Because a homogenous ATM network is connection-oriented, instead of just addressing a packet and sending it, a station requests that a call be made and provides the ATM address of the target machine or adapter, along with service access point (SAP) information, which identifies a particular application running on the target machine. So a connection is made from a particular application on one machine to a specified application on another machine, through the ATM cards on both machines.
This virtual circuit (VC) allows ATM to provide services such as QoS, because the VC allows applications to specify the guarantees they want. When these guarantees are made, the local adapter card, any switches in the path, whether local or wide area, and finally the adapter and application on the other machine agree to provide that QoS, such as a minimum bandwidth.