Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site encore.UUCP Path: utzoo!linus!encore!bartlett From: bartlett@encore.UUCP (John Bartlett) Newsgroups: net.dcom,net.lan Subject: Re: DELNI trees Message-ID: <320@encore.UUCP> Date: Thu, 10-Jul-86 11:18:02 EDT Article-I.D.: encore.320 Posted: Thu Jul 10 11:18:02 1986 Date-Received: Fri, 11-Jul-86 07:22:49 EDT References: <1057@ogcvax.UUCP> <218@cfa.UUCP> <145@ima.UUCP> <12250@amdcad.UUCP> Reply-To: bartlett@encore.UUCP (John Bartlett) Organization: Encore Computer, Marlboro MA Lines: 48 Xref: linus net.dcom:1764 net.lan:1399 Summary: 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. 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. 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. DEC recommends not cascading DELNI because they have to insure your system will work in any configuration. It would be very difficult to specify how to figure out if you can have x levels of cascading for a y length network etc. etc. The Encore Annex has a little single line DELNI that allows one other station to use the same transceiver that it is using. The way I specified the network configuration issue for the Annex was to coorelate the Annex to a certain amount of transceiver cable. In other words, the unit cascaded through the Annex cannot be more than 50 meters from the transceiver (Ethernet spec) and the Annex itself looks like 4 meters of cable. Hence the combined transceiver cables between the Annex and the Transceiver, and the second unit and the Annex cannot exceed (50 -4) 46 meters. John Bartlett {ihnp4,decvax,allegra,linus}!encore!bartlett Encore Computer Corp. 257 Ceder Hill Street Marlboro, Mass. 01752 (617) 460-0500 Opinions are not necessarily those of Encore Computer Corp.