cmpe 150- introduction to computer networks 1 cmpe 150 fall 2005 lecture 9 introduction to computer...

1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 9

Introduction to Computer Networks

2CMPE 150- Introduction to Computer Networks

Announcements• 2nd. lab today at 4pm BE 168.• If you cannot make it to a lab session, let us know

and we’ll schedule a make up slot.• Hw. 2 is up on the Web page.

• Career Center Job and Internship Fair!– When? Tue, Oct. 18th. 9am-noon.

– Where? University Center.

• CEFULs: CE ugrad lunch.– Meet faculty.

– Talk about important topics.

– Free lunch!

3CMPE 150- Introduction to Computer Networks

Today

• PHY (cont’d).– Wireless transmission (cont’d).

– Mobile telephony.

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Wireless Transmission

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Wireless Transmission

• Electron movement: electromagnetic waves that propagate through space.

T R

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Propagation

• Maximum speed: speed of light, c, 3*108 m/s.• In vacuum, all EM waves travel at the same

speed c.• Otherwise, propagation speed is function of

frequency (c = * f), where f is frequency (Hz) and is wavelength (m).

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

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Radio Transmission

• (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. E.g., AM radio uses MF.

• (b) In the HF and VHF bands, they bounce off the ionosphere. E.g., Hams and military.

~1Km

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

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Microwave Transmission

• Above 100MHz.• Waves travel in straight lines.• Directionality.– Better quality.– Space Division Multiple Access.– But, antennas need to be aligned, do not go

through buildings, multi-path fading, etc.• Before fiber, microwave transmission

dominated long-distance telephone transmission.

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Politics of the Electromagnetic Spectrum

• Need agreements to regulate access.– International and national.

• Local governments allocate spectrum for radio (AM and FM), TV, mobile phones, emergency services, etc.– In the US, FCC.

• World-wide, ITU-R tries to coordinate allocation so devices work everywhere.

• Separate frequency band that is unregulated.– ISM: Industrial, Scientific, and Medical.– Household devices, wireless phones, remote controls,

etc.

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ISM in the US

. Devices with power < 1W can use the ISM bands.

. 900 MHz is crowded and not available world-wide.

. At 2.4GHz, widely available but interference prone.. Bluetooth and some 802.11 WLANs.

. 5.7GHz is the next one to be populated.

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Spread Spectrum• Narrow frequency band -> good reception (power,

bandwidth).• But in some cases, wide band is used, aka, spread

spectrum.– Modulate signal to increase bandwidth of signal to

be transmitted.• 2 variations: – Frequency Hopping (FH).• Transmitter hops frequencies

– Direct Sequence (DS).• Use spreading code to convert each bit of the original

signal into multiple bits.

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

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Infrared Transmission

• Short range (e.g., remote controls).• Directional, cheap.• But, do not pass through obstacles.

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Lightwave Transmission

• Unguided optical transmission.• E.g., laser communication between two

buildings for LAN interconnection.• High bandwidth, low cost.• Unidirectionality.• Weather is a major problem (e.g., rain,

convection currents).

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Communication Satellites

• Weather balloons.

• The moon.

• Artificial satellites:–Geostationary.

–Medium-Earth Orbit.

–Low-Earth Orbit.

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Satellite Communications

SAT

ground stations

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Satellite Communications• Satellite-based antenna(e) in stable orbit

above earth.• Two or more (earth) stations

communicate via one or more satellites serving as relay(s) in space.

• Uplink: earth->satellite.• Downlink: satellite->earth.• Transponder: satellite electronics

converting uplink signal to downlink.

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Orbits

• Shape: circular, elliptical.• Plane: equatorial, polar.• Altitude: geostationary (GEO), medium earth

(MEO), low earth (LEO).

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Communication Satellites

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GEOs

• High-flying satellites.• Orbit at 35,863 Km above earth and rotates in

equatorial plane.• Many GEO satellites up there!

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GEO: Plus’s and minus’s• Plus’s:

– Stationarity: no frequency changes due to movement.– Tracking by earth stations simplified.– At that altitude, provides good coverage of the earth.

• Minus’s:– Weakening of signal.– Polar regions poorly served.– Delay!– Spectral waste for point-to-point communications.

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Principal Satellite Bands

. Downlink frequencies interfere with microwave.

. Internationally-agreed frequency bands.

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LEO Satellites

• Circular or slightly eliptical orbit under 2,000 Km.

• Orbit period: 1.5 to 2 hours.• Coverage diameter: 8,000 Km.• RTT propagation delay < 20ms (compared to

> 300ms for GEOs).• Subject to large frequency changes and

gradual orbit deterioration.

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LEO Constellations

• Advantages over GEOs:– Lower delay, stronger signal, more localized

coverage.

• But, for broad coverage, many satellites needed.

• Example: Iridium (66 satellites).

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LEOs

SAT

ground stations

SAT

SAT

constellation

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Low-Earth Orbit SatellitesIridium

• (a) The Iridium satellites from six necklaces around the earth.

• (b) 1628 moving cells cover the earth.

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In Summary…

• GEOs– Long delay - 250-300 ms.

• LEOs– Relatively low delay - 40 - 200 ms.

– Large variations in delay - multiple hops/route changes, relative motion of satellites, queuing.

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Satellite Data Rates

• Satellite has 12-20 transponders, each ranging from 36-50 Mbps.

• T1: 1.54 Mbps.• T2: 6.312 Mbps.• T3: 44.736 Mbps.• T4: 274.176 Mbps.

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The Mobile Telephone System

• First-Generation Mobile Phones: Analog Voice

• Second-Generation Mobile Phones: Digital Voice

• Third-Generation Mobile Phones:Digital Voice and Data

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The “Cell” Concept

• (a) Frequencies not reused in adjacent cells.• (b) To add more users, smaller cells.

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Mobile Phone System Structure

• Hierarchy.• Base station.• Mobile Switching Center (MSC).• MSCs connected through PSTN.

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Handoffs

• As mobile phones move, they switch cells, and thus base stations.

• Soft versus hard handoffs.– Two base stations while handoff is in

progress.

– Hard handoff.

• Roaming.

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Cable Television

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Community Antenna Television

• An early cable television system.

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Internet over Cable

• Cable television

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DSL

• The fixed telephone system.

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ADSL versus Internet over Cable

• Both uses fiber in the backbone.• ADSL uses twisted pair and IoC uses coax on

the edge.• Coax has higher capacity but shared with TV.• IoC’s capacity is unpredicatble as it depends

on how many users/traffic.