Introduction to Multiple Access Protocol.pptx

Data link layer is divided into two sub layers:
Logical link control (LLC) layer: The upper sub layer is responsible for data link control i.e. for flow and error control.
Media access control (MAC) layer: The lower sub layer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sub layer.

1. Multiple Access Protocol DR. UPAMA DAS SHARAD INSTITUTE OF TECHNOLOGY COLLEGE OF ENGINEERING, YADRAV

2. INTRODUCTION Data link layer is divided into two sub layers: Logical link control (LLC) layer: The upper sub layer is responsible for data link control i.e. for flow and error control. Media access control (MAC) layer: The lower sub layer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sub layer.

3. Classification of Multiple Access Protocol

4. RANDOM ACCESS PROTOCOLS First, there is no scheduled time for a station to transmit. Transmission is random among the stations. That is why these methods are called random access. Second, no rules specify which station should send next. Stations compete with one another to access the medium. That is why these methods are also called contention methods .

5. ALOHA ALOHA, the earliest random access method was developed at the University of Hawaii in early 1970. It was designed for a radio (wireless) LAN, but it can be used on any shared medium. It is obvious that there are potential collisions in this arrangement

6. Variations of ALOHA protocol Pure ALOHA : The original ALOHA protocol is called pure ALOHA . This is a simple, but elegant protocol. The idea is that each station sends a frame whenever it has a frame to send. However, since there is only one channel to share, there is the possibility of collision between frames from different stations.

7. Pure ALOHA

8. The pure ALOHA relies on acknowledgments If ACK does not arrive after a time-out period, resends the frame. If all stations try to resend their frames after the time-out, the frames will collide again. Each station waits a random amount of time before resending its frame. The randomness will help avoid more collisions. T his time is the back-off time TB . After a maximum number of retransmission attempts Kmax a station must give up and try later. Procedure for pure ALOHA protocol

9. Procedure for pure ALOHA protocol

10. Vulnerable time is the Length of time, in which there is a possibility of collision. Propagation time Tp is the time needed for a signal to propagate from one end of the medium to the other. The waiting time for which the station waits before retransmitting the data is called as back off time. Ethernet is defined as a networking technology that includes the protocol, port, cable, and computer chip needed to plug a desktop or laptop into a local area network (LAN) for speedy data transmission via coaxial or fiber optic cables. 

11. Slotted ALOHA Slotted ALOHA was invented to improve the efficiency of pure ALOHA. In slotted ALOHA the time is divided into slots (time to send each fixed size frame) and force the station to send only at the beginning of the time slot. If a station misses this moment, it must wait until the beginning of the next time slot. Still the possibility of collision is present if two stations try to send frame at the beginning of the same time slot .

12. Slotted ALOHA

13. Carrier Sense Multiple Access (CSMA) CSMA is based on the principle "sense before transmit" or "listen before talk." CSMA can reduce the possibility of collision, but it cannot eliminate it. Persistence Methods: 1-persistent method, Non-persistent method, and P-persistent method.

14. 1-Persistent Method: In this method, after the station finds the line idle, it sends its frame immediately. This method has the highest chance of collision because two or more stations may find the line idle and send their frames immediately. Ethernet, a LAN standard uses this method Non-persistent Method : If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time. T wo or more stations will not wait the same amount of time. T his method reduces the efficiency.

15. p-Persistent Method: C ombines the other two strategies .

16. CSMA/CD (Collision Detection) It was widely used in Early Ethernet technology/LANs when there used to be shared Bus Topology and each node were connected By Coaxial Cables. Step 1:  Check if the sender is ready for transmitting data packets. Step 2:  Check if the transmission link is idle .  Sender has to keep on checking if the transmission link/medium is idle. For this, it continuously senses transmissions from other nodes. Sender sends dummy data on the link. If it does not receive any collision signal, this means the link is idle at the moment. If it senses that the carrier is free and there are no collisions, it sends the data. Otherwise, it refrains from sending data.

17. Step 3:  Transmit the data & check for collisions.   Sender transmits its data on the link. CSMA/CD does not use an ‘acknowledgment’ system. It checks for successful and unsuccessful transmissions through collision signals. During transmission, if a collision signal is received by the node, transmission is stopped. The station then transmits a jam signal onto the link and waits for random time intervals before it resends the frame. After some random time, it again attempts to transfer the data and repeats the above process. Step 4:  If no collision was detected in propagation, the sender completes its frame transmission and resets the counters

18. Flow Diagram of CSMA/CD

19. CSMA/CA ( Collision Avoidance) In contrast to CSMA/CD (Carrier Sense Multiple Access/Collision Detection) that deals with collisions after their occurrence, CSMA/CA prevents collisions prior to their occurrence. When a frame is ready, the transmitting station checks whether the channel is idle or busy. If the channel is busy, the station waits until the channel becomes idle. If the channel is idle, the station waits for an Inter-frame gap (IFG) amount of time and then sends the frame. After sending the frame, it sets a timer . The station then waits for acknowledgement from the receiver. If it receives the acknowledgement before expiry of timer, it marks a successful transmission. Otherwise, it waits for a back-off time period and restarts the process.

20. Interframe space : In this method, the station waits for the channel to become idle, and if it gets the channel is idle, it does not immediately send the data. Instead of this, it waits for some time, and this time period is called the  Interframe   space or IFS . However, the IFS time is often used to define the priority of the station . Contention window : In the Contention window, the total time is divided into different slots. When the station/ sender is ready to transmit the data frame, it chooses a random slot number of slots as  wait time . If the channel is still busy, it does not restart the entire process, except that it restarts the timer only to send data packets when the channel is inactive . Acknowledgment : In the acknowledgment method, the sender station sends the data frame to the shared channel if the acknowledgment is not received ahead of time. M ethods used in the CSMA/ CA

21. CONTROLLED ACCESS METHODS In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. There are three popular controlled-access methods: Reservation Polling Token passing

22. Reservation In the reservation method, a station needs to make a reservation before sending data . Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. If there are N stations in the system, there are exactly N reservation mini-slots in the reservation frame. When a station needs to send a data frame, it makes a reservation in its own mini-slot. The stations that have made reservations can send their data frames after the reservation frame.

23. Polling O ne device is designated as a primary sta tion and the other devices are secondary stations. All data exchanges must be made through the primary device. The primary device controls the link; the secondary devices follow its instructions. If the primary wants to receive da ta, it asks the secondary if they have anything to send; this is called poll function. If the primary wants to send data , it tells the secondary to get ready to receive; this is called select function.

24. Select and Poll functions in Polling access method

25. Token Passing In token passing scheme, the stations are connected logically to each other in form of ring and access to stations is governed by tokens. A token is a special bit pattern or a small message , which circulate from one station to the next in some predefined order . In Token ring, token is passed from one station to another adjacent station in the ring whereas incase of Token bus, each station uses the bus to send the token to the next station in some predefined order.

26. In both cases, token represents permission to send . If a station has a frame queued for transmission when it receives the token, it can send that frame before it passes the token to the next station. If it has no queued frame, it passes the token simply. After sending a frame, each station must wait for all N stations (including itself) to send the token to their neighbors and the other N – 1 stations to send a frame, if they have one. There exists problems like duplication of token or token is lost or insertion of new station, removal of a station, which need be tackled for correct and reliable operation of this scheme.

27. REFERENCES https://www.geeksforgeeks.org/multiple-access-protocols-in-computer-network / https:// www.javatpoint.com/multiple-access-protocols https://byjus.com/gate/multiple-access-protocols-notes / https://www.scaler.com/topics/multiple-access-protocols / https:// www.tutorialspoint.com/multiple-access-protocols-in-computer-networks https://www.geeksforgeeks.org/carrier-sense-multiple-access-csma / https:// www.studytonight.com/computer-networks/multiple-access-in-data-link-layer