- Unshielded Twisted Pair
-
- Because
networking evolved over severeal decades, many different cabling
solutions have been used. But today, virtually all VDV copper cabling
in the US has moved to unshielded twisted pair (UTP) as specified in
the EIA/TIA 568 standard (ISO/IEC 11801 worldwide) because it has been
inexpensive, may already be in place, is familiar to installers and is
simple to install. (The cost and simplicity of installation has changed
a bit, however, with Cat 5e/6/6A, see below.) Some users, mostly
outside the US, use shielded twisted pair (STP) with shields over each
pair in the cable or screened twisted pair (ScTP) with an overall
shield around all the pairs. These cables can provide higher
performance but require more care in installation.
UTP
cable has been a standard cable for telephone systems for almost a
century, where it is generally used for analog phone systems (POTS or
plain old telephone service.) These cables are generally large
multipair cables with pair-count configurations of 25, 50, 100, 200
pairs or more. 
-
- Most
UTP cable used in structured cabling systems today is comprised of four
pairs of carefully twisted pairs of solid copper wire, insulated with
carefully chosen material to provide high bandwidth, low attenuation
and crosstalk. Cables permanently installed in premises cabling systems
use solid wire for higher performance but patchcords generally use
stranded wire for greater flexibility and ruggedness when handled.
- UTP cable works so well because it is used with
transmitters that work on "balanced transmission" as shown below.
They transmit equal but opposite signals on each wire of the
pair so each wire has only half the amplitude of the final signal.
The electrical and magnetic fields of each wire are opposite
and cancel out eachother, producing low electromagnetic emissions.
Likewise, electromagnetic pickup is the same on both wires so
they cancel out. The twists on the pair of wires mixes the emissions from the electrical signals so they cancel out.
-
Twisted Pair Trivia:
The two wires are referred to as "tip" and "ring" - but why? Does it
have something to do with the ringer on a phone? No! It refers to the
connection on a old phone plug from manual switchboards. The
white/stripe wire was connected to the "tip" of the plug and the solid
wire to the "ring" connector behind it!
-
- The
secret ingredient of Cat 5e/6 is the twists of the pairs! Below, you
can see the difference in the twists in each pair for different types
of cables. From the left, we have telephone wire with hardly any twists
in the pairs, then Category 3, Category 5e and Category 6 cables, with
each higher grade having more tightly twisted pairs. The Cat 6 cable
even has a separator between the pairs to reduce crosstalk at the
higher frequencies where it will be used.
- The pairs are
twisted tightly and very consistently, but each pair is twisted at a
different rate to "tune" them to different frequencies to prevent
crosstalk between the pairs. In order to maintain Cat 5e/6/6A performance, especially
crosstalk, you absolutely must keep the twists as close as possible to
the terminations- no more than 1/2 inch - 13 mm should be untwisted!
- UTP cable is terminated mostly
in jacks, connector receptacles that have punchdown terminations
on the backside and twists in the connectors inside to prevent crosstalk. See
Terminations. Some snap into work area outlets, others are
incorporated in rack mount patch panels. 568 allows many possible
cable configurations, including intermediate punchdowns, but
a direct run from a work area outlet to a patch panel will provide
the highest performance, likely necessary if upgrades to fast
networks like Gigabit Ethernet are contemplated.
-
- Patchcords for connecting network
equipment to the outlet or patchpanel are usually purchased factory-assembled.
The connector (plug) is properly called a "modular 8 pin connector"
but usually is referred to as a RJ-45, which is actually a specific
telco pin configuration on the same plug. They use stranded cable for flexibility
and require special connectors. Order them to proper length if
you can to prevent the mess that patch panels often become after
a few moves and changes.
Performance Grades- Higher
performance UTP cables were developed as computer network speeds
increased and needed higher bandwidth cables. The first graded cable,
Cat 3 was adequate for 10 Megabits/s Ethernet or 4 Mb/s Token Ring.
Token ring at 16 Mb/s required higher performance cable, so Cat 4 was
introduced, but neither network nor cable grade lasted long. Instead
Cat 5 was introduced to support "Fast Ethernet" at 100 Mb/s.
- When
Ethernet was upgraded to 1 gigabit per second (1000 Mb/s) , Cat 5 was
upgraded to "enhanced" Cat 5 or Cat 5e. Cat 6 was developed by cable
manufacturers as a higher performance cable with more headroom, but no
network was ever specified to use it. Instead, when 10 Gb/s Ethernet
was developed, an "augmented" Cat 6, Cat 6A, was required.
In
Europe, a shielded cable, Class F is used. Some US manufacturers offer
a version of this cable they call "Cat 7" but TIA has not considered
this cable part of TIA-568, so "Cat 7" is not its proper nomenclature.- You
can get hardware and cable rated for Cat 3, Cat 5e, Cat 6 or Cat 6A.
With cable, it's easy to see the difference; it's in the twists -
higher performance cables have more twists. But jacks are harder to
tell the differences. But they are different - inside jacks rated Cat 5
or above you will find internal twists to reduce crosstalk. If you
terminate Cat 5e
cable with Cat 3 jacks, you will get Cat 3 performance - no better!
When dealing with high performance UTP cabling like Category 5e, Cat 6
and Cat 6A designed to support
Gigabit Ethernet or faster, termination procedures become even more
complicated! It's vitally important to ensure all components are rated to the same high level in order to achieve that performance level.
Instead
of the "Categories" used in the US, in Europe and much of the rest
of the world they use "Classes" to designate performance. Here's a
table of performance and correllations to US standards. (Remember these
are the standards which specify minimum performance, so many
manufacturers will offer cables with frequency performance greater than
the standards for competitive reasons.)
| ISO/IEC
Class | US
Category | Frequency
(MHz) | |
A | | 0.1 | |
B | |
1 | |
C |
3 |
16 | |
D |
5e |
100 | |
E |
6 (6A) |
250 (500) | |
F |
- 7
- (Not currently under consideration)
|
600 |
-
- Here's
a guide to the diferences in the categories.
-
|
Characteristic
EIA/TIA(ISO)
|
Cat
5 |
- Cat 5e
- (Class D)
|
- Cat 6 (6A)
- (Class E)
|
- "Cat7"
- (Class F)
|
Fiber
Optics |
| Supports
networks |
100Base-T |
1000Base-T |
- 1000Base-T
- (10Gbase-T)
|
- ?
- None currently considered
|
- 10GBase-T
- and other networks at
up to 10 gigabits per second
|
| Test
Frequency |
100
MHz |
100MHz |
250
MHz
(500 MHz) |
600
MHz |
Terahertz |
| Length |
100
meters |
100
meters |
100
meters |
100
meters |
- 250m to 2km (MM)
- >20 km (SM)
|
| RJ-45
Compatible |
yes |
yes |
- yes
|
No |
NA |
| Field
Tester Requirement: |
Level
2 |
Level
2e |
Level
3 (3e) |
Unknown |
Optical Loss Test Set |
-
-
- Like
everything else that deals with computers and communications, the speed
of networks is going up. Cat 5e is OK to handle one gigabit networks,
but the EIA/TIA TR 42 committee that writes the cabling standards (the
same "568" we referred to earlier) approved the standard for Cat 6
cabling in June of 2002 after three years of debate, discussion and
testing. Unfortunately, network speeds jumped to 10 Gigabits/second,
requiring another redesign, leading to "augmented Cat 6" or Cat 6A.
-
- each new standard includes cables,
plugs and jack, patch panels and patch cords, in other words,
everything you need to install a complete cabling system. Performance
specifications for Cat 6A are for significant advances over Cat 6 or Cat
5e - with attenuation and crosstalk performance higher at every level.
-
- So using higher rated cables give you
more "headroom" - better signal to noise ratios - which
can mean more robust data transmission on slower Fast Ethernet and Gigabit
Ethernet (1000base-T) networks.
-
- Cat
6 never had a real reason for existence but many people installed it as
a higher performance cable than Cat 5e. Gigabit Ethernet ran fine on
Cat 5e (some manufcturers say it runs well on Cat 5 now) and when 10
Gigabit Ethernet came along, it could initially only run on fiber
optics. The copper suppliers would not allow their product to be
bypassed by technology, so an "augmented" Cat 6 was spec'ed for 10GbE
with approval in March, 2008. The problem with Cat 6A seems to be not
within the cable itself, but in crosstalk with adjacent cables, called
"alien" crosstalk and the high power consumed by Cat 6A transceivers to
send such high speed signals over copper. Fiber remains the most
reliable solution for GbE
and above.
-
- This leap in technological advances,
like all previous ones, comes with a cost. The goal of the committee
is that each generation of Category-rated cable be "backward
compatible" which means that any networking product that
works on Cat 3, 5 or 5e is supposed to work on Cat 6. The other
issue that the group wrestles with is "interoperability"
- mixing and matching components. This part of the standard states
that cable plant containing mixed categories (eg: Cat 5e patchcords
on a Cat 6 cabling systems) is supposed to work without compatibility
issues, but will only work at the level of the minimum component
specification (Cat 5e in this case).
-
- Finally, there is the issue
of compatibility among different manufacturers products. Practically
everybody told users installing "pre-standard" Cat
6 to stick to one manufacturer's products or those tested for
compatibility. The EIA/TIA press release on the Cat 6 standard
stated: "To ensure generic cabling system performance, Category
6 component requirements are specified to be interoperable when
products from different manufacturers are mated". The same issue probably exists with Cat 6A.
-
- Screened (ScTP) and shielded Twisted Pair (STP)
- Outside the US many countries, especially the Europeans, use Class F - a shielded 600 MHz cabling system, which the EIA/TIA TR42
committee has declined to consider such a standard in the US
due to lack of interest from the network development committees.
Internationally,
ScTP and STP cable is much more common, as concerns over
electromagnetic interference have affected standards more than in the
US. ScTP cable is simply a UTP cable with a metallic foil shield over
all 4 twisted pairs. Some manufacturers also call it FTP for
foil-shielded twisted pair. The foil shield is for noise immunity,
keeping outside noise out of the pairs and keeping emissions from the
pairs themselves from escaping the cable. ScTP is not the same as
shielded twisted pair which has a foil shield over each individual pair
and may also have a shield over all the shielded pairs under the cable
jacket.
Current ScTP cables are smaller than most UTP cables
that are rated as Cat 6A (augmented Category 6). The problems of
crosstalk between pairs in two separate cables that affects Cat 6A,
called alien crosstalk, is negated by the foil shield of ScTP. For UTP
Cat 6A cables, the solution to alien crosstalk was bigger cables with
thicker jackets, causing problems with the number of cables that could
be accommodated in conduits and cable trays. ScTP Cat 6A cables are no
larger than Cat 6 UTP cables, making them easier to fit in current
cabling hardware.
For termination, ScTP cables use shielded
modular 8-pin connectors just like UTP but with a shield. Terminating
the shield is pretty simple, just like adding a 9th wire to the cable.
One does have to worry about grounding, as ScTP cable must be tied into
the grounding busbar in the telecom room along with all the racks and
other metallic hardware.
The extra conductor in the cable offers
another benefit. It can be used for cable management systems where it
allows tracking cable connections using add-on electronic equipment
that automates the most time consuming part of any cabling project –
keeping documentation up to date.
- Cable Listings For Flammability
All
cables installed inside buildings must meet flammability requirements
of the building and electrical codes, which in the US means the NEC
(National Electrical Code) or CEC in Canada. Communications cable
will be rated per the following table, depending on its use.
| Cable Designation | Application | | CMX | General use for dwellings and raceways | | CMR | Riser rated for use in vertical shafts, rated to prevent carrying fire from floor to floor | | CMP | Plenum rated for use in environmental air spaces such as dropped ceiling spaces used for air return |
- CATV cable (CATV) and multipurpose cable (MP) are also covered under other sections of the electrical codes.
-
- Network Cable Handling
- The
performance of the cabling network is heavily dependent on the quality
of the installation. The components used in structured cabling
installation have been carefully designed and exhaustively tested to
meet or esceed the requirements of EIA/TIA 568 for performance at 100
MHz. If the cable is not properly installed, performance will be
degraded. In particular, one should avoid bending the cable too
tightly, kinking it or using tightly installed cable ties which can
crush the cables. Loosely bundling cables with "hook and loop" cable
ties is much better. See installation for more tips.
Test your comprehension with the section quiz. |
