Notes-Bianchi‘s Model for 802.11 Wireless LAN(DCF)

Reference Analysis-Bianchi model for IEEE 802.11

Preliminary

Bianchi Model is based on the Bi-dimensional Markov Chain and is used for performance evaluation of DCF in 802.11. 802.11 is one Medium Access Control(MAC) protocol for Wireless LAN,  DCF is its fundamental function, in which the Binary Exponential Backoff(BEB) rules and CSMA/CA based Access techniques are included. 

• BEB algorithm

BEB algorithm is used for avoiding collision when multi-users access to the same wireless channel. Within this algorithm, when the user detect the busy channel, it will delay some exponential backoff times before it transmits next time. The Backoff Counter 'W' is adapted for the every exponential backoff time, i.e., the backoff time is relected randomly between 0 to 2^W. The Backoff Counter 'W' is variable in a range[Wmin,Wmax], where m*Wmin=Wmax, the W will increase to its  double size when the user still detect the busy channel following a busy channel detection, otherwise it will decrease to the minimize size of W. 

• CSMA/CA

Carrier Sensing and Multiple Access and Collision Avoiding(CSMA/CA) used two access techniques. Basic mechanism and RTS/CTS mechanism. 

Basic mechanism:

RTS/CTS mechanism

2 mechanism in 802.11

If a user want to send uni-cast packets to AP, it must apart to the medium access process(competition for medium), in which two access mechanisms are applied. 

• Basic mechanism

1. Sender has to wait for at least DIFS(SIFS,PIFS) before sending data, after finishing that, if the channel is still busy, the BEB follows this process. 
2. Receiver acknowledged at once (after waiting for SIFS) if the packet was received correctly CRC.
3. Sender retransmits data packets in case of transmission errors.
4. Other stations defer access. 

Noted that sender can transmit data immediately without waiting for DIFS once the backoff counter reached 0. Immediate access when medium is free.

• RTS/CTS mechanism

1. Sender can send  RTS with reservation parameter after waiting for >=DIFS(reservation determines amount of time the data packet needs the medium).
2. Receiver acknowledgement via CTS after SIFS(if ready to receive).
3. Sender can now send data at once, acknowledgement via ACK.