Computer Networks

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Computer Networks
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What is a computer network

• A network consists of two or more computers that
  are linked in order to share resources , exchange
  files, or allow electronic communications.
• The computers on a network may be linked through
  cables, telephone lines, radio waves, satellites,
  or infrared light beams.

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Need of computer networks

• Resource sharing
• Peripheral Sharing
• Data sharing
• Co-operative Computation
• Sharing of Services
• Communication

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Examples of networks

• Internet
• College network
• Americal on line

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Network topology

• A network may be represented as a collection of
  nodes, some of which are connected by links.
• A given node may have links to many others.
• Network topology is determined only by the
  configuration of connections between nodes it is
  therefore a part of Graph theory.
• Distances between nodes, physical
  interconnections, (Communication by means of
  transmitted signals) transmission rates, and/or
  (Any communication that encodes a message) signal
  types are not a matter of network topology,
  although they may be effecton actual physical
  network.

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Network topology (cont.)

• Evaluation Criteria
• - Network performance and speed
• - Network reliability and redundancy
• - Security of the network
• - Acquisition and support costs
• - Criteria of the network bandwidth being met
• - Flexibility and enhanced future bandwidth
  capabilities

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Network Topology (cont.)

• Complete or fully connected
• Star
• Ring
• Bus
• Tree
• Line

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Complete Topology

• A fully connected or complete topology is a
  network topology in which there is a direct link
  between all pairs of nodes. In a fully connected
  network with n nodes, there are n(n-1)/2 direct
  links.
• Synonym fully connected mesh network.

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Complete topology
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Star topology

• A network topology in which peripheral nodes are
  connected to a central node, which rebroadcasts
  all transmissions received from any peripheral
  node to all peripheral nodes on the network,
  including the originating node. All peripheral
  nodes may thus communicate with all others by
  transmitting to, and receiving from, the central
  node only.

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Star topology (cont.)
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Ring Topology

• A network topology in which every node has
  exactly two branches connected to it. These nodes
  and branches form a ring. If one of the nodes on
  the ring fails then the ring is broken and cannot
  work. A dual ring topology has four branches
  connected to it, and is more resistant to
  failures.

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Ring topology (cont.)
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Linear Bus topology

• Bus topology is such that there is a single line
  to which all nodes are connected, and the nodes
  connect only to this bus.

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Bus topology (cont.)
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Layered Approach

• The OSI reference model is a hierarchical
  structure of seven layers that defines the
  requirements for communications between two
  computers.
• The model was defined by the International
  Standards Organization.
• It was conceived to allow interoperability
  across the various platforms offered by vendors.
  The model allows all network elements to operate
  together, regardless of who built them. By the
  late 1970's, ISO was recommending the
  implementation of the OSI model as a networking
  standard unfortunately, TCP/IP had been in use
  for years.
• Only a subset of the whole OSI model is used
  today.
• It is widely believed that much of the
  specification is too complicated and its full
  functionality has taken too long to implement,
  although there are many people that strongly
  support the OSI model.

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Purpose of OSI

• The OSI model divides the functions of a protocol
  into a series of layers.
• Each layer has the property that it only uses
  the functions of the layer below, and only
  exports functionality to the layer above.
• A system that implements protocol behavior
  consisting of a series of these layers is known
  as a 'protocol stack' or 'stack'.
• Protocol stacks can be implemented either in
  hardware or software, or a mixture of both.
• Typically, only the lower layers are implemented
  in hardware, with the higher layers being
  implemented in software.

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Physical Layer

• The physical layer defines all electrical and
  physical specifications for devices. This
  includes the layout of pins, voltages, and cable
  specifications.
• Hubs and repeaters are physical-layer devices.
  The major functions and services performed by the
  physical layer are
• o Establishment and termination of a
  connection to a communications medium.
• o Participation in the process whereby
  the communication resources are effectively
  shared among multiple users. For example,
  contention resolution and flow control.
• o Modulation, or conversion between the
  representation of digital data in user equipment
  and the corresponding signals transmitted over a
  communications channel.

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Data link layer

• The Data link layer provides the functional and
  procedural means to transfer data between network
  entities and to detect and possibly correct
  errors that may occur in the Physical layer. The
  addressing scheme is physical which means that
  the addresses are hard-coded into the network
  cards at the time of manufacture. The addressing
  scheme is flat. Note The best known example of
  this is Ethernet. Other examples of data link
  protocols are HDLC and ADCCP for point-to-point
  or packet-switched networks and LLC and Aloha for
  local area networks. This is the layer at which
  bridges and switches operate.

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Network Layer

• The Network layer provides the functional and
  procedural means of transferring variable length
  data sequences from a source to a destination via
  one or more networks while maintaining the
  quality of service requested by the Transport
  layer.
• The Network layer performs network routing, flow
  control, segmentation/desegmentation, and error
  control functions.
• The router operates at this layer -- sending data
  throughout the extended network and making the
  Internet possible, although there are layer 3 (or
  IP) switches.
• This is a logical addressing scheme - values are
  chosen by the network engineer.
• The addressing scheme is hierarchical.

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Transport Layer

• The purpose of the Transport layer is to provide
  transparent transfer of data between end users,
  thus relieving the upper layers from any concern
  with providing reliable and cost-effective data
  transfer.
• The transport layer controls the reliability of a
  given link. Some protocols are stateful and
  connection oriented. This means that the
  transport layer can keep track of the packets and
  retransmit those that fail. The best known
  example of a layer 4 protocol is TCP.

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Session Layer

• The Session layer provides the mechanism for
  managing the dialogue between end-user
  application processes.
• It provides for either duplex or half-duplex
  operation and establishes check pointing,
  adjournment, termination, and restart procedures.
  This layer is responsible for setting up and
  tearing down TCP/IP sessions.

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Presentation layer

• The Presentation layer relieves the Application
  layer of concern regarding syntactical
  differences in data representation within the
  end-user systems. MIME encoding, encryption and
  similar manipulation of the presentation of data
  is done at this layer. An example of a
  presentation service would be the conversion of
  an EBCDIC-coded text file to an ASCII-coded file.

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Application Layer

• This layer interfaces directly to and performs
  common application services for the application
  processes. The common application services
  provide semantic conversion between associated
  application processes. Examples of common
  application services include the virtual file,
  virtual terminal (for example, Telnet), and "Job
  transfer and Manipulation protocol" (JTM,
  standard ISO/IEC 8832).

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The OSI model in the reality

• Real-world protocol suites often do not strictly
  match the seven-layer model. There can be some
  argument as to where the distinctions between
  layers are drawn there is no one correct answer.
  However, most protocol suites share the concept
  of three general sections media, covering layers
  1 and 2 transport, covering layers 3 and 4, and
  application, covering layers 5 through 7.
• The DoD model, developed in the 1970s for DARPA,
  is a 4-layer model that maps closely to current
  common internet protocols. It is based on a more
  "pragmatic" approach to networking than OSI.

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Interfaces

• In addition to standards for individual protocols
  in transmission, there are also interface
  standards for different layers to talk to the
  ones above or below (usually operating-system-spec
  ific).
• Example Microsoft Windows' Winsock and Unix's
  Berkeley sockets and System V Streams are
  interfaces between applications (layers 5 and
  above) and the transport (layer 4). NDIS and ODI
  are interfaces between the media (layer 2) and
  the network protocol (layer 3).

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Examples of Layers

• Layer 7 (Application) HTTP , FTP , NFS , NTP
• Layer 6 ( Presentation) SSL ,TLS
• Layer 5 (Session) NetBios, TCP session
  establishment.
• Layer 4( Transport) NetBEUI, TCP, UDP
• Layer 3( Network) NetBEUI, IP, ICMP, IPsec,
  ARP.
• Layer 2(Data Link) Ethernet, Token Ring, PPP,
  HDLC, Frame Relay, ATM, Fibre Channel
• Layer 1( Physical ) RS-232, V.35, V.34, T1,
  10BASE-T, 100BASE-TX, ISDN, DSL

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Communicaion Methods

• A method of routing traffic between an
  originator and a destination through switching
  centers, from local users or from other switching
  centers, whereby a continuous electrical circuit
  is established and maintained between the calling
  and called stations until it is released by one
  of those stations.
• The method of establishing the connection and
  monitoring its progress and availability may
  utilize a separate control channel as in the case
  of ISDN or not as in the case of the Public
  Switched Telephone Network.

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Example switching techniques

• Circuit Switching
• Message Switching
• Packet Switching

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Circuit Switching

• Circuit switching is the most familiar technique
  used to build a communications network.
• It is used for usually for telephone networks.
• It allows communications equipment and circuits,
  to be shared among users.
• Each user has sole access to a circuit
  (functionally equivalent to a pair of copper
  wires) during network use.

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Circuit switching (cont.)

• Consider communication between two points A and D
  in a network. The connection between A and D is
  provided using (shared) links between two other
  pieces of equipment, B and C.

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Circuit switching (cont.)

• Network use is initiated by a connection phase,
  during which a circuit is set up between source
  and destination, and terminated by a disconnect
  phase. These phases, with associated timings.

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Message Switching

• Sometimes there is no need for a circuit to be
  established all the way from the source to the
  destination. Consider a connection between the
  users (A and D) in the figure below (i.e. A and
  D) is represented by a series of links (AB, BC,
  and CD).

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Message switching (cont.)

• For instance, when a telex (or email) message is
  sent from A to D, it first passes over a local
  connection (AB). It is then passed at some later
  time to C (via link BC), and from there to the
  destination (via link CD).
• At each message switch, the received message is
  stored, and a connection is subsequently made to
  deliver the message to the neighboring message
  switch.
• Message switching is also known as
  store-and-forward switching since the messages
  are stored at intermediate nodes en route to
  their destinations.

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Message Switching (cont.)

• Most message switched networks do not use
  dedicated point-to-point links and therefore a
  call must be set-up using a circuit switched
  network.

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Packet Switching

• Packet switching is similar to message switching
  using short messages.
• Any message exceeding a network-defined maximum
  length is broken up into shorter units, known as
  packets.
• For transmission the packets, each with an
  associated header, are then transmitted
  individually through the network.
• The fundamental difference in packet
  communication is that the data is formed into
  packets with a pre-defined header format (i.e.
  PCI), and well-known "idle" patterns which are
  used to occupy the link when there is no data to
  be communicated.

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Packet Switching (cont.)
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Types of networks

• LANs - Local Area Network, a computer network
  that spans a distance of tens of metres at the
  most. LANs are very common in offices but can
  also connect several offices together.
• WANs - Wide Area Network, a computer network that
  spans hundreds of metres to a few kilometres.
  Typically the campus spanning network that
  connects different departments in any University
  or larger company is called a WAN.
• MANs - Metropolitan Area Network, in essence a
  computer network that is meant to span a whole
  metropolitan area.
• The Internet - the term "The Internet" is used to
  describe the network of computer networks
  installed across the globe. They are
  interconnected and most computers connect to it
  via the Internet Service Providers.