randy@june.cs.washington.edu (William Randy Day) (03/20/88)
I have a question about thinwire ethernet. Is it possible
to insert a drop cable between the t-tap on the ethernet and the
transceiver tap on a controller's on-board trasceiver? If so,
what are the length limitations on this drop cable?
Thanks,
Randy Day.
Internet (ARPA): randy@dbnet.cs.washington.edu
CSNET: randy%washington@relay.cs.net
UUCP: {decvax|ihnp4}!uw-beaver!uw-june!randymwn@beach.cis.ufl.edu (Michael Nora) (03/22/88)
In article <4491@june.cs.washington.edu> randy@june.cs.washington.edu writes: > >I have a question about thinwire ethernet. Is it possible >to insert a drop cable between the t-tap on the ethernet and the >transceiver tap on a controller's on-board trasceiver? If so, >what are the length limitations on this drop cable? Yea, you can do that. I couldn't find the exact specification in the Digital/ Intel/Xerox v2.0 Ethernet Specifications Manual - all it talks about is the thick Etherhose. But from expirenece we have made drop cables up to about 8 feet long and they seem to be working okay . . . -- Michael Nora | Internet: mwn@beach.cis.ufl.edu University of Florida | UUCP: ...ihnp4!codas!uflorida!beach.cis.ufl.edu!mwn Data & Video Network | MaBellNet: (904) 335-8300
ron@topaz.rutgers.edu (Ron Natalie) (03/22/88)
No, you can not place cable between the transciever and the T-adapter. The T adapter needs to be directly connected to the BNC jack on the integral transciever in the computer. Not only does it violate the specification, it doesn't work at all. Generally a good, human readable, document for setting up Ethernet cables is the DEC Network Buyers Guide. I'm not recommending DEC equipment over anybody elses but the instructions for cabling and such are correct and you will not go wrong designing to their specifications. -Ron
mwn@beach.cis.ufl.edu (Michael Nora) (03/22/88)
In article <Mar.21.13.05.29.1988.1578@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes: >No, you can not place cable between the transciever and the T-adapter. >The T adapter needs to be directly connected to the BNC jack on the >integral transciever in the computer. Not only does it violate the >specification, it doesn't work at all. Wrong. You can place cable between the T-connector and the transceiver jack. On the workstation that I am typing this message from I just placed 12 feet of cable between the T-connector and the transceiver on the back of the unit and it is working just fine. All other units on the segment are working fine also. You just have to remember that if you do this the maximum length of your segment will have to be shorter than the maximum length allowed in the specs. We've done a lot of testing/playing with our segments here just to see how far we can push things before screw-ups occur. I've found that the specifications do have a little leeway - you can place cable between T-s and transceivers, you can go directly from thick-wire to thin-wire via N to BNC adaptors, you can use RG-58 cheapo cable with a velocity of propagation less than .77 - it's just that all these things will reduce the maximum allowable lengths of your segments. I'd suggest you experiment with your segment. Try placing cable between the T's and see if it affects performance. We have an Excelan LANalyzer here and I've looked at segments where there are quite a few workstations with cable between the T's and the transceivers, and there were no abnormalities. -- Michael Nora | Internet: mwn@beach.cis.ufl.edu University of Florida | UUCP: ...ihnp4!codas!uflorida!beach.cis.ufl.edu!mwn Data & Video Network | MaBellNet: (904) 335-8300
eshop@saturn.ucsc.edu (Jim Warner) (03/22/88)
In article <13364@uflorida.cis.ufl.EDU> mwn@beach.cis.ufl.edu () writes: >In article <Mar.21.13.05.29.1988.1578@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes: >>No, you can not place cable between the transciever and the T-adapter. >>The T adapter needs to be directly connected to the BNC jack on the >>integral transciever in the computer. Not only does it violate the >>specification, it doesn't work at all. > >Wrong. You can place cable between the T-connector and the transceiver jack. >On the workstation that I am typing this message from I just placed 12 feet >of cable between the T-connector and the transceiver on the back of the unit >and it is working just fine.... Using cable of any significant length violates the specification. Section 10.6.3 of 802.3 says: A BNC "T" adaptor provides a means of attaching a MAU to the coaxial cable. The connection shall not disturb the transmission line characteristics of the cable significantly; it shall present a low shunt capacitance and therefore a negligbly short stub length... Long (greater than 4 cm) connections between the coaxial cable and the input of the MAU jeopardize this objective. If you want to build and install a network that will work the first time without tuning, will work when you expand it to include the last station, and will continue to work after all the brand new components age and shift their values slightly, then you follow the rules. Tuning isn't, per se, bad. If you already own network diagnostic tools and can test your nonstandard configuration, and you decide this is the most cost effective solution to your networking needs, that's fine. My users want up front accurate estimates of how much they will pay me to install network access. For my estimates to be accurate, I need to know with high confidence that an installation will work the without doing a lot of tuning. So I opt to follow the rules. Beyond that, I want configuration rules that are the same for everybody. I'd be real interested to hear the results of a test where NM put 12 ft of stub coax between *every* station and the network cable. jim warner sr development engineer
ed@mtxinu.UUCP (Ed Gould) (03/22/88)
>>I have a question about thinwire ethernet. Is it possible >>to insert a drop cable between the t-tap on the ethernet and the >>transceiver tap on a controller's on-board trasceiver? If so, >>what are the length limitations on this drop cable? >Yea, you can do that. I couldn't find the exact specification in the Digital/ >Intel/Xerox v2.0 Ethernet Specifications Manual - all it talks about is the >thick Etherhose. But from expirenece we have made drop cables up to about >8 feet long and they seem to be working okay . . . It is a violation of the Ethernet specs to do so, although it may work in some circumstances. The spec is that the distance from the center conductor to the transciever (embedded in the controller in the case of T-tapped thinwire) must be less than some very small number (about 2cm if I remember correctly). The Ethernet spec is loaded with tradeoffs: The system is reasonably well guaranteed to work if the spec is followed carefully even when pushed to the limit. However, it's not often obvious what spec trades off against which other. Violating some specs might work; violating the wrong combination won't. -- Ed Gould mt Xinu, 2560 Ninth St., Berkeley, CA 94710 USA {ucbvax,uunet}!mtxinu!ed +1 415 644 0146 "`She's smart, for a woman, wonder how she got that way'..."
ted@blia.BLI.COM (Ted Marshall) (03/23/88)
The thing to remember is that the general rules were designed as a simple set of rules that, if followed, you would not exceed the actual design limits. Thus, you can break these rules SO LONG AS YOU MAKE THE NECESSARY ADJUSTMENTS ELSEWHERE. This is true of thick and thin wire. For example, with thickwire, you can place transceivers closer than 2.5m apart if you limit the total length of the segment (and maybe the number of transceivers). The misplaced transceivers will set up standing waves on the cable, but if you only have 100m of cable and 20 stations, the signal levels will be high enough to drown out the noise. So you may be able to add 8' drop cables to your thinwire without problems IN YOUR CURRENT CONFIGURATION. Later, if you extend the segment to full length or add a transceiver that is a little weak but within specifications, it may break. I have seen a number of sites that were very sloppy installing their network and it worked fine until the network grew near the limit specified by the rules. Then it fell apart. So the bottom line is: if you must break a rule, document that fact so that later, after you've left, the new network manager knows why it doesn't work even though he's within the specifications. If possible, get a Ethernet expert (no, I'm not that good) to figure out the trade-offs. -- Ted Marshall ...!ucbvax!mtxinu!blia!ted <or> mtxinu!blia!ted@Berkeley.EDU Britton Lee, Inc., 14600 Winchester Blvd, Los Gatos, Ca 95030 (408)378-7000 The opinions expressed above are those of the poster and not his employer.
soley@ontenv.UUCP (Norman S. Soley) (03/23/88)
In article <13328@uflorida.cis.ufl.EDU>, mwn@beach.cis.ufl.edu (Michael Nora) writes: > In article <4491@june.cs.washington.edu> randy@june.cs.washington.edu writes: > >I have a question about thinwire ethernet. Is it possible > >to insert a drop cable between the t-tap on the ethernet and the > >transceiver tap on a controller's on-board trasceiver? If so, > >what are the length limitations on this drop cable? > Yea, you can do that. I couldn't find the exact specification in the Digital/ > Intel/Xerox v2.0 Ethernet Specifications Manual - all it talks about is the > thick Etherhose. But from expirenece we have made drop cables up to about > 8 feet long and they seem to be working okay . . . The reason you can't find the it in the spec is because it is officially a no-no. But lots of people do no-no things and still have working networks, thats one of the nice things about Ethernet. I too use drops like this and 8 feet should be considered the absolute max that will work, shorter is better. Just don't go crying to the vendor if your network doesn't work and you've done things like this. -- Norman Soley - Data Communications Analyst - Ontario Ministry of the Environment UUCP: utzoo!lsuc!ncrcan!---\ VOICE: +1 416 323 2623 {utzoo,utgpu}!sickkids!ontenv!norm ENVOY: N.SOLEY {mnetor,utgpu}!ontmoh/
howell@ecsvax.UUCP (Doc A. Howell) (03/24/88)
If some people have done this and feel sure it works, could someone provide some basic specs on their configuration. Total distance of drops, number of drops per segment, total lengths of thick or thin media, type of equipment used. This type of information would help me out greatly. I currently have a new building that should have about 20 stations per thinwire segment. I plan to use the new 3Com multiconnect box as the ethernet driver. I have a wallplate with 1 (read that ONE) BNC connector in each office. Worse, I have the thin coax wired in a star, not a bus. I inherited this problem, and now I have to make it work. Plan 1 is to rewire the office clusters from the star to tie a group of 10 or so offices together and then connect these to a multiconnect thinwire driver. If drop cables work, then plan 2 will be to make short drops and tee off the wallplate. My max segment distance shouldn't be more than 300 feet. Plan 3 leave town if this doesn't work.
ward@cfa.harvard.EDU (Steve Ward) (03/24/88)
I have some DEC printed matter which says that it is permissible to use a thinwire ethernet tee connector stub cable of <== 27 inches. I am installing some thinwire connections to VS2000's and some PC Clones to talk to a VAX server. The statement is made in documents that came with the DEC hardware/software packages for the PC support, the names being something like "VMS Services for MSDOS" (server software which runs on VAX) and "PC Network Integration Package." I have installed a couple of dozen thinwire connections in an office environment using approx 12 inch tee connector stub cables, and this works fine. The reason I used the stub cables at all was to allow encasing the coaxial tee connector in a small plexiglass package that prevents accidental or deliberate disconnection in a casual way. The present scheme requires a screwdriver to get at the tee connector and there is a printed warning label stating it must not be opened, who to call in case of trouble, etc. This seems to be sufficient to prevent tampering or accidents at our site. The enclosure is simply two rectangular plexiglas plates screwed together with metal, threaded standoffs. The standoffs are (3 of 'em) located so as to secure the tee connector in the middle. A printed label is on the plexiglas plates. This is our simple solution to the hazards of thinwire and I suspect would be insufficient if public/student access to the thinwire cable was the case. Unfortunately it is easy to open up the thinwire ethernet cable with coaxial tee connectors at every connection. s
andrew@sask.UUCP (Derek Andrew) (03/25/88)
|| I have a question about thinwire ethernet. Is it possible || to insert a drop cable between the t-tap on the ethernet and the || transceiver tap on a controller's on-board trasceiver? If so, || what are the length limitations on this drop cable? | Yea, you can do that. I couldn't find the exact specification in the Digital/ | Intel/Xerox v2.0 Ethernet Specifications Manual - all it talks about is the | thick Etherhose. But from expirenece we have made drop cables up to about | 8 feet long and they seem to be working okay . . . No, you cannot do that... just because it works doesn't make it right! We accidentally did this and it worked fine while we had about 4 machines connected this way. The fifth connection took down the segment.
mangler@cit-vax.Caltech.Edu (Don Speck) (03/26/88)
Simple transmission line theory. When the wave travelling down the
Ether hits a tee connector, the energy splits, with 4/9 of it going
down the drop cable, 4/9 continuing on the main cable, and 1/9
reflecting back. The waveform loses 1/3 of its voltage.
When the energy going down the drop cable reflects off the end and
rejoins the main cable, the voltage goes back up. Thus the waveform
has a notch in it, whose duration is twice the round trip time of
the stub. Each time the waveform passes another stub, the notch
gets wider and deeper. When the width of the notch approaches half
a bit time (i.e. 50ns for Ethernet, 500ns for Starlan), the signal
will have reversed itself by the time the reflections add back in
(destructive interference), and things will go downhill fast from
there.
That means the combined length of all stubs is limited to about 3 meters.
If you have 100 taps on the cable, that's 3cm per stub (about what you
get from a single tee and BNC connector). With only 10 stations, you can
have one-foot drop cables.
It might help slightly to use high-impedance drop cables (video cable
or IBM 3270 cable) to reduce the depth of the notch if you're going to
cheat by hanging cables from a tee.
If you need a long drop cable someplace, make it one end of the main
cable and terminate it at the workstation. (Fortunately, cables have
two ends, so even if all your drop cables need to be long, you should
still be able to get two stations per thin ethernet segment).
Don Speck speck@vlsi.caltech.edu {amdahl,ames!elroy}!cit-vax!speckward@cfa.harvard.EDU (Steve Ward) (03/26/88)
In article <5948@cit-vax.Caltech.Edu>, mangler@cit-vax.Caltech.Edu (Don Speck) writes: > Simple transmission line theory. When the wave travelling down the > Ether hits a tee connector, the energy splits, with 4/9 of it going > down the drop cable, 4/9 continuing on the main cable, and 1/9 > reflecting back. The waveform loses 1/3 of its voltage. > When the energy going down the drop cable reflects off the end and > rejoins the main cable, the voltage goes back up. Thus the waveform > has a notch in it, whose duration is twice the round trip time of > the stub. Each time the waveform passes another stub, the notch > gets wider and deeper. When the width of the notch approaches half > a bit time (i.e. 50ns for Ethernet, 500ns for Starlan), the signal > will have reversed itself by the time the reflections add back in > (destructive interference), and things will go downhill fast from > there. > > That means the combined length of all stubs is limited to about 3 meters. > If you have 100 taps on the cable, that's 3cm per stub (about what you > get from a single tee and BNC connector). With only 10 stations, you can > have one-foot drop cables. > > It might help slightly to use high-impedance drop cables (video cable > or IBM 3270 cable) to reduce the depth of the notch if you're going to > cheat by hanging cables from a tee. > > If you need a long drop cable someplace, make it one end of the main > cable and terminate it at the workstation. (Fortunately, cables have > two ends, so even if all your drop cables need to be long, you should > still be able to get two stations per thin ethernet segment). > > Don Speck speck@vlsi.caltech.edu {amdahl,ames!elroy}!cit-vax!speck There seems to be a problem in the predictions/calculations based on the theory presented here based on my own recent experience and most certainly if the many who told of their experiences are accurately describing their drop cables, many seem to VASTLY EXCEEN 3 meters in total drop cable length. As I previously stated, I use drop cables about 12 inches in length, but admittedly the total drop cable length on the thin segment is right around 3 meters, not vastly exceeding it. The segment looks fine with TDR, E-net tester, and LAN analyzer shows that the segment is behaving normally (no pathologic repeat packets due to noise, point-to-point tests error free, etc). But some of the others posting to this discussion were talking about 3-12 foot long drop cables, with multiples of them on a segment, tallying what must be many, many times the 3 meter failure limit described above. What really bothers me is that I installed the 12 inch drop cables based on guidelines in some manufacturer-supplied documents. I guess I am now sufficiently motivated to go back and peruse the bookshelf and find the information. I will post quotations/citation in the next few days. As memory serves me the information stated a 27 inch limit on t-connector drop cables without mentioning any mandatory reduction in number of connections per segment, or reduction in segment length, which now seems incorrect in light of these discussions (the quoted message and others). Perhaps some of those who are using t-connector drop cables, particularly long ones, can add the total drop cable lengths, number of drops, and total thin E-net segment lengths and post this info. I will do this as well. It would be useful to compare notes here. Also, it would be useful to know if you analyzed/tested the segment with a TDR (time domain reflectometer) or other E-net cable tester and/or performed point-to-point testing on the segment and/or monitored the general traffic on the segment using some sort of LAN analyzer. d
miw@uqcspe.OZ (Mark Williams) (04/07/88)
I have been following this discussion with a certain amount of interest, since it is always interesting to see what people get to work. We, too have got a few rather dodgy configurations to work, but we always clean it up as soon as possible, as our network is expanding, and eventually things won't work. The more we play with the rules, the sooner that will be. One solution to the problem of drop cables (albeit an unacceptable one for some id the ThinWire Ethernet Station Adapder. DEC call their version of this the DESTA. It is a transceiver which plugs into the t-connector. That is, it is like a vampire tap that uses a straw. :-) You can then plug a normal transceiver cable (up to 40 metres long) into the 15-pin port on the DESTA. I realise that this is no comfort to those who have controllers with only bnc connectors, but for others it may be of some use. Mark Williams Prentice Computer Centre, University of Queensland, Australia ARPA: ccwilliams%wombat.decnet.uq.oz@uunet.uu.net -- DISCLAIMER: Whenever I tell them my opinions they fall asleep. It is better to have loved and lost, than to have spent your whole life winking