three part question:

1- is one of the conditions for a transmission to be considered "successfull" that the message returns to the sender;
2-if, this is so, is the "distance" between sender and recipient then always actually the circumference of the ring; that is, that it doesn't matter where the recipient is on the ring, since a message is not considered sent until it gets back to the sender;
3-why does the recipient continue to pass on the message even after its realized the message is for them?

thanks,
kate

I'm assuming all of your questions refer to ring networks (the first one could be more general, but..).

I'm also guessing that when you say "successful" you mean in terms of the way the second homework exercise uses that term (even though your question is in a "lectures" sub-topic).

1) In the homework assignment, I want you to consider a transmission to be successful as soon as the last bit of a message has been sent. This would be a reasonable assumption in a world where networks were very reliable and you could assume that every message sent got delivered. For the homework problem, I want you thinking about efficiency rather than reliability and this assumtion makes the problem a little simpler.

Real networks aren't actually that reliable.
To account for this, in most real ring protocols, every message sent makes a full round-trip eventually returning to the sender. This has the advantage of letting the sender know for sure that the message made it to the destination (instead of running into a cut wire somewhere). To make this useful, one would expect a station that did not eventually see one of its packets arriving after a round trip to try to send it again. In such a situation, it would make sense to say the transmission wasn't considered successful until the return trip was complete. For the problem, however, stick to the simpler version of "successful."

2) So, the distance between the sender and receiver can be any amount less than the full circumference of the ring. On the other hand, if you consider the transmission successful when the last bit is sent, you don't care when the last bit (or even the first bit) actually arrives.

3) The best answer to the "why continue to pass on the message" question is to ask "What if it they didn't?". The advantage might be that we wouldn't waste time sending all those bits around through all those machines that don't care about them. If you think about it, however, you will see that those stations would then have free time but nothing to do with it. They can't send anything else until they get the token and then can't get the token until the message that they would have forwarded would have passed by anyway.


Tom