The 7 Layers of the OSI Model
, 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 chan
nel to the
next station and back up the hierarchy.
This layer supports
Communication partners are identified, quality of service is
identified, user authentication and privacy are considered, and
any constraints on data
are identified. Everything at this
layer is application
specific. This layer provides application
, and other
are applications that exist entirely in
the application leve
Tiered application architectures are part of
This layer provides independence from differences in data
) by translatin
g 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
, providing freedom from compatibility problems.
sometimes called the syntax layer.
This layer establishes, manages and terminates connections
. The session layer sets up, coordinates, and
terminates conversations, exchanges, and dialogues between the
applications at each end.
It deals with session and connection
This layer provides transparent transfer of data between end
, and is responsible for end
It ensures complete data transfer.
This layer provides
logical paths, known as
, for transmitting data from
to node. Routing and forwarding are funct
ions of this layer,
as well as
, error handling,
control and packet sequencing.
At this layer, data packets are
and decoded into bits. It
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
) layer and the
(LLC) layer. The MAC sub layer controls how a computer
network gains access to the data and permission to
transmit it. The LLC layer controls frame
control and error checking.
This layer conve
ys the bit stream
electrical impulse, light or
at the electrical and
mechanical level. It provides the
means of sending and
receiving data on a carrier, including defining cables, cards and
with physical layer components.
The TCP/IP model
TCP/IP is based on a four
reference model. All protocols that belong to the TCP/IP
protocol suite are located in the top three layers of this model.
As shown in the following illustration, each layer of the TCP/IP model corresponds to one or
more layers of the seven
layer Open Syst
ems Interconnection (OSI) reference model
proposed by the International Standards Organization (ISO).
The types of services performed and protocols used at each layer within the TCP/IP model
are described in more detail in the following table.
Defines TCP/IP application protocols and how host
programs interface with transport layer services to
use the network.
HTTP, Telnet, FTP,
TFTP, SNMP, DNS,
communication session management
between host computers. Defines the level of
service and status of the connection used when
TCP, UDP, RTP
Packages data into IP datagrams, which contain
source and destination address informati
on that is
used to forward the datagrams between hosts and
Performs routing of IP datagrams.
IP, ICMP, ARP, RARP
Specifies details of how data is physically sent
through the network, including how bits are
ignaled by hardware devices that
interface directly with a network medium, such as
coaxial cable, optical fiber, or twisted
Ethernet, Token Ring,
FDDI, X.25, Frame