802.11 Networks
David Malone <David.Malone@nuim.ie>
Lesley Malone <lmalone@iol.ie>
27 February 2009
802.11 Networks
• Standard used for home/office wireless networks,
• Branded as WiFi,
• Uses ISM band at 2.4GHz (802.11b/g),
• Or maybe at 5GHz (802.11a),
• Transfers packets of data like Ethernet.
• Carrier sense multiple access protocol.
Aloha: Good Transmission
Send
Repeat
Aloha: Collision
Send
Send
*silence*
What About WiFi?
• After transmission choose rand(0,CW-1).
• Wait until medium idle.
• Count down in slots.
• Transmit when get to 0 (if you have a packet).
• If ACK then CW ← CWmin else CW ← 2CW .
Counting Down
(20us)
Someone else transmits,
stop counting!
Transmissions (~500us)
Data and then ACK
Collision followed by timeout
Previous work
• Radiofrequency Exposure from Wireless LANs Utilizing Wi-Fi
Technology, Kenneth R. Foster, Health Physics, March 2007,
93(3), 280–289.
• Exposure of the general public due to wireless LAN
applications in public places, G. Schmid et al, Radiation
Protection Dosimetry, 2007, 124(1), 48–52.
Impact of Collisions
• MAC is not perfect.
• For saturated network have Bianchi model.
• Use this to find probability of transmission.
• Then have binomial distribution of number transmitting.
• Use this to average energy per slot.
• Divide by average slot time to get power.
• Gives estimate of total network output.
Power for saturated network of n
0.25
60 byte packets
1500 byte packets
Total Transmitted Power (W)
0.2
0.15
0.14
0.12
0.1
0.1
0.08
0.06
0.05
0.04
0.02
0
0
5
10
15
20
25
30
0
0
50
100
150
Nominal power is 100mW.
200
250
300
Number of stations
350
400
450
500
Power for saturated broadcast network of n
3
60 byte packets
1500 byte packets
Total Transmitted Power (W)
2.5
2
1.5
0.25
0.2
1
0.15
0.1
0.05
0.5
0
0
5
10
15
20
25
30
0
0
50
100
150
Nominal power is 100mW.
200
250
300
Number of stations
350
400
450
500
Thanks!
Questions?