Telecommunication Systems Spring 2010

Telecommunication Systems Spring 2010

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

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

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)

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

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

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

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

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

ATM • Routing of ATM Cells !

ATM • Information transfer via ATM cells !

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

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

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)

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

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

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

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)

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

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

ATM • Safeguarding mechanisms !! • ATM does not support payload safeguarding, but it is performed in the terminal equipment

ATM • ATM Cell Types !!

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

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?

Thank You

Telecommunication Systems Spring 2010