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CNT 3004

Module 4

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Chapter 7

TransmissionMedia

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Chapter 7

Transmission Media

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Transmission medium and physical layer

The transmission medium carries information from a source node to a destination node.

The transmission medium is located below the physical layer.

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Classes of transmission media

A guided medium provides a physical conduit from one device to another. Twisted-pair cable, coaxial cable, and optical fiber are the most popular types of guided media.

An unguided medium is a free space (usually air) that transports electromagnetic waves without the use of a physical conductor. Radio waves, microwaves, and infrared signals are the most popular types of unguided media.

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Guided Media

Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

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Twisted-pair cable

Twisted-pair cable consists of two insulated copper wires, each with its own plastic insulation, twisted together.

One of the wires is used to carry signal to the receiver and the other wire is used as a ground reference. The receiver uses the signal difference between the two wires.

Interference and cross talk may affect both wires and create unwanted signals. Twisting allows both wires to be equally impacted by external noise and maintain a balance in the difference.

Twisted-pair cable is used in telephone lines for voice and data communications.

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UTP and STP

The most common twisted-pair cable used in communications is referred to as UTP (Unshielded Twisted Pair).

IBM has also produced a version of twisted-pair cable called STP (Shielded Twisted Pair) that has metal shield that encases the twisted pair wires.

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Categories of unshielded twisted-pair cablesCAT 1: UTP traditionally used in telephone lines; data rate less than

1 MbpsCAT 2: improved CAT 1 UTP; commonly used in T lines; data rate up

to 4 MbpsCAT 3: improved CAT 2; commonly used in 10 Mbps Ethernet LANs;

data rate up to 16 MbpsCAT 4: improved CAT 3; used in Token Ring networks, data rate up

to 20 MbpsCAT 5: improved CAT 4; used in 100 Mbps Ethernet LANs; data rate

up to 100 MbpsCAT 5e: enhanced CAT5 UTP; used in 100 Mbps and 1000 Mbps

Ethernet LANs; data rate up to 1000 MbpsCAT 6: enhanced CAT5 UTP used in Gigabit Ethernet; data rate up

to 10,000 Mbps

CAT5 has more twists per inch than CAT3 therefore can supporthigher speeds. The "twist" effect will cause any interferencepresented/picked up on one cable to be cancelled out by the othercable which twists around the initial cable.

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The most common UTP connector is a keyed connector called RJ45 (RJ stands for Registered Jack which is a standardized physical network interface).

UTP Cable/Connector

RJ11 (telephone line) is a registered jack standard for a modular plug connector using 2 conductors and 6 positions (6P2C)

RJ45 (data line) is a registered jack standard for a modular plug connector using 8 conductors and 8 positions (8P8C)

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A twisted-pair cable can pass a wide range of frequencies. However, the UTP attenuation measured in dB per kilometer (dB/km) sharply increases above 100 KHz.

The gauge is a measure of the thickness of the wire. The American wire gauge (AWG) is a standard for the diameters of round wires. The higher the gauge the smaller the diameter and the higher the value of electrical resistance.

The thickest wire is No. 0000 AWG which is 0.4600 inches in diameter and 0.1608 ohm/km in resistance. A very fine wire is No. 36 AWG which is 0.0050 inches in diameter and 1361 ohm/km in resistance.

UTP performance

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UTP performance

The attenuation sharply increases with frequencies above 100KHz.

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Coaxial cable

Coaxial cable (or coax) can carry signals of higher frequency ranges than twisted-pair cable.

Coaxial cable is used in cable TV networks and in bus-based Ethernet LANs.

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Coaxial cable has a metallic inner conductor (usually copper) enclosed in an an insulator covering, which is in turn encased in an outer conductor of metal foil (shield). The metallic shield is encased in an insulator covering. An outside plastic cover protects the coax wire.

The outer conductor serves both as a shield against noise and as a the second conductor that completes the circuit. It is the major factor that makes coaxial cable less susceptible to noise than twisted-pair cable.

Coaxial cable

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Categories of coaxial cables

RG= Radio Guide (cable designator)

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BNC connectors

BNC (Bayone-Neill-Concelman) connectors are used to connect coaxial cables to devices.

The BNC connector connects the end of the cable to the device (e.g., TV set). The BNC-T connector is used in Ethernet networks to connect a computer to the thin Ethernet cable. The BNC terminator is placed at the end of the Ethernet cable to prevent the reflection of the signal.

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Thin Ethernet

BNC-T connectors and BNC terminators are used in thin Ethernet

BNC connectors

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Coaxial cables can carry signals of higher frequency ranges than UTP. The attenuation of the signal increases with the increase of its frequency. The signal weakens rapidly and requires the frequent use of repeaters.

Coaxial cable performance

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Optical fiber

Case 1: If the angle of incidence I is less than the critical angle, the ray of light refracts and moves closer to the interface between the two types of glass.

Case 2: If the angle of incidence I is equal to the critical angle, the ray of light moves parallel to the interface between the two types of glass.

Case 3: If the angle of incidence I is greater than the critical angle, the ray of light reflects and travels again in the denser substance.

Case 1 Case 2 Case 3

Bending of light rays between two types of glass: high-density glass and low-density glass

The value of the critical angle depends on the two types of glass

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The core of dense glass is surrounded by a cladding of less dense glass.

Fiber-optic cables carry data signals in the form of light. The signal is propagated along the inner core by reflection.

Fiber-optic transmission is popular due to its noise resistance, low attenuation, and high-bandwidth capabilities.

Fiber-optic cable is used in backbone networks, cable TV networks, Fast Ethernet and Gigabit Ethernet networks.

Optical fiber

Light emitting diodeor laser diode

Photodetectoror photodiode

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Propagation modes

Signal propagation in optical fibers can be multimode(multiple beams from a light source) or single-mode(essentially one beam from a light source).

There are two subtypes of multimode fibers: step-index multimode and graded-index multimode.

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Multimode step-index fiber has the greatest distortion. Single mode fiber has the least distortion. Distortion of multimode graded-index fiber is in the middle.

Propagation modes

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increases the distortion of the signal. In multimode graded-index fiber, the core density

decreases with distance from the center, i.e., the core is a fiber glass with varying densities. This causes a curving of the light beams and reduces distortion.

The single-mode fiber uses a highly focused light source, a step-index fiber with smaller diameter (than multimode). The beams of propagated light are almost horizontal and arrive at the destination almost at the same time, which reduces distortion significantly.

In multimode step-index fiber, the core density is constant and the light beam changes direction suddenly at the interface between the core and the cladding. The sudden reflection

Propagation Modes

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Fiber types

Optical fibers are defined by the ratio of the diameter of the core to the diameter of the cladding.

Single mode optical fiber has lower attenuation thanmultimode fiber. Because of this, single mode fiber allowsthe light signal to travel longer distances before requiringamplification. Single mode fiber allows the signal to travelfaster and farther than multimode fiber.

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Fiber construction

Fiber-optic cables are composed of a glass or plastic inner core surrounded by cladding. The fiber is surrounded by plastic coating and strong Kevlar strands, all encased in an outside jacket (made of PVC or Teflon).

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Fiber-optic cable connectors

Fiber-optic cables use three different type of connectors: SC (subscriber channel) for cable TV, ST (straight-tip) with bayonet locking for networking devices, and MT-RJ which has the same size as RJ-45.

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Wavelength and period

Wavelength of a simple signal is the distance the signal travels in one period. Unlike the frequency of the signal, the value of the wavelength depends on the medium through which the signal travels. For example, the wavelength of green light in air is less than the wavelength of green light in a fiber-optic cable.

Wavelength = propagation speed period = c T = c / f

Wavelength = propagation speed / frequency

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ExerciseConsider a light signal whose wavelength is = 1550 nanometers. Calculate the frequency f of this signal assuming that the speed of light in vacuum is approximately c 300,000 km/sec (roughly 186,000 miles/sec).

Hint: apply the formula = c / f f = c /

Use an online frequency-wavelength calculator to verify your answer.

http://www.1728.org/freqwave.htm

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Optical fiber performance

= c / f Wavelength = propagation speed / frequency

• Attenuation in fiber-optic shows a flatter behavior than in twisted pair and coaxial cables. Due to its very low attenuation, optical fiber requires fewer repeaters than coax.

• Increasing the frequency (i.e., decreasing the wavelength) does not always increase attenuation.

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Wavelength-Division Multiplexing (WDM)• In fiber-optic communications, wavelength-division

multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry different signals.

• WDM allows for a multiplication in capacity that leads to huge increase in the bandwidth provided by a single strand of fiber.

• With WDM, a single strand of fiber can offer tera (10^12) bits/second capacity. Multi-fiber cables can offer peta (10^15) bits/second and even exa (10^18) bits/second infrastructure.

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DWDM = Dense Wavelength Division Multiplexing

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Advantages of Optical fiber• Higher bandwidth• Less signal attenuation• Immunity to electromagnetic interference• Resistance to corrosive materials• Light weight• Immunity to tapping

Disadvantages of Optical fiber• Higher cost for cables and interfaces/transceivers• Unidirectional light propagation, i.e., two fibers are usually

needed • More installation overhead

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Submarine CablesThe backbone of the nation’s and of the world’s information infrastructure is

now preponderantly composed of fiber optic cables. A critical element of that backbone is the world’s ever expanding network of submarine fiber optic cables.

A submarine communications cable is a cable laid beneath the sea to carry telecommunications between countries. Submarine cables are laid using special cable layer ships. Modern submarine cables use optical fiber technology to carry digital payloads for telephone traffic as well as Internet and private data traffic.

Modern submarine optical fiber cables use solid-state optical repeaters & amplifiers. Repeaters are powered by a constant direct current passed down the conductor near the center of the cable. Power feed equipment is installed at the terminal stations, with one end providing a positive voltage and the other a negative voltage. The amplifiers or repeaters derive their power from the potential difference drop across them.

The optic fiber used in undersea cables is chosen for its exceptional clarity, permitting runs of more than 100 kilometers between repeaters to minimize the number of amplifiers.

This topic is not covered in the textbook.

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Submarine Cables (continued)Originally, submarine cables were simple point-to-point connections. Modern

cable systems now usually have their fibers arranged in a self-healing ring to increase their redundancy, with the submarine sections following different paths on the ocean floor.

Dangers Facing Submarine Cables Cables can be broken by anchors, dredging fishing nets, earthquakes, undersea

avalanches, shark bites, and are sometimes cut by enemy forces in wartime. The cables need only be bent to suffer significant damage.

Protection of Submarine CablesCables in areas closer to the shore and whenever possible in other areas are

armored and/or buried some two to three feet deep in the ocean floor. Repair of Submarine Cables Shore stations can locate a break in a cable by electrical measurements. A

repair ship is sent to the location near the break. Several types of grapples can be used to lift the broken cable to the surface whereupon a new section is spliced in. The repaired cable is longer than the original, so the excess is deliberately laid in a 'U' shape on the seabed.

http://en.wikipedia.org/wiki/Submarine_communications_cable

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Repair of Submarine Cables

Special mechanical grapple

Human diver

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Submarine Cables (continued)Repair of Submarine Cables (continued)Typically 50 repairs a year are done in the Atlantic Ocean alone. There has

been significant breaks in 2006, 2008, 2009, 2011, 2012, and 2103 causing Internet outages in various regions of the world.

A number of ports near important cable routes became homes to specialized cable repair ships.

Example: Apollo Submarine Cable SystemThis system consists of the two most advanced transatlantic fiber optic

cables. Apollo North connects the UK and the USA and Apollo South connects France and the USA.

Apollo's customers include the worlds leading telecommunications and internet companies as well as a range of other bandwidth intensive companies.

Apollo offers point to point 10 Gbit/s between the major cities and carrier POPs on the US Eastern seaboard and Western Europe.

Note: POP = Point of Presence

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Submarine Cables (continued)Submarine Cables Maps

A number of submarine cable maps exist on the Internet.

http://www.submarinecablemap.com/

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Unguided Media: Wireless

Radio Waves

Microwaves

Infrared

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Electromagnetic spectrum for wireless communication

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Propagation methods

Wireless data is transmitted through ground propagation, sky propagation, or line-of-sight propagation.

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Propagation methods

In ground propagation, lower-frequency radio waves travel through the lowest portion of the atmosphere, hugging the earth. The signals emanate in all directions from the sending antenna and follow earth’s curvature. The distance traveled increases with the increase of the amount of power of the transmitted signal.

In sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they get reflected back to earth. Sky propagation can reach greater distance at lower output power.

In line-of-sight propagation, very high-frequency signals travel in straight lines directly from the sending antenna to the receiving antenna. The antenna must be directional and facing each other.

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Bands

This slide is for information only

Ground/Sky

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Wireless transmission waves

Wireless data can be classified as radio waves, microwaves, or infrared waves.

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Radio Waves

Although there is no clear-cut demarcationbetween radio waves and microwaves,electromagnetic waves ranging in frequenciesbetween 3 kHz and 1 GHz are normally calledradio waves; waves ranging in frequencies between1 and 300 GHz are called microwaves. However,the behavior of the waves, rather than thefrequencies, is a better criterion for classification.

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Radio waves are omnidirectional. The radio wave band is under government regulation.

Radio waves are used for multicast communications, such as radio and television, and paging systems.

Wireless signals with frequencies between 3 KHz to 1 GHz are generally considered to be radio waves.

Omnidirectional Radio Antennas

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Unidirectional Microwave Antennas

Microwaves are unidirectional; propagation is line of sight; frequency is between 1 GHz to 300 GHz.

Microwaves are used for satellite communications. The parabolic dish antenna and the horn antenna are used

for the reception and transmission of microwaves. The parabolic dish antenna has a parabola geometry. Every

line parallel to the line of sight reflects off the parabola at different angles such that all the reflected lines intersect in a common point called the focus. Hardware located at the focus is used to recover the signal.

Parabolic Dish Antenna

Horn Antenna

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• Infrared signals have frequencies in the range 300 Giga Hz (GHz) to 400 Tera Hz (THz).

• Because of their high frequencies, infrared signals cannot penetrate walls. They are therefore used in short-range communication in a closed area using line-of-sight propagation.

• Infrared devices (e.g., TV remote control) located in different rooms do not interfere with other infrared devices because the signals do not go beyond the walls surrounding each room.

Infrared Signals

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• IrDA (Infrared Data Association) developed standards for high-speed wireless communication between devices (e.g., keyboards, PCs, printers) with rates 4 Mbps, 16 Mbps, 100 Mbps.

• In April 2009, IrDA developed specification for 1 Giga-IR wireless communication of 1 Gbps speed.

• In December 2011, IrDA announced the creation of a New Working Group to extend the 1 Giga-IR and to achieve both 5 and 10 Gigabit per second optical wireless communications by using eye-safety compliant lasers.

• IrDA URL http://www.irda.org/

Infrared Signals

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From Wikipedia: Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or something similar.

Free-space point-to-point optical links can be implemented using infrared laser light. Free Space Optics are additionally used for communications between spacecrafts. Maximum range for terrestrial links is in the order of 2 to 3 km.

In outer space, the communication range of free-space optical communication is currently in the order of several thousand kilometers, but has the potential to bridge interplanetary distances of millions of kilometers.

This topic is not covered in the textbook.

Free Space Optical Communications and Antennas

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Free space laser communication is also used for data communication between high rise buildings, especially for the delivery of broadband data signals between commercial buildings.

Optical antennas, also called laser antennas or infrared antennas, are used to transmit light in free space for the purpose of data communications between two entities without using wires.

Free Space Optical Communications and Antennas