From: seifert@netcom.com (Rich Seifert)
Subject: Re: 10BaseT != 3-Com Multiconnect (interesting stuff)
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
Distribution: na
Date: Tue, 6 Oct 1992 17:40:09 GMT

In article <pwilson.718333138@charm.gandalf.ca>, pwilson@gandalf.ca (Paul Wilson(Toronto-Sales)) writes:
> In <1992Oct2.203308.2239@vogon1.pasen.gov> jdm1@vogon1.pasen.gov (Jonathan D McCown) writes:
> 
> >We have run across an interesting characteristic of the 3Com Multiconnect
> >repeater (10BaseT Hub extrordinaire).  Apparently the Multiconnect hub
> >was designed before the 10BaseT Standards were finalized, and while the
> >Multiconnect is compatible with 10BaseT: IT HAS PHENOMINAL RUN LENGTH
> >CHARACTERISTICS WHEN USED WITH 3COM NICS.  (much better than other
> >"standard 10BaseT" equipment that we've tested).
> 
> >Despite the  whining of our 3Com reps that we shouldn't mess with such
> >things, it is a fact of life that not all cable runs are short, nor is it
> >very easy to get permission to string cable in historical buildings.
> >We've driven runs up to 650' reliably (we use a router at one end) without
> >problems using Multiconnects-- futility incarnate with other gear.
> >It is also interesting that these runs are over telco 200 pair trunks :-)
> >The docs rate Multiconnect 10BaseT up to 200m.  (We need every inch) 
> 
> >The Multiconnect manual lists separate distance specs for "3Com" and
> >802.3 in all categories of Ethernet (base T,5,2 not BROAD) which basically
> >DOUBLE the distances set by the spec.  Maybe this is one for alt.conspiracy.
> 
> This raises an issue which I have wondered about for some time, because
> I am aware that other vendor's equipment (Gandalf, for example) exhibits
> similar characteristics.  But isn't there a tradeoff with propogation 
> delay?
> 
> Within ethernet, so I've always suspected, there are some hard and fast
> rules about length of segments of thicknet, numbers of nodes per segment,
> number of repeaters between two nodes, etc. and I always thought they 
> were there for a good reason, though it may not be apparent at the time.
> One such rule seems to relate to the length of cable/number of repeaters
> between the two furthest nodes, stating that it shall not exceed a certain
> distance, and this distance can ultimately be translated into propogation
> delay.  In summary, "Thou shalt not have two nodes a distance apart such
> that a signal shall exceed **** seconds propogation time."  Is my idea
> correct?
> 
> If this is so, then the above scenario is an acceptable one iff the network 
> trades off these extra long runs by not having more than, oh say 2 repeaters
> between the furthest nodes (just pulling numbers out of thin air for this).
> The only way to be really sure is to take the above **** time, and ensure
> that the propogation through the wire, plus the other wires, plus the 
> repeater propogation time, etc, doesn't exceed it.
> 
> While were on this topic, If my assumption is correct, where does the
> time value mentioned above come from?  I'd really like to know.
> 

Yes, it is possible for a vendor to make a product which allows stations
to be farther from the hub than the "vanilla standard". 3Com obviously
does this; I knwo that Cabletron did (and may still) offer such a product.

The main problem with 10BaseT and distance is the tradeoff among: 
signal levels, complexity of the receiver, radiated emissions (FCC),
noise susceptibility, error rate, and a few other physical layer
parameters. It is possible to design a system that "squeezes the
sausage" (i.e., provides longer distance at the expense of higher
errors, greater compelxity, higher cost, etc.) 

The problem is interoperability. It will not be possible to achieve
the added benefits if you also want to "mix and match" components
from various vendors. This is what standards are for. If you want
a proprietary solution, that is fine, but you are "stuck" with the one
vendor. I have no problem with this if the product solves a particular
problem that you hae, and the decision is made consciously.

As to the "total network length" restriction, this comes from a different
source. The distance from the station to the hub is a very small part
of the total network propagation budget. There is a maximum diameter
of an Ethernet/802.3 network allowed. This is 51.2 us max delay from
the beginning of a transmission until the guaranteed detection of
a collision on that frame. (Note that this time includes the maximum
length of a jam signal; the actual propagation time is 46.4 us, max).

The reason for this is to ensure that, for a minimum length frame
of 64 bytes (51.2 us), a station will be able to detect if a collision
occurred before finishing the frame transmission. Otherwise, it would not
back-off and retry, and the MAC algorithm would break down.


-- 
Rich Seifert                    Networks and Communications Consulting
seifert@netcom.com              (408) 996-0922
                                (408) 996-2860 FAX
"... specialists in Local Area Networks and Data Communications systems"