In discussing network cables and troubleshooting wiring problems, there are several important terms and concepts to understand. This section contains definitions of some of the terms used earlier in the chapter. This section should be
as an expanded glossary of terms associated with the Physical layer of the OSI model (discussed in Appendix A, "Overview of the OSI Seven-Layer Networking Reference Model"). In other chapters you will find references to these terms. Although it is not required reading for the casual reader, the following can serve as an invaluable reference when making purchasing decisions, as well as in the design phase of a new network or when upgrading an existing one.
Attenuation to Cross-Talk Ratio (ACR)
ACR is a critical factor in determining the capability of an
twisted-pair cable or
twisted-pair cable. Attenuation to cross-talk ratio (ACR) is the value of the attenuation less the cross-talk value, both
in decibels (db) at a particular frequency. This is a quality factor for cabling. Before you can understand this ratio, you need to understand what the
Attenuation is the decrease in magnitude of the signal as it travels through any transmitting medium, such as wire or glass. Attenuation is measured as a logarithm of the ratio between the input and the output power or between the input and the output voltage of the system. It's expressed in db. All good things must come to an end, and this is the case with electricity as well as light. As the signal
wire (or the fiber-
cable), some of the signal is lost. This is why it is necessary in a network topology to impose specific limits on the lengths of cable you can use. After you get past certain limits imposed by a particular topology, the signal becomes so degraded that the data transmitted cannot be reliably recovered at the destination.
Figure 6.3 shows that attenuation occurs as the signal travels down the wire. The amplitude of the electrical signal decreases the farther it travels from the transmitting side of the communications channel.
Figure 6.3. An electrical signal degrades as it travels through a copper cable (attenuation).
is the range of frequencies required for proper transmission of a signal. This is expressed in hertz (Hz) as a difference of frequencies. For example, the bandwidth used on a copper wire for voice communications (via the PSTN) is 4MHz. Because copper cables are capable of carrying frequencies well above this 4MHz limit, DSL transfers are possible via use of frequencies above the 4MHz used by voice communications.
Characteristic impedance is the value of impedance (a combination of resistance and reactance) of a transmission line measured over a specific frequency range. Impedance is expressed in units of Z, because it is a calculation based on both resistance and reactance of the network media. Whereas resistance is the capability of a medium to resist the transmission of electrons, reactance is another thing altogether. Reactance, for alternating current (AC), is the medium's tendency to store and then release the current as it flows through the medium.
Cables are made up of two or more copper wires that are bundled together with an outer cover so that it's easier to route them through the
that form the
your physical network takes. The coupling of signals from one pair of wires in a cable to another pair of wires in the same cable actually can cause the signals to interfere with each other. The electrical signal in a copper wire not only travels down that particular wire, but also
perpendicularly and can interfere with other copper wires in the same cable or bundle. This is called cross-talk. This coupling also can occur between wires of different cables that are close to one another. In Figure 6.4, you can see that some of the signal has radiated from one wire and produced noise on another.
Figure 6.4. A portion of an electrical signal radiates from an adjacent wire, producing interference (cross-talk).
To keep the individual copper wires separated from each other within a cable, an
is used to help prevent interference between two conductors. It can be a simple plastic nonconducting material, or a more complex formulation used in some high-capacity wire bundles.
As electrons move through a medium, two fields are associated with this movement: electric fields and magnetic fields. These fields exist at varying distances from the conductors (the wires) as they are brought closer together.
Electromagnetic interference (EMI) refers to the interference that electromagnetic signals produce by frequent changes of electrons moving through certain media. Network wiring and equipment can be very susceptible to EMI, and they also emit EMI.
Far-End Cross-Talk (FEXT)
Far-end cross-talk occurs between two twisted pairs of the cable at the far end (destination) of the cable from the measuring source. The transmitting end of a cable pair produces the stronger electrical signal (because the signal attenuates, or becomes weaker, as it
through the copper wire or fiber-optic cable), so FEXT can be a more difficult problem to tackle. However, you should be cognizant that connectors are properly created at the far end of a connection to prevent interference between copper wires at that point. The signal might be weaker at the end point of a connection, yet it still exists. This is why the specifications allow only a
very small amount of exposed copper wire
when connecting a cable to an actual connector (such as an RJ-45 jack).
is a measurement of the number of times a periodic action occurs in a measure of time. In terms of alternating current, this is the number of cycles per second and is usually expressed in hertz.
and Half-Duplex Communications
means that communications between two network nodes can occur in both directions simultaneously. Obviously, this is a communications method in which both transmitted and received signals are not simultaneously present. They alternate in time on the transmission medium. Another method for creating a full-duplex connection is to use separate wires for transmission and
. Using this method, both ends of a communications link can send or receive data
. Using half-duplex communications, only one side of the communications line can transmit at any point in time.
is the total resistance and reactance
by a circuit component. The units are expressed in ohms. The common symbol for impedance is the Greek letter zeta, or Z. This is a complex numerical value, mathematically expressed as either a complex number or the polar coordinate number.
is a condition in which the impedance of a device or wiring system is matched to another wiring system or device.
is the undesirable passage of current through an insulator or over the surface of a conductor. This can occur in older cable bundles in which the insulating material has become degraded over time and signals from one wire in the bundle interfere with signals in other wires in the same bundle. This could happen, for instance, if a small animal were to attempt to chew through a cable. Feel sorry for the rat, but feel sorrier for yourself when you have to replace the cable!
Near-End Cross-Talk (NEXT)
is cross-talk that occurs between two twisted pairs measured at the same location, and it usually occurs between wires in a twisted-pair cable. One of the conditions that can introduce this interference is a crushed cable, so care must be used when pulling network cabling and attaching connectors.
Nominal Velocity of Propagation
Nominal velocity of propagation
is the speed at which a signal travels through a medium expressed as a decimal fraction of the speed of light in a
is a measured parameter that includes the sum of the contributions of power from all pairs of a cable system, excluding the pair under test. This is done with all the other pairs of the cable having signals present.
are frequencies in the electromagnetic spectrum that are used for radio communications. These
occur above 300KHz.
Radio Frequency Interference (RFI)
Radio frequency interference
is electromagnetic interference at radio frequencies.
is a metallic foil or wire screen-mesh that encircles a cable or wires in a cable to prevent electromagnetic or radio frequency fields from entering or leaving a cable. This
interference with other cables in the same cable bundle or cables that are in close proximity to the shielded cable. You've probably
used in connection with network cables. This is an abbreviation for shielded twisted-pair cabling, typically found in Token-Ring networks. Although shielding cables were considered to be important in earlier network
, unshielded twisted-pair cables (in which the twisting of wires helps to reduce interference) are the norm today for cables connecting the work area to the telecommunications closet.
Time Domain Reflectometry (TDR)
Time domain reflectometry
is a method of measuring cable length or faults by timing the period between a test pulse and its reflection from an impedance discontinuity on the cable. A TDR measuring instrument can enable you to determine the approximate location of a problem on a cable. It can also be used to determine any defects in a spool of cable before you deploy it in your network. You can learn more about TDR in Chapter 53, "Network Testing and Analysis Tools."