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Wireless Access
• avoid collisions: 2+ nodes transmitting at same
time
• CSMA - sense before transmitting
– don’t collide with ongoing transmission by other node
• no collision detection!
– difficult to receive (sense collisions) when transmitting
(what you hear is not he hear)
– can’t sense all collisions in any case: hidden terminal,
fading
– The next best thing to do: avoid collisions
CSMA/C(ollision)A(voidance)
Collision Avoidance: RTS-CTS
exchange
A
AP
B
reservation collision
DATA (A)
time
defer
IEEE 802.11 Wireless LAN
• 802.11b
• 802.11a
– 2.4-5 GHz unlicensed
– 5-6 GHz range
radio spectrum
– up to 54 Mbps
– up to 11 Mbps
• 802.11g
– direct sequence
– 2.4-5 GHz range
spread spectrum
– up to 54 Mbps
(DSSS) in physical
• All use CSMA/CA for
layer
multiple access
– widely deployed, using
base stations
• Base-station and
ad-hoc network
versions
802.11 LAN architecture
Internet
AP
switch
or router
BSS 1
AP
BSS 2
• wireless host
communicates with base
station
– base station (BS) =
access point (AP)
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
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 AP’s
name (SSID) and MAC address
– selects AP to associate with
– may perform authentication
– will typically run DHCP to get IP address in AP’s subnet