Virtual Time Carrier Sensed Multiple Access (VT-CSMA)

Virtual Time Carrier Sensed Multiple Access (VTCSMA) is used mostly in hard real-time system. It is used in transferring signals simultaneously and efficiently in a real-time system for avoiding a collision. In a real-time system, CSMA can not be used but Virtual Time CSMA can be used for signal transfer. 

Every node in VTCSMA uses 2 types of clocks:

1. Real Clock called as RC: RC shows the real time which is synchronized for all the nodes.
2. Virtual Clock called as VC: VC is different for different nodes in the system.

The rate of VC is greater than the RC.

• If a node senses the communication channel to be busy, then the Virtual Clock or VC freezes and stops.
• If a node senses the communication channel to be free, the Virtual Clock of VC is reset.

Every node in the system decides a time, VSX(M) (virtual time to start transmission of message M) for the transmission of a message M. If VSX(m) <= VC, then the transmission signal is sent by the node and does not transfer otherwise. 

If many nodes transfer the message signal at the same time, then collision may occur. If a collision occurs while transferring the signal, then the next signal is transmitted using the probability of p or (1-p) after the channel is idle. 

Flow Diagram of Virtual Carrier Sensed Multiple Access:

The basic idea of using Virtual Time CSMA instead of CSMA is to incorporate the priority to the nodes or devices in the real-time system. Global ordering of priorities can be achieved using a priority function of the current time RC and VC and some other parameters, which makes Virtual Time CSMA better and efficient as compared to that of CSMA. 

VTCSMA is generally used in the Broadcast network.