Kalpana Devi, Assistant Professor, Department of Computer Science and Engineering, SRIT

Useful for students

1. IEEE 802.11 – MAC layer and Physical Layer

2. Communicates between station to station using AP

3. It senses the medium whether it is idle or busy When idle, waits for duration of time CTS- Clear to send RTS-Request to send distributed inter frame spacing- DIFS Short Interferences – SIFS( spacing between transmission and ACK) Network Allocation Vector(NAV)

4. PIFS-PCF Interframe Space B=Beacon Frame

5. AP will wait for certain period of time till the completion of PIFS. Later B frame is sent. B frame will not allow the other users to access . Only AP is allowed. AP will wait for shortest interference space (SIFS), then starts polling.   Polling - communications technique that determines when a terminal is ready to send data . A contention-based protocol (CBP) is a communications protocol for operating wireless telecommunication equipment that allows many users to use the same radio channel without pre-coordination. The "listen before talk" operating procedure in IEEE 802.11 is the most well known contention-based protocol . Contention-free service uses a centralized access control method. Access to the medium is restricted by the point coordinator, a specialized function implemented in access points. Associated stations can transmit data only when they are allowed to do so by the point coordinator.

6. Frame Control-FC

7. Indicates the current version of the protocol used. Types of information to be sent. 3 types are Management Frame Control Frame Data Frame Subtype – each of frame type is considered such as management, control and data frames. DS- Distributed Service, provides the address of DS More frag(1 bit)-fragmentation is done and the information is off 1 bit. Retry flag (0 or 1)- retransmission is done when it is 1 bit. Power Management – 1- the station is in power mode. More data- communication has not ended. WEP – Wired Equivalent privacy(Implementation of encryption) Reserved

8. D=Duration of network Address are dependent on (to and From)DS SC-sequence control (first 4 bits – fragment no and last 12 bits are sequence no) Frame body- actual data information from type and subtype. FCS=Frame correction =CRC 32 error detection sequence.

9. If 0,0 means the communication is not outside(with the help of DS). It is within BSS. If 0,1, the communication from the AP to BSS Address1 – address of the next device Address 2- addr of the previous device that the frame has left. Address3 – Final destination which is not defined in add1 and add2. Address 4- original source

10. Station A -> signal is given to both station B and C. When station A is communicating with station B, station C also receives the data. But if station C communicates then it occurs collision during A->B. But station C communicating to D (no issue).

11. FSK-Frequency Shift Key

12. PSK-Phase Shift Key

13. https://www.youtube.com/watch?v=Rp23mjuzBQQ