OLATUNDE OLYMPIO
ACSG 525
STUDY GUIDE FOR CHAPTER 8
Chapter 8
High-Speed Wireless Local Area Networks And WLAN Security
This chapter focuses on the study of two High Speed WLAN technologies and
standards (IEEE 802.11a, IEEE 802.11g) and security. Use the following questions
and remarks as a study guide to read and understand the contents of this chapter:
High-Speed WLAN:
802.11a
1) What is the IEEE 802.11a standard?
The IEEE 802.11a and the IEEE 802.11b were published at the same time but due
to high cost of component for IEEE 802.11a and technical issues, its products
only came to the market in late 2001. The IEEE 802.11a standard maintains the
same MAC layer functions as in IEEE 802.11b. The differences are found in the
PHY layer. 802.11a can reach higher speed and it is more flexible through a
higher frequency band, more transmission channel, its multiplexing technique,
and a more efficient error correction scheme.
2) What is the U-NII frequency band?
The U-NII (Unlicensed National Information Infrastructure) is an unlicensed band
intended for devices that provide short range, high speed wireless digital
communications. It has five frequency ranges: 5.15 – 5.25 GHz (U-NII Low
Band), 5.25 – 5.35 GHz (U-NII Middle Band), 5.47 – 5.725 GHz (New unnamed
band), and 7.725 – 5.825 GHz (U-NII High Band). The FCC has segmented the
total 555 MHz of the U-NII spectrum into four bands with maximum output
power limit as follow: U-NII Low Band with 40mW, U-NII Middle Band with
200 mW, New unnamed Band with 200 mW, and U-NII High Band with 800 mW
(only approved for outdoor use).
3) How can 802.11a devices transmit at faster rate than 802.11b devices?
A802.11a devices can transmit at faster rate because of the higher frequencies and
output power level at which they operate. The total bandwidth available for
802.11a is almost four time the total bandwidth of ISM that 802.11b uses.
4) How are channels allocated in 802.11a?
In 802.11a, eight frequency channels operate simultaneously in the Low Band and
Middle Band. Within each frequency channel there is a 20 MHz wide channel that
supports 52 carrier signals, with each signal 300 MHz wide. The center points for
the eight are 5.18, 5.20, 5.22, 5.24, 5.26, 5.28, 5.30 and 5.32 GHz.
5) What are the differences in the implementation of channel allocation for
802.11a and 802.11b with multiple APs?
In 802.11b WLAN, each AP can only use one of the three channels. If more
network bandwidth is required, a maximum of three APs can be installed in the
same area and the number of users can be distributed among the three. However,
even at 11 Mbps, this scheme only provides a maximum of 33 Mbps of
bandwidth. In 802.11a WLAN, each AP can use one of eight available channels,
and additional APs can be added, providing up to 432 Mbps of aggregated
bandwidth (8 x 54 Mbps) for wireless users in a single area.
6) What is the transmission technique used in 802.11a?
The transmission scheme used in 802.11a is the orthogonal frequency division
multiplexing (OFDM), in which the frequency band is divided into several subfrequencies that do not interfere with each other. 48 out the 52 sub-channels are
used for data.
7) What kind of modulation technique is used to encode the data in 802.11a?
The PSK is used for data rate of 6 Mbps. QPSK can double the amount of data
encoded over PSK to 250 Kbps per sun-channel resulting in 12Mbps. 16-QAM
can be used to transmit at 24 Mbps. Data rate of 54 Mbps are achieved by using
64-QAM.
8) How are errors handled in 802.11a?
Errors usually affect only one subchannel, and since transmission is over parallel
subchannels, radio interference from outside sources is minimized. Forward Error
Connection transmits a secondary copy along with the primary information, so if
a part of the transmission is lost, the secondary copy can be used to recover the
lost data through sophisticated algorithms.
9) What are the enhancements to 802.11a compared to 802.11 and 802.11b)?
The 802.11a standard made changes only to the PHY layer of the original 802.11
and 802.11b standards. The MAC layer remains the same. The 802.11a PHY layer
is divided into two sublayers: the PMD sublayer, which is responsible for the
characteristics of the wireless medium (such as OFDM) and defines the method
for transmitting and receiving data through that medium, and the PLCP sublayer
which is responsible for reformatting the data received from the MAC layer into
frames that the PMD can transmit. The PLCP for 802.11a is based on OFDM
instead of DSSS.
802.11g
10) What is the IEEE 802.11g?
The IEEE 802.11g standard operates in the same frequency band as 802.11b and
not the higher frequency band used by 802.11a. This standard was published in
June 2003.
11) What are the enhancements made in 802.11g?
IEEE 802.11g made changes only to the PHY layer to allow data rate between 1
and 11 Mbps. No changes were made to the MAC layer. The standard specifies
two mandatory transmission modes along with two optional modes. The first
required mode is the same mode used by 802.11b and must support data rate of 1,
2, 5.5 and 11 Mbps. The second required mode uses the same OFDM mode used
by 802.11a but in the same frequency band used by 802.11b. it uses the same 52
subchannels (48 data subchannels and 4 pilot subchannels) to provide rate of 6, 9,
12, 18, 24, 36, 48 and 54 Mbps.
12) How many channels are available in 802.11g compared to 802.11a??
The number of channels available with 802.11g is three compared to eight
channels for 802.11a. There are two optional transmission modes: the pPacket
Binary Convolutional Coding (PBCC) that can transmit at 22 or 33 Mbps, and the
DSSS-OFDM, which uses the standard DSSS preamble of 802.11b and transmits
the data portion of the frame using OFDM.
13) What are the two other important differences between 802.11g and 802.11b and
802.11a?
The two other important differences between 802.11g, 802.11b and 802.11a is
related to signal timing. 802.11g specifies that any time a device is transmitting at
a higher rate than 802.11b, a six microsecond quiet time of no transmission is
included at the end of the data portion of every frame, to allow additional
processing time for the receiver to decode the data. This requirement is required
for 802.11 b and 802.11g and not 802.11a because it is built with integrated
circuit for faster data rate. The other difference is the short interframe space
(SIFS) timing. In 802.11b time slots are 20 µs and the SIFS time is 10 µs.
802.11g has the same time slots but the SIFS time is different because the quiet
time is added (16 µs). This change in SIFS affects the performance of 802.11g.
While it can also transmit at 54 Mbps, the overall performance is lower than that
of 802.11a.
IEEE 802.11e
14) What is the IEEE 802.11e?
The IEEE 802.11e standard was approved for publication in November 2005. It
defines the enhancement to the MAC layer of 802.11 to expand support for LAN
application that require QoS and provides for improvements in the capabilities
and efficiency of the protocol.
15) What is the main difference in transmission acknowledgment between 802.11a ,
802.11g, and 802.11e
802.11a and 802.11g require an acknowledgment for each frame before a
transmission can continue but 802.11e allows a receipt of a burst of frames before
sending an acknowledgment. 802.11e prioritizes frames in DCF mode taking
advantage of the higher data rate of 802.11a and 802.11g.
16) What are the two new coordination functions implemented by 802.11e?
The two new coordination functions implemented by 802.11e are the enhanced
DCF (EDCF) and hybrid coordination function (HCF). In EDCF, a device with
higher priority traffic waits less to transmit, and therefore has a better chance of
getting its traffic through. HCF is a combination of DCF and point coordination
function. The interval between beacon frames is divided into contention free
period and a contention period. During the contention free period, the hybrid
coordinator control access to the medium and , based on information about the
traffic received from a client, will allocate more time slots to the station with
higher priority.
IEEE 802.11n
17) What is the IEEE 802.11n?
The project IEEE 802.11n is a proposed standard which goal is to provide data
rates higher than 100 Mbps using the 2.4 GHz ISM band. It will likely use the
OFDM or a variation as a transmission technique. 802.11n is fully backward
compatible with 802.11, 802.11b and 802.11g.
18) What is multiple-in, multiple out (MIMO) technology?
MIMO technology uses multiple antennas (usually 3 or 4) and the reflected
signals to extend the range of the WLAN by attempting to correctly decode frame
from multiple copies of it received at different times.
19) What is one of the limitation of 802.11n?
One of the limitations of 802.11n is that it uses the 2.4 GHz ISM band. Depending
on how many WLANs are in closed proximity of each other, there could be issues
of interference between them.
IEEE 802.11r
20) What is the IEEE 802.11r?
The project IEEE 802.11r was designed to solve the issues of the amount of time
required by 802.11 devices to associate/disassociate with one access point
(handoff), and the fact that the 802.11 protocol does not allow a device to find out
if the necessary QoS resources, such as the required number of time slot, are
available at a new AP, until the transmission from one to the other is completed.
21) Where are the changes made?
The changes are made in the MAC layer to provide a way for the client to
communicate with other APs and establish all the necessary parameters while still
connected to the original AP. Maintaining the original connection minimizes the
impact on the data stream since the client is not attempting to communicate with
more than one AP on different channels.
IEEE 802.11s
22) What is the IEEE 802.11s?
The project IEEE 802.11s is expected to provide wireless connectivity between
various APs in a mesh network.
HipperLAN/2
23) What is HiperLAN/2?
HiperLAN/2 is a high-speed WLAN technology similar to the IEEE 802.11a
standard. It was standardized in 2000 by the ETSI, the European
Telecommunication Standards Institute. HiperLAN/2 and 802,11a share many
characteristics but they are implemented differently.
WLAN Functionality
24) What is the purpose of a wireless bridge?
The wireless bridge is designed to connect two wired networks or to extend the
range of a WLAN. For 802.11b based bridges, the distance between connected
networks can be up to 18 miles transmitting at 11 Mbps, or up to 25 miles when
transmitting at 2 Mbps. 802.11a wireless bridges are approved for outdoor use
only by the FCC. Wireless bridges can also extend the range of a WLAN by
allowing the wireless bridge to connect to an AP as a repeater in point-tomultipoint mode.
25) What is a wireless switching?
A wireless switching devices help prevent the failure of a single AP from
crippling wireless access in an entire area to automatically increasing the transmit
power in one of the dumb radios to compensate for another radio failure.
26) What are other WLAN expansion Hardware?
The growth in the WLAN market has created an entirely new range of devices for
the wireless home and office such as the wireless presentation gateway, the
wireless media gateway, the wireless VoIP gateway, and the wireless gaming
adapter.
WLAN Security
27) What types of attacks can be generated against WLANS?
The most damaging attacks are hardware theft (the device may contain
information granting access to the network), AP impersonation (man-in-the
middle attack), passive monitoring (helps hackers built a profile of the network),
and denial of service (in DoS attack is used to flood the network with
transmissions and deny others access to the AP).
28) What type of security features are implemented in 802.11?
802.11 incorporates some basic security features such as authentication and
privacy processes. Authentication is a process that verifies that the client has
permission to access the network and privacy ensures that the transmission is not
read by unauthorized users, even if the transmissions fall into the wrong hand.
29) What is a Wired Equivalent Privacy (WEP)?
WEP is a specification for data encryption between wireless devices to prevent
eavesdropping. There are two versions: 64-bit encryption made of 40-bit key plus
a 24-bit initialization vector (IV), which is part of the encryption key that is sent
in clear text before encrypting the data; a 128-bit encryption made up with a 104bit key plus a 24-bit IV.
30) What are the five major characteristic of WEP?
The five major characteristic of a WEP is authentication, encryption, static keys,
single set of keys, and a 24-bit IV.
31) What is a Wi-Fi protected Access (WPA)?
WPA is a standard for network authentication and encryption introduced by the
Wi-Fi Alliance, to deal with the shortcomings of WEP. It can be implemented
with a simple software upgrade on the AP or residential wireless gateway. WPA
uses a 128-bit pre-shared key (PSK), which is also known as personal mode and it
is different for each device, packet, and session. A temporal key integrity protocol
(TKIP) is also used and it provides per packet key mixing. In addition, TKIP also
provides message integrity check (MIC), which uses a combination of variable
and static data item to ensure that the encrypted data has not been tampered with.
32) What are the standard 802.11i and 802.1X?
The 802.11i standard enhancement to 802.11 is the final result of a series of effort
to deal with the security weaknesses of the original WLAN standard. In
combination with 802.1X, 802.11i defines a robust security network association
(RSNA), which is a grouping of several functions that protects data frames by
providing mutual authentication between client devices and APs, controlled
access to the network, establishment of security keys, and key management.