Wlan无线网络

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

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

3. IEEE802.11 wireless communication have 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. Two 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 on a polling scheme controlled by the point coordinator.