ATM(Asynchronous Transfer Mode) , Why Cell size 53 Bytes ?,
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    ATM (Asynchronous Transfer Mode)



    Asynchronous Transfer Mode (ATM) is a standard switching technique designed to unify telecommunication and computer networks.

    ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.

    ATM is a packet network. it supports multiplexing of various signals over the same physical channel.

    It has been design to deliver voice,data and video information.


    Medium : ATM network is capable of supporting high data rates up to 155 Mbps(approx) so Transmission medium used for ATM is optical Fibers instead of coaxial cables in order to support high data rates.


    Limitations: ATM does not provide any error control or flow control at data link layer.


    Why ATM is used??

    AS asynchronous TDM is used in it so the empty slot does not appears on the output line of multiplexer.
    ATM provides Real time and Non real time services.
    Services provided as 1. service using the constant bit rate, 2. Compressed voice and video,3. IP based services available bit rate(ABR) and unspecified bit rate(UBR) services.
    Fixed packet size.By using small fixed length cells the ATM becomes so efficient that it can offer a constant data rate even though it is using packet switching techniques.






    ATM uses fixed length of packets that are known as cell.
    ATM cell packet size or cell size is 53 octets(bytes).In this Header is 5 bytes long and 48 bytes are reserved to carry the data.
    If the input is in the form of longs packets they are converted in small 53 bytes long ATM cells.
    one advantage of that is since all the packets are of same size, no packet has to wait ,this avoids the introduction of delays.


    Why cell size is 53 Bytes:

    During the standardization process a conflict arose within the CCITT as to the payload size within an ATM cell. The US wanted 64 byte payloads because it was felt optimal for US networks.The Europeans and Japanese wanted 32 payloads because it was optimal for them. In the end 48 bytes was chosen as a compromise. So 48 bytes payload plus 5 bytes header is 53 bytes total.




    ATM Protocol Architecture :

    The physical layer of the protocol involves the specifications of a transmission medium and signal encoding scheme.
    The date rate specified at this layer are b/w 25.6 Mbps and 622.08 Mbps,but it can be higher and lower than these possibly

    ATM Layer :
    This layer defines the transmission of data in fixed size cell and it also defines the logical connections.

    This layer is service dependent layer nd used for supporting the information transfer protocol not based on ATM.
    THE AAl maps the higher layer information into the ATM cell and cell is transported over the ATM network

    USER Plain :
    It is used for transferring user information along with associated control such as flow control, error control etc.

    Control Plain :
    It is supported to perform the call control and connection control functions.

    Management Plain Plain :
    It includes the management .The management plain performs management functions related to a function related to a system they includes-

    1.Provision of co-ordination b/w all plains.
    2.Layer Management.
    3.Management functions relating to resources and parameter in its protocol entities.


    ATM is a core protocol used over the SONET/SDH,backbone of the public switched telephone network (PSTN) and Integrated Services Digital Network (ISDN), but its use is declining in favour of All IP.

    It is also used in non-ISDN systems where the data rates are very high.