09-07-K.Steenhaut & J.Tiberghien - VUB 1 Telecommunications Concepts Chapter 1.3 Some Applications.

09-07-K.Steenhaut & J.Tiberghien - VUB1

TelecommunicationsConcepts

Chapter 1.3

Some Applications


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







Applications, File & Print

ServingPrint

serverFile

server

m4

Applicationsserver







email

a@m1 d@m4c@m4b@m2

m4

d@m4

INTERNET

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

Eudora Outlook ExchangeNetscapea@m1 d@m4c@m4b@m2

m4

SMTPpop3imap

(X400)

http

Pop/imap serverMail Transfer Agent

(X400)

UserAgent







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!


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


Internet Domains

uk

Root

ibm vrt

be

ac

ulb

dhondt

vub

mtv ieee

com orgedu

[email protected]

etro

tiberghien


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







Telnet

User of X

Application X

m4

Remote terminal user

m2

INTERNET

User of X

Local terminal user

Telnet


TelnetTo connect remote

users

User of X

ApplicationX

Remote terminal user

User of X

Local terminal user

TelnetTelnet

INTERNET

Applications

Operatingsystem







File Transfer Protocol

m4m2

Internet

Direct FTP userThird party FTP user







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.


Technically, what is the Web ?The WEB = Internet + HTML + URL + HTTP

• 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

• 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)

http: //infoweb.vub.ac.be:8080 /webpages/info.html


Inside the Web

HTTP Client(Web Browser)

HTTP Server

HTTP Server

HTTP Server

Other ServerHTTP Server

HTTP Linktransporting HTML-encoded hypertext

Other Data Link


Technically, what is the Web ?

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.

The WEB = Internet + HTML + URL + HTTP


HTTPHypertext Transfer

Protocol

Network Service

Transport entity

Transport entity

Connection oriented protocol

Client HTTP

Server HTTP

Connectionless protocol

Browser Web Server


HTTPis a Stateless

Protocol• Benefit : performance of server

• Penalty : Server can not keep information about user.

Client HTTP

Server HTTP

Connectionless protocol

Browser Web Server

> show me my bank transactionsGive your name and password <

> XYZ, ******OK, what do you want <

> show me my bank transactionsGive your name and password <

....


HTTPis a Stateless

ProtocolLogging in a stateless server :

Client HTTP

Browser

Server HTTP

Web Server> 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







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


Fourier AnalysisAny time dependant signal can be

decomposed in a series of sine functions

T

5.2..tsin51

T3.2..tsin

31

T2..t1 sinSquareWave


Sampled Signals

F = 2.5 kHz Sampling Frequency = 8 kHz

Nyquist sampling theorem : Fs > 2 * F


Sampled Signals

F = 2.5 kHz Sampling Frequency = 8 kHz

Nyquist sampling theorem : Fs > 2 * F


Aliasing

F1 = 2.5 kHz

F2 = 5.5 kHzSampling frequency = 8 kHz

Undersampling causes irrecoverable frequency shiftscalled “aliasing errors”.


A to D conversion

x

8 kHz sampling clock

AD

Anti-aliasing

filterCut-off

frequency< 4 KHz


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.


Sound Communications

Example : MP3 Music Server

http and/or FTP

Transmission delays cause no harm !


Internet Telephony(Skype)

Internet

- 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

Pots Gateway

PSTN







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

– [email protected]

– [email protected]

• 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


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.


Session Initiation [email protected]

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


Home network

Visited network

SIP in Redirection mode

Callers networkSIP Redirection server

Mobility management

Session set-upActual

Data transfer

• Most efficient data transfer• Caller has access to actual

called address


Home network

Visited network

SIP in Proxy mode

Callers networkSIP Proxy server

Mobility management

Session set-up

ActualData transfer

• Longer transmission delays• SIP server can be bottleneck• Caller has no access to the

actually called address







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).


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

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, ...







frequency code nbr of bits code nbr of bits

red 50% 00 1000 0 500

yellow 12.5% 11 250 111 375

blue 25% 01 500 10 500

green 12.5% 10 250 110 375

Data Compression

total 2000 1750

Four colors encoding

1000 colors, non-uniform distribution


frequency code nbr of bits code nbr of bits

red 25% 00 500 0

yellow 25% 11 500 111

blue 25% 01 500 10

green 25% 10 500 110

250

750

500

750

Data Compression

total 2000 2250

Four colors encoding

1000 colors, uniform distribution


Data CompressionShannon’s Law

Minimal Number of bits per symbol :

nmin = - ipi log2 pi

In the four colors example:

Non-uniform:

Uniform:

0.5 + 0.5 + 0.375 + 0.375 = 1.75

0.5 + 0.5 + 0.5 + 0.5 = 2.00


Data Compression

• 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

• 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


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

09-07-K.Steenhaut & J.Tiberghien - VUB 1 Telecommunications Concepts Chapter 1.3 Some Applications.

Documents

tiberghien vub

steenhaut j

person communications

etro tiberghien slide

domain dns address

mail domain

internet slide

m4 internet m4