[辅助资料]UTP 接口线序

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Key words:
        10BaseT - Unshielded Twisted Pair (UTP) - RJ45
        110 Blocks - Category 5 - Level 5 - EIA 568B
        Straight Through - Reversed - Cross-over
        MDI, MDI-X Hub Ports
Wiring Tutorial for 10BaseT Unshielded Twisted Pair (UTP)
One of the most common and most puzzling problems a network
engineer/technician may face is what is the PROPER way to make up a 10BaseT
cable.  Usually, to confound the learning process, someone introduces the
need for a reversed or cross-over cable at the same time.  
What these are and how to make them is the subject of this on-line tutorial.
Selection of Cabling Category
Since the overwhelming bulk of network cabling done today uses Unshielded
Twisted Pair (UTP) wiring that is what we will discuss.  The process begins
with the selection of the proper wiring level or category.  Today it is
basically inexcusable to use or install anything at less than Level V or
Category 5.
While technically Category 5 and Level V are
not the same, they are identical in practice.  Both support upto 100
megabit per second data transmission, and their physical cable assembly
requirements are the same.  Throughout this tutorial we will refer to them
both as CAT5.
When you order CAT5 unshielded twisted pair (UTP) cable you will receive a
cable containing 4 twisted pairs of wires, a total of 8 wires.  The strands
that constitute each wire will either be a single strand or multiple strands,
usually referred to as solid or flex.  Typically the solid is
used to run through walls and ceilings and the flex is used to make drop
cables (the cable from the wall plate to the desktop computer)  and patch
cables (the cable from the patch panel to the hub).  Whether the exterior
portion of the cable that contains the 4 twisted pairs, the jacket, is
Plenum grade or Non-plenum grade is very important, it refers
to the Fire Codes, but is outside the scope of this tutorial.
Ordering Pairs
The pairs of wires in UTP cable are colored so that you can identify the
same wire at each end.  Furthermore, they are usually color coded by pair
so that the pairs can also be identified from end to end.  Typical CAT5 UTP
cables contain 4 pairs made up of a solid color and the same solid color
striped onto a white background.  The most common color scheme is the one
that corresponds to the Electronic Industry Association/Telecommunications
Industry Association's Standard 568B.
The following table demonstrates the proper color scheme.
Wire pair #1:
White/Blue
Blue

Wire pair #2:
White/Orange
Orange
Wire pair #3:
White/Green
Green
Wire pair #4:   
White/Brown
Brown
Connectors
The cable connectors and jacks that are most commonly used with CAT5 UTP
cables are RJ45.  The RJ simply means Registered Jack and the
45 designation specifies the pin numbering scheme.  The connector is
attached to the cable and the jack is the device that the connector plugs
into, whether it is in the wall, the network interface card in the computer,
or the hub.  
Now that we are ready to insert the cable into the RJ45 plug the wire number and
color sequence becomes more complicated.
The IEEE Specification for Ethernet 10BaseT requires that two twisted pairs be
used and that one pair is connected to pins 1 and 2, and that the second pair
is connected to pins 3 and 6.  Yes that is right - pins 4 and 5 are skipped
and are connected to one of the remaining twisted pairs.

According to the EIA/TIA-568B RJ-45 Wiring Scheme:
It gets even more odd because wire Pair#2 (white/orange, orange) and
Pair#3 (white/green, green) are the only two pairs used for 10BaseT data.
      Pair#2 is connected to pins 1 and 2 like this:
      Pin 1     wire color:        white/orange
      Pin 2     wire color:              orange
Pair#3 is connected to pins 3 and 6 like this:
        Pin 3   wire color:  white/green
        Pin 6   wire color:        green
The remaining two twisted pairs are connected as such:
Pair#1
        Pin 4   wire color:              blue
        Pin 5   wire color:              white/blue
Pair#4
        Pin 7   wire color:              white/brown
        Pin 8   wire color:        brown
This is illustrated in the following diagram:

Now the wires forming the pairs must be gathered together and trimmed so that they can
be inserted into the RJ45 plug.  
The pairs gathered and trimmed is illustrated in the following diagram:

Then when the pairs are inserted into the RJ45 plug they should look like this:

Crossover Cables
In order to make what is commonly referred to as a "Crossover" cable one
must change the pinout connections on ONE end of the
cable.  If you do it on both ends of the cable you have crossed-over the
crossover and now have a straight-through cable, albeit a very non-standard
one.  In this case two negatives do make a positive.
You need to make a cable where pins 1 & 2 from one end are connected to pins
3 & 6 on the other end, and pins 3 & 6 from the first end are connected to
pins 1 & 2 on the other end.
Pins 4 & 5 and 7 & 8 are unchanged.
The two ends look like this:
     Standard End                          Crossover End
       Pin 1 White/Orange                   Pin 1 White/Green
       Pin 2 Orange                         Pin 2 Green
       Pin 3 White/Green                    Pin 3 White/Orange
       Pin 4 Blue                           Pin 4 Blue
       Pin 5 White/Blue                     Pin 5 White/Blue
       Pin 6 Green                          Pin 6 Orange
       Pin 7 White/Brown                    Pin 7 White/Brown

       Pin 8 Brown                          Pin 8 Brown
The following is the proper pin out and cable pair/color order for the
"crossover" end.
      Pair#2 is connected to pins 1 and 2 like this:
      Pin 1     wire color:        white/green
      Pin 2     wire color:              green
Pair#3 is connected to pins 3 and 6 like this:
        Pin 3   wire color:  white/orange
        Pin 6   wire color:        orange
The crossover pairs are illustrated in the following diagram:

Then when the pairs are inserted into the RJ45 plug they should look like this:

Note:  
Even though we are only interested in attaching the connectors to the cable
in this tutorial, we must take into account the wiring of the jacks as well
so that we connect the proper wires from the cable to the proper pins in the
connectors.  And that is determined by the wiring in the jack the connectors
will be plugged into.
Work in progress...
Copyright 1996, Robert Kerr
NetSpec, Inc.
All Rights Reserved.
               
               
               
               
               
               
               
               
               
               
               

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