WIFI

1
WI-FI

• CPSC 441 Computer Communications

2
WI-FI

• Wireless Fidelity
• Use of radio frequency for propagation of data
• Owned by the WI-FI Alliance
• Consortium of companies
• Agrees on a set of standards based on IEEE802.11

3
IEEE 802.11

• Wireless standards developed by IEEE WAN/LAN
  Standards Committee
• Uses the 5GHz and 2.4GHz spectrum

4
(No Transcript)
5
Pros and Cons

• Pro
• High bandwidth (up to 11 Mbps)
• Two modes of operation infrastructure vs. ad hoc
• Con
• Incompatibility between old and new cards
• Signal blocked by reinforced concrete or tinted
  glass
• High channel BER can degrade performance (lots!)
• No standard for hand-off between base stations
• Some channel numbers overlap spectrum
• High power consumption in laptops

6
CSMA/CA

• Carrier Sensing Medium Access with Collision
  Avoidance
• Wireless adapters cannot sense while sending
• Half-Duplex property
• Unable to detect collision

7
DCF

• 802.11 uses Distributed Coordination Function
  (DCF) for medium access control
• DCF uses
• CSMA/CA (physical carrier sensing)
• RTS/CTS (virtual carrier sensing, optional)

8
DCF Physical Carrier Sensing

• Listen to channel before transmitting
• If idle, wait for DCF inter-frame space (DIFS),
  then transmit
• If busy, wait for end of current transmission,
  then wait for additional DIFS backoff delay
• Receiver waits for short inter-frame space (SIFS)
  and sends ACK
• no collision detection, need to know if frame
  transmission
• SIFS ltlt DIFS
• If within SIFS transmitter does not receive ACK,
  will resend (channel capture until successful
  transmission)

9
DCF Physical Carrier Sensing
source
others
destination
Air is free for DIFS time period
DIFS
data
send frame
All other devices must defer while air is busy
NAV defer access
SIFS
Receive ACK back that frame was received intact!
ack
10
Exponential Backoff

• Backoff delay chosen randomly in 0, W-1
• W contention window, doubled after every
  unsuccessful transmission
• Max W is limited
• Reset to 0 after successful transmission
• Delay is used to countdown amount of idle channel
  time to wait before transmission
• If channel is busy while waiting, timer is frozen
  

11
Virtual Carrier Sensing

• RTS Request to Send
• CTS Clear to Send

A
B
Sender
Receiver
RTS
DATA
Sender
CTS
ACK
Receiver
NAV (RTS)
A
NAV (CTS)
B
Image Source http//wireless.cs.jhu.edu/class-sli
des/wireless-medium-access.ppt
12
Hidden Node Problem
RTS
CTS
B
A
C
13
References/Additional Reading

• Carey Williamsons CPSC641 Slides on wireless
  networking
• http//pages.cpsc.ucalgary.ca/carey/CPSC641/
• Additional Reading
• http//wireless.cs.jhu.edu/class-slides/wireless-m
  edium-access.ppt
• http//www.cs.ucla.edu/classes/winter04/cs117/chap
  ter7wlan.ppt
• http//www.research.rutgers.edu/xili/cs352/wirele
  ss.pdf
• http//www.business.uca.edu/kgriffin/MIS4363Power
  pointFiles/ch07.ppt

14
Questions?