Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

58 CHAPTER 4 Wireless Networking
■ Direct sequence spread spectrum (DSSS) DSSS
works by dividing the data and simultaneously
transmitting on as many frequencies as possible
within a particular frequency band (also known as
a channel). DSSS adds redundant bits of data
known as chips to the data to represent binary 0s
or 1s. The ratio of chips-to-data is known as the
spreading ratio: the higher the ratio, the more
immune to interference the signal is, because if
part of the transmission is corrupted, the data can
still be recovered from the remaining part of the
chipping code. Additionally, DSSS protects against
data loss through the redundant, simultaneous
transmission of data.
WIRELESS NETWORK ARCHITECTURE
Wireless networks operate at the physical and data link layers of the open system
interconnection (OSI) model. Both FHSS and DSSS are implemented at the
physical layer of the OSI model. The data link layer is divided into two sublayers:
the media access control (MAC) and logical link control (LLC) layers.
The MAC layer is responsible for such things as framing data, error control, synchronization,
and collision detection and avoidance. Wireless networks utilize
carrier sense multiple access with collision avoidance (CSMA/CA) method for
protecting against data loss. Using CSMA/CA, a wireless workstation first tries to
detect if any other device is communicating on the network. If it senses it is clear
to send, it initiates communication.
Wireless Network Protocols and Operation
WLANs are covered by the IEEE 802.11 standards.
In addition to providing a solution to the problems created by collisions that
occur on a wireless network, the 802.11 standard must deal with other issues
specific to the nature of wireless devices and wireless communications in general.
WIRELESS APPLICATION PROTOCOL
■ An open specification designed to enable mobile wireless users to easily
access and interact with information and services.
■ WAP is designed for hand-held digital wireless devices.
■ WAP browsers in a wireless client are analogous to the standard Internet
browsers on computers.
■ WAP uniform resource locators (URLs) are the same as those defined for
traditional networks and are also used to identify local resources in the
WAP-enabled client.
■ The WAP specification added two significant enhancements to the above
programming model: push and telephony support (wireless telephony
application [WTA]).
■ WAP also provides for the use of proxy servers, as well as supporting servers
that provide functions such as public key infrastructure support, user profile
support, and provisioning support.