Sharing resources¶
Multiple choice questions¶
- Which of the following affirmations are valid for the ALOHA Medium Access Control scheme ? Select all the valid ones.
The retransmission timer has a random part to prevent synchronisation among the communicating hosts.
ALOHA uses acknowledgements to confirm the correct reception of frames.
In ALOHA, a collision can occur when several hosts transmit almost at the same time.
With ALOHA, a host always listens before transmitting its frame.
When using ALOHA, a host always listens to the communication channel while transmitting to detect collisions.
ALOHA only supports fixed-length frames.
- Which of the following affirmations are valid for the CSMA Medium Access Control scheme ? Select all the valid ones.
With persistent CSMA, a host sends its frame as soon as the communication channel becomes free.
With non-persistent CSMA, a host checks at random times whether the communication channel when it needs to transmit.
With CSMA, collisions are detected thanks to the expiration of a retransmission timer.
When using CSMA, a host always listens to the communication channel while transmitting to detect collisions
With CSMA, acknowledgements are not needed since each host senses whether the communication channel is free before transmitting.
- Which of the following affirmations are valid for CSMA/CD ? Select the correct ones.
With CSMA/CD, there is no need to use acknowledgements to detect collisions.
CSMA/CD relies on explicit acknowledgements to detect collisions.
When using CSMA/CD, a host must check that the communication channel is free before transmitting data.
To be able to detect all collisions, CSMA/CD must enforce a minimum size for all transmitted frames.
To be able to detect all collisions, CSMA/CD must enforce a maximum size for all transmitted frames.
- Which of these Medium Access Control algorithms rely on explicit acknowledgements to detect collisions ? Select all the valid answers.
ALOHA
slotted ALOHA
CSMA/CD
CSMA
CSMA/CA
- Consider a network that is using CSMA/CD. An important parameter in such networks is the minimum frame size. Which of the following affirmations is true ?
If the shared medium has a bandwidth of 100 Mbps and a length of 10 kilometers, then the minimum frame size is 10000 bits.
If the shared medium has a bandwidth of 10 Mbps and a length of 2 kilometers, then the minimum frame size is 200 bits.
If the shared medium has a bandwidth of 100 Mbps and a length of 1 kilometer, then the minimum frame size is 10000 bits.
The minimum frame size is always set to 512 bits or 64 bytes.
If the shared medium has a bandwidth of 100 Mbps and a length of 1 kilometer, then the minimum frame size is 500 bits.
- CSMA/CA is a Medium Access Control scheme used in wireless networks. Only a few of the affirmations below concerning CSMA/CA are correct. Which ones ?
CSMA/CA uses explicit acknowledgements to ensure that the frames are correctly received by their destination.
CSMA/CA hosts must listen to the communication channel before transmitting.
The CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) enables the hosts to prevent all collisions.
A host using CSMA/CA can transmit its frame as soon as it detects that the communication channel is free.
When using RTS/CTS, four frames in total are required to exchange a single data frame.