dv@well.UUCP (David W. Vezie) (01/12/89)
Can someone give me the definitive word on how long Ethernet
can be? I've looking at the possibility of running standard thick
yellow ethernet cable between two buildings, with a sun acting as a
router on either side. I've read some info which says that the max
is 500M, while other info says that the max is 1000M, and the distance
is between the two (probably about 550-600M). Also, what would happen
if I exceeded the length? Would it not work, or would it be flakey,
or ???
Please reply to me directly, as I don't normally read this
newsgroup (maybe I should :-).
--
David Vezie
{hplabs|lll-crg}!well!dvkwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) (01/12/89)
In article <10315@well.UUCP> David Vezie (dv@well.UUCP) writes: > > Can someone give me the definitive word on how long Ethernet >can be? I've looking at the possibility of running standard thick >yellow ethernet cable between two buildings, with a sun acting as a >router on either side. I've read some info which says that the max >is 500M, while other info says that the max is 1000M, and the distance >is between the two (probably about 550-600M). Also, what would happen >if I exceeded the length? Would it not work, or would it be flakey, >or ??? > The maximum round trip delay between any two transmitters on an Ethernet is 464 bit times. This is equivalent to about 5000 m of coax cable or about 4.3 miles of free space (like microwave). I don't know off-hand how much fiber that is equivalent to, but it should be something like 2000-5000 m. Repeaters and multi-port transceivers add to this delay factor. Multiport transceivers are equivalent to about 50 m of coax. The transceiver on a coax cable is only capable of driving a signal down about 500 m of thick coax, 185 m of thin coax. Fiber optic semiconductors are able to drive standard multi-mode fiber about 2000 m. DEC recommends no more than 1000 m of total fiber path in any repeatered network, but this is a delay limit. So path lengths are limited by delay or by signal attenuation, whichever comes first. If you violate delay, you will have undetected collisions on the net, which will cause packets to be lost unknowingly to the transmitter. This will definitely reduce throughput for some stations on your net. If you violate the signal specs, you will not be able to make the net work at all between certain nodes. In general, these problems are hard to find, since the effects vary according to the traffic patterns. Another point you should keep in mind is the grounding requirement for baseband coax cable. Ground a thick cable in *exactly* one location. If you run thick coax between two buildings whose AC power feeds result in different ground potentials, you will have ground loops, and your Ethernet will not work. However, many people have blissfully installed thick between buildings (including me) without any problem whatsoever. It is definitely economical in some cases. Kent England, Boston University (Was that as good an answer as Jim Warner could give? :-)
karn@jupiter..bellcore.com (Phil R. Karn) (01/14/89)
> However, many >people have blissfully installed thick between buildings (including >me) without any problem whatsoever. Your bliss may end the next time you have a nearby lightning strike. For a few microseconds, you may find yourself with a 10kv potential difference between your building grounds. Ask the network people at the University of Delaware. I *strongly* advise against running coax outside of a building for this reason. Go with fiber. Phil
jqj@oregon.uoregon.edu (J Q Johnson) (01/16/89)
In article <13433@bellcore.bellcore.com>, Phil R. Karn writes: >> However, many >>people have blissfully installed thick between buildings (including >>me) without any problem whatsoever. > Your bliss may end the next time you have a nearby lightning strike. Kent and I agree with Phil in principal. In practice, you might be lucky. At Cornell we installed a thickwire Ethernet between 5 buildings in 1984. Ithaca NY has lots of lightning storms, but I don't think they've blown a transceiver yet (I left before they realized the hole I'd helped dig for them :-).
cuacw@warwick.ac.uk (Malcolm Barker) (01/18/89)
In article <164@oregan.uoregon.edu>, J Q Johnson writes: >Kent and I agree with Phil in principal. In practice, you might be lucky. >At Cornell we installed a thickwire Ethernet between 5 buildings in 1984. >Ithaca NY has lots of lightning storms, but I don't think they've blown >a transceiver yet (I left before they realized the hole I'd helped dig for >them :-). It's my understanding that lightning is not the only hazard to be considered when LAN cables are routed between buildings. If buildings A and B have different high voltage transformers to supply mains power, and an earth fault develops on the high voltage feed to building B, a fault current flows which can result in a high potential difference between the two local building earths. If an Ethernet cable is earthed in building B and is routed between A and B then the high voltage between the building earths also exists between the cable screen and the local earth in building A. Any personnel touching the cable screen and local earth in building A during the fault clearance period could be subjected to a serious hazard. - Malcolm Barker