The TX/RX "loopback" is inside the NIC, from what I understand. I don't think the senders see each other's transmissions even though their TX wires end up being the same wire past the splitter. But I always try to work above the physical layer and may be missing something.
Well, it's time to move on. Enjoy your Miller. ;-) Priscilla s vermill wrote: > > 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=55851&t=55667 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
