Chapter 15, Layer 7: Application Layer
application layer of the OSI model is the layer closest to the end system.
This determines whether sufficent
resources exist for communication between systems.
application layer does not provide support for any other OSI layer.
Most applications that work in a networked enviroment are classified as client-server applications.
applications, such as FTP, web browsers, and e-mail, all have two components,
which allow tham to function:
1. The Client Side
2. The Server Side
World Wide Web (www), and the web browsers netscape and internet explorer
are probably the most
commonly used network applications.
is a protocol that works with computer operating systems and network clients
instead of specific
a client wants to save a file from a word processor to a network, the redirector
enables the word processing
application to become a client.
network redirector enables data to be sent to to a device that is not directly
connected to the computer.
Two Types of Communication
Make a connection - break a connection and Termination determined by client.
communication activity falls into one of these two categories.
Make a Connection
Break a Connection
common cycle; make a connection - break a connection, between server and
client, underlies all Web Page
downloading a webpage, the connection to the remote serverwill be maintained
only long enough to process
Client Determines Termination
is illustrated by the Telnet and FTP examples, which establish a connection
to the server, and maintain that
connection unitl all processing has been performed.
client terminates the connection when the user determines that he/she is
internet is built on a hierarchial dressing scheme. This allows for
routing that is based on classes of addresses,
as opposed to individual addresses.
A domain is a group of computers that are associated by their geographical location or their business type.
A domain is a string of characters and/or numbers that reprsents the numeric address of an Internet site.
the URL http://www.cisco.com, the "cisco.com" part defines the domain.
DNS - Domain Name Server
DNS server manages domain names and responds to requests from clients to
translate a domain name into
the associated IP addresss.
E-mail cliant applications, such as Eudora, Outlook Express, and Netscape Mail, work with the POP3 protocol.
security, when message recipients check their e-mail they are often prompted
for a password. The password can
also be saved in many programs.
Terminal Emulation (Telnet) software provides the abiltiy to remortely access another computer.
telnet application works mainly on the application, resentation, and session
layers of the OSI model.