Definition: Learn what the Open Systems Interconnection (OSI)
reference model is and how its seven layers of functions provide vendors and
developers with a common language for discussing how messages should be
transmitted between any two points in a telecommunication network
OSI (Open Systems Interconnection) is reference model for
how applications can communicate over a network. A reference model is a
conceptual framework for understanding relationships.
The
purpose of the OSI reference model is to guide vendors and developers so the
digital communication products and software programs they create will Inter-operate,
and to facilitate clear comparisons among communications tools. Most vendors
involved in telecommunications make an attempt to describe their products and
services in relation to the OSI model. And although useful for guiding
discussion and evaluation, OSI is rarely actually implemented, as few network
products or standard tools keep all related functions together in well-defined
layers as related to the model. The TCP/IP protocols, which define the
Internet, do not map cleanly to the OSI model.
The seven Open Systems Interconnection layers are:
Layer 7: The application layer. This is
the layer at which communication partners are identified (Is there someone to
talk to?), network capacity is assessed (Will the network let me talk to them
right now?), and that creates a thing to send or opens the thing received.
(This layer is not the application itself, it is the set
of services an application should be able to make use of directly, although
some applications may perform application layer functions.)
Layer 6: The presentation layer. This
layer is usually part of an operating system and converts incoming and outgoing
data from one presentation format to another (for example, from
clear text to encrypted text at one end and back to clear text at the other).
Layer 5: The session
layer. This layer sets up, coordinates and terminates
conversations. Services include authentication and reconnection after an
interruption.
Layer 4: The
transport layer. This layer manages packetization of
data, then the delivery of the packets, including checking for errors in the
data once it arrives.
Layer 3: The network
layer. This layer handles the addressing and routing
of the data (sending it in the right direction to the right destination on
outgoing transmissions and receiving incoming transmissions at the packet
level). IP is the network layer for the Internet.
Layer 2: The
data-link layer. This layer sets up links across the
physical network, putting packets into network frames. This layer has two
sub-layers, the Logical Link Control Layer and the Media Access
Control Layer. Ethernet is the main data link layer in use.
Layer 1: The
physical layer. This layer conveys the bit streamthrough
the network at the electrical, optical or radio level. It provides the hardware means
of sending and receiving data on a carrier network.
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