The ATM Model

lThree layers in the ATM networking model map roughly to the physical and data-link layers of the OSI reference model:

lATM physical layer

lATM layer

lATM adaptation layer

ATM physical layer

lThe ATM physical layer is concerned with transmission of bit-stream data on the physical media.

lThis layer is divided into logical sublayers.

lThe physical medium sublayer commands the sending and receiving of the bit stream and uses timing information to synchronize the transmitted data.

lThe physical medium sublayer is dependent on the type of cabling used. ATM can transmit across SONET and T3/E3, fiber optics, shielded twisted pair (STP), and unshielded twisted pair (UTP).

The ATM layer

lCreates the virtual connection between the sending and receiving node and then switches the ATM cells through the virtual path.

lWhen two virtual connections temporarily share the same path, the ATM layer will multiplex the cells to be able to transmit them as a single bit stream, and then demultiplex the cells when the virtual connections split off into two different directions

The ATM adaptation layer

lReceives the packets from upper-layer processes and translates them into ATM cells.

lThese upper-layer packets can come from AppleTalk, TCP/IP, or IPX/SPX.

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