MAC Random Access- Aloha

Today, we focus on Random Access ALOHA protocols for meeting access control. We saw how important delay-bandwidth product is in the performance and management of the ARQ protocols as well as in immediate access control protocols. In case of the scheduling approach to media access control, if the reaction time is large by the time scheduling takes effect, the state of the system might have changed significantly. Random access has no scheduling overhead. Random access, media access control schemes assume a uniform frame size. ALOHA is a first random access, media access control protocol. It was originated as University of Hawaii needed a means to interconnect terminals at campuses located in different islands to the host computer at the main campus. It is a simple solution. A station transmits whenever it has data to transmit. If more than one frames are transmitted in overlapping manner, they collide with each other and therefore, all are lost. If the acknowledgement had not received within the timeout, ALOHA scheme recalls station to use a backoff algorithm to retransmit which choses a random number in a certain retransmission time interval. So randomization is intended to reduce the likelihood of additional collisions between the station retransmissions. The first frame transmission is done without any scheduling. Note that information about the outcome of the transmission is obtained at the earliest after the round trip time or two propagation delays. Let's take a look at the ALOHA performance model. We assume all frames have same lengths and the same transmission time X. Consider a reference frame that is transmitted starting at a time t_0 and completed the time t_0 plus X, this frame will be transmitted successfully if no other frame collide with it. Any frame that begins its transmission during the interval t_0 to t_0 plus X will collide with this reference frame. Further, any frame that begins its transmission in the prior X seconds will also collide with the reference frame. Therefore, the probability of a successful transmission is a probability but that there are no additional frame transmissions in the vulnerable period which is two X seconds. The system's throughput is a system workload in terms of number of transmission attempts per unit of time, multiply the probability of frame transmission is successful. Abramson used an approach to find the probability that there is no collision with the reference frame. The assumption is that effect or backoff scheme is that frame arrivals are equally likely to occur at any time interval. He derived the probability of a successful frame transmission approximately as e_power_negative_2G where G is the average number of frame arrivals per frame transmission time. The throughput is equal to the total arrival rate G times the probability of a successful transmission. The performance of ALOHA scheme can be improved by reducing the probability of collisions. If we reduce the vulnerable period, we can r