WLAN Wireless Network

1. Hidden terminal: A, B and C, A and C cannot hear each other, if they both send packets to B, the collision wil L happen.

2. exposed terminal: A and B can hear each other, but actually B can send packets to D.

3. IEEE802.11 Wireless communication has no collision detection. Thus, firstly, use the ACK frame to comfirm the correct frame, and it's short interframe space (SIFS); Then use rts/cts to reserve the channel for large frames, a duration field in rts/cts frames;

4. Ways to sense the carrier

• Physical
• Virtual (NAV)-duration field known from rts/cts

5. Priority Levels:sifs<pifs<difs (time duration)

• SIFS is used for ACK, data response, rts/cts response
• PIFs is used for PCF response
• DIFS is used for DFS response

6. Csma/ca collision avoidance, sense the carrier

• If Idle, wait a DIFS then transmit
• If busy, wait channel to idle+wait a difs+wait random backoff time, then transmit

7. Supperframe:a Contention-free burst occurs at the beggining, followed by A contention period.

8. The PCF protocol is based in a polling scheme controlled by the point Coordinator.

WLAN Wireless Network