Derive the efficiency of Pure ALOHA protocol

The Word Aloha generally means Hello in the Japanese Language. It works under the MAC layer of the OSI model. Multiple access protocol is a set of protocols that are used for avoiding cross-talks and to have an efficient transmission. When the data is transmitted between hosts that do not have any particular channel, then there may be high chances of data collision and crosstalk. In that case, ALOHA which is a Random Access Protocol is used to get more efficiency.

ALOHA in Computer System

ALOHA is a multiple-access protocol. Using this protocol, any number of data streams from different nodes is sent through the transmission channel. In ALOHA protocol, each data is sent without checking whether the transmission medium is busy or not. If more than two data try to occupy the same channel, then a collision occurs and the process stops and is retransmitted to the host.

Guidelines for ALOHA protocol

Any Host Station can send data to the transmission channel at any time. By using multiple channels, the collision and loss of data packets happen very easily. The Time taken for data initiation which is the carrier sense is nil, as the data are transmitted without any checking process. Collision detection is also not available in this protocol and the frame retransmits after a particular period.

Types of Aloha

There are namely two types of ALOHA protocols,

1. Slotted ALOHA

Like the ALOHA protocol, any station can send the data to the Transmission channel but only for the slotted time. So it follows discrete time and is globally synchronized.

2. Pure ALOHA

When the host wants data transmission in the station, it is permitted at any time. After the data is transmitted once, the station waits for some time to get the acknowledgment from the receiver end. This method is followed to avoid the collision of data. There may be two cases during the data transmission,

• Data Received − It waits for the response from the receiver for the given time, if it is not received and again it waits for a random time called the back off time to get the response.

• Data Lost − If still the acknowledgment is not received, the station considers that the data packet may be lost and again it retransmits the data packets.

Back off Time of Pure ALOHA

This strategy plays a vital role in the Pure ALOHA protocol. When the data packet acknowledgment is not received, the station waits for some more random time, or the waiting time is called the Back off strategy.

Back off Time (Tb) = k * RTT (Round Trip Time)

Where k is the number chosen by the station and one RTT equals the one-time slot.

Vulnerable Time of pure ALOHA

Vulnerable time is the time in which the collision of data packets occurs. If the first frame ‘A’ is sent at any particular time t, before the data is transmitted completely and the other frame ‘B’ starts before the completion of frame ‘A’ will lead to a collision.

The vulnerable time when the collision occurs (Vt) = 2 * Tt

Where Tt is the Transmission time of the frame.

Vulnerable time is also referred to as the propagation time or total time taken to transmit the complete data packet from the station.

$$\mathrm{Vulnerable\:time\:=\:\frac{(Message\:Length)}{(Transmission\:channel\:bandwidth)}}$$

Channel Throughput of Pure ALOHA

Here ‘G’ is the average number of frames transmitted through the channel during a period of one frame transmission. When the data packet is successfully or completely transmitted to the receiver end, the probability of the data packet is given below,

Maximum Efficiency

To find the maximum efficiency of the station during the data packet transmission, let us take the value of an average number of frames transmitted to be 1 and 1/2.

Throughput of Successful pure ALOHA or Efficiency, $\mathrm{S\:=\:G\:*e^{-2G}}$

To remove the exponential, differentiating on both sides concerning G,

$$\mathrm{\frac{\partial\:S}{\partial\:G}\:=1.(0)(e^{-2G})}$$

$$\mathrm{\frac{\partial\:S}{\partial\:G}\:=\:0}$$

So, the maximum efficiency occurs when G=1/2 and substituting the G value in the throughput equation.

$$\mathrm{S\:=\:\frac{1}{2}\:*e^{-2(\frac{1}{2})}}$$ [Since the value of e^(-1) is 0.367879]

$$\mathrm{S\:=\:\frac{1}{2}\:*e^{(-1)}}$$

$$\mathrm{S\:=\:\frac{1}{2}\:*\:0.367879}$$

$$\mathrm{S= 0.1839395}$$

The Efficiency of Pure ALOHA in percentage is 18.39% and is very less due to the number of collisions.

Conclusion

In computer networks, Access control is the process of controlling the stations during transmission. ALOHA is a Random access protocol composed of two types. From that pure ALOHA has more advantages over slotted ALOHA due to its simplicity and implementation. The number of collisions of data packets is less in slotted ALOHA and more in pure ALOHA.