Basics of Wi-Fi

WiFi stands for Wireless Fidelity.

It is Based on 802.11 and primarily a LAN Technology. Wi-Fi operates at the physical and data link layers of the OSI model

  • Wi-Fi is Half Duplex.
  • Ethernet is more secure than WiFi
  • There is Collision avoidance in wifi and collision detection in Ethernet.

Distributed Network in WiFi

Important Points

  • Devices in Same BSS can connect Directly.
  • Devices in different BSS can connect through Access Point.
  • 802.11a uses the 5 GHz U-NII band, which offers at least 23 non-overlapping channels rather than the 2.4 GHz ISM frequency band which offer only three non-overlapping channels.
  • RTS and CTS mechanism is used for Collision Avoidance.
  • RTS and CTS Also solve hidden node problem or hidden terminal problem.
  • Access point consisting of antenna and routers are main source that transmit and receive radio waves.

Bandwidth : 

  • 25 MHZ for 802.11b
  • 20 MHz for 802.11a

According to distributed control function , a wifi station will transmit only when channel is clear. All transmissions are Acknowledged. So, if a station does not receive an ACK , it assume collision and retry after random amount of time.


There are 2 General type of Wifi Transmission

  1. DCF (Distributed Coordination Function)
  2. PCF (Point coordination Function) – Not much used

There are 2 Problem that makes wireless different than Wired :

  1. Hidden Substation Problem
  2. High Error Rate

Important Notes

  • Every packet sent is positively acknowledged by receiver. Next packet is not sent until receiving a positive acknowledgment for previous Packet.
  • A Transmission begin with a RTS (Request to send) & Receiver respond with CTS (Clear to send). then the data packet flow. This help in hidden substation problem.

A group of corresponding station is called a BSS (Basic Service set).

Independent BSS, or ad hoc The network is only the members of the BSS, they talk between themselves directly, they self-organize, there is no central autority.

Infrastructure BSS :  The BSS is organized around an Access Point which can bridge traffic out the BSS onto a distribution network. Members of the BSS talk to the AP(Access Point) only.  A BSS (data link layer) is defined by who will hear a broadcast from the AP (but not by a station which is not an AP, because of the hidden station problem!).

ESS (Extended Service Set) :  A bunch of BSS’s connected by a distribution network. The distribution network connects the Access Points. WiFi doesn’t specify the protocol that builds ESS’s.


When a mobile host move beyond the range of one base station and into range of another. It will change its point of attachment into larger network . This is Handoff .


References

  • Wikipedia
  • http://www.cs.miami.edu/home/burt/learning/Csc524.052/notes/wifi.html
  • Kurose

Check Also : IPv4 Vs IPv6

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