Virtual channel sensing using CSMA/CA

Virtual channel sense or virtual carrier sense is a mechanism to predict future traffic in wireless networks that uses carrier sense multiple access with collision avoidance (CSMA/CA). It is implemented in wireless network protocols, IEEE 802.11 and IEEE 802.16, which operates in the medium access control (MAC) layer.

In virtual channel sensing, a timer mechanism is used that is based upon information of durations of previous frame transmission in order to predict future traffic in the channel. It uses network allocation vector (NAV), which can be considered as a counter that counts down to zero.

Technique

Virtual channel sensing mechanism initialises a NAV for implementing timer mechanism. The maximum NAV duration is the transmission time required by frame, which is the the time for which the channel will be busy. At the start of transmission of a frame, the NAV value is set to its maximum. A non-zero value indicates that the channel is busy, and so no station contends for it. When the NAV value decrements to 0, it indicates that the channel is free and the other stations can contend for it.

He following diagram shows virtual channel sensing using CSMA/CA −

The steps in transmission as depicted in the above diagram are −

• The transmitting station waits for a time equal to distributed inter – frame space (DIFS) and issues a request to send (RTS) if the channel is clear.
• After sending RTS, a NAV (RTS) is initialized, so that no other station attempts to transmit.
• The receiving station waits for a short inter – frame space (SIFS) and issues a clear to send (CTS).
• With the CTS, a NAV (CTS) is initialized. The sender waits for a SIFS and transmits its data frame.
• On receiving the data frame, the receiver waits for a SIFS and sends an acknowledgement frame (ACK).
• Both the NAV values decrements to 0 during this time period.
• The stations wait for a SIFS and a backoff period before contending for the channel.