ng the ones and zeros over the network. The ATM Layer An ATM switch performs activities at the ATM layer. It performs four main functions: switching, routing, congestion management, and multiplexing. The ATM Adaptation Layer The ATM adaptation layer (AAL) is the segmenta- tion and reassembly (SAR) layer. The native stream (whether it's real-time, analog, voice, MPEG-2 compressed video, or TCP/IP) goes through the adaptation layer, where it is segmented into 48-byte cells. Those 48-byte cells are then passed up to the first ATM switch in the network, which applies the header information that Assembles and disassembles broadband services into a stream of cells Adaptation Layer Each cell has a header that contains routing information Switches the cells around the network based on Layer routing information Physical Provides the physical transportation Layer of cells across the network Figure 7.18 ATM layers Chapter 7 Wide Area Networking defines on which path and which channel the conversation is to take place. (This speaks, again, to the connection orientation of ATM.) At the onset of the call, there is a negotiation phase, and each switch that's required to complete the call to the destination gets involved with determining whether it has a path and channel of the proper QoS to deliver on the requested call. If it does, at that time it makes a table entry that identifies what path and channel the call will take between the two switches. If along the way one of the switches can't guarantee the QoS being requested, the session is denied. ATM pro- vides an end-to-end view of the network and an assurance that all along the way, the proper QoS can be met. Again, the AAL segments the information into 48-byte cells, and each switch, in turn, applies the headers that contain the routing infor- mation, and at the receiving end, the AAL again reassembles the cells into the native stream that is understood by the end device. There are adaptation layers for various traffic types-for real-time traffic, for connection-oriented d