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WHAT IS IT??
• Open Systems Interconnect (OSI) Model : International
Standards Organization (ISO)
• “Open” means the concepts are nonproprietary; can be used by
anyone
• OSI is a model for understanding and designing a network
architecture that is flexible and robust
• The OSI model describes how data flows from one computer,
through a network to another computer
• 7 layers
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Layered Approach
• The process of breaking up the functions or tasks of networking
into layers reduces complexity.
• Each layer provides a service to the layer above it in the protocol
specification.
• Each layer communicates with the same layer’s software or
hardware on other computers.
• Data is Encapsulated with the necessary information as it moves
down the layers before network transit.
• Layer architecture simplifies the network design.
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Layered Approach
• It is easy to debug network applications in a layered
architecture network.
• The network management is easier due to the layered
architecture.
• Network layers follow a set of rules, called protocol.
• The protocol defines the format of the data being
exchanged, and the control and timing for the handshake
between layers.
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OSI in Action
• A message begins at the top application layer and moves down the
OSI layers to the bottom physical layer.
• As the message descends, each successive OSI model layer adds a
header to it.
• A header is layer‐specific information that basically explains what
functions the layer carried out.
• Conversely, at the receiving end, headers are striped from the
message as it travels up the corresponding layers.
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Service Primitives
A service is formally specified by a set of primitives (basic
operations) available to a user or other entity to access the
service.
Example: five service primitives for implementing a simple
connection-oriented service.
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Protocol Suite
• Networking model offers a generic means to separate
computer networking functions into multiple layers.
• Each of these layers relies on the layers below it to provide
supporting capabilities and performs support to the layers
above it.
• Such a model of layered functionality is also called a
“protocol stack” or “protocol suite”.
