MOLTA@VAXB.ACS.UNT.EDU (02/01/90)
>This is taken from "International Standard, ISO 8802-3, ANSI/IEEE Std >802.3, First edition 1989-02-24", Section 10 on Type 10BASE2. Under >10.7.2 Transmission System Requirements, "In order to maintain reflections >at an acceptable level, the minimum length cable section shall be 0.5m." Thanks for the authoritative answer. Does this mean that there needs to be a .5m stub tacked on to the last PC on a segment. We've been in the (possibly bad) habit of attaching a terminator directly to the T-connector on the last PC attached to an Ethernet segment. We haven't noticed any problems, but every once in a while, there are unsolved mysteries on our networks. I'd also be curious to hear about people's experiences in violating thin Ethernet cable distance restrictions, which officially call for segments of 200 meters (about 650 feet). There are several vendors, including 3Com, that claim to support segments of 1000 feet. While we try to keep within spec, I know we have several thinwire Ethernets on campus that are in the 700-800 ft. range. How far as anybody tried to push these limits? We've run RS-232 cable 1000 ft. at 9600 baud even though the spec says 50 ft. Is there any comparison? Dave Molta University of North Texas
ABTSACS@UWLAX.BITNET (Dan Abts - UWL System/Network Manager) (02/01/90)
>>This is taken from "International Standard, ISO 8802-3, ANSI/IEEE Std >>802.3, First edition 1989-02-24", Section 10 on Type 10BASE2. Under >>10.7.2 Transmission System Requirements, "In order to maintain reflections >>at an acceptable level, the minimum length cable section shall be 0.5m." >Thanks for the authoritative answer. Does this mean that there needs to be >a .5m stub tacked on to the last PC on a segment. We've been in the (possibly >bad) habit of attaching a terminator directly to the T-connector on the last PC >attached to an Ethernet segment. We haven't noticed any problems, but every >once in a while, there are unsolved mysteries on our networks. We have 1 meter between each station and the terminator directly on the T. This is what 3com recommended 6 years ago. >I'd also be curious to hear about people's experiences in violating thin >Ethernet cable distance restrictions, which officially call for segments of >200 meters (about 650 feet). There are several vendors, including 3Com, that > claim to >support segments of 1000 feet. While we try to keep within spec, I know we >have several thinwire Ethernets on campus that are in the 700-800 ft. range. >How far as anybody tried to push these limits? We've run RS-232 cable 1000 ft. >at 9600 baud even though the spec says 50 ft. Is there any comparison? > >Dave Molta >University of North Texas We have all 3com boards and have gone over 1200 feet with no problems. Dan Abts UW-La Crosse
JRD@USU.BITNET (Joe Doupnik) (02/01/90)
Dave,
Ethernet cabling discussion. Normally the separation between thin or
thick wire cable taps is 2.5 meters. The reason is to break up the small
reflections from each tap and is related to the propagation velocity of the
signal in the cable.
The final terminator can be attached to the T connector on the final
PC, no extra short wire is needed.
If you use one of the vendor's "bright ideas" on longer length runs
then beware that all (and I do mean all) the boards must be set for the
same peculiar signaling method because otherwise the collision detection
circuitry will declare collisions nearly all the time (the signal level is
outside the standard ranges for 0V and -2V levels). Try this: regular signaling,
put a scope on the last PC, then remove the terminator. The signal level jumps
up by a factor of 2 from the reflection. That's the level for these extended
length ideas.
The official max thin wire segment is 185 meters and is based on the
attenuation of the signal through typical cable. With better quality foam
dielectric and double shielded thin wire, such as made by Belden for Digital
at $210/1000 feet, that attenuation is slightly less and hence the cable can
be lengthened by say 10% or so (which we all do anyway, given how wires get
run). What we must never do is make the electrical length of the cable longer
than 256 bit times (Ethernet 512 bit slot time / 2) because efficient
collison detection requires that the sender be able to hear a collision while
it is transmitting. The worst case senario is PCs at the far ends transmit
almost simultaneously, A going before B by the cable propagation delay. B
starts nanoseconds before A's signal gets to it, B sees a collision and quits,
A must see that tiny collision fragment and also declare an error BEFORE it
finishs the current packet. With the 512 bit min packet size that sets the
max cable length to 256 bits.
In other words, yes, a little stretch is ok but more is worse.
RS232 and Ethernet have no, repeat no, relation about lengths. The
underlying princples are different.
Ethernet repeaters ($$$) help with long lines, and help in a subtle
way of cleaning up gitter of the individual bits from reflections. Two of
these fellows per cable, max. After that one needs a MAC level bridge ($1800,
or an el cheapo PC with PCBridge software and consequent management headaches).
Finally, just a word on cable types for those listening to previous
discussions on that matter. RG58 is a generic identification, like Chevy.
Real plain RG58 is 53.5 ohms impedance stuff; not what we need. RG58A is
50 ohms. Cable of the RG description is ok for short runs but specially made
low loss cable for Ethernet is better all around. As mentioned above, DEC
sells the grey jacket, double shielded, stranded center conductor, foam
dielectric thin wire, made by Belden, for about $210/1000 feet. Good price for
a good cable. Another point is that the connectors should be the crimp type,
never the "easy on" or "twist on" or other trash, and the center conductor
pin gets crimped too. A good crimp tool for RG58 gauge cable costs around
$40 to $80; expensive but worth it, especially the latter kind. Wire is the
cheapest part of the whole installation so don't cut corners there. Connectors
are expensive ($2+ apiece) but the good ones don't require servicing at
embarassing times.
Joe D.cfinney@UMAXC.WEEG.UIOWA.EDU (Charles Finney) (02/02/90)
The specification do not say as regards the .5m stub. I haven't worked through the spec to determine if it is a requirement or not. We terminate our machines without a stub, but the question has come up here because of intermittent problems. As for lengths over 185m, some boards will work and some won't. We have a seriously mixed bag of adapters, with a Sniffer it became apparent that not all adapters packets would reach the other end of the cable intact. Shortened the cables to spec using multiport repeaters and the dropped packet problems disappeared. Another potential problem is maximum end to end delay is 950 ns. Collision detection depends on this timing, 185m is the limit at .65c for propagation rate. Unless you are willing to live with the increased error ratesand dropped packets, etc. it's not a great idea. Charles E. Finney University of Iowa