Similarly, if any other station transmits a frame between the time interval t 0+t and t 0+2t again, it will result in a garbage frame due to collision with the reference frame. If, any other station sends a frame between t 0 and t 0+t then the end of the frame will collide with that earlier sent frame. Let a frame is that transmitted at time t 0 and t be the time required for its transmission. We will explain this with the help of the concept of vulnerable period as shown in Figure. Hence, to present a frame from colliding, no other frame should be sent within its transmission time. Pure ALOHAДЄs in the case of Pure ALOHA protocol frames can be sent any time so, the probability of collision will be very high. Here, we are assuming that the size of the frame will be constant, and hence, it will take constant time t= L/R for transmission of each packet. Let R be the bit rate of the transmission channel and L be the length of the frame. And if, after a number of repeats if it does receive its own pocket then the station gives up and stops retransmitting the same frame. If the sender station does not receive its own frame during this time limit then, it retransmits this frame by using the Backoff algorithm that we will discuss later on. Once one station sends a frame (when it receives its own frame and assumes that the destination has received it) after 2 times the maximum propagation time. Here, no station is constrained any station that has data /frame to transmit can transmit at any time. So, if two stations transmit a frame at an overlapping time then, collision will occur in the system. Here, the medium is shared between the stations. If, any of the stations want to transmit data among themselves, then, the station sends the data first to the central station, which broadcasts it to all the stations. In an ALOHA network, one station will work as the central controller, and the other station will be connected to the central station. Station accesses the channel and when their frames are ready. But, in case of allocating a single channel among N uncoordinated competing users, then the probability of collision will be high.

For the same, we discussed some retransmission strategies to deal with the error. If one node sends a frame to another node, there can be some error in the frame. In Slotted ALOHA, slots have been made, so that every frame transmission starts at the beginning of the slot and throughput is increased by a factor of 2. In Pure ALOHA no slotting was done but the efficiency was poor. There are two versions of ALOHA that is Pure ALOHA and Slotted ALOHA.

Explain Back off Algorithm and give one example of where it is used.ДЇor a large number of users the ALOHA protocol is considered.

How does Slotted ALOHA improve the performance of the system over Pure ALOHA?.

Download pure aloha program in c