Pure Aloha Adalah

Maximum Throughput of Pure ALOHA

The maximum throughput occurs when G=0.5

Smax=0.5 × e-1 ≈ 0.184

This means the maximum throughput of Pure ALOHA is approximately 18.4%. In other words, only about 18.4% of the time is used for successful transmissions, and the rest is lost due to collisions.

Throughput of Pure ALOHA

Let 𝑇 be the frame time, i.e. the time required for 1 frame to be transmitted.

Let G be the number of transmission attempts per frame time.

The probability that 𝑘 frames are generated during the frame time is given by the Poisson distribution−

$$P(k)=\frac{G^k e^{-G}}{k!}$$

From this we can say that the probability that 0 frames are generated ( 𝑘k = 0 ) during the frame time is $e^{-G}$.

In case of pure ALOHA, the vulnerable time period so that collision does not occur between two frames is equal to two frame times, i.e. 2T𝑇. In 2T time,average number of transmission attempts is 2G.

The probability that 0 frames are initiated in the vulnerable time period will be −

The throughput, 𝑆, is calculated as the number of transmission attempts per frame time, 𝐺, multiplied by the probability of success, 𝑃(0).

Key Features of Pure ALOHA

For more details please refer Derive the efficiency of Pure ALOHA protocol and Differences between Pure and Slotted Aloha article.

Pure ALOHA is an early and simple method for sending data over a shared network, where devices transmit whenever they have data to send. It doesn’t check if the channel is free, leading to frequent collisions when two devices send data at the same time. This results in a maximum efficiency of about 18.4%, meaning many transmissions are lost due to these collisions.

What is the use of ALOHA?

ALOHA is used to transmit data over a public network channel.

What is the full form of ALOHA?

The full form of ALOHA is Advocates of Linux Open-source Hawaii Association.

ALOHA is a medium access control (MAC) protocol for transmission of data via a shared network channel. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel. There are two types of ALOHA protocols – Pure ALOHA and Slotted ALOHA.

In pure ALOHA, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.

After transmitting a frame, a station waits for a finite period of time to receive an acknowledgement. If the acknowledgement is not received within this time,the station assumes that the frame has been destroyed due to collision and resends the frame.

A collision occurs if more than one frame tries to occupy the channel at the same time. The situation is depicted in the following diagram−

What is ALOHA and its type?

ALOHA is a basic method for sending data over a shared network. It was created to manage how data is transmitted and to deal with collisions when two devices send data at the same time.

There are two types of ALOHA:

Offered load and throughput calculations

Using NetSim, the attempts per packet time (G) can be calculated as follows.

$$\ \ G = \ \ \frac{Number\ of\ packets\ transmitted \times PacketTime(s)\ }{SimulationTime\ (s)}$$

where, G is Attempts per packet time. We derive the above formula keeping in mind that (i) NetSim's output metric, the number of packets transmitted, is nothing but the number of attempts, and (ii) since packets transmitted is computed over the entire simulation time, the number of "packet times" would be $\frac{SimulationTime(s)}{PacketTransmissionTime(s)}$ , which is in the denominator. Note that in NetSim the output metric Packets transmitted is counted at link (PHY layer) level. Hence MAC layer re-tries are factored into this metric.

The throughput (in Mbps) per packet time can be obtained as follows.

$$\ \ S = \ \ \ \ \frac{Number\ of\ packets\ successful \times PacketTime(s)}{SimulationTime\ (s)}$$

where, S = Throughput per packet time. In case of slotted aloha packet (transmission) time is equal to slot length (time). The packet transmission time is the PHY layer packet size in bits divided by the PHY rate in bits/s. Considering the PHY layer packet size as 1500B, and the PHY rate as 10 Mbps, the packet transmission time (or packet time) would be $\frac{1500 \times 8}{10 \times 10^{6}} = 1200\ \mu s$.

In the following experiment, we have taken packet size as 1460 B (Data Size) plus 28 B (Overheads) which equals 1488 B. The PHY data rate is 10 Mbps and hence packet time is equal to 1.2 milliseconds.

Open NetSim and click on Experiments> Legacy Networks> Throughput versus load for Pure and Slotted Aloha> Pure Aloha then click on the tile in the middle panel to load the example as shown in below Figure 15‑1.

Figure 15‑1: List of scenarios for the example of Throughput versus load for Pure and Slotted Aloha

NetSim UI displays the configuration file corresponding to this experiment as shown below Figure 15‑2.

Figure 15‑2: Network set up for studying the Pure aloha

Pure Aloha: Input for 10-Nodes sample

Step 1: Drop 10 nodes (i.e. 9 Nodes are generating traffic.)

Node 2, 3, 4, 5, 6, 7, 8, 9, and 10 generates traffic. The properties of Nodes 2, 3, 4, 5, 6, 7, 8, 9, and 10 which transmits data to Node 1 are given in the below table.

Step 2: Wireless Node Properties

Wireless Node Properties

Interface1_Wireless (PHYSICAL_LAYER)

Data Rate (Mbps) 10

Interface1_Wireless (DATALINK_LAYER)

Retry_Limit 0

MAC_Buffer FALSE

Slot Length(µs) 1200

Table 15‑1: Wireless Node Properties

(Note: Slot Length(µs) parameter present only in Slotted Aloha Wireless Node Properties Interface_1 (Wireless))

Step 3: In Adhoc Link Properties, channel characteristics is set as No Path Loss.

Step 4: Application Properties

Application_1 Properties

Application Method Unicast

Application Type Custom

Source_Id 2

Destination_Id 1

Transport Protocol UDP

Packet Size Distribution Constant

Inter Arrival Time Distribution Exponential

Table 15‑2: For Application_1 Properties

Step 5: Plots are enabled in NetSim GUI.

Step 6: Simulation Time- 10 Seconds

Note: Obtain the values of Total Number of Packets Transmitted and Collided from the results window of NetSim.

Input for 20-Nodes sample

Step 1: Drop 20 nodes (i.e., 19 Nodes are generating traffic.)

Nodes 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 transmit data to Node 1.

Continue the experiment by increasing the number of nodes generating traffic as 29, 39, 49, 59, 69, 79, 89, 99, 109, 119, 129, 139, 149, 159, 169, 179, 189 and 199 nodes.

Slotted ALOHA: Input for 10-Nodes sample

Step 1: Drop 20 nodes (i.e., 19 Nodes are generating traffic.)

Nodes 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 transmit data to Node 1 and set properties for nodes and application as mentioned above.

Continue the experiment by increasing the number of nodes generating traffic as 39, 59, 79, 99, 119, 139, 159, 179, 199, 219, 239, 259, 279, 299, 319, 339, 359, 379, and 399 nodes.

Comparison Table: The values of Total Number of Packets Transmitted and Collided obtained from the network statistics after running NetSim simulation are provided in the table below along with Throughput per packet time& Number of Packets Transmitted per packet time.

+--------+----------+-------+---------+---------+----------+---------+ | Number | Total | Total | Suc | A | Th | Thr | | of | number | n | cessful | ttempts | roughput | oughput | | nodes | of | umber | Packets | per | per | per | | gene | Packets | of | ( | packet | packet | packet | | rating | Tra | Pa | Packets | time(G) | time(S) | time. | | t | nsmitted | ckets | Trans | | | Theo | | raffic | | Col | mitted- | | | retical | | | | lided | Packets | | | | | | | | Co | | | (S | | | | | llided) | | | =$\math | | | | | | | | bf{\ }\ | | | | | | | | mathbf{ | | | | | | | | G}\math | | | | | | | | bf{*}\m | | | | | | | | athbf{e | | | | | | | | }^{\mat | | | | | | | | hbf{- 2 | | | | | | | | }\mathb | | | | | | | | f{G}}$) | +========+==========+=======+=========+=========+==========+=========+ | 9 | 494 | 60 | 434 | 0.05928 | 0.05208 | 0.05265 | +--------+----------+-------+---------+---------+----------+---------+ | 19 | 978 | 187 | 791 | 0.11736 | 0.09492 | 0.09281 | +--------+----------+-------+---------+---------+----------+---------+ | 29 | 1482 | 415 | 1067 | 0.17784 | 0.12804 | 0.12461 | +--------+----------+-------+---------+---------+----------+---------+ | 39 | 1991 | 700 | 1291 | 0.23892 | 0.15492 | 0.14816 | +--------+----------+-------+---------+---------+----------+---------+ | 49 | 2443 | 1056 | 1387 | 0.29316 | 0.16644 | 0.16311 | +--------+----------+-------+---------+---------+----------+---------+ | 59 | 2907 | 1429 | 1478 | 0.34884 | 0.17736 | 0.17363 | +--------+----------+-------+---------+---------+----------+---------+ | 69 | 3434 | 1874 | 1560 | 0.4122 | 0.19212 | 0.18075 | +--------+----------+-------+---------+---------+----------+---------+ | 79 | 3964 | 2377 | 1587 | 0.47568 | 0.19044 | 0.18371 | +--------+----------+-------+---------+---------+----------+---------+ | 89 | 4468 | 2909 | 1559 | 0.53616 | 0.18792 | 0.18348 | +--------+----------+-------+---------+---------+----------+---------+ | 99 | 4998 | 3468 | 1530 | 0.59976 | 0.1836 | 0.18073 | +--------+----------+-------+---------+---------+----------+---------+ | 109 | 5538 | 4073 | 1465 | 0.66456 | 0.1758 | 0.17592 | +--------+----------+-------+---------+---------+----------+---------+ | 119 | 6023 | 4574 | 1449 | 0.72276 | 0.17388 | 0.1703 | +--------+----------+-------+---------+---------+----------+---------+ | 129 | 6503 | 5102 | 1401 | 0.78036 | 0.16812 | 0.16386 | +--------+----------+-------+---------+---------+----------+---------+ | 139 | 6992 | 5650 | 1342 | 0.83904 | 0.16104 | 0.15668 | +--------+----------+-------+---------+---------+----------+---------+ | 149 | 7481 | 6208 | 1273 | 0.89772 | 0.15276 | 0.14907 | +--------+----------+-------+---------+---------+----------+---------+ | 159 | 7998 | 6787 | 1211 | 0.95976 | 0.14532 | 0.14078 | +--------+----------+-------+---------+---------+----------+---------+ | 169 | 8507 | 7341 | 1166 | 1.02084 | 0.13992 | 0.13252 | +--------+----------+-------+---------+---------+----------+---------+ | 179 | 9008 | 7924 | 1084 | 1.08096 | 0.13008 | 0.12442 | +--------+----------+-------+---------+---------+----------+---------+ | 189 | 9486 | 8483 | 1003 | 1.13832 | 0.12036 | 0.11682 | +--------+----------+-------+---------+---------+----------+---------+ | 199 | 10025 | 9093 | 932 | 1.203 | 0.11184 | 0.10848 | +--------+----------+-------+---------+---------+----------+---------+

Table 15‑3: Total No. of Packets Transmitted, Collided, Attempts per packet time and throughput per packet time for Pure Aloha.

Table 15‑4: Total No. of Packets Transmitted, Collided, Throughput per packet time and throughput per packet time for Slotted Aloha

Thus, the following characteristic plot for the Pure ALOHA and Slotted ALOHA is obtained, which matches the theoretical result.

Figure 15‑3: Throughput vs offered load for pure Aloha

Figure 15‑4: Throughput vs. Offered load for Slotted Aloha

Maximum Throughput of Pure ALOHA

The maximum throughput occurs when G𝐺=0.5.

The maximum throughput is thus−

$S_{max}$𝑎𝑥 = $0.5𝑒e^{-2*0.5}$ = $\frac{1}{2e}= 0.184$

Thus, it can be seen that the maximum throughput is only 18.4% in pure ALOHA.

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The ALOHA protocol was first developed at the University of Hawaii in the early 1970s for packet radio networks. However, it can be used in any situation where multiple devices share a common communication channel. This protocol allows devices to transmit data at any time, without a set schedule. This is known as a random access technique, and it is asynchronous because there is no coordination between devices. When multiple devices attempt to transmit data at the same time, it can result in a collision, where the data becomes garbled. In this case, each device will simply wait a random amount of time before attempting to transmit again. The basic concept of the ALOHA protocol can be applied to any system where uncoordinated users are competing for the use of a shared channel.

ALOHA is an early computer networking method created at the University of Hawaii in the early 1970s. It’s a straightforward way to send data over a shared medium, like a wireless or wired network. The main idea of ALOHA is how it handles collisions, which happen when two devices try to send data at the same time, causing interference.

There are two main versions of the ALOHA protocol:

In this article we going to discuss pure ALOHA in detail.

Vulnerable period for packet A

Throughput of Pure ALOHA

The probability of successful transmission (S) can be derived from the probability that no other packets are sent during the vulnerable time period. This is given by: