Understanding the
Host-to-Host
Communications Model
Building a Simple Network
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-1
Understanding Host-to-Host
Communications
Older model
– Proprietary
– Application and combinations software controlled by one
vendor
Standards-based model
– Multivendor software
– Layered approach
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-2
Why a Layered Network Model?
Reduces complexity
Standardizes interfaces
Facilitates modular engineering
Ensures interoperable
technology
Accelerates evolution
Simplifies teaching and learning
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-3
The Seven Layers of the OSI Model
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The Seven Layers of the OSI Model (Cont.)
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The Seven Layers of the OSI Model (Cont.)
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The Seven Layers of the OSI Model (Cont.)
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The Seven Layers of the OSI Model (Cont.)
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The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-9
The Seven Layers of the OSI Model (Cont.)
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Data Encapsulation
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Data De-Encapsulation
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Peer-to-Peer Communication
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TCP/IP Stack
Defines four layers
Uses different names for Layers 1
through 3
Combines Layers 5 through 7 into
single application layer
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TCP/IP Stack vs. the OSI Model
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-15
Summary
The OSI reference model defines the network functions that occur
at each layer.
The physical layer defines the electrical, mechanical, procedural,
and functional specifications for activating, maintaining, and
deactivating the physical link between end systems.
The data link layer defines how data is formatted for transmission
and how access to the physical media is controlled.
The network layer provides connectivity and path selection
between two host systems that may be located on geographically
separated networks.
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Summary (Cont.)
The transport layer segments data from the system of the sending
host and reassembles the data into a data stream on the system
of the receiving host.
The session layer establishes, manages, and terminates sessions
between two communicating hosts.
The presentation layer ensures that the information sent at the
application layer of one system is readable by the application
layer of another system.
The application layer provides network services to the
applications of the user, such as e-mail, file transfer, and terminal
emulation.
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-17
Summary (Cont.)
The information sent on a network is referred to as data or data
packets. If one computer wants to send data to another computer,
the data must first be packaged by a process called
encapsulation.
When the remote device receives a sequence of bits, the physical
layer at the remote device passes the bits to the data link layer for
manipulation. This process is referred to as de-encapsulation.
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-18
Summary (Cont.)
TCP/IP is now the most widely used protocol for a number of
reasons, including its flexible addressing scheme, its usability by
most operating systems and platforms, its many tools and utilities,
and the need to use it to connect to the Internet.
The components of the TCP/IP stack are the network access,
Internet, transport, and application layers.
The OSI model and the TCP/IP stack are similar in structure and
function, with correlation at the physical, data link, network, and
transport layers. The OSI model divides the application layer of
the TCP/IP stack into three separate layers.
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-19
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-20
