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 
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 
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 
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 
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 
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 
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 
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.
