Data Communications: Use the Right Medium for your Message

This chapter is from the book

Understanding Data Communications, 7th Edition

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This chapter is from the book

This chapter is from the book 

Understanding Data Communications, 7th Edition

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Bounded Medium

The physical channels (the media) that carry data are of two types: bounded and unbounded. In a bounded medium, the signals are confined to the medium and do not leave it (except for smaller leakage amounts). A pair of wires, coaxial cable, waveguide, and optical-fiber cable are examples of bounded media.

Wire Pairs

The simplest type of a bounded medium is a pair of wires providing go and return paths for electrical signals. Early telegraph systems used the Earth itself rather than a wire for one of the paths, as shown in Figure 3.3a. Repeaters were inserted along the line to reduce the effects of noise and attenuation (loss of signal strength). This scheme did not work well, however, because the Earth is not always a good conductor, and the path was susceptible to large noise currents induced by lightning. Losses were reduced by using two wires, as shown in Figure 3.3b, but because the line was still unbalanced to ground, it was subject to noise from almost every noise-producing device. Finally, the balanced two-wire line, shown in Figure 3.3c, was used to greatly reduce noise pickup.

The most common type of bounded medium consists of wire pairs twisted together and made into cables of from 4 to 3,000 pairs. Because a wire acts as an antenna, several techniques are used to reduce electromagnetic interference (EMI). Most wires are shielded, and some wires are also twisted at 90º angles every so often. The twists additionally suppress EMI. The size of the wire used varies from 16 AWG (American Wire Gauge) with a wire diameter of 0.05082 inch to 26 AWG with a diameter of 0.01594 inch. AWG wire sizes are inversely proportional to the diameter of the wire. That is, the lower the AWG is, the thicker the wire is, whereas a higher AWG indicates a thinner wire. In modern cables, each wire is insulated with a polyethylene or polyvinyl chloride (PVC) jacket; however, a large quantity of older cable is still in use in which the insulation for each wire is paper.

Table 3.1 provides a representative comparison of the characteristics of AWG cables from gauge number 10 through 26 in even increments. Note that, as you might expect, the resistance in ohms per kilometer increases. This is because the cable diameter is inversely proportional to the gauge number. Hence, the wire becomes smaller, which makes it more difficult for electrons to flow, and it can be viewed similar to the effect of different diameters of hoses on the flow of water.

Open-wire lines have a low attenuation of voice frequencies due to the large size of the wire and the relatively large distance between the two wires when mounted on the crossarm of a utility pole. A typical value of attenuation for 104-mil (0.104 inch) diameter open wire lines is 0.07 decibel (dB) per mile, whereas 19-gauge (0.03589-inch diameter) twisted-wire pairs in a multipair cable have a voice frequency attenuation of about 1dB per mile.

Figure 3.3 Types of transmission circuits.

Table 3.1 Representative American Wire Gauge (AWG) Characteristics

The attenuation of twisted-wire pairs rises rapidly with increasing frequency, and the amount of crosstalk between adjacent pairs also increases with frequency. The maximum usable frequency for wire pairs in cables is around 1MHz without special treatment.

The Effect of Inductance

A concept that might not be obvious about paired wire circuits is that the addition of inductance in the line can help reduce attenuation at voice frequencies. The line impedance (AC resistance) is increased so that a given amount of power can be transmitted with less current but at a higher voltage. The result is a reduction in the series losses and an increase in the shunt losses. Because the series losses are usually more severe, there is a net reduction in attenuation until the inductance rises to the point where series and shunt losses are equal.

Adding inductance to wire pairs is called loading, and a circuit to which inductance has been added is called a loaded line or loaded circuit. The effect of loading is illustrated in Figure 3.4. The typical frequency-versus-attenuation performance is shown for a nonloaded 19-gauge cable pair and a 19-gauge cable pair loaded with 88 millihenrys of inductance every 6000 feet. (The standard notation for this is 19H-88 loaded pair.) Figure 3.4 shows that the attenuation of the loaded circuit is less than that of the unloaded one and that it changes very little with increasing frequency, up to a certain point above 3KHz. This point is called the cutoff point or cutoff frequency.

Loading was introduced around 1900 on long-distance open-wire lines to reduce losses due to attenuation because there were no amplifiers for the signals.

Electronic Amplifiers

When the use of the DeForest triode as an amplifier began in 1914, it was no longer necessary to load long-distance circuits because the losses in the line could be compensated for by amplification. However, loading is still used on longer local loops (from the telephone office to the customer) because it is cheaper than adding active components for amplification. The presence of loading on local circuits has a considerable effect on their capability to carry high-frequency data signals, which causes problems for some new types of telephone service.

In 1883, Thomas Edison discovered the rectifying properties of the thermionic vacuum tube. A thermionic vacuum tube is one in which a stream of electrons is emitted by an incandescent substance. Edison's device was a two-element tube (called a diode) consisting of a cathode (the incandescent substance) and an anode.

The principle of thermionic vacuum tubes lay unused, however, until 1904. At that time, Sir John Ambrose Fleming, an English physicist and engineer, adapted the diode for use as a demodulator (detector) of radiotelegraph signals for the Marconi Wireless Telegraph Co. In 1906, Lee DeForest introduced a third element, called a control grid, to the diode and created the triode. By 1912, the triode and its associated circuits were developed for use as an amplifier.

Figure 3.4 Effect of inductance loading.

Twisted-Pair Wire

A special type of wire pair that deserves a degree of elaboration is twisted-pair wire. This type of wiring represents the vast majority of cabling used in the local loop from a telephone company central office to a subscriber, as well as in local area networks.

There are two general types of twisted-pair cable, shielded (STP) and unshielded (UTP). The former is a type of cable with a thin foil of lead wrapped around each pair to provide a degree of immunity to electromagnetic interference (EMI). When LANs were initially developed, STP cabling was primarily used; however, a large degree of twists in cable have the effect of canceling out electromagnetic interference. Because UTP is considerably less expensive than STP cabling, today the vast majority of LAN cabling is unshielded twisted-pair cabling. In comparison, because twisted-pair wire used for voice dates to the 18th century, telephone wire is unshielded.

In any communications system that will include twisted-pair cabling, the bit error rate depends upon the ability of a receiver to distinguish a signal from noise. The ratio of signal strength to noise is the well-known signal-to-noise ratio (S/N). In a twisted-pair cabling system, the S/N ratio is highly dependent upon near-end crosstalk (NEXT) and the attenuation of the cable.

NEXT

Near-end crosstalk represents the electromagnetic coupling between a transmit pair and a receive pair. That is, as data is transmitted on one wire pair, a small portion of the transmitted signal flows onto the receive pair, interfering with the received signal. Because the transmit signal is strongest at its source, a majority of crosstalk will occur where a LAN adapter is connected to a cable via a modular jack, but it will decrease in intensity as the signal traverses the cable. This explains the term "near-end" in NEXT. In a voice environment, NEXT can adversely affect a conversation. This is because frayed wiring at the base of a handset will allow a portion of a conversation to "bleed" over to the wiring connected to the receiver in a handset.

Figure 3.5 illustrates how NEXT is generated. NEXT is defined mathematically as follows:

 NEXT = 20 log10 Transmit voltage 
     Coupled voltage

Here, the coupled voltage is the voltage flowing on the receive pair when a transmit signal is placed on the transmit pair.

Attenuation

Attenuation on a twisted-pair cable is the same as on any medium: It represents the loss of signal power as a signal flows from transmitter to receiver.

Attenuation on twisted-pair is measured in decibels, as follows:

 Attenuation = 20 log10 Transmit voltage 
     Receive voltage

Coaxial Cable

To make telephone service economical, more than one conversation had to be put on a channel. This problem drove the evolution of coaxial cables. Indeed, the invention of the telephone arose out of Alexander Bell's experiments on a "harmonic telegraph," an attempt to put more than one telegraph signal on a channel. Putting more conversations or more data on a single channel requires a larger bandwidth (capability to carry more frequencies), which, as a practical matter, means higher frequencies. Because the practical frequency limit for wire pairs is around 1MHz, some other method had to be developed.

Some significant and interesting effects occur in the vicinity of a wire carrying an alternating current signal. One of these effects is that both an electric field and a magnetic field are created around the conductor. The magnetic field can induce the signal that it is carrying into adjacent conductors. (In communications, the induced and unwanted signal is called crosstalk.) However, if one conductor of the pair is the ground side of the circuit and is made to surround the other conductor, both the radiated electric field and the magnetic field can be confined within the tube formed by the outer conductor, as illustrated in Figure 3.6.

Figure 3.5 Signal coupled from transmit pair to receive pair.

This medium is called a coaxial cable because the two conductors have a common axis. At frequencies higher than about 100KHz, the self-shielding works well; at lower frequencies, however, the "skin depth" of the current is comparable to the thickness of the outer conductor, and the shielding becomes ineffective. The resistive loss of coaxial cable increases in proportion to the square root of the frequency, making coaxial cable generally usable at frequencies of up to 2000MHz, although some types can be used up to 10000MHz.

Waveguide

If the frequency of transmission is high enough, the electric and magnetic components of a signal can travel through free space, requiring no solid conductor. However, to avoid interference and losses due to signal spreading and to be able to route the signal as desired, it is sometimes useful to confine these waves to another bounded medium called a waveguide.

Waveguides are commonly used at frequencies from 2000MHz up to 110000MHz to connect microwave transmitters and receivers to their antennas. Waveguides are pressurized with dry air or nitrogen to drive out moisture from inside the waveguide because moisture attenuates the microwaves. Older waveguides were constructed with a rectangular cross section, but common practice today is to make the guides circular, as shown in Figure 3.7. Waveguides remain in use as a conductor of high-power, high-frequency signals, but optical-fiber cables are being used primarily in newer systems.

Fiber-Optic Systems

The capacity of a transmission system is a direct function of the highest frequency that it can carry. Progress in transmission technology has therefore been measured by the bandwidth of the media available to carry signals. Recent developments in the use of glass fibers to carry binary signals have shown these systems to be extremely well suited to high-data-rate applications.

Figure 3.6 Structure of coaxial cable.

Figure 3.7 Circular waveguides.

Fiber-optic systems are attractive for several reasons:

• The low transmission loss, as compared with wire pairs or coaxial cable, allows much greater separation between repeaters. A fiber-optic system with no repeaters has been demonstrated that transmits 40 gigabits per second (Gbps) over a span of 75 miles with an error rate several times lower than that of high-quality coaxial cable systems.

• Because the optical fibers carry light rays, the frequency of operation is that of light. The transmission wavelength used for current single-mode fibers is 1.2 micrometers, equivalent to a frequency of around 800 terahertz (800 trillion hertz). Such frequencies allow data transmission rates of 20000Mbps at distances up to approximately 100km.

• Optical-fiber cables do not radiate energy, do not conduct electricity, and are noninductive. They are essentially free from crosstalk and the effects of lightning-induced interference, and they present no security problem from an inductively coupled "wire tap."

• Because optical-fiber cables transport light energy, they can be routed through most hazardous areas, such as oil refineries, grain elevators, and similar locations where the use of cables carrying electricity either is barred or represents a potential danger.

• Optical-fiber cables are smaller, lighter, and cheaper than metallic cables of the same capacity. It is economically feasible to provide several unused fibers in a cable for spares and for future growth.

A cross-section of a typical optical-fiber cable is shown in Figure 3.8.

Figure 3.8 A typical optical five-fiber cable for direct burial.

One of the earliest fiber-optic systems to be placed into commercial service was the AT&T FT3 lightwave system, which carried up to 80,000 two-way voice conversations at the same time. The cable for this system was one-half inch in diameter and contained 144 fibers. Each fiber pair operated at 90Mbps for a total data rate of about 6000Mbps. The system provided one spare fiber for every operating one, and switchover to a spare was automatic upon loss of signal in a fiber. A more detailed discussion of fiber-optic systems is given in Chapter 7, "Fiber-Optic and Satellite Communications."

Building Cabling Standards

Although American Wire Gauge numbering governs the thickness of a conductor, other properties govern the capability of twisted pair to transport information. Recognizing the requirement to standardize cabling used within buildings for the transportation of voice, as well as for data on local area networks, the Electronic Industries Association (EIA) and the Telecommunications Industry Association (TIA) jointly developed a standard that specifies various building cabling parameters. Formally referred to as the EIA/TIA-568 standard, this standard specifies cabling parameters ranging from backbone cabling used to connect a building's telecommunications closets to an equipment room, to horizontal cabling used to cable individual users to an equipment closet. This standard defines the performance characteristics of both backbone and horizontal cables, as well as different types of connectors used with different types of cable.

Backbone Cabling

Four types of media are recognized by the EIA/TIA-568 standard for backbone cabling. Table 3.2 summarizes the backbone media cabling options to include the maximum cabling distance support by each type of media.

Backbone cabling is commonly used to connect network devices known as hubs to form the backbone of a local area network. That connection can occur between hubs located on different floors or hubs located on the same floor. Thus, backbone cabling can consist of vertical and horizontal cabling. In comparison, restrictive horizontal cabling provides the connection from individual workstations to the ports on a hub. Because most hubs are located in a telecommunications or wiring closet, the EIA/TIA-568 standard for horizontal cabling governs the cabling from a telecommunications closet to a user's work area. Figure 3.9 illustrates the relationship between backbone and horizontal cabling, as well as their uses for connecting hubs and workstations.

Table 3.2 EIA/TIA-568 Backbone Cabling Media Options

Figure 3.9 Building cable relationship.

Horizontal Cabling

Under the EIA/TIA-568 standard, horizontal cable connects equipment in a telecommunications or wiring closet to a user's work area. The media options supported for horizontal cabling are the same as those specified for backbone cabling, with the exception of coaxial cable, for which 50-ohm thin cable is specified. However, the cabling distance for each type of cable is restricted to 90 meters in length from equipment in the telecommunications or wiring closet to a telecommunications outlet located in a user's work area. This cabling distance permits the use of a patch cord or drop cable up to 10 meters in length to connect a workstation to the outlet. The resulting total length of horizontal cabling is restricted to a maximum of 100 meters.

This 100-meter horizontal cabling restriction is associated with many LAN technologies that are based on the use of unshielded twisted-pair (UTP) cabling. Recognizing that different types of LANs use different signaling rates, the EIA/TIA-568 standard classified UTP cable into five categories. Table 3.3 outlines those categories and their suitability for different types of voice and data applications.

Table 3.3 EIA/TIA-568 UTP Cable Categories

When examining the entries in Table 3.3, note that UTP Categories 3, 4, and 5 support transmission with respect to indicated signaling rates. This means that the capability of those categories of UTP to support different types of LAN transmission depends on the signaling method used by different LANs. Category 3 cable is typically used for Ethernet and 4Mbps token-ring LANs. Category 4 is normally used for 16Mbps token-ring LANs, and Category 5 cable supports both versions of Ethernet to include 10Mbps and 100Mbps operations and the emerging ATM to the desktop at a 155Mbps operating rate. (Chapter 11, "Local Area Networks" contains a detailed discussion of LANs.)

The frequency or signaling rate supported by each cable category enables the cable to limit NEXT and attenuation. Table 3.4 summarizes the limits for Categories 3, 4, and 5 cable for attenuation and NEXT over frequency in dB.

When examining the entries in Table 3.4, note that because Category 3 cable supports signaling rates only up to 16MHz, there are no entries in the attenuation and NEXT columns for that type of cable beyond that frequency. Similarly, Category 4 cable supports transmission up to a 20MHz signaling rate, which explains the absence of entries for attenuation and NEXT beyond a 20MHz frequency.

Table 3.4 Categories 3, 4, and 5 Attenuation and NEXT Limits

The literal need for speed resulting from the development of Gigabit Ethernet operating at 1000Mbps provided a driving force for the development of several new categories of cable. In May 2000, the TIA published a new standard titled "Transmission Performance specifications for 4-Pair 100 W Cabling—Addendum 5." This standard defines an enhancement to Category 5 and is commonly referred to as Category 5e or Enhanced Category 5 cable.

Category 5e cable has different limits for NEXT and attenuation than Category 5 cable. For example, at 10MH, Enhanced Category 5 cable limits NEXT to 6.3dB, while at 100MHz, the limit is 21.60dB. In comparison, in Table 3.4, Category 5 cable permits NEXT of 7.0dB at 10MHz and 24.0dB at 100MHz. Concerning attenuation, Category 5 cable permits a higher limit than Category 5 cable. At 10MHz, 47.30dB is permitted, while at 100MHz, 30.0dB is permitted. Whereas Category 5 cable is suitable for voice and data applications up to 155Mbps, Category 5e is suitable for supporting applications up to 250Mbps. In a Gigabit Ethernet environment, four pairs of Category 5e cable are used to support a 1Gbps operating rate.

Two additional evolving cable standards that warrant attention are Category 6 and Category 7. Category 6, like its predecessors, represents a standard for unshielded twisted-pair cable. In comparison, efforts at developing a Category 7 standard are focused upon shielded twisted-pair (STP) cable.

Under Category 6, crosstalk and attenuation are specified at a signal rate up to 200MHz. Under Category 7, a signal rate of 600MHz will be defined in a draft standard; one vendor using a special connector currently supports signaling via STP up to 1GHz. For the vast majority of businesses, Category 6 cable, which supports 200MHz signaling, should be more than adequate for most applications when the standard is finalized and cable is fabricated based upon the resulting specification.

Table 3.5 provides a comparison of the major characteristics of Category 5e, 6, and 7 cable standards. In examining the entries in Table 3.5, note that the term PS in PS NEXT represents power sum, a mathematical addition of noise from multiple sources of disturbance. When applied to NEXT, it represents the sum of the individual NEXT effects upon each wire pair by the other three pairs, and it is a critical parameter in full-duplex Gigabit Ethernet operations. Also note that although parameters remain to be defined (TBD) for Category 7 cable, the specifications will extend to 600MHz when defined.

Table 3.5 Comparing Evolving Cable Standards

Category 5e Category 6 Category 7