CIS-325: Data Communications 1 CIS-325 Data Communications Dr. L. G. Williams, Instructor

CIS-325: Data Communications 2 Chapter Thirteen Distributed Applications

CIS-325: Data Communications3 Key Distributed Applications ä Electronic Mail ä Electronic Data Interchange (EDI) ä Web-Based Applications

CIS-325: Data Communications4 Public ä Public is provided by a vendor, generally via a dial-up network ä Messages can be sent to any other registered user ä Gateway required to send messages to users outside the system ä e.g. AOL, MCI Mail

CIS-325: Data Communications5 Private ä Integrated with the user’s computer system ä Can run on a central host, or as part of a LAN environment ä Host examples: DEC All-In-One, IBM PROFS ä LAN examples: CC:Mail, QuickMail ä Owned and operated by an organization for internal messaging requirements

CIS-325: Data Communications6 Public vs. Private ä Features and services can be quite similar ä Private systems involve higher initial cost, low or no transaction costs ä Public systems involve little initial cost, ongoing transaction costs ä Private offers better integration with installed systems ä Public systems offer wider range of delivery options

CIS-325: Data Communications7 Internet ä Does not fit directly into either category ä Transfer mechanism for exchanging mail among systems, rather than a mail system itself

CIS-325: Data Communications8 Single System ä Only allows users of a shared system to exchange messages ä Each user has unique identifier and mailbox ä Sending a message simply puts it into recipients’ box ä Recipient fetches or reads message when desired

CIS-325: Data Communications9 Multiple Systems ä Distributed system enables mail servers to connect over a network to exchange mail ä Functions split ä User agent handles preparation, submission, reading, filing, etc ä Transfer agent receives mail from user, determines routing, communicates with remote systems ä Interconnection requires standards

CIS-325: Data Communications10 Simple Mail Transfer Protocol (SMTP) ä Standard for TCP/IP mail transfer, defined in RFC 821 RFC 821RFC 821 ä Concerned addressing and delivery, not content, with two exceptions ä Character set standardized as 7-bit ASCII ä Adds log information to message that indicates message path

CIS-325: Data Communications11 Basic Operation ä User creates message with user agent program ä Text includes RFC 822 header and body of message ä List of destinations derived from header ä Messages are queued and sent to SMTP sender program running on a host

CIS-325: Data Communications12 SMTP Mail Flow ä SMTP server transmits messages to appropriate hosts via TCP ä Multiple messages to same host can be sent on one connection ä Errors handling necessary for faulty addresses and unreachable hosts ä SMTP protocol attempts to provide error-free transmission, but does not provide end-to-end acknowledgement ä SMTP receiver accepts messages, places it in mailbox or forwards

CIS-325: Data Communications13 Limitations of SMTP and RFC822 ä Cannot transmit executables or binary files without conversion into text through non-standard programs (e.g. UUENCODE) ä Cannot transmit diacritical marks ä Transfers limited in size ä Gateways do not always map properly between EBCDIC and ASCII ä Cannot handle non-text data in X.400 messages ä Not all SMTP implementations adhere completely to RFC821 (tabs, truncation, etc)

CIS-325: Data Communications14 MIME (Multipurpose Internet Mail Extensions) ä Intended to resolve problems with SMTP and RFC822 ä Specifies five new header fields, providing info about body of message ä Defines multiple content formats ä Defines encodings to enable conversion of any type of content into transferable form

CIS-325: Data Communications15 Electronic Data Interchange (EDI) ä Direct, computer-to-computer exchange of business data ä Replaces use of paper documents ä Requires two participants to agree on electronic format for the data ä Two parties within a company (depts) ä Companies and customers ä Multiple companies

CIS-325: Data Communications16 Benefits of EDI ä Cost savings ä Speed ä Reduction of errors ä Security ä Integration with office automation ä Just-in-time delivery

CIS-325: Data Communications17 EDI Layered Architecture ä Semantic Layer: describes the business application (e.g. procurement) ä Standard Layer: X.12 from ANSI, EDIFCT from UN ä Transport Layer: , Point to Point, WWW ä Physical Layer: Dial-up lines, Internet, WANs

CIS-325: Data Communications18 EDI v ä EDI ä Typically no human involvement in processing the information; interface is software-to-software ä ä Data not necessarily structured for software processing. Human-to- software exchange is usually involved on at least one end.

CIS-325: Data Communications19 World Wide Web ä Developed by Tim Berners-Lee in 1991 at the European Particle Physics Laboratory (CERN) to allow physicists around the world to share information ä Marc Andreessen, employee at NCSA, introduced the GUI Web browser, Mosaic, in 1993.

CIS-325: Data Communications20 World Wide Web Technology ä Based on the concept of hypertext ä Single interface to a variety of protocols and standards to access the information on the Internet ä Has become the predominant Internet application ä 1993: annual rate of growth is 341,634% ä 1995: surpassed all other applications in amount of data being transferred

CIS-325: Data Communications21 World Wide Web Documents ä Documents have embedded selectable links that point to other documents ä Documents can be text or non-textual information such as audio, video, or multimedia documents ä Multimedia nature of the web allows non- text information to be embedded into documents, not just linked separately

CIS-325: Data Communications22 World Wide Web Functions ä Display HTML pages ä Retrieve files (often using FTP) ä Access Gopher sites ä Read newsgroups ä Search for information or people ä Display still images and video ä Listen to sound files ä Activate telnet connections

CIS-325: Data Communications23 World Wide Web Operation ä Data stored on servers ä Users access data with browsers ä GUI browser (e.g, Mosaic, Netscape, Internet Explorer, etc.) requires a direct Internet connection (dial-up or LAN) ä Browser requests access to a page ä Server transfers copy of page to requestor

CIS-325: Data Communications24 Uniform Resource Locator (URL) ä provide a standard means of locating Internet documents ä simple addressing scheme unifies a wide variety of disparate protocols ä specify the three pieces of information ä the protocol to be used (e.g., http, ftp, gopher, telnet, etc.) ä the server (and optional port) ä the file path to retrieve ä e.g., ftp://ftp.netscape.com/ telnet://wally.rit.edu/ gopher://gopher.cni.org/

CIS-325: Data Communications25 HyperText Markup Language (HTML) ä Derived from SGML (Standard Generalized Markup Language) ä Uses tags to indicate formatting characteristics them with a specific format ä Tags are defined functionally, not visually ä Tags can indicate a link to another document or resource ä Tagged text can be displayed by choosing “View Source” in most browsers

CIS-325: Data Communications26 HyperText Transfer Protocol (HTTP) ä the primary protocol used to distribute information within WWW ä a “connectionless” protocol ä limited to one request per connection ä the connection is broken after each request