CECS 474 Computer Network Interoperability Ethernet Supplement Tracy

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 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 • 1. Examples: ALOHA, 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

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

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:

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

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