CIS-325 Data Communications

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CIS-325Data Communications

• Dr. L. G. Williams, Instructor

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Chapter Thirteen

• Distributed Applications

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Key Distributed Applications

• Electronic Mail
• Electronic Data Interchange (EDI)
• Web-Based Applications

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Public E-Mail

• Public is provided by a vendor, generally via a
  dial-up network
• Messages can be sent to any other registered user
• Gateway required to send messages to users
  outside the system
• e.g. AOL, MCI Mail

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Private E-Mail

• Integrated with the users computer system
• Can run on a central host, or as part of a LAN
  environment
• Host examples DEC All-In-One, IBM PROFS
• LAN examples CCMail, QuickMail
• Owned and operated by an organization for
  internal messaging requirements

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Public vs. Private

• Features and services can be quite similar
• Private systems involve higher initial cost, low
  or no transaction costs
• Public systems involve little initial cost,
  ongoing transaction costs
• Private e-mail offers better integration with
  installed systems
• Public systems offer wider range of delivery
  options

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Internet E-Mail

• Does not fit directly into either category
• Transfer mechanism for exchanging mail among
  systems, rather than a mail system itself

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Single System E-Mail

• Only allows users of a shared system to exchange
  messages
• Each user has unique identifier and mailbox
• Sending a message simply puts it into recipients
  box
• Recipient fetches or reads message when desired

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Multiple Systems E-Mail

• Distributed system enables mail servers to
  connect over a network to exchange mail
• Functions split
• User agent handles preparation, submission,
  reading, filing, etc
• Transfer agent receives mail from user,
  determines routing, communicates with remote
  systems
• Interconnection requires standards

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Simple Mail Transfer Protocol (SMTP)

• Standard for TCP/IP mail transfer, defined in RFC
  821
• Concerned addressing and delivery, not content,
  with two exceptions
• Character set standardized as 7-bit ASCII
• Adds log information to message that indicates
  message path

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Basic E-Mail Operation

• User creates message with user agent program
• Text includes RFC 822 header and body of message
• List of destinations derived from header
• Messages are queued and sent to SMTP sender
  program running on a host

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SMTP Mail Flow

• SMTP server transmits messages to appropriate
  hosts via TCP
• Multiple messages to same host can be sent on one
  connection
• Errors handling necessary for faulty addresses
  and unreachable hosts
• SMTP protocol attempts to provide error-free
  transmission, but does not provide end-to-end
  acknowledgement
• SMTP receiver accepts messages, places it in
  mailbox or forwards

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Limitations of SMTP and RFC822

• Cannot transmit executables or binary files
  without conversion into text through non-standard
  programs (e.g. UUENCODE)
• Cannot transmit diacritical marks
• Transfers limited in size
• Gateways do not always map properly between
  EBCDIC and ASCII
• Cannot handle non-text data in X.400 messages
• Not all SMTP implementations adhere completely to
  RFC821 (tabs, truncation, etc)

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MIME (Multipurpose Internet Mail Extensions)

• Intended to resolve problems with SMTP and RFC822
• Specifies five new header fields, providing info
  about body of message
• Defines multiple content formats
• Defines encodings to enable conversion of any
  type of content into transferable form

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Electronic Data Interchange (EDI)

• Direct, computer-to-computer exchange of business
  data
• Replaces use of paper documents
• Requires two participants to agree on electronic
  format for the data
• Two parties within a company (depts)
• Companies and customers
• Multiple companies

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Benefits of EDI

• Cost savings
• Speed
• Reduction of errors
• Security
• Integration with office automation
• Just-in-time delivery

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EDI Layered Architecture

• Semantic Layer describes the business
  application (e.g. procurement)
• Standard Layer X.12 from ANSI, EDIFCT from UN
• Transport Layer E-mail, Point to Point, WWW
• Physical Layer Dial-up lines, Internet, WANs

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EDI v E-Mail

• EDI
• Typically no human involvement in processing the
  information interface is software-to-software

• E-Mail
• Data not necessarily structured for software
  processing. Human-to-software exchange is usually
  involved on at least one end.

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World Wide Web

• Developed by Tim Berners-Lee in 1991 at the
  European Particle Physics Laboratory (CERN) to
  allow physicists around the world to share
  information
• Marc Andreessen, employee at NCSA, introduced the
  GUI Web browser, Mosaic, in 1993.

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World Wide Web Technology

• Based on the concept of hypertext
• Single interface to a variety of protocols and
  standards to access the information on the
  Internet
• Has become the predominant Internet application
• 1993 annual rate of growth is 341,634
• 1995 surpassed all other applications in amount
  of data being transferred

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World Wide Web Documents

• Documents have embedded selectable links that
  point to other documents
• Documents can be text or non-textual information
  such as audio, video, or multimedia documents
• Multimedia nature of the web allows non-text
  information to be embedded into documents, not
  just linked separately

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World Wide Web Functions

• Display HTML pages
• Retrieve files (often using FTP)
• Access Gopher sites
• Read newsgroups
• Search for information or people
• Display still images and video
• Listen to sound files
• Activate telnet connections

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World Wide Web Operation

• Data stored on servers
• Users access data with browsers
• GUI browser (e.g, Mosaic, Netscape, Internet
  Explorer, etc.) requires a direct Internet
  connection (dial-up or LAN)
• Browser requests access to a page
• Server transfers copy of page to requestor

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Uniform Resource Locator (URL)

• specify the three pieces of information
• the protocol to be used (e.g., http, ftp, gopher,
  telnet, etc.)
• the server (and optional port)
• the file path to retrieve
• e.g., http//www.rit.edu/ellics/index.htmlhttp/
  /www.itcs.com80/elawley/ftp//ftp.netscape.com/
  telnet//wally.rit.edu/gopher//gopher.cni.org/

• provide a standard means of locating Internet
  documents
• simple addressing scheme unifies a wide variety
  of disparate protocols

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HyperText Markup Language (HTML)

• Derived from SGML (Standard Generalized Markup
  Language)
• Uses tags to indicate formatting characteristics
  them with a specific format
• Tags are defined functionally, not visually
• Tags can indicate a link to another document or
  resource
• Tagged text can be displayed by choosing View
  Source in most browsers

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HyperText Transfer Protocol (HTTP)

• the primary protocol used to distribute
  information within WWW
• a connectionless protocol
• limited to one request per connection
• the connection is broken after each request