2.6 The ATM Layer

As illustrated in Figure 2.15, the ATM layer provides the interface between the AAL and the physical layer. The ATM layer is responsible for relaying cells both from the AAL to the physical layer and to the AAL from the physical layer. The actual method by which the ATM layer performs this function depends on its location within an ATM network. Because an ATM network consists of endpoints and switches, the ATM layer can reside at either location. Similarly, a physical layer is required at both ATM endpoints and ATM switches.

Because a switch examines the information within an ATM cell to make switching decisions, it does not perform any adaptation functions. Thus, the ATM switch operates at layers 1 and 2, while ATM endpoints operate at layers 1 through 3 of the ATM protocol stack as shown in Figure 2.16.

Figure 2.16: The ATM Protocol Stack within a Network

When the ATM layer resides at an endpoint, it will generate idle or "empty" cells whenever there is no data to send, a function not performed by a switch. Instead, in the switch, the ATM layer is concerned with facilitating switching functions, such as examining cell header information, which enables the switch to determine where each cell should be forwarded to. For both endpoints and switches, the ATM layer performs a variety of traffic management functions, to include buffering incoming and outgoing cells as well as monitoring the transmission rate and conformance of transmission to service parameters that define a Quality of Service (QoS). At endpoints, the ATM layer also indicates to the AAL whether or not there was congestion during transmission, thus permitting higher layers to initiate congestion control.