Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site amdcad.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!amdcad!phil From: phil@amdcad.UUCP (Phil Ngai) Newsgroups: net.dcom,net.lan Subject: Re: DELNI trees Message-ID: <12250@amdcad.UUCP> Date: Tue, 8-Jul-86 21:47:48 EDT Article-I.D.: amdcad.12250 Posted: Tue Jul 8 21:47:48 1986 Date-Received: Wed, 9-Jul-86 06:49:46 EDT References: <1057@ogcvax.UUCP> <218@cfa.UUCP> <145@ima.UUCP> Reply-To: phil@amdcad.UUCP (Phil Ngai) Organization: AMD, Sunnyvale, California Lines: 112 Xref: watmath net.dcom:2025 net.lan:1584 In article <145@ima.UUCP> johnl@ima.UUCP (John R. Levine) writes: >In article <218@cfa.UUCP> wyatt@cfa.UUCP (Bill Wyatt) writes: >>Remember that a DELNI takes the place of a transceiver. Transceivers do >>not pass carrier sense through to an interface. Some of those ten wires in a >>transceiver cable are input, others are output. See my original article on >>the difference between an isolated DELNI and a connected one. > >Wrong. Incorrect. Untrue. (Sorry to be rude, but it dismays me to see >people firmly explaining their misconceptions.) The DELNI is not a >transceiver, it is a transceiver multiplexer. All of the stations >downstream from a DELNI need transceivers of their own. A connected DELNI >acts like a repeater from the downstream stations to the upstream cable, and >I presume nobody denies the existence of Ethernet repeaters. I am very surprised to see John Levine passing around such misinformation. I had come to expect much higher quality from him. Let me quote from EK-DELNI-TM-001, "DELNI Ethernet Local Network Interconnect Technical Manual": "1.1.1 Communication Interfaces The DELNI unit provides nine communication interface. These are: o Eight (8) local connectors (labeled 1 through 8) and o One (1) global connector (labeled with a square) 2.1.3 Connected DELNI LAN In a Connected DELNI LAN application (Figure 2-3), the stations are connected to the local connectors of a single DELNI unit operating in the GLOBAL mode. The global connector of the DELNI unit is connected to an Ethernet transceiver." Figure 2-3 confirms what should be obvious from this description, that the stations are directly connected to the DELNI without transceivers. The DELNI itself uses a transceiver to access the main Ethernet coax. Also, the DELNI does not act like a repeater in the Ethernet sense of the word. It does not retime or regenerate preamble. >The real problem with cascaded DELNIs is that the DELNI introduces no >delay to a packet that's passed through it, but it takes a moment to >realize when a packet has started, so a little bit of the packet preamble >is nibbled off. After several levels of DELNI, you lose enough of the >preamble that some stations can't recognize the packet. The specs are that the squelch (both TRANSMIT and RECEIVE) is from 100 to 250 nS and the propagation delay is from 10 to 25 nS. This means that an extra level of DELNI cascading would eat as much as 5 bits from the 64 bit preamble. This assumes an extra level up to the coax and an extra level down from the coax but that only transmitter squelch applies on the way up and receiver squelch on the way down. 250 plus 250 equals 500 nS or 5 bits. Compare this to the requirements on the MAU imposed by 802.3 (Ethernet 2 is identical): 8.2.1.1 Transmit Function Requirements At the start of a frame transmission on the coaxial cable, no more than 2 bits of information may be received from the DO circuit (transmit circuit of the transceiver cable) and not transmitted onto the coaxial medium. In addition it is permissible for the first bit sent to contain invalid data. 8.2.1.2 Receive Function Requirements At the start of a frame transmission from the coaxial cable, no more than 5 bits of information may be received from the coaxial cable and not transmitted onto the DI circuit (receive circuit of the transceiver cable). In addition it is permissible for the first bit sent over the receive circuit to contain invalid data. So we have a total of 3 plus 6 bits gobbled by the transceiver/MAU path (one into and one out of the coaxial cable). Section 7.5.1.2 of the Ethernet 2 spec requires the decoder of the station to operate with only 16 bits. I don't know if 802.3 has a similar requirement. For a non-DELNI system, there is 64 - 9 - 16 = 39 bits of preamble margin. For a single level DELNI system, there is 64 - 9 - 16 - 5 = 34 bits of margin. For a two level DELNI system (illegal by DEC configuration rules) there is 29 bits of margin. (fudge the above numbers by a couple of bits to allow for seeing the 11 which ends the preamble) I may have missed something here but it doesn't seem to me that truncated preamble is a reason for limiting Connected DELNIs to one level. Despite having just argued to the effect that a cascaded Connected DELNI should work, I would not recommend the use of such a configuration. There may be other reasons DEC does not support such a configuration. >I have personally lashed up 3-deep connected DELNIs and it worked, but I >was lucky because all of the transceivers involved could deal with the >truncated preamble. Actually, it's the station that has to deal with truncated preamble since it has to generate a clock out of the incoming bit stream. Yes, it may work at the time when you install it but the next time you expand the network, will it still work? I must also confess to being confused by Bill's statement that "transceivers do not pass carrier sense through to an interface". Assuming he means a station when he says "interface", is not the presence of signal on Circuit DI (received data) of the AUI cable an indication of carrier? And what are the implications of this with regard to cascaded DELNIs? -- Bring back The Phone Company! Phil Ngai +1 408 749 5720 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!phil ARPA: amdcad!phil@decwrl.dec.com