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Computer Communication & Networks

Computer Communication & Networks. Lecture 12 Datalink Layer: Multiple Access http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp. Waleed Ejaz waleed.ejaz@uettaxila.edu.pk. Data Link Layer. Data Link Layer Topics to Cover. Error Detection and Correction. Data Link Control and Protocols.

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Computer Communication & Networks

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  1. Computer Communication & Networks Lecture 12 Datalink Layer: Multiple Access http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp Waleed Ejaz waleed.ejaz@uettaxila.edu.pk

  2. Data Link Layer

  3. Data Link Layer Topics to Cover Error Detection and Correction Data Link Control and Protocols Multiple Access Local Area Networks Wireless LANs

  4. Multiple access problem • Example: • Cocktail party – many people gather together in a large room • Broadcast medium – air • Another example: a classroom • Human protocols: • “Give everyone a chance to speak” • “Don’t speak until you are spoken to” • “Don’t monopolize the conversation” • “Raise your hand if you have a question” • “Don’t interrupt when someone is speaking” • “Don’t fall asleep when someone else is talking”

  5. Multiple access protocols • In LANs, WiFi, satellite networks, cocktail party • If more than 2 users send @ the same time - collision • All collided packets are lost -> waste of bandwidth • Ideally, the MAC protocol for a broadcast channel with the bit-rate R bps should satisfy: • if only 1 node is sending than the throughput is R • when M nodes have data to send than the throughput is R/M • decentralized protocol – no master • simple & inexpensive to implement

  6. Taxonomy of Multiple-Access Protocols

  7. Random Access Protocols • In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send.

  8. ACK ACK ACK ACK ALOHA Network • Developed by Norm Abramson at the Univ. of Hawaii • the guy had interest in surfing and packet switching • mountainous islands → land-based network difficult to install • fully decentralized protocol

  9. Pure Aloha • The node immediately transmits its frame completely • If the frame is collided it retransmits the frame again (after completely transmitting its collided frame) with the probability p

  10. Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Slotted Aloha Operation • when node obtains fresh frame, it transmits in next slot • no collision, node can send new frame in next slot • if collision, node retransmits frame in each subsequent slot with prob. p until success

  11. Frames in a Slotted ALOHA Network

  12. Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Slotted Aloha Cons • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to transmit packet • clock synchronization

  13. Multiple Access Protocols ALOHA

  14. Carrier Sense Multiple Access • Invented to minimize collisions and increase the performance • A station now “follows” the activity of other stations • Simple rules for a polite human conversation • Listen before talking • If someone else begins talking at the same time as you, stop talking • CSMA: • A node should not send if another node is already sending • carrier sensing • CD (collision detection): • A node should stop transmission if there is interference • collision detection

  15. channel propagation delay Carrier Sense Multiple Access • If everyone is sensing the medium how come that collisions still occur?

  16. CSMA (cnt’d) • Reduces the chance of collisions • reduces the efficiency • increases the chance for collisions • 1-persistant • p-persistant • Decreases the chance for collisions • Improves efficiency

  17. Multiple Access Protocols ALOHA

  18. CSMA with Collision Detection (CSMA/CD) • CSMA/CD can be in one of the three states: contention, transmission, or idle. • Example of CSMA/CD: Ethernet • How long does it take before stations realize that there has been a collision?

  19. Collision Detection • How the station detects a collision? • There are many collision detection methods! • Most of them are analog processes. • Examples: • detecting voltage level on the line • detecting power level • detecting simultaneous transmission & reception

  20. Multiple Access Protocols ALOHA

  21. CSMA with Collision Avoidance -CSMA/CA- • no collisions

  22. Readings • Chapter 12 (B.A Forouzan) • Section 12.1 • (Cover only those contents which are related to topics covered in class)

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