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  • pg 1
									Telecommunication Systems
Spring 2010




                            1
                   ATM
 ATM Cell Structure !


   Payload contains the useful information of
    various applications
   In certain cell-types, the payload may
    contain signaling information or data
    required for network support


                                            2
                   ATM
 ATM Cell Structure !




   ATM is only responsible for the payload
    transfer, it is not involved in safeguarding
    the contents of the payload
   All ATM cells have a uniform structure

                                               3
                     ATM
 ATM Header Structure !!
  The header information routes the ATM cell
   through the network, however it does not
   contain the destination address


  VPI (Virtual Path Identifier) indicates the
   virtual path a particular cell refers to. VPI
   value may range (0-4096)
  VCI (Virtual Channel Identifier) indicates
   the virtual channel a particular cell refers to.
   VPI values may range (0-65536)               4
                    ATM
 ATM Header Structure !!
  PTI (Payload Type Indicator) is used to
   distinguish between cells carrying user
   information and cells carrying signaling
   information.
  CLP (Cell Loss Priority) decides whether the
   cell should be discarded, if the network is
   over loaded.
  HEC (Header Error Control) is used for
   safeguarding the header information
                                            5
                    ATM
 Virtual Paths and Virtual Channels !
   Physical paths between nodes of the ATM
    network are known as connection elements
   Connection elements are divided into two
    logical connections, virtual channels (VC)
    and virtual paths (VP)
   VC is a unidirectional channel for
    transporting the ATM cells across the
    connection element from node to node
                                            6
                   ATM
 Virtual Paths and Virtual Channels !
   Series of virtual channels between the source and
    destination is known as Virtual Channel
    Connection (VCC)
   The virtual path (VP) is a collection or bundle of
    virtual channels


   The cells may arrive at ATM switch on one VC and
    leave the ATM switch on another VC
   The channels are bundled into larger groups to
    allow for efficient switching of virtual channels
    with the same destination                         7
                  ATM
 Virtual Paths and Virtual Channels !
   Each virtual path and virtual channel
    has an identifier known as VPI and VCI
    respectively
   All VCIs and VPIs change at each
    network node and have therefore
    significance only across a particular
    connection element

                                       8
               ATM
The relationship between connection
  element, virtual paths and virtual
              channels !




                                 9
                  ATM
 Routing of ATM Cells !




                           10
                 ATM
 Information transfer via ATM cells !




                                     11
                   ATM
 Information transfer via ATM cells !
   The information transfer example of a video
    (the connection is already established)
   The higher layers perform the compression
    and encoding of the video signals
   ATM Adaptation Layer (AAL) sub divides
    the application layer data streams into
    segments


                                           12
                   ATM
 Information transfer via ATM cells !
   The ATM layer adds the header to the
    payload, this header contains the address
    information (VPI,VCI etc.), necessary for
    transport through the ATM network
   The physical layer ensures transmission by
    inserting a checksum in HEC



                                            13
                   ATM
 Information transfer via ATM cells !
   Physical layer incorporates the ATM cells in
    a transmission frame (e.g. SDH, PDH)




   Physical layer prepares the cells for
    transmission via a physical medium (e.g.
    electrical to optical conversion)

                                               14
                   ATM
 Information transfer via ATM cells !
   On the received side, (on the physical layer)
    ATM cells are regenerated from the received
    bit stream and the error free transmission is
    controlled via HEC field




                                             15
                   ATM
 Information transfer via ATM cells !
   ATM Layer of transmitting NT
   Multiplexes ATM cells coming in from the
    terminating devices
   ATM Layer ATM Switch
   Switches the ATM cells by means of
    VPI/VCI through the ATM switch and
    reassigns the corresponding VPI/VCI values
   ATM Layer of receiving NT
  Distributed the incoming ATM cells stream
    via VPI/VCI to the terminating equipment
                                           16
                   ATM
 Information transfer via ATM cells !
   ATM Layer of Receiving Terminal Equipment
   Recognizes its incoming cell and hands it over to
    the next higher layer
   ATM Adaptation Layer of Receiving Terminal
    Equipment
   Header and trailer are evaluated and the
    individual segments are integrated into one data
    stream
   Higher Layer of Receiving Terminal Equipment
  Video signal is recovered from the compressed and
    encoded data stream
                                                 17
                     ATM
 PTI
  Indicates in the first bit whether the cell
   contains user data or control data. If the cell
   contains user data, the bit is set to 0, if it
   contains control data, it is set to 1
  The second bit indicates congestion (0 = no
   congestion, 1 = congestion)
  The third bit indicates whether the cell is
   the last in a series of cells that represent a
   single frame (1 = last cell for the frame)
                                              18
                   ATM
 CLP
  Indicates whether the cell should be
   discarded if it encounters extreme
   congestion as it moves through the network
  If the CLP bit equals 1, the cell should be
   discarded in preference to cells with the
   CLP bit equal to 0



                                           19
                ATM
 Safeguarding mechanisms !!
  Because of the high transmission rates
   in ATM, complex error correction
   measures would cause considerable
   delay, this is why only the ATM header
   is safeguarded
  Safeguarding is performed with HEC
   byte which is calculated on the basis of
   the remaining 4 header octets
  Only the single bit error can be
   corrected , the rest can only be detected
                                        20
                     ATM
 Safeguarding mechanisms !!
   ATM does not support payload safeguarding,
    but it is performed in the terminal equipment




                                             21
                      ATM
 ATM Cell Types !!




                            22
                      ATM
 ATM Cell Types !!
   Valid/ Assigned Cells are assigned to
    existing connections and transmit pure
    information
   Metasignaling Cells are used for connection
    setup
   Invalid cells have multiple bit errors in
    header and are therefore discarded
   Resource Management Cells are used to
    control network resources, e.g. to adjust the
    transmit rate to the available bandwidth
                                              23
                      ATM
 ATM Cell Types !!
   OAM Cells are used for transmission
    safeguarding (e.g. error management in
    case of transmission route failure)
   Idle/unassigned Cells are inserted if there is
    no useful information to be transmitted, to
    ensure a continuous cell stream
   Resource management cell?


                                              24
Thank You




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