From: seifert@netcom.com (Rich Seifert)
Subject: Re: Who says DELNIs and DEMPRs alter the specs.
Organization: Networks & Communications Consulting
Date: Sun, 24 Jan 1993 00:29:03 GMT

In article <93019.171229DART@SLACVM.SLAC.STANFORD.EDU>, Martin L. Dart (415) 926-2745, <DART@SLACVM.SLAC.STANFORD.EDU> writes:
> I am soliciting comments and specific information about limits
> which are to be imposed on Ethernet segments with DEMPRs and DELNIs
> on them.  Please don't reply to this note with the standard "...these
> devices cause delays on the network.", because I don't believe it.  I
> have always believed that discrete components  could
> outperform a bunch of standalone units connected to a big antenna, aka
> Ethernet cabling.   Will someone who has actual test data please respond.
> Alternatively, if you have knowledge of tests that have been performed
> to substantiate your claims that could be useful too.
>

I don't need performance tests, I helped with the design of the DELNI and 
am familiar with the "innards". 

A DELNI haas a complete AUI cable interface circuit for each port, including
the port for connecting to a backbone. This circuit introduces "Ethernet
standard" delays" (3 bit times on transmit, carrier sense, and collision
presence detect). In addition there are some synchronizers, and a small
state machine (actually, just some discrete logic). The total delay through
a DELNI is on the order of bit times (100's of nanoseconds).

Doesn't sound like much, but unfortunately, every nanosecond of system
delay is accounted for in the Ethernet/802.3 specifications, and the DELNI
is not included there. Sooo, if you have DELNIs, you must make sure that
the system delays are not near worst-case (i.e., no max length cables, 
etc.).

As to the DEMPR, that is a complete repeater, and has all of the delays
associated with normal repeaters. This includes the DELNI-like delays
above, plus decoder delay, FIFO delays, etc. The 802.3 specification
gives a thorough presentation of system delays for repeaters.
 
> I'm sorry for sounding snooty, but I've read several articles talking about
> the necessity of imposing further limitations on Ethernet LANs.  The reason
> always being because they cause 'delays', but never a mention of how long
> the delay is...10, 20, 90 microseconds, how about a minute or two?  Also,
> these articles frequently state that removing a DELNI or DEMPR and replacing
> them with something else fixed the problem so a huge leap of faith is made
> to the conclusion which states that '...they were causing excessive delays.'.
> I say I have a very busy network, with many DEMPR and DELNI type devices
> connected to it in a way which violates these pseudo-specs yet the problems
> of 'delays' has not surfaced.
>

More often than not, the solution of problems with/without DELNIs has
to do with the implementation of heartbeat in the DELNI. A shortcut
was taken which can cause problems with some controller implementations.

 

-- 
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"
Newsgroups: comp.dcom.lans.ethernet