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.

Frequently Asked Questions on Pure ALOHA – FAQs

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 full form of ALOHA?

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

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).

What is the use of ALOHA?

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

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

Difference between Pure aloha and Slotted aloha

In this article, we will discuss the comparison between Pure aloha and Slotted aloha along with their separate discussion. Aloha is the random access protocol having two categories that are pure aloha and slotted aloha.

Pure aloha is used whenever data is available for sending over a channel at stations, whereas slotted aloha is designed to overcome the problem of pure aloha because there is a high possibility of frame hitting in pure aloha. Similarly, we will see the comparison chart between pure aloha and slotted aloha. So, without any delay, let's start the topic.

Before discussing the types of aloha, let's first see a brief description of aloha.

Aloha is designed for wireless LAN (Local Area Network) but can also be used in a shared medium to transmit data. In aloha, any station can transmit data to a channel at any time. It does not require any carrier sensing.

Pure aloha is used when data is available for sending over a channel at stations. In pure Aloha, when each station transmits data to a channel without checking whether the channel is idle or not, the chances of collision may occur, and the data frame can be lost.

When a station transmits the data frame to a channel without checking whether the channel is free or not, there will be a possibility of the collision of data frames. Station expects the acknowledgement from the receiver, and if the acknowledgement of the frame is received at the specified time, then it will be OK; otherwise, the station assumes that the frame is destroyed. Then station waits for a random amount of time, and after that, it retransmits the frame until all the data are successfully transmitted to the receiver.

There is a high possibility of frame hitting in pure aloha, so slotted aloha is designed to overcome it. Unlike pure aloha, slotted aloha does not allow the transmission of data whenever the station wants to send it.

In slotted Aloha, the shared channel is divided into a fixed time interval called slots. So that, if a station wants to send a frame to a shared channel, the frame can only be sent at the beginning of the slot, and only one frame is allowed to be sent to each slot. If the station is failed to send the data, it has to wait until the next slot.

However, there is still a possibility of a collision because suppose if two stations try to send a frame at the beginning of the time slot.

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:

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.

Get certified by completing the course

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