Network Sales and Services Handbook (Cisco Press Networking Technology)

ATM is a connection-oriented technology supporting point-to-point (unicast) and point-to-multipoint (multicast) connections. Regardless of the application or the upper-layer protocols (such as TCP/IP), ATM cells are received in the same order in which they are sent.

ATM does not guarantee data delivery across the WAN because of possible congestion in the network backbone. ATM determines the handling of such congestion issues based on the Class of Service (CoS) to which the customer is subscribed.

There are two types of ATM connections: virtual paths (VPs), which contain virtual channels (VCs). A virtual channel connection (VCC) (or virtual circuit) is the basic unit, which carries a single stream of cells, in order, from user to user. A collection of virtual circuits can be bundled together into a virtual path. VPs can be created end-to-end across an ATM WAN. In this case, the ATM network does not route cells belonging to a particular virtual circuit. All cells belonging to a particular virtual path are routed the same way through the ATM network, thus resulting in faster recovery in case of major failures.