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Ethernet Supplement

CECS 474 Computer Network Interoperability. Ethernet Supplement. Tracy Bradley Maples, Ph.D. Computer Engineering & Computer Science Cal ifornia State University, Long Beach. Notes for Douglas E. Comer, Computer Networks and Internets (5 th Edition) . LANs

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Ethernet Supplement

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  1. CECS 474 Computer Network Interoperability Ethernet Supplement Tracy Bradley Maples, Ph.D. Computer Engineering & Computer Science Cal ifornia State University, Long Beach Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)

  2. LANs • Local area networks (LANs) depend on multiple access protocols to regulate the sharing of the transmission medium. • There are two primary categories of multiple access protocols: • Contention Access --> MANAGE COLLISIONS • Examples: ALOHA, CSMA, CSMA/CD • Controlled Access --> COLLISION FREE • A. Predetermined Assignment: Static • • Examples: FDMA (Frequency Division Multiple Access), • TDMA (Time Division Multiple Access) • B. Demand Assignment: Based on Demand • • Examples: Circuit-oriented assignment, Packet-oriented assignment

  3. Contention Access • Advantages: • short delay for bursty traffic • simple (do to distributed control) • flexibility in number of users • fairness • Disadvantages: • low channel efficiency • not good for stream traffic (e.g., voice) • cannot support priority function • high variance in transmission delays

  4. CSMA/CD • Most popular contention access scheme is • CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection • CSMA/CD Transmission • In its most basic form, as in First and Second Generation Ethernet:

  5. CSMA/CD Transmission Algorithm The logic inside a network interface (NIC) using CSMA/CD:

  6. CSMA/CD Transmission A begins transmission to B. Just before A’s frame arrives, B begins transmission. COLLISION! B detects the collision and sends a JAM signal. A detects the collision and receives the JAM signal. JAM JAM Conclusion: The amount of time it takes to detect a collision is twice the propagation delay.

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