Computer Technologies Computer Technologies

Computer Communications

Computer systems(H/W, S/W)

Multimedia

Why should we know “The

Computer Technologies and Trends”

Why should we know “The

Computer Technologies and Trends”

To understand current status of them.

To predict future status of them.

- The future.

- The direction to which the future society goes

Eventually to figure out the future education system & the future school system.

Why should we know “The

Computer Technologies and Trends”

Why should we know “The

Computer Technologies and Trends”

전체적인 현재의 상황 이해의 요체

교육정보화의 현황 이해의 요체

교육정보화의 미래 청사진 작성의 요체

우연성 + 필연성

국내외의 정보통신정책 국내외의 정보통신정책

정보통신정책

정보산업의 방향

국가정보화 ( 교육정보화 , .......)

Copyright, 1998 © Young W. LEE.

Computer Communications

ComputerNetworks

정의 : Computer Network

An interconnected collection of autonomous computers

By AST, 1996

• Circuit Switching

• Packet Switching

• Frame Switching(Relay)

Topics

Topics ( 계속 )

• Local Area Network

• Metropolitan Area Network

• Wide Area Network

• Internetworking => Internet

Topics ( 계속 )

• Local Area Networks– Ethernet(CSMA/CD), Fast Ethernet– Token Ring– High Speed LAN : Fast Ethernet, Gigabit Ethernet,

HIPPI, FDDI, ...

Topics ( 계속 )

• N-ISDN : Narrowband Integrated Service Digital Network

• B-ISDN : Broadband ISDN

• ATM : Asynchronous Transfer Mode

• X.25

Topics ( 계속 ) : Protocols

• 7 Layer Protocols

• TCP/IP (5 Layer Protocols)

Topics ( 계속 )

• Example Networks– LAN : Novell Netware, TCP/IP (Protocols)– MAN : DQDB(Distributed Queue Dual Bus)– WAN : The ARPANET, NSFNET, X.25– GAN : Internet– Gigabit Networks– ............

Topics ( 계속 )

• Wireless Networks– Paging Systems– Mobile phones : Cordless Telephones– Mobile phones : Analog Cellular Telephones– Mobile phones : Digital Telephones – Personal Communication Services : (PCN :

Personal Communication Network)– PDA– Communication Satellites

Topics ( 계속 )

• Transmission Media : Fiber optics, Twisted pair, Baseband Coaxial Cable, Broadband Coaxial Cable.

• Modulation/Demodulation : Modem

• Multiplexing : FDM, TDM

• Network Security Issues

Topics ( 계속 )

• Framing• Error Control• Flow Control• Slip/PPP

Topics ( 계속 )

• Bridges

• Routing• Congestion Control

Topics ( 계속 )

• Domain Name Server

• Simple Network Management Protocol• Electronic Mail : SMTP & MIME• USENET News• World Wide Web

Topics ( 계속 )

• URL : Uniform Resource Locators

• URI : Uniform Resource Indentifiers• Hypertext Transfer Protocols : HTTP

Recommended Texts

• Computer Networks, A.S.Tanenbaum, 3rd Ed., 1996, Prentice Hall

• Data and Computer Communications, W, Stallings, 5th Ed., 1997, Prentice Hall

Wireless Communication

( 무선 , 이동통신 )

Wireless Communication

( 무선 , 이동통신 )

Wireless Communication Wireless Communication

Topics (Topics (계속계속))•• Wireless Netw orksWireless Netw orks

–– Paging S ystemsPaging S ystems

–– Mobi le phones : Cordles s T elephonesMobi le phones : Cordles s T elephones

–– Mobi le phones : Analog CellularMobi le phones : Analog CellularTelephonesTelephones

–– Mobi le phones : Digital TelephonesMobi le phones : Digital Telephones

–– Personal Communication Services :Personal Communication Services :(PCN : P ersonal Communication(PCN : P ersonal CommunicationNetw ork)Netw ork)

–– PDA(P ers onal Digi tal Assistant)PDA(P ers onal Digi tal Assistant)

–– Communication S atel litesCommunication S atel lites

The Principle The Principle

“When electron moves, they create electromagnetic waves

that can propagate through space (even in a vacuum)”

The waves were predicted by James Clerk Maxwell(U.K.) in 1865.

The waves were produced and observed by Heinrich Hertz

(Germany) in 1887. => Hz (Frequency : f) Wavelength * Frequency = Constant

The Electromagnetic Spectrum The Electromagnetic Spectrum

Radio(Hz) : 104 - 108

Microwave(Hz) : 108 - 1010

Infrared(Hz) : 1012 - 1014

Visible Light(Hz) : 1014 - 1014+Q

UV(Ultra-Violet)(Hz) : 1014+Q - 1016

X-ray(Hz): 1016- 1022

Gamma-Ray(Hz) 1022 -

Refer to AST’s Fig 2-11.

Why not higher frequency? Why not higher frequency?

Higher frequency,

Harder to produce,

Harder to modulate,

More difficult to propagate,

More dangerous to living things.

Paging Systems: Pager, 삐삐(Beeper) Paging Systems: Pager, 삐삐(Beeper)

The First Primitive Paging System : announcement using

loudspeakers in hospitals, stations, airport, .....

Now broadcasting radio stream and the beeper detects its

unique number from the radio stream, beeps and displays

the number to be called or messages(new type)

One way communication system

30 bytes /call? => 24 0,000 Pages per miniute via 1Mbps

satellite channel.

Old systems : 150-174 Mhz, New systems : 930-932 MHz

Cordless Telephones (I) Cordless Telephones (I)

Typically in a house or an office.

A base station + a telephone

Low power radio : 100 - 300 meters (900Mhz : 5km)

Old models : fixed frequency => 이웃과 혼선

New models : select the best transmission frequency.

=> no interference/conflict and better sound quality.

Receiving only => Calling as well

Cordless Telephones (II) Cordless Telephones (II)

First generation : analog (poor reception and no security)

In U.S.A. : CT-1

In Europe : CEPT-1

Second Generation : CT-2 (Digital)

Third Generation (1992) : CT-3 or DECT 900Mhz 5Km

Analog Telephones Analog Telephones

First Usage for martime and militar communication during

early 20th Century

The car-based telephone system

- First on in St. Louis in 1946.

- Push-to-talk systems

- A single channel shared for sending and receiving.

- A single large transmitter on top of a tall building

- Used in several cities beginning in the late 1950s.

- Still used for taxis, police cars, ........

IMTS(Improved Mobile Telephone System) IMTS(Improved Mobile Telephone System)

In the 1960s, 200 Watt transmitter on top of a hill.

Two refrequencies : one for sending, one for receiving.

=> no push-talk button

( 쌍방향 , 동시통화 가능 )

23 Channels : 150 Mhz - 450 Mhz

Cellular Phone System : AMPS (I) Cellular Phone System : AMPS (I)

AMPS : Advances Mobile Phone Systems. (Analog)

Bell Labs invented (TACS in U.K., MCS-L1 in Japan)

First installed in 1982

Cell : 10 - 20 km diameter

- Low transmitting power : (0.6 watt in handphone,

3(max. In FCC) watts in cars)

- Frequency reusable : no interference

Base station antennas(center): Ex) Roman Catholic Church

More users => split cells into sub-cells.

Cellular Phone System : AMPS (II) Cellular Phone System : AMPS (II)

The Base station = A computer + transmitter/receiver +

Antenna.

MTSO(Mobile Telephone Switching Office) = MSC

(Mobile Switching Center) :

- heads of all base stations.

- assign channels.

- first level MSC, second level MSC, .....

Handoff (300 msec): power fading away => the owning station talks to surrounding base stations. => radio relay.

Channels (I) Channels (I)

AMPS : 832 full-duplex channels.

Cf) a full duplex channel = a pair of simple channels.

832 simple transmission channels : 824 - 849 Mhz

832 simple receive channels : 869 - 894 Mhz.

(30khz wide / each simplex channel)

FDM (Frequency Division Multiplexing) used to separate

the channels.

Channels (II) Channels (II)

Echo Effect and Signal Distortion

40meter / wave => travel in straight line

=> Some directly arrived, some absorbed &

some bounded and arrived later.

May hear others talk bounced several time far away

In U.S.A., FCC controls the channels

- 832 channels in each city : half for the local telehpone

comapny, the wireline carrier, B-side carrier, the other

half for A side carrier (new comers) for competition.

Channels (III) Channels (III)

4 categories of the 832 channels

- 21 channels for control (base to mobile) to manage

the system

- Paging (base to mobile) to alert mobile user to calls

for them.

- Access (bidirectional) for call setup and channel

assignment.

- Data (bidirectional) for voice(typically 45 channels),

fax, or data.

What will come next time? What will come next time?

Call Managemenet

Security Issues

Personal Communications Services

Communication Satellites

Call Management (I) : Ex of AMPS Call Management (I) : Ex of AMPS

The PROM of each handphone has

- a 32 bit serial number :

- a 10 digit telephone number :

area code (3 자리 , 10bits) + number (7 자리 ,24bits)

Call Management (II) : Ex of AMPS Call Management (II) : Ex of AMPS

How to know where is each mobile phone.

When a phone is swithed on,

=> it scans a preprogrammed list of 21 control channels

=> it finds the most powerful signal.

=> The control channel informs the number of the

paging and access channels

=> The phone broadcasts its serial number and telephone

number. (in digital form, multiple times, with an

error recording code)

Call Management (V) : Ex of AMPS Call Management (V) : Ex of AMPS

How to process incoming calls (I)

=> All idle phones continuously listen to the paging channel

to detect messages directed at them.

=> If a call is placed to one of customers, a packet is sent to

the callee’s home MTSO to finds out where it is.

=> The MTSO sends to the current base station, a packet

and the base station broadcase the packet of “ Are

you there, Unit 14?

=> The customer phone responds wth “Yes” on the control

channel.

Call Management (III) : Ex of AMPS Call Management (III) : Ex of AMPS

=> The base station hears the announcement.

=> it tells the MTSO

=> The MTSO records the existence of its new customer.

=> It also informs the owner’s home MTSO of his

current location.

=> During normal operation, the handphone reregisters

about every 15 mins.

Call Management (IV) : Ex of AMPS Call Management (IV) : Ex of AMPS

How to make a call, after a phone is swithed on,

=> The user enter the called number and hit SEND button.

=> The phone send the # & its own id on the access channel.

=> The base station gets the request and informs the MTSO.

=> If a valid user, the MTSO looks for an idle channel.

=> The found channel number is sent back on the control

channel. => If collision occurs, it tries again later.

=> The mobile phone automatically switches to the selected

voice channel & waits until the called phone is responded.

Call Management (V) : Ex of AMPS Call Management (V) : Ex of AMPS

How to process incoming calls (II)

=> The base station says , “Unit 14, call for you on

channel 3.”

=> The customer phone swithces to channel 3

& starts making ringing sounds.

Security Security

Analog cellular phones : not secure at all

- all-band radio receiver (scanner) : Princess Di

- Theft using a computer : theft calling & even selling

Antenna amd base station

Digital Cellular Telephones (I) Digital Cellular Telephones (I)

1st generation cellular system : analog.

- In U.S.A. : AMPS only

- In Europe : 5 companies.

2nd generation : digital

- In U.S.A. : 4 => 2 : (IS-95), (IS-54, IS-135)

- In Europe : GSM (Global Systems for Mobile comm.)

* deployed before U.S.A.

- In Japan :

- In Korea :

Digital Cellular Telephones (II) Digital Cellular Telephones (II)

IS-54

- dual mode : analog & digital

- the same 30 Mhz channel as that of AMPS.

- 48.6 kbps / channel , 3 shared users / channel

GSM

- 1.8 Ghz & 900 Mhz : 50 200 khz

- smart card

- encryption

Personal Communications Services Personal Communications Services

Same telephone number anywhere in the world.

PCS(U.S.A.) / PCN (Personal Comm. Network)

Microcells : 50 -100 meters diameter (AMPS 20 km wide)

- low power possible (0.25 watt) => small, light phones

- 1/200 diameter => 40,000 times as many cells as

- the same 30 Mhz channel as that of AMPS.

Telepoints : small base stations.

Auction of the PCS spectrum in U.S.A.

- 1.7 - 2.3 Ghz : 94 -95

Communication Satellites Communication Satellites

Primitive trials in 1950s & early 1960s

- metallized weather balloons & the moon.

The 1st artificial satellite in 1962

- with amps

Big microwave repeaters : transponders

- receive => amplify => broadcast

Upward beam :

Downward Beam : Broad or Narrow as wanted.

Geosynchronous Satellites (I) Geosynchronous Satellites (I)

Kepler’s law : the orbital period of a satellite varies as the

orbital radius to the 3/2 power.

Low-orbit satellite (ex: 90min) :

Geosynchronous satellite (24 hours) at 36,000 Km above the equator.

- 2 degree spacing => 180 GSs

- different frequencies => more GSs

Geosynchronous Satellites (II) Geosynchronous Satellites (II)

Full duplex band frequency assignment.

Band Frequencies Downlinks Uplinks Problems

(Ghz) (Ghz)

C 4/6 3.7-4.2 5.925-6.425 Terrestrial

Interference

Ku 11/14 11.7-12.2 14.0-14.5 Rain

Ka 20/30 17.7-21.7 27.5-30.5 Rain :

Equipment cost

Geosynchronous Satellites (III) Geosynchronous Satellites (III)

C Band : Overcrowded now.

Ku Band & Ka Band :

- not yet crowded ( 1 degree space)

- how to avoid rain absorbing effect.

=> several widely separated ground stations.

=> expensive solution yet.

Geosynchronous Satellites (IV) Geosynchronous Satellites (IV)

A Typical satellite : 12-20 transponders, each with a

36-50 Mhz bandwidth (50Mhx or 800 64 kbps, ...)

A single spatial beam : up the entire earth

Spot beams : hundreds of km

Ex : 1 wide beam for 48 states, 2 spot beams for A & H.

Geosynchronous Satellites (V) Geosynchronous Satellites (V)

VSAT (Very Small Aperture Terminals)

- low cost microstations : 1m antenna, 1watt power

- good for 19.2 kbps in the uplink

- good for 512kbps in the downlink

- hub

- cheaper end-user-station but a longer delay.

Low-Orbit Satellites (I) Low-Orbit Satellites (I)

In 1990, Motorola announced the Iridium Project Proposal

77 low orbit sattllites => Iridium (element 77)

Later revised to be 66 satellites. => Dysprosium

Based on ideas from Cellular radio(mobile base stations).

Spot beams (max 48 beams / LOS => 1628 cells)

750Km above, necklace like movement,

every 32 degrees of latitude.

174 full duplex channels/LOS => 283,272 channels in all.

L band (1.6 GHz : 1.4 up/ 1.8 down) => small battery

Low-Orbit Satellites (II) Low-Orbit Satellites (II)

Ka Band between LOSs

3 dollars / min.

세계적으로 년말 까지 10 만대 예상

국내 : 11/1 일 부터 서비스 시작

- 30 만원 가입비 , 40 만원 보증보험

- 국제통화 : 분당 4-6$ (6 천 4 백 20 원 )

- 국내통화 : 분당 1.48$ ( 약 2 천원 )

- 월기본료 15 만원

- 단말기 : 대당 4 백 53 만원 , 약 400g (6*20*7cm)