. The fixed size of cells allows the switching function to be removed from software into hardware with a dramatic increase in switching speed. ATM thus provides the benefits of circuit- and packet-switched net- works, hence allowing all types of traffic to be integrated onto a single net- work. Many network operators use ATM technology in their core network. In ATM networks the switches are usually configured to provide semiperma- nent data connections. By semipermanent, we mean that these connections are not dialed up by users, but controlled from the network management center by a network operator. 6.8.1 Protocol Layers of ATM ATM networks can be considered as a number of layers providing different functions. The ATM stack consists of a physical layer, ATM cell layer, and ATM adaptation layer, as shown in Figure 6.59. They do not correspond to the three lowest OSI layers. ATM networks are connection oriented, which means that there is a connection establishment phase followed by a data transfer phase. During the connection establishment phase, a path (virtual circuit) through the net- work is built up and all cells of this call then use this path. This principle was Introduction to Telecommunications Network Engineering Upper layers Layer 3, adaptation layer Layer 2, ATM (cell) layer Layer 1, physical layer Network Network UNI = User network interface system system NNI = Network node interface Figure 6.59 The protocol layers of ATM. explained in Section 6.2.4. ATM thus provides guaranteed cell sequencing but some cells in a data sequence may be lost. The cells with errors are dis- carded by the network, and it is up to the end systems to detect and recover from a cell loss. If the network supports dial-up connections, the control of virtual paths and circuits is carried out by signaling on the subscriber inter- face called the user network interface (UNI). If dial-up connections are not supported, virtual paths are set up to each network node by the network operator. The interfaces between nodes in the network 