25-02-2013, 10:53 AM
Wireless Networks
Wireless Networks.ppt (Size: 790 KB / Downloads: 461)
Wireless?
A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier.
The last link with the users is wireless, to give a network connection to all users in a building or campus.
The backbone network usually uses cables
Common Topologies
Complete Wireless Networks
The physical size of the network is determined by the maximum reliable propagation range of the radio signals.
Referred to as ad hoc networks
Are self-organizing networks without any centralized control
Suited for temporary situations such as meetings and conferences.
How do wireless LANs work?
Wireless LANs operate in almost the same way as
wired LANs, using the same networking protocols
and supporting the most of the same
applications.
Integration With Existing Networks
Wireless Access Points (APs) - a small device that bridges wireless traffic to your network.
Most access points bridge wireless LANs into Ethernet networks, but Token-Ring options are available as well.
Security
In theory, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used
The IEEE 802.11 standard specifies optional security called "Wired Equivalent Privacy" whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures.
Transmission
Most wireless LAN products operate in unlicensed radio bands
2.4 GHz is most popular
Available in most parts of the world
No need for user licensing
Most wireless LANs use spread-spectrum radio
Resistant to interference, secure
Two popular methods
Frequency Hopping (FH)
Direct Sequence (DS)
Frequency Hopping Vs. Direct Sequence
FH systems use a radio carrier that “hops” from frequency to frequency in a pattern known to both transmitter and receiver
Easy to implement
Resistance to noise
Limited throughput (2-3 Mbps @ 2.4 GHz)
DS systems use a carrier that remains fixed to a specific frequency band. The data signal is spread onto a much larger range of frequencies (at a much lower power level) using a specific encoding scheme.
Much higher throughput than FH (11 Mbps)
Better range
Less resistant to noise (made up for by redundancy – it transmits at least 10 fully redundant copies of the original signal at the same time)