D-Link DES-3250TG User Manual page 60

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide
Layer 7
This layer, known as the application layer, provides access to
either the end user application software such as a database.
Users communicate with the application, which in turn delivers
data to the transport layer. Applications do not usually
communicate directly with lower layers. They are written to use
a specific communication library, like the popular WinSock
library.
Software developers must decide what type of transport
mechanism is necessary. For example, Web access requires
reliable, error-free access, and would demand TCP.
Multimedia, on the other hand, requires low overhead and
latency and commonly uses UDP.
TCP/IP
The TCP/IP protocol suite is a set of protocols that allow
computers to share resources across a network. TCP and IP are
only two of the Internet suite of protocols, but they are the best
known and it has become common to refer the entire family of
Internet protocols as TCP/IP.
TCP/IP is a layered set of protocols. An example, such as
sending e-mail, can illustrate this. There is first a protocol for
sending and receiving e-mail. This protocol defines a set of
commands to identify the sender, the recipient, and the content
of the e-mail. The e-mail protocol will not handle the actual
communication between the two computers, this is done by
TCP/IP. TCP/IP handles the actual sending and receiving of the
packets that make up the e-mail exchange.
TCP makes sure the e-mail commands and messages are
received by the appropriate computers. It keeps track of what
is sent and what is received, and retransmits any packets that
are lost or dropped. TCP also handles the division of large
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