INTRODUCTION.
The purpose of this article is to show you how to make the two kinds of
cables which can be used to network two or more computers together to
form quick and simple home or small office local area networks
(LANs). These instructions can also be used to make patch cables
for networks with more complex infrastructure wiring.
The two most common unshielded twisted-pair (UTP)
network standards are the10 Mhz 10BASE-T Ethernet and the 100Mhz
100BASE-TX Fast Ethernet. The 100BASE-TX standard is
quickly becoming the predominant LAN standard. If you are
starting from scratch, to build a small home or office network, this
is clearly the standard you should choose. This article will
show you how to make cables which will work with both standards.
LANS SIMPLIFIED.
A LAN can be as simple as two
computers, each having a network interface card (NIC) or network
adapter and running network software, connected together with a crossover
cable.
The next step up would be a network
consisting of three or more computers and a hub. Each of the
computers is plugged into the hub with a straight-thru cable
(the crossover function is performed by the hub).
There
are several classifications of cable used for twisted-pair
networks. I'll skip right over them and state that I use and
recommend Category 5 (or CAT 5) cable for all new installations.
Likewise, there are several fire code classifications for the outer
insulation of CAT 5 cable. I use CMR cable, or "riser
cable," for most of the wiring I do. You should also be
aware of CMP or plenum cable (a plenum is used to distribute air in a
building). You may be required by local, state or national codes
to use the more expensive plenum-jacketed cable if it runs through
suspended ceilings, ducts, or other areas, if they are used to
circulate air or act as an air passage from one room to another.
If in doubt, use plenum. CMR cable is generally acceptable for
all applications not requiring plenum cable.
CAT
5 wire is available in reel-in-box packaging. This is very handy for
pulling the wire without putting twists in it. Without this kind
of package or a cable reel stand, pulling wire is a two-person
job. Before the advent of the reel-in-box, we used to put a reel
of wire on a broom handle to pull it. One person would hold the
broom handle and the other would pull and measure the cable. You
will produce a tangled mess, if you pull the wire off the end of the
reel.
Stranded wire patch cables are often specified
for cable segments running from a wall jack to a PC and for patch
panels. They are more flexible than solid core wire.
However, the rational for using it is that the constant flexing of
patch cables may wear-out solid core cable--break it. I don't
think this is a real concern in the average small network.
For example, I have one solid core cable going to my work bench.
It has probably flexed and average person's lifetime of flexes from
the many many times I have connected customer computers to my
network. Also, stranded cable is susceptible to
degradation from moisture infiltration, may use an alternate color
code, and should not be used for cables longer than 3 Meters (about 10
feet).
Most of the wiring I do simply connects
computers directly to other computers or hubs. Solid core cable
is quite suitable for this purpose and for many home and small
business networks. I find it also quite acceptable for use
as patch cables. You might consider a stranded wire patch cable
if you have a notebook computer you are constantly moving around.
CAT 5 cable has four twisted pairs of wire
for a total of eight individually insulated wires. Each
pair is color coded with one wire having a solid color (blue, orange,
green, or brown) twisted around a second wire with a white background
and a stripe of the same color. The solid colors may have
a white stripe in some cables. Cable colors are commonly
described using the background color followed by the color of the
stripe; e.g., white-orange is a cable with a white background and an
orange stripe.
CONNECTORS.
The straight through and cross-over patch cables
discussed in this article are terminated with CAT 5 RJ-45 modular
plugs. RJ-45 plugs are similar to those you'll see on the end of
your telephone cable except they have eight versus four or six
contacts on the end of the plug and they are about twice as big.
Make sure they are rated for CAT 5 wiring. (RJ means
"Registered Jack"). Also, there are RJ-45 plugs
designed for both solid core wire and stranded wire. Others are
designed specifically for one kind of wire or the other.
Be sure you buy plugs appropriate for the wire you are going to
use. I use plugs designed to accommodate both kinds of wire.
Modular Plug Crimp Tool
You will need a modular crimp
tool. This one is very similar to the one I have been using for
many years for all kinds of telephone cable work and it works just
fine for Ethernet cables. You don't need a lot of bells and
whistles, just a tool which will securely crimp RJ-45
connectors. This one is made by Eclipse
Enterprises, Inc. Even though the crimper has cutters
which can be used to cut the cable and individual wires, and possibly
stripping the outer jacket, I find that the following tools are
better for stripping and cutting the cable...
Universal UTP Stripping Tool (Eclipse). I recently bought one of
these tools and it works slick, and it makes a much neater cut.
I recommend that you purchase one if you will be making many cables.
Diagonal Cutters It is easier to use
diagonal cutters ("diags" or "dikes") to cut the
cable off at the reel and to fine tune the cable ends during
assembly. Also, if you don't have a stripper, you can strip the
cable by using a small knife (X-acto, utility, etc.) to
carefully slice the outer jacket longitudinally and use the diags to
cut it off around the circumference.
Now, bear with me, you need to understand some
of this stuff...
The 10BASE-T and 100BASE-TX Ethernets consist of
two transmission lines. Each transmission line is a pair
of twisted wires. One pair receives data signals and the
other pair transmits data signals. A balanced line driver or
transmitter is at one end of one of these lines and a line receiver is
at the other end. A (much) simplified schematic for one of these
lines and its transmitter and receiver follow:
Pulses of energy travel down the
transmission line at about the speed of light (186,000
miles/second). The principal components of one of these pulses
of energy is the voltage potential between wires and current flowing
near the surface of the wires. This energy can also be
considered as residing in the magnetic field which surrounds the wires
and the electric field between the wires. In other words, an
electromagnetic wave which is guided by, and travels down the wires.
The main concern is the transient magnetic
fields which surrounds the wires and the magnetic fields generated
externally by the other transmission lines in the cable, other network
cables, electric motors, fluorescent lights, telephone and electric
lines, lightning, etc. This is known as noise. Magnetic
fields induce their own pulses in a transmission line which may
literally bury the Ethernet pulses, the conveyor of the information
being sent down the line.
The twisted-pair Ethernet employs two principle
means for combating noise. The first is the use of balanced
transmitters and receivers. A signal pulse actually consists of
two simultaneous pulses relative to ground: a negative pulse on one
line and a positive pulse on the other. The receiver detects the
total difference between these two pulses. Since a pulse of
noise (shown in red in the diagram) usually produces pulses of the
same polarity on both lines one pulse is essentially canceled by out
the other at the receiver. Also, the magnetic field surrounding
one wire from a signal pulse is a mirror of the one on the other wire.
At a very short distance from the two wires the magnetic fields are
opposite and have a tendency to cancel the effect of each other
out. This reduces the line's impact on the other pair of wires
and the rest of the world.
The second and the primary means of reducing
cross-talk--the term cross-talk came from the ability to (over) hear
conversations on other lines on your phone--between the pairs in the
cable, is the double helix configuration produced by twisting the
wires together. This configuration produces symmetrical
(identical) noise signals in each wire. Ideally, their
difference, as detected at the receiver, is zero. In actuality
it is much reduced.
COLOR-CODE STANDARDS
Let's start with
simple pin-out diagrams of the two types of UTP Ethernet cables and watch
how committees can make a can of worms out of them. Here are the
diagrams:
Note that the TX (transmitter) pins are
connected to corresponding RX (receiver) pins, plus to plus and minus to
minus. And that you must use a cossover cable to connect units
with identical interfaces. If you use a straight-through cable, one
of the two units must, in effect, perform the cross-over function.
Two wire color-code standards apply: EIA/TIA 568A
and EIA/TIA 568B. The codes are commonly depicted with
RJ-45 jacks as follows:
If we apply the 586A color code and show all eight
wires, our pin-out looks like this:
Note that pins 4, 5, 7, and 8 and the blue and brown
pairs are not used in either standard. Quite contrary to what you
may read elsewhere, these pins and wires are not used or required to
implement 100BASE-TX duplexing--they are just plain wasted.
However, the actual cables are not physically that
simple. In the diagrams, the orange pair of wires are not
adjacent. The blue pair is upside-down. The right ends match
RJ-45 jacks and the left ends do not. If, for example, we invert the
left side of the 586A "straight"-thru cable to match a 586A
jack--put one 180° twist in the entire cable from end-to-end--and twist
together and rearrange the appropriate pairs, we get the following
can-of-worms:
This
further emphasizes, I hope, the importance of the word
"twist" in making network cables which will work. You
cannot use an flat-untwisted telephone cable for a network cable.
Furthermore, you must use a pair of twisted wires to connect a set of
transmitter pins to their corresponding receiver pins. You cannot
use a wire from one pair and another wire from a different pair.
Keeping the above principles in mind,
we can simplify the diagram for a 568A straight-thru cable by
untwisting the wires, except the 180° twist in the entire cable,
and bending the ends upward. Likewise, if we exchange the green and
orange pairs in the 568A diagram we will get a simplified diagram for a
568B straight-thru cable. If we cross the green and orange pairs in
the 568A diagram we will arrive at a simplified diagram for a crossover
cable. All three are shown below.
There are only two unique cable
ends in the preceding diagrams. They correspond to the 568A and 568B
RJ-45 jacks and are shown to the right.
Again, the wires with colored
backgrounds may have white stripes and may be donated that way in
diagrams found elsewhere. For example, the green wire may be
labeled Green-White--I don't bother. The background color is
always specified first.
Now, all you need to remember, to
properly configure the cables, are the diagrams for the two cable ends
and the following rules:
It makes no functional difference
which standard you use for a straight-thru cable. You can
start a crossover cable with either standard as long as the other
end is the other standard. It makes no functional
difference which end is which. Despite what you may have read
elsewhere, a 568A patch cable will work in a network with 568B wiring
and 568B patch cable will work in a 568A network. The electrons
couldn't care less.
My preference is to use the
568A standard for straight-thru cables and to start crossover cables
with a 568A end. That way all I have to remember is the diagram
for the 568A end, that a straight-thru cable has two of them, and that
the green and orange pairs are swapped at the other end of a crossover
cable.
Let's Make Some Cable
1. Pull the cable off the
reel to the desired length and cut. I have a box of cable at one
end of my shop and a mark on the floor 10' away. For cable
lengths which are a fraction of ten feet, I eye-ball the length
as I pull the cable out of the box (also, my feet are
about one foot long). For longer cables, I pull it out to the
ten foot mark and go back to the box and pull the remaining fraction
or another ten feet. If you are pulling cables through walls, a
hole in the floor, etc., it easier to attach the RJ-45 plugs after the
cable is pulled. The total length of wire segments between a PC
and a hub or between two PC's cannot exceed 100 Meters (328 feet or
about the length of a football field) for 100BASE-TX and 300 Meters
for 10BASE-T.
2.
Strip one end of the cable with the stripper or a
knife and diags. If you are using the stripper, place the cable
in the groove on the blade (left) side of the stripper and align the
end of the cable with the right side of the stripper. This will
strip about 1/2" of the jacket off the cable. Turn
the stripper about 1 1/4 turns and pull. If you turn it more, you will
probably nick the wires. If you are using a knife and diags,
carefully slit the cable for about an inch or so and neatly trim
around the circumference of the cable with diags to remove the jacket.
3. Inspect the wires for
nicks. Cut off the end and start over if you see
any. You may have to adjust the blade with the screw at the
front stripper. Cable diameters and jacket thicknesses vary.
4.
Spread and arrange the pairs roughly in the order
of the desired cable end.
5. Untwist the pairs and
arrange the wires in the order of the desired cable end. Flatten
the end between your thumb and forefinger. Trim the ends of the wires
so they are even with one another. It is very important
that the unstripped (untwisted) end be slightly less than 1/2"
long. If it is longer than
1/2" it will be out-of-spec and susceptible to crosstalk.
If it less than slightly less than 1/2" it will not be
properly clinched when RJ-45 plug is crimped on.. Flatten
again. There should be little or no space between the wires.
6.
Hold the RJ-45 plug with the clip facing down or away from you.
Push the wire firmly into the plug. Now, inspect
the darn thing... before crimping and wasting the plug!
Looking through the bottom of the plug, the wire on the far left side
will have a white background. The wires should alternate light
and dark from left to right. The furthest right wire is
brown. The wires should all end evenly at the front of the
plug. The jacket should end just about where you see it in the
diagram--right on the line. Aren't you glad you didn't crimp the
plug?
7. Hold the wire near the RJ-45 plug with
the clip down and firmly push it into the left side of the front of
the crimper (it will only go in one way). Hold the wire in place
squeeze the crimper handles quite firmly. This is what will
happen:
(Crimp it once.) The crimper pushes two
plungers down on the RJ-45 plug. One forces what amounts to a
cleverly designed plastic plug/wedge onto the cable jacket and very
firmly clinches it. The other seats the "pins," each
with two teeth at its end, through the insulation and into the
conductors of their respective wires.
8. Test the crimp... If done properly an
average person will not be able to pull the plug off the cable with
his or her bare hands. And that quite simply, besides lower
cost, is the primary advantage of twisted-pair cables over the older
thinwire, coaxial cables. In fact, I would say the RJ-45 and
ease of its installation is the main reason coaxial cable is no
longer widely used for small Ethernets. But, don't pull that
hard on the plug. It could stretch the cable and change its
characteristics. Look at the side of the plug and see if it
looks like the diagram and give it a fairly firm tug to make sure it
is crimped well.
9. Prepare the other end of the cable so
it has the desired end and crimp.
10. If both ends of the cable are within
reach, hold them next to each other and with RJ-45 clips facing
away. Look through the bottom of the plugs. If the plugs
are wired correctly, and they are identical, it is a straight-thru
cable. If they are wired correctly and they are different, it is
a crossover cable.
11. If you have an operational network,
test the cable. Copy some large files.
12. If the cable doesn't work, inspect the
ends again and make sure you have the right cable and that it is
plugged into the correct units for the type of cable. Try
power-cycling (cold booting) the involved computers.
13. If you have many straight-thru cables
and a crossover cable in your system, you should consider labeling the
crossover cable or using a different colored cable for the crossover
cable so you don't mix them up. I do not recommend implementing
the crossover function, as recommended elsewhere, with two RJ-45
jacks, appropriately wired back to back, and two straight-thru
cables. This method costs noticeably more, introduces more than
the necessary number of components and connections, increases the
complexity and time of assembly, and decreases reliability.
CABLING RULES
1. Try to avoid running cables parallel to power
cables.
2. Do not bend cables to less than four
times the diameter of the cable.
3. If you bundle a group of cables
together with cable ties (zip ties), do not over-cinch them.
It's okay to snug them together firmly; but don't tighten them so much
that you deform the cables.
4. Keep cables away from devices which can
introduce noise into them. Here's a short list: copy machines,
electric heaters, speakers, printers, TV sets, fluorescent lights,
copiers, welding machines, microwave ovens, telephones, fans,
elevators, motors, electric ovens, dryers, washing machines, and shop
equipment.
5. Avoid stretching UTP cables (tention
when pulling cables should not exceed 25 LBS).
6. Do not run UTP
cable outside of a building. It
presents a very dangerous lightning hazard!
7. Do not use a stapler to secure UTP
cables. Use telephone wire/RJ6 coaxial wire hangers which are available at most
hardware stores.
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