Chapter 3 OSI Model

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Chapter 3OSI Model

• The model
• Functions of the layers

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3.1 The Model

• OSI (Open Systems Interconnections) Model
• Allows complete transparency between otherwise
  incompatible systems.
• Layered Architecture
• Peer-to-peer processes
• The processes on each machine that communicate at
  a given layer.
• Control data added to the beginning or end of a
  data parcel.
• Headers Layers 6, 5, 4, 3, and 2
• Trailers Layer 2
• Interface between layers
• Interface between each pair of adjacent layers
  defines what information and services a layer
  must provide for the layer above it.
• Organization of the layers
• Network support layers physical, data link, and
  network layers
• User support layers session, presentation, and
  application layers
• Transport layer end-to-end reliable data
  transmission.

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Figure 3-1
OSI Model
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OSI Layers
Figure 3-2
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An Exchange Using the OSI Model
Figure 3-3
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3.2 Functions of the Layers

• Physical Layer
• Transmit a bit stream over a physical medium.
• Physical characteristics of interfaces and media
  (Ch. 7)
• Define types of transmission medium.
• Representation of bits
• Define the type of encoding.
• Data rate (Transmission rate bits/sec.)
• Define duration of a bit.
• Synchronization of bits
• Tx and rx are synchronized at the bit level (cf.
  clock synch.)
• Line configuration
• Point-to-point and multipoint
• Physical topology
• Mesh, star, tree, bus, and ring
• Transmission mode
• Simples, half-duplex, and full-duplex

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Figure 3-4
Physical Layer
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• Data Link Layer
• Transforms the physical layer to a reliable data
  link.
• Is responsible for node-to-node delivery.
• Framing
• L2 data unit frame
• Physical addressing
• Sender and receiver are interconnected to the
  same network.
• Flow control
• Control the volume of data considering the
  receivers situation.
• Error control
• Detect and retransmit damaged or lost frames
  (Trailer CRC bits)
• Access control
• Determine which device has control over the link
  at any given time (Medium access control, shared
  medium).

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Figure 3-5
Data Link Layer
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Figure 3-6
Data Link Layer (Example 3.1)
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Network Layer

• Responsible for the source-to-destination
  delivery of a packet possibly access multiple
  networks (links).
• Logical addressing (IPv4 address)
• Routing (cf. Router or Gateway)

Figure 3-7
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Network Layer (Example 3.2)
Figure 3-8
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Figure 3-8-continued
Network Layer (Example 3.2)
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• Transport Layer
• Responsible for source-to-destination
  (end-to-end) delivery of the entire message. gt
  Ensure that the whole message arrive intact and
  in order.
• May create a connection the two end ports gt
  connection establishment, data transfer, and
  connection release.
• Service point addressing
• L2 data unit frame
• Segmentation reassembly
• Connection control
• Connectionless and connection-oriented transport
  layer (UDP and TCP)
• Flow control
• End-to-end flow control (TCP flow control)

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Transport Layer
Figure 3-9
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Figure 3-10
Transport Layer (Example 3.3)
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Figure 3-10-continued
Transport Layer (Example 3.3)
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• Session Layer
• Dialog controller establishment, maintenance,
  and synchronization for the interaction between
  communicating systems.
• Dialog control
• Allow two systems to enter into a dialog.
• Allow the communication between two processes to
  take place either in half-duplex or full-duplex.
• Synchronization
• Allow to add checkpoints into a stream of data
  (cf. file transfer).

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Figure 3-11
Session Layer
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• Presentation Layer
• Connect with the syntax and semantics of the
  information exchanged between two systems.
• Translation
• Responsible for interoperability between these
  different encoding methods.
• Encryption Decryption
• Compression
• Reduce the number of bits to be transmitted
  (text, audio, video)

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Figure 3-12
Presentation Layer
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• Application Layer
• Enable the user (human S/W) to access the
  network
• Network virtual terminal
• S/W version of a physical terminal (log on to a
  remote host)
• File transfer, access, and management (FTAM)
• Mail services
• X.400 (message-handling services)
• Directory services
• X.500 distributed database sources

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Figure 3-13
Application Layer
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Figure 3-14
Summary of Layer Functions
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3.3 TCP/IP Protocol Suite

• Used in the Internet
• Transmission Control Protocol/Internetworking
  Protocol
• TCP/IP protocol suite does not match exactly with
  those in the OSI model
• Transport layer TCP UDP (User Datagram
  Protocol)
• Network layer IP (Internetworking Protocol)

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Figure 3.15
TCP/IP and the OSI Model
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3.6 Summary

• OSI model
• Network support layer
• Physical layer Transmit a bit stream over a
  physical medium
• Data link layer Responsible for node-to-node
  delivery.
• Network layer Responsible for the
  source-to-destination delivery of a packet
  possibly access multiple networks (links
• Transport layer
• Responsible for source-to-destination
  (end-to-end) delivery of the entire message.
• Use support layer
• Session layer Establishment, maintenance, and
  synchronization for the interaction between
  communicating systems
• Presentation layer Connect with the syntax and
  semantics of the information exchanged between
  two systems.
• Application layer Enable the user (human S/W)
  to access the network
• TCP/IP suite

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