COMMUNICATION LINE PROBLEMS

Communication line problems fall into three general categories: excessive noise, cabling, and backbone connections. With proper testing and precautions, these problems can be taken care of before they happen.

EXCESSIVE NOISE

Noise is the term for random electrical signals that become part of a transmission, and that serve to make the signal (information) component of the transmission more difficult to identify. Noise can take various forms, including the following:

l Impulse noise: voltage increases that last for just a short period, usually for only a few milliseconds.

l White noise: random background noise.

l Crosstalk: interference on one wire from another.

There are limits set on the allowable levels for each of these types of noise. A noise filter can be used to remove random noise from a signal.

In a transmission, signal-to-noise ratio (SNR) is the ratio between the signal and noise levels at a given point, usually at the receiving end of the transmission. The SNR value is generally expressed in dB.

The SNR can be used to determine how long a cable segment can be before the signal loss is unacceptably high. The SNR also helps to determine whether a particular type of cable will work for the intended use. Cable testers can help determine whether a particular type of cable is appropriate in a specific environment.

In general, digital signals have a much higher SNR than analog signals. Because analog signals in a broadband network must be confined to a portion of the total bandwidth, filtering and other signal-cleaning measures are necessary This confinement makes the signal more delicate and subject to distortion.

Several types of filtering maybe used to help clean a broadband transmission. The filters are distinguished by the filtering technique they use as well as by where in the transmission process they are applied.

For example, filters applied early in the transmission, prior to modulation, are known as baseband or premodulation filters. Those applied after the modulation are known as passband or postmodulation filters.

CABLING

Cables are good media for signals, but they are not perfect. The signal at the end of the cable should be as loud and clear as at the beginning, but this will not be true.

Any transmission consists of signal and noise conponents. Even a digital signal degrades when transmitted over a wire. This is because the binary information must be converted to electrical form for transmission, and because the shape of the electrical signal changes over distance.

Signal quality degrades for several reasons, including attenuation, crosstalk, and impedance.

Attenuation

Attenuation is the decrease in signal strength, measured in decibels (dB) per 100 feet. Such loss happens as the signal travels over the wire. Attenuation occurs more quickly at higher frequencies and when the cable's resistance is higher.

In networking environments, repeaters are responsible for cleaning and boosting a signal before passing it on. Many devices are repeaters without explicitly saying so. For example, each node in a token-ring network acts as a repeater. Since attenuation is sensitive to frequency, some situations require the use of equalizers to boost different-frequency signals the appropriate amount.

Crosstalk

Crosstalk is interference in the form of a signal from a neighboring cable or circuit; for example, signals on different pairs of twisted wires in a twisted pair cable may interfere with each other. A commonly used measure of this interference in twisted-pair cable is near-end crosstalk (NEXT), which is represented in dB. The higher the dB value, the less crosstalk and the better is the cable.

Additional shielding between the carrier wire and the outside world is the most common way to decrease the effects of crosstalk.

Impedance

Impedance, which is a measure of electrical resistance, is not directly a factor in a cable's performance. However, impedance can become a factor if it has different levels at different locations in a network. In order to minimize the disruptive effects of different impedances in a network, special devices, called baluns, are used to equalize impedance at the connection.

Impedance does reflect performance indirectly. The higher the impedance, the higher is the resistance; the higher the resistance, the greater is the attenuation at higher frequencies.

Line Conditioning

Line conditioning tries to eliminate the effects of certain types of distortions on the signal. It becomes 3-6 more necessary as transmission speeds increase. Two types of line conditioning are available:

C conditioning tries to minimize the effects of distortion related to signal amplitude and distortion due to envelope delay.

D conditioning tries to minimize the effects of harmonic distortion in addition to the amplitude and envelope delay distortions handled by type C conditioning.

A line driver is a component that includes a transmitter and a receiver; it is used to extend the transmission range between devices that are connected directly to each other. In some cases a line driver can be used in place of a modem, for short distances of 10 miles or less.

To test a particular section of cable, you can use a line-testing tool. A line monitor is a low-end line-testing tool that tells you if the line is intact. A high-end line-testing tool can do very precise measurements using time domain reflectometry (TDR). A TDR is a device used to test the integrity of a section of cable before the cable is even unwound. This diagnostic method uses a signal of a known amplitude and duration, which is sent along a stretch of cable. Depending on the amount of time the signal takes to return and on the cable's nominal velocity of propagation, the TDR can determine the distance the signal traveled and whether there are any shorts or opens in the cable.