Media Assignment

ASSIGNMENT 

 

Q1. What is transmission media? Describe different types of transmission media with the help of examples?

TRANSMISSION MEDIA:

The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.

• Bounded/Guided Media
• Unbounded/Unguided Media

1.      BOUNDED MEDIA:

Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are

• Coaxial Cable
• Twisted Pairs Cable
• Fiber Optics Cable

COAXIAL CABLE:

Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.

Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.

Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.

Outside the copper mesh is the final protective cover. (as shown in Fig)

The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.

Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.

Q2. Describe different types of cable medium?

Network media is the actual path over which an electrical signal travels as it moves from one component to another. This chapter describes the common types of network media, including twisted-pair cable, coaxial cable, fiber-optic cable, and wireless.

TWISTED-PAIR CABLE:

Twisted-pair cable is a type of cabling that is used for telephone communications and most modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields. Twisting the wires can enhance this cancellation effect. Using cancellation together with twisting the wires, cable designers can effectively provide self-shielding for wire pairs within the network media.

Two basic types of twisted-pair cable exist: unshielded twisted pair (UTP) and shielded twisted pair (STP). The following sections discuss UTP and STP cable in more detail.

UTP CABLE:

UTP cable is a medium that is composed of pairs of wires (see Figure 8-1). UTP cable is used in a variety of networks. Each of the eight individual copper wires in UTP cable \is covered by an insulating material. In addition, the wires in each pair are twisted around each other.

Figure 8-1 Unshielded Twisted-Pair Cable

UTP cable relies solely on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by electromagnetic interference (EMI) and radio frequency interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. UTP cable must follow precise specifications governing how many twists or braids are permitted per meter (3.28 feet) of cable.

UTP cable often is installed using a Registered Jack 45 (RJ-45) connector (see Figure 8-2). The RJ-45 is an eight-wire connector used commonly to connect computers onto a local-area network (LAN), especially Ethernets.

Figure 8-2 RJ-45 Connectors

When used as a networking medium, UTP cable has four pairs of either 22- or 24-gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms; this differentiates it from other types of twisted-pair wiring such as that used for telephone wiring, which has impedance of 600 ohms.

UTP cable offers many advantages. Because UTP has an external diameter of approximately 0.43 cm (0.17 inches), its small size can be advantageous during installation. Because it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when installing a network in an older building. UTP cable is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. And because UTP can be used with most of the major networking architectures, it continues to grow in popularity.

Disadvantages also are involved in using twisted-pair cabling, however. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber-optic cables.

Although UTP was once considered to be slower at transmitting data than other types of cable, this is no longer true. In fact, UTP is considered the fastest copper-based medium today. The following summarizes the features of UTP cable:

·         Speed and throughput—10 to 1000 Mbps

·         Average cost per node—Least expensive

·         Media and connector size—Small

·         Maximum cable length—100 m (short)

Commonly used types of UTP cabling are as follows:

·         Category 1—Used for telephone communications. Not suitable for transmitting data.

·         Category 2—Capable of transmitting data at speeds up to 4 megabits per second (Mbps).

·         Category 3—Used in 10BASE-T networks. Can transmit data at speeds up to 10 Mbps.

·         Category 4—Used in Token Ring networks. Can transmit data at speeds up to 16 Mbps.

·         Category 5—Can transmit data at speeds up to 100 Mbps.

·         Category 5e —Used in networks running at speeds up to 1000 Mbps (1 gigabit per second [Gbps]).

·         Category 6—Typically, Category 6 cable consists of four pairs of 24 American Wire Gauge (AWG) copper wires. Category 6 cable is currently the fastest standard for UTP.

SHIELDED TWISTED-PAIR CABLE:

Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire twisting. Each pair of wires is wrapped in a metallic foil (see Figure 8-3). The four pairs of wires then are wrapped in an overall metallic braid or foil, usually 150-ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the cable (EMI and RFI). STP usually is installed with STP data connector, which is created especially for the STP cable. However, STP cabling also can use the same RJ connectors that UTP uses.

Figure 8-3 Shielded Twisted-Pair Cable

Although STP prevents interference better than UTP, it is more expensive and difficult to install. In addition, the metallic shielding must be grounded at both ends. If it is improperly grounded, the shield acts like an antenna and picks up unwanted signals. Because of its cost and difficulty with termination, STP is rarely used in Ethernet networks. STP is primarily used in Europe.

The following summarizes the features of STP cable:

·         Speed and throughput—10 to 100 Mbps

·         Average cost per node—Moderately expensive

·         Media and connector size—Medium to large

·         Maximum cable length—100 m (short)

When comparing UTP and STP, keep the following points in mind:

·         The speed of both types of cable is usually satisfactory for local-area distances.

·         These are the least-expensive media for data communication. UTP is less expensive than STP.

·         Because most buildings are already wired with UTP, many transmission standards are adapted to use it, to avoid costly rewiring with an alternative cable type.

Q3. Describe different types of wireless media with the help of example?

Transmission of waves take place in the electromagnetic (EM) spectrum. The carrier frequency of the data is expressed in cycles per second called hertz(Hz). Low frequency signals can travel for long distances through many obstacles but cannot carry a high bandwidth of date while high frequency signals can travel for shorter distances through few obstacles and carry a narrow bandwidth. Also the noise effect on the signal is inversely proportional to the power of the radio transmitter. The three broad categories of wireless media are: Radio - 10 Khz to 1 Ghz. It is broken into many bands including AM, FM, and VHF bands. The Federal communications Commission (FCC) regulates the assignment of these frequencies. Frequencies for unregulated use are: 902-928Mhz - Cordless phones, remote controls. 2.4 Ghz 5.72-5.85 Ghz Microwave Terrestrial - Used to link networks over long distances but the two microwave towers must have a line of sight between them. The frequency is usually 4-6GHz or 21-23GHz. Speed is often 1-10Mbps. The signal is normally encrypted for privacy. Two nodes may exist. Satellite - A satellite orbits at 22,300 miles above the earth which is an altitude that will cause it to stay in a fixed position relative to the rotation of the earth. This is called a geosynchronous orbit. A station on the ground will send and receive signals from the satellite. The signal can have propagation delays between 0.5 to 5 seconds due to the distances involved. The transmission frequency is normally 11-14GHz with a transmission speed in the range of 1-10Mbps. Infared - Infared is just below the visible range of light between 100Ghz and 1000Thz. A light emitting diode (LED) or laser is used to transmit the signal. The signal cannot travel through objects. Light may interfere with the signal. The types of infared are Point to point - Transmission frequencies are 100GHz-1,000THz . Transmission is between two points and is limited to line of sight range. It is difficult to eavesdrop on the transmission. The speed is 100Kbps to 16Mbps broadcast - The signal is dispersed so several units may receive the signal. The unit used to disperse the signal may be reflective material or a transmitter that amplifies and retransmits the signal. Normally the speed is limited to 1Mbps. The transmission frequency is normally 100GHz-1,000THz with transmission distance in 10's of meters. Installation is easy and cost is relatively inexpensive for wireless. Terms: Wireless bridge - Microwave or infrared is used between two line of site points where it is difficult to run wire. CDPD - Cellular Digital Packet Data will allow network connections for mobile users using satellites.

CATEGORIES OF LAN RADIO COMMUNICATIONS:

• Low power, single frequency - Distance in 10s of meters. Speed in 1-10Mbps. Susceptible to interference and eavesdropping.
• High power, single frequency - Require FCC licensing and high power transmitter. Speed in 1-10Mbps. Susceptible to interference and eavesdropping.
• Spread spectrum - It uses several frequencies at the same time. The frequency is normally 902-928MHz with some networks at 2.4GHz. The speed of 902MHz systems is between 2 and 6Mbps. If frequency-hopping is used, the speed is normally lower than 2Mbps. Not as susceptible to EMI, but still susceptible. Two types are:
1. Direct sequence modulation - The data is broken into parts and transmitted simultaneously on multiple frequencies. Decoy data may be transmitted for better security. The speed is normally 2 to 6 Mbps.
2. Frequency hopping - The transmitter and receiver change predetermined frequencies at the same time (in a synchronized manner). The speed is normally 1Gbps.

WIRELESS MEDIA COMPARISONS

Media	Speed	Approx Cost/station
900 MHz Spread spectrum	2-6Mbps	$5000
6 GHz Spread Spectrum	2-6Mbps	$1000
23Ghz Microwave	6Mbps at 50 kilometers	$15000
Intra building Infrared	10Mbps	$400
Medium distance inter building Infrared	10Mbps at 500 meters	$5000
Long distance inter building Infrared	155Mbps at 500 meters, Less than 20Mbps at 1200 meters.	$8000

Q4. Describe different types of wireless media with the help of example?

INTRODUCTION TO NAVAL WEAPONS ENGINEERING

Propagation of Waves:

        The process of communication involves the transmission of information from one location to another.  As we have seen, modulation is used to encode the information onto a carrier wave, and may involve analog or digital methods. It is only the characteristics of the carrier wave which determine how the signal will propagate over any significant distance.  This chapter describes the different ways that electromagnetic waves propagate.

Basics:

        An electromagnetic wave is created by a local disturbance in the electric and magnetic fields.  From its origin, the wave will propagate outwards in all directions. If the medium in which it is propagating (air for example) is the same everywhere, the wave will spread out uniformly in all directions.

Figure 1

Far from its origin, it will have spread out enough that it will appear have the same amplitude everywhere on the plane perpendicular to its direction of travel (in the near vicinity of the observer). This type of wave is called a plane wave. A plane wave is an idealization that allows one to think of the entire wave traveling in a single direction, instead of spreading out in all directions.

Figure 2

Electromagnetic waves propagate at the speed of light in a vacuum. In other mediums, like air or glass, the speed of propagation is slower. If the speed of light in a vacuum is given the symbol c₀, and the speed in some a medium is c, we can define the index of refraction, n as: 

n = c₀ / c

Here's a short table of the indices of refraction for common media:

Substance		Index Of Refraction
Vacuum		1
Air		1.0003
Water		1.33
Glass		1.55

Reflection:

        When a plane wave encounters a change in medium, some or all of it may propagate into the new medium or be reflected from it. The part that enters the new medium is called the transmitted portion and the other the reflected portion.  The part which is reflected has a very simple rule governing its behavior.  Make the following construction:

Figure 3

Angle of Incidence = the angle between the direction of propagation and a line perpendicular to the boundary, on the same side of the surface. Angle of Reflection = the angle between the direction of propagation of the reflected wave and a line perpendicular to the boundary, also on the same side of the surface. Then the rule for reflection is simply stated as: 

The angle of reflection = The angle of incidence

Figure 4

If the incident medium has a lower index of refraction then the reflected wave has a 180⁰ phase shift upon reflection. Conversely, if the incident medium has a larger index of refraction the reflected wave has no phase shift.

Refraction:

        When the wave enters the new medium, the speed of propagation will change. In order to match the incident and transmitted wave at the boundary, the transmitted wave will change its direction of propagation.  For example, if the new medium has a higher index of refraction, which means the speed of propagation is lower, the wavelength will become shorter (frequency must stay the same because of the boundary conditions).  For the transmitted wave to match the incident wave at the boundary, the direction of propagation of the transmitted wave must be closer to perpendicular. The relationship between the angles and indices of refraction is given by Snell's Law:

n_i sin_I = n_t sin_t

        When the direction of propagation changes, the wave is said to refract. It is most useful to know in which direction the wave will refract, not necessarily by how much.

Figure 5

The transmitted wave will bend more towards the perpendicular when entering a medium with higher index of refraction (slower speed of propagation). Example: Why a pool is deeper than it looks. 

When you look into a pool, the light from the bottom is refracted away from the perpendicular, because the index of refraction in air is less than in water.  To the observer at the side of the pool, the light appears to come from a shallower depth. For the same reason, when you look at objects underwater through a mask, they will appear to be larger than they really are.  The light from the object is spread outwards at the water-air interface of your mask.  To you it will appear the object is closer or larger.

Interference

        All electromagnetic waves can be superimposed upon each other without limit. The electric and magnetic fields simply add at each point.  If two waves with the same frequency are combined there will a be a constant interference pattern caused by their superposition.  Interference can either be constructive, meaning the strength increases as result, or destructive where the strength is reduced. The amount of interference depends of the phase difference at a particular point. It can be shown that constructive interference occurs for phase differences of 0-120⁰, and 240-360⁰.   Thus destructive interference occurs from 120-240⁰.  For two identical waves, no phase shift results in total constructive interference, where the strength

is maximum and 180⁰ phase shift will create total destructive interference (no signal at all).

The phase shift that causes the interference can occur either due to a difference in path length, x, or a difference in the arrival time, t. The amount of phase shift, , can be computed for these two cases by: Example: Omega is a radio navigation system that used the phase difference in the same signal from two fixed transmitters to determine a line-of-position. The same phase difference corresponded to multiple lines of position separated by a distance equivalent to 360⁰ of phase shift. Since the frequency was 10.2 kHz, the wavelength corresponding to 360⁰ phase shift was 16 miles, which was the lane width on an Omega overprinted chart. Loran-C also has a phase-difference mode with a lane width of only 3000 m since it operates at 100 kHz. 

Diffraction

        Recall that the idealized plane wave is actually infinite in extent.  If this wave passes through an opening, called an aperture, it will diffract, or spread out, from the opening.  The degree to which the cropped wave will spread out depends on the size of the aperture relative to the wavelength.  In the extreme case where the aperture is very large compared to the wavelength, the wave will see no effect and will not diffract at all.  At the other extreme, if the opening is very small, the wave will behave as if it were at its origin and spread out uniformly in all directions from the aperture.  In between, there will be some degree of diffraction.

Figure 6

First consider a circular aperture. If a wave with wavelength  encounters an opening with diameter D, the amount of diffraction as measured by the angle,  ,at which the new wave diverges from the opening, measured from edge to edge, will be approximated by 

 = /D

Comments on this formula:

The symbol is used to indicate an approximate result, as opposed to an exact relationship. This result is only valid for relatively small angles, something less than about 20⁰.

There is some ambiguity in what is meant by this angle. In actuality, the wave does not simply end at this boundary, but falls off smoothly. The exact point which defines the extent of the wave is a matter of definition and there are two standard conventions: the one used in this book is the ½ power or -3 dB definition. The factor of 2 is also an approximation. A more accurate description requires detailed knowledge about the shape of the aperture The angle  must be in radians! This formula will be quite handy for the study of beam-forming, and is worth committing to memory.  It has applications to antennas used in communications and radar.  It also applies to sonar arrays and electro-optics.

Antennas

        Antennas couple the current flowing in wires or waveguides into electromagnetic waves in the air. The most basic form of the antenna is the dipole antenna.

The Dipole Antenna

        This is nothing more than a straight piece wire.  When voltage is applied to the wire, current flows and the electrical charges pile up in either end.  A balanced set of positive and negative charges separated by some distance is called a dipole. The dipole moment is equal to the charge times the distance by which it is separated.

Figure 7

When an alternating voltage is applied the antenna, dipole moment oscillates up and down on the antenna, corresponding to the current. The oscillating current creates oscillating electric (E) and magnetic (H) fields which in turn generate more electric and magnetic fields. Thus a outward propagating electromagnetic wave is created. The electric field is oriented along the axis of the antenna and the magnetic field is perpendicular to both the electric field and the direction of propagation. The orientation of the fields is called the polarization.

Figure 8

When the electromagnetic wave passes over a conducting material, it will create an oscillating current in it. Therefore, the same design (i.e. the dipole antenna) can be used to extract signals from the air as current in a circuit. For a center-fed dipole antenna to work most effectively, it should be exactly one-half wavelength long. Receiving antennas which do not require high sensitivity need not follow this rule. Transmitting antennas on the other generally do, except at very low frequencies. When the antenna is placed in the ground, called a ground-plane antenna, the optimum size is reduced by half again, due to signal reflection at the ground plane. This appears to make an image antenna of equal size below the ground which reduces the actual antenna requirement. So for ground-plane antennas, the optimum size is one-quarter wavelength.

Figure 9

Example: Find the optimum antenna size for a ground-plane dipole used to broadcast commercial AM radio (approximately 1 MHz). The wavelength at 1 MHz is 300m, so the optimum antenna should be about 75 m tall. 

Polarization

        In the case of a dipole antenna, the electric field in aligned with the antenna axis and remains so as it propagates.  When the field remains in a particular direction the wave is considered to be linearly polarized.  For practical reasons, its orientation is usually resolved into a vertical and horizontal component. A vertical dipole antenna will create a vertical, linearly polarized electromagnetic wave.  A receiving antenna that is also aligned vertically will generate the largest current when exposed to the electromagnetic wave.  Therefore it is imperative to known the polarization type and direction of the transmitting antenna when trying to receive the signal.  

Example:  Commercial radio broadcasts come from large vertically oriented antennas. Therefore they are linearly vertically polarized signals and are best received by a vertical antenna.  So to maximize reception of a radio signal, hold the antenna upright.

Linear Polarization

        Linear polarization is not the only possibility.  Another type is circular polarization.  The best way to visualize this is like a corkscrew.  The electric field rotates as it travels along.  If the rotation is clockwise as seen looking in the direction of propagation, it is called right-hand circular polarization (RHCP). The other possibility is LHCP. Transmitting antennas for circular polarization are generally look like corkscrews.  Circular polarization is often used in satellite communications because it is not required to know the orientation of the satellites antenna (called skew).  Linear polarized signal from space are also subject to a rotation caused by the Earth's magnetic field called Faraday Rotation.  Circular polarization is not affected.

Figure 10

A wave need not be polarized. For example, sunlight is a homogeneous mixtures of waves will all orientations. It is said to be un-polarized. It can however, become polarized either by filtering and on reflection from a flat surface. Example: polarized sunglasses. 

        When sunlight is reflected off the road it appears as glare.  In the process of reflection the light becomes horizontally polarized.  Sunglasses with vertical polarization block this component and therefore reduce glare.  These glasses can easily be checked to see if they are polarized by holding two pair at right angles.  In this case, all possible orientations of linear polarization will be blocked and the lenses will appear opaque.

Antenna Beam-Forming

        The dipole antenna we have been discussing radiates energy in all directions perpendicular to its axis.  There is no signal coming from the ends.  In this sense, the dipole antenna has some directionality, or preferred direction.  In cases where high sensitivity is required or when it is necessary to exclude transmission or reception from unnecessary directions, antennas can be made even more directional. The process of creating directionality is called beam-forming.  It has applications in radar and sonar as well.  

        Beam-forming should be understood as the exploitation of interference.  For Example, consider two identical receiving dipole antennas, both oriented vertically in the ground.  Neither one will receive signals at any significant vertical angle. However, each one by itself, has no preferred direction in the horizontal plane. Suppose now that they are separated by exactly one-half of a wavelength of the signal they are receiving.  If the signal comes from a direction along the line that connects them, there will be a 180⁰ phase shift inserted between them, which will cause complete destructive interference.  Therefore they cannot receive signals along the line connecting them.

Figure 11

If the signal is coming from a direction perpendicular to the line connecting them, there will be equal path lengths and therefore complete constructive interference. They will be more sensitive along the directions perpendicular to the line connecting them. Therefore, the two-dipole antenna array is directional in the horizontal plane. As it turns out, the three-dipole antenna linear array is even more directional. There is no reception from the directions along the axis, and a more narrow region perpendicular to the array from which they receive strongly. The width of good reception is called the beam width. For a many-dipole linear array, the beam width gets smaller proportionally. If the overall array length is L, the beam width can be predicted theoretically:  /L. This looks just like the diffraction theory. In fact, it should. The model for deriving diffraction is to sum the results from many little "antennas" across the aperture. In the limit of many antennas this is exactly the linear array. These results may be immediately extended to any antenna shape. The beam width is simply found from the diffraction theory.

Figure 12

Example: Direct Satellite TV. This system uses an 18" dish to receive signals from a geo-synchronous satellites which are located at 101 W near the equator. The signal is Ku-band at about 12.5 GHz (2.4 cm wavelength), and is circularly polarized. The beam width of the 18" (44 cm) is 2.4/44 = 0.05 radians or 3⁰. This would imply that the antenna should be positioned within less than three degrees of the line of sight to the satellite. The beam width is made as small as possible to maximize the sensitivity of the antenna. A larger dish would have smaller beam width and therefore would be more sensitive but would require a more accurate aim.

Modes of Propagation in Air

The Electromagnetic Spectrum

Frequency Range	Band Designation
30-3000 Hz	ELF
3-30 kHz	VLF
30-300 kHz	LF
300-3000 kHz	MF
3-30 MHz	HF
30-300 MHz	VHF
300-3000 MHz	UHF
3-30 GHz	SHF
30-300 GHz	EHF

Ground Wave

        Radio waves in the VLF band propagate in a ground, or surface wave.  The wave is connected at one end to the surface of the earth and to the ionosphere at the other. The ionosphere is the region above the troposphere (where the air is), from about 50 to 250 miles above the earth.  It is a collection of ions, which are atoms that have some of their electrons stripped off leaving two or more electrically charged objects. The sun's rays cause the ions to form which slowly recombine.  The propagation of radio waves in the presence of ions is drastically different than in air, which is why the ionosphere plays an important role in most modes of propagation.  Ground waves travel between two limits, the earth and the ionosphere, which acts like a duct.  Since the duct curves with the earth, the ground wave will follow.  Therefore very long range propagation is possible using ground waves.

Figure 13

Sky Waves

        Radio waves in the LF and MF ranges may also propagate as ground waves, but suffer significant losses, or are attenuated, particularly at higher frequencies. But as the ground wave mode fades out, a new mode develops: the sky wave. Sky waves are reflections from the ionosphere.  While the wave is in the ionosphere, it is strongly bent, or refracted, ultimately back to the ground. From a long distance away this appears as a reflection.  Long ranges are possible in this mode also, up to hundreds of miles.  Sky waves in this frequency band are usually only possible at night, when the concentration of ions is not too great since the ionosphere also tends to attenuate the signal.  However, at night, there are just enough ions to reflect the wave but not reduce its power too much.

Figure 14

The HF band operates almost exclusively with sky waves. The higher frequencies have less attenuation and less refraction in the ionosphere as compared to MF. At the high end, the waves completely penetrate the ionosphere and become space waves. At the low end, they are always reflected. The HF band operates with both these effects almost all of the time. The characteristics of the sky wave propagation depend on the conditions in the ionosphere which in turn are dependent on the activity of the sun. The ionosphere has several well-defined regions in altitude.

Figure 15

D-region: about 75-95 km. Relatively weak ionization. Responsible for strong absorption of MF during daylight E-region: 95-150 km. An important player in ionospheric scatter of VHF. F-region: 150-400 km. Has separate F1 and F2 layers during the day. The strongest concentration of ions. Responsible for reflection of HF radio waves. Since the propagation characteristics depend on frequency, several key frequencies can de defined: Critical frequency: The minimum frequency that will penetrate the ionosphere at vertical incidence. The critical frequency increases during the daylight and decrease at night. At other angles, the wave will be reflected back. At frequencies above the critical frequency, some range of waves from vertical incidence and down will become space waves. This will cause a gap in coverage on the ground known as a skip zone. In figure xx, the skip zone extends to about 1400 miles. The transmitted frequency was 5 MHz and the critical frequency was 3 MHz in this example. Maximum Useable Frequency (MUF): defined for two stations. The maximum frequency that will reflect back to the receiving station from the transmitter. Beyond the MUF, the wave will become a space wave. At MUF the skip zone extends to just short of the receiver. In figure xx, the MUF for a receiver at 1400 miles is 5 MHz. Lowest Useable Frequency (LUF): again defined for two stations. At low frequencies, the signal will be attenuated before it can be reflected. The LUF increases with sunlight and is a maximum near noon. Optimum Frequency for Traffic (OFT): for two stations, taking into account the exact conditions in the ionosphere, there will be the perfect frequency that gives the strongest signal. This can be predicted by powerful modeling programs and is the best guarantee of success in HF. The diurnal variation if HF propagation is characterized a simple rule-of-thumb: the frequency follows the sun. At noon, the OFT is generally higher than at night.

Line of Sight:

In the VHF band and up, the propagation tends to straighten out into line-of-sight(LOS) waves.  However the frequency is still low enough for some significant effects. Ionospheric scatter. The signal is reflected by the E-region and scattered in all directions. Some energy makes it back to the earth's surface. This seems to be most effective in the range of 600-1000 miles.

Figure 16

Tropospheric scatter. Again, the wave is scattered, but this time, by the air itself. This can be visualized like light scattering from fog. This is a strong function of the weather but can produce good performance at ranges under 400 miles.

Figure 17

Tropospheric ducting. The wave travels slower in cold dense air than in warm air. Whenever inversion conditions exist, the wave is naturally bent back to the ground. When the refraction matches the curvature of the earth, long ranges can be achieved. This ducting occurs to some extend always and improves the range over true the line-of-sight by about 10 %.

Diffraction. When the wave is block by a large object, like a mountain, is can diffract around the object and give coverage where no line-of-sight exists.

        Beyond VHF, all the propagation is line-of-sight.  Communications are limited by the visual horizon.  The line-of-sight range can be found from the height of the transmitting and receiving antennas by:

where h_t and h_r are the heights of the antennas in meters, and R will be in km (the

conversion factor is already taken into account in the factor 13).

Q5.Describe different types of wireless media with the help of example?

In telecommunications and computer networks, multiplexing (known as muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire. It originated in telegraphy, and is now widely applied in communications. The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side. A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX). Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original

data stream.

Techniques of Multiplexing:

A multiplexing technique may be further extended into a multiple access method or channel access method, for example TDM into Time-division multiple access (TDMA) and statistical multiplexing into carrier sense multiple access (CSMA). A multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity. Multiplexing is provided by the Physical Layer of the OSI model, while multiple access also involves a media access control protocol, which is part of the Data Link Layer.

Q6.Describe FDM with the help of example?

Frequency Division Multiplexing:

Horak (1996) [2] explain that Frequency Division Multiplexing (FDM) can be described
by dividing the single high-capacity channel into several smaller-capacity
channels (sub-channel). Each sub channel transmits data simultaneously using
different frequency so that each sub-channel has its own frequency to use and
is not affecting other subchannels.

A radio is A good example to explain how FDM works. Note, that we
are only using one broad range of radio frequency and there are several radio
stations broadcasting its service using different frequency. All we need to do is
to adjust the radio to catch only certain radio broadcast on certain frequency.
(Fitzgerald 1002, p.96) [1].

According to Horak (1996) [2] FDM has drawback by dedicating such frequency
to several smaller circuits even though the designated channel is not
using it.

Figure 2 illustrate how FDM works by dividing one channel into several frequencies including the Guard-band act as delimiter for each logical sub-channel
so that the interference from other sub-channel using the same physical circuit
can be minimized. For example, in Figure 2, the multiplexed circuit is divided
into 4 frequencies. Channel #1 using 0-800 Hz for its data transfer and delimited
by 200 Hz Guard-band. Channel #2 using 1000-1800 Hz and delimited by
200 Hz too; and so on.

Figure 2: Frequency Division Multiplexing

In regards to speed, we simply need to divide the main circuit amongst
the available sub channels. For example, if we have a 64 Kbps physical circuit
and wanted to use 4 sub-channel, each sub-channel will have 16 Kbps. However,
Guard-band is also using this 64 Kbps physical circuit and therefore each channel
will be using only 15 Kbps with 4 Guard-bands (1 Kbps per Guard-band).
This calculation is subject to change while there are many ways to define the
bandwidth for sub channels and Guard-bands. (Fitzgerald 1002, pp.96-97) [1].

Horak (1996) [2] give the example of FDM usage in digital communication
area. It is currently use on broadband Local Area Network, cellular radio, and
some digitized voice applications.

 

Time Division Multiplexing:

Time-division multiplexing (TDM) is a method of putting multiple data
streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled
at the receiving end based on the timing.

~~~ THE END ~~~

Posted in: