Networks III

The OSI Reference Model


To understand how this process works, you need to have a solid
understanding of the OSI Reference Model. The OSI Reference Model was
designed by delegates from major computer and telecom companies back
in 1983. The goal was to design a network communications model that
was modular so that products from different vendors could
interoperate. Prior to this, network- ing solutions tended to be
proprietary, forcing implementers to purchase all of their components
from the same vendor. By defining the OSI Reference Model, the
industry created a standard that allows you to pick and choose
components from a variety of vendors.



The OSI Reference Model divides the communication process between two
hosts into layers. This is shown in Figure 15-1.



These layers break down the overall communication process into
specific tasks. Information flows down through the layers on the
sending system and then is transmitted on the network medium. The
information then flows up the layers on the receiving side.



The OSI Reference Model has seven layers:



• Physical 

Transmits electrical signals between hosts.


• Data Link 

Defines the rules and procedures for accessing the
physical layer. It defines how hosts are identified on the network and
how the network medium is accessed. It also specifies how to verify
that the data received from the physical layer doesn’t have any
errors. Information received from upper layers is organized into
datagrams.


• Network 

Enables the routing of the data. It specifies how to
recognize the address of neighboring nodes and routers. It also
specifies how to determine the next network point to which a packet
should be forwarded toward its destination. The Internet Protocol (IP)
operates at this layer, as does the Internet Control Message Protocol
(ICMP).


• Transport 

On the sending host, the Transport layer receives
information from the upper layers of the OSI model and divides it into
small, transmittable chunks called packets. On the receiving host, the
Transport layer reassembles packets from datagrams received from lower
layers of the OSI model. The Transport layer provides error-checking
mechanisms to ensure that data arrives at the destination host intact.
The Transmission Control Protocol (TCP) and User Datagram Protocol
(UDP) operate at this layer.


• Session

Responsible for establishing and maintaining connections
between source and destination network hosts. These connections are
called sessions.


• Presentation 

Responsible for ensuring that information passing
through the OSI layers is formatted correctly for the application on
the destination system.


• Application 

Responsible for providing applications with a way to
access the network.












LX0-104 Exam Objectives (R)