[SI-LIST] : Screened/Shielded UTP Cabling

craig stein (craig_stein@hp-cupertino-om6.om.hp.com)
Sun, 03 May 1998 19:06:24 -0700

I have some screened UTP cabling questions for anyone knowledgeable
about 100 ohm screened UTP cabling. Below I have enclosed cabling
configurations that a United Kingdom customer has implemented. They
desire to mix UTP with screened UTP in the same domain.

ISO/IEC 11801 specifies chassis ground for screened UTP cables at both
ends. The cables must have the grounding clamp on the RJ45 connector.
This will eliminate the large current loops, causing the return current
to flow back through the shield, limiting the noise voltage in series
with the signal. I am not sure, however, about mixing the balanced UTP
connectors with the grounded S/UTP parts.

I am attempting to determine what mixed (UTP/S-UTP) configurations we
can formally support. Below, it shows that separating a continuos S/UTP
plant with a UTP portion at the patch panel functions fine. But two UTP
segments in a the domain with two S/UTP does not work at certain
lengths.

The National PHY documents the exact problem that we are noticing over
35 to 41 meters of cable. They state that a link will not be established
because their PHY misinterprets the scrambled idles as FLPs causing a
link failure. I do not understand why providing a solid ground path for
the S/UTP seems to solve this problem.

See Below:

Madge MultiNet LANswitch LET36, LFE4004 modules.

Various cables.
C
---------- |---| ----------
| | A _ B | | D | |
| 10/100 |-------|_|---------------|_| |_|--------| Madge |
|UNIX BOX| | switch |
|AUTO-NEG| | 100 Mb/s
---------- ----------
Original cabling
================

A = UTP cable
B = ITT ISCS20003DSC Shielded cable
C = UTP patch cable
D = ITT ISCS20003DSC Shielded cable
Three 808 patch connector blocks, between A & B, B & C and C & D.

Test 1
======

After a reboot using these cables lan Card failed (orange led on). The
card did not seem to be properly interpreting the link pulse. The other
cards, lan1 and lan2 also connected from the box to the switch had green
LEDs, but ping showed up to 95% packet loss.

Using the Wirescope end to end on these cables gave the following
results:

lan0 cable total length 36 meters.
Next pair 2 and 3 56dB at 7.3Mhz Attenuation
pair 2 7.0dB at 100Mhz Attenuation
pair 3 7.3dB at 100Mhz Loop resistance
6 ohms both pairs

lan1 cable total length 36 metres.
Next pair 2 and 3 37dB at 57.75Mhz Attenuation
pair 2 7.8dB at 100Mhz Attenuation
pair 3 8.3dB at 100Mhz Loop resistance
6 ohms both pairs

lan2 cable total length 36 metres.
Next pair 2 and 3 39dB at 56Mhz Attenuation
pair 2 7.6dB at 100Mhz Attenuation
pair 3 8.0dB at 100Mhz Loop resistance
6 ohms both pairs

Test 2
=======

A = ITT ISCS20003DSC Shielded cable
B = ITT ISCS20003DSC Shielded cable
C = ITT ISCS20003DSC Shielded cable
D = ITT ISCS20003DSC Shielded cable

using the same structured wiring links as in test 1 but replacing the
UTP cable at A and C with Shielded cable. The cable length remained at
36 metres, but the problem was no longer evident.

Test 3
=======
A = original UTP cable
B = ITT ISCS20003DSC Shielded cable
C = ITT ISCS20003DSC Shielded cable
D = ITT ISCS20003DSC Shielded cable

using the same structured wiring links as in test 1 but replacing the
UTP cable at C with Shielded cable. The cable length remained at 36
metres, but the problem was no longer evident.

Test 4
=======
A = ITT ISCS20003DSC Shielded cable
B = ITT ISCS20003DSC Shielded cable
C = original UTP cable
D = ITT ISCS20003DSC Shielded cable

using the same structured wiring links as in test 1 but replacing the
UTP cable at A with Shielded cable. The cable length remained at 36
metres, but the problem was no longer evident.

Test 5
=======
A = original UTP cable
B = ITT ISCS20003DSC Shielded cable
C = original UTP cable
D = ITT ISCS20003DSC Shielded cable

re-testing using the same structured wiring links as in test 1. Cable
length remaining at 36 metres. This failed as in test 1.

Test 6
=======
A = original UTP cable
B = ITT ISCS20003DSC Shielded cable
C = original UTP cable
D = ITT ISCS20003DSC Shielded cable

re-testing using the same structured wiring links as in test 1. This
time we earth grounded the patch panels where patch cable C (UTP) is
connected. This worked fine.

Test 7
=======
A = original UTP cable
B = ITT ISCS20003DSC Shielded cable
C = original UTP cable
D = ITT ISCS20003DSC Shielded cable

same configuration, but removing the ground cable we could not get
another failure.