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Communication Security: Wireless • Chapter 4 175

for wireless networks because it relies on the fact that every workstation can hear
all the other workstations on a cable segment to determine if there is a collision.
In wireless networks, usually only the AP can hear every workstation that is
communicating with it (for example, workstations A and B may be able to commu-
nicate with the same AP, but may be too far apart from each other to hear their
respective transmissions).Additionally, wireless networks do not use full-duplex
communication, which is another way of protecting data against corruption and
loss as a result of collisions.

NOTE
APs are also referred to as wireless access points. This is a more precise
term that differentiates them from other network access points (such as
dial-in remote access points) but in this chapter, we will use the acronym
AP to avoid confusion with the Wireless Application Protocol (also
known as WAP).

CSMA/CA solves the problem of potential collisions on the wireless network
by taking a more active approach than CSMA/CD, which kicks in only after a col-
lision has been detected. Using CSMA/CA, a wireless workstation first trys to
detect if any other device is communicating on the network. If it senses it is clear
to send, it initiates communication.The receiving device sends an acknowledgment
(ACK) packet to the transmitting device indicating successful reception. If the
transmitting device does not receive an ACK, it assumes a collision has occurred
and retransmits the data. However, it should be noted that many collisions can
occur and that these collisions can be used to compromise the confidentiality of
Wired Equivalent Privacy (WEP) encrypted data.
CSMA/CA is only one way in which wireless networks differ from wired net-
works in their implementation at the MAC layer. For example, the IEEE standard
for 802.11 at the MAC layer defines additional functionality, such as virtual colli-
sion detection (VCD), roaming, power saving, asynchronous data transfer, and
encryption.
The fact that the WEP protocol is defined at the MAC layer is particularly
noteworthy and has significant consequences for the security of wireless networks.
This means that data at the higher levels of the OSI model, particularly
Transmission Control Protocol/Internet Protocol (TCP/IP) data, is also encrypted.
Because much of the TCP/IP communications that occur between hosts contain a

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