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Path: utzoo!utcs!mnetor!seismo!ll-xn!cit-vax!amdahl!amdcad!phil
From: phil@amdcad.UUCP
Newsgroups: net.dcom,net.lan
Subject: Re: DELNI trees
Message-ID: <12275@amdcad.UUCP>
Date: Fri, 11-Jul-86 19:11:16 EDT
Article-I.D.: amdcad.12275
Posted: Fri Jul 11 19:11:16 1986
Date-Received: Sat, 12-Jul-86 02:01:01 EDT
References: <1057@ogcvax.UUCP> <218@cfa.UUCP> <145@ima.UUCP> <12250@amdcad.UUCP> <320@encore.UUCP>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: AMD, Sunnyvale, California
Lines: 68
Xref: utcs net.dcom:2010 net.lan:1565

Well, I hate to disagree with anyone who makes a terminal server as
nice as Encore's Annex but I do have to inject some references into
this discussion.

In article <320@encore.UUCP> bartlett@encore.UUCP (John Bartlett) writes:
>
>There are a number of parameters that I am sure DEC is considering
>when they disallow cascaded DELNI, and the comments from Phil on
>lost preamble bits are one of the issues.
>
>The thing Phil did not address in his calculation of preamble bits
>is the presense of other bit eaters in the ethernet network such as
>repeaters.  In a less than full sized network, preamble eating is
>not a problem.

Ethernet Version 2.0, November, 1982, section 7.6.4.1, page 83:

"The repeater must transmit 64 preamble bits onto the outgoing segment"

ANSI/IEEE Std 802.3-1985, section 9.1.2.3, page 126:

"PREAMBLE INSERTION. The repeater unit shall output at least 56 bits
of preamble followed by the start frame delimiter."

(IEEE decided to be byte oriented and broke up the Ethernet 64 bit
preamble into a 56 bit "preamble" and an 8 bit SFD. When you string
all these bits together, the result looks just like a 64 bit Ethernet
preamble.)

>The second issue is the total round trip delay time of the network.
>Each DELNI does add propagation time delay to the network, and the
>total amount of network delay should not be violated because it is
>tied to the slot time and used in the CSMA/CD protocol.  In any
>specific application this may not be an issue, because again a full
>sized network may not be implemented.  Remember, a full sized
>network (from a delay point of view) has as much as 2.5 KM of length
>including a 1KM repeater link.  If you are operating with a single
>500M segment, delay won't be much of an issue.

A DELNI has a delay of 25 nS each way, maximum. (reference
EK-DELNI-TM-001 "DELNI Ethernet Local Network Interconnect Technical
Manual".) 1000 meters of point to point repeater link has a delay of
5,000 nS each way. 1500 meters of coax has a delay of 6,500 nS each
way. This is straining at gnats and swallowing camels if you think the
propagation delay of a DELNI is an issue. Well, if you had 460 DELNIs
in series you might want to worry. I don't generally deal with things
less than 1%.

>The last issue is signal jitter.  Jitter defines how far in or out
>of a bit window a signal can transition, and still be properly
>detected by the receiving logic.  Each thing (active or passive) in
>the network adds jitter to the signals (including the DELNI) and
>degrades the signal quality.

This could be a reason. The DELNI has 1 to 2 nS of timing asymmetry.
The entire system jitter budget is only 18 nS, including margin, in
802.3.  Of that 18 nS, 16.5 nS is already allocated. So the use of a
DELNI or two already puts the system on shaky ground with regard to
jitter margin. Cascading them could very well be a problem because of
this. By way of comparison, the MAU is allowed 2 nS in the transmit
direction and 1 nS in the receive direction.

-- 
 Why did VD become STDs?

 Phil Ngai +1 408 749 5720
 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!phil
 ARPA: amdcad!phil@decwrl.dec.com