CSMA/CD

Ethernet implementation:

1. If NIC receives datagram from network layer, creates frame

2. If senses channel

   Idle: Transmit

   Busy: Wait

3. If transmit with no collision, NIC is done with frame

4. If detects another transmission while sending, abort send jam

5. After aborting, NIC chooses $K$ randomly from $\{0,..., 2^m - 1\}$ and wait $K\times$512 bit times.

Other MAC protocols

Primary/Secondary: uses polling, and when will send (invites secondaries)

Token passing: Control token passed around, can only send when has

Summary

• Channel partitioning
• Random Access
• Polling, token passing

Local Area Networks

Uses 48-bit MAC addresses as link-layer identifiers.

ARP: Address Resolution Protocol

<IP address, MAC address, TTL>

TTL is used since IP addresses can change

Broadcast ARP with query

Reply with ARP back