Mac mode of communication and Inter-frame space

MAC modes of communication:

• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)

Distributed Coordination Function (DCF)

• Based on CSMA/CA
• This simply means that each station is responsible for coordinating the sending of its data.    
• Contention based same as Ethernet.
• All stations are equal
• Listen first before send .
• Send when no other station transmitting
• Begin to transmit data after a random back-off time (contention)
• If some station is transmitting then wait politely.
• DCF may use the CTS/RTS clearing technique to further reduce the possibility of collisions. 
• There are no Quality of Service (QoS) guarantees.

Backoff timer: 

To listen, the client chooses a random number and begins a countdown process, this process is called a backoff timer. 

Network allocation vector (NAV) 
 : 

is a reservation of the medium that includes the amount of time to send its frame, wait for the SIFS, and then receive an ACK from the AP.  (Data Send time + SIFS + ACK From AP time)

Contention window :
The total time that Station waits before sending is called the Contention window    

Point Coordination Function (PCF)

• which means the AP is responsible for coordination of data transmission. 
• Time is split into a contention-free period and a contention period
• During the contention-free polling periods, a station transmits data under the control of the AP.
• Supports time-bounded services. (Can give priority based on requirement)
• Coordinated by Point Coordinator (PC), typically collocated with the AP.
• PCF has higher priority than the DCF.
• The PCF is not widely implemented.
• PCF is restricted to infrastructure networks.
• Allows for a better management of QoS

IFS (Interframe space):
IFS is a period that a station has to wait before it can send. IFS ensure that the medium is clear, it ensures that frames are not sent so close together that they are misinterpreted.

Types :
1.SIFS - Short Interframe Space : For highest priority frames and mostly used for ACKs
2.Distributed-coordination interframe space (DIFS) : Used for data frames and is the normal spacing between frames
3.Point-coordination interframe space (PIFS) : Used when an AP is going to control the network. (Medium Priority time –critical frames)
4.RIFS were introduced with 802.11n to improve efficiency for transmissions (The length is always 2 microseconds)
5.AIFS - Arbitration Interframe Space : Used by APs and STAs that support 802.11e QoS