TRANSMISSION MEDIA

TRANSMISSION  MEDIA

Transmission medium is the physical path between transmitter and receiver

in a data transmission system. Transmission media can be classified

as guided or unguided. In both cases, communication is in the form of electromagnetic

waves. With guided media, the waves are guided along a solid medium,

such as copper twisted pair, copper coaxial cable, and optical fiber. The atmosphere

and outer space are examples of unguided media that provide a means of transmitting

electromagnetic signals but do not guide them; this form of transmission is usually

referred to as wireless transmission.

The characteristics and quality of a data transmission are determined both by

the characteristics of the medium and the characteristics of the signal. In the case of

guided media, the medium itself is more important in determining the limitations of

transmission.

For unguided media, the bandwidth of the signal produced by the transmitting

antenna is more important than the medium in determining transmission characteristics.

One key property of signals transmitted by antenna is directionality. In

general, signals at lower frequencies are omnidirectional; that is, the signal propagates

in all directions from the antenna. At higher frequencies, it is possible to focus

the signal into a directional beam.

In considering the design of data transmission systems, a key concern, generally,

is data rate and distance: the greater the data rate and distance, the better. A

number of design factors relating to the transmission medium and to the signal

determine the data rate and distance:

Bandwidth. All other factors remaining constant, the greater the bandwidth

of a signal, the higher the data rate that can be achieved.

Transmission impairments. Impairments, such as attenuation, limit the distance.

For guided media, twisted pair generally suffer more impairment than

coaxial cable, which in turn suffers more than optical fiber.

Interference. Interference from competing signals in overlapping frequency

bands can distort or wipe out a signal. Interference is of particular concern for

unguided media, but it is also a problem with guided media. For guided

media, interference can be caused by emanations from nearby cables. For

example, twisted pair are often bundled together, and conduits often carry

multiple cables. Interference can also be experienced from unguided transmissions.

Proper shielding of a guided medium can minimize this problem.

a Number of receivers. A guided medium can be used to construct a point-topoint

link or a shared link with multiple attachments. In the latter case, each

attachment introduces some attenuation and distortion on the line, limiting

distance and/or data rate.

Figure 3.1 depicts the electromagnetic spectrum and indicates the frequencies

at which various guided media and unguided transmission techniques operate. In

this lesson, we examine these guided and unguided alternatives. In all cases, we

describe the systems physically, briefly discuss applications, and summarize key

transmission characteristics.

GUIDED TRANSMSSION MEDIA

For guided transmission media, the transmission capacity, in terms of either data

rate or bandwidth, depends critically on the distance and on whether the medium is

point-to-point or multipoint, such as in a local area network (LAN). Table 3.1 indicates

the type of performance typical for the common guided medium for longdistance

point-to-point applications; we defer a discussion of the use of these media

for LANs to Part 111.

The three guided media commonly used for data transmission are twisted pair,

coaxial cable, and optical fiber (Figure 3.2). We examine each of these in turn.

Twisted Pair

The least-expensive and most widely-used guided transmission medium is twisted

pair.

 

Physical Description

A twisted pair consists of two insulated copper wires arranged in a regular spiral

pattern. A wire pair acts as a single communication link. Typically, a number of

these pairs are bundled together into a cable by wrapping them in a tough protective

sheath. Over longer distances, cables may contain hundreds of pairs. The twisting

tends to decrease the crosstalk interference between adjacent pairs in a cable.

Neighboring pairs in a bundle typically have somewhat different twist lengths to

reduce the crosstalk interference. On long-distance links, the twist length typically

varies from two to six inches. The wires in a pair have thicknesses of from 0.016 to

0.036 inches.

Applications

By far the most common transmission medium for both analog and digital signals is

twisted pair. It is the most commonly used medium in the telephone network as well

as being the workhorse for communications within buildings.

In the telephone system, individual residential telephone sets are connected to

the local telephone exchange, or "end office," by twisted-pair wire. These are

referred to as subscriber loops. Within an office building, each telephone is also connected

to a twisted pair, which goes to the in-house private branch exchange (PBX)

system or to a Centrex facility at the end office. These twisted-pair installations

were designed to support voice traffic using analog signaling. However, by means of

a modem, these facilities can handle digital data traffic at modest data rates.

Twisted pair is also the most common medium used for digital signaling. For

connections to a digital data switch or digital PBX within a building, a data rate of

64 kbps is common. Twisted pair is also commonly used within a building for local

area networks supporting personal computers. Data rates for such products are typically

in the neighborhood of 10 Mbps. However, recently, twisted-pair networks

with data rates of 100 Mbps have been developed, although these are quite limited

in terms of the number of devices and geographic scope of the network. For longdistance

applications, twisted pair can be used at data rates of 4 Mbps or more.

Twisted pair is much less expensive than the other commonly used guided

transmission media (coaxial cable, optical fiber) and is easier to work with. It is

more limited in terms of data rate and distance.

Transmission Characteristics

Twisted pair may be used to transmit both analog and digital signals. For analog signals,

amplifiers are required about every 5 to 6 km. For digital signals, repeaters are

required every 2 or 3 km.

Compared to other commonly used guided transmission media (coaxial cable,

optical fiber), twisted pair is limited in distance, bandwidth, and data rate. As Figure

3.3 shows, the attenuation for twisted pair is a very strong function of frequency.

Other impairments are also severe for twisted pair. The medium is quite susceptible

to interference and noise because of its easy coupling with electromagnetic

fields. For example, a wire run parallel to an ac power line will pick up 60-Hz

energy. Impulse noise also easily intrudes into twisted pair. Several measures are

taken to reduce impairments. Shielding the wire with metallic braid or sheathing

reduces interference. The twisting of the wire reduces low-frequency interference,

and the use of different twist lengths in adjacent pairs reduces crosstalk.

For point-to-point analog signaling, a bandwidth of up to about 250 kHz is

possible. This accommodates a number of voice channels. For long-distance digital

point-to-point signaling, data rates of up to a few Mbps are possible; for very short

distances, data rates of up to 100 Mbps have been achieved in commercially available

products.

Unshielded and Shielded Twisted Pair

Twisted pair comes in two varieties: unshielded and shielded. Unshielded twisted

pair (UTP) is ordinary telephone wire. Office buildings, by universal practice, are

pre-wired with a great deal of excess unshielded twisted pair, more than is needed

for simple telephone support. This is the least expensive of all the transmission

media commonly used for local area networks, and is easy to work with and simple

to install.

Unshielded twisted pair is subject to external electromagnetic interference,

including interference from nearby twisted pair and from noise generated in the

environment. A way to improve the characteristics of this medium is to shield the

twisted pair with a metallic braid or sheathing that reduces interference. This

shielded twisted pair (STP) provides better performance at lower data rates. However,

it is more expensive and more difficult to work with than unshielded twisted

pair.

Category 3 and Category 5 UTP

Most office buildings are prewired with a type of 100-ohm twisted pair cable commonly

referred to as voice-grade. Because voice-grade twisted pair is already

installed, it is an attractive alternative for use as a LAN medium. Unfortunately, the

data rates and distances achievable with voice-grade twisted pair are limited.

In 1991, the Electronic Industries Association published standard EIA-568,

Commercial Building Telecommunications Cabling Standard, that specified the use

of voice-grade unshielded twisted pair as well as shielded twisted pair for in-building

data applications. At that time, the specification was felt to be adequate for the

range of frequencies and data rates found in office environments. Up to that time,

the principal interest for LAN designs was in the range of data rates from 1 Mbps

to 16 Mbps. Subsequently, as users migrated to higher-performance workstations

and applications, there was increasing interest in providing LANs that could operate

up to 100 Mbps over inexpensive cable. In response to this need, EIA-568-A was

issued in 1995. The new standard reflects advances in cable and connector design

and test methods. It covers 150-ohm shielded twisted pair and 100-ohm unshielded

twisted pair.

EIA-568-A recognizes three categories of UTP cabling:

Category 3. UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 16 MHz.

Category 4. UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 20 MHz.

Category 5. UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 100 MHz.

Of these, it is Category 3 and Category 5 cable that have received the most

attention for LAN applications. Category 3 corresponds to the voice-grade cable

found in abundance in most office buildings. Over limited distances, and with

proper design, data rates of up to 16 Mbps should be achievable with Category 3.

Category 5 is a data-grade cable that is becoming increasingly common for preinstallation

in new office buildings. Over limited distances, and with proper design,

data rates of up to 100 Mbps should be achievable with Category 5.

A key difference between Category 3 and Category 5 cable is the number of

twists in the cable per unit distance. Category 5 is much more tightly twistedtypically

3 to 4 twists per inch, compared to 3 to 4 twists per foot for Category 3.

The tighter twisting is more expensive but provides much better performance than

Category 3.

Table 3.2 summarizes the performance of Category 3 and 5 UTP, as well as the

STP specified in EIA-568-A. The first parameter used for comparison, attenuation,

is fairly straightforward. The strength of a signal falls off with distance over any

transmission medium. For guided media, attenuation is generally logarithmic and is

therefore typically expressed as a constant number of decibels per unit distance.

Attenuation introduces three considerations for the designer. First, a received signal

must have sufficient magnitude so that the electronic circuitry in the receiver

can detect and interpret the signal. Second, the signal must maintain a level sufficiently

higher than noise to be received without error. Third, attenuation is an

increasing function of frequency.

Near-end crosstalk, as it applies to twisted pair wiring systems, is the coupling

of the signal from one pair of conductors to another pair. These conductors may be

the metal pins in a connector or the wire pairs in a cable. The near end refers to coupling

that takes place when the transmit signal entering the link couples back to the

receive conductor pair at that same end of the link; in other words, the near-transmitted

signal is picked up by the near-receive pair.

Coaxial Cable

Physical Description

Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently

to permit it to operate over a wider range of frequencies. It consists of a

hollow outer cylindrical conductor that surrounds a single inner wire conductor

(Figure 3.2b). The inner conductor is held in place by either regularly spaced insulating

rings or a solid dielectric material. The outer conductor is covered with a

jacket or shield. A single coaxial cable has a diameter of from 0.4 to about 1 in.

Because of its shielded, concentric construction, coaxial cable is much less susceptible

to interference and crosstalk than is twisted pair. Coaxial cable can be used over

longer distances and supports more stations on a shared line than twisted pair.

Applications

Coaxial cable is perhaps the most versatile transmission medium and is enjoying

widespread use in a wide variety of applications; the most important of these are

8 Television distribution

c Long-distance telephone transmission

e Short-run computer system links

e Local area networks

Coaxial cable is spreading rapidly as a means of distributing TV signals to individual

homes-cable TV. From its modest beginnings as Community Antenna Television

(CATV), designed to provide service to remote areas, cable TV will eventually

reach almost as many homes and offices as the telephone. A cable TV system

can carry dozens or even hundreds of TV channels at ranges up to a few tens of

miles.

Coaxial cable has traditionally been an important part of the long-distance

telephone network. Today, it faces increasing competition from optical fiber, terrestrial

microwave, and satellite. Using frequency-division multiplexing (FDM, see

Lesson 7), a coaxial cable can carry over 10,000 voice channels simultaneously.

Coaxial cable is also commonly used for short-range connections between

devices. Using digital signaling, coaxial cable can be used to provide high-speed 110

channels on computer systems.

Another application area for coaxial cable is local area networks (Part

Three). Coaxial cable can support a large number of devices with a variety of data

and traffic types, over distances that encompass a single building or a complex of

buildings.

Transmission Characteristics

Coaxial cable is used to transmit both analog and digital signals. As can be seen

from Figure 3.3, coaxial cable has frequency characteristics that are superior to

those of twisted pair, and can hence be used effectively at higher frequencies and

data rates. Because of its shielded, concentric construction, coaxial cable is much

less susceptible to interference and crosstalk than twisted pair. The principal constraints

on performance are attenuation, thermal noise, and intermodulation noise.

The latter is present only when several channels (FDM) or frequency bands are in

use on the cable.

For long-distance transmission of analog signals, amplifiers are needed every

few kilometers, with closer spacing required if higher frequencies are used. The

usable spectrum for analog signaling extends to about 400 MHz. For digital signaling,

repeaters are needed every kilometer or so, with closer spacing needed for

higher data rates.

Optical Fiber

Physical Description

An optical fiber is a thin (2 to 125 pm), flexible medium capable of conducting an

optical ray. Various glasses and plastics can be used to make optical fibers. The lowest

losses have been obtained using fibers of ultrapure fused silica. Ultrapure fiber

is difficult to manufacture; higher-loss multicomponent glass fibers are more economical

and still provide good performance. Plastic fiber is even less costly and can

be used for short-haul links, for which moderately high losses are acceptable.

An optical fiber cable has a cylindrical shape and consists of three concentric

sections: the core, the cladding, and the jacket (Figure 3.2~)T. he core is the innermost

section and consists of one or more very thin strands, or fibers, made of glass

or plastic. Each fiber is surrounded by its own cladding, a glass or plastic coating

that has optical properties different from those of the core. The outermost layer,

surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed

of plastic and other material layered to protect against moisture, abrasion, crushing,

and other environmental dangers.

Applications

One of the most significant technological breakthroughs in data transmission has

been the development of practical fiber optic communications systems. Optical

fiber already enjoys considerable use in long-distance telecommunications, and its

use in military applications is growing. The continuing improvements in performance

and decline in prices, together with the inherent advantages of optical fiber,

have made it increasingly attractive for local area networking. The following characteristics

distinguish optical fiber from twisted pair or coaxial cable:

Greater capacity. The potential bandwidth, and hence data rate, of optical

fiber is immense; data rates of 2 Gbps over tens of kilometers have been

demonstrated. Compare this capability to the practical maximum of hundreds

of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or

up to 100 Mbps over a few tens of meters for twisted pair.

Smaller size and lighter weight. Optical fibers are considerably thinner than

coaxial cable or bundled twisted-pair cable-at least an order of magnitude

thinner for comparable information-transmission capacity. For cramped conduits

in buildings and underground along public rights-of-way, the advantage

of small size is considerable. The corresponding reduction in weight reduces

structural support requirements.

Lower attenuation. Attenuation is significantly lower for optical fiber than for

coaxial cable or twisted pair (Figure 3.3) and is constant over a wide range.

Electromagnetic isolation. Optical fiber systems are not affected by external

electromagnetic fields. Thus, the system is not vulnerable to interference,

impulse noise, or crosstalk. By the same token, fibers do not radiate energy,

thereby causing little interference with other equipment and thus providing a

high degree of security from eavesdropping. In addition, fiber is inherently

difficult to tap.

Greater repeater spacing. Fewer repeaters means lower cost and fewer

sources of error. The performance of optical fiber systems from this point of

view has been steadily improving. For example, AT&T has developed a fiber

transmission system that achieves a data rate of 3.5 Gbps over a distance of

318 km [PARK921 without repeaters. Coaxial and twisted-pair systems generally

have repeaters every few kilometers.

Five basic categories of application have become important for optical fiber:

Long-haul trunks

Metropolitan trunks

Rural-exchange trunks

Subscriber loops

Local area networks

Long-haul fiber transmission is becoming increasingly common in the telephone

network. Long-haul routes average about 900 miles in length and offer high

capacity (typically 20,000 to 60,000 voice channels). These systems compete economically

with microwave and have so underpriced coaxial cable in many developed

countries that coaxial cable is rapidly being phased out of the telephone network

in such areas.

Metropolitan trunking circuits have an average length of 7.8 miles and may

have as many as 100,000 voice channels in a trunk group. Most facilities are installed

in underground conduits and are repeaterless, joining telephone exchanges in a

metropolitan or city area. Included in this category are routes that link long-haul

microwave facilities that terminate at a city perimeter to the main telephone

exchange building downtown.

Rural exchange trunks have circuit lengths ranging from 25 to 100 miles that

link towns and villages. In the United States, they often connect the exchanges of

different telephone companies. Most of these systems have fewer than 5,000 voice

channels. The technology in these applications competes with microwave facilities.

Subscriber loop circuits are fibers that run directly from the central exchange

to a subscriber. These facilities are beginning to displace twisted pair and coaxial

cable links as the telephone networks evolve into full-service networks capable of

handling not only voice and data, but also image and video. The initial penetration

of optical fiber in this application is for the business subscriber, but fiber transmission

into the home will soon begin to appear.

A final important application of optical fiber is for local area networks.

Recently, standards have been developed and products introduced for optical fiber

networks that have a total capacity of 100 Mbps and can support hundreds or even

thousands of stations in a large office building or in a complex of buildings.

The advantages of optical fiber over twisted pair and coaxial cable become

more compelling as the demand for all types of information (voice, data, image,

video) increases.

Transmission Characteristics

Optical fiber systems operate in the range of about 1014 to 1015 Hz; this covers portions

of the infrared and visible spectrums. The principle of optical fiber transmission

is as follows. Light from a source enters the cylindrical glass or plastic core.

Rays at shallow angles are reflected and propagated along the fiber; other rays are

absorbed by the surrounding material. This form of propagation is called multimode,

referring to the variety of angles that will reflect. When the fiber core radius

is reduced, fewer angles will reflect. By reducing the radius of the core to the order

of a wavelength, only a single angle or mode can pass: the axial ray. This singlemode

propagation provides superior performance for the following reason: With

multimode transmission, multiple propagation paths exist, each with a different

path length and, hence, time to traverse the fiber; this causes signal elements to

spread out in time, which limits the rate at which data can be accurately received.

Because there is a single transmission path with single-mode transmission, such distortion

cannot occur. Finally, by varying the index of refraction of the core, a third

type of transmission, known as multimode graded index, is possible. This type is

intermediate between the other two in characteristics. The variable refraction has

the effect of focusing the rays more efficiently than ordinary multimode, also known

as multimode step index. Table 3.3 compares the three fiber transmission modes.

Two different types of light source are used in fiber optic systems: the lightemitting

diode (LED) and the injection laser diode (ILD). Both are semiconductor

devices that emit a beam of light when a voltage is applied. The LED is less costly,

operates over a greater temperature range, and has a longer operational life. The

ILD, which operates on the laser principle, is more efficient and can sustain greater

data rates.

There is a relationship among the wavelength employed, the type of transmission,

and the achievable data rate. Both single mode and multimode can support

several different wavelengths of light and can employ laser or LED light source. In

optical fiber, light propagates best in three distinct wavelength "windows," centered

on 850,1300, and 1550 nanometers (nm). These are all in the infrared portion of the

frequency spectrum, below the visible-light portion, which is 400 to 700 nm. The loss

is lower at higher wavelengths, allowing greater data rates over longer distances

(Table 3.3). Most local applications today use 850-nm LED light sources. Although

this combination is relatively inexpensive, it is generally limited to data rates under

100 Mbps and distances of a few kilometers. To achieve higher data rates and longer

distances, a 1300-nm LED or laser source is needed. The highest data rates and

longest distances require 1500-nm laser sources.