Priscilla Oppenheimer wrote: > > s vermill wrote: > > > > Priscilla Oppenheimer wrote: > > > > > > Back to the Ethernet question. Does the splitter simply take > > > the four wires that 10BaseT uses and make 2 wires out of > each, > > > sending one of each to each port? What an awful thing to do > to > > > an Ethernet! You bad boys. ;-) > > > > Quite devious indeed! And yes, the splitter has one male > RJ-45 > > and a modular body that has two female RJ-45s pointing in the > > opposite direction. Pin 1 from the male goes to pin 1 of both > > females, etc. > > > > > > > > As Scott mentioned, some books make it sound like the sender > > > loops back what it sends so that it can compare that with > what > > > it receives back from the hub, sort of implying that the hub > > > sends back the transmitter's bits to the transmitter. A hub > > > doesn't do that. And the loopback isn't used to do a > bit-wise > > > comparison with what the hub is sending, like some books > > imply. > > > That would be computationally expensive and also isn't > > > necessary. Simply receiving while you are sending means a > > > collision occurred. > > > > I gave this some thought on my drive home. I've read that > NICs > > internally bridge tx to rx. According to this theory, a > > comparator circuit outputs zero as long as what is on tx is > > also on rx. If someone else collides, the comparator outputs > > something other than zero, because what is on rx is now a > > combination of the colliding signal and what tx was > > outputting. Does that make sense? I realize this may be > urban > > myth > > I've seen this in books also. It may be true. But I also > noticed that Odom backed off (so to speak) on how he explains > this. I used to have an old copy of his CCNA book for teaching. > In the new edition, he has changed that discussion. > > > (especially since, as you pointed out, this is a lot more > > expensive than just declaring a collision if you rx while > > tx'ing), but it would be interesting if some or all NICs > > actually did this. Because then, although CSMA/CD still > > wouldn't work for the reasons already mentioned, the collision > > between the two stations would be detected and backoff would > > take place. Otherwise, it would be up to upper layer > protocols > > to retrans. > > The two stations still wouldn't see each other.
Consider that the two tx leads physically tie together. So if both stations were to transmit simultaneously, each would have a comparator that is expecting just the one transmitted signal. What would show up on the bridged rx lead would be the mess that the colliding signals created. They otherwise wouldn’t "see" one another. But we digress. Hubs are, of course, on the way out the door and this is a bad practice anyway. Before traveling, I used to upload all of my in-progress files to a network share and then log my laptop in using the splitter just long enough to pull them back down (i.e. I was lazy). I would then do the inverse upon return. Just goes to show what happens when you let a WAN jock play on the LAN! (I'm semi-reformed at this stage and acknowledge my debt to society) > > The books that are wrong, by the way, make it sound like the > hub sends back to the transmitter, which it doesn't. Are you > implying that it would in this case? I don't think it would. No, that wouldn't make any sense. Regardless of how NICs determine a collision condition, it wouldn't work that a hub repeat back on the transmitting port. I was thinking outlout a post or two back. Miller time(r)... Scott Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=55839&t=55667 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
