Local Area Networks (LANs)

Shared Media

• Not Point-to-Point Links
• Must share a common resource/channel
• How to Share a Wire: CSMA/CD

Carrier Sense

• POLITENESS: If you hear some one talking, wait until s/he finishes.
• PERSISTENT: Start immediately after line becomes free. This leads to COLLISION.
• NON-PERSISTENT: Wait for some (random) time, before trying!

Collision Detection

• Station must listen when transmitting
• Minimum transmission time needed to detect collision
• $2 * \tau$ where $\tau$ is time for signal between farthest stations
• On 10 Mbps Ethernet 64 bytes (512 bit time) needed
• Binary Exponential Backoff