Media Access Control

This is the way we control multiple transmissions on the same line, as transmitting multiple messages at the same time corrupts our data.

Improves utilization - Active transmissions not disrupted by collisions - Only the new sender backs-off if the channel is active

Used by: Ethernet, Wi-Fi

Process

• When propagation delay low, listen before sending
• If link is idle, send data immediately
• If another transmission is active, or if collision occurs, stop sending, wait, then retransmit
  • Wait time should be random – to avoid deterministic repeated collisions; pick a random initial back-off interval of $x$ seconds $\pm 50\%$
  • Wait time should increase with number of collisions – repeated collisions signal congestion; reduce transmission rate allows network to recover; each repeated collision before success, $x\to 2x$

Side Note:

Why does propagation delay matter? Case: A starts transmitting B listens, hears no traffic (message from A hasn’t reached it yet) B starts transmitting And then: Collision occurs, as messages overlap in transit;

So: Smaller propagation delay $\to$ less likely to occur