Wireless Networks

number of different wireless specs
wireless media –> multiple access
overlapping signals can be a problem
- transmission distance
- transmission rates
bluetooth:
- 10 meters, 2mbps
WiFi
- 100 meters, 100-450mbps
4G
- kilometers
- 1-5 mbps
wireless medium has challenges
- sharing efficiently
- signal attenuation
allocations determined by FCC
shared spectrum - spread spectrum over multiple frequencies
frequency hopping for security
direct sequence for fault tolerance using data, random sequence of pseudo random bits, and the XOR of the streams.
wireless networks typically have base station connected to wired network
usually there is a “fixed’ portion of wireless network
- mesh/ad-hoc networking w/o

802.11

defines physical layers operating in frequency bands at different data rates
original standard with frquency hopping on 1MHz wide bands
2nd using direct-sequence with 11-bit chipping
802.11b for 11mbps
802.11a for 54mbps (5GHz)
802.11g for 54 mbps (2.4GHz)
also support for lower data rates (6, 9, 12, 18, etc…)
systems try to pick optimal rate based on environment noise
wireless collision is a difficult problem
- hidden node problem - A and C signals collide at node B, but never reach one another
CSMA/CA (collision avoidance)
RTS-CTS (ready to send, clear to send)
- on collisions, wait a random amount of time before retrying (exponential distribution)
packet data transmitted after successful RTS-CTS exchange
most 802.11 deployments use a base stations
- split routing and wireless signal
allow devices to roam between APs
- each device associated with an AP
selecting AP is scanning
- send probe frame
- all APs respond with probe Response
- node selections AP with association request
- AP replies with association response
4 address in 802.11 frame to identify original sender receiver and possible forwarding sender/receiver