Data communications

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Data communications

• By
• Prof. Raghuveer.K
• Department of Computer Science Engg
• The National Institute of Engineering,
• Mysore

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Peer-to-peer file sharing

• File sharing applications such as Napster and
  Gnutella are popular for sharing the MP3 audio
  and other files.
• Here clients can also acts as transient server
  while the application is activated.
• When a peer is interested in finding a certain
  file, it sends a query. The response provides a
  list of peer s that have the file and additional
  information such as the speed of each peers
  connection to the Internet.
• The requesting peer can then set up a TCP
  connection to one of the peers in the list and
  proceeded to retrieve the file.

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The OSI reference model

• The model proposed by ISO OSI ( Open System
  Interconnection) reference model because it
  deals with connecting open systems that is
  systems that are open for communication with
  other systems.
• The OSI model has seven layers.

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The OSI reference model

• The concept related to layers are
• i) A layer should be created where a different
  level of abstraction is needed.
• ii) Each layer should perform a well defined
  function
• Iii) The function of each layer should be chosen
  with an eye toward defining internationally
  standardized protocols.
• Iv) The layer boundaries should be chosen to
  minimise the information flow across the
  interface
• v) The number of layers should be larger enough
  that distinct functions need not be thrown
  together in the same layers out of necessity and
  small enough that the architecture does not
  become unwieldy.

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OSI reference model

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OSI reference model

• How to remember the layers
• Please Do Not Touch Sitas Pet Animal

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OSI reference model

• The physical layer deals with the transfer of
  bits over a communication channel.
• Physical layer concerned with the representation
  of signals.
• This layer is concerned with the procedures to
  set up and release the physical connections.

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OSI reference model

• The Data link layer provides the frames (blocks
  of information) across a transmission link that
  directly connects two nodes.
• Data link layer inserts a framing information in
  the sequence of transmitted bits to handle
  boundaries.
• Data link layer inserts control and address
  information in the header and check bits to
  enable recovery from transmission errors and flow
  control.

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OSI reference model

• High level data link control (HDLC) and
  Point-to-point protocol (PPP) are the two
  standard data link protocol that are wide in use.
• A flat addressing space is used to enable
  machines to listen and recognize frames that are
  destined to them.

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OSI reference model

• The network layer provides for the transfer of
  data in the form of packets across a
  communication network.
• It uses the hierarchical way of addressing
  scheme.
• Routing protocol is the procedure that is used to
  select paths across a network. The nodes in the
  network must work together to perform the routing
  effectively.
• Network layer is responsible for dealing with
  congestion that occurs from time to time in the
  network.

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OSI reference model

• When the two machines are connected to the same
  packet-switching network, single address space
  and routing procedure are used.

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OSI reference model

Courtesy Data communication network by Alberto
Leon-Garcia
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OSI reference model

• If two machines are connected to the different
  networks, the transfer of data must traverse two
  or more networks and hence internetworking
  protocols are necessary to route the data between
  gateways/routers that connects the intermediate
  networks.

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OSI reference model

Courtesy Data communication network by Alberto
Leon-Garcia
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OSI reference model

• The internetworking protocols must understand the
  differences in addressing and differences in the
  size of the packets that are handled within each
  network.

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OSI reference model

• The transport layer is responsible for the
  end-to-end transfer of messages from a process in
  the source machine to a process in the
  destination machine.
• The transport layer protocol accepts messages
  from its higher layers and prepares blocks of
  information called segments or datagram's for
  transfer between end machines.

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OSI reference model

• The transport layer provides number of services
• Transport layer may provide a connection-oriented
  service that involves the error-free transfer of
  a sequence of bytes or messages.
• The associated protocol carries out error
  detection and recovery and sequence and flow
  control.
• The transport layer is responsible for setting up
  and releasing connections across a network.

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OSI reference model

• Top four layers are end to end and involve the
  interaction of peer processes across the network.
• Lower two layers involves the interaction of
  peer-to-peer processes across a single hop.

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OSI reference model

• The session layer can be used to control the
  manner in which data are exchanged.
• Certain applications require half-duplex dialog
• Certain applications requires the introduction of
  synchronization points that can be used to mark
  the progress of an interaction and can serve as
  points from which error recovery can be initiated.

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OSI reference model

• The presentation layer provide the application
  layer with independence from difference in the
  representation of data.
• Presentation layer should convert the
  machine-dependent information provided by
  application A into a machine-dependent form
  suitable for application B.
• Because different computers use different codes
  for representing characters and integers.

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OSI reference model

• The application layer is to provide services that
  are frequently required by applications that
  involve communications.
• For example browser application use HTTP
  application layer protocol to access a WWW
  document.
• Application layers protocols have been developed
  for file transfer, virtual terminal, electronic
  mail, name service, network management and other
  applications.

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OSI reference model

• In general each layer adds a header and possibly
  a trailer to the block of information it accepts
  from the layer above.
• The next figure shows the header and trailers
  that are added as a block of application data
  works its way down the seven layers.

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OSI reference model

Courtesy Data communication networks by Alberto
Leon -Garcia
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Protocol layers in the ISO Open Systems
Interconnection (OSI) model
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OSI protocol summary
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OSI reference model

• Summary ( why layered network)
• Reduces complexity
• Standardizes interfaces
• Facilitates modular engineering
• Ensures interoperable technology
• Accelerates evolution

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Unified view of layers, protocols and services

• The development of OSI reference model leads the
  world to the development of unified view of
  layers, protocols and services.
• In each layer a process on one machine carries
  out a conversation with a peer process on the
  other machine across a peer interface. This is
  shown in the next slide.

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Courtesy Data communication networks by Alberto
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Unified view of layers, protocols and services

• The processes at layer n are referred to as layer
  n entities.
• Layer n entities communicate by exchanging
  protocol data units (PDUs). Each PDU contains
  header, which contains protocol control
  information and usually user information.
• The communication between peer processes is
  usually virtual in the sense that no direct
  communication link exists between them.

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Unified view of layers, protocols and services

• For communication to take place, the layer n1
  entities make use of the services provided by
  layer n.
• The transmission of the layer n1 PDU is done by
  passing a block of information from layer n1 to
  layer n through a software port called the layer
  n service access point (SAP) across a service
  interface.
• SAP is identified by a unique identifier.

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Unified view of layers, protocols and services

• The block of information passed between layer n
  and layer n1 entities consists of control
  information and a layer n service data unit
  (SDU), which is the layer n1 PDU itself.
• The layer n SDU, which is the layer n1 PDU, is
  encapsulated in the layer n PDU.
• The service provided by layer n typically
  accepting a block of information from layer n1,
  transferring the information to its peer process.

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Unified view of layers, protocols and services

• The service provided by the layers can be
  connection oriented or connectionless.
• A connection-oriented service has three phases
• Step 1 Establishing a connection between two
  layer n SAPs. The setup involves negotiating
  parameters like sequence numbers, flow control
• Step 2 Transferring n-SDUs using the layer n
  protocol
• Step 3 Tearing down the connection and releasing
  the various resources allocated to the connection.

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Unified view of layers, protocols and services

• In connectionless service which does not require
  a connection setup and each SDU is transmitted
  directly through the SAP.

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Unified view of layers, protocols and services

• Suppose a layer n SDU is too large to be handled
  by the layer n-1 and so segmentation and
  reassembly are applied.
• The layer n SDU is segmented into multiple layer
  n PDUs that are then transmitted using the
  services of layer n-1.
• The layer n entity at the other side must
  reassemble the original layer n SDU from the
  sequence of layer n PDUs it receives.

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Unified view of layers, protocols and services

Courtesy Data communication networks by Alberto
Leon -Garcia
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Unified view of layers, protocols and services

• It is also possible that the layer n SDUs are so
  small as to result in inefficient use of the
  layer n-1 services and so blocking and unblocking
  applied.
• The layer n entity may block several layer n SDUs
  into a single layer n PDU . The layer n entity on
  the other side must then unblock the received PDU
  into the individual SDUs.

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Unified view of layers, protocols and services

Courtesy Data communication networks by Alberto
Leon -Garcia
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Unified view of layers, protocols and services

• Multiplexing involves the sharing of a layer n
  services by multiple layer n1 users.
• N1 layer users passes its SDUs for transfer
  using the services of a single layer n entity.
• Demultiplexing is carried out by the layer n
  entity at the other end

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Unified view of layers, protocols and services

Courtesy Data communication networks by Alberto
Leon -Garcia