THE OSI & TCP/IP MODELS

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THE OSI TCP/IP MODELS

• Tan Alam
• School of IT
• Bond University

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The OSI Model
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Link

• http//topicmaps.bond.edu.au/mda/internet/osi-mode
  l/_at_/users/rho/InTechI/

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Physical Layer

• Protocols at this layer generate and detect
  voltage (or in the case of fiber optic
  transmission, pulses of light) so as to transmit
  and receive signals carrying data.
• The Physical layer sets the data transmission
  rate and monitors data error rates, though it
  does not provide error correction services.
• Physical network problems, such as a severed wire
  and noise interference, affect the Physical layer.

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Data Link Layer

• Controls communications between the Network layer
  and the Physical layer.
• Frame - a structured package for moving data that
  includes not only the raw data, or "payload," but
  also
• the current senders and receivers local network
  addresses (their physical address on this
  network) and
• error checking and control information.

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Data Link Layer
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Network Layer

• Looks at the original source and final
  destination Network addresses (commonly Internet
  address) and decides on the next best step to
  route the data from the source to the
  destination.
• To do this the Network layer is responsible for
  passing the data from the current source to the
  next best step destination.
• Network layer addresses,which reside at the
  Network layer of the OSI model, follow a
  hierarchical addressing scheme and can be
  assigned through operating system software.
• Network Addresses can enable a node to be
  reachable via the Internet or only within their
  own network.

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Transport Layer

• Primarily responsible for ensuring that data is
  transferred from Process A on Node A to
  Process B on Node B reliably, in the correct
  sequence, and without errors.
• Transport protocols also handle flow control, or
  the method of gauging the appropriate rate of
  transmission based on how fast the recipient can
  accept data.
• Segmentation - the process of chopping up a
  large message into smaller chunks (segments) so
  that they can move from one network to another
  network (networks can handle different size data
  units)

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Session Layer

• Session - a connection for data exchange between
  two parties.
• The Session layers functions include
• Establishing and keeping alive the
  communications link for the duration of the
  session
• Synchronizing the dialog between the two nodes
• Determining whether communications have been cut
  off, and, if so, figuring out where to restart
  transmission

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Presentation Layer

• Serves as a translator between the application
  and the network.
• At the Presentation layer, data becomes formatted
  in a schema that the network can understand this
  format varies with the type of network used.
• Encryption - the use of a mathematical routine to
  scramble data so that it can only be read by
  reversing the formula.

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Application Layer

• Provides the interface for user software to
  interact with the operating system and network
  software that enable programs to use network
  services.
• Network Protocol Suites come with application
  programs that enable the node to access and
  manage the network.
• OSI model also has the Application program
  interfaces (API) (a set of instructions that
  allows a program to interact with the operating
  system) as part of the application layer.

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Applying the OSI Model
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Hierarchy of the OSI Model
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TCP/IP Protocol Suite compared to the OSI Model

• The TCP/IP suite includes the following
• Application layer - provides authentication and
  compression services, and is roughly equivalent
  to the Application, Presentation, and Session
  layers of the OSI model.
• Transport layer - roughly corresponds to the
  Transport layer of the OSI model.
• Network layer - equivalent to the Network layer
  of the OSI model. (sometimes called the
  Internetwork or Internet Layer)
• Link layer - roughly equivalent to the Data Link
  and Physical layers of the OSI model.
• These two are frequently listed separately as DLL
  (Data Link Layer) and Physical Layer (this is
  sometimes separated out from the other protocols).

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TCP/IP Roughly Compared to the OSI Model
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Internet Protocol (IP)

• IP belongs to the Network layer of the OSI model
  and to the Internetwork layer of the TCP/IP
  suite.
• It provides information about how and where data
  should be delivered enabling it to travel through
  many Networks (over the Internetwork/Internet).
• Datagram (or Packet) - the entire information
  passed by the Network layer to the Data Link
  Layer (max length 65,535bytes). This information
  includes the portion passed down by the Transport
  layer (segment) and the IP header information.
• The IP header acts as an envelope for the segment
  and encapsulates it with information necessary
  for routers to transfer data between networks or
  subnets (see next slide).
• IP is an unreliable, connectionless protocol,
  which means that it does not guarantee delivery
  of data its main job is to route data along the
  best path.

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Hierarchy of Networks
INTERNET
Network_Z
Network_A
Network_B
.
Subnet_Aa
Subnet_Za
Subnet_Zb
Subnet_Ab
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Internet Protocol (IP)
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Transmission Control Protocol (TCP)

• Provides reliable delivery of data by
• Connection-oriented a transmission connection
  is established between source and destination
  before any data starts being sent.
• Reliable - It provides error-checking and
  retransmission notification.
• Flow control involving the rate of data being
  sent and received.
• Sequencing information to enable large messages
  to be split into smaller segments.

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Transmission Control Protocol (TCP)
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User Datagram Protocol (UDP)

• Provides unreliable delivery of data by
• Connection-less no connection is established,
  information is just packaged and sent to the
  destination without checking there are available.
• Unreliable - It provides minimal error-checking
  (of the single message) and each datagram is a
  separate message so no retransmissions are
  supported.
• Flow-control and sequencing are not needed.
• Limited size each datagram is independent so
  the size of the message being carried cannot be
  larger then one IP packet can carry

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The TCP/IP Application Layer Protocols

• Dynamic Host Configuration Protocol (DHCP) - an
  automated means of assigning a unique Network
  layer (or IP) address to nodes on a network.
• File Transfer Protocol (FTP) - used to send and
  retrieve files between nodes.
• Hypertext Transfer Protocol (HTTP) - protocol
  that enables Web browsers to issue requests to
  Web servers and interpret the response.

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The TCP/IP Application Layer Protocols

• Simple Mail Transfer Protocol (SMTP) - the
  protocol responsible for moving mail messages
  from one mail server to another over the Internet
  and other TCP/IP-based networks.
• Telnet - a terminal emulation protocol used to
  log into remote hosts using the TCP/IP protocol
  suite.

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Addressing in TCP/IP