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Business Data Communications & Networking
Key Distributed Applications
Electronic MailElectronic Data Interchange (EDI)Web-Based Applications
Business Data Communications & Networking
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
Business Data Communications & Networking
Private E-Mail
Integrated with the user’s 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: CC:Mail, QuickMail
Owned and operated by an organization for internal messaging requirements
Business Data Communications & Networking
Public v Private
Features and services can be quite similarPrivate systems involve higher initial cost,
low or no transaction costsPublic systems involve little initial cost,
ongoing transaction costsPrivate e-mail offers better integration
with installed systemsPublic systems offer wider range of
delivery options
Business Data Communications & Networking
Internet E-Mail
Does not fit directly into either category
Transfer mechanism for exchanging mail among systems, rather than a mail system itself
Business Data Communications & Networking
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
Business Data Communications & Networking
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
Business Data Communications & Networking
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
Business Data Communications & Networking
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
Business Data Communications & Networking
SMTP Mail FlowSMTP 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
Business Data Communications & Networking
SMTP Connection Setup
Sender opens TCP connection to receiverReceiver acknowledges connection with
“220 Service Ready” or “421 Service Not Available”
If connection is made, sender identifies itself with the “HELO” command
Receiver accepts identification with “250 OK”
Business Data Communications & Networking
SMTP Mail Transfer
MAIL command identifies originator, provides reverse path for error reporting
RCPT commands identify recipient(s) for message Receiver has several positive or negative
responses to RCPT Sender will not send message until it is sure
at least one copy can be deliveredDATA command transfers message
Business Data Communications & Networking
Sample SMTP Exchange
S: MAILFROM:<[email protected]>R: 250 OK
S: RCPT TO:<[email protected]>R: 250 OK
S: RCPT TO:<[email protected]>R: 550 No such user here
S: DATAR: 354 Start mail input; end with <CRLF>.<CRLF>S: Blah blah blah….S:…etc. etc. etc.S: <CRLF>.<CRLF>R: 250 OK
Business Data Communications & Networking
SMTP Connection Closing
Sender sends a QUIT command to initiate TCP close operation
Receiver sends a reply to the QUIT command, then initiates its own close
Business Data Communications & Networking
RFC 822
Defines format for text messages via electronic mail
Used by SMTP as accepted mail formatSpecifies both envelope and contentsIncludes a variety of headers that can
be included in the message header lines
Business Data Communications & Networking
Limitations of SMTP and RFC822Cannot transmit executables or binary files
without conversion into text through non-standard programs (e.g. UUENCODE)
Cannot transmit diacritical marksTransfers limited in sizeGateways do not always map properly between
EBCDIC and ASCIICannot handle non-text data in X.400 messagesNot all SMTP implementations adhere completely
to RFC821 (tabs, truncation, etc)
Business Data Communications & Networking
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
Business Data Communications & Networking
MIME Header Fields
MIME-Version: Indicates compliance with RFCs 1521 and 1522
Content-Type: Describes data in sufficient detail for receiver to pick method for representation
Content-Transfer-Encoding: Indicates type of transformation used to represent content
Content-ID: Used to uniquely identify MIME entities
Content-Description: Plain text description for use when object is not readable
Business Data Communications & Networking
MIME Content Types
Seven major types: Text, Multipart, Message, Image, Video, Audio, Application
Fourteen subtypes: See page 384 for detailsText provides only plain subtype, but a
richtext subtype is likely to be addedMultipart indicates separate parts, such as
text and an attachmentMIME types are used by web servers, as well
Business Data Communications & Networking
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
Business Data Communications & Networking
Benefits of EDI
Cost savingsSpeedReduction of errorsSecurityIntegration with office automationJust-in-time delivery
Business Data Communications & Networking
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