Lecture 13 - MAC Layer Protocols

Question 1

List the steps involved when transmitting using the CSMA/CD protocol [3]

Answer 1

1. Sense the medium. If it is free, transmit the packets. Otherwise, wait for a random amount of time (known as backoff delay) and repeat this step.
2. If collision occurs (this is detected through lack of ACK), send a jam signal.
3. Wait for a random amount of time (exponentiation backoff) and repeat step 1.

Question 2

List the steps involved when transmitting using the PURE ALOHA protocol [3]

Answer 2

1. Nodes transmit on a common channel
2. When data is received, an ACK is sent
3. If collision occurs, no ACK is received within a set time period. The packet is therefore sent again after a random delay

Question 3

What are the advantages of PURE ALOHA?

Answer 3

• It works better than fixed assignment when there is a large number of bursty stations
• Adapts to varying number of stations.

Question 4

What are the disadvantages of PURE ALOHA?

Answer 4

• Theoretically proven throughput maximum of 18.4%.

* Requires queueing buffers for retransmission of packets.

Question 5

Explain the SLOTTED ALOHA protocol [3]

Answer 5

1. Packets are of equal sizes
2. Nodes can only transmit a packet at the start of an interval
3. If a collision occurs, complete collision will take place

Question 6

What are the advantages of SLOTTED ALOHA? [2]

Answer 6

1. It doubles the efficiency of PURE ALOHA

2. It can adapt to a changing number of stations

Question 7

What are the disadvantages of SLOTTED ALOHA? [5]

Answer 7

1. It has a maximum throughput of 36.8%
2. It requires queuing buffers for retransmission
3. Synchronisation is required
4. If there is collision, a complete slot is wasted
5. Bandwidth is wasted as there are a lot of idle slots