Notes to Chapter 4

pages 93 - 95
telephone 25-pair cable

 Here's a good way to remember the order of the first group of colors:
 

Blue    the blue sky above
Orange   the orange sun below
Green the green grass below that
Brown   brown dirt below the grass
Slate   slate (gray) rocks below the earth 

The second group, which doesn't come with a clever way to remember the order,  is:
 

White
Red
Black
Yellow
Violet

If you know of a way to remember the order of this group, or come up with your own, let us all know via the discussion board.

Telephone cable typically comes in 25-pair (50-wire) bundles.  Once you know the order of the color identifying groups, you can quickly locate a pair within the overall set.  This system is used for data communication purposes other than POTS.  Look at the table below to follow the patterns in determining the pair numbers.  (Note:  some web sites and references number the wires, as opposed to the pairs, which can be confusing.)  The primary colors go in a horizontal sequence, and the secondary colors run in a vertical sequence.  The numbers are the pair numbers.  Look at the chart and see that pair number 1 consists of a blue wire with a white stripe and a white wire with a blue stripe.  Similarly, pair 19 is made up of a brown wire with a yellow stripe and a yellow wire with a brown stripe.  Refer to the External Links button to find a color version of the table.
 
 

 
Blue
Orange
Green
Brown
Slate
White
1
Blue/White
White/Blue
2
Orange/White
White/Orange
3
Green/White
White/Green
4
Brown/White
White/Brown
5
Slate/White
White/Slate
Red
6
Blue/Red
Red/Blue
7
Orange/Red
Red/Orange
8
Green/Red
Red/Green
9
Brown/Red
Red/Brown
10
Slate/Red
Red/Slate
Black
11
Blue/Black
Black/Blue
12
Orange/Black
Black/Orange
13
Green/Black
Black/Green
14
Brown/Black
Black/Brown
15
Slate/Black
Black/Slate
Yellow
16
Blue/Yellow
Yellow/Blue
17
Orange/Yellow
Yellow/Orange
18
Green/Yellow
Yellow/Green
19
Brown/Yellow
Yellow/Brown
20
Slate/Yellow
Yellow/Slate
Violet
21
Blue/Violet
Violet/Blue
22
Orange/Violet
Violet/Orange
23
Green/Violet
Violet/Green
24
Brown/Violet
Violet/Brown
25
Slate/Violet
Violet/Slate

Which one of the pair is the tip, and which is the ring?  Tip is the primary, and ring is the secondary.  Just like before, red = ring, and red is in the list of secondary colors.

When there are more than 25 wire pairs in a cable, sets of 25 pairs (50 wires) are isolated by colored "threads" that follow the color sequence of the first 25 pairs.  By looking at the binder thread you can tell that you are in the first set of 25 pairs (1-25), second set of 25 pairs (26-50), third set of pairs (51-75), etc.  This goes all the way up to 24 binders, or 600 pairs.
 

page 95
Why the twist?

When an electric current travels down a wire, it creates a magnetic field.  Similarly, a magnetic field around a wire creates an electric current.  This makes it possible for the current wire to create a magnetic field, which in turn creates an electric current in another wire. When the wires are closely-packed telephone wires, the secondary current is the low-level "cross-talk" where you can just barely hear someone else's conversation on your line.  Cross-talk on data lines can shift 1's to 0's, which can produce all sort of bad consequences like getting the wrong amount on your paycheck.

Twist counteracts the interference that produces cross-talk, since it makes the magnetic fields cancel each other out. The twists must be kept to a very specific number of twists per inch, and each wire pair in a bundle must have very specific twist numbers.  The spacing and quality control of the wires is what makes them able to handle higher and higher data rates without interference-caused cross-talk.  The different wire types such as Category 3, 5, 5e, and 6 are all inherently the same, but with better and better-controlled interference.

UTP and STP

The most common type of data cable in use today is "unshielded twisted pair" or UTP.  It consists of four twisted pairs of insulated wires inside of an exterior insulation.  Shielded twisted pair, or STP, is similar to UTP except that it has a wire mesh around the pairs to minimize interference.  STP is not commonly used in the USA because it is more difficult to install and more expensive, even though it has inherently less interference.

page 96
CATx cable

Table 4-1 doesn't show CAT 5e cable.  Refer to the TIA website for details.  It doesn't show CAT 1, 2, or 4 because they were standards that never went into use.  It is important to note that the bandwidth values listed are the ones specified by the standards.  Manufacturers, though, often exceed the standards, so it is possible to see a catalog listing of 250-MHz Cat 5 cable, even though this is not officially possible.  Manufacturers were selling CAT 6 cable well before the standard was approved, assuming that their product was going to exceed the eventual standard.

page 97
interconnecting systems

Table 4-2 is useful only in the event that you need to connect LAN wiring with telephone wiring.

pages 101-102
microwave

Microwave transmission antennas must be able to "see" each other because microwaves do not bend around the curve of the earth or bounce off parts of the atmosphere.

Why does rain affect microwave transmissions?  Ask any Physics 157 student; it's one of the topics covered in that class.   Microwave cooking relies on the fact that microwaves get absorbed by the water in food and heat it up.  The water vapor in clouds and the liquid water in rain do the same thing, and unfortunately absorb the data encoded on the microwave transmissions.

page 104
optical fiber

Optical fibers work by using the principle of refraction of light -- denser materials slow down the passage of light through them and cause the light to bend.  The classic example of refraction takes place when you look at a stick that is part in the air and part in water -- the stick appears to bend when it goes into the water.  Refraction is also responsible for mirages and the fact that moon looks bigger when it is low on the horizon that when it high in the sky.

If light enters an optical fiber at a shallow angle, an effect called "total internal reflection" keeps the light bouncing down the length of the fiber, rather than going out the edge.  You can see this effect if you sit at the bottom of a swimming pool. If you look straight up, you can see whatever is above you.   If you look at an angle close to the horizontal, though, you will not see anything that is out of the water, but will see the reflection of the bottom of the pool.

Optical fibers are made in different configurations. Some fibers have the same index of refraction everywhere in the fiber, and some vary from the center on outward (graded-index and step-index).  To get the best data transmission, optical fiber should have all the light that enters the fiber reach the other end at the same time.  "Multi-mode" fiber allows some light to take a longer path, and thus take longer to reach the other end, while the light in "single-mode" fiber all reaches the far end at the same time.  Multi-mode fiber is cheaper, but only good for short distances, such as between buildings on a college campus.  Single-mode fiber is used for longhair data transmission.