MAC • Now consider broadcast channels. In broadcast channels/multiaccess channels/random access channels, multiple sources may compete for a shared channel. • The biggest problem now is how to determine who gets the channel. • With point-to-point link, this problem does not exist. • After figuring this out, you can run some point-to-point protocol to make sure that things got there right; or, you don’t.
How would you design a MAC protocol? • The simplest. Got data, send immediately. • But, what if there is collision? • Immediately resend? • Wait for a constant time and resend?
ALOHA • Each station sends when data is available. • Each station can detect if the frame it sent collided with frames from other stations. If yes, wait for a random time, resend.
ALOHA • What is the performance likely to be? • At low traffic load, everything will get through. • At high traffic load, very few frames can get through.
Slotted ALOHA • Time is divided into slots and each station sends at the beginning of time slots. • Intuitively, why this is better? • Can we do even better than that?
Beyond Aloha – Carrier Sense Protocols • CSMA – Carrier Sense Multiple Access • Why call it carrier?
Carrier Sense • 1-persistent. Station listens to the channel, if busy, wait until idle and send immediately (1-persistent). If collision, wait for a random time and starts over again. • Better than ALOHA because at least if someone is sending, won’t send • Problems • Two stations wait for channel to be idle and will send at the same time • If propagation delay is long, B does not know A has started sending
Carrier Sense • Non-persistent. If no one else is sending, send. Else, wait for a random time and check the channel again.
Carrier sense • P-persistent. Slotted channel. If busy, wait to the next time slot. If idle, send with probability p. With probability q=1-p, defer to the next slot. Until he sends or he senses that another station has started sending (considered a collision). If collision (either real or ``considered’’), wait for a random time and start the process again.
What next? • What can we do beyond carrier sense?
CSMA with collision detection • CSMA/CD. Improves CSMA by listening to the channel and abort immediately when there is a collision. • Used by Ethernet.