ATM is said to support variable bandwidth circuits. Although each cell is always sent at the same rate on the physical networks, by sending a varying number of cells every second, a circuit can send vary the number of bits sent. Data in an ATM network is transmitted in fixed sized packets called cells. ATM is circuit oriented. Before a computer on an ATM network can send any data, it must first establish a virtual circuit connecting it to the destination.
Virtual circuits are point to point and bi-directional.
Virtual circuits are established within virtual paths. A series of virtual paths can be connected to form a virtual circuit. Many circuits can be supported by one virtual path. Many virtual paths can exist over a single transmission path. ATM switches perform the routing in an ATM network.