Application Layer

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Lecture 2

• Application Layer

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Lecture 2 Outline

• Application Architectures
• Host-Based, Client-Based, and Client-Server
  Architectures
• Choosing Architectures
• World Wide Web
• How the Web Works
• Inside an HTTP Request HTTP Response
• Electronic Mail
• How E-Mail Works, Inside an SMTP Packet
• Listserv, Attachments in MIME
• Other Applications
• FTP, Telnet, Instant Messaging, Videoconferencing

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Multi-layer Models
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Application Layer - Introduction
Application Layer
Applications (e.g., email, web, word processing)
Transport Layer
Network Layer

• Functions of Application Layer
• Data storage
• Storing of data generated by programs (e.g.,
  files, records)
• Data access logic
• Processing required to access stored data (e.g.,
  SQL)
• Application logic
• Business logic such as word processors,
  spreadsheets
• Presentation logic
• Presentation of info to user and acceptance of
  user commands

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Application Architectures

• Determined by how functions of application
  programs are spread among clients and servers
• Host-based Architectures
• Server performs almost all functions
• Client-based architectures
• Client performs most functions
• Client-server architectures
• Functions shared between client and server

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Application Architectures

• Determined by how functions of application
  programs are spread among clients and servers
• Host-based Architectures
• Server performs almost all functions
• Client-based architectures
• Client performs most functions
• Client-server architectures
• Functions shared between client and server

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Host-Based Architectures
or a PC running a terminal emulation program

• Client captures key strokes then sends them to
  the mainframe
• Client displays information according to the
  servers instructions

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Host-based Architecture Problems

• Host becoming a bottleneck
• All processing done by the host, which can
  severely limit network performance
• Host upgrades typically expensive and lumpy
• Available upgrades require large scale and often
  costly jumps in processing and memory
• Network demand grows more incrementally than does
  the host capacity
• May see poor fit (too much or too little) between
  host performance and network demand

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Application Architectures

• Determined by how functions of application
  programs are spread among clients and servers
• Host-based Architectures
• Server performs almost all functions
• Client-based architectures
• Client performs most functions
• Client-server architectures
• Functions shared between client and server

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Client-Based Architectures
Example Using a word processing package on a PC
and storing data files on a server
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Client-Based Architecture Problems

• Data traffic must travel back and forth between
  server and client
• Example when the client program is making a
  database query, the ENTIRE database must travel
  to the client before the query can be processed
• Often the large file sizes moving across the LAN
  can yield a poor result in network performance

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Application Architectures

• Determined by how functions of application
  programs are spread among clients and servers
• Host-based Architectures
• Server performs almost all functions
• Client-based architectures
• Client performs most functions
• Client-server architectures
• Functions shared between client and server

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Client-Server Architectures
Server (PC, mini, mainframe)
Client (PC)
Example Using a Web browser to obtain web pages
uses logic balanced between the client and server
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Client-Server Architectures

• Advantages
• More efficient because of distributed processing
• Allow hardware and software from different
  vendors to be used together
• Disadvantages
• Difficulty in getting software from different
  vendors to work together smoothly
• May require Middleware, a third category of
  software

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Middleware
client application programs
a standard way of translating between software
from different vendors

• Manages message transfers
• Insulates network changes from the clients (e.g.,
  adding a new server)

Middleware
server application programs

• Examples of standards for Middleware
• Distributed Computing Environment (DCE)
• Common Object Request Broker Architecture (CORBA)
• Open Database Connectivity (ODBC)

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Multi-tier Architectures

• Involve more than two computers in distributing
  application program logic
• 2-tier architecture
• Uses clients and servers in a balance, very
  popular approach in simple LANs
• 3-tier architecture
• 3 sets of computers involved
• N-tier architecture
• More than three sets of computers used, more
  typical across complex organizations
• Allows load balancing across servers

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1-tier architecture Example Mainframe
architecture
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2-tier Architecture
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3-tier Architecture
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3-tier Architecture
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3-tier Architecture
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N-tier Architecture
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Multi-tier Architectures

• Advantages
• Better load balancing
• More evenly distributed processing. (e.g.,
  application logic distributed between several
  servers.)
• More scalable
• Only servers experiencing high demand need be
  upgraded
• Disadvantages
• Heavily loaded network
• More distributed processing necessitates more
  data exchanges
• Difficult to program and test due to increased
  complexity

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Thin and Thick Clients

• Classification depends on how much of the
  application logic resides on the client or server
• Thin client
• Little or no application logic on client
• Becoming popular because easier to manage, (only
  the server application logic generally needs to
  be updated)
• The best example World Wide Web architecture
  (uses a two-tier, thin client architecture)
• Thick client
• All or most of the application logic resides on
  the client

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Thin-Client Example Web Architecture
Web Server (PC, mini, mainframe)
Client (PC)
Presentation logic
Application Logic Data Access logic Data Storage
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Criteria for Choosing Architecture

• Infrastructure Cost
• Cost of servers, clients, and circuits
• Mainframes very expensive terminals, PCs
  inexpensive
• Development Cost
• Mainly cost of software development
• Software expensive to develop off-the-shelf
  software inexpensive
• Scalability
• Ability to increase (or decrease) in computing
  capacity as network demand changes
• Mainframes not scalable PCs highly scalable

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Choosing an Architecture
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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging

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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging

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World Wide Web
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World Wide Web

• Web began with two innovative ideas
• Hypertext
• A document containing links to other documents
• Uniform Resource Locators (URLs)
• A formal way of identifying links to other
  documents
• Invention of WWW (1989)
• By Tim Berners-Lee at CERN in Switzerland
• First graphical browser, Mosaic, (1993)
• By Marc Andressen at NCSA in USA later founded
  Netscape

CERN - Conseil Européen pour la Rechèrche
Nucléaire (Berners-Lee, T. (2000) Weaving the
Web. New York HarperCollins. P. 4) NCSA -
National Center for Supercomputing Applications
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HTTP vs HTML

• HTML hypertext markup language
• Definitions of tags that are added to Web
  documents to control their appearance
• HTTP hypertext transfer protocol
• The rules governing the conversation between a
  Web client and a Web server

Both were invented at the same time by the same
person
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Reminder What is a protocol?

• In diplomatic circles, a protocol is the set of
  rules governing a conversation between people
• We have seen that the client and server carry on
  a machine-to-machine conversation
• A network protocol is the set of rules governing
  a conversation between a client and a server
• There are many protocols, HTTP is just one

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A HTTP conversation
Client
Server

• OK
• Send page or error message
• OK

• I would like to open a connection
• GET ltfile locationgt
• Display response
• Close connection

HTTP is the set of rules governing the format and
content of the conversation between a Web client
and server
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An HTTP example

• The message requesting a Web page must begin with
  the work GET and be followed by a space and the
  location of a file on the server, like this

GET /fac/lpress/shortbio.htm
The protocol spells out the exact message format,
so any Web client can retrieve pages from any Web
server.
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How the Web Works
Main Web communications protocol HTTP
Hypertext Transfer Protocol
Server Computer
HTTP Request
Client Computer
HTTP Response
A request-response cycle includes multiple
steps since web pages often contain embedded
files, such as graphics, each requiring a
separate response.
Clicking on a hyperlink or typing a URL into a
browser starts a request-response cycle
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HTTP Request Message
Request line (command, URL, HTTP version number)
required
(If the user types in the URL by themselves, then
the referring page is blank.)
Request header (information on the browser,
date, and the referring page )
optional
Request body (information sent to the server,
such as from a form)
optional
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Example of an HTTP Request
Note that this HTTP Request message has no Body
part.
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HTTP Response Message
Response status (http version number, status
code, reason)
optional
Response header (information on the server, date,
URL of the page retrieved, format used )
optional
Response body (requested web page)
required
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Example of an HTTP Response
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http viewer http//www.rexswain.com/httpview.html

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Web sniffer http//web-sniffer.net/
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HTML - Hypertext Markup Language

• A protocol used to format Web pages
• Also developed at CERN (initially for text files)
• Tags are embedded in HTML documents
• include information on how to format the file
• Extensions to HTML needed to format multimedia
  files
• XML - Extensible Markup Language
• A new markup language becoming popular

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HTTP is an application layer protocol

• The Web client and the Web server are application
  programs
• Application layer programs do useful work like
  retrieving Web pages, sending and receiving email
  or transferring files
• Lower layers take care of the communication
  details
• The client and server send messages and data
  without knowing anything about the communication
  network

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The application layer is boss the top layer
Layer Function
Application Do useful work like Web browsing, email, and file transfer
Lower layers Handle communication between the client and server

• Your boss says Send this package to Miami -- I
  don't care if you use Federal Express, UPS, or
  any other means. Also, let me know when it
  arrives or if it cannot be delivered for some
  reason.
• The application program says Send this request
  to the server -- I don't care how you do it or
  whether it goes over phone lines, radio, or
  anything else about the details. Just send the
  message, and let me know when it arrives or if it
  cannot be delivered for some reason.

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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging

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E-mail Standards
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E-mail Standards

• SMTP - Simple Mail Transfer Protocol
• Main e-mail standard for
• Originating user agent and the mail transfer
  agent
• Between mail transfer agents
• Originally written to handle only text files
• Usually used in two-tier client-server
  architectures
• Post Office Protocol (POP) and Internet Mail
  Access Protocol (IMAP)
• Main protocols used between the receiver user
  agent and mail transfer agent
• Main difference with IMAP, messages can be left
  at the server after downloading them to the
  client
• Other competing standards
• Common Messaging Calls (CMC), X.400

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Two-Tier E-mail Architecture

• User agent is another word for an e-mail client
  application
• Run on client computers
• Send e-mail to e-mail servers
• Download e-mail from mailboxes on those servers
• Examples Eudora, Outlook, Netscape Messenger
• Mail transfer agent is another word for the mail
  server application
• Used by e-mail servers
• Send e-mail between e-mail servers
• Maintain individual mailboxes.

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Host Based e-mail Architectures

• An old method used on UNIX based hosts
• Similar to client-server architecture, except
• Client PC replaced by a terminal (or terminal
  emulator)
• Sends all keystrokes to the server
• Display characters received from the server
• All software resides on the server
• Takes client keystrokes and understand users
  commands
• Creates SMTP packets and sends them to next mail
  server
• Useful when traveling in locations with poor
  internet facilities

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Web-based e-mail
Server computer with
Web server software
HTTP request
Client computer with
LAN
SMTP packet
Web browser
Server computer with
email server software
HTTP response
SMTP packet
Internet
SMTP packet
Server computer with
e-mail server software
HTTP request
Client computer with
Web browser
LAN
IMAP or POP packet
Server computer with
HTTP response
IMAP packet
Web server software
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Sample SMTP Message
Note that this SMTP message has no attachments.
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MIME

• Multipurpose Internet Mail Extension
• A graphics capable mail transfer agent protocol
  (to send graphical information in addition to
  text)
• SMTP was designed years ago for text transfer
  only
• MIME software is included as part of an e-mail
  client
• Translates graphical information into text
  allowing the graphic to be sent as part of an
  SMTP message (as a special attachment)
• Receivers e-mail client then translates the MIME
  attachment from text back into graphical format

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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging
• ListServ Forums and discussion group lists

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Listserv Discussion Groups

• Mailing lists of users who join to discuss some
  special topic (e.g., cooking, typing, networking)
• Some permit any member to post messages, some are
  more restricted
• Parts of listserv
• Listserv Processor
• Process commands ( subscriptions, etc,)
• Listserv Mailer
• Receive a message and resend it to everyone
• To subscribe
• Send an email to Listserv processor (address of
  the processor is different than the address of
  mailer)

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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging

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File Transfer Protocol (FTP)

• Enables sending and receiving files over the
  Internet
• Requires an application program on the client
  computer and a FTP server program on a server
• Commonly used today for uploading web pages
• Many packages available using FTP
• WS-FTP (a graphical FTP software)
• FTP sites
• Closed sites
• Requires account name and password
• Anonymous sites
• Account name anonymous password email address

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Upload my website files
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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging
• telNet

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telnet
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Telnet

• Allows one computer to log into another computer
• Remote login enabling full control of the host
• Requires an application program on the client
  computer and a Telnet server program on the
  server
• Client program emulates a dumb terminal off the
  server
• Many packages available conforming Telnet
• EWAN
• Requires account name and password
• Anonymous sites similar to FTP approach
• Account name anonymous password email address

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telNet from a webbrowser
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Connecting to the Library of Congress
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type 1, press ENTER
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Connecting to Weather Underground
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Connecting to National Institutes of Health
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main menu
L
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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging

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Instant Messaging (IM)

• A client-server program that allows real-time
  typed messages to be exchanged
• Client needs an IM client software
• Server needs an IM server package
• Some types allow voice and video packets to be
  sent
• Like a telephone
• Examples include AOL, ICQ, Live messenger, Yahoo
  messenger, Skype, etc.
• Two step process
• Telling IM server that you are online
• Chatting

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How Instant Messaging Works
When the sender types in text, the IM client
sends the text in a packet to the IM server which
relays it to the receiver.
IM packet
LAN
IM packet
Sender sends a request to the IM server telling
it that sender is online. If a friend connects,
the IM server sends a packet to senders IM
client and vice versa.
LAN
IM packet
Internet
If a chat session has more than two clients,
multiple packets are sent by the IM server. IM
servers can also relay information to other IM
servers.
IM packet
LAN
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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging

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Videoconferencing

• Provides real time transmission of video and
  audio signals between two or more locations
• Allows people to meet at the same time in
  different locations
• Saves money and time by not having to move people
  around
• Typically involves matched special purpose rooms
  with cameras and displays
• Desktop videoconferencing
• Low cost application linking small video cameras
  and microphones together over the Internet
• No need for special rooms
• Example Net Meeting software on clients
  communicating through a common videoconference
  server

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Videoconferencing Standards

• Proprietary early systems
• Common standards in use today
• H.320
• Designed for room-to-room videoconferencing over
  high-speed phone lines
• H.323
• Family of standards designed for desktop
  videoconferencing and just simple audio
  conferencing over Internet
• MPEG-2
• Designed for faster connections such as LAN or
  privately owned WANs

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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging
• Webcasting

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Webcasting

• Special type of uni-directional videoconferencing
• Content is sent from the server to users
• Process
• Content created by developer
• Downloaded as needed by the user
• Played by a plug-in to a Web browser
• No standards for webcasting yet
• Defacto standards products by RealNetworks

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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging
• Webcasting
• Remote desktop
• Other applications

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Diagram of Remote Desktop?
Windows XP Professional Computer
User at Remote Computer
Remote Desktop is single session.
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How Do I Enable Remote Desktop?

• Enabled by selecting a check box (open My
  Computer and click the Remote tab). See next
  slide.
• By default, Administrators can remotely connect.
• No reboot required.

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UI to Enable Remote Desktop and Add Users
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Remote Desktop Connection UI

• Click Start, point to All Programs, point to
  Accessories, point to Communications, click
  Remote Desktop Connection.

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What Can You Do with Remote Desktop?

• File system redirection (drive mapping)
• File copy redirection
• Local and network printer redirection
• Audio redirection
• High color support

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What Can You Do with Remote Desktop? (2)

• Have local computer keyboard shortcuts sent to
  remote computer
• Smart card support
• Ports redirection
• Access all your installed applications

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Connection Settings

• Default.rdp is a hidden file in My Documents that
  contains the settings for Remote Desktop
  Connection.
• Stored passwords are encrypted.
• In Windows 95, Windows 98, Windows Me, and
  Windows NT 4.0 clients, we never store the
  passwords.
• Can save the .rdp files to your desktop and treat
  them like shortcuts.

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Windows Key Combinations and Audio

• Three options for Windows key combinations
• On Local Computer
• On Remote Computer
• In Full Screen Mode only
• So no need to remember a different set of
  shortcuts like in Windows 2000.
• Audio Redirection
• Leave at remote computer
• Do not play
• Bring to local computer

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Local Resources Tab

• Under Local Devices, Smart Cards will appear if
  Smart Cards are installed on local computer.

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Audio Redirection

• Tuned to minimize the performance impact on
  network bandwidth.
• This is negotiated "on the fly".
• Does not redirect MIDI sound.

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Experience Tab for Low Bandwidth Connections

• Used to optimize connection experience.
• By default, Remote Desktop optimized for 56 Kbps
  connections.
• Several predefined settings or choose your own.

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Where Can I Get Remote Desktop Connection?

• From the Windows XP CD Take the CD with you,
  insert it in a down-level computer, and click
  "Perform Additional Tasks"
• or
• Download from http//www.microsoft.com/windowsxp/r
  emotedesktop/
• or
• Use Remote Desktop Web Connection

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Remote Desktop Web Connection

• ActiveX control that runs inside of Internet
  Explorer 4.0 or later.
• Installed by going to
• Add/Remove Programs
• Windows Components
• Internet Information Server Details
• World Wide Web Service Details
• Remote Desktop Web Connection
• Finally navigate to computer with Internet
  Explorer to http//ltmycomputergt/tsweb/.

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Get Access to Your Desktop Through Internet
Explorer
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Applications Layer Examples

• World Wide Web
• E-mail
• File Transfer
• Videoconferencing
• Instant Messaging
• Webcasting
• Other applications

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Implications for Management

• Network may be used to provide a worry-free
  environment for applications
• Network is the critical infrastructure over which
  the wide variety of strategic applications enable
  an organization to compete in its environment
• The applications running on the network have the
  potential for changing the organization
• Dramatic increase in number and type of
  applications
• Rapid growth in amount and type of networking
  traffic over time
• Different implication on network design and
  management
• Increased operating cost for the network function