Telecommunications

Chapter 1.3 Some Applications TelecommunicationsConcepts

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 & PrintServing File server Applications server Print server m4

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)

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

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

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

Root be edu com org ac ibm vub etro tiberghien Internet Domains uk ieee mtv vrt ulb dhondt tiberghien@info.vub.ac.be

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

Application X m4 m2 INTERNET User of X User of X Local terminal user Remote terminal user Telnet Telnet

INTERNET TelnetTo connect remote users Telnet Telnet Application X Applications Operating system User of X User of X Local terminal user Remote terminal user

m4 File Transfer Protocol m2 Internet Direct FTP user Third party FTP user

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

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

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 HTTP Server HTTP Server HTTP Client (Web Browser) HTTP Server HTTP Link transporting HTML- encoded hypertext Other Data Link Other Server HTTP Server

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

HTTPHypertext Transfer Protocol Web Server Browser Connectionless protocol Server HTTP Client HTTP Connection oriented protocol Transport entity Transport entity Network Service

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 < ....

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 < ....

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

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

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

F = 2.5 kHz Sampling Frequency = 8 kHz Sampled Signals Nyquist sampling theorem : Fs > 2 * F

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 AD x Anti- aliasing filter Cut-off frequency < 4 KHz 8 kHz sampling clock

Quantization errors 0101 0100 0011 0010 0001 0000 1111 1110 1101 1100 1011

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

http and/or FTP Sound CommunicationsExample : MP3 Music Server Transmission delays cause no harm !

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

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

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 Cathy@info.vub.ac.be The SIP server can handle both diversity in terminals and mobility of terminals

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 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

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 • 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

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

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

Telecommunications