#)ATM standards define protocols that operate at Layer 2 (the data link layer) of the International Standards Organization (ISO) seven-layer Open Systems Interconnection (OSI) reference model.

#)The diagram below shows the layered architecture of the OSI model.

#)The data link layer is concerned with data transmission between two network switches.

#)This layer is not concerned with the transmission of an entire message between a source and a destination switch--this responsibility belongs to Layer 3 (the network layer).

#)Rather, the data link layer transports portions of messages (cells, in the case of ATM) between two points in the network.

#)These points may be the source and the destination of the message, or they may be only intermediate hops between the source and the destination.

#)The data link layer may divide higher level data into smaller units (cells, in this case), whose sizes are compatible with overall network requirements.

#)Layer 2 data units contain a cell header, an information field, and some method of checking for transmission errors.

Placing User Data into ATM Cells

#)Before frames can be transported across an ATM network, they must be divided into ATM cells.

#)The processes that divide the frames into cells occur at the data link layer (Layer 2).

#)Layer 2 is divided into two parts: the ATM adaptation layer (AAL) and the ATM layer. Once frames are divided into ATM cells, the cells can be transferred to Layer 1, the physical layer