Steve Dispensa wrote: > > > A loop could exist at the physical layer too. A newbie could > connect the > > hubs in such a way that there was a loop. And it could indeed > cause problems > > heh... I just did this last weekend at a local high school i > volunteer at sometimes, and I've been doing this a while. The > hubs were old and didn't have any error detection/avoidance > circuitry, so it took me a minute to figure out what had > happened... > > While we're on the topic of physical ethernet design, don't > forget the 5/4/3 rule for 10Mbps. Also, IIRC, the 100Mbps spec > requires not more than 2 >100m segments between layer 3 > devices. Anyone remember the details?
Of course I do remember the details of scaling shared Ethernet. Sigh. ;-) The round-trip propagation delay in one collision domain must not exceed 512 bit times. This is a requirement for collision detection to work correctly. This rule means that the maximum round-trip delay for 10-Mbps Ethernet is 51.2 microseconds. One way to make this work is by following the 5-4-3 "rule." The maximum round-trip delay for 100-Mbps Ethernet is only 5.12 microseconds because the bit time on 100-Mbps Ethernet is 0.01 microseconds as opposed to 0.1 microseconds on 10-Mbps Ethernet. To make 100-Mbps Ethernet work, there are much more severe distance limitations than those required for 10-Mbps Ethernet. The general "rule" is that a 100-Mbps Ethernet has a maximum diameter of about 200 meters when UTP cabling is used. Theoretical discussions always talk about the two classes of repeaters for 100 Mbps, although, as far as I know, all vendors use Class II repeaters, but just in case, here's the info. Note that the real concern is propagation delay and repeaters (as well as cabling) introduce some delay, so they must be taken into account. In the IEEE 100BaseT specification, two types of repeaters are defined: * Class I repeaters have a latency of 0.7 microseconds or less. Only one repeater hop is allowed. * Class II repeaters have a latency of 0.46 microseconds or less. One or two repeater hops are allowed. Of course, none of this is all that relevant IF: * You use fiber-optic cabling (which has less delay than copper) * Your 100 Mbps Ethernet isn't shared (which most aren't today) I wrote the "rules" up at one point. I'll see if I can find a URL. Here we go: http://www.priscilla.com/enetscales.htm Priscilla > > -sd > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=58862&t=58783 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
