Using a Single Horizontal Cable Run for Two 10Base-T Connections

Let's face it, you will sometimes fail to run enough cable to a certain room. You will need an extra workstation in an area, and you won't have enough connections. Knowing that you have a perfectly good four-pair UTP cable in the wall, and that only two of those pairs are in use, makes your mood even worse. Modular Y-adapters can come to your rescue.

Several companies make Y-adapters that function as splitters. They take the four pairs of wire that are wired to the jack and split them off into two separate connections. The Siemon Company makes a variety of modular Y-adapters (see Figure 1) for splitting 10Base-T, Token Ring, and voice applications. This splitter will split the four-pair cable so that it will support two separate applications, provided that each application requires only two of the pairs. You must specify the type of splitter you need (voice, 10Base-T, Token Ring, etc.). Don't forget, for each horizontal cable run you will be splitting, you will need two of these adapters: one for the patch-panel side and one for the wall plate.

Figure 1: A modular Y-adapter for splitting a single four-pair cable into a cable that will support two separate applications

Warning

Many cabling professionals are reluctant to use Y-adapters because the high-speed applications such as 10Base-T Ethernet and Token Ring may interfere with one another if they are operating inside the same sheath. Certainly you should not use Y-adapters for applications such as 100Base-TX. Furthermore, Y-adapters eliminate any chance of migrating to a faster LAN system that may utilize all four pairs.

ANSI/TIA -568-C Wiring Schemes T568A and T568B

ANSI/TIA-568-C does not sanction the use of the USOC scheme. Instead, two wiring schemes are specified, both of which are suitable for either voice or high-speed LAN operation. These are designated as T568A and T568B wiring schemes.

Both T568A and T568B are universal in that all LAN systems and most voice systems can utilize either wiring sequence without system errors. After all, the electrical signal really doesn't care if it is running on pair 2 or pair 3, as long as a wire is connected to the pin it needs to use. The TIA/EIA standard specifies eight-position, eight-contact jacks and plugs and four-pair cables, fully terminated, to facilitate this universality.

The T568B wiring configuration was at one time the most commonly used scheme, especially for commercial installations; it is shown in Figure 1. The TIA/EIA adopted the T568B wiring scheme from the AT&T 258A wiring scheme.

 
Figure 1: The T568B wiring pattern

The T568A scheme (shown in Figure 10.9) is well suited to upgrades and new installations in residences because the wire-termination pattern for pairs 1 and 2 is the same as for USOC. Unless a waiver is granted, the U.S. government requires all government cabling installations to use the T568A wiring pattern. The current recommendation according to the standard is for all new installations to be wired with the T568A scheme.

 
Figure 2: The T568A wiring pattern

The wire colors and the associated pin assignments for the T568B wiring scheme look like this:

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

The pin assignments for the T568A wiring schemes are identical to the assignments for the T568B pattern except that wire pairs 2 and 3 are reversed. The T568A pattern looks like this:

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

Note that when you buy eight-position modular jacks, you may need to specify whether you want a T568A or T568B scheme because the jacks often have IDC connections on the back where you punch the pairs down in sequence from 1 to 4. The jacks have an internal PC board that takes care of all the pair splitting and proper alignment of the cable conductors with the pins in the jack. Most manufacturers now provide color-coded panels on the jacks that let you punch down either pinout scheme, eliminating the need for you to specify (and for them to stock) different jacks depending on which pinout you use.

Tip

Whichever scheme you use, T568A or T568B, you must also use that same scheme for your patch panels and follow it in any cross-connect blocks you install. Consistency is the key to a successful installation.

Be aware that modular jacks pretty much look alike even though their performance may differ dramatically. Be sure you also specify the performance level (e.g., Category 3, Category 5e, Category 6, Category 6A, etc.) when you purchase your jacks.

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Tips for Terminating UTP Connectors

Keep the following points in mind when terminating UTP connectors:

• When connecting to jacks and plugs, do not untwist UTP more than 0.5″ for Category 5e and not more than 0.375″ for Category 6.
• Always use connectors, wall plates, and patch panels that are compatible (same rating or higher) with the grade of cable used.
• To "future-proof" your installation, terminate all four pairs, even if the application requires only two of the pairs.
• Remember that the T568A wiring scheme is compatible with USOC wiring schemes that use pairs 1 and 2.
• When terminating ScTP cables, always terminate the drain wire on both ends of the connection.

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When working with ScTP wiring, the drain wire makes contact with the cable shield along its entire length; this provides a ground path for EMI energy that is collected by the foil shield. When terminating ScTP, the drain wire within the cable is connected to a metal shield on the jack. This must be done at both ends of the cable. If left floating or if connected only on one end, instead of providing a barrier to EMI the cable shield becomes a very effective antenna for both emitting and receiving stray signals.

In a cable installation that utilizes ScTP, the plugs, patch cords, and patch panels must be shielded as well.

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.