The basics of data copper communications

As we've already seen, different types of cable can carry signals at different speeds. Bandwidth, pulling characteristics, connection requirements, UL ratings, insulation ratings, and shielding are also critical factors you must carefully consider and coordinate in planning any installation.During the actual installation, you'll rough-in the various cables in much the same manner as common electrical

As we've already seen, different types of cable can carry signals at different speeds. Bandwidth, pulling characteristics, connection requirements, UL ratings, insulation ratings, and shielding are also critical factors you must carefully consider and coordinate in planning any installation.

During the actual installation, you'll rough-in the various cables in much the same manner as common electrical cables. But, you must follow the manufacturer's instructions precisely. The use of the proper connectors and fittings is also important since the placement of the appropriate fittings is crucial to final equipment connections. When pulling main runs of cables, you have to meet additional conditions. In general, you accomplish the installation of these conductors by the same methods as with standard electrical wires, but with extra care. Because datacom cable has far less copper, it has reduced tensile strength. Sidewall pressure also carries the risk of damaging the insulation of individual conductors. Both of these characteristics put the performance of the product at risk.

Here are some general rules you should follow:

Do not exceed a pulling tension of 20% of the ultimate breaking strength of the cable (these figures are available from the cable maker).

Lubricate the raceway generously with a suitable pulling compound. (Check with the manufacturer for types of lubricants best suited to the type of cable).

Use pulling eyes with swivels for longer pulls.

For long underground runs, pull the cable both ways from a centrally located manhole to avoid splicing. Use pulling eyes on each end.

Do not bend, install, or rack any cable in an arc of less than 12 times the cable diameter.

Components Using Cat 5, 6, or 7 cable alone doesn't guarantee a high performance network. Once you install the cable, you should terminate it with appropriate connectors and use factory recommended procedures. Then, use only properly rated (Cat 5, 6, or 7) punchdown blocks, patch panels, and patch cords to complete the cable plant. Unless you use all the appropriate components, the system cannot properly be called Category 5, 6, or 7.

Roughing and trimming Like power wiring, the installation of data cabling consists of two primary phases: Roughing in the wiring, then trimming it later.

During the rough-in phase, it's important that all of cables are put in their proper places and installed carefully (not bent too tightly, pulled too hard, skinned, or otherwise damaged). At this time, it's also important you consider the routing of the cables, especially if they're unshielded. You should never place unshielded copper cables too closely to sources of electromagnetism, such as motor windings, transformers or ballasts.

You also should consider fire stopping. Note where the fire barriers in the structure are, and make sure you make proper allowances for crossing them.

There's another factor that's different for roughing in data cable: You must protect them during the construction process (while you're not there). This is critical in situations where there'll be a long time between the initial cable installation and the jack installations. Here, it's up to you to protect your cables using any way that will work. If you don't protect them, they may be pulled and twisted by accident. This will damage the cables, and this damage will not show until you get a tester on them. In other situations, such as when you have a complete raceway system to use, there may be very little time between the cable installation and the wiring of the jacks.

Also like power wiring, it's important you leave enough extra cable at each outlet point. The recommended lengths are a minimum of 3 meters in telecommunications closets for both twisted-pair and fiber cable, 1 meter for fiber, and 30 cm for twisted-pair cable at the outlet. (Note: When you move from power wiring to data cabling, the units of measurement switch from English to metric.) Also, remember to check your specifications for requirements on extra cable.

Trimming data cabling is pretty much the same as trimming power wiring (strip the cables, install the devices and plates, etc.), except you need a lot more testing. When trimming power wiring, we generally test by flipping a switch or hitting the outlet with a voltage tester. Either power is present, or it's not. Testing data cabling is not so simple. Remember, you need to test not only for the presence of the signal, but also for the quality of the signal.

Remember: you'll be spending serious time testing your cables, and documenting those test results, so get used to the idea.

Wiring layouts In last month's article, we explained topology-the patterns in which data networks connect computers. Topology decides how you'll run your cables. In most cases, this will be a star pattern, meaning that every data outlet gets its own home run, as shown in Fig. 1 and 2. (This is the routing for the EIA/TIA 568 standard, which almost all new computer networks follow.) Under this system, a Cat 5 (or now, level 6 or 7) cable runs from the outlet to a communications closet. Usually, the communications closet is a telephone closet with a little extra equipment added to it. This can make for space problems, especially in pre-existing buildings. During your estimating and planning, bear in mind that the closets may be over-crowded. (Do something about this beforehand if at all possible.)

At the wiring closet, your cable home runs will connect to a punch-down block. (The standard method of connecting communications conductors is at a multi-terminal assembly of self-stripping, crimp connections. This component is called a punch-down block, 66 block or 110 block. "Punch-down block" is the generic name; 66 blocks are designed especially for voice conductors, and 110 blocks are designed especially for data conductors.)

You use patch panels and punch-down blocks for testing, additions to the system, and modifications to the cable plant (cabling system). From the punch-down block, you run short patch cables to a patch panel, and from there to a hub. A hub is an electronic device that takes the signals from each of the cables and puts them into a backbone cable. The backbone cable runs between floors of the building and connects several hubs together. You may also hear hubs referred to as concentrators.

The outlet itself will almost always be one or two RJ-45 jacks, as shown in Fig. 3, mounted on a single-gang plate. (A RJ-45 is an 8-pin modular phone plug that's nearly universally used for data networks.)

Cable colors While the color coding of data cabling is not as well known (or indeed, followed) the color code for power conductors, does exist and should be followed. (In power wiring, things explode if you don't use the color code. In data cabling, they simply don't work.)

The cabling administration standard (EIA-606) lists the colors and functions of data cabling as:

Blue: horizontal voice cables

Brown: inter-building backbone

Gray: second-level backbone

Green: network connections & auxiliary circuits

Orange: demarcation point, telephone cable from Central Office

Purple: first-level backbone

Red: key-type telephone systems

Silver or White: horizontal data cables, computer & PBX equipment

Yellow: auxiliary, maintenance & security alarms

Separation from sources of interference Remember that you should not install unshielded data cables near sources of electromagnetism. EIA/TIA-569, the cabling pathways standard, specifies these distances for structured data cabling systems:

Minimum bending radii According to a draft version of EIA-568, the minimum bend radius for UTP is four times the outside cable diameter, or about 1 inch. For multi-pair cables, the minimum bending radius is 10 times the outside diameter. The minimum bend radius for Type 1A STP cable (100 Mb/s STP) is 7.5 cm (3 inches) for non-plenum cable and 15 cm (6 inches) for the stiffer plenum-rated kind.For optical cables not under tension, the minimum bend radius is 10 times the cable diameter. For cables under tension, no less than 20 times the cable diameter. The standard also states no optical cable should be bent on a radius less than 3.0 cm (1.18 inch).

A different standard, ISO DIS 11801 (essentially a parallel standard to the one mentioned above), for 100 ohm and 120 ohm balanced cable lists three different minimum bend radii:

Minimum for pulling during installation is eight times cable diameter.

Minimum installed radius is six times for riser cable.

Minimum installed radius is four times cable diameter for horizontal runs.

For fiber optic cables, the requirements are the same as those stated above. Some manufacturers recommendations differ from the above, so it's worth checking the spec sheet for the cable you plan to use.

Testing It's important for the impedance of a cable to be uniform throughout the cable's length. Cable faults (problem areas in the cable, like ground faults) change the impedance of the cable at the point where the fault lies, resulting in reflected signals. Cable testers use this method to find cable faults. For example, a break in a wire creates an "open circuit," or infinitely high impedance at that point. When a high frequency signal emitted from a cable tester encounters this high impedance, it will reflect back towards the tester like an ocean wave bouncing off a seawall. Similarly, a short circuit represents zero impedance, which will also reflect a high frequency.

A cable testing device called a time domain reflectometer (TDR) is capable of calculating about how far down the cable the fault lies. The formula for this feature uses a cable value known as nominal velocity of propagation (NVP), which is the rate at which a current can flow through the cable, expressed as a percentage of light speed (0.8 C, for example, being 8/10ths of the speed of light, or 240,000 km per second). The cable tester multiplies the speed of light by the cable's NVP and by the total time it takes the pulse to reach the fault and reflect back to the tester. It then divides this number by two, for the one-way distance.

The same concept is used to check the electrical length of a cable installation. In this case, you must make sure not to terminate one end of the cable. The open end will register as infinite impedance and reflect a pulseback to the tester. Again, this response time is plugged into the formula to estimate the overall electrical length of the wire.

Such cable testers cannot check the first 20 ft or so of a cable. The reason for this blind spot is that a pulse transmitted by the tester will be reflected back to the device before it is entirely transmitted. Thus, the tester can't get an accurate reading.

Interference Noise can jeopardize the transmission of a signal because it can introduce false signals or noise spikes at different frequencies on a wire. A load may interpret a noise spike as part of a digital signal, distorting the original content of the signal. Common sources of noise spikes include ac lines, telephones, and devices such as radios, microwave ovens, and motors. Some cable testers test for noise, running tests at different frequencies. (See table for minimum separation distances from power sources.)

Another type of interference is called crosstalk, or more specifically, near-end crosstalk (NEXT). As mentioned earlier, when a current moves through a wire, it creates an electromagnetic field. This field can interfere with signals traveling on an adjacent wire. To reduce the effect of NEXT, wires are twisted (thus the name twisted pair). The twisting allows the wires to cancel each other's noise. The risk of NEXT is highest at the ends of a cable because wire pairs generally don't have twists at their ends, where they enter connectors. If the untwisted end length is too long, NEXT can distort data signals. Also, due to attenuation, signals are strongest when they are transmitted, and weakest when they arrive at their destination. So, the magnetic field of a signal being transmitted from a device on one wire may overwhelm a signal arriving at the same device on the wire's pair.

NEXT is measured in decibels, which represent a ratio of a signal's strength to the noise generated by crosstalk. (See sidebar "Understanding the Decibel"). The stronger the signal and weaker the noise, the higher the NEXT value. For this reason, a high NEXT reading is good. Low NEXT readings, which indicate high crosstalk interference, can mean the cable is terminated improperly.

When to test and what to use You should test network cables both during the installation process and upon completion of the system. Testing during the installation process helps catch problems while they're still simple to fix. Testing the system upon completion is not only a good practice but is even required by law for communications systems.

The most common testing tools for copper data cabling are the following:

Digital voltmeter (DVM): Measures volts.

Digital multimeter (DMM): Measures volts, ohms, capacitance, and (some) frequency.

Time domain reflectometer (TDR): Measures cable lengths and locates impedance mismatches.

Tone generator and inductive amplifier: Used to trace cable pairs, follow cables hidden in walls or ceiling. The tone generator typically will put a 2 kHz audio tone on the cable under test. The inductive amp detects and plays this through a built-in speaker.

Wiremap tester: Checks a cable for open or short circuits, reversed pairs, crossed pairs and split pairs.

Noise testers, 10Base-T: The standard sets limits for how often noise events can occur, and their size, in several frequency ranges. Various handheld cable testers are able to perform these tests. Butt set: A telephone handset that when placed in series with a battery (such as the one in a tone generator) allows voice communication over a copper cable pair. Can be used for temporary phone service in a wiring closet.

Testing UTP cables. Many of the problems encountered in UTP cable plants are a result of miswired patch cables, jacks, and cross-connects. Horizontal and riser distribution cables and patch cables are wired straight through end-to-end, so that Pin 1 at one end is connected to Pin 1 at the other. (Crossover patch cables are an exception to this rule). Normally, jacks and cross-connects are designed so that the installer always punches down the cable pairs in a standard order, from left to right: Pair 1 (Blue), Pair 2 (Orange), Pair 3 (Green) and Pair 4 (Brown). The white striped lead is usually punched down first, followed by the solid color. The jack's internal wiring connects each pair to the correct pins, according to the assignment scheme for which the jack is designed: EIA568A and 568B (Fig. 4), or USOC.

One common source of problems is an installation in which USOC jacks are mixed with EIA-568A or 568B. When this is done, everything appears to be punched down correctly, but some cables will work and others will not.

Wiremapping. Wiremap tests will check all lines in the cable for all of the following errors:

Open: Lack of continuity between pins at both ends of the cable.

Short: Two or more lines short-cir-cuited together.

Crossed pair: A pair is connected to different pins at each end (example: Pair 1 is connected to Pins 4 and 5 at one end and Pins 1 and 2 at the other).

Reversed pair: The two lines in a pair are connected to opposite pins at each end of the cable. For example: the line on Pin 1 is connected to Pin 2 at the other end; the line on Pin 2 is connected to Line 1. This is also called a polarity reversal or tip-and-ring reversal.

Split pair: One line from each of two pairs is connected as if it were a pair. For example, the Blue and White-Orange lines are connected to Pins 4 and 5, White-Blue and Orange to Pins 3 and 6. The result is excessive Near End Crosstalk (NEXT), which wastes 10Base-T bandwidth and usually prevents 16 Mb/s token-ring from working at all.

Length tests. You usually check cable length with a TDR, which transmits a pulse down the cable and measures the elapsed time until it receives a reflection of the signal from the far end of the cable. Each type of cable transmits signals at something less than the speed of light. This factor is called the nominal velocity of propagation (NVP), expressed as a decimal fraction of the speed of light. (UTP has an NVP of approximately 0.59 to 0.65). From the elapsed time and the NVP, the TDR calculates the cable's length. A TDR may be a special-purpose, or may be built into a handheld cable tester.

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