ATM Layer

The ATM layer performs multiplexing, switching and control actions based upon the header; passing cells to and from the AAL Layer. This layer is also responsible for creation of the VCI's and VPI's.

In all, the layer is responsible for:

• Cell Construction
• Cell Reception & Header Validation
• Cell Relaying, Forwarding and Copying
• Cell Multiplexing
• Payload Type Discrimination
• Interpretation of predefined Reserved Header Values
• Cell Loss Priority Coding
• Support for multiple QoS classes
• Usage Parameter Control
• Explicit Forward Congestion Indication
• Generic Flow Control
• Connection Assignment and Removal

A virtual connection is established by finding many connection points, and linking them into a single, cohesive connection.

The ATM layer actually creates two unique versions of the cell header depending on whether the cell is part of the UNI (user-network-interface) or NNI (network-network-interface). The UNI cell header is as described before. An NNI cell header does not use the Generic Flow Control field, and instead, allocates those 4 bits to the VPI field. This means a NNI cell has a 12-bit VPI field, and allows for more Virtual Paths.

Generic Flow Control is currently defined by the standards as an "uncontrolled mode", where the GFC= 0x0. If too many non-zero GFC's are received, layer management is notified, and corrective action taken.

The Payload Type is a 3-bit field. The 1st bit is the AAL (ATM Adaptation Layer), used by AAL5 to identify the last cell in a series. The 2nd bit indicates some amount of upstream congestion. The 3rd discriminates between data and operation cells.

Payload types carrying user information may also indicate whether congestion was experienced by the Explicit Forward Congestion Indication or whether the cell contains information for the AAL.