A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
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3.3.2.3. Hidden node <strong>and</strong> CSMA/CA<br />
When a station is communicating <strong>wi</strong>th another station, there is an existing risk that the<br />
packets may collide. It is due to the hidden node problem. Each station has limited coverage, so<br />
stations may not be aware <strong>of</strong> the other nearby stations. Thus if there are more than two stations<br />
that try to transmit a signal to the same station at the same time, <strong>and</strong> then the collision <strong>wi</strong>ll<br />
happen. In Figure 3-15, station A <strong>and</strong> C are hidden nodes to each other. If station A is sending a<br />
signal, station C cannot be aware <strong>of</strong> it. Since most stations are half-duplex (cannot transmit <strong>and</strong><br />
listen at the same time), the collision is dif<strong>fi</strong>cult to be detected. In Figure 2-17, it shows that only<br />
station B knows collisions happened. [17][55]<br />
Figure 3-15 The Hidden Node Problem<br />
To deal <strong>wi</strong>th this problem, 802.11 WG uses a different method from Ethernet. With <strong>wi</strong>red<br />
transmission they use CSMA/CD (Carrier Sense Multiple Access/Collision Detection), but since<br />
the <strong>wi</strong>reless transmission has very limited resource <strong>and</strong> is not as reliable as Ethernet, 802.11 WG<br />
turns to use collision avoidance (CSMA/CA).<br />
There are two technologies used in CSMA/CA, physical channel sensing <strong>and</strong> virtual channel<br />
sensing. [17][55]<br />
Physical channel sensing<br />
When a station wants to send signals, for example, it senses the channel <strong>fi</strong>rst. If the channel<br />
is idle, it just sends. During the transmission, it <strong>wi</strong>ll not sense the channel at the time, but keeps<br />
sending signals. However if the channel is busy, it <strong>wi</strong>ll wait until the channel is idle <strong>and</strong> then<br />
start the transmission. Once a collision occurs, the stations <strong>wi</strong>ll wait for certain time, by using the<br />
Ethernet binary exponential back <strong>of</strong>f algorithm, <strong>and</strong> then try again later. [17][55]<br />
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