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OSI-MODEL
The OSI, or Open System Interconnection, model defines a networking framework
for implementing protocols in seven layers. Control is passed from one layer to
the next, starting at the application layer in one station, proceeding to the
bottom layer, over the channel to the next station and back up the hierarchy.
Layer 7: Application -
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.
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.
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.
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.
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.
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.
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. Presentation (Layer 6)
Session (Layer 5)
Transport (Layer 4)
Network (Layer 3)
Data Link (Layer 2)
Physical (Layer 1)
