| Initially, the Asynchronous Transfer Mode (ATM) was introduced to provide a uniform protocol for the transmission of voice and data, offering guarantees for the required QoS (Quality of Service) parameters (such as data rate and delay) [McSp95]). In contrast to initial expectations and forecasts, the ATM network technology has not established itself in end systems, but it is widely used in core networks. First of all, ATM offers a uniform concept to support QoS (Quality of Service) in networks; QoS was attempted much later in IP-based networks. The ATM network technology is connection-oriented, which means that a connection has to be established before data can be transmitted. There are two types of connections: In a Permanent Virtual Connection (PVC), the connection throughout the network is established by the network management; a network management station extends the forwarding tables within the forwarding nodes between two endpoints of an ATM connection so that the ATM cells created by the endpoints are forwarded to the other endpoint. The second type of ATM connection is a Signaled Virtual Connection (SVC); in this connection type, the connection is established by the communicating end systems, which send connection requests and respond to such requests. In ATM jargon, packets are called cells. In contrast to IP protocol data units, an ATM cell has a fixed size, 53 bytes: 5 bytes for the packet header, 48 bytes for the payload. The 5-byte packet header includes forwarding information, as for IP frames, which allocates a cell to a connection. The ATM network technology uses a hierarchical connection concept, which distinguishes between paths and channels. Each cell is allocated to exactly one virtual path, and to exactly one virtual channel within that path, as shown in Figure 10-1. This allocation to a path and a channel is specified in two bit fields in the cell header: an 8-bit field for the Virtual Path Identifier (VPI), and a 16-bit field for the Virtual Channel Identifier (VCI). Figure 10-1. Virtual paths and channels in the ATM network technology. 
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