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Back Computer Computer Engineering Dictionary Computer Networks Multiple Access Protocols - Pure Aloha vs Slotted Aloha & Throughput

Multiple Access Protocols - Pure Aloha vs Slotted Aloha & Throughput

MAC Aloha

For accessing a channel we need to know some rules. These rules are used to access a channel as they serve as a communication source and facilitates data transfer and identification of device. Many algorithmic variations are possible for channel access. The following is a short list.

  1. ALOHA
  2. Carrier Sense Multiple Access Protocols
  3. Collision-Free Protocols
  4. Wireless LAN Protocols

ALOHA :

Aloha is a computer networking system which was introduced in the early 1970 by Norman Abramson and his colleagues at university of Hawaii to solve the channel allocation problem. On the basis of global time synchronization. Aloha is divided into two different versions or protocols. i.e Pure Aloha and Slotted Aloha.

Pure Aloha:

Pure Aloha does not require global time synchronization. The basic idea of pure aloha system is that it allows its users to transmit whenever they have data.A sender just like other users can listen to what it is transmitting, and due to this feedback broadcasting system is able to detect collision, if any. If the collision is detected the sender will wait a random period of time and attempt transmission again. The waiting time must not be the same or the same frames will collide and destroyed over and over. Systems in which multiple users share a common channel in a way that can lead to conflicts are widely known as contention systems.

Efficiency of Pure Aloha :

Let "T" be the time needed to transmit one frame on the channel, and "frame-time" as a unit of time equal to T. Let "G" refer to the mean used in the Poisson distribution over transmission-attempt amounts that is, on average, there are G transmission-attempts per frame-time. Let "t" be the time at which the sender wants to send a frame. We want to use the channel for one frame-time beginning at t, and so we need all other stations to refrain from transmitting during this time. Moreover, we need the other stations to refrain from transmitting between t-T and t as well, because a frame sent during this interval would overlap with our frame.EFFICIENCY OF MAC ALOHA Vulnerable period for the shaded frame is 2t, if t is the frame time. A frame will not collide if no other frames are sent within one frame time of its start, before and after. For any frame-time, the probability of there being k transmission-attempts during that frame-time is: {G^k e^{-G}} / {k!} If throughput (number of packets per unit time) is represented by S, under all load, S =GPo, where Po is the probability that the frame does not suffer collision. A frame does not have collision if no frames are send during the frame time. Thus, in t time Po=(e)^(-G). In 2t time Po=e^(-2G), as mean number of frames generated in 2t is 2G. From the above, throughput in 2t time S=G*(Po)=G*e^(-2G)

Slotted Aloha Channel :

Slotted Aloha does require global time synchronization.

Efficiency of Slotted Aloha Channel :

Assume that the sending stations has to wait until the beginning of a frame time (one frame time is one time slot) and arrivals still follow Poisson Distribution, where they are assumed probabilistically independent: In this case the vulnerable period is just t time units. Then the Probability that k frames are generated in a frame time is effective:-

Pk=(G^k)*(e^-G)/k! In t time, the probability of zero frames, Po=e^(-G) From the above throughput becomes:

S=GPo=G*(e^-G)

Comparison Of Pure Aloha And Slotted Aloha:

Pure Aloha vs Slotted Aloha

Throughput versus offered traffic for pure ALOHA and slotted ALOHA systems, ie, plot of S against G, from S=Ge^(-2G) and S=Ge^(-G) formulas.

CSMA: CSMA is a set of rules in which the devices attached to a network first determines whether the channel or carrier is in use or free and then act accordingly. As in this MAC protocol,the network devices or nodes before transmission senses the channel,therefore, this protocol is known as carrier sense multiple access protocol. Multiple Access indicates that many devices can connect to and share the same network and if a node transmits anything, it is heard by all the stations on the network.

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