Network Architectures

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					  Network Architectures

Week 3 – OSI and The Internet
What do we want from a
communications network?

   We want to
       transfer messages from a process in one
       to a process in another computer
       reliably
       quickly
       and in an understandable form.
We also know that:
   Messages have to pass through a number of
    switching nodes from one host to another, and thus
    the network has to route messages correctly
   The various links may be of different media at
    different bandwidths
   The signals carrying data are subject to interference
    and degradation, and thus messages may be
    corrupted or lost
   The byte coding structure in one computer may be
    different from the other
    We need a series of protocols
    to address these issues

   Remember the definition of a protocol:
   “A protocol defines the format and order of
    messages exchanged between two
    communicating entities, and the actions taken
    on receipt or transmission of a message.”
   Those “communicating entities” may be as
    limited as the two devices on either end of a
    physical link.
    Network Architectures
   Data communication developed in the 60’s
   By early 70s suppliers were developing own
    architectures – IBM/SNA DEC/DECnet
   But these did not help inter-organisation data
   Two approaches:
      OSI model

      Internet’s TCP/IP
OSI – Open Systems Interconnect

   Developed by the International
    Standards Organisation in 1974
   It is a reference model
   Describes a network and a framework
    for developing network protocols
   Incorporates work done by a number of
    organisations, particularly DEC
   Not ever fully implemented
    OSI Continued
   Its real benefit is that is defines a
    layered architecture and thus the model
    is still used
   The objectives of the model were:
       Must provide a high degree of connectivity
       Must be reliable
       Must be easy to implement, to use and to
         OSI Layers
        7   – Application
        6   – Presentation
        5   – Session
        4   – Transport
        3   – Network
        2   – Data link
        1   - Physical
As a layered architecture, each layer is isolated from the others,
And thus its internals can be modified or replaced.
    Key points about a layered
   The source process only wants to know that the
    message it sent gets to the destination process
    – it does not need to know how
   The Network stack is implemented in the
    operating system of both hosts and the various
   As such the software in each operating system
    will be different code – but implementing the
    same protocol
    Application – Layer 7
   Provides an application or a service to
    the user application
   Examples are
          FTAM – File transfer Access Mode
          X.400 – message handling
          X.500 – network directory services
    Presentation – Layer 6
   Structures data in agreed format
   Carries out code conversion (ASCII to
   Carries out data compression
   Carries out data encryption
    Session – Layer 5
   Co-ordinates connection &
    disconnection of dialogs between
   Synchronises the flow of data -
   Re-establishes the connection if it fails
    Transport – Layer 4
   Provides end-to-end, error free delivery
    of messages, based on level of service
   These include:
       Error control
       Flow control
       Partitioning and reassembling messages
Network – Layer 3
   Responsible for end-to-end routing of
    data packets across the network
   Logical addressing
   Routing
   Performs network management
   Formats packages
Data Link – layer 2
   Responsible for reliable transfer of data
    across a link in frames
   Provides for error detection and control
   Organises data into frames
   Provides flow control – if receiver
    slower than transmitter
   Negotiates access control between the
    two devices
    Physical – Layer 1
   Transmits bits across the physical
   Accepts data in frames and translates
    into signals on the medium
   Concerned about the medium being
    used , the signalling scheme and the
    Why is OSI not the prevailing
   A case of De facto prevailing over De jure
   While OSI was defined, manufacturers were slow to
    implement it
   Europeans were keen, but the US had TCP/IP
   US government proclaimed GOSIP (Government OSI
    Profile) in 1992
   But TCP/IP took off with the Internet in the early 90s
   It worked, was common, and most organisations just
    accepted it
TCP/IP, Internet & The WWW
   TCP/IP are two protocols at the
    Transport and Network levels
   The Internet is a “Network of Networks”
    that use TCP/IP as key layers in its
    protocol stack
   The World Wide Web is an application
    that runs on the Internet
   TCP (Transmission Control Protocol
   IP (Internet Protocol)
   Developed at the same time as OSI, but as a
    product not an international model
   Developed for the ARPANET – Dept of
    Defence, defence contractors, Universities
    and the Military
   To enable communication across analogue
    lines, packet radios and Ethernet networks
   To be a Network of networks
TCP/IP (cont.)
   Developed by Vint Cerf & Robert Kahn
   Uni of California included it is BSD UNIX
   National Science Foundation mandated it and
    ran the backbone 1985
   While the requirements were much the same
    as for OSI, it was not built as a layered
   Many of its attributes reflect the environment
    it was developed in
   Commercial services started in the 1980’s and
    NSF stopped providing the backbone in 1995.
New Data link level protocol
added in 1990

       RFC 1149: A Standard for the Transmission of IP
        Datagrams on Avian Carriers.
   Later followed up with
       RFC 2549: IP over Avian Carriers with Quality of
    The Principles set out for
   Autonomy – a network should be able to
    work on its own without change
   Best effort service – Lost messages would be
   Stateless servers – Routers should not need
    to maintain the state of a connection
   Decentralised control – No global control over
    the Internet
Comparing The Internet & OSI
      Internet “layers”

     Application – layer 5   Message
     Transport – layer 4     Segment
     Network – layer 3       Datagram
     Data Link – layer 2     Frame
     Physical – layer 1      Bit

PDU (Protocol Data Unit)
Layers as per Kurose & Rose
    Generic functions that may be
    at each layer
   Error control
   Flow or congestion control
   Segmentation & re-assembly
   Multiplexing – higher level sessions
    sharing a single lower level connection
   Connection set-up

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