Wireless LANs include mechanisms to improve the reliability of the packet
transmissions to be at least the same level as wired Ethernet. Using the TCP/IP
protocols will help protect the network against any loss or corruption of data
over the air. Most WLAN systems use spread-spectrum technology or orthogonal
frequency-division multiplexing (OFDM).
The two types of spread-spectrum
radio are direct sequence (DSSS) and frequency hopping (FHSS). Both are shown
in Figure
. They are based
on the idea that a signal that is spread widely or that is quickly moving from
channel to channel will be difficult to detect and hard to interfere with. DSSS
generates a redundant bit pattern called a chip or chipping code, for each bit
to be transmitted. FHSS uses a narrowband carrier that changes frequency in a
pattern known to both the transmitter and the receiver. If everything stays
properly synchronized this creates a single logical channel, even though the
frequency is constantly changing. Early implementations of 802.11 used FHSS,
however 802.11b standardized on DSSS.
Currently the 802.11a and 802.11g
standards, operating up to 54 Mbps, use OFDM instead of DSSS. OFDM limits the
crosstalk or interference of transmitting channels. OFDM is used in European
digital audio broadcast services. Compared to DSSS, OFDM allows more speed.
OFDM does not give up distance. In fact it facilitates longer distance
capability. OFDM does require more processing power in the radio.
In
addition to taking care to match transmission technologies, wireless network
administrators must be aware that connection issues can also exist in changing
environments where obstacles may block, reflect, or impede signals. Antenna
choice and mounting location must be carefully considered when designing WLANs
in order to avoid future interference. The connection will usually not be lost
even though the available bandwidth may drop very low. Lack of guaranteed
bandwidth is of particular concern for many companies.