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Theron Hicks wrote:
> Dan,
>     You are _extremely_ lucky.  The "f" type connector is a 75 ohm connector.
> Coaxial ethernet (10 base-2) type connections are 50 ohm connections.  If you wire
> a 50 ohm ethernet connection with 75 ohm cable it should _not_ work.  If you are
> lucky it may function but not on a consistent basis.  I expect that you are
> dropping packets like crazy.  Lest someone claim that I am crazy I can state that I
> have had to trouble shoot networks where someone has used 75 ohm cable.  The
> problem is very difficult to locate as the 2 cables look quite similar.  In you
> case the section of your system which is 75 ohm may be short enough (in length)
> that it manages to work but this is _not_ a recomended practice.
> 
> Daniel Erskine wrote:
> > Greetings:

>>> <<< SNIP >>>

> > If you try this, you *might* want to use differing color CATV plates, so
> > that you want accidentally plug a TV, descrambler, VCR, etc. into it ;(
> > You will want this setup to NOT connect into your existing CATV lines, as
> > something will probably get FRIED!

>>> <<< SNIP >>>

> > Hope this helps,
> > Daniel W. Erskine            "mailto:erskined@gte.net"
> > (314) 515-1406       "mailto:netsup75@edwardjones.com"

Greetings:
  I wouldn't think this (above) would make me "_extremely_ lucky", just
*VERY*
cautious!  I NEVER connected the 50ohm 10b2 into the 75ohm CATV network! 
Both
"networks" are self-contained, or atleast the PC network is ;)
  Based on your observation, YES combining both networks should not work at
all!
A friend of mine told me one of infamous "school-of-hard-knocks"
war-stories
about doing this exact same thing... Aaa-ack!

Sorry, I did not clarify this better!
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(314) 515-1406       "mailto:netsup75@edwardjones.com"
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Daniel Erskine wrote:
> 
> I've reviewed all the previous responses.  So far, I haven't seen
> this option presented.  Radio Shack sells "BNC"-to-"F" cable
> connectors/adapters. You will need a few for this scenerio.  You will
> also need a few standard CATV outlet plates, and a "little bit of
> elbow-grease" ;)  

If you are going to expend any "elbow grease" you should be stepping up
to CAT-5 not down to CATV.

> I've tried this setup in a previous apartment to connect (2) adjoining
> rooms.  In my example, I used an existing cable TV connection in a
> wall separating (2) adjoining rooms, 

If the wire is already in place and can be dragged through the wall it
would provide a great wire pull for some CAT-5. If it can't be pulled 
out conviniently then you might think about using it.

The last time I priced BNC to RJ-45 converters they were in the $75 
each range.  For $75 I can get 1000 ft of CAT-5, for another $75 I can
get a crimper, a 100 RJ45 ends, and 4-6 wall plates.  For 2 PC's
it's a wash $ wise.  FOr 3 or more you'll need a hub and if we are 
talking 10BaseT you are pulling out in front on the $'s.  If we are
talking 100BaseT you win with the 2 PC's ... unless there have been
some developments in CATV speeds.

> you could "run" your cable from room-to-room, from the "outside" of
> your house by going through the 2-by-4 wall studs!

If you can do this then you could even consider fiber! *8^)

Paul
-- 
ISDN is almost like actually being connected to the net.
Wonder what xDSL will be like?

In article <35DB3C67.1041534F@edwardjones.com>,
Daniel Erskine  <netsup75@edwardjones.com> wrote:

>  I wouldn't think this (above) would make me "_extremely_ lucky", just
>*VERY*
>cautious!  I NEVER connected the 50ohm 10b2 into the 75ohm CATV network! 
>Both
>"networks" are self-contained, or atleast the PC network is ;)
>  Based on your observation, YES combining both networks should not work at
>all!
>A friend of mine told me one of infamous "school-of-hard-knocks"
>war-stories
>about doing this exact same thing... Aaa-ack!

>Sorry, I did not clarify this better!


The basic issue with respect to mixing the 75 Ohm CATV cable with the 50
Ohm Ethernet cable is that because of the different impedances, there
will be signal reflections at the junction.  These reflections can cause
various problems, including such things as (apparent) collisions.

The phenomenon in question has nothing to do with whether or not there
actually is a TV signal on the CATV wire.  Rather, it's similar to the
effect when you look through a window:  the glass has a different index
of refraction from the air (or whatever medium you're looking through,
unless you're in some rather strange circumstances) -- actually, "glass"
is a very loose category of things with a considerable range of indices
of refraction.  Anyway, depending on ambient lighting &c., you'll often
see a reflection in the glass.

If you could "adjust" the index of refaction of the window or the
intervening medium (or both), you would find that the intensity of the
reflection would be minimal when the indices of refraction match.

Similarly, in an electrical circuit, when a signal gets to a point where
there's a (sudden) change in the circuit's impedance, part of the signal
will continue on through, but part will be reflected.

BTW, this is also why coax Ethernet uses terminating resistors -- the
end of the wire is an exceedingly high "impedance mismatch".  :-}  The
resistors' function is to absorb any signal that gets that far (and
there should be a fair amount of that!) to minimize the reflections at
the end.  And this also applies to other terminators, such as those on
SCSI chains....

So:  sometimes you can get "lucky" in spite of impedance mismatches; the
busier the circuit, the less lucky you're likely to be.

Hope this helps clarify what's going on,
david
-- 
David H. Wolfskill				david@dhw.vip.best.com
As a computing professional, I believe it would be unethical for me to
advise, recommend, or support the use (save possibly for personal
amusement) of any product that is or depends on any Microsoft product.

Hmmm, if you gotta ask... :)

comp.home.automation is a good place to start. There are constant
discussions of interior wiring, both before and after a house is
constructued. I live in a 2 story house with a finished basement, and
have had to run cable through *lots* of walls, cubbyholes, plenums,
etc. I've got a Merlin Plus phone switch (4-pair per extension, cable
hog!), Appletalk and single-lines/fax (cat-3), plus video and cable
modems (RG-6 quad-shield for video/data cable), and I've wired just
about every room in the house (including bathrooms), so I know the
*insides* of the house better probably then the builders. The real
trick is getting between floors - sometimes that means going form the
basement to the house attic, just to wire the 2nd floor. The trick is
to get to know the 'cubby-holes', such as the attic, basement,
crawlspaces, etc. Just be patient, find the proper runs, install good
outlet boxes to the studs, do *NOT* under *ANY* circumstances run high
(power) and low (data) voltage cable together, and it'll all work out.

On Tue, 18 Aug 1998 20:35:03 GMT, kanryo@juno.com wrote:

>I want to run cabling between 2 PCs for networking purposes.  Unfortunately
>the 2 PCs are in different parts and floors of a large house.  The most
>direct pathway would be out one sliding window/external wall and into
>another.  But it seems that cold air will seep through any holes are that
>made or used.  Is there a way to feed the cabling out and back into the house
>without enabling cold air to seep in?  Or is there a way to somehow feed the
>cabling through the central air/heat ducts?  Thanks for any
>advice/suggestions.
>
>-----== Posted via Deja News, The Leader in Internet Discussion ==-----
>http://www.dejanews.com/rg_mkgrp.xp   Create Your Own Free Member Forum

On Wed, 19 Aug 1998 21:54:21 -0700, newhouse at rockhead dot com
<Dontspamme@nojunk.net.crap> wrote:

> If you are going to expend any "elbow grease" you should be stepping up
> to CAT-5 not down to CATV.

Honestly!  He should get a hub and pull multiple runs of CAT5.  If going
outside, use PVC electrical conduit.  It's cheap stuff and will protect
the cables for years and years.

> If the wire is already in place and can be dragged through the wall it
> would provide a great wire pull for some CAT-5. If it can't be pulled 
> out conviniently then you might think about using it.

I'll bet any existing coax is either RG6 or RG59 (75ohm CATV COAX)
with F connectors.  It's probably also stapled to the studs inside
the wall!  If he really wants to use 10-BASE-2, then he wants RG58
(50ohm COAX) with BNC connectors.  Using existing 75ohm coax as if
it was 50ohm coax (with F to BNC adapters) is asking for trouble.

- Dan

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Dan Cook wrote:
> On Wed, 19 Aug 1998 21:54:21 -0700, newhouse at rockhead dot com
> <Dontspamme@nojunk.net.crap> wrote:
> 
> > If you are going to expend any "elbow grease" you should be stepping up
> > to CAT-5 not down to CATV.
> 
> Honestly!  He should get a hub and pull multiple runs of CAT5.  If going
> outside, use PVC electrical conduit.  It's cheap stuff and will protect
> the cables for years and years.
> 
> > If the wire is already in place and can be dragged through the wall it
> > would provide a great wire pull for some CAT-5. If it can't be pulled
> > out conviniently then you might think about using it.
> 
> I'll bet any existing coax is either RG6 or RG59 (75ohm CATV COAX)
> with F connectors.  It's probably also stapled to the studs inside
> the wall!  If he really wants to use 10-BASE-2, then he wants RG58
> (50ohm COAX) with BNC connectors.  Using existing 75ohm coax as if
> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> 
> - Dan

Greetings:
  Thank you for the information!  Your idea of PVC conduit is a pretty
good idea, although it might look a little gaudy ;)
  And I have to agree!  AFAIK, CATV lines are *usually* stapled to the
wall studs UNLESS the house did not have preexisting wiring!  In that
case, the CATV lines were probably run along the outside walls.
  I'll inquire with my network staff for more research material, as
I'm curious as to why 75ohm cable would be that much of a problem?!?

Thanx, again :)
------------------------------------------------------
    THE PRECEDING REFLECTS THE VIEWS OF THE AUTHOR
   AND NOT, NECESSARILY, THE VIEWS OF EDWARD JONES.
------------------------------------------------------
Daniel W. Erskine            "mailto:erskined@gte.net"
(314) 515-1406       "mailto:netsup75@edwardjones.com"
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On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
<netsup75@edwardjones.com> said:

>> Using existing 75ohm coax as if
>> it was 50ohm coax (with F to BNC adapters) is asking for trouble.

>  I'll inquire with my network staff for more research material, as
>I'm curious as to why 75ohm cable would be that much of a problem?!?

I wonder as well.

For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
make a difference.  That measurement is a measure of the cable's
impedance over some distance (1000 ft?), so for a short bit of maybe
50-100 ft, the difference *may* be neglible.

Of course, this all depends on how senstive the receiving equipment is,
and I can't make any claims to know how it would really affect the
performance.

The reason you *should* use properly rated cable is because with
impedance matching you can achieve maximum power output.

In article <35e6319c.53011606@news.lakwod1.co.home.com>,
Aaron Blosser <blosser@home.com> wrote:
>>  I'll inquire with my network staff for more research material, as
>>I'm curious as to why 75ohm cable would be that much of a problem?!?

>I wonder as well.

>For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
>make a difference.  That measurement is a measure of the cable's
>impedance over some distance (1000 ft?), so for a short bit of maybe
>50-100 ft, the difference *may* be neglible.

>Of course, this all depends on how senstive the receiving equipment is,
>and I can't make any claims to know how it would really affect the
>performance.

>The reason you *should* use properly rated cable is because with
>impedance matching you can achieve maximum power output.

And, as I wrote a few days ago in this thread, the basic issue isn't the
length of the cable -- that's a concern for other reasons -- but the
transition-points.

	You get reflections at the points where there's an
	impedance mismatch.

The same kind of effect may be seen by looking at an object with or
without a pane of glass intervening -- the thickness of the pane isn't
all that much at issue; that it's there at all, and has an index of
refraction different from the surrounding medium (air, usually), causes
observable reflections if there's a signal coming from the observer's
vantage-point.

Reflections not only represent some signal lost, but also increase the
probability of a reflection being interpreted as something that could
contribute to an Ethernet collision.

david
-- 
David H. Wolfskill				david@dhw.vip.best.com
As a computing professional, I believe it would be unethical for me to
advise, recommend, or support the use (save possibly for personal
amusement) of any product that is or depends on any Microsoft product.

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David Wolfskill wrote:
> And, as I wrote a few days ago in this thread, the basic issue isn't the
> length of the cable -- that's a concern for other reasons -- but the
> transition-points.
> 
>         You get reflections at the points where there's an
>         impedance mismatch.
> 
> The same kind of effect may be seen by looking at an object with or
> without a pane of glass intervening -- the thickness of the pane isn't
> all that much at issue; that it's there at all, and has an index of
> refraction different from the surrounding medium (air, usually), causes
> observable reflections if there's a signal coming from the observer's
> vantage-point.
> 
> Reflections not only represent some signal lost, but also increase the
> probability of a reflection being interpreted as something that could
> contribute to an Ethernet collision.
> 
> David H. Wolfskill                              david@dhw.vip.best.com

Greetings:
  This sounds logical to me :)  I'm not a network topologist yet, so
whatever I learn is always appreciated :)

Thanx,
------------------------------------------------------
    THE PRECEDING REFLECTS THE VIEWS OF THE AUTHOR
   AND NOT, NECESSARILY, THE VIEWS OF EDWARD JONES.
------------------------------------------------------
Daniel W. Erskine            "mailto:erskined@gte.net"
(314) 515-1406       "mailto:netsup75@edwardjones.com"
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With apologies to the other contributors on this thread:

I was wondering if the larger issue here is finding the right diameter bit
for glass .  . . .     :-)       (sorry, sort of)   Regards,  Jim

In <35e6319c.53011606@news.lakwod1.co.home.com>, blosser@home.com (Aaron Blosser) writes:
>On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
><netsup75@edwardjones.com> said:
>
>>> Using existing 75ohm coax as if
>>> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
>
>>  I'll inquire with my network staff for more research material, as
>>I'm curious as to why 75ohm cable would be that much of a problem?!?
>
>I wonder as well.
>
>For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
>make a difference.  That measurement is a measure of the cable's
>impedance over some distance (1000 ft?), so for a short bit of maybe
>50-100 ft, the difference *may* be neglible.
>
>Of course, this all depends on how senstive the receiving equipment is,
>and I can't make any claims to know how it would really affect the
>performance.
>
>The reason you *should* use properly rated cable is because with
>impedance matching you can achieve maximum power output.

With 10Base2 proper impedance matching is critical to prevent reflections 
-- which cause errors and false collisions.  However, for reasonably small
nets you should be able to use 75 ohm cable **IF** you use 75 ohm 
terminators.  The drivers and receivers should be able to tolerate the 
difference in current and voltage.

Dan Hicks
Hey!!  My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks

Are you certain that to use short runs of 75 ohm cables terminated with 75
ohms is better than using short runs of 75 ohm cable terminated with what
ethernet expects to see 50 ohms?

It's just that the current drive for the ethernet will put out and then
look for a collision.  The current drive will go out to the 75 ohm load
and produce 50% higher voltage than expected (half way to a collision)
then all the ringing will get added to that.  

Versus, taking your lumps at the cable mismatch but making certain the
ethernet drive only drives 50 ohms.  

It seems qualititatively that 50 ohms would buy you more margin for error
since at short run lengths there would be absolutely no ringing from
mismatch and the system would work perfectly.  

                                                  - Robert -

PS  of course if the runs were into the 100 ft + this might all change,
but then I'd just model the system's components and run it through PSpice
demo to see which way works best.



On 29 Aug 1998 danhicks@millcomm.com wrote:

> In <35e6319c.53011606@news.lakwod1.co.home.com>, blosser@home.com (Aaron Blosser) writes:
> >On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> ><netsup75@edwardjones.com> said:
> >
> >>> Using existing 75ohm coax as if
> >>> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> >
> >>  I'll inquire with my network staff for more research material, as
> >>I'm curious as to why 75ohm cable would be that much of a problem?!?
> >
> >I wonder as well.
> >
> >For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> >make a difference.  That measurement is a measure of the cable's
> >impedance over some distance (1000 ft?), so for a short bit of maybe
> >50-100 ft, the difference *may* be neglible.
> >
> >Of course, this all depends on how senstive the receiving equipment is,
> >and I can't make any claims to know how it would really affect the
> >performance.
> >
> >The reason you *should* use properly rated cable is because with
> >impedance matching you can achieve maximum power output.
> 
> With 10Base2 proper impedance matching is critical to prevent reflections 
> -- which cause errors and false collisions.  However, for reasonably small
> nets you should be able to use 75 ohm cable **IF** you use 75 ohm 
> terminators.  The drivers and receivers should be able to tolerate the 
> difference in current and voltage.
> 
> Dan Hicks
> Hey!!  My advice is free -- take it for what it's worth!
> http://www.millcomm.com/~danhicks
> 
> 
> 
> 

    Scrap the 10Base2.  

    Get a hub.

    run 10BaseT.

					-Matt

-- 
    Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
                    Communications
    <dillon@best.net> (Please include original email in any response)

In <6sannp$ofm$1@flea.best.net>, dillon@best.net (Matt Dillon) writes:
>    Scrap the 10Base2.  
>
>    Get a hub.
>
>    run 10BaseT.

Agreed.  10BaseT is much easier to deal with, plus once you have Cat-5 
cable installed you can go to 100BaseT if you ever want to.  The only 
place to use 10Base2 is in a situation where everything is in one room.

Dan Hicks
Hey!!  My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks

In article <35e6319c.53011606@news.lakwod1.co.home.com>,
Aaron Blosser <blosser@home.com> wrote:
>On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
>For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
>make a difference.  That measurement is a measure of the cable's
>impedance over some distance (1000 ft?), so for a short bit of maybe
>50-100 ft, the difference *may* be neglible.

Oh, it does make a difference.  Even a two-inch length of the wrong
cable causes reflections.  The characteristic impedance of a cable
is the same as value of the resistor that can terminate the cable without
causing reflections or anti-reflections; it has nothing to due
with the length of the cable.
	-Joe
-- 
INWAP.COM is Joe and Sally Smith, John and Chris O'Halloran and our cats
See http://www.inwap.com/ for "ReBoot", PDP-10, and Clan MacLeod.

In <6sdefd$jqn$1@shell3.ba.best.com> inwap@best.com (Smith and
O'Halloran) writes:

>The characteristic impedance of a cable
>is the same as value of the resistor that can terminate the cable without
>causing reflections or anti-reflections; it has nothing to due
>with the length of the cable.

At any given frequency, yes.
-- 
Rahul Dhesi <dhesi@spams.r.us.com>

danhicks@millcomm.com writes:

> In <35e6319c.53011606@news.lakwod1.co.home.com>, blosser@home.com (Aaron Blosser) writes:
> >On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> ><netsup75@edwardjones.com> said:
> >
> >>> Using existing 75ohm coax as if
> >>> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> >
> >>  I'll inquire with my network staff for more research material, as
> >>I'm curious as to why 75ohm cable would be that much of a problem?!?
> >
> >I wonder as well.
> >
> >For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> >make a difference.  That measurement is a measure of the cable's
> >impedance over some distance (1000 ft?), so for a short bit of maybe
> >50-100 ft, the difference *may* be neglible.
> >
> >Of course, this all depends on how senstive the receiving equipment is,
> >and I can't make any claims to know how it would really affect the
> >performance.
> >
> >The reason you *should* use properly rated cable is because with
> >impedance matching you can achieve maximum power output.
> 
> With 10Base2 proper impedance matching is critical to prevent reflections 
> -- which cause errors and false collisions.  However, for reasonably small
> nets you should be able to use 75 ohm cable **IF** you use 75 ohm 
> terminators.  The drivers and receivers should be able to tolerate the 
> difference in current and voltage.

This is not correct. Even if you use 75 ohm coax, you MUST terminate
with 50 ohm terminators. On coax the collision detection works on DC
signaling which is sourced as a current. If the DC resistance of the
cable is wrong (37.5 ohms vs. 25 ohms), collision detection breaks.

75 ohm cable will sort of work (even in long runs), but you may see
weird behavior due to standing waves and performance will probably be
sub-optimal. Try very hard to avoid mixed runs of 50 and 75 ohm cable
as each transition will create reflections and the chances of getting
reasonable performance will approach nil.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net				Phone: +1 510 486-8634

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Aaron Blosser wrote:
> On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> <netsup75@edwardjones.com> said:
> 
> >> Using existing 75ohm coax as if
> >> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> 
> >  I'll inquire with my network staff for more research material, as
> >I'm curious as to why 75ohm cable would be that much of a problem?!?
> 
> I wonder as well.
> 
> For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> make a difference.  That measurement is a measure of the cable's
> impedance over some distance (1000 ft?), so for a short bit of maybe
> 50-100 ft, the difference *may* be neglible.
> 
> Of course, this all depends on how senstive the receiving equipment is,
> and I can't make any claims to know how it would really affect the
> performance.
> 
> The reason you *should* use properly rated cable is because with
> impedance matching you can achieve maximum power output.

Greetings:
  Here's another thought!  Is there a 50ohm-to-75ohm converter
or filter on the market?  If so, is it bi-directional?  Maybe
buying something like this would cut this news-thread short ;)

Thanx...
------------------------------------------------------
    THE PRECEDING REFLECTS THE VIEWS OF THE AUTHOR
   AND NOT, NECESSARILY, THE VIEWS OF EDWARD JONES.
------------------------------------------------------
Daniel W. Erskine            "mailto:erskined@gte.net"
(314) 515-1406       "mailto:netsup75@edwardjones.com"
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--------------46B17197B9E8D1930020AAFD--

Some people don't even like mixing the same cable type from different
vendors be cause of the possible (but slight) impedence mismatches.

Regards
Tom

Smith and O'Halloran wrote:
> 
> In article <35e6319c.53011606@news.lakwod1.co.home.com>,
> Aaron Blosser <blosser@home.com> wrote:
> >On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> >For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> >make a difference.  That measurement is a measure of the cable's
> >impedance over some distance (1000 ft?), so for a short bit of maybe
> >50-100 ft, the difference *may* be neglible.
> 
> Oh, it does make a difference.  Even a two-inch length of the wrong
> cable causes reflections.  The characteristic impedance of a cable
> is the same as value of the resistor that can terminate the cable without
> causing reflections or anti-reflections; it has nothing to due
> with the length of the cable.
>         -Joe
> --
> INWAP.COM is Joe and Sally Smith, John and Chris O'Halloran and our cats
> See http://www.inwap.com/ for "ReBoot", PDP-10, and Clan MacLeod.

    I think you people are all nuts.  How much time do you 
    want to waste trying to make odd coax configurations 
    work?  It's hard enough even when you use the right
    type of coax and connectors!  Bad crimps, too short
    cables, incorrect T junction spacing, bad terminators,
    etc etc etc.

    Scrap the coax.  Really.  Throw it away.  Get a small
    10BaseT HUB and some cat-5 cables and you are done.

				-Matt

-- 
    Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
                    Communications
    <dillon@best.net> (Please include original email in any response)

Yes, length makes a difference.  WHEN the rise time of the signal is
included.  

For example a very slowly changing signal doesn't care whether you used 50
ohm, 75 ohm, or even romex.  

The rise times of ethernet are slow enough that a 2 inch, or 2 foot
mismatch doesn't matter much.  

You may not know this but almost all tees used in 75 ohm systems are
actually 50 ohm tees.  They're so short and most video is too slow to care
about that miniscule mismatch.

                                    - Robert -


On 30 Aug 1998, Smith and O'Halloran wrote:

> In article <35e6319c.53011606@news.lakwod1.co.home.com>,
> Aaron Blosser <blosser@home.com> wrote:
> >On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> >For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> >make a difference.  That measurement is a measure of the cable's
> >impedance over some distance (1000 ft?), so for a short bit of maybe
> >50-100 ft, the difference *may* be neglible.
> 
> Oh, it does make a difference.  Even a two-inch length of the wrong
> cable causes reflections.  The characteristic impedance of a cable
> is the same as value of the resistor that can terminate the cable without
> causing reflections or anti-reflections; it has nothing to due
> with the length of the cable.
> 	-Joe
> -- 
> INWAP.COM is Joe and Sally Smith, John and Chris O'Halloran and our cats
> See http://www.inwap.com/ for "ReBoot", PDP-10, and Clan MacLeod.
> 
> 

Yes there are converters.

They take the form of "lossy" networks like resistors, or non lossy ones
like transformers.

But neither one will adequately pass the DC levels required for the proper
operation of ethernet.  

                                      - Robert -

On Mon, 31 Aug 1998, Daniel Erskine wrote:

> Aaron Blosser wrote:
> > On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> > <netsup75@edwardjones.com> said:
> > 
> > >> Using existing 75ohm coax as if
> > >> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> > 
> > >  I'll inquire with my network staff for more research material, as
> > >I'm curious as to why 75ohm cable would be that much of a problem?!?
> > 
> > I wonder as well.
> > 
> > For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> > make a difference.  That measurement is a measure of the cable's
> > impedance over some distance (1000 ft?), so for a short bit of maybe
> > 50-100 ft, the difference *may* be neglible.
> > 
> > Of course, this all depends on how senstive the receiving equipment is,
> > and I can't make any claims to know how it would really affect the
> > performance.
> > 
> > The reason you *should* use properly rated cable is because with
> > impedance matching you can achieve maximum power output.
> 
> Greetings:
>   Here's another thought!  Is there a 50ohm-to-75ohm converter
> or filter on the market?  If so, is it bi-directional?  Maybe
> buying something like this would cut this news-thread short ;)
> 
> Thanx...
> ------------------------------------------------------
>     THE PRECEDING REFLECTS THE VIEWS OF THE AUTHOR
>    AND NOT, NECESSARILY, THE VIEWS OF EDWARD JONES.
> ------------------------------------------------------
> Daniel W. Erskine            "mailto:erskined@gte.net"
> (314) 515-1406       "mailto:netsup75@edwardjones.com"

Especially with the cost of 10BT dropping like the DOW ....

Best Regards,

Mike.

In article <6serpe$d8n$1@flea.best.net>,
  dillon@best.net (Matt Dillon) wrote:
>     I think you people are all nuts.  How much time do you
>     want to waste trying to make odd coax configurations
>     work?  It's hard enough even when you use the right
>     type of coax and connectors!  Bad crimps, too short
>     cables, incorrect T junction spacing, bad terminators,
>     etc etc etc.
>
>     Scrap the coax.  Really.  Throw it away.  Get a small
>     10BaseT HUB and some cat-5 cables and you are done.
>
> 				-Matt
>
> --
>     Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
>                     Communications
>     <dillon@best.net> (Please include original email in any response)
>


--
==========
Sorry for the dejanews address.  The spam
has finally gotten to me....

-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/rg_mkgrp.xp   Create Your Own Free Member Forum

In article <35EACDBE.237D36B8@edwardjones.com>,
Daniel Erskine  <netsup75@edwardjones.com> wrote:
>Greetings:
>  Here's another thought!  Is there a 50ohm-to-75ohm converter
>or filter on the market?  If so, is it bi-directional?  Maybe
>buying something like this would cut this news-thread short ;)

Yep.  It's called a 10BASET twisted-pair hub.  You can get them for
about $30, and it eliminates just about all the problems you see with coax.

As Matt Dillon mentioned in another reply, the best thing to do in the long
run is to rip out all the 10BASE2 (thin-net) coax and switch to CAT-5 twisted
pair wiring and hubs.  That alone will make your ethernet more reliable.
(BT, DT, need to get a custom t-shirt.)
	-Joe
-- 
INWAP.COM is Joe and Sally Smith, John and Chris O'Halloran and our cats
See http://www.inwap.com/ for "ReBoot", PDP-10, and Clan MacLeod.

c.c.eiftj@08.usenet.us.com (Rahul Dhesi) writes:
>What sort of reliability problems have people observed with properly
>installed thinnet?  I'm guessing that most problems are caused by using
>out-of-spec parts.

At my previous job (about four years ago) I became responsible for the network
after the last competent Sys Admin quit.  Even though I had segmented the
five+ year old thinnet network into lots of short, manageable segments,
having a piece of it go down was a weekly occurance.  Most of the problems
were due to old T-connectors that had become loose.  By just pushing on a
T where it connected to a transceiver you could make the whole segment
go down.  I wasted a lot of time figuring this out.  I did a lot of
binary searches locating bad connectors.

When you figure a half-dozen $100K+/year engineers aren't getting anything
done when the network is down, it was pretty easy to convince the Exec VP
of the company to pop for $6k to pull four-six runs of Cat-5 to each office,
plus install, hubs, and new transceivers when we moved into new office space.

With Cat 5 twisted pair you can easily migrate to 100BASE-T, full-duplex,
switched Ethernet, FDDI over copper, 155Mbps ATM, etc.

-Jonathan		jhue@geocities.com

Bob O`Brien wrote:
 
> My own LAN is mixed 10bT and 10b2, but only because I'm too lazy to
> buy another hub to replace a working segment.  But if anything goes
> wrong with the thinwire part, it's history.

I think this is getting closer to the correct way to evaluate this.
Incremental investments to keep a older network functional is quite a
bit different than starting from (near) scratch and building an
antiquated network.  Personally I think 10B2 is antiquated and not worth
the effort/expense to setup from scratch.

Then there's the otherside of the coin, I have a 5-port fiber switch
(half duplex ... sigh) and I keep thinking about stringing fiber around
the house.  BUT, at ~$220/NIC it's >100% beyond my finacial pain 
threshold ... now if it were full duplex ... hmmmm??

If I recall correctly, the original poster was talking about stringing
"new wire" and was looking for ideas about how to run it outside and 
then back inside and asked about plenum spaces.  In this situation
where "new wire" is the option I recommend 4-pair Cat-5 (in conduit if
feasible, after all, next year fiber *might* make sense?).

Paul
-- 
ISDN is almost like actually being connected to the net.
Wonder what xDSL will be like?

jhue@geocities.com (Pond Scum) writes:

>When you figure a half-dozen $100K+/year engineers aren't getting anything
>done when the network is down, it was pretty easy to convince the Exec VP
>of the company to pop for $6k to pull four-six runs of Cat-5 to each office,
>plus install, hubs, and new transceivers when we moved into new office space.

Plus, in the second half of 98, there's no excuse not to be switched.

I use 10Base2 connectors to make monsters out of, now.

Karl 

In <sreyas5rvgt.fsf@ptavv.es.net>, Kevin Oberman <oberman@es.net> writes:
>> With 10Base2 proper impedance matching is critical to prevent reflections 
>> -- which cause errors and false collisions.  However, for reasonably small
>> nets you should be able to use 75 ohm cable **IF** you use 75 ohm 
>> terminators.  The drivers and receivers should be able to tolerate the 
>> difference in current and voltage.
>
>This is not correct. Even if you use 75 ohm coax, you MUST terminate
>with 50 ohm terminators. On coax the collision detection works on DC
>signaling which is sourced as a current. If the DC resistance of the
>cable is wrong (37.5 ohms vs. 25 ohms), collision detection breaks.

Yeah, I'd forgotten about how Ethernet does collision detection.  Best bet
might be to split the difference and go with terminators in the 65-ohm 
region.  But really, folk, 10Base2 is a dead-end technology.  The far 
better strategy would be to pull in Cat-5 twisted pair cable and run 
10BaseT.  Pulling new cable sometimes takes creativity, but there's 
(almost) always a way.

Dan Hicks
Hey!!  My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks

I have to agree.  I had to troubleshoot a problem where a length of 75 ohm
cable was used to connect a new machine in my lab.  There were 3 machines
in a row on a bench.  2 and 3 could talk to each other, but not to
anything else on the network, and 1 could talk to the rest of the network
and 3, but not 2.  We wasted a LOT of time trying to figure this one out.

It's not worth the hassle for the small amount of money you'll save.  Coax
is a pain to run in general, use Cat5 10 or 100 base T.

: In article <6serpe$d8n$1@flea.best.net>,
:   dillon@best.net (Matt Dillon) wrote:
:>     I think you people are all nuts.  How much time do you
:>     want to waste trying to make odd coax configurations
:>     work?  It's hard enough even when you use the right
:>     type of coax and connectors!  Bad crimps, too short
:>     cables, incorrect T junction spacing, bad terminators,
:>     etc etc etc.
:>
:>     Scrap the coax.  Really.  Throw it away.  Get a small
:>     10BaseT HUB and some cat-5 cables and you are done.
:>
:> 				-Matt
:>
:> --
:>     Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
:>                     Communications
:>     <dillon@best.net> (Please include original email in any response)
:>


: --
: ==========
: Sorry for the dejanews address.  The spam
: has finally gotten to me....

: -----== Posted via Deja News, The Leader in Internet Discussion ==-----
: http://www.dejanews.com/rg_mkgrp.xp   Create Your Own Free Member Forum

-- 
Do not use if brain is raw, blistered, or oozing.

Matt Dillon wrote:
> 
>     Scrap the 10Base2.
> 
>     Get a hub.
> 
>     run 10BaseT.

I heard that somewhere?
-- 
ISDN is almost like actually being connected to the net.
Wonder what xDSL will be like?

In <6sfb3d$1k3$1@shell3.ba.best.com> inwap@best.com (Smith and
O'Halloran) writes:

>As Matt Dillon mentioned in another reply, the best thing to do in the long
>run is to rip out all the 10BASE2 (thin-net) coax and switch to CAT-5 twisted
>pair wiring and hubs.  That alone will make your ethernet more reliable.

What sort of reliability problems have people observed with properly
installed thinnet?  I'm guessing that most problems are caused by using
out-of-spec parts.
-- 
Rahul Dhesi <dhesi@spams.r.us.com>

In <UwUG1.35$kE2.252043@news.shore.net> holmes73@hotmail.com writes:

>I have to agree.  I had to troubleshoot a problem where a length of 75 ohm
>cable was used to connect a new machine in my lab.  There were 3 machines
>in a row on a bench.  2 and 3 could talk to each other, but not to
>anything else on the network, and 1 could talk to the rest of the network
>and 3, but not 2.  We wasted a LOT of time trying to figure this one out.

This is an argument against thinnet.  Really, it's an argument against
out-of-spec coax.

You can easily create the same scenario by using out-of-spec twisted
pair wiring.
-- 
Rahul Dhesi <dhesi@spams.r.us.com>

In article <6shoi6$eu4$1@samba.rahul.net>, c.c.eiftj@08.usenet.us.com (Rahul Dhesi) wrote:

>What sort of reliability problems have people observed with properly
>installed thinnet?  I'm guessing that most problems are caused by using
>out-of-spec parts.

I've run a small 3/4 node network at home on thin-wire for several
years. I've had two NIC failures (both 3C509-Combo's) and no other
problems. I've also moved twice during that period. I have no reason
to think that the NIC failures would NOT have occurred with CAT-5
and a hub.

OTOH, I have also worked at a larger industrial installation with
several hundred nodes on thin-wire. That was a disaster. 

IMO, thin-wire *can* work well for small networks. I wouldn't use
if for more than a few nodes - troubleshooting is just too
troublesome.

-- 
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| Malcolm Hoar           "The more I practice, the luckier I get". |
| malch@malch.com                                     Gary Player. |
| http://www.malch.com/               Shpx gur PQN.                |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Rahul Dhesi <c.c.eiftj@08.usenet.us.com> wrote:
> inwap@best.com (Smith and O'Halloran) writes:

>>As Matt Dillon mentioned in another reply, the best thing to do in the long
>>run is to rip out all the 10BASE2 (thin-net) coax and switch to CAT-5 twisted
>>pair wiring and hubs.  That alone will make your ethernet more reliable.
>
>What sort of reliability problems have people observed with properly
>installed thinnet?  I'm guessing that most problems are caused by using
>out-of-spec parts.

While I agree with you 100%, and have never heard of any problems
with thinnet that can't be traced to poor workmanship or parts, this
thread had already digressed into 75 Ohm coax and mismatched connectors.

It's not much of a stretch to assume that what Matt meant included
  "unless you're doing thinnet *right*,
   there's no point in doing it at all."
That would certainly be _my_ advice.

My own LAN is mixed 10bT and 10b2, but only because I'm too lazy to
buy another hub to replace a working segment.  But if anything goes
wrong with the thinwire part, it's history.


	Bob O`Bob
-- 
+ email replies without this line will be discarded (expires 12oct98) wd3gefrb +

Have Mallet
Will Traceroute

:In article <6shp0b$f0e$1@samba.rahul.net>,
:Rahul Dhesi <c.c.eiftj@08.usenet.us.com> wrote:
:>In <UwUG1.35$kE2.252043@news.shore.net> holmes73@hotmail.com writes:
:>
:>>I have to agree.  I had to troubleshoot a problem where a length of 75 ohm
:>>cable was used to connect a new machine in my lab.  There were 3 machines
:>>in a row on a bench.  2 and 3 could talk to each other, but not to
:>>anything else on the network, and 1 could talk to the rest of the network
:>>and 3, but not 2.  We wasted a LOT of time trying to figure this one out.
:>
:>This is an argument against thinnet.  Really, it's an argument against
:>out-of-spec coax.
:>
:>You can easily create the same scenario by using out-of-spec twisted
:>pair wiring.
:>-- 
:>Rahul Dhesi <dhesi@spams.r.us.com>

    IMHO, I've had to run a hellofalot of coax in my life, and with BEST I've
    run a hellofalot of cat-5 cable to HUBs and switches.

    The difference between maintaining a coax-based network and a hub'd or
    switched network is the difference between night and day.  Literally.

					-Matt

-- 
    Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
                    Communications
    <dillon@best.net> (Please include original email in any response)

In <6shrbr$1m2$1@nntp1.ba.best.com> malch@malch.com (Malcolm Hoar) writes:

>IMO, thin-wire *can* work well for small networks. I wouldn't use
>if for more than a few nodes - troubleshooting is just too
>troublesome.

Why is this so?  The only reason for better troubleshooting that I have
heard described is that with 10BaseT you can easily isolate a node by
unplugging its wire.

But with Thinnet you can easily isolate a node by unplugging its
T-connector.  The only difference is that with 10BaseT you cao do this
while standing near the hub, and with Thinnet you have to be standing
near the node.  Which doesn't seem to be that big a deal -- you are
already wandering around checking everything anyway.

The only real advantages of 10BaseT that I see are that the wiring is
easily pulled through conduit and that it's easy to make everything
terminate in a single closet, so you have to walk less.  Which reasoning
makes sense if you have a low-traffic network with hundreds of nodes on
the same 10 Mbps segment.  But with contemporary machines you can only
put a few on the same 10Mbps segment before you max out the bandwidth, so
you need a switch anyway.  So I don't see a realistic scenario where you
have hundreds of feet of coax that is impossible to trace.  More likely
you have 50 feet of coax terminating at a switch, which gives you back
the star configuration that 10BaseT users like.

My guess is that everybody is remembering the old days when a single 10
Mbps segment was enough for a 1000-employee company, with low-power
machines barely using any Ethernet bandwidth.  In those days it was
definitely possible to have a single Thinnet cable (joined with many
T-connectors, of course) winding its way through the whole company, and
perhaps people are remembering how hard it was to isolate pieces of that
long cable.

Now if you are comparing 100BaseTX with Thinnet it's a different story.
100BaseTX wins of course, since it's much faster.
-- 
Rahul Dhesi <dhesi@spams.r.us.com>

In <6shsde$rnu$1@flea.best.net> dillon@best.net (Matt Dillon) writes:

>    The difference between maintaining a coax-based network and a hub'd or
>    switched network is the difference between night and day.  Literally.

Coax vs hubs is a comparison I can understand, but coax vs switched
network doesn't seem like a valid dichotomy.  Whether or not you use
coax, you will need to go to a switched network once you begin maxing
out the bandwidth.
-- 
Rahul Dhesi <dhesi@spams.r.us.com>

In article <6shurm$g49$1@samba.rahul.net>, c.c.eiftj@08.usenet.us.com (Rahul Dhesi) wrote:
>In <6shrbr$1m2$1@nntp1.ba.best.com> malch@malch.com (Malcolm Hoar) writes:
>
>>IMO, thin-wire *can* work well for small networks. I wouldn't use
>>if for more than a few nodes - troubleshooting is just too
>>troublesome.
>
>Why is this so?  The only reason for better troubleshooting that I have
>heard described is that with 10BaseT you can easily isolate a node by
>unplugging its wire.

From practical experience at one place I worked:

Around every two weeks someone would add/remove/relocate a node
on the segment and break all or part of it by breaking the daisey
chain and/or leaving it unterminated.

It took a fair amount of time just to establish that there was a
segment-specific problem (the NOC and servers were in another
building). A Technician would be dispatched to track down the
problem. The cable runs had evolved over time and well, ..... 
can you say, ratsnest ..... :-)

Bottom line: an hour or so of downtime every couple of weeks
on some segments. Virtually all of which could have been avoided
with a 10BaseT and hub configuration.

But, as I said before, thin-wire works just great on my small
home network. 

-- 
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| Malcolm Hoar           "The more I practice, the luckier I get". |
| malch@malch.com                                     Gary Player. |
| http://www.malch.com/               Shpx gur PQN.                |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

:In article <6shurm$g49$1@samba.rahul.net>,
:Rahul Dhesi <c.c.eiftj@08.usenet.us.com> wrote:
:>In <6shrbr$1m2$1@nntp1.ba.best.com> malch@malch.com (Malcolm Hoar) writes:
:>
:>>IMO, thin-wire *can* work well for small networks. I wouldn't use
:>>if for more than a few nodes - troubleshooting is just too
:>>troublesome.
:>
:>Why is this so?  The only reason for better troubleshooting that I have
:>heard described is that with 10BaseT you can easily isolate a node by
:>unplugging its wire.
:>
:>But with Thinnet you can easily isolate a node by unplugging its

    No, it's not that easy.  With thinnet, it isn't necessarily the node
    that's broken.  Someone might have stepped on the coax, or someone might
    have pulled another cable through the conduit and notched your coax
    at a bend.  Sometimes problems are intermittent, sometimes problems effect
    a portion of the wire protocol (such as messing up the collision detection)
    and isn't the fault of any single node.  This brings back memories (that
    I'd rather forget) of spending hours trying track coax problems down with
    a time domain reflectometer.  Even with the reflectometer it's a bitch.

    10BaseT is a whole different ballgame.  Short of breaking the copper,
    you can put those cables through a lot of abuse without screwing anything
    up.

:>The only real advantages of 10BaseT that I see are that the wiring is
:>easily pulled through conduit and that it's easy to make everything
:>terminate in a single closet, so you have to walk less.  Which reasoning

    The advantage is that when something breaks, you can usually isolate it
    in a minute or two with 10BaseT, verses (sometimes) hours trying to
    isolate a problem on a large 10Base2 network.  Even with a small network, 
    the maintainance time required to mess with 10BaseT is easily an order of 
    magnitude *LESS* then the maintenance time required to mess with 10Base2.

    This isn't even mentioning the bandwidth and scaleability advantages you
    have with twisted pair.

						    -Matt

-- 
    Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
                    Communications
    <dillon@best.net> (Please include original email in any response)

Rahul Dhesi wrote:
> 
> 
> My guess is that everybody is remembering the old days when a single 10
> Mbps segment was enough for a 1000-employee company, with low-power
> machines barely using any Ethernet bandwidth.  In those days it was
> definitely possible to have a single Thinnet cable (joined with many
> T-connectors, of course) winding its way through the whole company, and
> perhaps people are remembering how hard it was to isolate pieces of that
> long cable.
> 
	As long as the company was 
	
	1/ small...10Base2 segment length limited to 185m max
	2/ had 30 connected machines or less ...max nodes per segment 30
	
	of course with repeaters and bridging you could build a *big* Ethernet ;-)

	10Base5 (Thick Ethernet): 500m segment, 100 nodes/segment 

	BTW, TimeDomainReflectometry used to work relatively well to locate
	breaks (or unterminated connectors) in the cable.

	Remember those days well...
-- 

      OLTECO                         Ari Ollikainen 
      P.O. BOX 3688                  Networking Technology and Architecture  
      Stanford, CA                   Ari@OLTECO.com           
      94309-3688                     415.517.3519

In <6shoi6$eu4$1@samba.rahul.net>, c.c.eiftj@08.usenet.us.com (Rahul Dhesi) writes:
>In <6sfb3d$1k3$1@shell3.ba.best.com> inwap@best.com (Smith and
>O'Halloran) writes:
>
>>As Matt Dillon mentioned in another reply, the best thing to do in the long
>>run is to rip out all the 10BASE2 (thin-net) coax and switch to CAT-5 twisted
>>pair wiring and hubs.  That alone will make your ethernet more reliable.
>
>What sort of reliability problems have people observed with properly
>installed thinnet?  I'm guessing that most problems are caused by using
>out-of-spec parts.

The main thing is that a 10BaseT terminal can be disconnected without 
bringing down the whole network.  Yes, this is possible with 10Base2, but 
only with special extra-cost connectors.  In large networks, 10BaseT can 
have managed hubs that automatically disconnect failed NICs and otherwise
keep small problems from becoming large ones.  Plus you could reliably run
10BaseT over cheap zip cord if you had to, since it's quite insensitive to
impedance and reflections.

Dan Hicks
Hey!!  My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks

Daniel Erskine <netsup75@edwardjones.com> wrote in article
<35EACDBE.237D36B8@edwardjones.com>...
> Aaron Blosser wrote:
> > On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> > <netsup75@edwardjones.com> said:
> > 
> > >> Using existing 75ohm coax as if
> > >> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> > 
> > >  I'll inquire with my network staff for more research material, as
> > >I'm curious as to why 75ohm cable would be that much of a problem?!?
> > 
> > I wonder as well.
> > 
> > For short runs of cable, I highly doubt that using 50ohm vs. 75ohm
would
> > make a difference.  That measurement is a measure of the cable's
> > impedance over some distance (1000 ft?), so for a short bit of maybe
> > 50-100 ft, the difference *may* be neglible.

Not so.  A cable's impedance is the ratio of the magnitude of a voltage
wave form
travelling down the cable, to the magnitude of the corresponding current
wave.  Nothing
to do with distance.  But everything to do with reflections at junctions
where there is 
impedance mismatch.  

> > Of course, this all depends on how senstive the receiving equipment is,
> > and I can't make any claims to know how it would really affect the
> > performance.
> > 
> > The reason you *should* use properly rated cable is because with
> > impedance matching you can achieve maximum power output.

Not really.  Maybe you're thinking about amplifier power output, which
peaks into
load, when load impedance is equal to effective internal series resistance
of amp
final stage?  In fact, having impedance matching in a transmission line
does two
things: maximizes strength of received signal at receiver(s), minimizes
signal 
reflected back to source from junctions (which degrades ability to detect
signal.)

> Greetings:
>   Here's another thought!  Is there a 50ohm-to-75ohm converter
> or filter on the market?  If so, is it bi-directional?  Maybe
> buying something like this would cut this news-thread short ;)
> 
> Thanx...
> ------------------------------------------------------
>     THE PRECEDING REFLECTS THE VIEWS OF THE AUTHOR
>    AND NOT, NECESSARILY, THE VIEWS OF EDWARD JONES.
> ------------------------------------------------------
> Daniel W. Erskine            "mailto:erskined@gte.net"
> (314) 515-1406       "mailto:netsup75@edwardjones.com"

With coax so cheap, I'd get the best available cable.
HTH,
John

In article <01bdd698$fe212500$8028a8c0@jb.zedak.com>, jb@zedak.keepyourspam.com 
writes...
> Daniel Erskine <netsup75@edwardjones.com> wrote in article
> <35EACDBE.237D36B8@edwardjones.com>...
> > Aaron Blosser wrote:
> > > On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> > > <netsup75@edwardjones.com> said:
> > > 
> > > >> Using existing 75ohm coax as if
> > > >> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> > > 
> > > >  I'll inquire with my network staff for more research material, as
> > > >I'm curious as to why 75ohm cable would be that much of a problem?!?
> > > 
> > > I wonder as well.
> > > 
> > > For short runs of cable, I highly doubt that using 50ohm vs. 75ohm
> would
> > > make a difference.  That measurement is a measure of the cable's
> > > impedance over some distance (1000 ft?), so for a short bit of maybe
> > > 50-100 ft, the difference *may* be neglible.
> 
> Not so.  A cable's impedance is the ratio of the magnitude of a voltage
> wave form
> travelling down the cable, to the magnitude of the corresponding current
> wave.  Nothing
> to do with distance.  But everything to do with reflections at junctions
> where there is 
> impedance mismatch.  
> 
> > > Of course, this all depends on how senstive the receiving equipment is,
> > > and I can't make any claims to know how it would really affect the
> > > performance.
> > > 
> > > The reason you *should* use properly rated cable is because with
> > > impedance matching you can achieve maximum power output.
> 
> Not really.  Maybe you're thinking about amplifier power output, which
> peaks into
> load, when load impedance is equal to effective internal series resistance
> of amp
> final stage?  In fact, having impedance matching in a transmission line
> does two
> things: maximizes strength of received signal at receiver(s), minimizes
> signal 
> reflected back to source from junctions (which degrades ability to detect
> signal.)


The reason that using correct cable is important are as follows:

1 - Terminators and NIC's are 75 ohm, if the cable is not, you end up
    with reflections that play hell with the waveforms and thus, data 
    integrity.

2 - The reason distance is a factor (and why the IEEE specifies a 
    maximum length for 10base2 segments) is due to the rate of signal
    propagation.  Since 802.3 collision detection relies on a known rate
    of propagation and maximum segment length (tailoring it's "collision
    listen window" accordingly), if you run out-of-spec cabling you run
    the risk that collision-detection no longer works correctly, which
    causes a host of problems including runt packets, late-collisions, 
    etc.



Phil


-- 
Philip J. Koenig         The Electric Kahuna Organization          [see below]
-----------------Computers & Communications for the New Millenium-------------
References to my email address in this message have been modified to foil
address-collection robots.  If you wish to send email, use the following 
address by removing numbers and spaces:  pjkunet64 @  ekahuna27 . c o m

:In article <MPG.10572b78acbc04c39896ca@ispnews.walltech.com>,
:Philip J. Koenig <For_Email@ddress.see.below_plea.se> wrote:
:>In article <01bdd698$fe212500$8028a8c0@jb.zedak.com>, jb@zedak.keepyourspam.com 
:>writes...
:>> Daniel Erskine <netsup75@edwardjones.com> wrote in article
:>> <35EACDBE.237D36B8@edwardjones.com>...
:..
:>
:>The reason that using correct cable is important are as follows:
:>
:>1 - Terminators and NIC's are 75 ohm, if the cable is not, you end up
:>    with reflections that play hell with the waveforms and thus, data 
:>    integrity.

    10base2 uses 50 ohm coax and 50 ohm termination.

:>2 - The reason distance is a factor (and why the IEEE specifies a 
:>    maximum length for 10base2 segments) is due to the rate of signal
:>    propagation.  Since 802.3 collision detection relies on a known rate
:>    of propagation and maximum segment length (tailoring it's "collision
:>    listen window" accordingly), if you run out-of-spec cabling you run
:>    the risk that collision-detection no longer works correctly, which
:>    causes a host of problems including runt packets, late-collisions, 
:>    etc.

    The collision window is not the maximum segment length,  It's the 
    preamble period plus I think the MAC header (don't quite remember but
    I think it's the first 64 bit times or something like that).  If
    a collision occurs after that point, you get a 'late collision' which,
    which detectable, is a very serious error.

						-Matt

:>Phil
:>
:>
:>-- 
:>Philip J. Koenig         The Electric Kahuna Organization          [see below]


-- 
    Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet
                    Communications
    <dillon@best.net> (Please include original email in any response)

For_Email@ddress.see.below_plea.se (Philip J. Koenig) writes:

> The reason that using correct cable is important are as follows:
> 
> 1 - Terminators and NIC's are 75 ohm, if the cable is not, you end up
>     with reflections that play hell with the waveforms and thus, data 
>     integrity.

Terminators and NICs are 50 ohms for 10Base2 (Ethernet). 

> 2 - The reason distance is a factor (and why the IEEE specifies a 
>     maximum length for 10base2 segments) is due to the rate of signal
>     propagation.  Since 802.3 collision detection relies on a known rate
>     of propagation and maximum segment length (tailoring it's "collision
>     listen window" accordingly), if you run out-of-spec cabling you run
>     the risk that collision-detection no longer works correctly, which
>     causes a host of problems including runt packets, late-collisions, 
>     etc.

The reason for the limits on coax length is resistance. The time
budget is real, but only applies to repeated (multi-segment) networks
and fiber nets. Neither coax nor UTP lengths will allow for single
segments approaching the Ethernet time budget. (Thin net (10Base2) is
limited to 185 meters and thick net (10Base5) to 500
meters.)

While Vp varies from medium to medium, it is pretty safe to say that
time budget will allow for about a 2500 meter network diameter.

If you really want to get into all of this, read up on transmission
lines and see http://www.ots.utexas.edu/ethernet/. If you want to get
deeper into it, I recommend Charles Spurgeon's book, "Practical
Networking with Ethernet", ISBN: 1-85032-885-4.

And if you plan to make a living on Ethernet, get a copy of IEEE
802.3, the actual standard. (You need to be serious about Ethernet to
get that because it is NOT an easy read, assumes a fair amount of EE
background,  and costs about $200.)
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net				Phone: +1 510 486-8634

In article <sren28ijnot.fsf@ptavv.es.net>,
Kevin Oberman  <oberman@es.net> wrote:
>For_Email@ddress.see.below_plea.se (Philip J. Koenig) writes:
>
>> The reason that using correct cable is important are as follows:
>> 
>> 1 - Terminators and NIC's are 75 ohm, if the cable is not, you end up
>>     with reflections that play hell with the waveforms and thus, data 
>>     integrity.
>
>Terminators and NICs are 50 ohms for 10Base2 (Ethernet). 

Yep.

What also comes into it, and most people ignore it, is that there are
multiple budgets in thinnet Ethernets:

1.	The total cable length and propagation velocity through both the
	wire and repeaters (the rule being that a transmission of the
	preamble from one end must reach the other end BEFORE the preamble
	length in bit-times expires, or you can't detect a collision).

2.	The length of the *STUBS*.  Each NIC and connector to it is in fact
	a stub.  Each stub generates a reflection of the signal which is, in
	part, based upon its length (and other factors way too complicated
	to worry about).  Even though you don't think about a T-connector as
	being a "stub", it *IS* one, and so is the physical connector on the
	NIC.  Exceed this limit *in total* and the network starts acting
	VERY strange (certain hosts can't see each other, etc).

3.	ANY discontinuities (impedence mismatches) between pieces of wire
	(and there is a small but present one at each connector!) cause
	further reflections due to impedence mismatch.

That's for starters.  

Frankly, thinnet is a pain in the ass.  ThickNet (10Base5) with Vampire taps
at least works reliably.  Thinnet, in my experience, NEVER does across large
installations (hundreds of hosts).  All it takes is ONE person with a
desktop machine that moves it and "stretches" the cable a bit, causing a
loss of good continuity in one of the connectors (an impedence mismatch) 
and you have trouble.  Frequently the trouble will show up somewhere
*different* than where the actual cable problem is.  

The only way to KNOW where the discontinuity happens to be is to have both 
a network map (with cable lengths documented for each piece of wire -
literally) and a TDR.  Given those two things you can find and fix Thinnet
problems.  The TDR has to be used during installation of the cable to get
the lengths (one function of a TDR is to give you the length of the cable
from one end to another, given a proper terminator on one end and the TDR on
the other).  With that, and the use of the TDR when you have trouble, you
can pinpoint the bad connection.

Without those *TWO* things you'll be chasing your tail on thinnet problems
forever.  I know of a few people who have both TDRs and a Thinnet network -
I know of VERY few who took the trouble to map the lengths of all the 
segments from tap-to-tap during installation, and without that information 
a TDR doesn't do much when troubleshooting (ie: the impedance discontinuity
is 57' down that wire - which of the following six workstations is exactly
57' from where the TDR is on the wire right now?!).

These days UTP is so cheap to install that you have to be smoking funny
things to use anything else.

--
-- 
Karl Denninger (karl@MCS.Net)
Voice: 312-803-6271 x219     

...and to think the poor guy *only* asked if he could run cable out the
window and back in! <LOL>

Karl Denninger wrote in message <6sk9pt$llg$1@Mars.mcs.net>...
>In article <sren28ijnot.fsf@ptavv.es.net>,
>Kevin Oberman  <oberman@es.net> wrote:
>>For_Email@ddress.see.below_plea.se (Philip J. Koenig) writes:
>>


<Real Good Tech Stuff Snipped>

>These days UTP is so cheap to install that you have to be smoking funny
>things to use anything else.
>

THIS statement is the bottom line. ^^^^^^^^^^^^^ (Randy)

>--
>--
>Karl Denninger (karl@MCS.Net)
>Voice: 312-803-6271 x219

In article <6sk60s$j7d$1@flea.best.net>, dillon@best.net writes...
> :In article <MPG.10572b78acbc04c39896ca@ispnews.walltech.com>,
> :Philip J. Koenig <For_Email@ddress.see.below_plea.se> wrote:
> :>In article <01bdd698$fe212500$8028a8c0@jb.zedak.com>, jb@zedak.keepyourspam.com 
> :>writes...
> :>> Daniel Erskine <netsup75@edwardjones.com> wrote in article
> :>> <35EACDBE.237D36B8@edwardjones.com>...
> :..
> :>
> :>The reason that using correct cable is important are as follows:
> :>
> :>1 - Terminators and NIC's are 75 ohm, if the cable is not, you end up
> :>    with reflections that play hell with the waveforms and thus, data 
> :>    integrity.
> 
>     10base2 uses 50 ohm coax and 50 ohm termination.


GACK... sorry my mistake.  Too many years working with cable TV stuff
I guess..  :)



> :>2 - The reason distance is a factor (and why the IEEE specifies a 
> :>    maximum length for 10base2 segments) is due to the rate of signal
> :>    propagation.  Since 802.3 collision detection relies on a known rate
> :>    of propagation and maximum segment length (tailoring it's "collision
> :>    listen window" accordingly), if you run out-of-spec cabling you run
> :>    the risk that collision-detection no longer works correctly, which
> :>    causes a host of problems including runt packets, late-collisions, 
> :>    etc.
> 
>     The collision window is not the maximum segment length,  It's the 
>     preamble period plus I think the MAC header (don't quite remember but
>     I think it's the first 64 bit times or something like that).  If
>     a collision occurs after that point, you get a 'late collision' which,
>     which detectable, is a very serious error.
> 
> 						-Matt
> 


Yes, "collision window" is not the maximum segment length.

To be more specific, the time allotted for listening for collisions is 
specific (I don't remember, possibly 64 bit times as you say) and it is
a direct function of A) the velocity of propagation in the coax, which
is specified in the 802.3 spec to be within a certain known range and 
B) the length of the segment.

Given standard "thinnet" cable, if the length is kept within the 802.3-
specified maximums, the propagation (or "round-trip") time is such 
that it does not exceed the "collision listen window".  The risk is of
course if the cable is too long, the transceivers on the wire will stop
listening for collisions on the wire when they may still be occurring.

Some NIC's had/have a mode for "extended segment length", it is not
technically 802.3 compliant.  And by the same token (no pun intended), 
if you really wanted to run a longer segment length, you could also run 
cable that had a faster velocity of propagation than the 802.3 standard
originally called for.

However in the absence of a thorough knowledge of the physics and 
transmission-line theory involved (and requisite test equipment), the
obvious solution is simply to use 802.3 compliant cable and adhere to
802.3-recommended maximum cable lengths. (and other recommendations
like maximum 30 taps per segment, no short stubs, etc.. :-)




Phil

-- 
Philip J. Koenig         The Electric Kahuna Organization          [see below]
-----------------Computers & Communications for the New Millenium-------------
References to my email address in this message have been modified to foil
address-collection robots.  If you wish to send email, use the following 
address by removing numbers and spaces:  pjkunet64 @  ekahuna27 . c o m

In <6shurm$g49$1@samba.rahul.net>, c.c.eiftj@08.usenet.us.com (Rahul Dhesi) writes:
>In <6shrbr$1m2$1@nntp1.ba.best.com> malch@malch.com (Malcolm Hoar) writes:
>
>>IMO, thin-wire *can* work well for small networks. I wouldn't use
>>if for more than a few nodes - troubleshooting is just too
>>troublesome.
>
>Why is this so?  The only reason for better troubleshooting that I have
>heard described is that with 10BaseT you can easily isolate a node by
>unplugging its wire.
>
>But with Thinnet you can easily isolate a node by unplugging its
>T-connector.  The only difference is that with 10BaseT you cao do this
>while standing near the hub, and with Thinnet you have to be standing
>near the node.  Which doesn't seem to be that big a deal -- you are
>already wandering around checking everything anyway.

One significant advantage is that 10BaseT is largely insensitive to 
mismatched impedance and related problems.  Minor irregularities due 
to corroded connectors, etc, won't faze it.  Plus when there is a problem
it will only take out the one terminal, not the entire network.

Dan Hicks
Hey!!  My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks

In <35EACDBE.237D36B8@edwardjones.com>, Daniel Erskine <netsup75@edwardjones.com> writes:
>Greetings:
>  Here's another thought!  Is there a 50ohm-to-75ohm converter
>or filter on the market?  If so, is it bi-directional?  Maybe
>buying something like this would cut this news-thread short ;)

There are such things, frequently mislabeled "baluns".  They would in 
theory work, but are not perfect.  

Dan Hicks
Hey!!  My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks