Computer Networks - part III (Networking Media)

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Objectives

• Identify general cabling characteristics applied to physical media

• Describe the primary cable types used in networking

• Identify the components in a structured cabling installation

• Describe wireless transmission techniques used in LANs and WANs

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Network Cabling: Tangible Physical Media

• The interface between a computer and the medium to which it attaches defines the translation from a computer’s native digital information into the form needed to send outgoing messages– Because all media must support the basic tasks of sending

and receiving signals, you can view all networking media as doing the same thing; only the methods vary

– You need to know the physical characteristics and limitations of each kind of network media so that you can make the best use of each type

• Each has a unique design and usage, with associated cost, performance, and installation criteria

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General Cable Characteristics

• The following characteristics apply network cabling:– Bandwidth rating– Maximum segment length– Maximum number of segments per internetwork– Maximum number of devices per segment– Interference susceptibility– Connection hardware– Cable grade– Bend radius– Material costs– Installation costs

Factors to Consider When Choosing a Medium

• Data transfer speed• Use in specific network topologies• Distance requirements• Cable and cable component costs• Additional network equipment that might be

required• Flexibility and ease of installation• Immunity to interference from outside sources• Upgrade options

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Primary Cable Types

• All forms of cabling are similar, in that they provide a medium across which network information can travel in the form of a physical signal, whether electrical or light pulses

• The primary cable types are:– Coaxial cable– Twisted-pair– Fiber-optic cable

Communications Media Types

• Coaxial cable– Based on copper wire construction

• Twisted-pair cable– Based on copper wire construction

• Fiber-optic cable– Glass (usually), or plastic

• Wireless technologies– Radio or microwaves

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Coaxial Cable• Was the predominant form of network cabling• Shielding: protective layer(s) wrapped around cable to

protect it from external interference• Copper core surrounded by insulation• Insulation surrounded by another conducting material,

which is covered by an outer insulating material• Types:

– Thick coax cable (thickwire or thicknet)– Thin coax cable

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Thick Coax Cable

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Connecting to Thick Coax Cable

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Thick Coax Cable Properties

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Cable Specifications

• 10BASE5 – speed of transmission at 10 Mbps– type of transmission is baseband– 5 represents the capability of the cable to allow the signal to travel for

approximately 500 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received.

– often referred to as Thicknet

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Thin Coax Cable

• 10BASE2 – speed of transmission at 10 Mbps– type of transmission is baseband– The 2, in 10BASE2, represents the capability of the cable to allow the

signal to travel for approximately 200 meters, before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE2 is often referred to as Thinnet.

Thin Coax Cable

• Attaches to a bayonet nut connector (BNC)

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Thin Coax Cable Properties

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Coaxial Cable• Advantages:

– Requires fewer repeaters than twisted pair

– Less expensive than fiber – It has been used for many

years for many types of data communication, including cable television

• Disadvantages:– More expensive and more

difficult to install than twisted pair

– Needs more room in wiring ducts than twisted pair

Twisted-Pair Cable

• Flexible cable that contains pairs of insulated copper wires that are twisted together for reduction of EMI and RFI and covered with an outer insulating jacket

• Typically used on LANs to bring network to desktop

• Connects to network devices with RJ-45 plug-in connectors

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Twisted-Pair Cable

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Types of Twisted-Pair Cable

• Shielded twisted-pair (STP) cable– Pairs of insulated wires that are twisted together,

surrounded by shielding material for added EMI and RFI protection, all inside a protective jacket

• Unshielded twisted-pair (UTP) cable– No shielding material between pairs of insulated

wires twisted together and cable’s outside jacket

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STP and UTP Cable

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Twisted-Pair Cable Standards

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Twisted-Pair Cable (continued)

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Unshielded Twisted Pair (UTP)

• Unshielded twisted-pair cable (UTP) is a four-pair wire medium used in a variety of networks.

• TIA/EIA-568-A contains specifications governing cable performance. • RJ-45 connector • When communication occurs, the signal that is transmitted by the source needs to

be understood by the destination. • The transmitted signal needs to be properly received by the circuit connection

designed to receive signals. • The transmit pin of the source needs to ultimately connect to the receiving pin of

the destination.

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Unshielded Twisted Pair (UTP)

Straight-through Cross-over Rollover

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UTP Straight-through Cable

• The cable that connects from the switch port to the computer NIC port is called a straight-through cable.

Host or RouterHub or Switch

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UTP Straight-through Cable

Host or RouterHub or Switch

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UTP Cross-over Cable

• The cable that connects from one switch port to another switch port is called a crossover cable.

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UTP Cross-over Cable

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UTP Rollover Cable

• The cable that connects the RJ-45 adapter on the com port of the computer to the console port of the router or switch is called a rollover cable.

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UTP Rollover Cable

Rollover cable

Console port

Com1 or Com2 serial port

Terminal or a PC with terminal emulation software

Router

Fiber-Optic Cable• Fiber optic cable is made of a light conducting glass or

plastic core surrounded by more glass, called cladding, and a tough outer sheath

• The center core provides the light path while the cladding is composed of varying layers of reflective glass designed to reflect light back into the core

• Usually uses infrared light for signal transmission• Used to connect

networks on LANsand to connect LANs into WANs

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Fiber-Optic Cable• Advantages

– Able to sustain transmissions over long distances due to high bandwidth and low attenuation

– No EMI or RFI problems– Difficult to place unauthorized taps

• Disadvantages– Very fragile– Relatively expensive– Requires specialized training to install

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How does fiber work?

• Light travels in a straight line as long as it is moving through a single uniform substance.

• If a ray of light traveling through one substance suddenly enters another (less or more dense) substance, its speed changes abruptly, causing the ray to change direction. This change is called refraction.

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Refraction

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Critical angle•If the angle of incidence increases, so does the angle of refraction.•The critical angle is defined to be an angle of incidence for which the angle of refraction is 90 degrees.

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Reflection

• When the angle of incidence becomes greater than the critical angle, a new phenomenon occurs called reflection.

• Light no longer passes into the less dense medium at all.

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Critical Angle

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• Optical fibers use reflection to guide light through a channel.• A glass or core is surrounded by a cladding of less dense

glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being into it.

• Information is encoded onto a beam of light as a series of on-off flashes that represent 1 and 0 bits.

How does fiber work?

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Fiber construction

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Benefits of Optical Fiber

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Fiber-Optic Cable Modes• Step-Index MultiMode: most economical, suffers from

modal dispersion, limited transmission rate, short links • Step-Index Single-Mode: small core diameter causes all

light to travel in one mode with no modal dispersion, most expensive, high transmission rate, long distance links

• Graded-Index MultiMode: good compromise

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Optical Fiber Transmission Modes

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• The purpose of fiber-optic cable is to contain and direct a beam of light from source to target.

• The sending device must be equipped with a light source and the receiving device with photosensitive cell (called a photodiode) capable of translating the received light into an electrical signal.

• The light source can be either a light-emitting diode (LED) or an injection laser diode.

Light sources for optical fibers

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Transmitting Devices

• The transmitter converts the electronic signals into their equivalent light pulses. • There are two types of light sources used to encode and transmit the data

through the cable: – A light emitting diode (LED) producing infrared light. – Light amplification by stimulated emission radiation (LASER) a light source

producing a thin beam of intense infrared light usually with wavelengths of 1310nm or 1550 nm.

– Lasers are used with single-mode fiber over the longer distances involved in WANs or campus backbones.

Fiber-optic cable connectors

The subscriber channel (SC) connector is used in cable TV. It uses a push/pull locking system. The straight-tip (ST) connector is used for connecting cable to networking devices. MT-RJ is a new connector with the same size as RJ45.

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Signals and noise in optical fibers

• Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end.

• Although fiber is the best of all the transmission media at carrying large amounts of data over long distances, fiber is not without problems. When light travels through fiber, some of the light energy is lost.

• The most important factor is scattering. – The scattering of light in a fiber is caused by microscopic non-uniformity

(distortions) in the fiber that reflects and scatters some of the light energy.

Properties of Single-Mode Fiber-Optic Cable

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Properties of Multimode Fiber-Optic Cable

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• Single cable sheath containing a combination of fibers and copper cables in different combinations for different implementations

• Full HFC system can deliver:– Plain Old Telephone Service (POTS)– Up to 37 analog TV channels– Up to 188 digital TV channels– Up to 464 digital point channels– High-speed, two-way digital data link for PCs

Hybrid Fiber/Coax Cables (HFC)

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Hybrid Fiber/Coax Cables (HFC)

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Fiber-Optic Cable (continued)

Summary of the Characteristics of Conducted Media

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Comparison of Cable Media

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Wireless Networking: Intangible Media

• Wireless technologies play an increasing role in all kinds of networks– Since 1990, the number of wireless options has

increased, and the cost continues to decrease– Wireless networks can now be found in most towns

and cities in the form of hot spots, – More home users have turned to wireless networks

• Wireless networks are often used with wired networks to interconnect geographically dispersed LANs or groups of mobile users with stationary servers and resources on a wired LAN

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Example: Wireless network at Home

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Wireless Applications Wireless applications:• Ready access to data for mobile professionals• Delivery of network access into isolated facilities or disaster-

stricken areas• Access in environments where layout and settings change

constantly• Improved customer services in busy areas, such as check-in or

reception centers• Network connectivity in structures where in-wall wiring would

be impossible to install or too expensive• Home networks where the installation of cables is inconvenient

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Wireless LAN Components• NIC attaches to an antenna and an emitter, rather than

to a cable• An access point (AP) is installed to translate between

the wired and wireless networks– An AP includes an antenna and a transmitter to send and

receive wireless traffic, but also connects to the wired side of the network

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Wireless LAN Transmission (2)• Commonly used frequencies for wireless data

communications– Radio —10 KHz (kilohertz) to 1 GHz (gigahertz)– Microwave —1 GHz to 500 GHz

• Wireless LANs make use of four primary technologies for transmitting and receiving dataa) Infraredb) Laserc) Narrowband (single-frequency) radiod) Spread-spectrum radio

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Wireless LAN Transmission (1)

Wireless LANs send/receive signals Broadcast through the air:• Waves in the electromagnetic spectrum• Frequency: cycles per second (measured in Hz)

– Frequency affects the amount and speed of data transmission.

e.g., electrical power 60 Hz telephone 0 – 3 kHz

– Lower-frequency transmissions can carry less data more slowly over longer distances

– Higher-frequency technologies often use tight-beam broadcasts and require a clear line of sight between sender and receiver

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Laser-Based LAN Technologies

Laser-based transmissions require a clear line of sight between sender and receiver• Any solid object or person blocking a beam blocks

data transmissions• To protect people from injury and avoid excess

radiation, – laser-based LAN devices are subject to many of the

same limitations as infrared, – but aren’t as susceptible to interference from visible

light sources

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Narrowband Radio LAN TechnologiesNarrowband radio (single-frequency radio): use low-powered, two-way radio communication• Receiver and transmitter must be tuned to the same frequency• Require no line of sight between sender and receiver• Broadcast range: 50 – 70 m (164 – 230 ft)• Federal Communications Commission (FCC) regulates radio

frequencies– Organizations must complete a time-consuming and expensive

application process before being granted the right of exclusive use certain frequency in specific locale

– Some frequencies are set aside for unregulated use• Remote control, many WLANs

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Narrowband Radio LAN Technologies

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Spread-Spectrum LAN Technologies• Spread-Spectrum LAN uses multiple frequencies

simultaneously– Improve reliability– Reduce susceptibility to interference– Enhance security

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Spread-Spectrum LAN Technologies

Two types of Spread-Spectrum communication• Frequency hopping

– Switch data among multiple frequencies at regular intervals– Using one frequency at a time, bandwidth 1 – 2 Mbps– Transmitter and receiver must be synchronized

• Direct-sequence modulation– Break data into fixed-size segments: chips– Transmit the data on several different frequencies at the same

time– The receiver monitors frequencies and reassembles the data,

bandwidth 2 – 6 Mbps

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802.11 Wireless Networking• 802.11 standard is also referred to as Wireless Fidelity

(Wi-Fi)– 802.11b and 802.11g running at a 2.4 GHz frequency (11

Mbps and 54 Mbps, respectively), – 802.11a specifies a bandwidth of 54 Mbps at a 5 GHz

frequency• 802.11 wireless is an extension to Ethernet using

airwaves as the medium; – most 802.11 networks incorporate wired Ethernet

segments– Networks can extend to several hundred feet– Many businesses are setting up Wi-Fi hot spots

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Wireless MAN: The 802.16 Standard

• One of the latest wireless standards, 802.16 Worldwide Interoperability for Microwave Access (WiMax)– 802.16-2004 (previously named 802.16a), or fixed WiMax– 802.16e, or mobile WiMax

• 802.16 standards promise wireless broadband to outlying and rural areas– last-mile wired connections are too expensive or impractical because of

rough terrain– Delivers up to 70 Mbps of bandwidth at distances up to 30 miles– Operates in a wide frequency range (2 -- 66 GHz)

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Fixed WiMax: 802.16-2004• Fixed WiMax is being used to

deliver wireless Internet access to entire metropolitan areas rather than the limited-area hot spots available with 802.11

• Fixed WiMax can blanket an area up to a mile in radius, compared to just a few hundred feet for 802.11– E.g., Los Angeles has begun

implementing fixed WiMax in an area of downtown that encompasses a 10-mile radius

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Mobile WiMax: 802.16e

• 802.16e Standard promises to bring broadband Internet roaming to the public– Allow users to roam from area to area without losing the

connection, which offers mobility much like cell phone– The mobile WiMax standard is not yet finalized

• The future: – Fixed WiMax is expected to be the dominant technology

for the next several years,– Mobile WiMax will win out in the end

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Microwave Networking Technologies

• Microwave systems have higher bandwidth thanradio-based systems.

– Using high frequency need a clear line of sight between transmitter and receiver

– Require FCC approval and licensing more expensive than radio-based systems

• Two types of microwave systems: – Terrestrial microwave systems – Satellite microwave systems

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Microwave Networking Technologies

Terrestrial microwave systems • Use high-beam, high-frequency

signals to link.• Towers relay signal across

continent

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Microwave Networking TechnologiesSatellite microwave

systems:• Send / receive data from

geosynchronous satellites• The broadcast nature of this system

requires all data are encrypted