lfm@ukc.UUCP (L.Marshall) (03/13/84)
Anyone got experience of using wet string, washing line etc., as substitute
for the extruded gold that must be used inside most manufacturers ethernet
cables?
Thanks in advance.
Lindsay F. Marshall
uucp : ...!{mcvax,vax135}!ukc!lfm
ARPA : Lindsay_Marshall%NEWCASTLE@MIT-MULTICS
post : Computing Laboratory, U of Newcastle upon Tyne, U.K.
+44 - 632 - 329233 xtn 212rpw3@fortune.UUCP (03/16/84)
#R:ukc:-412600:fortune:11600074:000:1838
fortune!rpw3 Mar 16 02:54:00 1984
Nothing's free. Triple-shielding plus double foil shielding plus silver
plating the center conductor does cost money. For the budget minded, you can
use virtually any 50 ohm cable of an appropriate diameter to mate with your
Ethernet transceivers, BUT... you lose some (or a lot) of:
- maximum distance (due to increased attenuation)
- maximum number of stations (attenuation and impedance variations)
- EMI immunity (poorer shieding) [stuff getting IN]
- RFI protection (shielding again) [stuff getting OUT]
- long term mechanical stability (poorer quality insulation)
- fire hazard protection (non-Teflon cable)
- fire code approval (if non-Teflon and used in HVAC plenums)
For a small number (two dozen?) of stations with a short cable (100m ?)
under no fire code or electrical code restrictions and in an electrically
"clean" environment where you're not going to zap anybody's radio reception,
you can get away with standard RG-8/U cable (Radio Shack, among others).
Some companies are also using small diameter cable with their products
(notably 3-Com's "thin ethernet"). Again, degradation depends on the quality
of the cable. RG-58/U isn't half as good as RG-8/U, but there's an RG-1xx/U
(sorry, don't know the number) that is supposed to be relatively good.
But why gamble? Standard conforming cable is NOT that expensive, if you shop
around. You can buy it directly from Belden, Brand-Rex, and other fine cable
manufacturers for under $2.00/meter (retail is 3 times that). One hundred
meters is no more than ONE transceiver costs (roughly). By the time your net
is big enough to worry about the cost of the cable, you NEED that cable!
(Catch-22)
Rob Warnock
UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3
DDD: (415)595-8444
USPS: Fortune Systems Corp, 101 Twin Dolphin Drive, Redwood City, CA 94065dmmartindale@watcgl.UUCP (Dave Martindale) (03/20/84)
If you use anything other than "real ethernet cable", make sure that the transceiver physically attaches to it correctly before you buy it. Transceivers tend to bolt directly onto the cable, making their electrical connection through a drilled hole. A change in outside diameter of the cable, thickness of insulation, or diameter of inner conductor could cause a non-existent or poor connection. Also, the Ethernet specs say what the velocity factor and resistive losses of the cable should be to guarantee that the system will operate properly out to its distance limits. Either buy cable which is within the specs, or make SURE you understand how it will affect the network you can build. Finally, real ethernet cables have marks on them telling you where to install the transceivers so that the impedance discontinuities produced by the transceivers produce reflections which tend to cancel each other rather than adding. If you buy cable which isn't marked in this way, measure it for yourself.