USOC Wiring Scheme

The Bell Telephone Universal Service Order Code (USOC) wiring scheme is simple and easy to terminate in up to an eight-position connector; this wiring scheme is shown in Figure 1. The first pair is always terminated on the center two positions. Pair 2 is split and terminated on each side of pair 1. Pair 3 is split and terminated on each side of pair 2. Pair 4 continues the pattern; it is split and terminated on either side of pair 3. This pattern is always the same regardless of the number of contacts you populate. You start in the center and work your way to the outside, stopping when you reach the maximum number of contacts in the connector.

 
Figure 1: The Universal Service Order Code (USOC) wiring scheme

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Tip and Ring Colors

When looking at wiring schemes for modular plugs and jacks, you may see the letters T and R used, as in Figure 1. The T identifies the tip color, and the R identifies the ring color. In a four-pair cable, the cable pairs are coded in a standard color coding, which is on the insulation of the individual wires. In a four-pair cable, the tip is the wire that is predominantly white, and the ring identifies the wire that is a predominantly solid color.

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The wire colors and associated pin assignments for USOC look like this:

Pin	Wire Color
1	White/brown
2	White/green
3	White/orange
4	Blue
5	White/blue
6	Orange
7	Green
8	Brown

Warning

Do not use the USOC wiring scheme for systems that will support data transmission.

USOC is used for analog and digital voice systems but should never be used for data installations. Splitting the pairs can cause a number of transmission problems when used at frequencies greater than those employed by voice systems. These problems include excessive crosstalk, impedance mismatches, and unacceptable signal-delay differential.

Solid- vs. Stranded-Conductor Cables

UTP and ScTP cables have either solid copper conductors or conductors made of several tiny strands of copper. Solid conductors are very stable geometrically and, therefore, electrically superior, but they will break if flexed very often. Stranded conductors are very flexible and resistant to bend-fatigue breaks, but their cross-sectional geometry changes as they are moved, and this can contribute to electrical anomalies. Stranded cables also have a higher attenuation (signal loss) than solid-conductor cables.

Note

Solid-conductor cables are usually used in backbone and horizontal cabling where, once installed, there won't be much movement. Stranded-conductor cables are used in patch cords, where their flexibility is desirable and their typically short lengths mitigate transmission problems.

The differences in conductors mean a difference in IDC types. You have to be careful when you purchase plugs, wall plates, and patch panels because they won't work interchangeably with solid- and stranded-core cables—the blade designs are different.

Warning

Using the wrong type of cable/connector combination can be a major source of intermittent connection errors after your system is running.

With a solid-conductor IDC, you are usually forcing the conductor between two blades that form a V-shaped notch. The blades slice through the plastic and into the copper conductor, gripping it and holding it in place. This makes a very reliable electrical contact. If you force a stranded conductor into this same opening, contact may still be made. But, because one of the features of a stranded design is that the individual copper filaments can move (this provides the flexibility), they will sort of mush into an elongated shape in the V. Electrical contact may still be made, but the grip on the conductor is not secure and often becomes loose over time.

The blade design of IDC connectors intended for stranded-core conductors is such that forcing a solid-core conductor onto the IDC connector can break the conductor or fail to make contact entirely. Broken conductors can be especially problematic because the two halves of the break can be close enough together that contact is made when the temperature is warm, but the conductor may contract enough to cause an open condition when cold.

Some manufacturers of plugs advertise that their IDC connectors are universal and may be used with either solid or stranded conductors. Try them if you like, but if you have problems, switch to a plug specifically for the type of cable you are using.

Jacks and termination blocks are almost exclusively solid-conductor devices. You should never punch down on a 66, 110, or modular jack with stranded conductors.

Advantages of Biscuit Jacks

Biscuit jacks offer a few advantages in your structured-cabling design. First of all, they are very inexpensive compared to other types of surface-mount wiring systems, which is why many houses that had the old four-pin telephone systems now have biscuit jacks—you could buy 20 of them for around $25. Even the biscuits that support multiple jacks are still fairly inexpensive.

Another advantage of biscuit jacks is their ability to work in situations where standard modular or fixed-design wall plates won't work and other types of surface-mount wiring are too bulky. The best example of this is office cubicles (i.e., modular furniture). A biscuit jack has an adhesive tab on the back that allows it to be mounted anywhere, so you can run a telephone or data cable to a biscuit jack and mount it under the desk where it will be out of the way.

Finally, biscuit jacks are easy to install. The cover is removed with one screw. Inside many of the biscuit jacks are screw terminals (one per pin in each jack), as shown in Figure 1. To install the jack, you just strip the insulation from each conductor and wrap it clockwise around the terminal and between the washers and tighten the screw. Repeat this process for each conductor in the cable. These jacks are not high-speed data compatible and are capable of Category 3 performance at best.

 
Figure 1: Screw terminals inside a biscuit jack

Note

Not all biscuit jacks use screw terminals. The more modern data communications jacks use IDC connectors to attach the wire to the jack.

Disadvantages of Biscuit Jacks

The main disadvantage to biscuit jacks is that the older biscuit jacks are not rated for high-speed data communications. Notice the bunch of screw terminals in the biscuit jack shown in Figure 1. When a conductor is wrapped around these terminals, it is exposed to stray electromagnetic interference (EMI) and other interference, which reduces the effective ability of this type of jack to carry data. At most, the older biscuit jacks with the screw terminals can be rated as Category 3 and are not suitable for the 100Mbps and faster communications today's wiring systems must be able to carry.

Types of Biscuit Jacks

No discussion of wall plates would be complete without a discussion of biscuit jacks, or surface-mount jacks that look like small biscuits (see Figure 1). They were originally used in residential and light commercial installations for telephone applications. In fact, you may have some in your home if it was built before 1975. Biscuit jacks are still used when adding phone lines in residences, especially when people can't put a hole in the wall where they want the phone jack to go.

 
Figure 1: An example of a biscuit jack

Types of Biscuit Jacks

The many different types of biscuit jacks differ primarily by size and number of jacks they can support. The smaller type measures 2.25 inches wide by 2.5″ high and is mainly used for residential-telephone applications. The smaller size can generally support up to a maximum of two jacks.

The larger-sized biscuit jacks are sometimes referred to simply as surface-mount boxes because they don't have the shape of the smaller biscuit jacks. These surface-mount boxes are primarily used for data communications applications and come in a variety of sizes. They also can have any number or type of jacks and are generally modular. Figure 2 shows an example of a larger biscuit jack that is commonly used in surface-mount applications.

 
Figure 2: Example of a larger biscuit jack

Note

Generally speaking, the smaller biscuit jacks are not rated for Category 5e (or any higher categories). They must be specifically designed for a Category 5e application. Some companies offer a modular-design biscuit jack that lets you snap in high-performance, RJ-45-type jacks for Category 5e and better compliance.

Labeling Modular Wall Plates

Just like fixed-design wall plates, modular wall plates use labels to differentiate the different jacks by their purpose. In fact, modular wall plates have the widest variety of labels—every modular wall plate manufacturer seems to pride itself on its varied colors and styles of labeling. However, as with fixed-design plates, the labels are either text (e.g., LAN,Phone) or pictures of their intended use, perhaps permanently molded in the plate or on the jack.

Cabling @Work: We Don't Know What Tomorrow Will Bring, but Adding Some Flexibility Today May Make You More Prepared

A few years ago during a hotel's remodeling project, the hotel wired its rooms with only the minimum network connection points using fixed wall plates for its new local area network. Later that year the hotel had to rewire each room to accommodate an IP phone system upgrade, and to offer in-room fax machines and additional telephones on additional lines. Each room had to have additional cabling installed as well as new wall plates.

Though no precise figures have been calculated to see exactly how much would have been saved by installing modular wall plates initially instead of fixed wall plates, it was determined that the fixed wall plate design would not enable the hotel to keep up with the ever-growing number of additional network points and media types that its guests require.