mythbusting: shielded cabling - bicsi · – the shield around a shielded cable is just like ......
TRANSCRIPT
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MYTHBUSTING Takes on Shielded Cabling
Herb BrianCongdon Davis
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Objectives• Take on myths of shielded cabling and use
modern-day science to show what's real and what's fiction - through trial and error actually demonstrate it.
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Background• Shielded is century-old technology (1880)
– Dating back to early coaxial cabling
• Shielded Twisted-pair Cabling is over 25 years old– Dating back to IBM Type I Cabling
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Background• Wide industry experience with IBM Type 1
– That experience was, admittedly, mostly negative
• Partly why the industry preference moved to unshielded in North America (‘cheap and easy’ over ‘high performance’)
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Background• Shielded technology has innovated,
evolved and developed– Evidence of that has not been widely seen in
North America due to the limited use of shielded cabling
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Background• So, the previous convictions of IBM Type 1
are still applied to all shielded cabling regarding:
– Cable Termination– The Antenna Effect– Ground Loops– The Baked Potato Effect
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Cable Termination
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Cable Termination• Myth:
– Terminating a shielded cable with a shielded connector takes twice as long as terminating an unshielded cable with an unshielded connector.
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Research• IBM Type 1 Cable
– Two shielded twisted pairs• not four pair
– 22 AWG solid copper conductors– Braided shield
• IBM Data Connector– Hermaphroditic– 4 pole– Large
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Research• Data Connector Termination Procedure:
13 steps!– snip & cut, slide, twist, cut & trim, clamp, slide,
insert (x4), pull, align & squeeze, ground, bend, slide, cover & snap (x2) !!
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Research• Today
– New cable types• Without braid• Unshielded pairs• Four-pair (not 2-pair)
– New connector types• Die-cast (without a “can”)
– New installation tools• Simplify procedure• Faster
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Experiment - Subjects• Shielded Termination
– IBM Type 1• 2-pair• S/FTP
– Category 6A• 4-pair• F/UTP
• Unshielded Termination– Category 6
• 4-pair• U/UTP
– Category 6A• 4-pair• U/UTP
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Experiment - Setup• Experienced Technicians• Measure Time For
– Cable Prep– Connector Prep– Termination
• Dependent Variables– Cabling Type– Termination Tool
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Termination Tools• Punchdown Tool
– Each wire individually• Installation Tool
– All wires simultaneously
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Results – Category 6 Unshielded
• Two times myth plausible for IBM Type 1 cable termination– Maybe even 4 times
• four pairs in over 7 minutes!
Cable Type Tool Type Avg. Install Time
Cat6 U/UTP Punchdown 2 minutes
IBM Type 1 Pliers/Snips 3 min 45 sec (2pr)
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Results – Category 6A Punchdown
• Two times myth plausible for punchdowncable termination– More of a Cat 6A issue than shielded issue
• Tighter twists, larger wires, more fillers
Cable Type Tool Type Avg. Install Time
Cat6 U/UTP Punchdown 2 minutes
Cable Type Tool Type Avg. Install Time
Cat6A U/UTP Punchdown 3 min 15sec
Cat6A F/UTP Punchdown 3 min 20sec
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Results – Category 6A Installation Tool
Cable Type Tool Type Avg. Install Time
Cat6 U/UTP Punchdown 2 minutes
Cat6A U/UTP Punchdown 3 min 15 sec
Cat6A U/UTP Install Tool 1 min 25 sec
Cat6A F/UTP Install Tool 1 min 30 sec
• Even less than Category 6 punchdown!
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Result: The Termination Time Myth
BUSTED!!
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Additional Considerations• Field Testing – could make installation
time worse for unshielded– depends on users choice of ANEXT testing
options• F/UTP vs. SF/FTP (Category 7)
– Category 7 (SF/FTP) would take longer• Bonding and Grounding
– No additional time compared to unshielded• Except for pulling back shield foil
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The Antenna Effect
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The Antenna Effect• Myth
– The shield of a shielded cable acts like and antenna and collects radio frequency noise if it isn’t grounded on both ends
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Experiment - Subjects• U/UTP Cable
• F/UTP Cable
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Experiment - Setup• Noise Source
– WAP (2GHz)– Laptop PC
• U/UTP Cable• F/UTP Cable• Network Analyzer
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Results – Noise on Unshielded Cable
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Results – Noise on F/UTP Cable with Both Ends Grounded
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Results – Noise on F/UTP Cable with Single-ended Ground
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Results – Noise on Ungrounded F/UTP Cable
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Result: The Antenna Effect
BUSTED!!
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Additional Considerations• All metallic elements act as antennas to
some extent – even pairs in an unshielded cable
• The shield dramatically reduces the effects of RFI on the pairs, even if grounded on only one end or if ungrounded!– But ground it anyway!
• A shield reduces the effects of induced noise for signals > 30 MHz– Shield thickness property
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Ground Loops
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Ground Loops• Myth
– Conductive loops transmit spurious current (noise) if there is any variation in voltage along the loop
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Research• Myth probably outgrowth of “mains hum”
– Two pieces of audio equipment plugged into different power outlets with different ground potentials
– This potential difference causes a spurious current to flow through the cables, creating an audible buzz
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Experiment - Subjects• F/UTP Cable
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Experiment - Setup• Apply 60 Hz AC current
directly onto shield of F/UTP cable– Not an induced current
– a placed current• Test signal integrity on
balanced twisted-pairs for presence of 60Hz influence
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Results• No influence
on signal integrity at 60Hz
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Result: Ground Loops
BUSTED!!
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Additional Considerations• Basically proves that balance works, at
least for 60Hz (low frequency)– Audio cables tend not to be balanced or
shielded• Higher frequency effects reduced by the
shield
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The Baked Potato Effect
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The Baked Potato Effect• Myth
– The shield around a shielded cable is just like aluminum foil around a baked potato – it traps generated heat and cooks the cable
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Research• Wrapping the potato in aluminum foil
before cooking will help to retain moisture• When cooking over an open fire or in
coals, a jacket of foil prevents burning• Aluminum foil is designed to keep
moisture in the baked potato – not to retain heat
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Research• Power over Ethernet
– Data cables used for signal and for power– Power current creates heat via I2R losses
• Power over Ethernet Plus– More power – more current – more heat
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Experiment - Subjects• U/UTP Cable
• F/UTP Cable
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Experiment - Setup• Cable bundles
– In insulated conduit, with firestopping
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Experiment - Setup• Worst case
– Place 720 mA current (original PoEP objective)
• The objective for PoEP is currently only 600mA– 57 Vdc measured at output end of cable (not
source)– Power on all four pairs– Pairs wired in series
Input
Output
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Experiment - Setup
Thermocouple #1Center of Cable Core
Thermocouple #2Under Cable Jacket
Thermocouple #4Cable on Outer Layer of Bundle
Thermocouple #3Cable on Inner Layer
of Bundle
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Results – F/UTP Shielded Cable
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.00.
25
0.75
1.25
1.75
2.25
2.75
3.25
3.75
Time [hrs]
Tem
pera
ture
Incr
ease
(C)
CoreUnder JacketCenter CableOuter Cable
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Results – F/UTP and U/UTP Cable
T-Rise measurements Ambient Temp. = 22.5 °C
0
2
4
6
8
10
Time
Temp
eratur
e Inc
rease
(°C)
Center cable T-Rise Outer cable T-Rise
T-Rise measurementsAmbient Temp. = 21.7 °C
0
2
4
6
8
10
TimeTe
mpera
ture I
ncrea
se (°
C)
Center cable T-Rise Outer cable T-Rise
F/UTP Cable U/UTP Cable
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Results – 720mA
Cable Type Steady-state Temperature Rise
F/UTP 6°C
U/UTP 7°C
• Both show increase• Shielded compares favorably
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Result: The Baked Potato Effect
BUSTED!!
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Additional Considerations• Unlikely to see that kind of temperature
rise in the field– Lower current than POEP objective– Violation of fill ratios– All four pairs carrying max power– Max power measured at device end, not
source end
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Conclusions
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ReviewMyth Conclusion
Termination Time = x2 BUSTED !!
Antenna Effects = Bad BUSTED !!
Ground Loops = Bad BUSTED !!
Baked Potato Effect = Hot BUSTED !!
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Thank You!
Remember: These are trained professionals – do not try this at home!
No animals (or potatoes) were harmed in this production