Transmission media.........................................................................
Connecting Devices........................................................................
Error detection and correction........................................................
Flow and Error Control...................................................................
Piggy backing.................................................................................
IEEE standard.................................................................................
Transmission Media:
Media are roughly grouped into guided media, such as copper wire
and fiber optics, and unguided media, such as terrestrial wireless, satellite, and
lasers through the air.
Guided Transmission Media
The purpose of the physical layer is to transport bits from one machine to another. Various physical media can be used for the actual transmission.
Magnetic Media
Fig:- Magnetic Media – Audio Casette and Floppy Disk
One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or removable media.
e.g., recordable DVDs. physically transport the tape or disks to the destination machine, and read them back in again. Although this method is not as sophisticated as using a geosynchronous communication satellite, it is often more cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor.
A simple calculation will make this point clear. An industry-standard Ultrium tape can hold 800 gigabytes. A box 60 × 60 × 60 cm can hold about 1000 of these tapes, for a total capacity of 800 terabytes, or 6400 terabits (6.4 petabits). The effective bandwidth of this transmission is 6400 terabits/86,400 sec, or a bit over 70Gbps. If the destination is only an hour away by road, the bandwidth is increased to over 1700 Gbps. No computer network can even approach this. Of course, networks are getting faster, but tape densities are increasing too.
Twisted Pair Wire
Fig :- Unshielded Twisted Pair and Shielded Twisted Pair Wire
Although the bandwidth characteristics of magnetic tape are excellent, the de lay characteristics are poor. Transmission time is measured in minutes or hours, not milliseconds. For many applications an online connection is needed. One of the oldest and still most common transmission media is twisted pair. A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form, just like a DNA molecule. Twisting is done because two parallel wires constitute a fine antenna. When the wires are twisted,the waves from different twists cancel out, so the wire radiates less effectively. Asignal is usually carried as the difference in voltage between the two wires in thepair. This provides better immunity to external noise because the noise tends toaffect both wires the same, leaving the differential unchanged.The most common application of the twisted pair is the telephone system. Nearly all telephones are connected to the telephone company (telco) office by a twisted pair. Both telephone calls and ADSL Internet access run over these lines.
Twisted pairs can run several kilometers without amplification, but for longer dis-tances the signal becomes too attenuated and repeaters are needed. When many twisted pairs run in parallel for a substantial distance, such as all the wires coming from an apartment building to the telephone company office, they are bundled together and encased in a protective sheath. The pairs in these bundles would interfere with one another if it were not for the twisting. In parts of the world where telephone lines run on poles above ground, it is common to see bundles several centimeters in diameter.
Twisted pairs can be used for transmitting either analog or digital information.The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec can be achieved for a few kilometers in many cases. Due to their adequate performance and low cost, twisted pairs are widely used and are likely to remain so for years to come.
Twisted-pair cabling comes in several varieties. The garden variety deployed in many office buildings is called Category 5 cabling, or ‘‘Cat 5.’’ A category 5 twisted pair consists of two insulated wires gently twisted together. Four such pairs are typically grouped in a plastic sheath to protect the wires and keep them together. Different LAN standards may use the twisted pairs differently. For example, 100-Mbps Ethernet uses two (out of the four) pairs, one pair for each direction.
#Note: Shielded Twisted Pair wire has an additional inner Copper shield for Protection. Unshielded Twisted Pair wire has only outer PVC Shield.
Coaxial Cable
Fig:- Co-Axial Cable
Another common transmission medium is the coaxial cable It has better shielding and greater bandwidth than unshielded twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the start. The other kind, 75-ohm cable, is commonly used for analog transmission and cable television. This distinction is based on historical, rather than technical,factors (e.g., early dipole antennas had an impedance of 300 ohms, and it was easy to use existing 4:1 impedance- matching transformers). Starting in the mid-1990s, cable TV operators began to provide Internet access over cable, which hasmade 75-ohm cable more important for data communication.
A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely woven braided mesh. The outer conductor is covered in a protective plastic sheath. The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable quality and length. Modern cables have a bandwidth of up to a few GHz. Coaxial cables used to be widely used within the telephone system for long-distance lines but have now largely been replaced by fiber optics on long haul routes. Coax is still widely used for cable television and metropolitan area networks, however.
Power Lines
Fig:- PowerLine for signal transmission
The telephone and cable television networks are not the only sources of wiring that can be reused for data communication. There is a yet more common kind of wiring: electrical power lines. Power lines deliver electrical power to houses,and electrical wiring within houses distributes the power to electrical outlets.The use of power lines for data communication is an old idea. Power lines have been used by electricity companies for low-rate communication such as remote metering for many years, as well in the home to control devices (e.g., the X10 standard). In recent years there has been renewed interest in high-rate communication over these lines, both inside the home as a LAN and outside the home for broadband Internet access. We will concentrate on the most common scenario:using electrical wires inside the home.
The convenience of using power lines for networking should be clear. Simply
plug a TV and a receiver into the wall, which you must do anyway because they need power, and they can send and receive movies over the electrical wiring. There is no other plug or radio.The data signal is superimposed on the low-frequency power signal (on the active or ‘‘hot’’wire) as both signals use the wiring at the same time.
The difficulty with using household electrical wiring for a network is that itwas designed to distribute power signals. This task is quite different than distributing data signals, at which household wiring does a horrible job. Electrical signals are sent at 50–60 Hz and the wiring attenuates the much higher frequency(MHz) signals needed for high-rate data communication. The electrical properties of the wiring vary from one house to the next and change as appliances are turned on and off, which causes data signals to bounce around the wiring. Transient currents when appliances switch on and off create electrical noise over a wide range of frequencies. And without the careful twisting of twisted pairs, electrical wiring acts as a fine antenna, picking up external signals and radiating signals of its own.
This behavior means that to meet regulatory requirements, the data signal must exclude licensed frequencies such as the amateur radio bands.
Despite these difficulties, it is practical to send at least 100 Mbps over typical
household electrical wiring by using communication schemes that resist impaired frequencies and bursts of errors. Many products use various proprietary standards for power-line networking, so international standards are actively under development.
Fibre Optic Cable
Fig:- Fibre Optic Cable
Fiber optic cables are similar to coax, except without the braid. shows a single fiber viewed from the side. At the center is the glass core through which the light propagates. In multimode fibers, the core is typically 50 microns in diameter, about the thickness of a human hair. In single-mode fibers, the core is 8 to 10 microns. The core is surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the core. Next comes a thin plastic jacket to protect the cladding. Fibers are typically grouped in bundles, protected by an outer sheath. Terrestrial fiber sheaths are normally laid in the ground within a meter of the surface, where they are occasionally subject to attacks by backhoes or gophers. Near the shore, transoceanic fiber sheaths are buried in trenches by a kind of seaplow. In deep water, they just lie on the bottom, where they can be snagged by fishing trawlers or attacked by giant squid. Fibers can be connected in three different ways.
First, they can terminate in connectors and be plugged into fiber sockets. Connectors lose about 10 to 20% of the light, but they make it easy to reconfigure systems.
Second, they can be spliced mechanically. Mechanical splices just lay the two carefully cut ends next to each other in a special sleeve and clamp them in place. Alignment can be improved by passing light through the junction and then making small adjustments to maximize the signal. Mechanical splices take trained personnel about 5 minutes and result in a 10% light loss.
Third, two pieces of fiber can be fused (melted) to form a solid connection. A fusion splice is almost as good as a single drawn fiber, but even here, a small amount of attenuation occurs.
For all three kinds of splices, reflections can occur at the point of the splice,
and the reflected energy can interfere with the signal. Two kinds of light sources are typically used to do the signaling. These are LEDs (Light Emitting Diodes) and semiconductor lasers. They have different properties, as shown in Fig. 2-9. They can be tuned in wavelength by inserting Fabry-Perot or Mach-Zehnder interferometers between the source and the fiber. Fabry-Perot interferometers are simple resonant cavities consisting of two parallel mirrors. The light is incident perpendicular to the mirrors. The length of the cavity selects out those wavelengths that fit inside an integral number of times.
Mach-Zehnder interferometers separate the light into two beams. The two beams
travel slightly different distances. They are recombined at the end and are in
phase for only certain wavelengths.The receiving end of an optical fiber consists of a photodiode, which gives off an electrical pulse when struck by light. The response time of photodiodes, which convert the signal from the optical to the electrical domain, limits data rates to about 100 Gbps. Thermal noise is also an issue, so a pulse of light must carry enough energy to be detected. By making the pulses powerful enough, the error rate can be made arbitrarily small.
Wireless Transmission
Three general ranges of frequencies are of interest in our discussion of wireless transmission. Frequencies in the range of about 1 GHz 1gigahertz = 10 9 Hertz2 to 40 Ghz are referred to as microwave frequencies.At these frequencies, highly directional beams are possible, and microwave is quite suitable for point-to-point transmission. Microwave is also used for satellite communications. Frequencies in the range of 30 MHz to 1 Ghz are suitable for omnidirectional applications. We refer to this range as the radio range. Another important frequency range, for local applications, is the infrared portion of the spectrum. This covers, roughly, from 3 * 10 11 to 2 * 10 14 Hz. Infrared is useful to local point-to-point and multipoint applications within confined areas, such as a single room.
For unguided media, transmission and reception are achieved by means of an
antenna. Before looking at specific categories of wireless transmission, we provide a
brief introduction to antennas.
Antennas
An antenna can be defined as an electrical conductor or system of conductors usedeither for radiating electromagnetic energy or for collecting electromagnetic energy.For transmission of a signal, radio-frequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water). For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio-frequency
electrical energy and fed into the receiver.
In two-way communication, the same antenna can be and often is used for
both transmission and reception. This is possible because any antenna transfers
energy from the surrounding environment to its input receiver terminals with the
same efficiency that it transfers energy from the output transmitter terminals into
the surrounding environment, assuming that the same frequency is used in both
directions. Put another way, antenna characteristics are essentially the same whether
an antenna is sending or receiving electromagnetic energy.
An antenna will radiate power in all directions but, typically, does not perform
equally well in all directions. A common way to characterize the performance of an
antenna is the radiation pattern, which is a graphical representation of the radiation
properties of an antenna as a function of space coordinates. The simplest pattern is
produced by an idealized antenna known as the isotropic antenna. An isotropic
antenna is a point in space that radiates power in all directions equally. The actual
radiation pattern for the isotropic antenna is a sphere with the antenna at the center.
Parabolic Reflective Antenna
An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. The fixed point is called the focus and the fixed line is called the directrix If a parabola is revolved about its axis, the surface generated is called a paraboloid. A cross section through the paraboloid parallel to its axis forms a parabola and a cross section perpendicular to the axis forms a circle. Such surfaces are used in headlights, optical and radio telescopes, and microwave antennas because of the following property: If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the paraboloid. In theory, this effect creates a parallel beam without dispersion. In practice, there will be some dispersion, because the source of energy must occupy more than one point. The larger the diameter of the antenna, the more tightly directional is the beam. On reception, if incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus.
Antenna Gain
Antenna gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna).A concept related to that of antenna gain is the effective area of an antenna.The effective area of an antenna is related to the physical size of the antenna and toits shape. The relationship between antenna gain and effective area is
G= 4 ∏ Ae = 4 ∏ Æ’ 2Ae
λ2 C2
G = Antenna gain
Ae = Effective Area
Æ’ = Carrieer Frequency
C = Speed of Light (approx. 3 * 10 8 m/s2)
λ = Carrier Wavelength
Terrestrial Microwave
Physical Description
The most common type of microwave antenna is the parabolic “dish.” A typical size is about 3 m in diameter. The antenna is fixed rigidly and focuses a narrow beam to achieve line-of-sight transmission to the receiving antenna. Microwave antennas are usually located at substantial heights above ground level to extend the range between antennas and to be able to transmit over intervening obstacles. To achieve long-distance transmission, a series of microwave relay towers is used, and point-to-point microwave links are strung together over the desired distance.
Fig:- Parabolic Dish
Applications
The primary use for terrestrial microwave systems is in long-haul telecommunications service, as an alternative to coaxial cable or optical fiber. The
microwave facility requires far fewer amplifiers or repeaters than coaxial cable over
the same distance but requires line-of-sight transmission. Microwave is commonly
used for both voice and television transmission. Another increasingly common use of microwave is for short point-to-point links between buildings. This can be used for closed-circuit TV or as a data link between local area networks. Short-haul microwave can also be used for the so called bypass application. A business can establish a microwave link to a long distance telecommunications facility in the same city, bypassing the local telephone company.
Transmission Characteristics
Microwave transmission covers a substantial portion of the electromagnetic spectrum. Common frequencies used for transmission are in the range 1 to 40 GHz. The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate. As with any transmission system, a main source of loss is attenuation. For microwave (and radio frequencies), the loss can be expressed as
L=10 log 4∏d 2dB
λ
where,
d is the distance
λ is the wavelength.
The most common bands for long-haul telecommunications are the 4-Ghz to 6-GHz bands. With increasing congestion at these frequencies, the 11-GHz band is now coming into use. The 12-GHz band is used as a component of cable TV systems. Microwave links are used to provide TV signals to local CATV installations, the signals are then distributed to individual subscribers via coaxial cable. Higher frequency microwave is being used for short point-to-point links between buildings typically, the 22-GHz band is used. The higher microwave frequencies are less useful
for longer distances because of increased attenuation but are quite adequate for shorter distances. In addition, at the higher frequencies, the antennas are smaller and cheaper.
Satellite Microwave
Physical Description
A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground-based microwave transmitter/receivers, known as earth stations, or ground stations. The satellite receives transmissions on one frequency band (uplink), amplifies or repeats the signal , and transmits it on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called transponder channels, or simply transponders.
In the first, the satellite is being used to provide a point-to-point link between two distant ground-based antennas. In the second, the satellite provides communications between one ground-based transmitter and a number of ground-based receivers. For a communication satellite to function effectively, it is generally required that it remain stationary with respect to its position over the earth. Otherwise, it would not be within the line of sight of its earth stations at all times. To remain stationary, the satellite must have a period of rotation equal to the earth’s period of rotation. This match occurs at a height of 35,863 km at the equator.
Two satellites using the same frequency band, if close enough together, will
interfere with each other. To avoid this, current standards require a 4° spacing
(angular displacement as measured from the earth) in the 4/6-GHz band and a 3°
spacing at 12/14 GHz. Thus the number of possible satellites is quite limited.
Applications
The following are among the most important applications for satellites:
Television distribution
Long-distance telephone transmission
Private business networks
Global positioning
Broadcast Radio
Physical Description The principal difference between broadcast radio and
microwave is that the former is omnidirectional and the latter is directional. Thus
broadcast radio does not require dish-shaped antennas, and the antennas need not
be rigidly mounted to a precise alignment.
Applications Radio is a general term used to encompass frequencies in the range
of 3 kHz to 300 GHz. We are using the informal term broadcast radio to cover the
VHF and part of the UHF band: 30 MHz to 1 GHz. This range covers FM radio and
UHF and VHF television. This range is also used for a number of data networking
applications.
Transmission Characteristics
The range 30 MHz to 1 GHz is an effective one for broadcast communications. Unlike the case for lower-frequency electromagnetic waves, the ionosphere is transparent to radio waves above 30 MHz. Thus transmission is limited to the line of sight, and distant transmitters will not interfere with each other due to reflection from the atmosphere. Unlike the higher frequencies of the microwave region, broadcast radio waves are less sensitive to attenuation from rainfall. Because of the longer wavelength, radio waves suffer relatively less attenuation. A prime source of impairment for broadcast radio waves is multipath interference. Reflection from land, water, and natural or human-made objects can create multiple paths between antennas. This effect is frequently evident when TV reception displays multiple images as an airplane passes by.
Infrared
Infrared communications is achieved using transmitters/receivers (transceivers)that modulate noncoherent infrared light. Transceivers must be within the line ofsight of each other either directly or via reflection from a light-colored surface suchas the ceiling of a room.One important difference between infrared and microwave transmission is that the former does not penetrate walls. Thus the security and interference problems encountered in microwave systems are not present. Furthermore, there is no frequency allocation issue with infrared, because no licensing is required.
Wireless Propagation
A signal radiated from an antenna travels along one of three routes: ground wave,sky wave, or line of sight (LOS). Table 4.7 shows in which frequency range each predominates. In this book, we are almost exclusively concerned with LOS communication, but a short overview of each mode is given in this section.
Fig:-Wireless Propagation modes
Ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz. Several factors account for the tendency of electromagnetic wave in this frequency band to follow the earth’s curvature. One factor is that the electromagnetic wave induces a current in the earth’s surface, the result of which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the earth’s curvature. Another factor is diffraction, which is aphenomenon having to do with the behavior of electromagnetic waves in the presence of obstacles. Electromagnetic waves in this frequency range are scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere.
The best-known example of ground wave communication is AM radio.
Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere(ionosphere) back down to earth. Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact caused by refraction. Refraction is described subsequently. A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth’s surface. With this propagation mode, a signal can be picked up thousands of kilometers from the transmitter.
Line-of-Sight Propagation Above 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must be by line of sight. For satellite communication, a signal above 30 MHz is not reflected by the ionosphere and therefore a signal can be transmitted between an earth station and a satellite overhead that is not beyond the horizon. For ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other. The term effective is used because microwaves are bent or refracted by the atmosphere. The amount and even the direction of the bend depends on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight.
Refraction occurs because the velocity of an electromagnetic wave is a function ofthe density of the medium through which it travels. In a vacuum, an electromagnetic wave (such as light or a radio wave) travels at approximately 3 * 10 8 m/s. This is the constant, c, commonly referred to as the speed of light, but actually referring to the speed of light in a vacuum. 2 In air, water, glass, and other transparent or partiallytransparent media, electromagnetic waves travel at speeds less than c. When an electromagnetic wave moves from a medium of one density to a medium of another density, its speed changes. The effect is to cause a one-time bending of the direction of the wave at the boundary between the two media. Moving from a less dense to a more dense medium, the wave will bend toward the more dense medium. This phenomenon is easily observed by partially immersing a stick in water.
Optical and Radio Line of Sight With no intervening obstacles, the optical
line of sight can be expressed as
d = 3.572h
where d is the distance between an antenna and the horizon in kilometers and h is
the antenna height in meters. The effective, or radio, line of sight to the horizon is
expressed as
d = 3.57 2Kh
Fig:- Optical and Radio Horizon
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