Networking Fundas
Dr. Deepak C. Karia,
S.P.I.T.
WLAN LAN: Incomplete/ Infrastructure Wireless
Network
Access point.
Repeaters.
WLAN WLAN: Complete/Ad-hoc Wireless Network
size of the network.
Suited for temporary
situations such as meetings
and conferences.
What is infrastructure and ad-hoc modes in
wireless networking?
bridges a wireless network to a wireless devices directly
wired Ethernet network.
communicates with each
other.
A wireless access point (AP) is Does not require AP’s.
required for infrastructure
mode wireless networking.
Mobile communication?
Wireless vs. mobile Examples
stationary computer
laptop (portable)
wireless LAN in buildings
Personal Digital Assistant (PDA)
What is 802.11 (Wi-fi)?
A family of WLAN specifications developed by IEEE.
Defines standard for WLANs using the following four
technologies:
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)
Infrared (IR)
Orthogonal Frequency Division Multiplexing (OFDM)
Versions: 802.11a, 802.11b, 802.11g, 802.11e, 802.11f,
802.11i.
802.11 - 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, secured
Two popular methods
Frequency Hopping (FH)
Direct Sequence (DS)
Frequency Hopping SS
FH systems uses 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)
Types:
Direct Sequence SS
DS systems uses 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 (redundancy –
it transmits at least 10 fully redundant
copies of the original signal at the
same time)
802.11a
Employs Orthogonal Frequency Division Multiplexing
(OFDM: method of encoding digital data on multiple
carrier frequencies.)
Offers higher bandwidth than that of 802.11b.
Operates in the 5 GHz range.
Advantages
Ultra-high spectrum efficiency
5 GHz band is 300 MHz (vs. 83.5 MHz @ 2.4 GHz)
More data can travel over a smaller amount of
bandwidth
High speed
Up to 54 Mbps
Less interference
Fewer products using the frequency
2.4 GHz band shared by cordless phones,
microwave ovens, Bluetooth, and WLANs
Disadvantages
Standards and Interoperability
Standard not accepted worldwide
No interoperability certification available
for 802.11a products
Not compatible or interoperable with 802.11b
Legal issues
License-free spectrum in 5 GHz band not
available worldwide
Market
Beyond LAN-LAN bridging, there is limited interest for
5 GHz adoption.
Cost
2.4 GHz will still has >40% cost advantage.
Range
At equivalent power, 5 GHz range will be ~50% of 2.4
GHz.
Power consumption
Higher data rates and increased signal requires more
power
OFDM is less power-efficient then DSSS.
Applications
Building-to-building connections
Video, audio conferencing/streaming video,
and audio
Large file transfers, such as engineering
CAD drawings
Faster Web access and browsing
High worker density or high throughput scenarios
Numerous PCs running graphics-intensive applications
802.11a vs. 802.11b
Frequency
802.11b transfers at 2.4 gigahertz
802.11a transfers at 5 gigahertz
Coverage Distance
802.11b goes about 400 feet indoors
802.11a goes about 60 feet indoors
Need more access points to cover a location
Compatible to each other?
Not yet.
802.11g
802.11g is a high-speed extension to 802.11b
Compatible with 802.11b
High speed up to 54 Mbps
2.4 GHz (vs. 802.11a, 5 GHz)
Using OFDM for backward compatibility.
Pros of 802.11g - fast maximum speed; signal range is good and not
easily obstructed
Cons of 802.11g - costs more than 802.11b; appliances may interfere
on the unregulated signal frequency.
Performance
802.11a offers speeds with a theoretically maximum rate
of 54Mbps in the 5 GHz band
802.11b offers speeds with a theoretically maximum rate
of 11Mbps at in the 2.4 GHz spectrum band
802.11g is a new standard for data rates of up to a
theoretical maximum of 54 Mbps at 2.4 GHz.
Choose the right technology
Usually IEEE 802.11b or 802.11a
802.11b offers interoperability (WECA Wi-Fi Certification
Program)
802.11a offers higher data rates (up to 54 mbps) ->
higher throughput per user. Limited interoperability.
Data rates
Data rates affect range
802.11b 1 to 11 Mbps in 4 increments
802.11a 6 to 54 Mbps in 7 increments
The minimum data rate must be determined at
design time
Selecting only the highest data rate will require a
greater number of APs to cover a specific area
Compromise between data rates and overall
system cost
