Curt Tanner posted an update 3 days, 16 hours ago
? At time stamp 0, which is vehicle x sets its LI to 9. Other vehicles in the remaining orbitals are still in steady state hence they ask1 inhibitor have an arbitrary value of LI.? At 100 ms the vehicles in first orbital (1 hop) receive the message and set their LI to 9.? At 200 ms enters state and decrements its LI to 7. However, is still in state and also enters the same state by setting its LI to 9. This is the most crucial part of CAVI-MAC. Observe that at time stamp 200 ms switches to SCH after and other orbitals to also switch to SCH after their specified . In this case, and get exclusive access for a fraction of time on CCH i.e., . During this time there is no interference from its one hop neighbours () and also hidden terminals (). Hence, and get a fraction of time in CCH which is virtually an interference free environment. This fraction of time is sufficient for transmitting event driven messages from to .? Now at 300 ms, sets its LI to 5 and has spent 200 ms in . Hence moves to state and sets its LI to 7. Now, enters state so and now experience virtual interference free environment for a fraction of time in CCH.? At 600 ms completes the state and moves to and this gets repeated for all other orbitals.Table 3. (Length Index, State) of orbitals at various time stamps.t (ms) 0 () () () () () ()100 () () () () () ()200 () () () () () ()300 () () () () () ()400 () () () () () ()500 () () () () () ()600 () () () () () ()At every 100 ms from the time an event occurs, a set of adjacent orbitals experience a virtual interference free environment. In such a scenario the vehicles in inner orbitals and outer orbitals will switch to SCH. This helps in propagation of event driven messages in a deterministic way. At time stamp 500 ms, there is a guarantee that vehicles in the outer most orbitalare aware of the event. A typical vehicle transmission range of 250–500 m will have a five hop length of 1.75–2.5 km in both the directions as shown in Fig. 7. Typically, hop bound in case of VANETs is considered to be five. In order to consider five hops the statedecrements the value of LI by 2 and not by 1.5. Analytical modelLet L denote the length of a packet and D be the data rate. Time required to send a packet is given by Equation (7) and the amount of time spent in collision is given by Equation (8) where SIFS is Short Inter Frame Space, AIFS is Arbitration Inter Frame Space and EIFS is Error Inter Frame Space as given in Equations (5) and (6) respectively . We assume that events occur at discrete time intervals of time over the specified channel and refer this interval as slot time represented by ρ.(5)?(6)(7)(8)(9)The probabilitywhere and as computed in Equation (9) represents the probability that out of n vehicles and w contention slots, k vehicles transmitting in slot with slots already passed before the first transmission attempt as given in . is modeled as a Bernoulli process with as the probability of uniformly choosing any slot out of available slots. The first term in Equation (9) gives the probability of no transmission in the first slots, the second term is the number of ways we can choose k vehicles out of n vehicles. The third term is the probability of k vehicles choosing uniformly at random the slot for transmission. The final term gives the probability for the remaining vehicles not choosing to transmit in theslot.5.1. Packet success probability ()Letrepresent the set of vehicles that are at j hops from the vehicle. First hop () and second hop () vehicles are prevented from transmission since Golgi complex cause interference. It is safe for vehicles from three hop () distance and further to transmit without causing interference. Thus, at each stage the number of successful transmissions is . Equation (10) represents the success probability of n vehicles transmitting data in w contention slots with t time remaining in CCH as derived from . The first term α gives the probability that among n vehicles, only vehicle and all its three hop neighbours (), choosing backoff from w slots, successfully transmits in the slot. Here, is the mean number of successful transmissions in the remaining slots. The second term β refers to the probability of remaining vehicles incurring a collision in the slot.