Essentially it is two protocols, a data transmission checking protocol and a collision protocol.

When a computer wishes to transfer data, it will check the bus backbone to see if it is busy. If it is not, it will transfer a frame of data to its recipient. It will keep doing this until a collision is detected.

When a collision is detected, the computer will send a jamming signal to all the other computers on the bus network to tell them to stop transferring. It will also stop transferring.

Some time later said computer will try transferring a frame of data again and the process will repeat.

Hope that helps! :smile:

Reply 427

LlamaStealer

1

Original post by akashpatel

Could you explain CSMA/CD to me please?

when a node wants to transmit it checks if the bus is clear
if it is and they are the only one sending then that's fine, but several check at once, and all the transmit at once, there's a collision

in this event, all computers in transmission emit a jamming signal to indicate that computers should cease transmission
then all the computers wait a random amount of time (some number of milliseconds) before trying to transmit again, so there is a low chance of another collision

if for some reason the network is overloaded and there are lots of collisions, an error is output to the user

Reply 428

dominicwild

6

Original post by AlecRobertson

For June 2012, question 5bi and 5bii, isn't the mark scheme the wrong way round?

Looks the right way around to me.

Reply 429

treedough

Original post by AlecRobertson

For June 2012, question 5bi and 5bii, isn't the mark scheme the wrong way round?

no

Reply 430

akashpatel

1

Original post by AlecRobertson

Essentially it is two protocols, a data transmission checking protocol and a collision protocol.

When a computer wishes to transfer data, it will check the bus backbone to see if it is busy. If it is not, it will transfer a frame of data to its recipient. It will keep doing this until a collision is detected.

When a collision is detected, the computer will send a jamming signal to all the other computers on the bus network to tell them to stop transferring. It will also stop transferring.

Some time later said computer will try transferring a frame of data again and the process will repeat.

Hope that helps! :smile:

Thank you very much! That was very helpful :smile:

Reply 431

Gigabit

14

Original post by dominicwild

Looks the right way around to me.

I'm sorry but I don't get it!

In the question it says private key but in the answer it says public key?!?

Reply 432

LlamaStealer

1

Original post by AlecRobertson

I'm sorry but I don't get it!

In the question it says private key but in the answer it says public key?!?

It shouldn't be encrypted with A's public key, as then it would only be able to be decrypted with A's private key - this being only available to A, no one would be able to see it at all

It shouldn't be encrypted with A's private key, as then it would only be able to be decrypted with A's public key - this being publicly available, anyone would be able to see it

the desired case is so that only B can see it, so the sender, A, would use B's public key to encrypt it. This could only be decrypted with B's private key, which only B has access too, so no one else would be able to see the message

Reply 433

akashpatel

1

Original post by AlecRobertson

I'm sorry but I don't get it!

In the question it says private key but in the answer it says public key?!?

To decrypt something encrypted by A's Private key, the only way to decrypt it is by using A's Public Key. However, A's public key is Public lol therefore anybody could access it and decrypt the information and gain access to the sensitive data get it?

Reply 434

donutellme

18

lol im so fuked

Posted from TSR Mobile

Reply 435

Async

19

Original post by AlecRobertson

I'm sorry but I don't get it!

In the question it says private key but in the answer it says public key?!?

The question is talking about asymmetric encryption.

Asymmetric encryption is a two way thing, you encrypt the data withy a public key (which everyone can see) and you decrypt it with a private key.

Therefore, in this STEM question.
5b) i
A wants to send data to B. So if A encrypts the data As public key, then that means only A can decrpyt the message, therefore B can't decrypt it.

for
5b) ii

If the data is encrypted using A's private key then that means it can only be decrpyted using his public key. Therefore anybody can read the message.

Watch this to help you understand the two types of encryption you need to know for the exam.
https://www.youtube.com/watch?v=ERp8420ucGs

Reply 436

dominicwild

6

Original post by AlecRobertson

I'm sorry but I don't get it!

In the question it says private key but in the answer it says public key?!?

If a message is encrypted with a public key, it can only be decrypted with the corresponding private key. This works the other way around to, a message encrypted with a private key, can only be decrypted the corresponding public key.
In this instance, the corresponding keys are A's (public or private) keys.

The question is why not to encrypt the data with A's public key. As the mark scheme says, only A has A's private key, therefore B cannot decrypt it like that.

The other reason why not to encrypt with A's private key, is because A's public key, is well, public. Anyone can have it, anyone can decrypt the message and therefore the message can not be securely sent to B. As the mark scheme says.

Reply 437

Shaney96

7

Well, I'm off to bed I guess, it'll probably take an hour to get to sleep due to nerves but meh. Probably won't be on tomorrow morning before the exam, so see you all on the other side and good luck to you all!