CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 9 Introduction to Computer Networks.

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.

4CMPE 150- Introduction to Computer Networks

Wireless Transmission

5CMPE 150- Introduction to Computer Networks

Wireless Transmission

• Electron movement: electromagnetic waves that propagate through space.

T R

6CMPE 150- Introduction to Computer Networks

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).

7CMPE 150- Introduction to Computer Networks

The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

8CMPE 150- Introduction to Computer Networks

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

9CMPE 150- Introduction to Computer Networks

The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

10CMPE 150- Introduction to Computer Networks

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.

11CMPE 150- Introduction to Computer Networks

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.

12CMPE 150- Introduction to Computer Networks

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.

13CMPE 150- Introduction to Computer Networks

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.

14CMPE 150- Introduction to Computer Networks

The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

15CMPE 150- Introduction to Computer Networks

Infrared Transmission

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

16CMPE 150- Introduction to Computer Networks

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).

17CMPE 150- Introduction to Computer Networks

Communication Satellites

• Weather balloons.

• The moon.

• Artificial satellites:–Geostationary.

–Medium-Earth Orbit.

–Low-Earth Orbit.

18CMPE 150- Introduction to Computer Networks

Satellite Communications

SAT

ground stations

19CMPE 150- Introduction to Computer Networks

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.

20CMPE 150- Introduction to Computer Networks

Orbits

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

(MEO), low earth (LEO).

21CMPE 150- Introduction to Computer Networks

Communication Satellites

22CMPE 150- Introduction to Computer Networks

GEOs

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

equatorial plane.• Many GEO satellites up there!

23CMPE 150- Introduction to Computer Networks

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.

24CMPE 150- Introduction to Computer Networks

Principal Satellite Bands

. Downlink frequencies interfere with microwave.

. Internationally-agreed frequency bands.

25CMPE 150- Introduction to Computer Networks

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.

26CMPE 150- Introduction to Computer Networks

LEO Constellations

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

coverage.

• But, for broad coverage, many satellites needed.

• Example: Iridium (66 satellites).

27CMPE 150- Introduction to Computer Networks

LEOs

SAT

ground stations

SAT

SAT

constellation

28CMPE 150- Introduction to Computer Networks

Low-Earth Orbit SatellitesIridium

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

• (b) 1628 moving cells cover the earth.

29CMPE 150- Introduction to Computer Networks

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.

30CMPE 150- Introduction to Computer Networks

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.

31CMPE 150- Introduction to Computer Networks

The Mobile Telephone System

• First-Generation Mobile Phones: Analog Voice

• Second-Generation Mobile Phones: Digital Voice

• Third-Generation Mobile Phones:Digital Voice and Data

32CMPE 150- Introduction to Computer Networks

The “Cell” Concept

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

33CMPE 150- Introduction to Computer Networks

Mobile Phone System Structure

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

34CMPE 150- Introduction to Computer Networks

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.

35CMPE 150- Introduction to Computer Networks

Cable Television

36CMPE 150- Introduction to Computer Networks

Community Antenna Television

• An early cable television system.

37CMPE 150- Introduction to Computer Networks

Internet over Cable

• Cable television

38CMPE 150- Introduction to Computer Networks

DSL

• The fixed telephone system.

39CMPE 150- Introduction to Computer Networks

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.