Why is Vulnerable Time Equal to 2*Tp Where Tp is Transmission Time?

Answer: Collision of one node with another node could be possible if any node had started transmission in Tp time before that node’s transmission or any node would start transmission in Tp time after that node’s transmission. Hence Vulnerable time is 2*Tp.

Pure-ALOHA is one of the basic Random Access Protocols. Here any device can start transmitting at any point of time. So if there is no such rule for transmission then there will be a high chance of collision.

1. Transmission Time: It is the time required to transmit any data.

2. Vulnerable Time: It is the time in which any other node will start transmission then collision will happen. When any node starts transmitting data then these are two possible cases in which collision can happen:

• A node starts transmitting and it takes Tp(Transmission time) to send that data. Before completion of its transmission, any other node will start transmission then collision will happen. So suppose it starts transmitting at time x. So in time x to x+Tp if any other node starts transmission then the collision will occurred.
• A node starts transmitting but before starting of its transmission, if any node already started transmission then also collision will happen. Suppose our node will start transmission at x. If any node in time x-Tp to x had started transmission then also collision will happen.

So if our node transmits at x then any node transmits in between x-Tp to x the there will be collision and if any node transmits between x to x+Tp then there will be also collision.

So Vulnerable time will be = (x-Tp to x) + (x to x+Tp) = Tp + Tp= 2*Tp