The data link layer is used in a computer network to transmit the data between two devices or nodes. It divides the layer into parts such as data link control and the multiple access resolution/protocol. The upper layer has the responsibility to flow control and the error control in the data link layer, and hence it is termed as logical of data link control. Whereas the lower sub-layer is used to handle and reduce the collision or multiple access on a channel. Hence it is termed as media access control or the multiple access resolutions. ALOHA, an acronym for Advocates of Linux Open-source Hawaii Association, is a multiple access protocol that allows data to be transmitted over a public network channel. It operates within the Medium Access Control (MAC) sublayer of the Open Systems Interconnection (OSI) model.When a sender and receiver have a dedicated link to transmit data packets, the data link control is enough to handle the channel. Suppose there is no dedicated path to communicate or transfer the data between two devices. In that case, multiple stations access the channel and simultaneously transmits the data over the channel. It may create collision and cross talk. Hence, the multiple access protocol is required to reduce the collision and avoid crosstalk between the channels. ALOHA is a multiple access protocol for transmission of data via a shared network channel. It operates in the medium access control sublayer (MAC sublayer) of the open systems interconnection (OSI) model. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel. this protocol, all the station has the equal priority to send the data over a channel. In random access protocol, one or more stations cannot depend on another station nor any station control another station. Depending on the channel's state (idle or busy), each station transmits the data frame. However, if more than one station sends the data over a channel, there may be a collision or data conflict. Due to the collision, the data frame packets may be lost or changed. And hence, it does not receive by the receiver end. In ALOHA, each node or station transmits a frame without trying to detect whether the transmission channel is idle or busy. If the channel is idle, then the frames will be successfully transmitted. If two frames attempt to occupy the channel simultaneously, collision of frames will occur and the frames will be discarded. These stations may choose to retransmit the corrupted frames repeatedly until successful transmission occurs 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. Slotted ALOHA reduces the number of collisions and doubles the capacity of pure ALOHA. The shared channel is divided into a number of discrete time intervals

1. VISVESVARAYA TECHNOLOGICAL UNIVERSITY
BELAGAVI – 590018
GOVERNMENT ENGINEERING COLLEGE,
KUSHALNAGARA 571234
Department of Electronics And Communication Engineering
SUBJECT: Computer Communication Network [21EC53]
TOPIC : Slotted Aloha and CSMA
PRESENTED BY;
Akshatha M
USN:4GL21EC002
PRESENTED TO;
PROF.Roopa

2. INDEX
 MULTIPLE Access protocol
 Aloha
 Pure Aloha
 Slotted aloha
 CSMA protocol
 Types of CSMA protocol

3. Multiple Access Protocol
Random Access
Protocols
Controlled Access
Protocols
Channelization
Protocols

4. ALOHA
What is aloha?
 Aloha is a random access protocol.
 It was actually designed for WLAN but it is also applicable for shared
medium
 In this, mulyiple stations can transmit data at the same time and can
hence lead to collision & data being garbled
TYPES
 Pure Aloha
 Slotted Aloha

5. PURE ALOHA

6. SLOTTED ALOHA
It was developed just to improve the efficiency of pure aloha as the
chances for collision in pure aloha are high.
The time of the shared channel is divided into discrete time
intervals called slots.
Sending of data is allowed only at the beginning of these slots.
If a station misses out the allowed time, it must wait for the next
slot. This reduces the probability of collision.

7. Vulnerable Time= Frame Transmission Time
Throughput = G x e^-G; Where G is the number of stations wish to
transmit in the same time.
Maximum Throughput = 0.368 gor G=1.

8. SLOTTED ALOHA

9. PURE ALOHA V/S SLOTTED ALOHA
Pure Aloha Slotted Aloha
Any Station can transmit the data at any time. Any station can transmit the data at the beginning
of any time slot.
The time is continuous and not globally
synchronized.
The time is discret and globally synchronized.
Vulnerable time in which collision may occur=2 x Tfr Vulnerable time in which collision may occur = Tfr
Probability of successful transmission of data packet
=G x e^-2G
Probability of successful transmission of data
packet=G x e^-G
Maximum efficiency=18.4%(occurs at G=1/2) Maximum efficiency = 36.8% (occurs at G=1)
Main advantage: Simplicity in implementation. Main advantage : It reduces the number of
collisions to half and doubles the efficiency of pure
aloha

10. CSMA PROTOCOL
• CSMA Stands for Carrier Sense Multiple Access.
• To minimize the chance of collision and, therefore, increase the
performance, the CSMA method was developed.
• Principle of CSMA :”Sense before transmit” or “listen before talk”.
• Carrier busy = Transmission is taking place.
• Carrier idle = No transmission currently taking place.
• The possibility of collision still exist because of propagation delay; a station
may sense the medium and find it idle,only because the first bit sent by
another station has not yet been received.

11. Space /time model of collision in CSMA

12. Vulnerable time in CSMA

13. What are the types of CSMA?
1-persistent CSMA
• The 1-persistent method is simple and straightforward.
• In this method, after the station finds the line idle, it sends its frame immediately
with probability 1.
• This method has the highest chance of collision because two or more stations may
find the line idle
Nonpersistent
• In the nonpersistent method, a station that has a frame to send senses the
line.
• If the line is idle, it sends immediately.
• If the line is not idle, it waits a random amount of time and then senses the
line again.
• The nonpersistent approach reduces the chance of collision
• This method reduces the efficiency of the network

14. p-Persistent
• The p-persistent method is used if the channel has time slots with a slot
duration equal to or greater than the maximum propagation time.
• It reduces the chance of collision and improves efficiency.
• In this method, after the station finds the line idle it follows these steps:
• 1. With probability p, the station sends its frame.
2. With probability q = 1 − p, the station waits for the beginning of the next
time slot and checks the line again.
a. If the line is idle, it goes to step 1.
b. If the line is busy, it acts as though a collision has occurred and uses the
backoff procedure.

15. Behaviour of three persistence methods

16. Flow diagrams of three persistence methods

17. THANK YOU