Computer Networks
LAB MANUAL
Instructors: Dr. Vijay Raisinghani, Prof. Sneha Deshmukh
2021-2022
B.Tech (IT) 2nd year, Sem iV
6. Plot the characteristic curve throughput versus offered traffic for a Pure
ALOHA and Slotted ALOHA.
Part A
Aim: To Plot the characteristic curve throughput versus offered traffic for a Pure ALOHA and Slotted
ALOHA.
Software used: Netsim
Theory:
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.
Pure ALOHA:
The first version of the protocol (now called "Pure ALOHA", and the one implemented in ALOHAnet)
was quite simple:
 If you have data to send, send the data
 If, while you are transmitting data, you receive any data from another station, there has been a
message collision. All transmitting stations will need to try resending "later".
Note that the first step implies that Pure ALOHA does not check whether the channel is busy before
transmitting. Since collisions can occur and data may have to be sent again, ALOHA cannot use 100%
of the capacity of the communications channel. How long a station waits until it transmits, and the
likelihood a collision occurs are interrelated, and both affect how efficiently the channel can be used.
This means that the concept of "transmit later" is a critical aspect: the quality of the backoff scheme
chosen significantly influences the efficiency of the protocol, the ultimate channel capacity, and the
predictability of its behavior.
Slotted ALOHA:
In slotted Aloha, time is divided up into discrete intervals, each interval corresponding to one frame.
In Slotted ALOHA, a computer 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
Where, S (frames per frame time) is the mean of the Poisson distribution with which frames being
generated. For reasonable throughput S should lie between 0 and 1. 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 co It is easy to note that Slotted ALOHA peaks at
G=1, with a throughput of S=1/e or about 0.368. It means that if the system is operating at G=1, the
probability of an empty an slot is 0.368
Calculations used in NetSim to obtain the plot between S an
Using NetSim, the attempts per packet time (G) can be calculated as follows;
G= Number of packets Transmitted∗PT
ST∗1000
Where, G = Attempts per packet time
Computer Networks
LAB MANUAL
Instructors: Dr. Vijay Raisinghani, Prof. Sneha Deshmukh
2021-2022
B.Tech (IT) 2nd year, Sem iV
PT = Packet time (in seconds)
ST = Simulation time (in seconds)
The throughput (mbps) per frame time can be obtained as follows:
Where, S = Throughput per frame time
PT = Packet time (in milli seconds)
PS = Packet size (in bytes)
Calculations for the packet time:
In the following experiment, we have taken packet size=1472 (Data Size) + 28 (Overheads) = 1500
bytes
Bandwidth is 10 Mbps and hence, packet time comes as 1.2 milliseconds.
Procedure:
To Create Scenario for Aloha:
 Create Scenario: “New Simulation ->Legacy Networks ->Aloha”. Obtain the values of Throughput
and Total Attempts from the statistics of NetSim simulation for various numbers of traffic
generators.
Sample Inputs:
Input for Sample 1: Node 1 transmits data to Node 2.
Simulation Time - 10 Seconds
(Note: The Simulation Time can be selected only after doing the following two tasks,
• Set the properties of Nodes
• Then click on the Validate & Simulate button).
Input for Sample 2: Node 1 transmits data to Node 2, and Node 2 transmits data to Node 1.
Computer Networks
LAB MANUAL
Instructors: Dr. Vijay Raisinghani, Prof. Sneha Deshmukh
2021-2022
B.Tech (IT) 2nd year, Sem iV
Simulation Time- 10 Seconds
(Note: The Simulation Time can be selected only after doing the following two tasks,
NetSim simulation for various numbers of traffic generators.
Input for Sample 2: Node 1 and Node 2 both generate traffic. Node 1 transmits data to Node 2 & Node
2 transmits data to Node 1.The properties of Node 1 and Node 2 are set as shown in Sample 1.
Input for Sample 3: 3 Nodes are generating traffic. Node 1 transmits data to Node 2, Node 2 transmits
data to Node 3 and Node 3 transmits data to Node 1.
And so on continue the experiment by increasing the number of nodes generating traffic as 4, 6, 8, 10,
15, 20 22 and 24 nodes.
Similarly for Slotted Aloha for the steps in slotted Aloha.pdf.
Computer Networks
LAB MANUAL
Instructors: Dr. Vijay Raisinghani, Prof. Sneha Deshmukh
2021-2022
B.Tech (IT) 2nd year, Sem iV
Part B: Report to be submitted:
Aim:
Software used:
Observations:
No
of Throughput
nodes
(mbps)
generatin
g traffic
Pure Slotted
Aloha Aloha
2
4
6
8
10
15
20
22
24


Plot the graph curve throughput versus offered traffic for a Pure ALOHA and Slotted ALOHA.
Comment on throughput obtain in Aloha and Slotted Aloha
Conclusion: