# For those Network techie geeks

Discussion in 'NZ Computing' started by Bok, Jan 6, 2004.

1. ### BokGuest

Is this some kind of trick question?

You haven't specified the nature of the in-wall cabling. If the in wall cabling is 10base2 (thin net), then it
doesn't sound valid to me (but then again I'm not a network techie geek
)
If the in-wall cable is 10base2, then my answer to this is close to ZERO -
the center of the connector T should be connected as close to the NIC as
possible for reliable operation.

The situation will be different if there are active devices in the wall
such as a media converter (tranceiver) or repeater of some form - please
clarify.

Bok, Jan 6, 2004

2. ### TEXGuest

System running 10base2, BNC interconnect. Computers are connected from
the nic to the wall via a single cable. NOT a T connector. Basically you
are extending the center of the T connector in a normal simple
configuration.

How long must the cable be from the wall to the computer???

TEX

TEX, Jan 7, 2004

3. ### LennierGuest

1 x D' + 3" + H where D = the distance between the computer and the wall
and H = the distance between the floor and the plug on the wall...

;o)

Lennier

Lennier, Jan 7, 2004
4. ### TEXGuest

No this is not a trick question. Just an interesting thing I discovered
today. A friend was try to explain to me the finer details...but got
lost half way through it. Thought someone might actaully know WTF I am
taking about and actually explain to me in laymans terms. I know it is
old technology now, but was an interesting learning experience.
The inwall cabling is 10base2, and it utilises a "AMP" thinnet TAP
connector to interface to the NICS. Effectivly the the thinnet TAP is a
T connector, in which the center of the T is cabled to the NIC. Saves
the hassle and tangle of dual leads and T on the end of each NIC

As you have said the the center of the T has to be as close to the NIC
as possible for reliable operation, but since I have a 6ft long thinnet
tap cable, this answer is not quiet right.

TEX

TEX, Jan 7, 2004
5. ### Richard Malcolm-SmithGuest

I have seen a system in some place where we were ripping it out where the wall
plate allowed for a plug to be removed and bridged the connection when it was
taken out, and the plug actually ran 2 pieces of coax up to the PC where they
terminated in a single BNC plug. Could this be what you have there?

Richard Malcolm-Smith, Jan 7, 2004
6. ### TEXGuest

Nope. The setup was originally used for a Novell LAN, years and years
ago. Definatly a single core coax cable connected to the NIC.

TEX

TEX, Jan 7, 2004
7. ### Uncle StoatWarblerGuest

It sounds like the old Novell 75 ohm 2Mb/s standard.

Uncle StoatWarbler, Jan 7, 2004
8. ### BokGuest

The individual lengths of the drop cables in the AMP system are not
critical. Take a look at the description, it's not a single coax. Quoting
from the above PDF
"The drop cables are made from dual thinwire cable with an
overall PVC sheath".
This makes all the difference - each side of the coax within the drop
cable is essentially inlined on the main cable. This is not the same
as adding a single coax extension between a T adaptor and NIC where the
length would be critical (to avoid inteference from reflected signal back
on the main bus).

However, because it's dual coax, you need to allow for double the drop
length when computing the total bus length, to stay within the recommended
recommended 185meter. limit for 10base2.

To emulate this electically with single core coax you need to run two
lengths of coax from the wall to a T connector connected directly to the
NIC.

The advantage of the AMP system is that you can plug and unplug the
drop cable without disrupting the LAN because the AMP connector is make
before break, which retains continuity.

Bok, Jan 7, 2004
9. ### bokGuest

The inwall cabling is 10base2, and it utilises a "AMP" thinnet TAP
That's quite different - we had that type of cabling in one of the buildings
here at work years ago. The drop cables for thinnet taps consisted of TWO
coax cables, which loops the signal to the node, then back to the other side
of the tap. There is a microswitch in the tap assembly that allows the two
sides of the coax to be inserted creating an unbroken network which is
'electrically equivalent' to connecting with a traditional BNC T connector.
If it's lan tap, then you are not connecting to a T connector in the wall.
Thinnet tap drop cables usually had a square looking connector into the wall
NOT a BNC connector. It's not quite right to say:

"Basically you are extending the center of the T connector in a normal
simple configuration."

This is not a valid 10base2 configuration and will be unreliable.

cheers

bok, Jan 7, 2004
10. ### bokGuest

If it's lan tap, then you are not connecting to a T connector in the wall.
Just to clarify this a bit. The tap assembly plus the drop cable are
equivalent to a T connector. It is invalid to lengthen the drop cable with a
length of coax and BNC coupler, which would be the same as inserting a
length of coax between the centre of a T adaptor and the network node (NIC).

hope this helps

bok, Jan 7, 2004
11. ### bokGuest

Sorry, I replied to your other post before I read this. I thought you might
have the same thinnet tap technology Richard referred to, which is the
variety I'm familiar with. Was the wall connection BNC? If it was, then this
cabling doesn't conform to most 10base2 standards I've come across.

bok, Jan 7, 2004
12. ### bokGuest

Nope. The setup was originally used for a Novell LAN, years and years
Could be, but that's not 10base2, which by definition is 10Mb/s.

bok, Jan 7, 2004
13. ### Uncle StoatWarblerGuest

The untrained eye wouldn't be able to tell the differnce.

Uncle StoatWarbler, Jan 7, 2004
14. ### Uncle StoatWarblerGuest

This _is_ the Novell 2Mb/s system.

Drop cables must be a multiple of 1/2 the wavelength, that way they
present the same impedance at the T as is on the end of the tail - and an
open circuit is seen as an open circuit.

Uncle StoatWarbler, Jan 8, 2004
15. ### TEXGuest

Correct. It is a square connector that plugs into the wall, with a bit
of cable and male BNC on one end. If you look inside the wall jack, it
isn't (as you correctly put it) a T connector, but is wired exactly the
same way.

When you say "The tap assembly plus the drop cable are equivalent to a T
connector", how would that be different to T with BNC coupler and
extension cable??

The situation was, that I inserted a T connector in the main line, and
extended the line of the center tap, with coupler and cable. (To emulate
the thinnet TAP configuration, since you can't get connectors any more)
But found that only certain lengths of cable would work.

After hours of mucking around, I was then told since, the cabling is
tuned to a freq, the length must be proportional to half the wavelength
of the travelling freq on the cable. Hence why some of the cables
worked, and some didn't.

Is this correct or totally wrong?? How does the thinnet tap system
differ from the T and coupling?? They are both electrically the same.

TEX

TEX, Jan 8, 2004
16. ### TEXGuest

Yes the system was used to run a novell network ages ago. It just
utilises the same 50ohm cable and 50ohm terminators as per 10base2 and
has been happily running 10mbs for the last 8 years, using the premade
drop cables. I had to buy a cable not too long ago, and they can in
lengths of 6ft and 8.2ft.

Yes my non computer techy friend (but electronics geek) said something
about if the cable is not 1/2 wavelength the impedance potential at the
junction (T) would appear as if not terminate. He then raved on about
something about calculating the length of the drop cable etc using a
formula, but by that stage I was lost.

He said this sort of thing is also important in arial tuning for TV's
and RF. Apparently it is a big field, but would never have thought it
would come into use in networking.

Would be keen on finding some more info on how this 2mbs novell system
works and how it differs from 10base2.

TEX

TEX, Jan 8, 2004
17. ### bokGuest

Correct. It is a square connector that plugs into the wall, with a bit
It is different! Take a look the "Thinnet Tap" system described at the

The pictures might help to determine if the system you are dealing with is
the same as this, which is a standard 10base-2 configuration.

"Never attempt to lengthen a Drop Cable with a length of coaxial cable and a
BNC coupler. This is the same as locating a "T" adapter away from the node,
and will cause major network problems!"
This isn't going to emulate a thinnet tap system, refer to web page.
That's correct, but if you don't use the correct drop cables it won't be a
standard 10base-2 configuration. If your system is as shown on the web page
above and you are running 10Mb/s you are better off trying to get the
correct drop cables from somewhere.
Not quite. With a T plus extension cable you will get standing waves
propagated on the extension that will reflect back on the main cable - the
inteference between the waves will result in a a suboptimal signal.

bok, Jan 8, 2004
18. ### Rudy SeoaGuest

Here's a data sheet on the AMP system:
http://www.videkonline.co.uk/pictures/pdfs/page094.pdf

The drop cables are only available in 2.5, 3.6, 4.8, 7.6m. These set
lengths have probably something to do with the RF-side of things, but
the theory to confirm this is beyond me.

Rudy Seoa, Jan 8, 2004
19. ### peteGuest

I know of some cabling similar - I think it was proprietary 3Com. At the
network card end the cable has a single BNC connector, with no T piece. At
the wall socket end is a odd looking plug. We have a few cables at work to
support a client who still uses this stuff. The benefit is that you can
remove a workstation without dropping the whole ine. The flylead cable in
effect is a very long T piece. It pretty old stuff now. And yes, this
client is using a Novell lan (the servers are 486 ICLs, the workstations
are a mix of 386 and 486 diskless PCs, and on the whole, it all works
pretty well)

pete, Jan 8, 2004
20. ### Uncle StoatWarblerGuest

This is one of the reasons the system was 2Mb/s.

It's actully more like ARCnet than Ethernet - but it's not ARCnet either.

Uncle StoatWarbler, Jan 8, 2004