Wireless Access

1
Wireless Access

• avoid collisions 2 nodes transmitting at same
  time
• CSMA - sense before transmitting
• dont collide with ongoing transmission by other
  node
• no collision detection!
• difficult to receive (sense collisions) when
  transmitting (what you hear is not he hear)
• cant sense all collisions in any case hidden
  terminal, fading
• The next best thing to do avoid collisions
  CSMA/C(ollision)A(voidance)

2
Collision Avoidance RTS-CTS exchange
A
B
AP
defer
time
3
IEEE 802.11 Wireless LAN

• 802.11b
• 2.4-5 GHz unlicensed radio spectrum
• up to 11 Mbps
• direct sequence spread spectrum (DSSS) in
  physical layer
• widely deployed, using base stations

• 802.11a
• 5-6 GHz range
• up to 54 Mbps
• 802.11g
• 2.4-5 GHz range
• up to 54 Mbps
• All use CSMA/CA for multiple access
• Base-station and ad-hoc network versions

4
802.11 LAN architecture

• wireless host communicates with base station
• base station (BS) access point (AP)

switch or router
BSS 1
BSS 2
5
802.11 Operating Modes

• DCF Distributed Coordination Function
• aka Ad-hoc mode
• CSMA/CA
• RTS/CTS signaling
• Two stations can communicate
• All stations have the same logic
• No infrastructure, Suitable for small area
• PCF Point Coordination Function
• Infrastructure Based
• Must have a base station (or access point)
• BS periodically broadcast a beacon
• MS can only transmit when it is being polled

6
802.11 Channels

• 802.11b Use 11 channels at different frequencies
• AP admin select operating frequency for AP
• interference possible channel can be same as
  that chosen by neighboring AP!
• Planning decide which AP use which channels to
  provide good coverage.
• host must associate with an AP
• scans channels, listening for beacon frames
  containing APs name (SSID) and MAC address
• selects AP to associate with
• may perform authentication
• will typically run DHCP to get IP address in APs
  subnet