Network cables and impedances that go bump in the night

[D Yuniskis]

Hi,

I often need to plug something in to the network temporarily
(e.g., to configure something or talk to a NAS box, etc.).
I have a switch in the office to serve the dozen or so nodes
there. So, there is always a spare port that I can use.

But, it's on the floor amid a tangle of power cords, network
cables, etc. I.e., I have to get out of my chair and *kneel*
on the floor to make the connection.

My knees don't like it when I do this. They go to great
pains to remind me of their displeasure!

I have these little "surface mount" (as in "surface of the wall")
boxes that are intended as four point drops. I.e., you feed
four cables into them "from behind" and have four connectors
available on the face of the box. Much like a wall plate
but this will mount *on* the wall instead of flush.

I had thought of installing three connectors in the box. Then
wiring connector 1 to connector 2 "straight thru" and then
chaining on connector 3 "with a twist". After mounting this
on the underside of one of the desktops, I could connect
connector 1 to the switch "permanently" with a standard patch
cord.

Connector 2 would give me a convenient "outlet" to which other
devices could be attached, temporarily. And, in those cases
where I need to connect "node to node", I could use connector
1 (or 2) with connector 3 (to get the "twist") without the need
of a crossover cable. I.e., I can keep one "regular" patch
cord handy and use it for all needs.

Neglecting the problems with trying to get two wires on
a single punchdown block... am I likely to have other
problems from the impedance bump that the "extra" connector
will present? Or, will it be insignificant enough (especially
since this is just a temporary connection, normally)?

Thx,
--don

 

[JosephKK]

Hi,

I often need to plug something in to the network temporarily
(e.g., to configure something or talk to a NAS box, etc.).
I have a switch in the office to serve the dozen or so nodes
there. So, there is always a spare port that I can use.

But, it's on the floor amid a tangle of power cords, network
cables, etc. I.e., I have to get out of my chair and *kneel*
on the floor to make the connection.

My knees don't like it when I do this. They go to great
pains to remind me of their displeasure!

I have these little "surface mount" (as in "surface of the wall")
boxes that are intended as four point drops. I.e., you feed
four cables into them "from behind" and have four connectors
available on the face of the box. Much like a wall plate
but this will mount *on* the wall instead of flush.

I had thought of installing three connectors in the box. Then
wiring connector 1 to connector 2 "straight thru" and then
chaining on connector 3 "with a twist". After mounting this
on the underside of one of the desktops, I could connect
connector 1 to the switch "permanently" with a standard patch
cord.

Connector 2 would give me a convenient "outlet" to which other
devices could be attached, temporarily. And, in those cases
where I need to connect "node to node", I could use connector
1 (or 2) with connector 3 (to get the "twist") without the need
of a crossover cable. I.e., I can keep one "regular" patch
cord handy and use it for all needs.

Neglecting the problems with trying to get two wires on
a single punchdown block... am I likely to have other
problems from the impedance bump that the "extra" connector
will present? Or, will it be insignificant enough (especially
since this is just a temporary connection, normally)?

Thx,
--don

Don't do this daisy chaining. It will break every rule of XBase-T
twisted pair network wiring. Put in little $15 switches instead.

 

[Tim Watts]

Adding yet another "small switch" means finding a way to mount that
switch someplace convenient (most of the 4 or 5 port switches I've seen
are intended as "desktop" devices -- not viable, here -- so I would have
to physically modify one in order to fasten it to the underside of the
work surface. And, then make room for yet another wall wart to power the
damn thing -- despite the fact that 95% of the time the switch will be
unused (so, even if I install a power switch *in* the switch, the wall
wart is still wasting power and taking up space).

Netgear 5 port metal cased jobbies have keyhole slots and are happy to be
mounted upside down on a couple of roundhead screws...

eg GS105, gig, 40 quid sterling.
or if you want to be cheap, FS105 for half the money 100/10. Bear in mind
that you might do well selling the former on ebay at end of use and be no
worst off than using the FS105.

 

[D Yuniskis]

Hi Joseph,

Don't do this daisy chaining. It will break every rule of XBase-T
twisted pair network wiring. Put in little $15 switches instead.

I'm aware that it "breaks the rules". The point of my query
was to determine how any problems would manifest themselves.
I.e., for what is admittedly a "temporary connection", I
can easily life with reduced bandwidth, etc. OTOH, if it
simply "didn't work", then it would be a wasted effort.

Adding yet another "small switch" means finding a way to
mount that switch someplace convenient (most of the 4 or 5 port
switches I've seen are intended as "desktop" devices -- not
viable, here -- so I would have to physically modify one
in order to fasten it to the underside of the work surface.
And, then make room for yet another wall wart to power the
damn thing -- despite the fact that 95% of the time the
switch will be unused (so, even if I install a power switch
*in* the switch, the wall wart is still wasting power and
taking up space).

 

[D Yuniskis]

Hi Paul,

Nope. This is done in many commercial Ethernet hubs. One port will be
brought out in two jacks, one wired as a crossover, so that hubs can be
daisy chained.

Yes, that was the inspiration for attempting this. But,
doing it in foil is considerably different than trying to
get it right with discrete wires (e.g., it is usually
a PITA just to keep the tight twists all the way up to
the punchdown blocks -- and I would have to keep them
*beyond* ... to the *next* punchdown block!)

Its possible that leaving a longish cord in the unused port might cause
reflections (from the open end) that will interfere with the port in
use. But I doubt an open stub of only a few inches length will produce
much of an effect.

Dunno. I don't know how to gauge what the impedance bump
will do to the "legitimate" data signals.

Don't go connecting both jacks to live equipment. That will bugger
things up. I can't remember how many times I've been asked to debug a
friend's office LAN and found a new PC plugged into the one remaining
open jack on the hub. I wish they'd make a little sliding door that
would block one or the other jack sharing that port.

Ah, that's a great idea!! Or, If they had specified *all*
cables needed a twist and done away with that "special"
connector entirely...

 

[JosephKK]

Hi Joseph,


I'm aware that it "breaks the rules". The point of my query
was to determine how any problems would manifest themselves.
I.e., for what is admittedly a "temporary connection", I
can easily life with reduced bandwidth, etc. OTOH, if it
simply "didn't work", then it would be a wasted effort.

Because of the conceptual differences and operational differences
between coax networking and twisted pair networking i bet there is a
odds on chance of not working. It may be possible with 1:1:1
transformers but i would still bet against it. The inevitable
impedance change may cause line levels to shift beyond acceptable
bounds. The bump from the daisy chain is not so much the issue,
unless you want 1GBASE-CX. 10BASE-T will not hardly notice, though
100BASE-TX probably will. All 3 of them may die if the parallel
connection is tried.

Adding yet another "small switch" means finding a way to
mount that switch someplace convenient (most of the 4 or 5 port
switches I've seen are intended as "desktop" devices -- not
viable, here -- so I would have to physically modify one
in order to fasten it to the underside of the work surface.
And, then make room for yet another wall wart to power the
damn thing -- despite the fact that 95% of the time the
switch will be unused (so, even if I install a power switch
*in* the switch, the wall wart is still wasting power and
taking up space).

You might try running two cables out of your existing switch.
And yes, i understand that pulling the new cable may well be a
real pain.

 

[D Yuniskis]

Hi Joel,

I haven't had a problem like this in a number of years now -- almost all
routers, switches, etc. that you see today implement auto-MDIX on the
Ethernet ports, in which case it doesn't matters whether or not your
cables perform the crossover.

I do still carry around one of those Ethernet crossover fobs just in
case I encounter someone's older network equipment, though.

One advantage of this approach is I can remove the cable to the
switch from the backside of this little box and then I essential
have that "crossover fob" sitting there under the desktop.
I.e., I could plug two devices directly together (via this "fob")
instead of having to dig up a crossover cable.