Short Notes: Basics of Wi-Fi | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

 Page 1


WiFi stands for Wireless Fidelity. WiFi is the marketing name for IEEE standard 
802.11. It is a standard for both Level 1 (physical) and Level 2 (data link) of a 
wireless data transmission protocol. It is primarily a local area networking (LAN) 
technology designed to provide in-building broadband coverage.
Current WiFi systems support a peak physical-layer data rate of 54 Mbps and 
typically provide indoor coverage over a distance of 100 feet.
WiFi offers remarkably higher peak data rates than do 3G systems, primarily since 
it operates over a larger 20 MHz bandwidth, but WiFi systems are not designed to 
support high-speed mobility.
WiFi interfaces are now also being built into a variety of devices, including personal 
data assistants (PDAs), cordless phones, cellular phones, cameras, and media 
players.
WiFi is Half Duplex: All WiFi networks are contention-based TDD systems, where 
the access point and the mobile stations all vie for use of the same channel. 
Because of the shared media operation, all Wi-Fi networks are half duplex.
There are equipment vendors who market Wi-Fi mesh configurations, but those 
implementations incorporate technologies that are not defined in the standards. 
Channel Bandwidth: The Wi-Fi standards define a fixed channel bandwidth of 25 
MHz for 802.11 b and 20 MHz for either 802.11 a or g networks.
Wi-Fi - IEEE Standards: The 802.11 standard is defined through several 
specifications of WLANs. It defines an over-the-air interface between a wireless
Page 2


WiFi stands for Wireless Fidelity. WiFi is the marketing name for IEEE standard 
802.11. It is a standard for both Level 1 (physical) and Level 2 (data link) of a 
wireless data transmission protocol. It is primarily a local area networking (LAN) 
technology designed to provide in-building broadband coverage.
Current WiFi systems support a peak physical-layer data rate of 54 Mbps and 
typically provide indoor coverage over a distance of 100 feet.
WiFi offers remarkably higher peak data rates than do 3G systems, primarily since 
it operates over a larger 20 MHz bandwidth, but WiFi systems are not designed to 
support high-speed mobility.
WiFi interfaces are now also being built into a variety of devices, including personal 
data assistants (PDAs), cordless phones, cellular phones, cameras, and media 
players.
WiFi is Half Duplex: All WiFi networks are contention-based TDD systems, where 
the access point and the mobile stations all vie for use of the same channel. 
Because of the shared media operation, all Wi-Fi networks are half duplex.
There are equipment vendors who market Wi-Fi mesh configurations, but those 
implementations incorporate technologies that are not defined in the standards. 
Channel Bandwidth: The Wi-Fi standards define a fixed channel bandwidth of 25 
MHz for 802.11 b and 20 MHz for either 802.11 a or g networks.
Wi-Fi - IEEE Standards: The 802.11 standard is defined through several 
specifications of WLANs. It defines an over-the-air interface between a wireless
client and a base station or between two wireless clients.
Specifications:
• 802.11 - This pertains to wireless LANs and provides 1- or 2-Mbps 
transmission in the 2.4-GHz band using either frequency-hopping spread 
spectrum (FHSS) or direct-sequence spread spectrum (DSSS).
• 802.11 a - This is an extension to 802.11 that pertains to wireless LANs and 
goes as fast as 54 Mbps in the 5-GHz band. 802.11 a employs the orthogonal 
frequency division multiplexing (OFDM) encoding scheme as opposed to 
either FHSS or DSSS.
• 802.11 b - The 802.11 high rate WiFi is an extension to 802.11 that pertains to 
wireless LANs and yields a connection as fast as 11 Mbps transmission (with 
a fallback to 5.5, 2, and 1 Mbps depending on strength of signal) in the 2.4- 
GHz band. The 802.11 b specification uses only DSSS. Note that 802.11 b was 
actually an amendment to the original 802.11 standard added in 1999 to 
permit wireless functionality to be analogous to hard-wired Ethernet 
connections.
• 802.11 g - This pertains to wireless LANs and provides 20+ Mbps in the 2.4- 
GHz band.
Wi-Fi Concepts: There are two general types of Wi-Fi transmission: DCF 
(Distributed Coordination Function) and PCF (Point Coordination Function). DCF is 
Ethernet in the air. It employs a very similar packet structure, and many of the same 
concepts. There are two problems that make wireless different then wired.
• The hidden substation problem.
• High error rate.
These problems demand that a DCF Wi-Fi be a CSMA/CA network (Collision 
Avoidance) rather than a CSMA/CD network (Collision Detect). The results are the 
following protocol elements,
• Positive Acknowledgement. Every packet sent is positively acknowledged by 
the receiver. The next packet is not sent until receiving a positive 
acknowledgement for the previous packet.
• Channel clearing. A transmission begins with a RTS (Request to Send) and the 
destination or receiver responds with a CTS (Clear to Send). Then the data 
packets flow. For the channel is cleared by these two messages.
• Channel reservation: Each packet has a NAV (Network Allocation Vector) 
containing a number X. The channel is reserved to the correspondents (the 
sender and receiver of this packet) for an additional X milliseconds after this 
packet. Once you have the channel, you can hold it with the NAV. The last ACK 
contains NAV zero, to immediately release the channel.