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.
What is the formula for Pure Aloha and Slotted Aloha?
Pure Aloha: S = G x e-2G
Slotted Aloha: S = G x e-G
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What is the advantage of Pure Aloha over Slotted Aloha?
The advantage of pure aloha over slotted aloha is that there is no fixed size, and it has the ability to start transmission at any particular time and no need for synchronization.
Differences Between Pure Aloha and Slotted Aloha
In Pure Aloha, the Probability of successful transmission of the data packet
In Slotted Aloha, the Probability of successful transmission of the data packet
In Pure Aloha, Maximum efficiency
In Slotted Aloha, Maximum efficiency
What is Slotted Aloha?
Since the pure aloha possesses a very high possibility of data frame hitting, the slotted aloha is used to overcome this issue. Unlike pure aloha, a slotted aloha would disallow data transmission when the station is willing to send it.
The shared channel in a slotted aloha gets divided into slots. These are basically fixed time intervals. Now, if a station is sending a frame to the destined channel, it can only send the data frame at the beginning of this slot. Added to this, the station can only send a single frame in one slot. Thus, if the station fails to transmit/ send the data frame and the channel on the receiver’s end doesn’t receive it, the sender station HAS TO wait for the next slot to begin.
This method reduces the chances of collision to a great extent, but collision may still occur if two stations are trying to send data frames at the beginning of the very same slot.
Pure aloha v/s slotted aloha
Now, let's see the comparison chart between pure aloha and slotted aloha. We are comparing both terms on the basis of characteristics to make the topic more clear and understandable.
From the above discussion, it can be said that slotted aloha is somewhat better than pure aloha. It is because there is less possibility of collision in slotted aloha.
So, that's all about the article. Hope it will be helpful and informative to you.
Plot the characteristic curve of throughput versus offered traffic for a Pure and Slotted ALOHA system
NOTE: NetSim Academic supports a maximum of 100 nodes and hence this experiment can only be done partially with NetSim Academic. NetSim Standard/Pro would be required to simulate all the configurations.
ALOHA provides a wireless data network. It is a multiple access protocol (this protocol is for allocating a multiple access channel). There are two main versions of ALOHA: pure and slotted. They differ with respect to whether or not time is divided up into discrete slots into which all frames must fit.
In Pure Aloha, users transmit whenever they have data to be sent. There will be collisions and the colliding frames will then be retransmitted. In NetSim's Aloha library, the sender waits a random amount of time per the exponential back-off algorithm and sends it again. The frame is discarded when the number of collisions a packet experiences crosses the the "Retry Limit" - a user settable parameter in the GUI.
Let ''frame time'' denotes the amount of time needed to transmit the standard, fixed-length frame. In this experiment point, we assume that the new frames generated by the stations are modeled by a Poisson distribution with a mean of N frames per frame time. If N > 1, the nodes are generating frames at a higher rate than the channel can handle, and nearly every frame will suffer a collision. For reasonable throughput, we would expect 0 \< N \< 1. In addition to the new frames, the stations also generate retransmissions of frames that previously suffered collisions.
The probability of no other traffic being initiated during the entire vulnerable period is given by$\ e^{- 2G}\ $which leads to $S = \ G \times e^{- 2G}$ where, S is the throughput and G is the offered load. The units of$\ S$ and $G$ is frames per frame time.
G is the mean of the Poisson distribution followed by the transmission attempts per frame time, old and new combined. Old frames mean those frames that have previously suffered collisions.
The maximum throughput occurs at $G = 0.5$, with $S = \frac{1}{2e}$, which is about 0.184. In other words, the best we can hope for is a channel utilization of 18%. This result is not very encouraging, but with everyone transmitting at will, we could hardly have expected a 100% success rate.
In slotted Aloha, time is divided up into discrete intervals, each interval corresponding to one frame. In Slotted Aloha, a node is required to wait for the beginning of the next slot in order to send the next packet. The probability of no other traffic being initiated during the entire vulnerable period is given by $e^{- G}$ which leads to $S = G \times e^{- G}$. It is easy to compute that Slotted Aloha peaks at G = 1, with a throughput of $s = \frac{1}{e}$ or about 0.368.
Differences between Pure and Slotted Aloha
Let us talk about the differences between Pure and Slotted Aloha. To make this topic more understandable and clear, we are comparing both of the terms based on their individual characteristics in a table.
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Pure VS. Slotted Aloha: Know the Differences between Pure and Slotted Aloha
The term Aloha refers to a random access protocol. It has two broad categories- pure and slotted. In this article, we will discuss the various differences between pure aloha and slotted aloha.
We utilise pure aloha when we have data available to be sent over any channel at any station. However, using a pure aloha poses the risk of data frame hitting. The slotted aloha, on the other hand, exists to overcome this problem. Let us discuss more on these two types of aloha. But before we do that, we will know more about what is aloha.
Aloha has basically been designed for WLAN or wireless Local Area Network- but we can also utilise it in a shared medium for data transmission. Multiple stations can easily transmit data and info to any channel in an aloha at any given time. Thus, aloha will never require carrier sensing.
We use pure aloha when we have data and info to be sent over a channel at a station. In the case of a pure aloha, every station performs data transmission to another channel without checking the availability status of the channel. It means that we don’t know if that particular channel is idle at the moment. Such a case increases the overall chances of collision during the transmission, and we may eventually lose the data frame at times.
When a station transmits the given data frame to any channel, it will expect some sort of acknowledgment from the receiver’s end. If it receives this acknowledgment within a specified time, then it’s fine. Else, the station would assume that the data frame has been destroyed during transmission.
Practically, the station would wait for some random time for the acknowledgment. After that, it will consider the data to be destroyed and then retransmit this frame in the very same channel. This will occur repeatedly until the receiver successfully receives the intended data and reverts back with an acknowledgment.
Differences between Pure and Slotted Aloha
Last Updated : 03 Oct, 2024
Pre-Requisite: Multiple Access Protocols
Aloha is a type of Random access protocol it was developed at the University of Hawaii in early 1970, it is a LAN-based protocol this type there are more chances of occurrence of collisions during the transmission of data from any source to the destination, Aloha has two types one Pure Aloha and another Slotted Aloha.
Pure Aloha can be termed as the main Aloha or the original Aloha. Whenever any frame is available, each station sends it, and due to the presence of only one channel for communication, it can lead to the chance of collision.
In the case of the pure aloha, the user transmits the frame and waits till the receiver acknowledges it, if the receiver does not send the acknowledgment, the sender will assume that it has not been received and sender resends the acknowledgment.
For more, refer to Pure Aloha.
Slotted Aloha is simply an advanced version of pure Aloha that helps in improving the communication network. A station is required to wait for the beginning of the next slot to transmit. The vulnerable period is halved as opposed to Pure Aloha.
Slotted Aloha helps in reducing the number of collisions by properly utilizing the channel and this basically results in the somehow delay of the users. In Slotted Aloha, the channel time is separated into particular time slots.
For more, refer to Slotted Aloha.
