Application
(Layer 7) 	
This layer supports application and end-user processes.
Communication partners are identified, quality of service is
identified, user authentication and privacy are considered,
and any constraints on data syntax are identified.
Everything at this layer is application-specific. This layer
provides application services for file transfers, e-mail,
and other network software services. Telnet and FTP are
applications that exist entirely in the application level.
Tiered application architectures are part of this layer.
 
Presentation
(Layer 6) 	
This layer provides independence from differences in data
representation (e.g., encryption) by translating from
application to network format, and vice versa. The
presentation layer works to transform data into the form
that the application layer can accept. This layer formats
and encrypts data to be sent across a network, providing
freedom from compatibility problems. It is sometimes called
the syntax layer.
 
Session
(Layer 5) 	
This layer establishes, manages and terminates connections
between applications. The session layer sets up,
coordinates, and terminates conversations, exchanges, and
dialogues between the applications at each end. It deals
with session and connection coordination.
 
Transport
(Layer 4) 	
This layer provides transparent transfer of data between end
systems, or hosts, and is responsible for end-to-end error
recovery and flow control. It ensures complete data transfer.
 
Network
(Layer 3) 	
This layer provides switching and routing technologies,
creating logical paths, known as virtual circuits, for
transmitting data from node to node. Routing and forwarding
are functions of this layer, as well as addressing,
internetworking, error handling, congestion control and
packet sequencing.
 
Data Link
(Layer 2) 	
At this layer, data packets are encoded and decoded into
bits. It furnishes transmission protocol knowledge and
management and handles errors in the physical layer, flow
control and frame synchronization. The data link layer is
divided into two sub layers: The Media Access Control (MAC)
layer and the Logical Link Control (LLC) layer. The MAC sub
layer controls how a computer on the network gains access to
the data and permission to transmit it. The LLC layer
controls frame synchronization, flow control and error checking.
 
Physical
(Layer 1)
 	This layer conveys the bit stream - electrical impulse,
light or radio signal -- through the network at the
electrical and mechanical level. It provides the hardware
means of sending and receiving data on a carrier, including
defining cables, cards and physical aspects. Fast Ethernet,
RS232, and ATM are protocols with physical layer components.