Each individual OSI
layer has a set of functions that it must perform in order for data packets
to travel from a source to a destination on a network. Below is a brief
description of each layer in the OSI reference model as shown in the
Figure.
Layer 7: The
Application Layer
![](./22_files/image008.gif)
The application layer is the OSI layer that is closest to the user; it
provides network services to the user's applications. It differs from the
other layers in that it does not provide services to any other OSI layer,
but rather, only to applications outside the OSI model. Examples of such
applications are spreadsheet programs, word processing programs, and bank
terminal programs. The application layer establishes the availability of
intended communication partners, synchronizes and establishes agreement on
procedures for error recovery and control of data integrity. If you want to
remember Layer 7 in as few words as possible, think of browsers.
Layer 6: The Presentation
Layer ![](./22_files/image009.gif)
The presentation layer ensures that the information that the application
layer of one system sends out is readable by the application layer of
another system. If necessary, the presentation layer translates between
multiple data formats by using a common format. If you want to think of
Layer 6 in as few words as possible, think of a common data format.
Layer 5: The Session
Layer![](./22_files/image010.gif)
As its name implies, the session layer establishes, manages, and terminates
sessions between two communicating hosts. The session layer provides its
services to the presentation layer. It also synchronizes dialogue between
the two hosts' presentation layers and manages their data exchange. In
addition to session regulation, the session layer offers provisions for
efficient data transfer, class of service, and exception reporting of session
layer, presentation layer, and application layer problems. If you want to
remember Layer 5 in as few words as possible, think of dialogues and
conversations.
Layer 4: The
Transport Layer![](./22_files/image011.gif)
The transport layer segments data from the sending host's system and
reassembles the data into a data stream on the receiving host's system. The
boundary between the transport layer and the session layer can be thought
of as the boundary between application protocols and data-flow protocols.
Whereas the application, presentation, and session layers are concerned
with application issues, the lower four layers are concerned with data
transport issues.
The transport layer
attempts to provide a data transport service that shields the upper layers
from transport implementation details. Specifically, issues such as how
reliable transport between two hosts is accomplished is the concern of the
transport layer. In providing communication service, the transport layer
establishes, maintains, and properly terminates virtual circuits. In
providing reliable service, transport error detection-and-recovery and
information flow control are used. If you want to remember Layer 4 in as
few words as possible, think of quality of service, and reliability.
Layer 3: The Network
Layer![](./22_files/image012.gif)
The network layer is a complex layer that provides connectivity and path
selection between two host systems that may be located on geographically
separated networks. If you want to remember Layer 3 in as few words as
possible, think of path selection, routing, and addressing.
Layer 2: The Data
Link Layer![](./22_files/image013.gif)
The data link layer provides reliable transit of data across a physical
link. In so doing, the data link layer is concerned with physical (as
opposed to logical) addressing, network topology, network access, error
notification, ordered delivery of frames, and flow control. If you want to
remember Layer 2 in as few words as possible, think of frames and media
access control.
Layer 1: The
Physical Layer![](./22_files/image014.gif)
The physical layer defines the electrical, mechanical, procedural, and
functional specifications for activating, maintaining, and deactivating the
physical link between end systems. Such characteristics as voltage levels,
timing of voltage changes, physical data rates, maximum transmission
distances, physical connectors, and other, similar, attributes are defined
by physical layer specifications. If you want to remember Layer 1 in as few
words as possible, think of signals and media.
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