Shared Media

Question 1

What is CSMA?

Answer 1

Carrier Sense Multiple Access (CSMA) checks whether the medium is active before sending a packet. Unlike polling/probing, a node with something to send doesn’t have to wait for a master, or for its turn in a schedule. This works well under low contention.

Question 2

Describe CSMA/CD

Answer 2

1) Is my frame ready for transmission? If yes, it goes on to the next point.
2) Is medium idle? If not, wait until it becomes ready
3) Start transmitting.
4) Did a collision occur? If so, go to collision detected procedure.
5) End Transmission

Collision detection usually involves exponential backoff.

Question 3

What is p-persistant CSMA/CD

Answer 3

When the channel is idle send with probability p.

Question 4

What is CSMA/CA

Answer 4

1. First check if the medium is busy.
2. If so, wait for the medium to become idle.
3. Set a contention timer, randomly chosen in the range [1,CW].
4. On timeout, send packet and wait for ACK.
5. If no ACK, assume packet it lost and try again, after doubling CW.
6. If another station transmits whilst counting down, freeze CW and unfreeze when packet completes transmission.

Question 5

What is the hidden terminal problem?

Answer 5

See notes for diagram.

Question 6

What is the exposed terminal problem?

Answer 6

See notes for diagram.

Question 7

What are CTS and RTS

Answer 7

Before sending to a node, a RTS is sent. Transmission only proceeds when a CTS is sent back. This avoids the hidden terminal problem.

Question 8

What is ALOHA? What is the optimisation that the slotted version makes?

Answer 8

• ALOHA is one of the earliest multiple access schemes. Data is simply sent. If no ACK is received, try again after a random time. Slotted ALOHA introduces discrete time intervals, allowing nodes to send only at the start of one of these intervals.