GUIDED MEDIAGuided media, which are those that provide a conduitfrom one device to another, include twisted-pair cable,coaxial cable, and fiber-optic cable.
Overview The transmission media that are used to convey information can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable, and optical fiber. Unguided media employ an antenna for transmitting through air, vacuum, or water. 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, key concerns are data rate and distance: the greater the data rate and distance the better.
Data Rate and Bandwidth Any transmission system has a limited band of frequencies This limits the data rate that can be carried
Design Factors Bandwidth higher bandwidth gives higher data rate Transmission impairments eg. attenuation Interference Number of receivers in guided media more receivers introduces more attenuation
Guided Media – Twisted-pair Cable Twisted-pair cable
Twisted pair One of the wires carries signal, the other is used only as a ground reference. The receiver uses the difference b/w the two levels. Twisting increases the probability that both wires are effected by the noise in the same manner, thus the difference at the receiver remains same. Therefore, number of twists per unit length determines the quality of the cable.
Twisted Pair - TransmissionCharacteristics analog needs amplifiers every 5km to 6km digital can use either analog or digital signals needs a repeater every 2-3km limited distance limited bandwidth (1MHz) limited data rate (100MHz) susceptible to interference and noise
Unshielded Versus Shielded Twisted-Pair Cable UTP and STP cables
Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference
Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy)
Near End Crosstalk coupling of signal from one pair to another occurs when transmit signal entering the link couples back to receiving pair ie. near transmitted signal is picked up by near receiving pair
Coaxial cable Inner conductor is a solid wire outer conductor serves both as a shield against noise and a second conductor
Coaxial Cable Applications Most versatile medium Television distribution Long distance telephone transmission Can carry 10,000 voice calls simultaneously Short distance computer systems links Local area networks
Coaxial Cable - TransmissionCharacteristics superior frequency characteristics to TP performance limited by attenuation & noise analog signals amplifiers every few km closer if higher frequency up to 500MHz digital signals repeater every 1km closer for higher data rates
Guided Media – Coaxial Cable Categories of coaxial cables
BNC connectors BNC = Bayone-Neill- Concelman BNC Connector is used to connect the end of the cable to a device BNC T is used in networks to branch out a cable for connection to a computer or other device BNC Terminator is used at the end of the cable to prevent the reflection of signal.
Guided Media – Coaxial Cable Applications: Analog telephone networks Cable TV networks Traditional Ethernet LAN – 10Base2, 10Base5
Guided Media – Fiber-Optic CableFiber-optic cable transmit signals in the form of light. Bending of light ray
Bending of light ray Angle of Incidence (I): the angle the ray makes with the line perpendicular to the interface between the two substances Critical Angle: the angle of incidence which provides an angle of refraction of 90-degrees.
Optical Fiber - Benefits greater capacity data rates of hundreds of Gbps smaller size & weight lower attenuation electromagnetic isolation greater repeater spacing 10s of km at least
Optical Fiber - Benefits The following characteristics distinguish optical fiber from twisted pair or coaxial cable: Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is immense; data rates of hundreds of Gbps over tens of kilometers have been demonstrated. Compare this to the practical maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to 100 Mbps to 10 Gbps over a few tens of meters for twisted pair. Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable or bundled twisted-pair cable. For cramped conduits in buildings and underground along public rights-of-way, the advantage of small size is considerable. The corresponding reduction in weight reduces structural support requirements. Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair, and is constant over a wide range. Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers do not radiate energy, so there is little interference with other equipment and there is a high degree of security from eavesdropping. In addition, fiber is inherently difficult to tap. Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error. The performance of optical fiber systems from this point of view has been steadily improving. Repeater spacing in the tens of kilometers for optical fiber is common, and repeater spacings of hundreds of kilometers have been demonstrated.
Optical Fiber - TransmissionCharacteristics uses total internal reflection to transmit light effectively acts as wave guide for 1014 to 1015 Hz can use several different light sources Light Emitting Diode (LED) cheaper, wider operating temp range, lasts longer Injection Laser Diode (ILD) more efficient, has greater data rate relation of wavelength, type & data rate
Guided Media – Fiber-Optic Cable Propagation Modes
Guided Media – Fiber-Optic Cable Propagation Modes
Wireless Transmission Frequencies 2GHz to 40GHz microwave highly directional point to point satellite 30MHz to 1GHz omnidirectional broadcast radio 3 x 1011 to 2 x 1014 infrared local
Broadcast Radio radio is 3kHz to 300GHz use broadcast radio, 30MHz - 1GHz, for: FM radio UHF and VHF television is omnidirectional still need line of sight suffers from multipath interference reflections from land, water, other objects
Unguided Media – Radio Waves Omnidirectional Antenna Frequencies between 3 KHz and 1 GHz. are used for multicasts communications, such as radio and television, and paging system.
Terrestrial Microwave used for long haul telecommunications and short point-to-point links requires fewer repeaters but line of sight use a parabolic dish to focus a narrow beam onto a receiver antenna 1-40GHz frequencies higher frequencies give higher data rates main source of loss is attenuation distance, rainfall also interference
Unguided Media – Microwaves Frequencies between 1 and 300 GHz. Used for unicast communication such as cellular phones, satellite networks and wireless LANs. Unidirectional Antenna
Satellite Microwave satellite is relay station typically requires geo-stationary orbit height of 35,784km spaced at least 3-4° apart typical uses television long distance telephone private business networks global positioning
Unguided Media – Infrared Frequencies between 300 GHz to 400 THz. Can not penetrate walls. Used for short-range communication in a closed area using line-of-sight propagation.
Infrared modulate noncoherent infrared light end line of sight (or reflection) are blocked by walls no licenses required typical uses TV remote control IRD port
Antennas electrical conductor used to radiate or collect electromagnetic energy transmission antenna radio frequency energy from transmitter converted to electromagnetic energy byy antenna radiated into surrounding environment reception antenna electromagnetic energy impinging on antenna converted to radio frequency electrical energy fed to receiver same antenna is often used for both purposes
Radiation Pattern power radiated in all directions not same performance in all directions as seen in a radiation pattern diagram an isotropic antenna is a (theoretical) point in space radiates in all directions equally with a spherical radiation pattern
Antenna Gain measure of directionality of antenna power output in particular direction verses that produced by an isotropic antenna measured in decibels (dB) results in loss in power in another direction effective area relates to size and shape related to gain
Line of Sight Transmission Free space loss loss of signal with distance Atmospheric Absorption from water vapour and oxygen absorption Multipath multiple interfering signals from reflections Refraction bending signal away from receiver
Comparison of Media Medium Cost Speed Atten Interfere Security UTP Low 1-100M High High Low STP Medium 1-150M High Medium Low Coax Medium 1M–1G Medium Medium Low Fibre High 10M–2G Low Low High Radio Medium 1-10M Varies High Low Microw High 1M–10G Varies High Medium Satellite High 1 M–10G Varies High Medium Cellular High 9.6–19.2K Low Medium Low