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842 WIRELESSINTERNETLANs, allowing for very simple bridging from wireless
to IEEE wireline networks, but the MAC is unique to
WLANs.
For 802.3 ethernet LANs, the carrier sense multiple ac-
cess with collision detection (CSMA/CD) protocol regu-
lates how ethernet stations establish access to the wire
and how they detect and handle collisions that occur when
two or more devices try to simultaneously communicate
over the physical medium. In an 802.11 WLAN, collision
detection is not possible due to the near/far problem. To
detect a collision, a station must be able to transmit and
listen at the same time, but in radio systems the transmis-
sion (near signal) drowns out the ability of the station to
detect a collision (far signal).
Since collision detection is not possible, the stations
only use collision avoidance; they sense the channel be-
fore transmitting. If the channel is busy, stations back off
and try again at a later time. If the channel is idle, a sta-
tion will transmit its frame. Since several stations may be
sensing the channel at the same time and all detect it to
be idle, they will start to transmit concurrently, thereby
causing collisions. Because stations are unable to detect
collisions, CSMA/CA systems need to use explicit packet
acknowledgments (ACK). In other words, an ACK packet
is sent by the receiving station to confirm that the data
packet arrived intact.
Another MAC-layer problem specific to wireless is the
hidden-terminal issue, in which two stations can both
hear activity from the access point, but not from each
other, usually due to distance or a physical obstruction. To
solve this problem, 802.11 specifies an optional request to
send/clear to send (RTS/CTS) protocol at the MAC layer.
When this feature is in use, a sending station transmits
an RTS and waits for the access point to reply with a
CTS. Since all stations in the network can hear the access
point, the CTS causes them to delay any intended trans-
missions, allowing the sending station to transmit and re-
ceive a packet acknowledgment without any chance of
collision. Since RTS/CTS adds additional overhead to the
network by temporarily reserving the medium, it is typi-
cally used only on the largest-sized packets, for which re-
transmission would be expensive from a bandwidth stand-
point.Security.802.11 provides for MAC layer (OSI Layer 2)
access control and encryption mechanisms, which are
jointly known as wired equivalent privacy (WEP), with
the objective of providing WLANs with security equiva-
lent to their wireline counterparts. For the access control,
the ESSID (also known as a WLAN service area ID) is pro-
grammed into each access point. A wireless client must
know the ESSID to associate with an access point. No
communication can occur unless there is an association
between a client and the access point. In addition, there
is provision for a table of MAC addresses called anaccess
control listto be included in the access point, restricting
access to only those clients whose MAC addresses are on
the list.
For data encryption, the standard provides for optional
encryption using a 40-bit shared-key RC4 PRNG algo-
rithm from RSA Data Security. All data sent and received
while the end station and access point are associated canbe encrypted using this key. In addition, when encryption
is in use, the access point will issue an encrypted chal-
lenge packet to any client attempting to associate with
it. The client must use its key to encrypt the correct re-
sponse in order to authenticate itself and gain network
access.
Unfortunately, beginning with an internal study in
2000 (Walker, 2000) to a highly publicized study in 2001
(Borisov, Goldberg, & Wagner 2001), WEP has been
shown to fall well short of accomplishing its security
goals. Some of the problems of WEP that have been iden-
tified by researchers include the following:WEP uses RC4, a synchronous stream cipher, but it is
difficult to ensure synchronization during a complete
session over the unreliable wireless link, leading to the
use of a separate key for each packet—a clear violation
of one of the most important requirements of RC4.
A very limited key-space is used, which is problematic
since a separate key is needed for each packet.
802.11 does not provide any mechanism for sharing keys
over an insecure channel.
There is no mechanism for a mobile to authenticate the
network.
Checksum (CRC-32) used for integrity check is linear;
thus, it is relatively easy to make undetected changes
in the message.Such weaknesses combine to result in a network that is
vulnerable to several types of attacks and intrusions.
There are several ongoing efforts to secure the 802.11
network, one of which is the robust security network
(RSN). In RSN, a recently proposed 802.1x standard
(Institute of Electrical and Electronics Engineers, 2001)
forms the basis for access control, authentication, and key
management. In addition, a number of protocols such as
extensible authentication protocol–transport layer secu-
rity (EAP-TLS) (Aboba & Simon 1999; Diersk & Allen,
1999; Zorn, 1999) are being considered to provide strong
authentication between the MS and the AP.Timing and Power Management
Synchronization of all clocks within a BSS is maintained
by periodic transmission of beacons containing times-
tamp information. In the infrastructure mode, the AP
serves as the timing master and generates all timing bea-
cons. Synchronization is maintained to within 4 ms plus
propagation delay.
Timing beacons also play an important role in power
management. There are two power saving modes defined:
awake and doze. In the awake mode, stations are fully
powered and can receive packets at any time. In the doze
mode, it is unable to transmit or receive data and con-
sumes very little power. A station must inform the AP that
it is entering the doze mode. The AP does not send packets
to stations in the doze mode, but instead buffers them for
transmission at a designated time.Comparison of 802.11b and 802.11a
Two advanced WLAN standards, 802.11b and 802.11a,
were developed by the IEEE’s 802.11 working group. At