The OSI Reference Model

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The OSI Reference Model

• Chapter 2

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International Organization for Standards (ISO)

• recognized the need to create a network model
  that would help network builders implement
  networks that could communicate and work
  together--OSI Model
• The OSI model is a layered communication process
  with each layer performing a specific task.

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The OSI Reference Model
The OSI model describes how information or data
moves from one computer through a network to
another computer.
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The OSI Reference Model
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Networking concepts . . .

• Networking is the interconnection of
  workstations, peripherals, and other devices.
• In networking, it is possible for different types
  of computer systems to communicate.
• In networking, what is important is that all
  devices speak the same language or use the same
  protocol (use same set of rules).

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Source, Destination, and Data Packets . . .

• Data is sent in bits, 1s and 0s.
• Data is not the information itself but an encoded
  form of information which is a series of
  electrical impulses into which information is
  transmitted for sending.
• Small, easily transmitted units into which
  computer data is broken for transmission across a
  network are called packets.

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Source, Destination, and Data Packets . . .

• Using the OSI model, data can be called packets,
  or frames or segments.
• Source address specifies the identity of the
  computer sending the packet.
• Destination address specifies the identity of the
  computer designated to receive the packet.

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Why Data Packets?

• Computers can take turns sending packets.
• If packet is lost, only small amount of data must
  be retransmitted.
• Data can take different paths.

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Different Media Types
AIR carries light, radio, microwave
Mediamaterial through which data packets travel
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The OSI Reference Model . . .

• uses a layered model because
• it breaks network communication into smaller,
  simpler parts that are easier to develop.
• it facilitates standardization of network
  components to allow multiple-vendor development
  and support.

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The OSI Reference Model

• allows different types of network hardware and
  software to communicate with each other.
• prevents changes in one layer from affecting the
  other layers so that they can develop more
  quickly.
• breaks network communication into smaller parts
  to make learning it easier to understand.

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KNOW THE CORRECT ORDER OF THE LAYERS!
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The OSI Reference Model
Each layer has a unique function.
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The OSI Reference Model
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The OSI Reference Model
The top 3 layers are known as the appli-cation
layers because they deal with the user interface,
data formatting, and the application access.
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The OSI Reference Model
Layers 1-4 are known as the data flow layers
because they control the physical delivery of
messages over the network.
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Keyword browsers
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The OSI Reference Model

• LAYER 7 THE APPLICATION LAYER
• closest to the user
• provides network services to user applications
• does not provide services to any other OSI layer
  but rather to application processes outside the
  scope of the OSI model

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The OSI Reference Model

• LAYER 7 THE APPLICATION LAYER
• identifies and establishes the availability of
  intended communication partners
• synchronizes cooperating applications
• establishes agreement on procedures for error
  recovery and control of data integrity

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Keywords common data format
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The OSI Reference Model

• LAYER 6 THE PRESENTATION LAYER
• ensures that information sent by the application
  layer of one system is readable by the
  application layer of another system
• translates between multiple data representation
  formats by using a common data representation
  format
• concerned with data structures and negotiation of
  data transfer syntax
• responsible for compression and encryption

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Keywords dialogues and conversations
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The OSI Reference Model

• LAYER 5 THE SESSION LAYER
• establishes, manages, and terminates sessions
  between communicating hosts
• synchronizes dialog between presentation layer
  entities and manages their data exchange
• offers provisions for efficient data transfer,
  class of service, and exception reporting of
  session, presentation, and application layer
  problems
• manages data exchange between presentation layer
  entities

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Keywords reliability flow control error
correction
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The OSI Reference Model

• LAYER 4 THE TRANSPORT LAYER
• segments and reassembles data into a data stream
• concerned with how reliable transport over an
  internetwork is accomplished
• responsible for reliable network communication
  between end nodes and provides mechanisms for the
  establishment, maintenance, and termination of
  virtual circuits, transport fault detection and
  recovery, and information flow control

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Keywords path selection routing addressing
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The OSI Reference Model

• LAYER 3 THE NETWORK LAYER
• provides connectivity and path selection between
  two end systems where routing occursthese may be
  located on geographically separated networks

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Keywords frames media access control
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The OSI Reference Model

• LAYER 2 THE DATA LINK LAYER
• provides reliable transit of data across a
  physical link
• is concerned with physical (as opposed to
  logical) addressing, network topology, network
  access, error notification, ordered delivery of
  frames, and flow control

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Keywords signals and media
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The OSI Reference Model

• LAYER 1 THE PHYSICAL LAYER
• defines the electrical and functional
  specifications for the link between end systems
  (including media)
• defines voltage levels, timing of voltage
  changes, physical data rates, maximum
  transmission distances, physical connectors, and
  other, similar, attributes

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How Data is Encapsulated

• Build the data.
• Package the data for end-to-end transport.
• Append (add) the network address to the header
  (includes control information).
• Append (add) the local address to the data link
  header.
• Convert to bits for transmission.

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The Importance of TCP/IP
The U.S. Department of Defense (DoD) created the
TCP/IP reference model because it wanted a
network that could survive any conditions, even a
nuclear war.
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The TCP/IP Model

• The DoD wants its packets to get through every
  time, under any conditions, from any one point to
  any other point. It was this very difficult
  design problem that brought about the creation of
  the TCP/IP model, and which has since become the
  standard on which the Internet has grown.

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The TCP/IP Model - Application Layer

• The designers of TCP/IP felt that the higher
  level protocols should include the session and
  presentation layer details. They simply created
  an application layer that handles high-level
  protocols, issues of representation, encoding,
  and dialog control.  The TCP/IP combines all
  application-related issues into one layer, and
  assures this data is properly packaged for the
  next layer.

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The TCP/IP Model - Transport Layer
This layer deals with the quality-of-service
issues of reliability, flow control, and error
correction. One of its protocols, the
transmission control protocol (TCP), provides
excellent and flexible ways to create reliable,
well-flowing, low-error network communications.
TCP is a connection-oriented protocol. It
dialogues between source and destination while
packaging application layer information into
units called segments. Connection-oriented does
not mean that a circuit exists between the
communicating computers (that would be circuit
switching). It does mean that Layer 4 segments
travel back and forth between two hosts to
acknowledge the connection exists logically for
some period. This is known as packet switching.
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The TCP/IP Model - Internet Layer
The purpose of the Internet layer is to send
source packets from any network on the
internetwork and have them arrive at the
destination independent of the path and networks
they took to get there. The specific protocol
that governs this layer is called the Internet
protocol (IP). Best path determination and packet
switching occur at this layer. Think of it in
terms of the postal system. When you mail a
letter, you do not know how it gets there (there
are various possible routes), but you do care
that it arrives.
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The TCP/IP Model Network Access Layer
The name of this layer is very broad and somewhat
confusing. It is also called the host-to-network
layer. It is the layer that is concerned with all
of the issues that an IP packet requires to
actually make a physical link, and then to make
another physical link. It includes the LAN and
WAN technology details, and all the details in
the OSI physical and data link layers.
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Common TCP/IP Protocols

• Application Layer

• Transport Layer

• Internet Layer

• Network Access

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Common TCP/IP Protocols

• FTP - File Transfer Protocol
• HTTP - Hypertext Transfer Protocol
• SMTP - Simple Mail Transfer protocol
• DNS - Domain Name System
• TFTP - Trivial File Transfer Protocol

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Common TCP/IP Protocols
The transport layer involves two protocols -
transmission control protocol (TCP) and user
datagram protocol (UDP).
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• Similarities
• both have layers
• both have application layers, though they include
  very different services
• both have comparable transport and network layers
  
• packet-switched (not circuit-switched) technology
  is assumed
• networking professionals need to know both

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• Differences
• TCP/IP combines the presentation and session
  layer issues into its application layer
• TCP/IP combines the OSI data link and physical
  layers into one layer
• TCP/IP appears simpler because it has fewer
  layers
• TCP/IP protocols are the standards around which
  the Internet developed, so the TCP/IP model gains
  credibility just because of its protocols. In
  contrast, typically networks aren't built on the
  OSI protocol, even though the OSI model is used
  as a guide. 

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Although TCP/IP protocols are the standards with
which the Internet has grown, this curriculum
will use the OSI model for the following reasons

• It is a worldwide, generic, protocol-independent
  standard.
• It has more details, which makes it more helpful
  for teaching and learning.
• It has more details, which can be helpful when
  troubleshooting.

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Lab Activities
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Check Your Understanding Journal