Transmission Media
Many different types of media can be used for the physical layer. For example, telephone twisted pair, coax cable, shielded copper cable and fiber optics are the main types used for LANs. Different transmission techniques generally categorized as baseband or broadband transmission may be applied to each of these media types.
OBJECTIVES OF THIS CHAPTER
After completing this chapter, the student should be able to:
- define transmission medium, electromagnetic spectrum
- identify guided medium such as coax, fiber or twisted pair cable
- measure performance of transmission medium
- identify unguided medium, such as infrared, microwave
- understand the working of satellite
4.1 INTRODUCTION
Transmission medium provides physical entity for the conveyance of signals.
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.
Number of receivers. A guided medium can be used to construct a point-to-point 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 4.1 depicts the electromagnetic spectrum and indicates the frequencies at which various guided media and unguided transmission techniques operate. In this chapter, we examine these guided and unguided alternatives.
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Figure 4.1 : Electromagnetic Spectrum with frequency ranges
Transmission media can be divided into two broad categories : Guided and Unguided.
Figure 4.2 : Types of transmission media
4.2 GUIDED MEDIA
Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.
Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only. Cable is the medium through which information usually moves from one network device to another.
Twisted pair cable and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light.
There four basic types of Guided Media :
- Open Wire
- Twisted Pair
- Coaxial Cable
- Optical Fiber
Contents
Figure 4.3 : Types of guided media
OPEN WIRE
[[Image:]]Open Wire is traditionally used to describe the electrical wire strung along power poles. There is a single wire strung between poles. No shielding or protection from noise interference is used. We are going to extend the traditional definition of Open Wire to include any data signal path without shielding or protection from noise interference. This can include multiconductor cables or single wires. This media is susceptible to a large degree of noise and interference and consequently not acceptable for data transmission except for short distances under 20 ft.
Figure4.4: Open wire
TWISTED-PAIR (TP) CABLE
Twisted pair cable is least expensive and most widely used. The wires in Twisted Pair cabling are twisted together in pairs. Each pair would consist of a wire used for the +ve data signal and a wire used for the -ve data signal. Any noise that appears on one wire of the pair would occur on the other wire. Because the wires are opposite polarities, they are 180 degrees out of phase When the noise appears on both wires, it cancels or nulls itself out at the receiving end. Twisted Pair cables are most effectively used in systems that use a balanced line method of transmission : polar line coding (Manchester Encoding) as opposed to unipolar line coding (TTL logic).
Physical description
- Two insulated copper wires arranged in regular spiral pattern.
- Number of pairs are bundled together in a cable.
- Twisting decreases the crosstalk interference between adjacent pairs in the cable, by using different twist length for neighboring pairs.
A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One of the wire is used to carry signals to the receiver, and the other is used only a ground reference.
Why the cable is twisted?
In past, two parallel flat wires were used for communication. However, electromagnetic interference from devices such as a motor can create noise over those wires.
If the two wires are parallel, the wire closest to the source of the noise gets more interference and ends up with a higher voltage level than the wire farther away, which results in an uneven load and a damaged signal. If, however, the two wires are twisted around each other at regular intervals, each wire is closer to the noise source for half the time and farther away for the other half. The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference. To further improve noise rejection, a foil or wire braid shield is woven around the twisted pairs.
Twisted pair cable supports both analog and digital signals. TP cable can be either unshielded TP (UTP) cable or shielded TP (STP) cable. Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP. Cables without a shield are called Unshielded Twisted Pair or UTP. Shielding means metallic material added to cabling to reduce susceptibility to noise due to electromagnetic interference (EMI).
IBM produced a version of TP cable for its use called STP. STP cable has a metal foil that encases each pair of insulated conductors. Metal casing used in STP improves the quality of cable by preventing the penetration of noise. It also can eliminate a phenomenon called crosstalk.
Crosstalk is the undesired effect of one circuit (or channel) on another circuit (or channel). It occurs when one line picks up some of the signal traveling down another line. Crosstalk effect can be experienced during telephone conversations when one can hear other conversations in the background.
Twisted-pair cabling with additional shielding to reduce crosstalk and other forms of electromagnetic interference (EMI). It has an impedance of 150 ohms, has a maximum length of 90 meters, and is used primarily in networking environments with a high amount of EMI due to motors, air conditioners, power lines, or other noisy electrical components. STP cabling is the default type of cabling for IBM Token Ring networks. STP is more expensive as compared to UTP.
UTP is cheap, flexible, and easy to install. UTP is used in many LAN technologies, including Ethernet and Token Ring.
In computer networking environments that use twisted-pair cabling, one pair of wires is typically used for transmitting data while another pair receives data. The twists in the cabling reduce the effects of crosstalk and make the cabling more resistant to electromagnetic interference (EMI), which helps maintain a high signal-to-noise ratio for reliable network communication. Twisted-pair cabling used in Ethernet networking is usually unshielded twisted-pair (UTP) cabling, while shielded twisted-pair (STP) cabling is typically used in Token Ring networks. UTP cabling comes in different grades for different purposes.
The Electronic Industries Association (EIA) has developed standards to classify UTP cable into seven categories. Categories are determined by cable quality, with CAT 1 as the lowest and CAT 7 as the highest.
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Table 4.1: Categories of UTP cable
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The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot.
Unshielded Twisted Pair Connector
The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector. A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.
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STP cabling comes in various grades or categories defined by the EIA/TIA wiring standards, as shown in the table 4.2
STP Cabling Categories :
| Category | Description |
| IBM Type 1 | Token Ring transmissions on AWG #22 wire up to 20 Mbps. |
| IBM Type 1A | Fiber Distributed Data Interface (FDDI), Copper Distributed Data Interface (CDDI), and Asynchronous Transfer Mode (ATM) transmission up to 300 Mbps. |
| IBM Type 2A | Hybrid combination of STP data cable and CAT3 voice cable in one jacket. |
| IBM Type 6A | AWG #26 patch cables. |
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Figure 4.7 : STP cable
Transmission characteristics
- Requires amplifiers for analog signals.
- Requires repeaters for digital signals.
- Attenuation is a strong function of frequency.
- Higher frequency implies higher attenuation.
- Susceptible to interference and noise.
- Improvement possibilities.
- Shielding with metallic braids or sheathing reduces interference.
- Twisting reduces low frequency interference.
- Different twist length in adjacent pairs reduces crosstalk.
Comparison of Unshielded and shielded twisted pairs
Unshielded twisted pair (UTP).
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- Ordinary telephone wire.
- Subject to external electromagnetic interference.
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Shielded twisted pair (STP)
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- Shielded with a metallic braid or sheath.
- Reduces interference.
- Better performance at higher data rates.
- More expensive and difficult to work compared to UTP.
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Applications of TP cable
- Most common transmission media for both digital and analog signals.
- TP cables are used in telephone lines to provide voice and data channels.
- The line that connects subscribers to the central telephone office is most commonly UTP cable.
- The DSL lines that are used by the telephone companies to provide high data rate connections also use high bandwidth capability UTP cable.
- Local Area Network (LAN) also uses twisted-pair cable.
COAXIAL CABLE
A form of network cabling used primarily in older Ethernet networks and in electrically noisy industrial environments. The name “coax” comes from its two-conductor construction in which the conductors run concentrically with each other along the axis of the cable. Coaxial cabling has been largely replaced by twisted-pair cabling for local area network (LAN) installations within buildings, and by fiber-optic cabling for high-speed network backbones.
Coaxial cable (or coax) carries signals of higher frequency ranges than twisted-pair cable. Instead of having two wires, coax has a central core conductor of solid or standard wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two (also usually copper).
Figure (a) Figure (b)
Figure 4.8 : Coaxial cable
The outer metallic wrapping serves both as a shield against and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
Coaxial cable supports both analog and digital signals.
Physical description
- Consists of two conductors with construction that allows it to operate over a wider range of frequencies compared to twisted pair.
- Hollow outer cylindrical conductor surrounding a single inner wire conductor.
- Inner conductor held in place by regularly spaced insulating rings or solid dielectrical material.
- Outer conductor covered with a jacket or shield.
- Diameter from 1 to 2.5 cm.
- Shielded concentric construction reduces interference and crosstalk.
- Can be used over longer distances and support more stations on a shared line than twisted pair.
Coaxial cable Standards
Although Coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between n/w devices than twisted pair cable.
Coaxial cabling comes in various types and grades. The most common are:
Thicknet cabling, which is an older form of cabling used for legacy 10Base5 Ethernet backbone installations. This cabling is generally yellow and is referred to as RG-8 or N-series cabling. Strictly speaking, only cabling labeled as IEEE 802.3 cabling is true thicknet cabling.
Figure 4.9 : Thicknet Coaxial Cable
Thinnet coaxial cabling, which is used in 10 Base2 networks for small Ethernet installations. This grade of coaxial cabling is generally designated as RG-58A/U cabling, which has a stranded conductor and a 53-ohm impedance. This kind of cabling uses BNC connectors for connecting to other networking components, and must have terminators at free ends to prevent signal bounce.
ARCNET cabling, which uses thin coaxial cabling called RG-62 cabling with an impedance of 93 ohms.
RG-59 cabling, which is used for cable television (CATV) connections.
Coaxial cables are categorized by radio government (RG) rating. Each RG number denotes a unique set of physical specifications, including the wire gauge (gauge is the measure of the thickness of the wire) of the inner conductor, the thickness and type of inner insulator, the construction o the shield, and the size and type of the outer casting.
- RG-8. Used in thick Ethernet.
- RG-9. Used in thick Ethernet.
- RG-11. Used in thick Ethernet.
- RG-58. Used in thin Ethernet.
- RG-59. Used in cable TV.
To connect coaxial cable to devices, we need coaxial connector. The most common type of connector used today is the Bayone-Neill-Concelman, or BNC connector.
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Figure 4.11: BNC connector
Transmission characteristics
- Used to transmit both analog and digital signals.
- Superior frequency characteristics compared to twisted pair.
- Can support higher frequencies and data rates.
- Shielded concentric construction makes it less susceptible to interference and crosstalk than twisted pair.
- Constraints on performance are attenuation, thermal noise, and intermodulation noise.
- Requires amplifiers every few kilometers for long distance transmission.
- Usable spectrum for analog signaling up to 500 MHz.
- Requires repeaters every few kilometers for digital transmission.
- For both analog and digital transmission, closer spacing is necessary for higher frequencies/data rates.
Application of Coaxial cable
- The use of coaxial cable started in analog telephone networks where a single coaxial network could carry 10,000 voice signals.
- Later it was used in digital telephone networks where a single coaxial cable could carry digital data up to 600 Mbps. (However, coaxial cable in telephone n/ws has largely been replaced toady with fiber-optic cable).
- Most common use is in cable TV.
- Coaxial cabling is often used in heavy industrial environments where motors and generators produce a lot of electromagnetic interference (EMI), and where more expensive fiber-optic cabling is unnecessary because of the slow data rates needed.
- Another common application of coaxial cable is in traditional Ethernet LANs. Because of its high bandwidth, and consequently high data rate, coaxial cable was chosen for digital transmission in early Ethernet LANs.
- 10Base-2, or Thin Ethernet, uses RG-58 coaxial cable with BNC connectors to transmit data at 10 Mbps with a range of 185 m.
- 10Base5, or Thick Ethernet, uses RG-11 to transmit 10 Mbps with a range of 5000 m.
FIBER-OPTIC CABLE
Fiber-optic is a glass cabling media that sends network signals using light. Fiber-optic cabling has higher bandwidth capacity than copper cabling, and is used mainly for high-speed network Asynchronous Transfer Mode (ATM) or Fiber Distributed Data Interface (FDDI) backbones, long cable runs, and connections to high-performance workstations. A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. Light is a form of electromagnetic energy. It travels at its fastest in a vacuum: 3,00,000 kilometers/sec. The speed of light depends on the density of the medium through, which it is traveling (the higher the density, the slower the speed). Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another (more or less dense), the ray changes direction. This change is called.
Refraction : The direction in which a light ray is refracted depends on the change in density encountered. A beam of light moving from a less dense into a denser medium is bent towards vertical axis.
When light travels into a denser medium, the angle of incidence is greater than the angle of refraction; and when light travels into a less dense medium, the angle of incidence is less than the angle of refraction.
Critical Angle : A beam of light moving from a denser into a less dense medium, as the angle of incidence increases the angle of refraction also increases.
Figure 4.12 : Critical Angle
At some point in this process, the change in the incident angle results in a refracted angle of 90 degrees, with the refracted beam now lying along with horizontal. The incident angle at this pt is known as the critical angle.
Reflection : When the angle of incidence becomes greater than the critical angle, a new phenomenon occurs called reflection. Light no longer passes into the less dense medium at all.
Optical fiber use reflection to guide light through a channel.
A glass or plastic core is surrounded by cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.
Information is encoded onto a beam of light as a series of on-off flashes that represents 1 and 0s.
Comparison of optical fiber with twisted pair and coaxial cable
1.Capacity
- Much higher bandwidth.
- Can carry hundreds of Gbps (Gigabit per second) over tens of Kilometers (kms).
2.Smaller size and lightweight
- Very thin for similar data capacity.
- Much lighter and easy to support in terms of weight (structural properties).
3.Significantly lower attenuation
4.EM isolation (Resistance to noise).
- Not affected by external EM (Electromagnetic) fields.
- Not vulnerable to interference, impulse noise, or crosstalk.
- No energy radiation; little interference with other devices; security from eavesdropping.
5.Greater repeater spacing
- Lower cost and fewer error sources.
6.Speed
- Fiber optic networks operate at high speeds - up into the gigabits.
7.Distance
- Signals can be transmitted further without needing to be "refreshed" or strengthened.
8.Maintenance
- Fiber optic cables costs much less to maintain.
The use of fiber-optics was generally not available until 1970 when Corning Glass Works was able to produce a fiber with a loss of 20 dB/km. It was recognized that optical fiber would be feasible for telecommunication transmission only if glass could be developed so pure that attenuation would be 20dB/km or less. That is, 1% of the light would remain after traveling 1 km. Today's optical fiber attenuation ranges from 0.5dB/km to 1000dB/km depending on the optical fiber used. Attenuation limits are based on intended application.
In recent years it has become apparent that fiber-optics are steadily replacing copper wire as an appropriate means of communication signal transmission. They span the long distances between local phone systems as well as providing the backbone for many network systems. Other system users include cable television services, university campuses, office buildings, industrial plants, and electric utility companies.
The applications of optical fiber communications have increased at a rapid rate, since the first commercial installation of a fiber-optic system in 1977. Telephone companies began early on, replacing their old copper wire systems with optical fiber lines. Today's telephone companies use optical fiber throughout their system as the backbone architecture and as the long-distance connection between city phone systems. Some 10 billion digital bits can be transmitted per second along an optical fiber link in a commercial network, enough to carry tens of thousands of telephone calls.
A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. Looking at the components in a fiber-optic chain will give a better understanding of how the system works in conjunction with wire based systems.
At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are funneled into the fiber-optic medium where they transmit themselves down the line,
Think of a fiber cable in terms of very long cardboard roll (from the inside roll of paper towel) that is coated with a mirror. If you shine a flashlight in one you can see light at the far end - even if bent the roll around a corner.
[[Image:]]Light pulses move easily down the fiber-optic line because of a principle known as total internal reflection. "This principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses.
Figure 4.14: Fiber Optic cable
The light is "guided" down the center of the fiber called the "core". The core is surrounded by an optical material called the "cladding" that traps the light in the core using an optical technique called "total internal reflection." The core and cladding are usually made of ultra-pure glass, although some fibers are all plastic or a glass core and plastic cladding. The fiber is coated with a protective plastic covering called the "primary buffer coating" that protects it from moisture and other damage.
Transparent glass or plastic fibers, which allows light to be guided from one end to the other with minimal loss.
Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a light-emitting diode (LED)) or a laser. The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signals.
While fiber optic cable itself has become cheaper over time - an equivalent length of copper cable cost less per foot but not in capacity. Fiber optic cable connectors and the equipment needed to install them are still more expensive than their copper counterparts.
The bandwidth of a fiber-optic cable depends on the distance as well as the frequency. Bandwidth is usually expressed in frequency distance form, for example in MHz-km. In other words, a 500-MHz-km fiber-optic cable can transmit a signal a distance of 5 kilometers at a frequency of 100 MHz (5 x 100 = 500), or a distance of 50 kilometers at a frequency of 10 MHz (50 x 10 = 500). In other words, there is an inverse relationship between frequency and distance for transmission over fiber-optic cables.
Propagation Mode
There are two different modes for propagating light along optical channels: multimode and single mode. There are two basic types of fiber: multimode fiber and single-mode fiber.
Figure 4.15: Propagation modes
Multimode
Multimode is so named because multiple beams from a light source move through the core in different paths. Multimode cable is made of glass fibers, with a common diameters in the 50-to-100 micron range for the light carry component (the most common size is 62.5).
Multimode fiber gives you high bandwidth at high speeds over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300 nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 meter), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission.
In multimode step-index fiber, the density of the core remains constant from the center to the edges.
Figure 4.16 : Multimode, Step-index fiber
A beam of light moves through this constant density in straight line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt change to a lower density that alters the angle of beam’s motion. The term step-index refers to the suddenness of this change.
Step-index multimode fiber has a large core, up to 100 microns in diameter. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance. It is less costly variety of multimode fiber, it uses a wide core with a uniform index of refraction, causing the light beams to reflect in mirror fashion off the inside surface of the core by the process of total internal reflection. Because light can take many different paths down the cable and each path takes a different amount of time, signal distortion can result when step-index fiber is used for long cable runs. Use this type only for short cable runs.
A second type of fiber, called multimode graded index fiber, decreases this distortion of the signal through the cable. The word index here refers to the index of refraction.
Figure 4.17 : Multimode, graded-index fiber
Index of refraction is related to density. A graded-index fiber, therefore, is one with varying density. Density is highest at the center of the core and decreases gradually to its lowest at the edge.
Graded-index multimode fiber contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Also, rather than zigzagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow, but straight rays in the core axis. The result, a digital pulse suffers less dispersion.
Single Mode
Single mode uses step-index fiber and a highly focused source of light that limits beams to small range of angles, all close to the horizontal.
Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission. Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550 nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. Single-mode fiber is also called as mono-mode optical fiber, single-mode optical waveguide, unimode fiber.
The single mode fiber is manufactured with a much smaller diameter than that of multimode fibers, and with substantially lower density (index of refraction).
The decrease in density results in a critical angle that is close enough to 90 degrees to make the propagation of beams almost horizontal.
Figure 4.18 : Single-mode fiber
Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.
Fiber Sizes
Optical fibers are defined by the ratio of the diameter of their core to the diameter of their cladding, both expressed in microns (micrometer).
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Table 4.3 : Fiber Types
The last size listed is used only for single mode. Single mode fiber has a very small core causing light to travel in a straight line and typically has a core size of 8 or 10 microns.
Multimode fiber supports multiple paths of light and has a much larger core and has a core size of 50 or 62.5 microns.
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Light Sources for Optical Fiber
The purpose of fiber-optic cable is to contain and direct a beam of light from source to destination.
For transmission to occur, the sending device must be equipped with a light source and the receiving device with a photosensitive cell (called a photodiode) capable of translating the received light into current usable by a computer.
The light source can be either a light-emitting diode (LED) or an injection laser diode (ILD).
Figure 4.19: Light source in fiber optic cable
LEDs are the cheaper source, but they provide unfocused light that strikes the boundaries of the channel at uncontrollable angles and diffuses over distance. For this reason, LEDs are limited to short-distance use. Modulation bandwidth of LED is up to 100–200 MHz.
Problems with LEDs : Light-emitting volume is large, poor coupling efficiency to fibers, low carrier density.
Lasers, on the other hand, can be focused to a very narrow range, allowing control over the angle of incidence. Laser signals preserve the character of the signal over considerable distances. Laser stands for Light Amplification by Stimulated Emission of Radiation (LASER).
Every laser has a range of optical wavelengths, and the speed of light in fused silica (fiber) varies with the wavelength of the light. Since a pulse of light from the laser usually contains several wavelengths, these wavelengths tend to get spread out in time after traveling some distance in the fiber. The refractive index of fiber decreases as wavelength increases, so longer wavelengths travel faster. The net result is that the received pulse is wider than the transmitted one, or more precisely, is a superposition of the variously delayed pulses at the different wavelengths.
Applications of fiber-optic cable
- Fiber-optic cable is often found in backbone networks because its wide b/w is cost-effective. SONET n/w provides such backbone.
- Some cable TV companies use a combination of optical-fiber and coaxial cable.
- Telephone companies also using optical-fiber cable.
- Local Area Networks (LANs) such as 100BaseFx network (Fast Ethernet) and 1000Base-X also use fiber-optic cable.
Advantages of fiber-optic cable
1.Higher Bandwidth : Higher data rate than TP & coaxial cable.
2.Less signal attenuation: Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for 50 km without requiring regeneration. We need repeaters after every 5km for coaxial or TP cable.
3.Noise resistance : Because fiber-optic transmission uses light rather than electricity, noise is not a factor. External light, the only possible interference, is blocked from the channel by the outer jacket.
4.Light weight : Fiber-optic cables are much lighter than copper cables.
5.More immune to tapping (or Security) : Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antennas that can easily be tapped.
6.Optical fiber can carry thousands of times more information than copper wire. For example, a single-strand fiber strand could carry all the telephone conversations in the United States at peak hour. Fiber is more lightweight than copper. Copper cable equals approximately 80 lbs/1000 feet while fiber weighs about 9 lbs/1000 feet.
7.Reliability : Fiber is more reliable than copper and has a longer life span.
8.Fiber optic cable can carry signals for longer distance without repeater than co-axial cable.
Disadvantages of fiber-optic cable.
- Installation/maintenance expertise : Installation and maintenance need expertise that is not yet available everywhere.
- Unidirectional : Propagation of light is unidirectional.
- Cost : Fiber-optic cable is more expensive.
- Fragility : Glass fiber is more easily broken than wire, making it less useful for applications where h/w portability is required.
- Limited physical arc of cable of cable. Bend it too much and it will break!
Trade-offs between electrical and optical cable
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- Electrical is cheaper, especially for short distances, because silicon circuits can send and receive over wires directly. Other semiconductor materials are required to implement the lasers for optical communication. Thus, optical requires multiple die, and has a higher base cost.
- Optical provides better performance at high-bandwidths and long distances. Glass propagates light better than copper propagates electrical currents.
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Following table shows the comparison of guided media w.r.t the bandwidth.
Table 4.5 : Cable types Vs bandwidth used
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4.3 PERFORMANCE OF TRANSMISSION MEDIUM
Transmission media are roads on which data travel. To measure the performance of transmission media three concepts are used : throughput, propagation speed, and propagation time.
Throughput
The throughput is the measurement of how fast data can pass through a point.
If we consider any point in the transmission medium as a wall through, which bits pass, throughput is the number of bits that can pass this wall in one second.
1111000001 111000101011110011100011
Propagation speed
Propagation speed measures the distance a signal or a bit can travel through a medium in one second. The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. For example, in a vacuum, light is propagated with a speed of 3 108 m/s. It is lower in air. It is much lower in a cable. In fiber-optic cable the speed is 2 108 m/s.
Propagation Time
Propagation time measures the time required for a signal (or a bit) to travel from one point of the transmission medium to another.
Propagation time is calculated as
Propagation time = Distance / Propagation speed
Time t1
Time t2
Figure 4.21 : Propagation time
4.4 UNDUIDED MEDIA
Unguided media, or wireless communication, transport electromagnetic waves without using a physical conductor. Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media.
Signals are broadcast through air and thus are available to anyone who has a device capable of receiving them. In wireless communication, transmission and reception are achieved using an antenna. Transmitter sends out the electromagnetic signal into the medium. Receiver picks up the signal from the surrounding medium.
Wireless transmission can be divided into three groups: radio waves, microwave, and infrared waves. The section of the electromagnetic spectrum defined as radio communication is divided into eight ranges, called bands.
These bands are rated form very low frequency (VLF) to extremely high frequency (EHF).
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Extremely High Frequency |
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Super High Frequency |
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Ultra High Frequency |
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Very High Frequency |
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High Frequency |
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Middle Frequency |
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Low Frequency |
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Very Low Frequency |
Figure 4.22 : Radio Communication band
Satellite communication systems use UHF (Ultra High Frequency) or SHF (Super High Frequency) microwaves.
RADIO WAVES
Radio wave transmission utilizes five different types of propagation: surface (or ground), tropospheric, ionospheric, line-of-sight, and space.
Radio technology considers the earth as surrounded by two layers of atmosphere : the troposphere and the ionosphere.
The troposphere is the portion of the atmosphere extending outward 30 miles from the earth’s surface. Clouds, wind, temperature variation, and whether in general occur in the troposphere. The ionosphere is the layer of atmosphere above troposphere but below space.
Surface (or ground) propagation : Radio waves travel through the lowest portion of the atmosphere, hugging the earth. Distance cover by these signals depends on the amount of power in the signal: the greater the power, the greater the distance. The radio wave travels along the Earth's surface as a result of currents flowing in the ground. This is the dominant mechanism at low frequencies. e.g. Radio 4 = 1500m 200kHz. Surface propagation uses VLF (Very Low Frequency) & LF (Low Frequency) bands.
Tropospheric Propagation: It can work two ways. Either a signal can be directed in a straight line from antenna to antenna (line-of-sight), or it can be broadcast at an angle into the upper layers of the troposphere where it is reflected back down to the earth’s surface. Tropospheric propagation uses MF (Middle Frequency) band.
Ionospheric Propagation : In ionospheric propagation, higher-frequency radio waves radiate upward into the ionosphere where they reflected back to earth. Radio waves can be reflected from the ionosphere. Example of total internal reflection, the refractive index gradually increases with height. The return wave can in turn be reflected back up again. The gap between the ionosphere and the ground acts as a waveguide. Inospheric propagation uses HF (High Frequency) band.
Line-of-Sight Propagation: In line-of-sight propagation, very high frequency signal are transmitted in straight line directly from antenna to antenna. Antennas must be directional, facing each other, and either tall enough or close enough together not to be affected by the curvature of the earth. Example of line-of sight system is microwave link using dishes and towers. A 60m-tower gives 60 km line of sight. e.g. Gas Board in Regent Road. Satellite communication is an extreme example of line-of-sight radio links. One tower is of height 35600km. Line – of – sight propagation uses VHF (Very High Frequency) & UHF (Ultra High Frequency) bands.
Space Propagation: Space propagation utilizes satellite relays in place of atmosphere refraction. A broadcast signal is received by an orbiting satellite, which rebroadcasts the signal to the intended receiver back on the earth.
Radio waves, particularly those waves that propagate in sky mode, can travel long distance. This makes radio waves a good candidate for long-distance broadcasting such, as AM, FM radio.
Applications of Radio waves
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- Radio waves are used for multicast communication in which there is one sender but many receivers, such as AM and FM radio, television, cordless phone, and paging system.
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MICROWAVE
Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves. Microwaves do not follow the curvature of the earth and therefore require line-of-sight transmission and reception equipment. The distance coverable by a line-of-sight signal depends to a large extent on the height of antenna : the taller the antennas, the longer the sight distance. Height allows the signal to travel farther without being stopped by the curvature of the planet and raises the signal above many surface obstacles, such as low hills and tall buildings. Typically, antennas are mounted on towers that are in turn often mounted on hills or mountains. Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused.
To increase the distance a system of repeaters can be installed with each antenna. A signal received by one antenna can be converted back into transmittable form and relayed to the next antenna.
Figure 4.23: Microwave
Antennas used in microwave communications
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- Microwaves need unidirectional antennas that send out signals in one direction.
- Two types of antennas are used for microwave communications : the parabolic dish and the horn.
- A parabolic dish antenna is based on the geometry of a parabola: every line parallel to the line of symmetry (line of sight) reflects off the curve at angles such that they intersect in a common point called the focus.
- The parabolic dish works as a funnel, catching a wide range of waves & directing them to a common point.
- A horn antenna looks like a gigantic scoop. Outgoing transmission are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head.
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Applications of Microwaves
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- Microwaves, due to their unidirectional properties, are very useful when unicasting (one-to-one) communication is needed between the sender and the receiver.
- Microwaves are used in cellular phones, satellite networks, and wireless LANs.
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Advantages of Microwave :
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- They require no right of way acquisition between towers.
- They can carry high quantities of information due to their high operating frequencies.
- Low cost land purchase: each tower occupies only a small area.
- High frequency/short wavelength signals require small antennae.
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Disadvantages of microwave:
- Attenuation by solid objects: birds, rain, snow and fog.
- Reflected from flat surfaces like water and metal.
- Diffracted (split) around solid objects.
- Refracted by atmosphere, thus causing beam to be projected away from receiver.
Advantages of microwave over fiber optics :
- No Need to dedicate complete physical path on land.
- Putting up simple "tower" cheaper than laying cables.
- Some frequency band do not need licensing to use.
INFRARED
Infrared signals, with frequencies from 300 GHz to 400 GHz can be used for short-range communication.
Infrared signals cannot penetrate walls. This advantageous characteristic prevents interference between one system and another: a short-range communication system in one room cannot be affected by another system in the next room.
When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors.
This characteristic makes infrared signals useless for long-distance communication.
We cannot use infrared waves outside a building because the sun’s rays contain infrared waves that can interfere with the communication.
No licensing is required for infrared signals, that is, no frequency allocation issues with infrared signals
- Infrared (or milimeter) waves characteristics :
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- Used by remote controls for TV, VCRs, etc.
- Cheap and easy to build.
- Straight line, no obstacles - even more so than microwaves.
- Used for wireless LANs within a room.
- Applications of Infrared
- Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.
- The infrared band, almost 400 THz, has an excellent potential for data transmission.
- The Infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established standard for using these signals for communications between devices such as keyboards, mice, PCs, and printers.
- For example, some manufactures provide a special port called the IrDA port that allows a wireless keyboard to communicate with a PC.
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- Satellite Communication
Not so long ago, satellites were exotic, top-secret devices. They were used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, television transmission by DIRECTV and the DISH Network, and everyday telephone calls. In many other instances, satellites play a background role that escapes our notice :
- Some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution.
- Before sending signals down the wire into our houses, cable television depends on satellites to distribute its transmissions.
- Guided Missiles use the satellite-based Global Positioning System (GPS) to track the proper destination.
- Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal.
What is a Satellite ?
Satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth's original, natural satellite, and there are many manmade (artificial) satellites, usually closer to Earth. The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee. Artificial satellites generally are not mass-produced. Most satellites are custom built to perform their intended functions. Exceptions include the GPS (Global Positioning System) satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).
Although anything that is in orbit around Earth is technically a satellite, the term "satellite" is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. We commonly hear about weather satellites, communication satellites and scientific satellites. The Soviet Sputnik satellite was the first to orbit Earth, launched on October 4, 1957.
Some Examples of Artificial satellites :
- 1957 - launch of SPUTNIK 1, Low Earth orbit (LEO), 200 to 600 km, period 90mins.
- 1958-64 - early developments mainly related to space race !
- TELSTAR I elliptical orbit 960 to 6080 km, period 2 hr 38 mins.
- 1965 - INTELSAT I (Early Bird). First geosynchronous satellite that provided a routine link between USA and Europe for 4 years
INTELSAT - International Telecommunications Satellite Organization. More than 110 countries are members of this organization. The INTELSAT is responsible for providing communication links between its members - hires out a service.
In satellite transmission signals travel in straight lines, the limitations imposed on distance by the curvature of the earth are reduced.
Satellite communication is an extreme example of line-of-sight radio links. One tower is of height 35600km.
Satellite relays allow microwave signals to span continents and oceans with a single bounce. Satellite communication systems use UHF (Ultra High Frequency) or SHF (Super High Frequency) microwaves. This ensures that they penetrate the ionosphere and provides a large bandwidth.
A satellite network is a combination of nodes that provides communication from one point on the earth to another. A node in the network can be satellite, an earth station, or an end-user terminal or telephone.
Although a real satellite, such as the moon, can be used as a relaying node in the network, the use of artificial satellite is preferred because we can install electronic equipment on the satellite to regenerate the signal that has lost its energy during travel. The relay function of the satellite communications system is to receive the up-link signal from the ground, amplify it, change its frequency and retransmit it to the ground. Another restriction on using natural satellites is their distances from the earth, which create a long delay in communication.
Satellite can provide transmission capability to and from any location on earth, no matter how remote. This advantage makes high quality communication available to undeveloped parts of the world without requiring a huge investment in ground-based infrastructure.
Physical description
- Communication satellite is a microwave relay station between two or more ground stations (also called earth stations).
- Satellite uses different frequency bands for incoming (uplink) and outgoing (downlink) data.
- A single satellite can operate on a number of frequency bands, known as transponder channels or transponders.
- Geosynchronous orbit (35,784 km).
- Satellites cannot be too close to each other to avoid interference : This limits the number of available satellites.
Transmission characteristics
- Optimum frequency range in 1 to 10 GHz.
- Below 1 GHz, significant noise from galactic, solar, and atmospheric noise, and terrestrial electronic devices.
- Most satellites use 5.925 to 6.425 GHz band for uplink and 4.2 to 4.7 GHz band for downlink.
- Propagation delay of about a quarter second due to long distance.
- Problems in error control and flow control.
- Inherently broadcast, leading to security problems.
Orbits
An artificial satellite needs to have an orbit, the path in which it travels around the earth.
Geosynchronous orbits (also called synchronous or equatorial-orbit) are orbits in which the satellite is always positioned over the same spot on Earth.
A geosynchronous orbit is one for which the orbital period of the spacecraft is the time taken for the Earth to complete 360o rotation.
Geostationary orbits
This is a special case of the geosynchronous orbit. In such an orbit the satellite remains above the same point on the ground all the time. Geostationary orbits are 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earth's rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out into space. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The "satellite parking strip" area over the equator is becoming congested with several hundred television, weather and communication satellites ! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite's signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.
The scheduled Space Shuttles use a much lower, asynchronous (or inclined) orbit, which means they pass overhead at different times of the day. Other satellites in asynchronous orbits average about 400 miles (644 km) in altitude.
In a polar orbit, the satellite generally flies at a low altitude and passes over the planet's poles on each revolution. The polar orbit remains fixed in space as Earth rotates inside the orbit. As a result, much of Earth passes under a satellite in a polar orbit. Because polar orbits achieve excellent coverage of the planet, they are often used for satellites that do mapping and photography
Artificial satellites which orbit the earth follow the same laws that govern the motion of the planets around the sun. Johannes Kepler (1571-1630) was derived law called as Kepler’s law, describes planetary motion. The period of a satellite, the time required for a satellite to make a complete trip around the earth, is determined by Kepler’s law, which defines the period as a function of the distance of the satellite from the center of the earth.
Period = C distance1.5
Where C is a constant approximately equal to 1 /100. The period is in seconds and the distance in kilometers.
- Example 1 : What is the period of the moon according to Kepler’s law ?
Solution :
The moon is located approximately 3,84,000 km above earth.
The radius of the earth is 6378 km.
Period = C distance1.5
=(1/100) (3,84,000 + 6378) 1.5
=24,39,090 sec
=1 month
- Example 2 : According to Kepler’s law, what is period of a satellite that is located at an orbit approximately 35,786 km above the earth?
Solution :
Period = C distance1.5
=(1/100) (35,786 + 6378) 1.5
=86,579 sec
=24 hrs
This means that a satellite located at 35,786 km has a period of 24 hrs, which is the same as the rotation period of the earth. A satellite like this is said to be stationary to the earth.
4.6 Geostationary Satellite
The point 36,000 km from the Earth's surface, the gravitational pull of the Earth and the centrifugal force of Earth's rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out into space. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The "satellite parking strip" area over the equator is becoming congested with several hundred televisions, weather and communication satellites ! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite's signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.
4.7 COMPARISONS
Comparison of Step Index and Graded Index Fiber
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Comparison of Satellite Communication and Optical Communication
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Comparison of Single mode and Multimode fiber
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Comparison of LED and Laser Diode
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Comparison of Guided medias
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Comparison of Wired and Wireless Media
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4.8 Summary
Transmission medium
- Physical path between transmitter and receiver.
- May be guided (wired) or unguided (wireless).
- Communication achieved by using EM waves.
- Characteristics and quality of data transmission.
- Dependent on characteristics of medium and signal.
Guided medium.
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- Medium is more important in setting transmission parameters.
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Unguided medium.
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- Bandwidth of the signal produced by transmitting antenna is important in setting transmission parameters.
- Signal directionality.
- Lower frequency signals are omnidirectional.
- Higher frequency signals can be focused in a directional beam.
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There 4 basic types of Guided Media : Open Wire, Twisted Pair, Coaxial Cable, and Optical Fiber. Coaxial cable (or coax) carries signals of higher frequency ranges than twisted-pair cable. A fiber-optic cable is made of glass or plastic and transmits signals in the form of light.
4.9 PRACTICE SET
Review Questions
- Is the transmission media a part of the physical media? Why or why not?
- Name two major categories of transmission media.
- How do guided media differs from unguided media?
- Write a short note on TP cable
- Write a short note on coaxial cable
- Write a short note on Fiber optic Cable.
- Explain Advantages and Disadvantages of TP, coaxial and Fiber optic cable.
- Give a use for each class of guided media.
- What is major advantages of STP over UTP ?
- What is the significance of the twisting in TP cable?
- Why is coaxial cable is superior to TP cable ?
- What is the cladding in an optical cable ?
- How does the sky propagation differ from line-of-sight propagation ?
- What is an IrDA port ?
- Explain various categories of UTP cable and their use.
- Write a short note on light sources and detectors used in optical cable.
- Write a short note on Kepler’s laws and orbital aspect.
- Write a short note on Geostationary Satellite.
- Explain different types of unguided media.
Multiple Choice Questions
- Transmission media are usually categorized as _______.
- fixed or unfixed
- guided or unguided
- determinate or indeterminate
- metallic or nonmetallic
- Transmission media lie below the _______ layer.
- Physical
- Data Link Layer
- Network
- Transport
- [[Image:]] In fiber optics, the signal is _______ waves.
- light
- radio
- infrared
- microwave
- In copper cable, the signal is --------- waves.
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- Light
- Electric
- Infrared
- microwave
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- Which of the following primarily uses guided media?
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- cellular telephone system
- local telephone system
- satellite communication
- radio broadcasting
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- Which of the following is not a guided medium?
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- twisted-pair
- coaxial
- fiber-optic
- atmosphere
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- [[Image:]]In an optical fiber, the inner core is _______ the cladding.
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- denser than
- less dense than
- same density
- another name for
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- The inner core of an optical fiber is _______ in composition.
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- glass or plastic
- copper
- liquid
- bimetallic
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- When a beam of light travels through media of two different densities, if the angle of incidence is greater than the critical angle, _______ occurs.
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- refraction
- reflection
- incidence
- criticism
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- _____ medium provides a physical conduit from one device to another.
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- guided
- unguided
- either (A) or (B)
- none of the above
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- ________ cable consists of two insulated copper wires twisted together.
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- coaxial
- fiber
- twisted pair
- none of the above
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- ______ cables are composed of a glass or plastic inner core surrounded by cladding, all encased in an outside jacket.
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- fiber
- coaxial
- twisted-pair
- none of the above
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- ______ cables carry data signals in the form of light.
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- fiber
- coaxial
- twisted-pair
- none of the above
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- In a fiber-optic cable, the signal is propagated along the inner core by _______.
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- refraction
- modulation
- reflection
- none of the above
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- _________ media transport electromagnetic waves without the use of a physical conductor.
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- guided
- unguided
- either a or b
- none of the above
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- ________ are used for short-range communications such as those between a PC and a peripheral device.
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- radio waves
- microwaves
- infrared waves
- none of the above
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