Data Communication and Computer Network 1 ณรงค์ฤทธิ์ มณีจิระปราการ สาขาคอมพิวเตอร์ธุรกิจภาควิชาพาณิชย์ศาสตร์มหาวิทยาลัยนเรศวร

Data Communication and Computer Network

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Introduction

The world of computer networks and data communications would not exist if there were no medium by which to transfer data.

The two major categories of media include: • Conducted media• Radiation or Wireless media


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Classes of Transmission Media

• Conducted or guided media– use a conductor such as a wire or a fiber

optic cable to move the signal from sender to receiver

• Wireless or unguided media– use radio waves of different frequencies

and do not need a wire or cable conductor to transmit signals


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Design Factors for Transmission Media

• Bandwidth: All other factors remaining constant, the greater the band-width of a signal, the higher the data rate that can be achieved.

• Transmission impairments. Limit the distance a signal can travel.

• Interference: Competing signals in overlapping frequency bands can distort or wipe out a signal.

• Number of receivers: Each attachment introduces some attenuation and distortion, limiting distance and/or data rate.


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Electromagnetic Spectrum for Transmission Media


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• Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint

• Examplestwisted pair wirescoaxial cablesoptical fiber

CoaxialCoaxial

Fiber opticFiber optic

Guided Transmission Media


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Twisted Pair Wire

Two or more pairs of single conductor wires that have been twisted around each other.

Twisted pair wire is classified by category. Twisted pair wire is currently Category 1 through Category 5e.

Twisting the wires helps to eliminate electromagnetic interference between the two wires.

Shielding can further help to eliminate interference.


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Twisted Pair Wires

• Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs

• Often used at customer facilities and also over distances to carry voice as well as data communications

• Low frequency transmission medium


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Types of Twisted Pair

• STP (shielded twisted pair)– the pair is wrapped with metallic foil or

braid to insulate the pair from electromagnetic interference

• UTP (unshielded twisted pair)– each wire is insulated with plastic wrap, but the pair is

encased in an outer covering


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Ratings of Twisted Pair• Category 3 UTP

– data rates of up to 10mbps are achievable

• Category 5 UTP– data rates of up to 100mbps are achievable– more tightly twisted than Category 3 cables– more expensive, but better performance

• STP– More expensive, harder to work with


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Twisted Pair Advantages

• Inexpensive and readily available

• Flexible and light weight

• Easy to work with and install


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Twisted Pair Disadvantages

• Susceptibility to interference and noise

• Attenuation problem– For analog, repeaters needed every 5-6km– For digital, repeaters needed every 2-3km

• Relatively low bandwidth (3000Hz)


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Coaxial Cable (or Coax)

• Used for cable television, LANs, telephony• Has an inner conductor surrounded by a

braided mesh• Both conductors share a common center

axial, hence the term “co-axial”


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Coaxial CableA single wire wrapped in a foam insulation surrounded by a braided metal shield, then covered in a plastic jacket. Cable can be thick or thin.

Baseband coaxial technology uses digital signaling in which the cable carries only one channel of digital data.

Broadband coaxial technology transmits analog signals and is capable of supporting multiple channels of data.


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Coax Layers

copper or aluminum conductor

insulating material

shield(braided wire)

outer jacket(polyethylene)


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Coax Advantages

• Higher bandwidth– 400 to 600Mhz– up to 10,800 voice conversations

• Can be tapped easily (pros and cons)• Much less susceptible to interference than

twisted pair


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Coax Disadvantages

• High attenuation rate makes it expensive over long distance

• Bulky


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Fiber Optic Cable

A thin glass cable approximately a little thicker than a human hair surrounded by a plastic coating and packaged into an insulated cable.

A photo diode or laser generates pulses of light which travel down the fiber optic cable and are received by a photo receptor.


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Fiber Optic Cable

• Relatively new transmission medium used by telephone companies in place of long-distance trunk lines

• Also used by private companies in implementing local data communications networks

• Require a light source with injection laser diode (ILD) or light-emitting diodes (LED)


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plastic jacket glass or plasticcladding

fiber core

Fiber Optic Layers

• consists of three concentric sections


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Fiber Optic Typesmultimode step-index fiber the reflective walls of the fiber move the light pulses

to the receiver multimode graded-index fiberacts to refract the light toward the center of the

fiber by variations in the density single mode fiberthe light is guided down the center of an

extremely narrow core


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fiber optic multimodestep-index

fiber optic multimodegraded-index

fiber optic single mode

Fiber Optic Signals


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Fiber Optic Advantages• greater capacity (bandwidth of up to 2 Gbps)

• smaller size and lighter weight

• lower attenuation

• immunity to environmental interference

• highly secure due to tap difficulty and lack of signal radiation


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Fiber Optic Disadvantages

• expensive over short distance• requires highly skilled installers• adding additional nodes is difficult


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It is very common to mix fiber with twisted pair in LANs.


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Conducted Media In Action

How do we wire a local area network?

Remember: using Category 5 unshielded twisted pair, the maximum segment length is 100 meters.

A wall jack is a passive device and does not regenerate a signal.

Hub to hub connections are often fiber optic cable.


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Wireless Media

Radio, satellite transmissions, and infrared light are all different forms of electromagnetic waves that are used to transmit data.

Note in the following figure how each source occupies a different set of frequencies.


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Wireless (Unguided Media) Transmission

• transmission and reception are achieved by means of an antenna

• Directional– transmitting antenna puts out focused

beam– transmitter and receiver must be aligned

• omnidirectional– signal spreads out in all directions– can be received by many antennas


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Wireless Examples

• terrestrial microwave

• satellite microwave

• broadcast radio

• infrared


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Radio

• radio is omnidirectional and microwave is directional

• Radio is a general term often used to encompass frequencies in the range 3 kHz to 300 GHz.

• Mobile telephony occupies several frequency bands just under 1 GHz.


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Terrestrial Microwave• used for long-distance telephone service• uses radio frequency spectrum, from 2 to 40

Ghz• parabolic dish transmitter, mounted high• used by common carriers as well as private

networks• requires unobstructed line of sight between

source and receiver• curvature of the earth requires stations

(repeaters) ~30 miles apart


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Terrestrial Microwave

Land-based, line-of-sight transmission

Approximately 40-88 Kms maximum between towers

Transmits data at hundreds of millions of bits per second

Popular with telephone companies and business to business transmissions


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Often the microwave antennas are on towers and buildings.


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Satellite Microwave Transmission

• a microwave relay station in space• can relay signals over long distances• geostationary satellites

– remain above the equator at a height of 22,300 miles (geosynchronous orbit)

– travel around the earth in exactly the time the earth takes to rotate


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Satellite Transmission Links

• earth stations communicate by sending signals to the satellite on an uplink

• the satellite then repeats those signals on a downlink

• the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming


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dish dish

uplink station downlink station

satellitetransponder

22,300 miles

Satellite Transmission Process


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Satellite Transmission Applications

• television distribution– a network provides programming from a

central location– direct broadcast satellite (DBS)

• long-distance telephone transmission– high-usage international trunks

• private business networks


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Satellite Microwave Applications

• Television distribution

• Long-distance telephone transmission

• Private business networks


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Microwave Transmission Disadvantages

• line of sight requirement

• expensive towers and repeaters

• subject to interference such as passing airplanes and rain


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Principal Satellite Transmission Bands

• C band: 4(downlink) - 6(uplink) GHz– the first to be designated

• Ku band: 12(downlink) -14(uplink) GHz– rain interference is the major problem

• Ka band: 19(downlink) - 29(uplink) GHz– equipment needed to use the band is still very

expensive


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Fiber vs Satellite


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Mobile Telephone

Wireless telephone service, such as cellular telephone, cell phone, and PCS.

To support multiple users in a metropolitan area (market), the market is broken into cells.

Each cell has its own transmission tower and set of assignable channels.


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Mobile TelephoneAMPS - Advanced Mobile Phone Service - First popular mobile phone service, uses analog signals and dynamically assigned frequency division multiplexing.

D-AMPS - Digital Advanced Mobile Phone Service - Applies digital time division multiplexing on top of AMPS.

PCS - Personal Communication Systems - Newer all-digital mobile phone service (2nd generation)


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Mobile Telephone

PCS phones come in three technologies:

TDMA - Time division multiple access

CDMA - Code division multiple access

GSM - Global system for mobile communications

3G – Third generation wireless (Internet/data access)


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Infrared• Uses transmitters/receivers (transceivers) that modulate noncoherent infrared light.

• Transceivers must be within line of sight of each other (directly or via reflection ).

• Unlike microwaves, infrared does not penetrate walls.

• Very common with remote control devices, but can also be used for device-to-device transfers, such as PDA to computer.


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Wireless LAN (IEEE 802.11)

This technology transmits data between workstations and local area networks using high speed radio frequencies.

Current technology (and protocol) allows for 11 Mbps data transfer at distances up to hundreds of feet.

More on this in Chapter Seven (LANs)


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Bluetooth

Bluetooth is a Radio Frequency specification for short-range, point-to-multipoint voice and data transfer.

Bluetooth can transmit through solid, non-metal objects.

Its typical link range is from 10 cm to 10 m, but can be extended to 100 m by increasing the power.


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Bluetooth

Bluetooth will enable users to connect to a wide range of computing and telecommunication devices without the need of connecting cables.

Typical uses include phones and pagers, modems, LAN access devices, headsets, notebooks, desktop computers, and PDAs.


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WAP (Wireless Application Protocol)

WAP allows wireless devices such as mobile telephones, PDAs, pagers, and two-way radios to access the Internet.

WAP is designed to work with small screens and with limited interactive controls.

WAP incorporates Wireless Markup Language (WML) which is used to specify the format and presentation of text on the screen.


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WAP (Wireless Application Protocol)

WAP may be used for applications such as:- travel directions- sports scores- e-mail- online address books- traffic alerts- banking- news

Possible short-comings of WAP include slow speeds, security, and a very small user interface.


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Media Selection Criteria

Cost – Initial cost, ROI, maintenance/support costSpeed – Data transfer speed, propagation speedDistance and expandabilityEnvironment – Noise levelSecurity – Wiretap possible? Need encryption?


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Interconnecting Two Buildings

Two buildings are separated by 400 meters. How do we interconnect them?

Twisted pair? (Do we even have access?)Coaxail cable?Fiber?Wireless?Other? (Chapter 12)


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Wireless Media In Action

DataMining Corporation has one office in Chicago and one in Los Angeles.

There is a need to transmit large amounts of data between the two sites.

DataMining is considering using a Very Small Aperture Terminal satellite system.


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Wireless Media In Action

Cost is proportional to high amount of traffic with very high reliability.

Speed is high enough to support company’s needs.

Distance can easily expand across the U.S.

Satellite systems are robust in most environments.

Security can be very good with encryption.

Documents

Data Communication and Computer Network 1 ณรงค์ฤทธิ์ มณีจิระปราการ สาขาคอมพิวเตอร์ธุรกิจภาควิชาพาณิชย์ศาสตร์มหาวิทยาลัยนเรศวร