1
Q
List the steps involved when transmitting using the CSMA/CD protocol [3]
A
- 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.
- If collision occurs (this is detected through lack of ACK), send a jam signal.
- Wait for a random amount of time (exponentiation backoff) and repeat step 1.
2
Q
List the steps involved when transmitting using the PURE ALOHA protocol [3]
A
- Nodes transmit on a common channel
- When data is received, an ACK is sent
- If collision occurs, no ACK is received within a set time period. The packet is therefore sent again after a random delay
3
Q
What are the advantages of PURE ALOHA?
A
- It works better than fixed assignment when there is a large number of bursty stations
- Adapts to varying number of stations.
4
Q
What are the disadvantages of PURE ALOHA?
A
- Theoretically proven throughput maximum of 18.4%.
* Requires queueing buffers for retransmission of packets.
5
Q
Explain the SLOTTED ALOHA protocol [3]
A
- Packets are of equal sizes
- Nodes can only transmit a packet at the start of an interval
- If a collision occurs, complete collision will take place
6
Q
What are the advantages of SLOTTED ALOHA? [2]
A
- It doubles the efficiency of PURE ALOHA
2. It can adapt to a changing number of stations
7
Q
What are the disadvantages of SLOTTED ALOHA? [5]
A
- It has a maximum throughput of 36.8%
- It requires queuing buffers for retransmission
- Synchronisation is required
- If there is collision, a complete slot is wasted
- Bandwidth is wasted as there are a lot of idle slots
