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Data Communications

Chapter 4

Media

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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• Wireless 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 6.

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

Shielding can further help to eliminate interference.

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Coaxial Cable

A 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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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

Two biggest problems with fiber is reflection and refraction.

Glass core

Cladding

Index of refraction = cos(A) / cos(B)If < 1 then less optically dense mediumIf > 1 then more optically dense medium

A A

B

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Fiber Optic Cable

The cladding is less dense than the core, thus causing the light to reflect back into the core.

Core

Cladding

Protective covering(jacket)

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Fiber Optic Cable

Thick cable (62.5/125 micron) causes more ray collisions, so you have to transmit slower. This is step index multimode fiber. Typically use LED for light source, shorter distance transmissions

Better – graded index multimode fiber – no clear difference between core and cladding

Best – single mode fiber – very thin (8.3/125 microns) – very little reflection, fast transmission, typically uses a laser, longer distance transmissions

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

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Data Communications and Computer Networks Chapter 3

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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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Data Communications and Computer Networks Chapter 3

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Terrestrial Microwave

Land-based, line-of-sight transmission

Approximately 20-30 miles 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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Data Communications and Computer Networks Chapter 3

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Often the microwave antennas are on towers and buildings.

Distance between towers d = 7.14 * SQRT(K * h) where K is an adjustment factor, usually 1.33 and h = tower height. (Two 100 m towers can be 82 km apart.)

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Typical Terrestrial Microwave Performance

Band Bandwidth (MHz) Data Rate (Mbps)

2 7 12 6 30 90 11 40 135 18 220 274

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Satellite Microwave

Similar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground station.

L band – mobile communicationsC band – TV, VSATKu band - HDTVKa band -

Satellites can be classified by how far out into orbit each one is (LEO, MEO, GEO, and HEO).

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Satellite Microwave

LEO - Low Earth Orbit - 100 miles to 1000 miles. Used for pagers, wireless e-mail, special mobile telephones, spying, videoconferencing.

MEO - Middle Earth Orbit - 1000 to 22,300 miles. Used for GPS and government.

GEO - Geosynchronous Orbit - 22,300 miles. Used for weather, television, and government operations.

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Satellite Microwave

HEO – Highly Elliptical Orbit

A fourth type of orbit used by the military for spying and by scientific organizations for photographing celestial bodies.

When satellite is far out into space, it takes photos. When satellite is close to earth, it transmits data.

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Satellite Microwave

Satellite microwave can also be classified by its configuration:

Bulk carrier configuration

Multiplexed configuration

Single-user earth station configuration (e.g. VSAT)

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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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Advanced Mobile Phone Service (AMPS) – IS-54

• Uses FDM with analog signals

• Bandwidth is divided into 30 kHz channels

• Each channel is used for either transmitting or receiving

• Technology almost dead in North America

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Digital Advanced Mobile Phone Service (D-AMPS) – IS-136

• Uses TDMA to break up each AMPS channel into 3 separate channels

• Same frequency ranges as AMPS

• Additional features such as caller ID and longer battery life

• Cingular Wireless and AT&T

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Code Division Multiple Access (IS-95a)

• Also known as TIA/EIA-95

• Based on direct sequence spread spectrum technology

• Developed by engineers at Qualcomm

• Verizon Wireless, PrimeCo, Sprint PCS

• Same frequencies as AMPS and D-AMPS

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GSM – Global System for Mobile Communications

• A TDMA technology

• Widely popular in Europe but minimal in North America

• Cingular Wireless and Voicestream

• Uses 880 – 915 MHz and 925-960 MHz

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iDEN (Integrated Dispatch Enhanced Network)

• Functionally the same as MIRS (Motorola Integrated Radio System)

• A high-capacity digital trunked radio system providing integrated voice and data services to its users

• Used by Nextel Communications

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GPRS (General Packet Radio Service)

• The 2.5G version of GSM

• Theoretically allows each user access to 8 GSM data channels at once, boosting data transfer speeds to more than 100 Kbps

• (30 Kbps in the real world since it only uses 2 GSM channels)

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UMTS (Universal Mobile Telecommunications System

• Also called Wideband CDMA

• The possible 3G version of GPRS

• UMTS is not backward compatible with GSM, so first UMTS phones will have to be dual-mode

• Based on TDMA, same as D-AMPS and GSM

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1xRTT (CDMA2000) 1x Radio Transmission Technology

• The 2.5G backwards compatible replacement for CDMA

• 1xRTT will replace GPRS

• 1x means that it requires only the same amount of spectrum as 2G networks based on CDMA (IS-95)

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1xEV (1x Enhanced Version)

• The 3G replacement for 1xRTT

• Will come in two flavors– 1xEV-DO for data only– 1xEV-DV for data and voice

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EDGE (Enhanced Data rates for Global Evolution)

• Further upgrade to GSM

• Possible 3G replacement for GPRS

• Uses improved modulation to triple the data rate where reception is clear

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Company Claimed U.S. Coverage

U.S. Customers

2G 2.5G (mid 2002)

3G (late 2003)

Verizon Wireless

90% 28 million IS-95a (CDMA)

1xRTT 1xEV

Cingular Wireless

70% 22 million GSM and D-AMPS

GPRS EDGE or UMTS

AT&T Wireless

98% 18 million D-AMPS GPRS EDGE or UMTS

Sprint PCS 83% 13 million IS-95a (CDMA)

1xRTT 1xEV

Nextel Communic

77% 8 million IDEN 1xRTT 1xEV

Voicestream

97% 6 million GSM GPRS EDGE or UMTS

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Cellular Digital Packet Data

Technology that supports a wireless connection for the transfer of computer data from a mobile location to the public telephone network and the Internet.

Can be used in conjunction with mobile telephones and laptop computers.

All digital transfer but relatively slow at 19,200 bps.

Emergency services make use of CDPD.

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Pagers

Typically one-way communication service that uses ground-based and sometimes satellite-based systems.

Some systems are two-way.

Some systems can transmit small text messages.

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Infrared Transmissions

Special transmissions that use a focused ray of light in the infrared frequency range.

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

Will infrared last?

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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.

Want to go to the movies?

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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 low speeds, security, and a very small user interface.

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Broadband Wireless Systems

Delivers Internet services into homes and businesses.

Designed to bypass the local loop telephone line.

Transmits voice, data and video over high frequency radio signals.

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Broadband Wireless Systems

Two basic technologies:

Multichannel multipoint distribution service (MMDS) – supports digital data, video, Internet access, millions bps, 2.5 GHz, 30-35 miles

Local multipoint distribution service (LMDS) – digital data, video, Internet access, millions bps, 28 GHz – 30 GHz, but only a few miles

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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 technologies (and protocols) allow up to 54 Mbps data transfer at distances up to hundreds of feet.

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Free Space Optics

Uses lasers, or more economically infrared transmitting devices

Line of sight between buildings

Typically short distances, such as across the street

Newer auto-tracking systems keep lasers aligned when buildings shake from wind, transportation going by

Current FSO speeds go from T-3 (45Mbps) up to OC-48 (2.5Gbps) with faster systems in the lab

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Free Space Optics

Major weakness is fog.

A typical FSO has a link margin of about 20 dB.

Under perfect conditions air reduces a system’s power by approx 1 dB/km.

Heavy fog can cause a loss of 400 dB/km (rendering 20 dB system to 50 meters.

Scintillation is also a problem (especially in hot weather).

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Ultra-Wideband

UWB not limited to a fixed bandwidth but broadcasts over a wide range of frequencies simultaneously.

Many of these frequencies are used by other sources, but UWB uses such low power that it “should not” interfere with these other sources.

Can achieve speeds up to 100 Mbps (unshared) but for small distances such as a wireless LAN.

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Ultra-Wideband

Proponents say UWB gets something for nothing, since it shares frequencies with other sources. Opponents say too much interferences.

Cell phone industry really against UWB because CDMA most susceptible to interference.

GPS (also uses CDMA) may be affected by UWB.

One solution may be to have one type of system indoors (stronger signals) and another type outdoors (1/10 the power).

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Conducted Media Example

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 Example

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 Example

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.

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Review Questions

• What are the limitations of Cat 5e/6?

• Two buildings in a city are separated by 100 meters. How do we interconnect them?

• Two buildings on a campus are separated by 100 meters. How do we interconnect them?

• Two buildings on a campus are separated by 400 meters. How do we interconnect them?