802.11 Technical Tutorial (PDF):
http://www.alvarion-usa.com/RunTime/Materials/KnowledgePoolFiles/
C3_IEEE_80211_Technical_Tutorial.pdf
4. Scanning 802.11 Networks:
http://msdn.microsoft.com/library/default.asp?url=/library/enus/network/hh/network/213_802.11_bb61b6ee-e9ab-4f03-aee3-c932fe2ea2a9.xml.asp
5. IEEE 802.11 Tutorial (PDF):
http://esoumoy.free.fr/telecom/tutorial/ieee-tutorial.pdf
6. Improving WLAN Performance with Fragmentation:
http://www.wi-fiplanet.com/tutorials/article.php/1468331
7. 802.11 Beacons Revealed:
http://www.wi-fiplanet.com/tutorials/article.php/1492071
8. Improving WLAN Performance with RTS/CTS:
http://www.wi-fiplanet.com/tutorials/article.php/1445641
9. Understanding 802.11 Frame Types:
http://www.wi-fiplanet.com/tutorials/article.php/1447501
10. Quality of Service Schemes for IEEE 802.11 Wireless LANs – An Evaluation (PDF):
http://www.cs.ucsb.edu/~ebelding/courses/595/s05/papers/qos_schemes
11. Mobile Networking Through Mobile IP:
http://www.computer.org/internet/v2n1/perkins.htm
12. Cisco Aironet: Performing Diagnostics:
http://www.cisco.com/en/US/products/hw/wireless/ps4555/products_installation_and_c
onfiguration_guide_chapter09186a0080154ed8.html
Key Terms
 802.11e: A IEEE draft outlining Quality of Service specifications for 802.11 WLANs.
 Access categories (AC): Streams of communication in an Enhanced Distributed
Channel Access (EDCA) protocol.
 Active scanning: The process in which a wireless device first sends out a management
probe request frame on each available channel and then waits for an answer.
 Ad hoc mode: A wireless network that does not use an access point. Also known as
peer-to-peer mode or Independent Basic Service Set (IBSS).
 Ad hoc traffic indication message (ATIM) window: A specific period of time at
which each ad hoc device must be awake.
 Association: The process of being accepted into the wireless network.
 Authentication: A process in which the AP accepts or rejects a wireless device.
 Backoff interval: The random amount of time that a device waits after a network
collision.
 Basic Service Area (BSA): The geographical area of coverage for a WLAN.
 Basic Service Set (BSS): A group of wireless devices that is served by a single access
point (AP).
CWNA Guide to Wireless LANs, Second Edition
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 Beaconing: The process of an AP in an infrastructure network or wireless device in an
ad hoc network sending a beacon frame to both announce its presence and to provide
the necessary information for other devices to join the network.
 Buffering: The process of an access point temporarily storing frames for wireless
devices that are asleep.
 Care-of address: A new IP address on a foreign network.
 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA): A protocol
designed to avoid collisions on a wireless medium.
 Carrier Sense Multiple Access with Collision Detection (CSMA/CD): A contentionbased channel access method used by 802.3 Ethernet networks.
 Carrier sensing: The process of listening on a network medium prior to transmitting.
 Challenge text: A block of text that must be encrypted by a wireless device in shared
key authentication.
 Channel access methods: Different ways of sharing a network medium.
 Contention: A channel access method in which devices compete with each other to use
the network medium.
 Continuous aware mode (CAM): The state of a laptop when it is connected to
standard alternating current (AC) for power.
 Control frame: MAC frame that provides assistance in delivering the frames that
contain the data.
 Data frame: MAC frame that carries the information to be transmitted to the
destination device.
 Digital certificate: Digital documents that associate an individual with a key value.
 Distributed Coordination Function (DCF): An IEEE 802.11 standard for sharing the
wireless medium.
 Distributed Coordination Function IFS (DIFS): An interframe space that is the
standard interval between the transmission of data frames.
 Dynamic rate shifting: The decrease in transmission speed as a mobile device moves
farther away from the access point.
 Enhanced Distributed Channel Access (EDCA): An IEEE 802.11e draft which is
contention-based yet supports prioritization of different types of traffic.
 Extended Service Set (ESS): Two or more BSS networks that are connected through a
common distribution system.
 Foreign agent: A forwarding mechanism in Mobile IP.
 Foreign network: Another network on a Mobile IP service.
 Fragmentation: The process of dividing data to be transmitted from one large frame
into several smaller ones.
 Frame: The formal name for a packet that has reached the MAC layer (or the Data Link
layer in the OSI model).
 Frame acknowledgment: A frame (abbreviated ACK) that is sent by the receiving
device back to the sending device to confirm that the data frame arrived intact.
 Handoff: The process of switching associations from one access point to another.
 Hidden node problem: A wireless device that can be seen by an access point but not
by all other wireless devices.
 Home agent: A forwarding mechanism in Mobile IP that keeps track of where a mobile
computer is located.
 Hybrid Coordination Function Controlled Channel Access (HCCA): A proposed
IEEE 802.11e protocol based upon polling.
CWNA Guide to Wireless LANs, Second Edition
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 Independent Basic Service Set (IBSS): A wireless network that does not use an access
point. Also known as ad hoc or peer-to-peer mode.
 Infrastructure mode: A group of wireless devices that is served by a single access
point (AP).
 Interframe spaces (IFS): Standard spacing intervals between the transmissions of the
data frames used for special types of transmissions.
 Management frame: A type of MAC frame used to set up the initial communications
between a device and the access point or between devices.
 Mobile IP: A mechanism within the TCP/IP protocol to support mobile computing.
 Net allocation vector (NAV): The location in an access point or wireless device that
contains the length of time a medium is reserved.
 Open system authentication: An authentication method in which a wireless device
sends a request to the access point. The most basic and often default authentication
method, it provides very little security.
 Passive scanning: The process of a wireless device listening for a beacon frame.
 Peer-to-peer mode: A wireless network that does not use an access point.
 Point Coordination Function (PCF): An optional 802.11 transmission process in
which the access point serves as the polling device or point coordinator.
 Point Coordination Function IFS (PIFS): An interframe space that is used by a
device to access the medium after it has asked for and then been given approval to
transmit.
 Polling: A channel access method in which each device is polled in sequence if it has
frames to send.
 Power management: An 802.11 standard that allows mobile devices to be off as much
as possible to conserve battery life but not miss data transmissions.
 Quality of Service (QoS): Prioritizing frames so that time-sensitive frames are
transmitted prior to data frames.
 Reassociation: The process in which a device drops a connection with one access point
and establishes a connection with another.
 Request to Send/Clear to Send (RTS/CTS): Also known as carrier sensing, a protocol
that reserves the wireless medium for transmissions so that no collisions may occur.
 Roaming: Movement between WLAN cells.
 RTS threshold: A limit on packet size of the RTS/CTS protocol.
 Scanning: The process of a receiving wireless device looking for beacons.
 Service Set Identifier (SSID): A unique identifier of up to 32 characters that serves as
a WLAN network name.
 Shared key authentication: An authentication method in which a wireless device must
encrypt challenge text before it can be authenticated. This method is more secure than
open system authentication, but still not entirely secure.
 Short IFS (SIFS): An interframe space that is used for immediate response actions
such as ACK.
 Slot time: The amount of time that a station must wait after the medium is clear.
 Traffic indication map (TIM): A list of the stations that have buffered frames waiting
at the access point.
 Virtual carrier sensing: Also known as RTS/CTS, the process of reserving the
wireless medium for transmissions so that no collisions may occur.
 Voice over IP (VoIP): A telephony service that uses IP-based data packet switching
networks to transmit voice communications.
 Wi-Fi Multimedia (WMM): A Wi-Fi Alliance specification for prioritizing frames.