 the packet backbone. It is used in data backbones and voice backbones. In the access network there is stiff competition at layer 2 from the cheaper Ethernet. ATM responds well to the requirements of a packet backbone. The equally long packets make it easier to work at high speed. The QoS capabilities were included from the start of system design. For IP over ATM traffic we need a definition of the encapsulation of IP packets in ATM cells, i.e., how to put IP data inside ATM cells. See Figure 12.13. Encapsulation is performed by an adaptation layer (AAL). Most IP packets are too large to fit in a 53- byte ATM cell. Therefore, IP packets must be cut into smaller pieces in a process called segmentation before they can be encapsulated and put in ATM cells. The last router on the path of IP packets must reassemble the fragments to reconstitute the original IP packets. Segmentation and reassembly (SAR) is a complex and time-consuming process. Edge network IP routers Core network ATM switches SONET/SDH transmission Figure 12.13 IP over ATM Packet Backbone 12.7.2 THE ATM NETWORK AND ITS INTERFACE PROTOCOLS See also Chapter 8, especially Figure 8.28. In a common design with a bearer signalling system called PNNI (see below), the ATM nodes are grouped into peer groups (PGs). Each peer group consists of a group of ATM nodes that serve a particular geographical area. In order to keep the network, signalling and routing simple, the maximum number of nodes in a peer group is typically set to 10. This number includes existing and planned nodes for the geographical areas. Each PG is identified with a peer group identifier (PGI). Within a peer group a peer group leader (PGL) is selected. A peer group hierarchy is shown in Figure 12.14. The ATM Forum defines a number of interface protocols, most of them previously introduced in Chapter 8, Section 8.7.2: private user-network interface (UNI) private network-to-network interface (PNNI) public UNI public NNI B-ISDN inter-carrier interface (B-ICI) ATM inter-network interface (AINI) Se