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

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: 1.Contention Access --> MANAGE COLLISIONS Examples: ALOHA, CSMA, CSMA/CD 1.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

4. 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

5. 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:

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

7. CSMA/CD Transmission Conclusion: The amount of time it takes to detect a collision is twice the propagation delay. 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