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contents
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
contents3
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
applications file print serving
Applications, File & PrintServing

File

server

Applications

server

Print

server

m4

contents5
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
email

INTERNET

m4

d@m4

a@m1

b@m2

c@m4

d@m4

email

m4 is a “pop server”, with a mailbox for each registered user.

Connection between pop server and users can be temporary

(direct or via internet)

email protocols

m4

b@m2

c@m4

d@m4

a@m1

Email Protocols

Pop/imap server

Mail Transfer Agent

(X400)

pop3

imap

http

SMTP

User

Agent

(X400)

Outlook

Exchange

Eudora

Netscape

contents8
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
hierarchical addresses
Addresses composed of meaningful fields

Address = hierarchical list of domain names

A domain is an organizational and/or geographic entity regrouping lower level domains or hosts.

Addresses belonging to a given domain are maintained by the authority of that domain.

Example : classical telephone numbers

32 2 629 2905, 32 475 819327

hierarchy jeopardized by number portability!

Hierarchical Addresses
flat addresses
Organizational and geographic hierarchy do not necessarily match

Address values contain no useful information

Example : Internet network ID’s

VUB = 134.184 ULB = 164.15

KUL = 134.58 KULAK = 193.190

RUG = 157.193

Using large set of flat addresses user-unfriendly

Solution :

Assign a flat and an hierarchical address to every user and maintain a database linking both

Flat Addresses
internet domains

Root

be

edu

com

org

ac

ibm

vub

etro

tiberghien

Internet Domains

uk

ieee

mtv

vrt

ulb

dhondt

tiberghien@info.vub.ac.be

domain name servers
In Each domain, there is at least one DNS

Name & DNS Address of all subdomains

Name & address of all hosts in the domain

DNS address of, at least, root of DNS system

Names & DNS addresses of frequently used domains

Each host must know the address of one DNS

Address resolution can be done recursively or by successive calls to different DNS

Local DNS is locally maintained

Caching greatly improves performance of DNS by keeping the translations of most recently accessed domains and hosts

Domain Name Servers
contents13
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
telnet

Application X

m4

m2

INTERNET

User of X

User of X

Local terminal user

Remote terminal user

Telnet

Telnet

telnet to connect remote users

INTERNET

TelnetTo connect remote users

Telnet

Telnet

Application

X

Applications

Operating

system

User of X

User of X

Local terminal user

Remote terminal user

contents16
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
file transfer protocol

m4

File Transfer Protocol

m2

Internet

Direct FTP user

Third party FTP user

contents18
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
world wide web
Uniform, hypertext based, user friendly interface for distributed databases.

Inexpensive, high quality, browsers available for almost all computers.

Well over 109 pages available worldwide, mainly for public relations, publicity and electronic commerce.

Electronic commerce still somewhat restrained by security concerns and by logistics.

World Wide Web
technically what is the web
HTML = Hypertext Mark-up language

Multimedia document

Normal text

Graphics and images (stored in separate files)

Sound (stored in separate files)

Executable programs ( = applets in Java code)

References to other hypertext documents = “Anchors”

“clickable” normal text or image (icon)

address (URL) where the corresponding document can be found

Technically, what is the Web ?

The WEB = Internet + HTML + URL + HTTP

technically what is the web21

http:

//infoweb.vub.ac.be:80

/webpages/info.html

Technically, what is the Web ?

The WEB = Internet + HTML + URL + HTTP

  • URL = Universal Resource Locator
  • Protocol used to communicate with resource

(each protocol further defines URL format)

  • Internet Resource Address
    • Domain name
      • Internet Protocol address of machine (134.121.023.001)
      • Domain Server Name of machine (infoweb.vub.ac.be)
    • Port number on machine
      • Optional : Each protocol has a default port number
  • Resource details (protocol specific)

(i.e. directory and filename of an HTML page)

inside the web
Inside the Web

HTTP Server

HTTP Server

HTTP Client

(Web Browser)

HTTP Server

HTTP Link

transporting HTML-

encoded hypertext

Other Data Link

Other Server

HTTP Server

technically what is the web23
HTTP :HyperText Transfer Protocol.

HTTP Client : a Web Browser

Presenting information encoded in HTML.

Translating user interactions into the HTTP.

HTTP Server : a process interpreting client requests

Sending pieces of hypertext stored in HTML

Translating information available in other formats

acquiring information from outside the Web.

Stateless (connectionless) communication protocol

Uses a connection oriented transport protocol.

Technically, what is the Web ?

The WEB = Internet + HTML + URL + HTTP

http hypertext transfer protocol
HTTPHypertext Transfer Protocol

Web Server

Browser

Connectionless protocol

Server

HTTP

Client

HTTP

Connection oriented protocol

Transport

entity

Transport

entity

Network Service

http is a stateless protocol
Benefit : performance of server

Penalty : Server can not keep information about user.

HTTPisa Stateless Protocol

Web Server

Browser

Connectionless protocol

Server

HTTP

Client

HTTP

> show me my bank transactions

Give your name and password <

> XYZ, ******

OK, what do you want <

> show me my bank transactions

Give your name and password <

....

http is a stateless protocol26
Logging in a stateless server :

Web Server

Browser

Server

HTTP

Client

HTTP

HTTPisa Stateless Protocol

> I want to log into the server

Give your name and password <

> XYZ, ******

OK, here is your key 478 <

> show me my bank records,

my key is 478.

Here are your bank records <

$$$$$$$$$ <

If you need more use key 953 <

....

search engines
Finding information becomes more and more difficult due to the amount of information.

Automated indexing services, searching all available databases on the Internet and setting up keyword databases are very popular.

Good ranking of keywords can be purchased from some indexing services.

Many sites use tricks to be favorably presented by search engines

Search Engines
contents28
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
digital sound
Sound = variation in air pressure

Air pressure = continuous time function

To digitize sound the pressure function is sampled periodically and the corresponding pressure memorized

Time functions = sum of sine functions

Human ear can perceive pressure variations in the frequency range 25 Hz to 20000 Hz

Perception proportional to logarithm of power

Digital Sound
fourier analysis

2.

1

3.2.

1

5.2.

.t

.t

.t

=

+

+

+

SquareWave

1sin

sin

sin

T

3

T

5

T

Fourier Analysis

Any time dependant signal can be

decomposed in a series of sine functions

sampled signals

F = 2.5 kHz

Sampling Frequency = 8 kHz

Sampled Signals

Nyquist sampling theorem : Fs > 2 * F

sampled signals32

F = 2.5 kHz

Sampling Frequency = 8 kHz

Sampled Signals

Nyquist sampling theorem : Fs > 2 * F

aliasing
Aliasing

F1 = 2.5 kHz

F2 = 5.5 kHz

Sampling frequency = 8 kHz

Undersampling causes irrecoverable frequency shifts

called “aliasing errors”.

a to d conversion
A to D conversion

AD

x

Anti-

aliasing

filter

Cut-off

frequency

< 4 KHz

8 kHz sampling clock

quantization errors
Quantization errors

0101

0100

0011

0010

0001

0000

1111

1110

1101

1100

1011

data rates for sound
Based on subjective quality tests

High quality stereo music (CD) : 1.5 Mb/s

Sampling = 44 100 Hz, 2 * 16 bits / sample.

Standard telephony (ISDN) : 64 Kb/s

Sampling = 8 000 Hz, 8 bits / sample.

Voice and music contain redundancies

Data-compression techniques applicable

Music MP3, CD quality = 128 Kb/s

GSM telephony = 13 Kb/s

Dedicated compression techniques cause serious distortion if applied to other signals.

Data rates for sound
internet telephony skype
Internet Telephony(Skype)

Internet

PSTN

Pots Gateway

  • Free if both ends on the Internet
  • Local call charges if one end on PSTN
  • Voice quality dependent on network delays
  • Acceptable if network not overloaded
contents39
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
names vs addresses
A name identifies a service

The mail service of a specific person (mailbox)

The web service of a specific company

An address specifies a host on which services are available

Several names can be linked with a single address

jacques.tiberghien@ulb.ac.be

tiberghien@vub.ac.be

Several addresses can be linked with a single name

Redundant web servers

DNS assumes stable links between names and addresses

Machines offering services have stable addresses

Names vs. Addresses
beyond dns
For some applications DNS is insufficient

Internet telephony = person to person service

At different times, people are at different locations

Personal computers seldom have a unique address

One person can be reached through different devices, with different service levels.

Session Initiation Protocol (SIP, RFC 3261)

A SIP server is an agent that handles multi-media services for one or several people:

As any server, it has a stable address

It knows the present address of the people it works for

It establishes and manages connections between multi-media communicating applications.

Beyond DNS
session initiation protocol
Session Initiation Protocol

Cathy@info.vub.ac.be

The SIP server can handle both diversity in terminals and mobility of terminals

sip in redirection mode
SIP in Redirection mode

Home network

SIP

Redirection server

Callers network

Session set-up

Actual

Data transfer

Mobility management

Visited network

  • Most efficient data transfer
  • Caller has access to actual called address
sip in proxy mode
SIP in Proxy mode

Home network

Session set-up

SIP

Proxy server

Callers network

Actual

Data transfer

Mobility management

Visited network

  • Longer transmission delays
  • SIP server can be bottleneck
  • Caller has no access to the actually called address
contents45
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
image sampling
Images decomposed into matrix of pixels

A pixel has a uniform color and luminosity

Size of pixel < size of smallest detail of image

Too large pixels => spatial aliasing

Finite resolution for color and luminosity => Quantization errors.

For moving pictures, spatial and temporal aliasing can occur (e.g. wheel revolving in wrong direction).

Image Sampling
bit maps
Bit Maps
  • Luminosity and color of each point of a regular raster is encoded
  • Very versatile but requires a lot of memory.
  • In MS Windows, files with a .bmp suffix are bit maps.

Size = 10 MBytes

geometric coding of images
Straight lines between two points

circle with given center, radius, color and intensity

Very efficient for computer generated images

Geometric coding of images

Size = 280 KBytes

Size = 13 KBytes

graphical encoding
Bit Maps (.bmp files in DOS)

Luminosity and color of each point of a regular raster is encoded

Very versatile but requires a lot of memory.

Geometric coding :

Straight lines between two points

circle with given center, radius, color and intensity

Very efficient for computer generated images

Geometric coding > bit maps : simple

Bit maps > Geometric coding : Complex feature extracting algorithms: jpeg, mpeg, ...

Graphical encoding
contents50
Data Communications Applications

File & print serving

Mail

Domain Name Server

Telnet

File Transfer Protocol

World Wide Web

Multi-media Applications

Voice

Images

Side Tracks

Data compression

Person to person communications

Contents
data compression
Data Compression

Four colors encoding

1000 colors, non-uniform distribution

frequency

code

nbr of bits

code

nbr of bits

red

50%

00

1000

0

500

blue

25%

01

500

10

500

green

12.5%

10

250

110

375

yellow

12.5%

11

250

111

375

total

2000

1750

data compression52

total

2000

2250

Data Compression

Four colors encoding

1000 colors, uniform distribution

frequency

code

nbr of bits

code

nbr of bits

red

25%

00

500

0

250

blue

25%

01

500

10

500

green

25%

10

500

110

750

yellow

25%

11

500

111

750

data compression shannon s law

Non-uniform:

0.5 + 0.5 + 0.375 + 0.375 = 1.75

Uniform:

0.5 + 0.5 + 0.5 + 0.5 = 2.00

Data CompressionShannon’s Law

Minimal Number of bits per symbol :

nmin = - ipi log2 pi

In the four colors example:

data compression54
Entropy coding :

based on symbol frequency

No loss of information

used for data compression and storage

Examples

Huffman coding : variable length codes

See colors example

run length encoding : repetition factors

123333320000000123333333311111112234

12#532#7012#83#712234

Example : Zip - Unzip

Data Compression
data compression55
Source coding :

based on meaning of data

acceptable loss of information

used for data compression and storage

spectacular compression rates

Examples

GSM vocoders : only for human voice

64 Kb/s > 13 Kb/s or 6.5 Kb/s

JPEG : still images, compression 1:20

MPEG : TV images, compression 1:100

Data Compression
introduced concepts
Distributed address management

Person to person vs. address to address

Universal Resource Locator

Hypertext

Fourier : any function = sum sine functions

Nyquist :

Fsampling >= 2 * Fmax In analog signal

Aliasing occurs if Fsampling < 2 * Fmax

Shannon :

Data compression possible if redundant information

Introduced concepts